16202 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Parts 50, 53, and 58 

[EPA–HQ–OAR–2015–0072; FRL–8635–02– 

OAR] 

RIN 2060–AV52 

Reconsideration of the National 

Ambient Air Quality Standards for 

Particulate Matter 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final rule. 

SUMMARY: Based on the Environmental 

Protection Agency’s (EPA’s) 

reconsideration of the air quality criteria 

and the national ambient air quality 

standards (NAAQS) for particulate 

matter (PM), the EPA is revising the 

primary annual PM2.5 standard by 

lowering the level from 12.0 mg/m3 to 

9.0 mg/m3. The Agency is retaining the 

current primary 24-hour PM2.5 standard 

and the primary 24-hour PM10 standard. 

The Agency also is not changing the 

secondary 24-hour PM2.5 standard, 

secondary annual PM2.5 standard, and 

secondary 24-hour PM10 standard at this 

time. The EPA is also finalizing 

revisions to other key aspects related to 

the PM NAAQS, including revisions to 

the Air Quality Index (AQI) and 

monitoring requirements for the PM 

NAAQS. 

DATES: This final rule is effective May 6, 

2024. 

ADDRESSES: The EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2015–0072. All 

documents in the docket are listed on 

the https://www.regulations.gov 

website. Although listed in the index, 

some information is not publicly 

available, e.g., CBI or other information 

whose disclosure is restricted by statute. 

Certain other material, such as 

copyrighted material, is not placed on 

the internet and will be publicly 

available only in hard copy form. 

Publicly available docket materials are 

available electronically through https:// 

www.regulations.gov. 

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: 

Table of Contents 

The following topics are discussed in 

this preamble: 

Executive Summary 

I. Background 

A. Legislative Requirements 

B. Related PM Control Programs 

C. Review of the Air Quality Criteria and 

Standards for Particulate Matter 

1. Reviews Completed in 1971 and 1987 

2. Review Completed in 1997 

3. Review Completed in 2006 

4. Review Completed in 2012 

5. Review Initiated in 2014 

a. 2020 Proposed and Final Decisions 

b. Reconsideration of the 2020 PM NAAQS 

Final Action 

D. Air Quality Information 

1. Distribution of Particle Size in Ambient 

Air 

2. Sources and Emissions Contributing to 

PM in the Ambient Air 

3. Monitoring of Ambient PM 

4. Ambient Concentrations and Trends 

a. PM2.5 Mass 

b. PM2.5 Components 

c. PM10

d. PM10–2.5

e. UFP 

5. Characterizing Ambient PM2.5

Concentrations for Exposure 

a. Predicted Ambient PM2.5 and Exposure 

Based on Monitored Data 

b. Comparison of PM2.5 Fields in 

Estimating Exposure and Relative to 

Design Values 

6. Background PM 

II. Rationale for Decisions on the Primary 

PM2.5 Standards 

A. Introduction 

1. Background on the Current Standards 

2. Overview of the 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. Public Health Implications and At-Risk 

Populations 

c. PM2.5 Concentrations in Key Studies 

Reporting Health Effects 

i. PM2.5 Exposure Concentrations 

Evaluated in Experimental Studies 

ii. Ambient PM2.5 Concentrations in 

Locations of Epidemiologic Studies 

d. Uncertainties in the Health Effects 

Evidence 

3. Summary of Exposure and Risk 

Estimates 

a. Key Design Aspects 

b. Key Limitations and Uncertainties 

c. Summary of Risk Estimates 

B. Conclusions on the Primary PM2.5

Standards 

1. CASAC Advice 

2. Basis for the Proposed Decision 

3. Comments on the Proposed Decision 

4. Administrator’s Conclusions 

C. Decisions 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 

2. Basis for the Proposed Decision 

3. Comments on the Proposed Decision 

4. Administrator’s Conclusions 

C. Decisions on the Primary PM10 Standard 

IV. Communication of Public Health 

A. Air Quality Index Overview 

B. Air Quality Index Category Breakpoints 

for PM2.5

1. Summary of Proposed Revisions 

a. Air Quality Index Values of 50, 100, and 

150 

b. Air Quality Index Values of 200 and 

Above 

2. Summary of Significant Comments on 

Proposed Revisions 

a. Air Quality Index Values of 50, 100, and 

150 

b. Air Quality Index Values of 200 and 

Above 

c. Other Comments 

3. Summary of Final Revisions 

C. Air Quality Index Category Breakpoints 

for PM10

D. Air Quality Index Reporting 

1. Summary of Proposed Revisions 

2. Summary of Significant Comments on 

Proposed Revisions 

3. Summary of Final Revisions 

V. Rationale for Decisions on the Secondary 

PM Standards 

A. Introduction 

1. Background on the Current Standards 

a. Non-Visibility Effects 

b. Visibility Effects 

2. Overview of Welfare Effects Evidence 

a. Nature of Effects 

i. Visibility 

ii. Climate 

iii. Materials 

3. Summary of Air Quality and 

Quantitative Information 

a. Visibility Effects 

i. Target Level of Protection in Terms of a 

PM2.5 Visibility Index 

ii. Relationship Between the PM2.5

Visibility Index and the Current 

Secondary 24-Hour PM2.5 Standard 

b. Non-Visibility Effects 

B. Conclusions on the Secondary PM 

Standards 

1. CASAC Advice 

2. Basis for the Proposed Decision 

3. Comments on the Proposed Decision 

4. Administrator’s Conclusions 

C. Decisions on the Secondary PM 

Standards 

VI. Interpretation of the NAAQS for PM 

A. Amendments to Appendix K: 

Interpretation of the NAAQS for 

Particulate Matter 

B. Amendments to Appendix N: 

Interpretation of the NAAQS for PM2.5

VII. Amendments to Ambient Monitoring and 

Quality Assurance Requirements 


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1The press release for this announcement is 

available at: https://www.epa.gov/newsreleases/epareexamine-health-standards-harmful-soot-previousadministration-left-unchanged. 

2 In 2021, the Administrator announced his 

decision to reestablish the membership of the 

CASAC. The Administrator selected seven members 

to serve on the chartered CASAC, and appointed a 

PM CASAC panel to support the chartered CASAC’s 

review of the draft ISA Supplement and the draft 

PA as a part of this reconsideration (see section 

I.C.6.b below for more information). 

3More information regarding the CASAC review 

of the draft ISA Supplement and the draft PA, 

including opportunities for public comment, can be 

found in the following Federal Register notices: 86 

FR 54186, September 30, 2021; 86 FR 52673, 

September 22, 2021; 86 FR 56263, October 8, 2021; 

87 FR 958, January 7, 2022. 

A. Amendment to 40 CFR Part 50 

(Appendix L): Reference Method for the 

Determination of Fine Particulate Matter 

as PM2.5 in the Atmosphere—Addition of 

the Tisch Cyclone as an Approved 

Second Stage Separator 

B. Issues Related to 40 CFR Part 53 

(Reference and Equivalent Methods) 

C. Changes to 40 CFR Part 58 (Ambient Air 

Quality Surveillance) 

D. Incorporating Data From NextGeneration Technologies 

VIII. Clean Air Act Implementation 

Requirements for the Revised Primary 

Annual PM2.5 NAAQS 

A. Designation of Areas 

B. Section 110(a)(1) and (2) Infrastructure 

SIP Requirements 

C. Implementing the Revised Primary 

Annual PM2.5 NAAQS in Nonattainment 

Areas 

D. Implementing the Primary and 

Secondary PM10 NAAQS 

E. Prevention of Significant Deterioration 

and Nonattainment New Source Review 

Programs for the Revised Primary 

Annual PM2.5 NAAQS 

F. Transportation Conformity Program 

G. General Conformity Program 

IX. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 14094: Modernizing Regulatory 

Review 

B. Paperwork Reduction Act (PRA) 

C. Regulatory Flexibility Act (RFA) 

D. Unfunded Mandates Reform Act 

(UMRA) 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

and Safety Risks 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution or Use 

I. National Technology Transfer and 

Advancement Act (NTTAA) 

J. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations and Executive Order 14096: 

Revitalizing Our Nation’s Commitment 

to Environmental Justice for All 

K. Congressional Review Act (CRA) 

References 

Executive Summary 

This document presents the 

Administrator’s final decisions for the 

reconsideration of the 2020 final 

decision on the primary (health-based) 

and secondary (welfare-based) National 

Ambient Air Quality Standards 

(NAAQS) for Particulate Matter (PM). 

More specifically, this document 

summarizes the background and 

rationale for the Administrator’s final 

decisions to revise the primary annual 

PM2.5 standard by lowering the level 

from 12.0 mg/m3 to 9.0 mg/m3; to retain 

the current primary 24-hour PM2.5

standard (at a level of 35 mg/m3); to 

retain the primary 24-hour PM10

standard; and, not to change the 

secondary PM standards at this time. In 

reaching his final decisions, the 

Administrator considered the currently 

available scientific evidence in the 2019 

Integrated Science Assessment (2019 

ISA) and the Supplement to the 2019 

ISA (ISA Supplement), quantitative and 

policy analyses presented in the 2022 

Policy Assessment (2022 PA), advice 

from the Clean Air Scientific Advisory 

Committee (CASAC), and public 

comments on the proposal. The EPA has 

established primary and secondary 

standards for PM2.5, which includes 

particles with diameters generally less 

than or equal to 2.5 mm, and PM10, 

which includes particles with diameters 

generally less than or equal to 10 mm. 

The standards include two primary 

PM2.5 standards: an annual average 

standard, averaged over three years, 

with a level of 12.0 mg/m3, and a 24- 

hour standard with a 98th percentile 

form, averaged over three years, and a 

level of 35 mg/m3. It also includes a 

primary PM10 standard with a 24-hour 

averaging time, and a level of 150 mg/ 

m3, not to be exceeded more than once 

per year on average over three years. 

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. 

The most recent of the PM NAAQS 

was completed in December 2020. In 

that review, the EPA retained the 

primary and secondary NAAQS, 

without revision (85 FR 82684, 

December 18, 2020). Following 

publication of the 2020 final action, 

several parties filed petitions for review 

and petitions for reconsideration of the 

EPA’s final decision. 

In June 2021, the Agency announced 

its decision to reconsider the 2020 PM 

NAAQS final action.1 The EPA decided 

to reconsider the December 2020 

decision because the available scientific 

evidence and technical information 

indicated that the current standards may 

not be adequate to protect public health 

and welfare, as required by the Clean 

Air Act. The EPA noted that the 2020 

PA concluded that the scientific 

evidence and information called into 

question the adequacy of the primary 

PM2.5 standards and supported 

consideration of revising the level of the 

primary annual PM2.5 standard to below 

the current level of 12.0 mg/m3 while 

retaining the primary 24-hour PM2.5

standard (U.S. EPA, 2020b). The EPA 

also noted that the 2020 PA concluded 

that the available scientific evidence 

and information did not call into 

question the adequacy of the primary 

PM10 or secondary PM standards and 

supported consideration of retaining the 

primary PM10 standard and secondary 

PM standards without revision (U.S. 

EPA, 2020b). 

The final decisions presented in this 

document on the primary PM2.5

standards have been informed by key 

aspects of the available health effects 

evidence and conclusions contained in 

the 2019 ISA and ISA Supplement, 

quantitative exposure/risk analyses and 

policy evaluations presented in the 2022 

PA, advice from the CASAC 2 and 

public comment received as part of this 

reconsideration.3 The health effects 

evidence newly available in this 

reconsideration, in conjunction with the 

full body of evidence critically 

evaluated in the 2019 ISA, supports a 

causal relationship between long- and 

short-term exposures and mortality and 

cardiovascular effects, and the evidence 

supports a likely to be a causal 

relationship between long-term 

exposures and respiratory effects, 

nervous system effects, and cancer. The 

longstanding evidence base, including 

animal toxicological studies, controlled 

human exposure studies, and 

epidemiologic studies, reaffirms, and in 

some cases strengthens, the conclusions 

from past reviews regarding the health 

effects of PM2.5 exposures. 

Epidemiologic studies available in this 

reconsideration demonstrate generally 

positive, and often statistically 

significant, PM2.5 health effect 

associations. Such studies report 

associations between estimated PM2.5

exposures and non-accidental, 

cardiovascular, or respiratory mortality; 

cardiovascular or respiratory 

hospitalizations or emergency room 

visits; and other mortality/morbidity 

outcomes (e.g., lung cancer mortality or 

incidence, asthma development). The 

scientific evidence available in this 

reconsideration, as evaluated in the 

2019 ISA and ISA Supplement, includes 


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a number of epidemiologic studies that 

use various methods to characterize 

exposure to PM2.5 (e.g., ground-based 

monitors and hybrid modeling 

approaches) and to evaluate associations 

between health effects and lower 

ambient PM2.5 concentrations. There are 

a number of recent epidemiologic 

studies that use varying study designs 

that reduce uncertainties related to 

confounding and exposure 

measurement error. The results of these 

analyses provide further support for the 

robustness of associations between 

PM2.5 exposures and mortality and 

morbidity. Moreover, the Administrator 

notes that recent epidemiologic studies 

strengthen support for health effect 

associations at lower PM2.5

concentrations, with these new studies 

finding positive and significant 

associations when assessing exposure in 

locations and time periods with lower 

annual mean and 25th percentile 

concentrations than those evaluated in 

epidemiologic studies available at the 

time of previous reviews. Additionally, 

the experimental evidence (i.e., animal 

toxicological and controlled human 

exposure studies) strengthens the 

coherence of effects across scientific 

disciplines and provides additional 

support for potential biological 

pathways through which PM2.5

exposures could lead to the overt 

population-level outcomes reported in 

epidemiologic studies for the health 

effect categories for which a causal 

relationship (i.e., short- and long-term 

PM2.5 exposure and mortality and 

cardiovascular effects) or likely to be 

causal relationship (i.e., short- and longterm PM2.5 exposure and respiratory 

effects; and long-term PM2.5 exposure 

and nervous system effects and cancer) 

was concluded. 

The available evidence in the 2019 

ISA continues to provide support for 

factors that may contribute to increased 

risk of PM2.5-related health effects 

including lifestage (children and older 

adults), pre-existing diseases 

(cardiovascular disease and respiratory 

disease), race/ethnicity, and 

socioeconomic status. For example, the 

2019 ISA and ISA Supplement conclude 

that there is strong evidence that Black 

and Hispanic populations, on average, 

experience higher PM2.5 exposures and 

PM2.5-related health risks than nonHispanic White populations. In 

addition, studies evaluated in the 2019 

ISA and ISA Supplement also provide 

evidence indicating that communities 

with lower socioeconomic status (SES), 

as assessed in epidemiologic studies 

using indicators of SES including 

income and educational attainment are, 

on average, exposed to higher 

concentrations of PM2.5 compared to 

higher SES communities. 

The quantitative risk assessment, as 

well as policy considerations in the 

2022 PA, also inform the final decisions 

on the primary PM2.5 standards. The risk 

assessment in this reconsideration 

focuses on all-cause or nonaccidental 

mortality associated with long- and 

short-term PM2.5 exposures. The 

primary analyses focus on exposure and 

risk associated with air quality that 

might occur in an area under air quality 

conditions that just meet the current 

and potential alternative standards. The 

risk assessment estimates that the 

current primary PM2.5 standards could 

allow a substantial number of PM2.5- 

associated premature deaths in the 

United States, and that public health 

improvements would be associated with 

just meeting all of the alternative (more 

stringent) annual and 24-hour standard 

levels modeled. Additionally, the 

results of the risk assessment suggest 

that for most of the U.S., the annual 

standard is the controlling standard and 

that revision to that standard has the 

most potential to reduce PM2.5

exposure-related risk. The analyses are 

summarized in this document and in 

the proposal and are described in detail 

in the 2022 PA. 

In its advice to the Administrator, in 

its review of the 2021 draft PA, the 

CASAC concurred that the currently 

available health effects evidence calls 

into question the adequacy of the 

primary annual PM2.5 standard. With 

regard to the primary annual PM2.5

standard, the majority of the CASAC 

concluded that the level of the standard 

should be revised within the range of 

8.0 to 10.0 mg/m3, while the minority of 

the CASAC concluded that the primary 

annual PM2.5 standard should be revised 

to a level of 10.0 to 11.0 mg/m3. With 

regard to the primary 24-hour PM2.5

standard, the CASAC did not reach 

consensus on the adequacy of the 

current standard. The majority of the 

CASAC concluded that the primary 24- 

hour PM2.5 was not adequate and that 

the level of the standard should be 

revised to within the range of 25 to 30 

mg/m3, while the minority of the CASAC 

concluded that the standard was 

adequate and should be retained, 

without revision. Additionally, in their 

review of the 2019 draft PA, the CASAC 

did not reach consensus on the 

adequacy of the primary annual PM2.5

standard, with the minority 

recommending revision and the 

majority recommending the standard be 

retained. In their review of the 2019 

draft PA, the CASAC reached consensus 

regarding the adequacy of the primary 

24-hour PM2.5 standard, concluding that 

the standard should be retained. 

In considering how to revise the suite 

of primary PM2.5 standards to provide 

the requisite degree of protection, the 

Administrator recognizes that the 

current annual standard and 24-hour 

standard, together, are intended to 

provide public health protection against 

the full distribution of short- and longterm PM2.5 exposures. Further, he 

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 longterm average and short-term peak PM2.5

concentrations. 

As in 2012, the Administrator 

concludes that the most effective way to 

reduce total population risk associated 

with both long- and short-term PM2.5

exposures is to set a generally 

controlling annual standard, and to 

provide supplemental protection against 

the occurrence of peak 24-hour PM2.5

concentrations by means of a 24-hour 

standard set at the appropriate level. 

Based on the current evidence and 

quantitative information, as well as 

consideration of CASAC advice and 

public comments, the Administrator 

concludes that the current primary 

annual PM2.5 standard is not adequate to 

protect public health with an adequate 

margin of safety. The Administrator 

notes that the CASAC was unanimous 

in its advice on the 2021 draft PA 

regarding the need to revise the annual 

standard. In considering the appropriate 

level for a revised annual standard, the 

Administrator concludes that a standard 

set at a level of 9.0 mg/m3 reflects his 

judgment about placing the most weight 

on the strongest available evidence 

while appropriately weighing the 

uncertainties. 

With regard to the primary 24-hour 

PM2.5 standard, the Administrator finds 

the available scientific evidence and 

quantitative information to be 

insufficient to call into question the 

adequacy of the public health protection 

afforded by the current 24-hour 

standard. He further notes that a more 

stringent annual standard set at a level 

of 9.0 mg/m3 is expected to reduce both 

average (annual) concentrations and 

peak (daily) concentrations. The 

Administrator also notes that, in their 

review of the 2021 draft PA, the CASAC 

did not reach consensus on whether 

revisions to the primary 24-hour PM2.5

standard are warranted at this time. He 

also notes that, in their review of the 

2019 draft PA, the CASAC did reach 

consensus that the primary 24-hour 

PM2.5 standard should be retained. The 

Administrator concludes that the 24- 

hour standard should be retained to 


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4Consistent with the 2016 Integrated Review Plan 

(U.S. EPA, 2016), other welfare effects of PM, such 

as ecological effects, are being considered in the 

separate, on-going review of the secondary NAAQS 

for oxides of nitrogen, oxides of sulfur and PM. 

Accordingly, the public welfare protection provided 

by the secondary PM standards against ecological 

effects such as those related to deposition of 

nitrogen- and sulfur-containing compounds in 

vulnerable ecosystems is being considered in that 

separate review. Thus, the Administrator’s 

conclusion in this reconsideration of the 2020 final 

decision is focused only and specifically on the 

adequacy of public welfare protection provided by 

the secondary PM standards from effects related to 

visibility, climate, and materials and hereafter 

‘‘welfare effects’’ refers to those welfare effects. 

continue to provide requisite protection 

against short-term peak PM2.5

concentrations, particularly when 

considered in conjunction with the 

protection provided by the suite of 

standards and the decision to revise the 

annual standard to a level of 9.0 mg/m3. 

The primary PM10 standard is 

intended to provide public health 

protection against health effects related 

to exposures to PM10–2.5, which are 

particles with a diameter between 10 mm 

and 2.5 mm. The final decision to retain 

the current 24-hour PM10 standard has 

been informed by key aspects of the 

available health effects evidence and 

conclusions contained in the 2019 ISA, 

the policy evaluations presented in the 

2022 PA, advice from the CASAC and 

public comments. Specifically, the 

health effects evidence for PM10–2.5

exposures is somewhat strengthened 

since past reviews, although the 

strongest evidence still only provides 

support for a suggestive of, but not 

sufficient to infer, causal relationship 

with long- and short-term exposures and 

mortality and cardiovascular effects, 

short-term exposures and respiratory 

effects, and long-term exposures and 

cancer, nervous system effects, and 

metabolic effects. In reaching his final 

decision on the primary PM10 standard, 

the Administrator recognizes that, while 

the available health effects evidence has 

expanded, recent studies are subject to 

the same types of uncertainties that 

were judged to be important in previous 

reviews. He also recognizes that, in their 

review of the 2019 draft PA and the 

2021 draft PA, the CASAC generally 

agreed that it was reasonable to retain 

the primary 24-hour PM10 standard 

given the available scientific evidence, 

including retaining PM10 as the 

indicator. He concludes that the newly 

available evidence does not call into 

question the adequacy of the current 

primary PM10 standard, and retains that 

standard, without revision. 

With respect to the secondary PM 

standards, this reconsideration focuses 

on visibility, climate, and materials 

effects.4 The Administrator’s final 

decision to not change the current 

secondary standards at this time has 

been informed by key aspects of the 

currently available welfare effects 

evidence as well as the conclusions 

contained in the 2019 ISA and ISA 

Supplement; quantitative analyses of 

visibility impairment; policy 

evaluations presented in the 2022 PA; 

advice from the CASAC; and public 

comments. Specifically, the welfare 

effects evidence available in this 

reconsideration is consistent with the 

evidence available in previous reviews 

and supports a causal relationship 

between PM and visibility, climate, and 

materials effects. With regard to 

visibility effects, the Administrator 

notes that he judges that the evidence 

supports a target level of protection of 

27 dv. He further notes that the results 

of quantitative analyses of visibility 

impairment suggest that in areas that 

meet the current secondary 24-hour 

PM2.5 standard that estimated light 

extinction in terms of a 3-year visibility 

metric would be at or well below the 

target level of protection. With regard to 

climate and materials effects, while the 

evidence has expanded since previous 

reviews, significant limitations and 

uncertainties remain in the evidence. 

While the evidence has expanded since 

previous reviews, the available 

scientific evidence remains insufficient 

to allow the Administrator to make a 

reasoned judgment about what specific 

standard(s) would be requisite to protect 

against known or anticipated adverse 

effects to public welfare from PM’s 

effects on materials damage or climate.- 

In their review of the 2019 draft PA and 

the 2021 draft PA, the CASAC did not 

recommend revising the secondary PM 

standards. In considering the available 

evidence and quantitative information, 

with its inherent uncertainties and 

limitations, the Administrator judges 

that it is appropriate not to change the 

secondary PM standards at this time. 

The final revisions to the primary 

annual PM2.5 NAAQS trigger a process 

under which States (and Tribes, if they 

choose) make recommendations to the 

Administrator regarding designations, 

identifying areas of the country that 

either meet or do not meet the new or 

revised PM NAAQS. Those areas that do 

not meet the revised PM NAAQS will 

need to develop plans that demonstrate 

how they will meet the standards. As 

part of these plans, states have the 

opportunity to advance environmental 

justice, in this case for overburdened 

communities in areas with high PM 

concentrations above the NAAQS, by 

using the tools described in the current 

PM NAAQS implementation guidance 

(80 FR 58010, 58136, August 25, 2016). 

The EPA is not making changes to any 

of the current PM NAAQS 

implementation programs in this final 

rulemaking. 

On other topics, the EPA is finalizing 

two sets of changes to the PM2.5 subindex of the Air Quality Index (AQI). 

First, the EPA is continuing to use the 

approach used in the revisions to the 

AQI in 2012 (77 FR 38890, June 29, 

2012) of setting the lower breakpoints 

(50, 100 and 150) based on the levels of 

the primary annual and 24-hour PM2.5

standards. In so doing, the EPA is 

revising the AQI value of 50 to 9.0 mg/ 

m3 and is retaining the AQI values of 

100 and 150 at 35.4 mg/m3 and 55.4 mg/ 

m3, respectively. Second, the EPA is 

revising the upper AQI breakpoints (200 

and above), and replacing the linearrelationship approach used in 1999 (64 

FR 42530, August 4, 1999) to set these 

breakpoints, with an approach that more 

fully considers the PM2.5 health effects 

evidence from controlled human 

exposure and epidemiologic studies that 

has become available in the last 20 

years. The EPA is also revising the AQI 

values of 200, 300 and 500 to 125.4 mg/ 

m3, 225.4 mg/m3, and 325.4 mg/m3, 

respectively. In addition, this final rule 

revises the daily reporting requirement 

from 5 days per week to 7 days per 

week, while also reformatting appendix 

G and providing clarifications. 

With regard to monitoring-related 

activities, the EPA finalizes revisions to 

data calculations and ambient air 

monitoring requirements for PM to 

improve the usefulness and 

appropriateness of data used in 

regulatory decision making and to better 

characterize air quality in communities 

that are at increased risk of PM2.5

exposure and health risk. These changes 

are found in 40 CFR part 50 (appendices 

K, L, and N), part 53, and part 58 with 

associated appendices (A, B, C, D, and 

E). These changes include addressing 

updates in data calculations, approval of 

reference and equivalent methods, 

updates in quality assurance statistical 

calculations to account for lower 

concentration measurements, updates to 

support improvements in PM methods, 

a revision to the PM2.5 network design 

to account for at-risk populations, and 

updates to the Probe and Monitoring 

Path Siting Criteria for NAAQS 

pollutants. 

In setting the NAAQS, the EPA may 

not consider the costs of implementing 

the standards. This was confirmed by 

the Supreme Court in Whitman v. 

American Trucking Associations, 531 

U.S. 457, 465–472, 475–76 (2001), as 

discussed in section II.A of this 

document. As has traditionally been 


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5The 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). 

6Under 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.’’ 

done in NAAQS rulemaking, the EPA 

prepared a Regulatory Impact Analysis 

(RIA) to provide the public with 

information on the potential costs and 

benefits of attaining several alternative 

PM2.5 standard levels. In NAAQS 

rulemaking, the RIA is done for 

informational purposes only, and the 

final decisions on the NAAQS in this 

rulemaking are not based on 

consideration of the information or 

analyses in the RIA. The RIA fulfills the 

requirements of Executive Orders 

14094, 13563, and 12866. The RIA 

estimates the costs and monetized 

human health benefits of attaining the 

revised and two alternative annual 

PM2.5 standard levels and one 

alternative 24-hour PM2.5 standard level. 

Specifically, the RIA examines the 

revised annual standard level of 9.0 mg/ 

m3 in combination with the current 24- 

hour standard of 35 mg/m3 (i.e., 9.0/35 

mg/m3), as well as the following less and 

more stringent alternative standard 

levels: (1) An alternative annual 

standard level of 10.0 mg/m3 in 

combination with the current 24-hour 

standard (i.e., 10.0/35 mg/m3), (2) an 

alternative annual standard level of 8.0 

mg/m3 in combination with the current 

24-hour standard (i.e., 8.0/35 mg/m3), 

and (3) an alternative 24-hour standard 

level of 30 mg/m3 in combination with 

an alternative annual standard level of 

10 mg/m3 (i.e., 10.0/30 mg/m3). The RIA 

presents estimates of the costs and 

benefits of applying illustrative national 

control strategies in 2032 after 

implementing existing and expected 

regulations and assessing emissions 

reductions to meet the current annual 

and 24-hour particulate matter NAAQS 

(12.0/35 mg/m3). 

I. Background 

A. Legislative Requirements 

Two sections of the Clean Air Act 

(CAA) govern the establishment and 

revision of the NAAQS. Section 108 (42 

U.S.C. 7408) directs the Administrator 

to identify and list certain air pollutants 

and then to issue air quality criteria for 

those pollutants. The Administrator is 

to list those pollutants ‘‘emissions of 

which, in his judgment, cause or 

contribute to air pollution which may 

reasonably be anticipated to endanger 

public health or welfare’’; ‘‘the presence 

of which in the ambient air results from 

numerous or diverse mobile or 

stationary sources’’; and for which he 

‘‘plans to issue air quality 

criteria. . . .’’ (42 U.S.C. 7408(a)(1)). 

Air quality criteria are intended to 

‘‘accurately reflect the latest scientific 

knowledge useful in indicating the kind 

and extent of all identifiable effects on 

public health or welfare which may be 

expected from the presence of [a] 

pollutant in the ambient air. . . .’’ (42 

U.S.C. 7408(a)(2)). 

Section 109 [42 U.S.C. 7409] directs 

the Administrator to propose and 

promulgate ‘‘primary’’ and ‘‘secondary’’ 

NAAQS for pollutants for which air 

quality criteria are issued [42 U.S.C. 

7409(a)]. Section 109(b)(1) defines 

primary standards as ones ‘‘the 

attainment and maintenance of which in 

the judgment of the Administrator, 

based on such criteria and allowing an 

adequate margin of safety, are requisite 

to protect the public health.’’ 5 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.’’ 6

In setting primary and secondary 

standards that are ‘‘requisite’’ to protect 

public health and welfare, respectively, 

as provided in section 109(b), the EPA’s 

task is to establish standards that are 

neither more nor less stringent than 

necessary. In so doing, the EPA may not 

consider the costs of implementing the 

standards. See generally Whitman v. 

American Trucking Associations, 531 

U.S. 457, 465–472, 475–76 (2001). 

Likewise, ‘‘[a]ttainability and 

technological feasibility are not relevant 

considerations in the promulgation of 

national ambient air quality standards.’’ 

American Petroleum Institute v. Costle, 

665 F.2d 1176, 1185 (D.C. Cir. 1981); 

accord Murray Energy Corporation v. 

EPA, 936 F.3d 597, 623–24 (D.C. Cir. 

2019). 

The requirement that primary 

standards provide an adequate margin 

of safety was intended to address 

uncertainties associated with 

inconclusive scientific and technical 

information available at the time of 

standard setting. It was also intended to 

provide a reasonable degree of 

protection against hazards that research 

has not yet identified. See Lead 

Industries Association v. EPA, 647 F.2d 

1130, 1154 (D.C. Cir. 1980); American 

Petroleum Institute v. Costle, 665 F.2d at 

1186; Coalition of Battery Recyclers 

Ass’n v. EPA, 604 F.3d 613, 617–18 

(D.C. Cir. 2010); Mississippi v. EPA, 744 

F.3d 1334, 1353 (D.C. Cir. 2013). Both 

kinds of uncertainties are components 

of the risk associated with pollution at 

levels below those at which human 

health effects can be said to occur with 

reasonable scientific certainty. Thus, in 

selecting primary standards that include 

an adequate margin of safety, the 

Administrator is seeking not only to 

prevent pollution levels that have been 

demonstrated to be harmful but also to 

prevent lower pollutant levels that may 

pose an unacceptable risk of harm, even 

if the risk is not precisely identified as 

to nature or degree. The CAA does not 

require the Administrator to establish a 

primary NAAQS at a zero-risk level or 

at background concentration levels, see 

Lead Industries Ass’n v. EPA, 647 F.2d 

at 1156 n.51, Mississippi v. EPA, 744 

F.3d at 1351, but rather at a level that 

reduces risk sufficiently so as to protect 

public health with an adequate margin 

of safety. 

In addressing the requirement for an 

adequate margin of safety, the EPA 

considers such factors as the nature and 

severity of the health effects involved, 

the size of the sensitive population(s), 

and the kind and degree of 

uncertainties. The selection of any 

particular approach to providing an 

adequate margin of safety is a policy 

choice left specifically to the 

Administrator’s judgment. See Lead 

Industries Ass’n v. EPA, 647 F.2d at 

1161–62; Mississippi v. EPA, 744 F.3d at 

1353. 

Section 109(d)(1) of the Act requires 

the review every five years of existing 

air quality criteria and, if appropriate, 

the revision of those criteria to reflect 

advances in scientific knowledge on the 

effects of the pollutant on public health 

and welfare. Under the same provision, 

the EPA is also to review every five 

years and, if appropriate, revise the 

NAAQS, based on the revised air quality 

criteria. Section 109(d)(1) also provides 

that the Administrator may review and 

revise criteria or promulgate new 

standards earlier or more frequently. 

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 


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7Prior to the review initiated in 2007 (see below), 

the AQCD provided the scientific foundation (i.e., 

the air quality criteria) for the NAAQS. Beginning 

in that review, the Integrated Science Assessment 

(ISA) has replaced the AQCD. 8PM10 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. 

9The 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). 

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. 

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. 

B. Related PM Control Programs 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once the 

EPA has established them. Under 

section 110, Part C, and Part D, Subparts 

1 and 4 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 the NAAQS for PM 

through control programs directed to 

sources of the pollutants involved. The 

States, in conjunction with the EPA, 

also administer the prevention of 

significant deterioration of air quality 

program that covers these pollutants 

(see 42 U.S.C. 7470–7479). In addition, 

Federal programs provide for or result 

in nationwide reductions in emissions 

of PM and its precursors under Title II 

of the Act, 42 U.S.C. 7521–7574, which 

involves controls for motor vehicles and 

nonroad engines and equipment; the 

new source performance standards 

under section 111 of the Act, 42 U.S.C. 

7411; and the national emissions 

standards for hazardous pollutants 

under section 112 of the Act, 42 U.S.C. 

7412. 

C. Review of the Air Quality Criteria and 

Standards for Particulate Matter 

1. Reviews Completed in 1971 and 1987 

The EPA first established NAAQS for 

PM in 1971 (36 FR 8186, April 30, 

1971), based on the original Air Quality 

Criteria Document (AQCD) (DHEW, 

1969).7 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, 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.8 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 the 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; 9 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 

PM NAAQS, petitions for review were 

filed by several parties, addressing a 

broad range of issues. In May 1999, the 

U.S. Court of Appeals for the District of 

Columbia Circuit (D.C. Circuit) upheld 

the EPA’s decision to establish fine 

particle standards and to regulate coarse 

particle pollution, but vacated the 1997 

PM10 standards, concluding that the 

EPA had not provided a reasonable 

explanation justifying use of PM10 as an 

indicator for coarse particles. American 

Trucking Associations, Inc. v. EPA, 175 

F. 3d 1027 (D.C. Cir. 1999). 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. 


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10Prior 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. 

11The history of the NAAQS review process, 

including revisions to the process, is discussed at 

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

12The EPA also eliminated the option for spatial 

averaging. 

The D.C. Circuit also addressed more 

general issues related to the NAAQS, 

including issues related to the 

consideration of costs in setting NAAQS 

and the EPA’s approach to establishing 

the levels of NAAQS. Regarding the cost 

issue, the court reaffirmed prior rulings 

holding that in setting NAAQS the EPA 

is ‘‘not permitted to consider the cost of 

implementing those standards.’’ 

American Trucking Associations v. 

EPA, 175 F. 3d at 1040–41. Regarding 

the levels of NAAQS, the court held that 

the EPA’s approach to establishing the 

level of the standards in 1997 (i.e., both 

for PM and for the ozone NAAQS 

promulgated on the same day) effected 

‘‘an unconstitutional delegation of 

legislative authority.’’ American 

Trucking Associations v. EPA, 175 F. 3d 

at 1034–40. Although the court stated 

that ‘‘the factors EPA uses in 

determining the degree of public health 

concern associated with different levels 

of ozone and PM are reasonable,’’ it 

remanded the rule to the EPA, stating 

that when the EPA considers these 

factors for potential non-threshold 

pollutants ‘‘what EPA lacks is any 

determinate criterion for drawing lines’’ 

to determine where the standards 

should be set. 

The D.C. Circuit’s holding on the cost 

and constitutional issues were appealed 

to the United States Supreme Court. In 

February 2001, the Supreme Court 

issued a unanimous decision upholding 

the EPA’s position on both the cost and 

constitutional issues. Whitman v. 

American Trucking Associations, 531 

U.S. 457, 464, 475–76. On the 

constitutional issue, the Court held that 

the statutory requirement that NAAQS 

be ‘‘requisite’’ to protect public health 

with an adequate margin of safety 

sufficiently guided the EPA’s discretion, 

affirming the EPA’s approach of setting 

standards that are neither more nor less 

stringent than necessary. 

The Supreme Court remanded the 

case to the D.C. Circuit for resolution of 

any remaining issues that had not been 

addressed in that court’s earlier rulings. 

Id. at 475–76. In a March 2002 decision, 

the D.C. Circuit rejected all remaining 

challenges to the standards, holding that 

the EPA’s PM2.5 standards were 

reasonably supported by the 

administrative record and were not 

‘‘arbitrary and capricious.’’ American 

Trucking Associations v. EPA, 283 F. 3d 

355, 369–72 (D.C. Cir. 2002). 

3. Review Completed in 2006 

In October 1997, the EPA published 

its plans for the third periodic review of 

the air quality criteria and NAAQS for 

PM (62 FR 55201, October 23, 1997). 

After the CASAC and public review of 

several drafts, the EPA’s National Center 

for Environmental Assessment (NCEA) 

finalized the AQCD in October 2004 

(U.S. EPA, 2004a). 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).10 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. The thenAdministrator 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. On 

February 24, 2009, the D.C. Circuit 

issued its opinion in the case American 

Farm Bureau Federation v. EPA, 559 F. 

3d 512 (D.C. Cir. 2009). The court 

remanded the primary annual PM2.5

NAAQS to the EPA because the Agency 

had failed to adequately explain why 

the standards provided the requisite 

protection from both short- and longterm exposures to fine particles, 

including protection for at-risk 

populations. Id. at 520–27. With regard 

to the standards for PM10, the court 

upheld the EPA’s decisions to retain the 

24-hour PM10 standard to provide 

protection from thoracic coarse particle 

exposures and to revoke the annual 

PM10 standard. Id. at 533–38. With 

regard to the secondary PM2.5 standards, 

the court remanded the standards to the 

EPA because the Agency failed to 

adequately explain why setting the 

secondary PM standards identical to the 

primary standards provided the 

required protection for public welfare, 

including protection from visibility 

impairment. Id. at 528–32. The EPA 

responded to the court’s remands as part 

of the next review of the PM NAAQS, 

which was initiated in 2007 (discussed 

below). 

4. Review Completed in 2012 

In June 2007, the EPA initiated the 

fourth periodic review of the air quality 

criteria and the PM NAAQS by issuing 

a call for information (72 FR 35462, June 

28, 2007). Based on the NAAQS review 

process, as revised in 2008 and again in 

2009,11 the EPA held science/policy 

issue workshops on the primary and 

secondary PM NAAQS (72 FR 34003, 

June 20, 2007; 72 FR 34005, June 20, 

2007), and prepared and released the 

planning and assessment documents 

that comprise the review process (i.e., 

Integrated Review Plan, (IRP; U.S. EPA, 

2008), Integrated Science Assessment 

(ISA; U.S. EPA, 2009a), Risk and 

Exposure Assessment (REA) planning 

documents for health and welfare (U.S. 

EPA, 2009b, U.S. EPA, 2009c), a 

quantitative health risk assessment (U.S. 

EPA, 2010a) and an urban-focused 

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

and a Policy Assessment (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 12 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 


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13Consistent with the primary standard, the EPA 

eliminated the option for spatial averaging with the 

annual standard. 

14Announcement available at: https://

www.regulations.gov/document/EPA-HQ-OAR2015-0072-0223. 

and annual PM2.5 standards 13 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). 

5. Review Initiated in 2014 

In December 2014, the EPA 

announced the initiation of the current 

periodic review of the air quality criteria 

for PM and of the PM2.5 and PM10

NAAQS and issued a call for 

information (79 FR 71764, December 3, 

2014). On February 9 to 11, 2015, the 

EPA’s NCEA and OAQPS held a public 

workshop to inform the planning for the 

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 

the 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 the 

review. This workshop provided for a 

public discussion of the key science and 

policy-relevant issues around which the 

EPA structured the review of the PM 

NAAQS and of the most meaningful 

new scientific information that would 

be available in the review to inform 

understanding of these issues. 

The input received at the workshop 

guided the EPA staff in developing a 

draft IRP, which was reviewed by the 

CASAC Particulate Matter Panel and 

discussed on public teleconferences 

held in May 2016 (81 FR 13362, March 

14, 2016) and August 2016 (81 FR 

39043, June 15, 2016). Advice from the 

CASAC, supplemented by the 

Particulate Matter Panel, and input from 

the public were considered in 

developing the final IRP (U.S. EPA, 

2016). The final IRP discusses the 

approaches to be taken in developing 

key scientific, technical, and policy 

documents in the review and the key 

policy-relevant issues that frame the 

EPA’s consideration of whether the 

primary and/or secondary NAAQS for 

PM should be retained or revised. 

In May 2018, the then-Administrator 

issued a memorandum announcing the 

Agency’s intention to conduct the 

review of the PM NAAQS in such a 

manner as to ensure that any necessary 

revisions were finalized by December 

2020 (Pruitt, 2018). Following this 

memo, on October 10, 2018, the thenAdministrator 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).14

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 then-Administrator dated 

April 11, 2019 (Cox, 2019a). The EPA 

addressed these comments in the final 

ISA, which was released in December 

2019 (U.S. EPA, 2019a). 

The EPA released the draft PA in 

September 2019 (84 FR 47944, 

September 11, 2019). The draft PA 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 

October 2019 (84 FR 51555, September 

30, 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 then-Administrator dated December 

16, 2019 (Cox, 2019b). 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. Some 

CASAC members expressed support for 

retaining the current primary annual 

PM2.5 standard while other members 

expressed support for revising that 

standard in order to increase public 

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

letter). These views are described in 

greater detail in the letter to the thenAdministrator (Cox, 2019b) and in the 

notice of final rulemaking (85 FR 

82706–82707, December 18, 2020), as 

well as below. With regard to the 

secondary standards, the CASAC 

recommended retaining the current 

standards. In response to the CASAC’s 

comments, the 2020 final PA 

incorporated a number of changes (Cox, 

2019b, U.S. EPA, 2020b), as described in 

detail in section I.C.5 of the 2020 

proposal document (85 FR 24100, April 

30, 2020). 

a. 2020 Proposed and Final Actions 

On April 14, 2020, the EPA 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’s final decision on the 

PM NAAQS was published in the 

Federal Register on December 18, 2020 

(85 FR 82684, December 18, 2020). In 

the 2020 rulemaking, the EPA retained 

the primary and secondary PM2.5 and 

PM10 standards, without revision. The 

then-Administrator’s rationale for his 

decisions is described in more detail in 

section II, III, and V below, and is 

briefly summarized here. 

In reaching his final decision to retain 

the primary annual and 24-hour PM2.5

standards, the then-Administrator 

considered the available scientific 

evidence, quantitative information, 

CASAC advice, and public comments in 

his supporting rationale in the 2020 

final action (85 FR 82714, December 18, 

2020). In so doing, he concluded that 

the available controlled human 

exposure studies did not provide 

support for additional public health 

protection against exposures to peak 

PM2.5 concentrations, beyond the 

protection provided by the combination 

of the current primary annual and 24- 

hour PM2.5 standards. He also noted that 

the available epidemiologic studies did 

not indicate that associations in those 

studies are strongly influenced by 

exposures to peak concentrations in the 

air quality distribution and thus did not 

indicate the need for additional 

protection against short-term exposures 

to peak PM2.5 concentrations. 

Accordingly, and taking into account 

consensus CASAC advice to retain the 

current primary 24-hour PM2.5 standard, 

the then-Administrator concluded the 

primary 24-hour PM2.5 standard should 

be retained. 

With respect to the annual PM2.5

standard, the then-Administrator 

recognized that important uncertainties 

and limitations that were present in 

epidemiologic studies in previous 


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15See California v. EPA, (D.C. Cir., No. 21–2014 

consolidated with Nos. 21–1027, 21–1054). 

16See https://www.whitehouse.gov/briefing-room/ 

presidential-actions/2021/01/20/executive-orderprotecting-public-health-and-environment-andrestoring-science-to-tackle-climate-crisis/. 

17See https://www.whitehouse.gov/briefing-room/ 

statements-releases/2021/01/20/fact-sheet-list-ofagency-actions-for-review/. 

18The press release for this announcement is 

available at: https://www.epa.gov/newsreleases/epareexamine-health-standards-harmful-soot-previousadministration-left-unchanged. 

19The list of provisionally considered studies is 

included in Appendix A to the 2020 Response to 

Comments document (U.S. EPA, 2020a). 

reviews remained in the evidence 

assessed in the 2019 ISA. In considering 

the epidemiologic evidence, the thenAdministrator noted that: (1) The 

reported mean concentration in the 

majority of the key U.S. epidemiologic 

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 primary annual PM2.5 standard 

of 12 mg/m3; (3) air quality analyses 

show the study means to be lower than 

their corresponding design by 10–20%; 

and (4) that these analyses must be 

considered in light of uncertainties 

inherent in the epidemiologic evidence. 

The then-Administrator further 

considered other available information, 

including the risk assessment, 

accountability studies, and controlled 

human exposure studies, and found 

that, in considering all of the evidence 

together along with advice from the 

CASAC, the suite of primary PM2.5

standards were requisite to protect 

public health with an adequate margin 

of safety, and should be retained, 

without revision. 

With regard to the primary PM10

standard, the then-Administrator noted 

that the expanded body of evidence has 

broadened the range of effects that have 

been linked with PM10–2.5 exposures. In 

light of that information, as well as 

continued uncertainties in the evidence 

and advice from the CASAC to retain 

the standard, the then-Administrator 

judged 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 (85 FR 82725, 

December 18, 2020). 

With regard to the secondary PM 

standards, the then-Administrator 

concluded that there was insufficient 

information available to establish any 

distinct secondary PM standards to 

address climate and materials effects of 

PM. For visibility effects, he found that 

in the absence of a monitoring network 

for direct measurement of light 

extinction, a calculated light extinction 

indicator that utilizes the IMPROVE 

algorithms continued to provide a 

reasonable basis for defining a target 

level of protection against PM-related 

visibility impairment. He further found 

that a visibility index with a 24-hour 

averaging time was reasonable based on 

its stability and suitability for 

representing subdaily periods, and a 

form based on the 3-year average of 

annual 90th percentile values was 

reasonable based on its stability and that 

it represents the median of the 20 

percent worst visibility days which are 

targeted under the Regional Haze 

program. With regard to the level of a 

visibility index, the then-Administrator 

judged it 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, taking into 

consideration the variability, limitations 

and uncertainties of the public 

preference studies. The thenAdministrator judged that the secondary 

24-hour PM2.5 standard with its level of 

35 mg/m3 would provide at least the 

target level of protection for visual air 

quality of 30 dv which he judged 

appropriate. Accordingly, taking into 

consideration the advice of the CASAC 

to retain the current secondary PM 

standards, the then-Administrator found 

the current secondary standards provide 

the requisite degree of protection and 

that they should be retained (85 FR 

82742, December 18, 2020). 

Following publication of the 2020 

final action, several parties filed 

petitions for review and petitions for 

reconsideration of the EPA’s final 

decision. The petitions for review were 

filed in the D.C. Circuit and the Court 

consolidated the cases.15 Following 

EPA’s decision to reconsider the 2020 

final decision, the Court ordered the 

consolidated cases to be held in 

abeyance. 

b. Reconsideration of the 2020 PM 

NAAQS Final Action 

Executive Order 13990 directed 

review of certain agency actions (86 FR 

7037, January 25, 2021).16 An 

accompanying fact sheet provided a 

non-exclusive list of agency actions that 

agency heads should review in 

accordance with that order, including 

the 2020 Particulate Matter NAAQS 

Decision.17

On June 10, 2021, the Agency 

announced its decision to reconsider the 

2020 PM NAAQS final action because 

the available scientific evidence and 

technical information indicate that the 

current standards may not be adequate 

to protect public health and welfare, as 

required by the Clean Air Act.18 The 

Administrator reached this decision in 

part based on the fact that the EPA 

noted that the 2020 PA concluded that 

the scientific evidence and information 

called into question the adequacy of the 

primary annual PM2.5 standard and 

supported revising the level to below 

the current level of 12.0 mg/m3 while 

retaining the primary 24-hour PM2.5

standard (U.S. EPA, 2020b). The EPA 

also noted that the 2020 PA concluded 

that the available scientific evidence 

and information supported retaining the 

primary PM10 standard and secondary 

PM standards without revision (U.S. 

EPA, 2020b). 

The EPA staff conclusions detailed in 

the 2020 PA in combination with the 

CASAC advice that informed the 

Administrator’s decisions regarding the 

2020 final action, studies highlighted by 

public comments on the 2020 proposal, 

and the numerous studies published 

since the literature cutoff date of the 

2019 ISA all informed the scope of the 

reconsideration. 

In its review of the 2019 draft PA, 

some members of the CASAC had 

recommended that greater attention 

should be given to accountability 

studies and epidemiologic studies that 

employ alternative methods for 

confounder control (also referred to as 

causal inference or causal modeling 

studies) in order to ‘‘more fully account 

for effects of confounding, measurement 

and estimation errors, model 

uncertainty, and heterogeneity’’ in 

epidemiologic studies (Cox, 2019b, p. 8 

of consensus responses). In addition, 

public commenters submitted a number 

of recent studies published after the 

literature cutoff date for the 2019 ISA 

that would have been considered within 

the scope of the 2019 ISA. While the 

EPA provisionally considered these 

studies in responding to public 

comments,19 it was determined that, at 

the time of the 2020 final action, these 

studies were generally consistent with 

the evidence assessed in the 2019 ISA 

(85 FR 82690, December 18, 2020; U.S. 

EPA, 2020a). As such, and consistent 

with previous NAAQS reviews, the EPA 

concluded that the new studies did 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, and therefore, reopening of the 

air quality criteria was not warranted 

(85 FR 82691, December 18, 2020). 

However, at that time, the EPA 


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20The press release for this announcement is 

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

administrator-regan-directs-epa-reset-criticalscience-focused-federal-advisory. 

21The press release for this announcement is 

available at: https://www.epa.gov/newsreleases/epaannounces-selections-charter-members-clean-airscientific-advisory-committee. 

22The list of members of the chartered CASAC 

and their biosketches are available at: https://

casac.epa.gov/ords/sab/r/sab_apex/casac/ 

mems?p14_

committeeon=2021%20CASAC%20PM%20Panel

&session=17433386035954. 

23The list of members of the PM CASAC panel 

and their biosketches are available at: https://

casac.epa.gov/ords/sab/ 

f?p=105:14:9979229564047:::14:P14_

COMMITTEEON:2021%20CASAC

%20PM%20Panel. 

24As described in section 1.2.1 of the ISA 

Supplement: ‘‘In considering the public health 

protection provided by the current primary PM2.5

standards, and the protection that could be 

provided by alternatives, [the U.S. EPA, within the 

2020 PM PA] emphasized health outcomes for 

which the ISA determined that the evidence 

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

relationship with PM2.5 exposures’’ (U.S. EPA, 

2020b). Although the 2020 PA initially focused on 

this broader set of evidence, the basis of the 

discussion on potential alternative standards 

primarily focused on health effect categories where 

the 2019 PM ISA concluded a ‘causal relationship’ 

(i.e., short- and long-term PM2.5 exposure and 

cardiovascular effects and mortality) as reflected in 

Figures 3–7 and 3–8 of the 2020 PA (U.S. EPA, 

2020b).’’ 

25As described in section 1.2.1 of the ISA 

Supplement: ‘‘The 2019 PM ISA concluded a 

‘causal relationship’ for each of the welfare effects 

categories evaluated (i.e., visibility, climate effects 

and materials effects). While the 2020 PA 

considered the broader set of evidence for these 

effects, for climate effects and material effects, it 

concluded that there remained ‘substantial 

uncertainties with regard to the quantitative 

relationships with PM concentrations and 

concentration patterns that limit[ed] [the] ability to 

quantitatively assess the public welfare protection 

provided by the standards from these effects’ (U.S. 

EPA, 2020b).’’ 

recognized that its ‘‘provisional 

consideration of these studies did not 

and could not provide the kind of indepth critical review’’ (85 FR 82690, 

December 18, 2020) that studies 

undergo in the development of an ISA. 

In preparing to reconsider the 2020 

final decision for the PM NAAQS, the 

Agency revisited the need to reopen the 

air quality criteria, given the amount of 

time that had passed since the literature 

cutoff date of the 2019 ISA (i.e., 

approximately January 2018) and the 

volume of literature that had become 

available, including those studies 

provisionally considered in responding 

to comments in 2020. In so doing, the 

EPA preliminarily concluded that at 

least some of these studies were likely 

to be relevant to its reconsideration of 

the air quality criteria and the PM 

NAAQS and that, in considering public 

comments on any proposed decisions 

for the reconsideration, these studies 

were likely to be raised by public 

commenters and would potentially 

warrant a reopening of the air quality 

criteria. For example, on February 16, 

2021, the EPA received two petitions to 

reconsider the PM NAAQS. One 

petition objected to the EPA’s 

provisional consideration of studies 

submitted in public comments on the 

2020 proposal and suggested that the 

provisional consideration was 

inadequate because the studies could be 

important in determining whether the 

existing standards are adequately 

protective. See, Petition for 

Reconsideration of National Ambient 

Air Quality Standards for Particulate 

Matter, submitted by American Lung 

Association, et al, dated Feb. 16, 2020. 

The other petition identified a number 

of new studies, including one 

epidemiologic study that was published 

after the provisional consideration was 

completed that could further inform the 

concern expressed by the CASAC that 

associations reported in epidemiologic 

studies do not adequately account for 

‘‘uncontrolled confounding and other 

potential sources of error and bias.’’ See 

Petition for Reconsideration of ‘‘Review 

of the National Ambient Air Quality 

Standards for Particulate Matter,’’ 

submitted by the State of California, 

dated Feb. 16, 2020. This was also an 

uncertainty noted by the thenAdministrator in the 2020 decision, who 

also recognized ‘‘that methodological 

study designs to address confounding, 

such as causal inference methods, are an 

emerging field of study.’’ Thus, the 

Agency concluded it was appropriate to 

reconsider not only the standards but 

also the air quality criteria, in light of 

public comments during the 2020 PM 

NAAQS proposal and recent studies 

published since the cutoff date of the 

2019 ISA, as reflected in petitions. In 

deciding to reopen the air quality 

criteria, the Agency concluded it was 

reasonable to focus on studies that were 

most likely to inform decisions on the 

appropriate standard, but not to reassess 

areas which, based on the assessment of 

available science published since the 

cutoff date of the 2019 ISA and through 

2021, were judged unlikely to have new 

information that would be useful for the 

Administrator’s decision making. The 

Agency accordingly announced that, in 

support of the reconsideration, it would 

develop a supplement to the 2019 ISA 

and a revised PA. 

The EPA also explained that the draft 

ISA Supplement and draft PA would be 

reviewed at a public meeting by the 

CASAC, and the public would have 

opportunities to comment on these 

documents during the CASAC review 

process, as well as to provide input 

during the rulemaking through the 

public comment process and public 

hearings on the proposed rulemaking. 

On March 31, 2021, the Administrator 

announced his decision to reestablish 

the membership of the CASAC to 

‘‘ensure the agency received the best 

possible scientific insight to support our 

work to protect human health and the 

environment.’’ 20 Consistent with this 

memorandum, a call for nominations of 

candidates to the EPA’s chartered 

CASAC was published in the Federal 

Register (86 FR 17146, April 1, 2021). 

On June 17, 2021, the Administrator 

announced his selection of the seven 

members to serve on the chartered 

CASAC.21 22 Additionally, a call for 

nominations of candidates to a PMspecific panel was published in the 

Federal Register (86 FR 33703, June 25, 

2021). The members of the PM CASAC 

panel were announced on August 30, 

2021.23

The draft ISA Supplement was 

released in September 2021 (U.S. EPA, 

2021a; 86 FR 54186, September 30, 

2021), and included a discussion of the 

rationale and scope of the Supplement. 

As explained therein, the ISA 

Supplement focuses on a thorough 

evaluation of some studies that became 

available after the literature cutoff date 

of the 2019 ISA that could either further 

inform the adequacy of the current PM 

NAAQS or address key scientific topics 

that have evolved since the literature 

cutoff date for the 2019 ISA. In selecting 

the health effects to evaluate within the 

ISA Supplement, the EPA focused on 

health effects for which the evidence 

supported a ‘‘causal relationship’’ 

because those were the health effects 

that were most useful in informing 

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

2022a, section 1.2.1).24 Consistent with 

the rationale for the focus on certain 

health effects, in selecting the nonecological welfare effects to evaluate 

within the ISA Supplement, the EPA 

focused on the non-ecological welfare 

effects for which the evidence 

supported a ‘‘causal relationship’’ and 

for which quantitative analyses could be 

supported by the evidence because 

those were the welfare effects that were 

most useful in informing conclusions in 

the 2020 PA.25 Specifically, for nonecological welfare effects, the focus 

within the ISA Supplement is on 

visibility effects. The ISA Supplement 

also considers recent health effects 

evidence that addresses key scientific 

topics where the literature has evolved 

since the 2020 review was completed, 


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26These key scientific topics include 

experimental studies conducted at near-ambient 

concentrations, epidemiologic studies that 

employed alternative methods for confounder 

control or conducted accountability analyses, 

studies that assess the relationship between PM2.5

exposure and severe acute respiratory syndrome 

coronavirus 2 (SARS–CoV–2) infection and 

coronavirus disease 2019 (COVID–19) death; and in 

accordance with recent EPA goals on addressing 

environmental justice, studies that examine 

disparities in PM2.5 exposure and the risk of health 

effects by race/ethnicity or socioeconomic status 

(SES) (U.S. EPA, 2022a, section 1.2.1). 

specifically since the literature cutoff 

date for the 2019 ISA.26

Building on the rationale presented in 

section 1.2.1, the ISA Supplement 

considers peer-reviewed studies 

published from approximately January 

2018 through March 2021 that meet the 

following criteria: 

• Health Effects 

Æ U.S. and Canadian epidemiologic 

studies for health effect categories 

where the 2019 ISA concluded a 

‘‘causal relationship’’ (i.e., short- and 

long-term PM2.5 exposure and 

cardiovascular effects and mortality). 

D U.S. and Canadian epidemiologic 

studies that employed alternative 

methods for confounder control or 

conducted accountability analyses (i.e., 

examined the effect of a policy on 

reducing PM2.5 concentrations). 

• Welfare Effects 

Æ U.S. and Canadian studies that 

provide new information on public 

preferences for visibility impairment 

and/or developed methodologies or 

conducted quantitative analyses of light 

extinction. 

• Key Scientific Topics 

Æ Experimental studies (i.e., 

controlled human exposure and animal 

toxicological) conducted at nearambient PM2.5 concentrations 

experienced in the U.S. 

Æ U.S.- and Canadian-based 

epidemiologic studies that examined the 

relationship between PM2.5 exposures 

and severe acute respiratory syndrome 

coronavirus 2 (SARS–CoV–2) infection 

and coronavirus disease 2019 (COVID– 

19) death. 

Æ At-Risk Populations. 

D U.S.- and Canadian-based 

epidemiologic or exposure studies 

examining potential disparities in either 

PM2.5 exposures or the risk of health 

effects by race/ethnicity or 

socioeconomic status (SES). 

Given the narrow scope of the ISA 

Supplement, it is important to recognize 

that the evaluation does not encompass 

the full multidisciplinary evaluation 

presented within the 2019 ISA that 

would result in weight-of-evidence 

conclusions on causality (i.e., causality 

determinations). The ISA Supplement 

critically evaluates and provides key 

study-specific information for those 

recent studies deemed to be of greatest 

significance for informing preliminary 

conclusions on the PM NAAQS in the 

context of the body of evidence and 

scientific conclusions presented in the 

2019 ISA. 

In developing a revised PA to support 

the reconsideration, the EPA considered 

the available scientific evidence, 

including the evidence presented in the 

2019 ISA and ISA Supplement. The 

2022 PA considered the quantitative 

and technical information presented in 

the 2020 PA, in addition to new and 

updated analyses conducted since the 

2020 final decision. For those health 

and welfare effects for which the ISA 

Supplement evaluated recently 

available studies (i.e., PM2.5-related 

health effects and visibility effects), new 

updated quantitative analyses were 

conducted as a part of the development 

of the 2022 PA. The newly available 

scientific and technical information 

presented in the 2022 PA were 

considered in reaching conclusions 

regarding the adequacy of the current 

standards and any potential alternative 

standards. For those health and welfare 

effects for which newly available 

scientific and technical information 

were not evaluated (i.e., PM10–2.5-related 

health effects and non-visibility welfare 

effects), the conclusions presented in 

the 2022 PA rely heavily on the 

information that supported the 

conclusions in the 2020 PA. 

The CASAC PM panel met at a virtual 

public meeting in November 2021 to 

review the draft ISA Supplement (86 FR 

52673, September 22, 2021). A virtual 

public meeting was then held in 

February 2022, and during this meeting 

the chartered CASAC considered the 

CASAC PM panel’s draft letter to the 

Administrator on the draft ISA 

Supplement (87 FR 958, January 7, 

2022). 

The chartered CASAC provided its 

advice on the draft ISA Supplement in 

a letter to the EPA Administrator dated 

March 18, 2022 (Sheppard, 2022b). In 

its review of the draft ISA Supplement, 

the CASAC noted that they found ‘‘the 

Draft ISA Supplement to be a wellwritten, comprehensive evaluation of 

the new scientific information 

published since the 2019 PM ISA’’ 

(Sheppard, 2022b, p. 2 of letter). 

Furthermore, the CASAC stated that 

‘‘the final Integrated Science 

Assessment (ISA) Supplement . . . 

deserve[s] the Administrator’s full 

consideration and [is] adequate for 

rulemaking’’ (Sheppard, 2022b, p. 2 of 

letter). The CASAC generally endorsed 

EPA’s decisions regarding the limited 

scope of the draft ISA Supplement, 

stating that ‘‘this limitation [on scope] is 

appropriate for the targeted purpose of 

the Draft ISA Supplement’’ although the 

CASAC noted it would not be 

appropriate for ISAs generally, and 

recommended that the EPA provide 

additional acknowledgment and 

explanation for the limited scope 

(Sheppard, 2022b, p. 2 of letter; see also 

pp. 2–3 of consensus responses). The 

EPA specifically noted in the final ISA 

Supplement, which was released in May 

2022 (U.S. EPA, 2022a; hereafter 

referred to as the ISA Supplement 

throughout this document) that the 

‘‘targeted approach to developing the 

Supplement to the 2019 PM ISA for the 

purpose of reconsidering the 2020 PM 

NAAQS decision does not reflect a 

change to EPA’s approach for 

developing ISAs for NAAQS reviews.’’ 

Thus, the evidence presented within the 

2019 ISA, along with the targeted 

identification and evaluation of new 

scientific information in the ISA 

Supplement, provides the scientific 

basis for the reconsideration of the 2020 

PM NAAQS final decision. 

The draft PA was released in October 

2021 (86 FR 56263, October 8, 2021). 

The CASAC PM panel met at a virtual 

public meeting in December 2021 to 

review the draft PA (86 FR 52673, 

September 22, 2021). A virtual public 

meeting was then held in February 2022 

and March 2022, and during this 

meeting the chartered CASAC 

considered the CASAC PM panel’s draft 

letter to the Administrator on the draft 

PA (87 FR 958, January 7, 2022). The 

chartered CASAC provided its advice on 

the draft PA in a letter to the EPA 

Administrator dated March 18, 2022 

(Sheppard, 2022a). The EPA took steps 

to address these comments in revising 

and finalizing the PA. The 2022 PA 

considers the scientific evidence 

presented in the 2019 ISA and ISA 

Supplement and considers the 

quantitative and technical information 

presented in the 2020 PA, along with 

updated and newly available analyses 

since the completion of the 2020 review. 

For those health and welfare effects for 

which the ISA Supplement evaluated 

recently available evidence and for 

which updated quantitative analyses 

were supported (i.e., PM2.5-related 

health effects and visibility effects), the 

2022 PA includes consideration of this 

newly available scientific and technical 

information in reaching preliminary 

conclusions. For those health and 

welfare effects for which newly 

available scientific and technical 


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27 In addition to the 2020 review’s opening ‘‘call 

for information’’ (79 FR 71764, December 3, 2014), 

the 2019 ISA identified and evaluated studies and 

reports that have undergone scientific peer review 

and were published or accepted for publication 

between January 1, 2009, through approximately 

January 2018 (U.S. EPA, 2019a, p. ES–2). References 

that are cited in the 2019 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. 

28As described above, the ISA Supplement 

represents an evaluation of recent studies that are 

of greatest policy relevance and utility to the 

reconsideration of the 2020 final decision on the 

PM NAAQS (U.S. EPA, 2022a). 

information were not evaluated (i.e., 

PM10–2.5-related health effects and nonvisibility effects), the conclusions 

presented in the 2022 PA rely heavily 

on the information that supported the 

conclusions in the 2020 PA. The final 

PA was released in May 2022 (U.S. EPA, 

2022b; hereafter referred to as the 2022 

PA throughout this document). 

Drawing from his consideration of the 

scientific evidence assessed in the 2019 

ISA and ISA Supplement and the 

analyses in the 2022 PA, including the 

uncertainties in the evidence and 

analyses, and from his consideration of 

advice from the CASAC, on January 5, 

2023, the Administrator proposed to 

revise the level of the primary annual 

PM2.5 standard and to retain the primary 

24-hour PM2.5 standard, the primary 24- 

hour PM10 standard, and the secondary 

PM standards. These proposed 

decisions were published in the Federal 

Register on January 27, 2023 (88 FR 

5558, January 27, 2023). The EPA held 

a multi-day virtual public hearing on 

February 21–23, 2023 (88 FR 6215, 

January 31, 2023). In total, the EPA 

received nearly 700,000 comments on 

the proposal from members of the 

public by the close of the public 

comment period on March 28, 2023. 

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’’ document), can be found in 

the docket for this rulemaking (Docket 

No. EPA–HQ–OAR–2015–0072). 

As in prior reviews, the EPA is basing 

its decision in this reconsideration on 

studies and related information in the 

air quality criteria, which have 

undergone CASAC and public review. 

These studies assessed in the 2019 

ISA 27 and ISA Supplement 28 and the 

2022 PA, and the integration of the 

scientific evidence presented in them, 

have undergone extensive critical 

review by the EPA, the CASAC, and the 

public. Decisions on the NAAQS 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. It is for this reason that the EPA 

preliminarily concluded that the 

scientific evidence available since the 

completion of the 2019 ISA, including 

those raised in public comments on the 

proposal in 2020, warranted a partial 

reopening of the air quality criteria and 

prepared an ISA Supplement to enable 

the EPA, the CASAC, and the public to 

consider them further. Some 

commenters have referred to and 

discussed additional individual 

scientific studies on the health effects of 

PM that were not included in the 2019 

ISA or ISA Supplement (‘‘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 2019 ISA and ISA Supplement. The 

EPA’s provisional consideration of these 

studies did not and could not provide 

the kind of in-depth critical review 

described above, but rather was focused 

on determining whether they warranted 

further 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 the studies and related 

information 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 6114, 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 review the O3 NAAQS, 

‘‘new’’ studies may sometimes be of 

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 13013–13014, 

March 9, 1993). In the present case, the 

EPA decided to partially reopen the air 

quality criteria and prepared an ISA 

Supplement as a part of the 

reconsideration to facilitate evaluation 

of these studies by the EPA, the CASAC, 

and the public. The narrow scope of the 

ISA Supplement is supported by EPA’s 

provisional consideration of ‘‘new’’ 

studies submitted in response to public 

comments on the 2020 proposal which 

concluded 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. Therefore, a full reopening of 

the air quality criteria was not 

warranted to assess the health and 

welfare effects of PM for purposes of the 

review. 

Accordingly, the EPA is basing the 

final decisions in this reconsideration 

on the studies and related information 

included in the PM air quality criteria 

(including the 2019 PM ISA and ISA 

Supplement) that have undergone 

rigorous review by the EPA, the 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 

reconsideration and which will provide 

the opportunity to fully assess these 

studies through a more rigorous review 

process involving the EPA, the CASAC, 

and the public. 

D. Air Quality Information 

This section provides a summary of 

basic information related to PM ambient 

air quality. It summarizes information 

on the distribution of particle size in 

ambient air (section I.D.1), sources and 

emissions contributing to PM in the 

ambient air (section I.D.2), monitoring 

ambient PM in the U.S. (section I.D.3), 

ambient PM concentrations and trends 

in the U.S. (I.D.4), characterizing 

ambient PM2.5 concentrations for 

exposure (section I.D.5), and 

background PM (section I.D.6). 

Additional detail on PM air quality can 

be found in Chapter 2 of the 2022 PA 

(U.S. EPA, 2022b). 

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, 2019a, 

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 


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29See also: https://www.usda.gov/sites/default/ 

files/documents/usda-epa-doi-cdc-mou.pdf. 

30See: https://www.epa.gov/air-quality-analysis/ 

final-2016-exceptional-events-rule-supportingguidance-documents-updated-faqs. 

31For PM2.5, neighborhood scale is defined at 40 

CFR part 58, appendix D, 4.7.1(c)(3) as follows: 

Measurements in this category would represent 

conditions throughout some reasonably 

homogeneous urban sub-region with dimensions of 

a few kilometers and of generally more regular 

shape than the middle scale. Homogeneity refers to 

the particulate matter concentrations, as well as the 

land use and land surface characteristics. Much of 

the PM2.5 exposures are expected to be associated 

with this scale of measurement. In some cases, a 

location carefully chosen to provide neighborhood 

scale data would represent the immediate 

neighborhood as well as neighborhoods of the same 

type in other parts of the city. PM2.5 sites of this 

kind provide good information about trends and 

compliance with standards because they often 

represent conditions in areas where people 

commonly live and work for periods comparable to 

those specified in the NAAQS. In general, most 

PM2.5 monitoring in urban areas should have this 

scale. 

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

ultrafine particles (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, 2019a, section 2.2). 

Atmospheric lifetimes are generally 

longest for PM2.5, which often remains 

in the atmosphere for days to weeks 

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

2022b, section 2.1). 

2. Sources and Emissions Contributing 

to PM in the Ambient Air 

PM is composed of both primary 

(directly emitted particles) and 

secondary particles. Primary PM is 

derived from direct particle emissions 

from specific PM sources while 

secondary PM originates from gas-phase 

precursor chemical compounds present 

in the atmosphere that have participated 

in new particle formation or condensed 

onto existing particles (U.S. EPA, 2019a, 

section 2.3). As discussed further in the 

2019 ISA (U.S. EPA, 2019a, section 

2.3.2.1), secondary PM is formed in the 

atmosphere by photochemical oxidation 

reactions of both inorganic and organic 

gas-phase precursors. Precursor gases 

include sulfur dioxide (SO2), nitrogen 

oxides (NOX), and volatile organic 

compounds (VOC) (U.S. EPA, 2019a, 

section 2.3.2.1). Ammonia also plays an 

important role in the formation of 

nitrate PM by neutralizing sulfuric acid 

and nitric acid. Sources and emissions 

of PM are discussed in more detail the 

2022 PA (U.S. EPA, 2022b, section 

2.1.1). Briefly, anthropogenic sources of 

PM include both stationary (e.g., fuel 

combustion for electricity production 

and other purposes, industrial 

processes, agricultural activities) and 

mobile (e.g., diesel- and gasolinepowered highway vehicles and other 

engine-driven sources) sources. Natural 

sources of PM include dust from the 

wind erosion of natural surfaces, sea 

salt, wildfires, primary biological 

aerosol particles (PBAP) such as bacteria 

and pollen, oxidation of biogenic 

hydrocarbons, such as isoprene and 

terpenes to produce secondary organic 

aerosol (SOA), and geogenic sources, 

such as sulfate formed from volcanic 

production of SO2. Wildland fire, which 

encompass both wildfire and prescribed 

fire, accounts for 44% of emissions of 

primary PM2.5 emissions (U.S. EPA, 

2021b). Emissions from wildfire 

comprises 29% of primary PM2.5

emissions. 

In recent years, the frequency and 

magnitude of wildfires have increased 

(U.S. EPA, 2019a). The magnitude of the 

public health impact of wildfires is 

substantial both because of the increase 

in PM2.5 concentrations as well as the 

duration of the wildfire smoke season, 

which is considered to range from May 

to November. Wildfire can make a large 

contribution to air pollution (including 

PM2.5), and wildfire events can threaten 

public safety and life. The impacts of 

wildfire events can be mitigated through 

management of wildland vegetation, 

including through prescribed fire. 

Prescribed fire (and some wildfires) can 

mimic the natural processes necessary 

to maintain fire-dependent ecosystems, 

minimizing catastrophic wildfires and 

the risks they pose to safety, property 

and air quality (see, e.g., 81 FR 58010, 

58038, August 24, 2016). The EPA views 

the strategic use of prescribed fire as an 

important tool for reducing wildfire risk 

and the severity of wildfires and 

wildfire smoke (88 FR, 54118, 54126, 

August 9, 2023).29 As noted in the PM 

NAAQS proposal, agencies have efforts 

in place to reduce the frequency and 

severity of human-caused wildfires (88 

FR 5570, January 27, 2023). 

Wildfire events produce high PM 

emissions that may impact the PM 

concentrations in ambient air to the 

extent that the concentrations result in 

an exceedance or violation which may 

affect the design value in a given area. 

The EPA’s Exceptional Events Rule (81 

FR 68216, October 3, 2016) describes 

the process by which air agencies may 

request to exclude ‘event-influenced’ 

data caused by exceptional events, 

which can include wildfires and 

prescribed fires on wildland. The EPA 

has issued guidance specifically 

addressing exceptional events 

demonstrations for both wildfires and 

prescribed fires on wildland. These 

documents are available on EPA’s 

Exceptional Events Program website.30

The EPA will develop fire-related 

exceptional events implementation 

tools, including updates as needed to 

existing guidance to facilitate more 

efficient processing of PM2.5-related 

exceptional events demonstrations for 

both the 24-hour and annual standards. 

3. Monitoring of Ambient PM 

To promote uniform application of 

the air quality standards set forth under 

the CAA and to achieve the degree of 

public health and welfare protection 

intended for the NAAQS, the EPA 

establishes PM Federal Reference 

Methods (FRMs) for both PM10 and 

PM2.5 in appendices J and L to 40 CFR 

part 50, both of which were amended 

following the 2006 and 2012 PM 

NAAQS reviews. The current PM 

monitoring network relies on FRMs and 

automated continuous Federal 

Equivalent Methods (FEMs) approved 

pursuant to 40 CFR part 53, in part to 

support changes necessary for 

implementation of the revised PM 

standards. Additionally, 40 CFR part 58, 

appendices A through E, detail the 

requirements to measure ambient air 

quality and report ambient air quality 

data and related information. More 

information on PM ambient monitoring 

networks is available in section 2.2 of 

the 2022 PA (U.S. EPA, 2022b). 

The PM2.5 monitoring program is one 

of the major ambient air monitoring 

programs with a robust, nationally 

consistent network of ambient air 

monitoring sites providing mass and/or 

chemical speciation measurements. 40 

CFR part 58, appendix D, section 4.7 

provides the applicable PM2.5 network 

design criteria. For most urban 

locations, PM2.5 monitors are sited at the 

neighborhood scale,31 where PM2.5

concentrations are reasonably 

homogeneous throughout an entire 

urban sub-region. In each CBSA with a 

monitoring requirement, at least one 

PM2.5 monitoring station representing 


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32 40 CFR part 58, app. D, 4.7.1(b)(2). 

33See https://www.epa.gov/air-trends/particulatematter-pm25-trends for up-to-date PM2.5 trends 

information. 34A design value is considered valid if it meets 

the data handling requirements given in appendix 

N to 40 CFR part 50. 

35The 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. 

area-wide air quality is sited in an area 

of expected maximum concentration.32

By ensuring the area of expected 

maximum concentration in a CBSA has 

a site compared to both the annual and 

24-hour NAAQS, all other similar 

locations are thus protected. Sites that 

represent relatively unique microscale, 

localized hot-spot, or unique middle 

scale impact sites are only eligible for 

comparison to the 24-hour PM2.5

NAAQS. 

Under 40 CFR part 50, appendix L, 

and 40 CFR part 53, and 40 CFR part 58 

appendix D there are three main 

methods components of the PM2.5

monitoring program: filter-based FRMs 

measuring PM2.5 mass, FEMs measuring 

PM2.5 mass, and other samplers used to 

collect the aerosol used in subsequent 

laboratory analysis for measuring PM2.5

chemical speciation. The FRMs are 

primarily used for comparison to the 

NAAQS, but also serve other important 

purposes, such as developing trends and 

evaluating the performance of FEMs. 

PM2.5 FEMs are typically continuous 

methods used to support forecasting and 

reporting of the Air Quality Index (AQI) 

but are also used for comparison to the 

NAAQS. Samplers that are part of the 

Chemical Speciation Network (CSN) 

and Interagency Monitoring of Protected 

Visual Environments (IMPROVE) 

network are used to provide chemical 

composition of the aerosol and serve a 

variety of objectives. More detail on of 

each of these components of the PM2.5

monitoring program and of recent 

changes to PM2.5 monitoring 

requirements are described in detail in 

the 2022 PA (U.S. EPA, 2022b, section 

2.2.3). 

4. Ambient Concentrations and Trends 

This section summarizes available 

information on recent ambient PM 

concentrations in the U.S. and on trends 

in PM air quality. Sections I.D.4.a and 

I.D.4.b summarize information on PM2.5

mass and components, respectively. 

Section I.D.4.c summarizes information 

on PM10. Sections I.D.4.d and I.D.4.e 

summarize the more limited 

information on PM10–2.5 and UFP, 

respectively. Additional detail on PM 

air quality and trends can be found in 

the 2022 PA (U.S. EPA, 2022b, section 

2.3). 

a. PM2.5 mass 

At monitoring sites in the U.S., 

annual PM2.5 concentrations from 2017 

to 2019 averaged 8.0 mg/m3 (with the 

10th and 90th percentiles at 5.9 and 

10.0 mg/m3, respectively) and the 98th 

percentiles of 24-hour concentrations 

averaged 21.3 mg/m3 (with the 10th and 

90th percentiles at 14.0 and 29.7 mg/m3, 

respectively) (U.S. EPA, 2022b, section 

2.3.2.1). The highest ambient PM2.5

concentrations occur in the western 

U.S., particularly in California and the 

Pacific Northwest (U.S. EPA, 2022b, 

Figure 2–15). 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, 2022b, section 2.3.2.1). 

Recent ambient PM2.5 concentrations 

reflect the substantial reductions that 

have occurred across much of the U.S. 

(U.S. EPA, 2022b, section 2.3.2.1). From 

2000 to 2019, national annual average 

PM2.5 concentrations declined from 13.5 

mg/m3 to 7.6 mg/m3, a 43% decrease 

(U.S. EPA, 2022b, section 2.3.2.1).33

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 declined 

significantly (U.S. EPA, 2022b, 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, 2022b, 

section 2.3.2.1). As noted below, some 

sites in the northwestern U.S. and 

California, where wildfire have been 

relatively common in recent years, have 

experienced high concentrations over 

shorter periods (i.e., 2-hour averages). 

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. For 2016–2018, Gantt 

et al. (2021) reported that 52% and 24% 

of the time near-road sites reported the 

highest annual and 24-hour PM2.5

design value 34 in the CBSA, 

respectively. Of the CBSAs with the 

highest annual design values at nearroad sites reported by Gantt et al. (2021), 

those design values were, on average, 

0.8 mg/m3 higher than at the highest 

measuring non-near-road sites (range is 

0.1 to 2.1 mg/m3 higher at near-road 

sites). Although most near-road 

monitoring sites do not have sufficient 

data to evaluate long-term trends in 

near-road PM2.5 concentrations, 

analyses of the data at one near-roadlike site in Elizabeth, NJ, 35 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, 2022b, section 

2.3.2.1). 

Ambient PM2.5 concentrations can 

exhibit a diurnal cycle that varies due 

to impacts from intermittent emission 

sources, meteorology, and atmospheric 

chemistry. The PM2.5 monitoring 

network in the U.S. has an increasing 

number of continuous FEM monitors 

reporting hourly PM2.5 mass 

concentrations that reflect this diurnal 

variation. The 2019 ISA describes a twopeaked diurnal pattern in urban areas, 

with morning peaks attributed to rushhour traffic and afternoon peaks 

attributed to a combination of rush hour 

traffic, decreasing atmospheric dilution, 

and nucleation (U.S. EPA, 2019a, 

section 2.5.2.3, Figure 2–32). Because a 

focus on annual average and 24-hour 

average PM2.5 concentrations could 

mask subdaily patterns, and because 

some health studies examine PM 

exposure durations shorter than 24- 

hours, it is useful to understand the 

broader distribution of subdaily PM2.5

concentrations across the U.S. The 2022 

PA presents information on the 

frequency distribution of 2-hour average 

PM2.5 mass concentrations from all FEM 

PM2.5 monitors in the U.S. for 2017– 

2019. At sites meeting the current 

primary PM2.5 standards, these 2-hour 

concentrations generally remain below 

10 mg/m3, and rarely exceed 30 mg/m3. 

Two-hour concentrations are higher at 

sites violating the current standards, 

generally remaining below 16 mg/m3 and 

rarely exceeding 80 mg/m3 (U.S. EPA, 

2022b, section 2.3.2.2.3). The extreme 

upper end of the distribution of 2-hour 

PM2.5 concentrations is shifted higher 

during the warmer months, generally 

corresponding to the period of peak 

wildfire frequency (April to September) 

in the U.S. At sites meeting the current 

primary standards, the highest 2-hour 

concentrations measured rarely occur 

outside of the period of peak wildfire 

frequency. Most of the sites measuring 


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36The form of the current 24-hour PM10 standard 

is one-expected-exceedance, averaged over three 

years. 

37For more information, see https://

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

trends#pmnat. 

38PM from dust emissions in the National 

Emissions Inventory (NEI) remain fairly consistent 

from year-to-year, except when there are severe 

weather incursions or there is a dust event that 

transports or causes major local dust storms to 

occur (particularly in the western U.S.). These dust 

events and weather incursions needed to effect dust 

emissions on a national level are not common and 

only seldomly occur. In the emissions trends 

analysis presented in the 2022 PA (U.S. EPA, 

2022b, section 2.1.1), dust is included in the NEI 

sector labeled ‘‘miscellaneous.’’ 

these very high concentrations are in the 

northwestern U.S. and California, where 

wildfires have been relatively common 

in recent years (see U.S. EPA, 2022b, 

Appendix A, Figure A–1). When the 

period of peak wildfire frequency is 

excluded from the analysis, the extreme 

upper end of the distribution is reduced 

(U.S. EPA, 2022b, section 2.3.2.2.3). 

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, 2022b, 

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, 

2019a, 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, 2019a, section 2.5.1.1.6; U.S. EPA, 

2022b, section 2.3.1). 

c. PM10

At long-term monitoring sites in the 

U.S., the 2017–2019 average of 2nd 

highest 24-hour PM10 concentration was 

68 mg/m3 (with 10th and 90th 

percentiles at 28 and 124 mg/m3, 

respectively) (U.S. EPA, 2022b, section 

2.3.2.4).36 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, 

2019a, 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, 

2022b, section 2.3.2.4). From 2000 to 

2019, 2nd highest 24-hour PM10

concentrations have declined by about 

46% (U.S. EPA, 2022b, section 

2.3.2.4).37 Analyses at individual 

monitoring sites indicate that annual 

average PM10 concentrations have 

generally 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 (U.S. 

EPA, 2022b, section 2.3.2.4). Annual 

2nd highest 24-hour 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, 2022b, 

section 2.3.2.4). 

Compared to previous reviews, data 

available from the NCore monitoring 

network in the current reconsideration 

allows a more comprehensive analysis 

of the relative contributions of PM2.5

and PM10–2.5 to PM10 mass. PM2.5

generally contributes more to annual 

average PM10 mass in the eastern U.S. 

than the western U.S. (U.S. EPA, 2022b, 

Figure 2–23). At most sites in the 

eastern U.S., the majority of PM10 mass 

is comprised of PM2.5. As ambient PM2.5

concentrations have declined in the 

eastern U.S. (U.S. EPA, 2022b, section 

2.3.2.2), the ratios of PM2.5 to PM10 have 

also declined. For sites with days 

having concurrently very high PM2.5 and 

PM10 concentrations (U.S. EPA, 2022b, 

Figure 2–24), the PM2.5/PM10 ratios are 

typically higher than the annual average 

ratios. This is particularly true in the 

northwestern U.S. where the high PM10

concentrations can occur during 

wildfires with high PM2.5 (U.S. EPA, 

2022b, section 2.3.2.4). 

d. PM10–2.5

Since the 2012 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 2022 PA 

(U.S. EPA, 2022b, 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. 

Due to the short atmospheric lifetime 

of PM10–2.5 relative to PM2.5, many of the 

high concentration sites are isolated and 

likely near emission sources associated 

with wind-blown and fugitive dust. The 

spatial distributions of annual average 

and 98th percentile concentrations of 

PM10–2.5 are more similar than that of 

PM2.5, suggesting that the same dustrelated emission sources are affecting 

both long-term and episodic 

concentrations (U.S. EPA, 2022b, Figure 

2–25). The highest concentrations of 

PM10–2.5 are in the southwest U.S. where 

widespread dry and windy conditions 

contribute to wind-blown dust 

emissions. 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, 2022b, Figure 2–25). The 

majority of PM10–2.5 sites in the U.S. do 

not have a concentration trend from 

2000–2019, reflecting the relatively 

consistent level of dust emissions across 

the U.S. during the same time period 

(U.S. EPA, 2022b, section 2.3.2.5).38

e. UFP 

Compared to PM2.5 mass, there is 

relatively little data on U.S. particle 

number concentrations, which are 

dominated by UFP. In the published 

literature, annual average particle 

number concentrations reaching about 

20,000 to 30,000 cm3 have been 

reported in U.S. cities (U.S. EPA, 

2019a). In addition, based on UFP 

measurements in two urban areas (New 

York City, Buffalo) and at a background 

site (Steuben County) in New York, 

there is a pronounced difference in 

particle number concentration between 

different types of locations (U.S. EPA, 

2022b, Figure 2–26; U.S. EPA, 2019a, 

Figure 2–18). Urban particle number 

counts were several times higher than at 

the background site, and the highest 

particle number counts in an urban area 

with multiple sites (Buffalo) were 

observed at a near-road location (U.S. 

EPA, 2022b, section 2.3.2.6). 

Long-term trends in UFP are not 

routinely available at U.S. monitoring 


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39For the annual PM2.5 standard, design values 

are calculated as the annual arithmetic mean PM2.5

concentration, averaged over 3 years. For the 24- 

hour standard, design values are calculated as the 

98th percentile of the annual distribution of 24- 

hour PM2.5 concentrations, averaged over three 

years (appendix N of 40 CFR part 50). 

40This analysis includes an updated version of 

the surface used in Di et al. (2016). Predictions in 

Di et al. (2016) were for 2000 to 2012 using a neural 

network model. The Di et al. (2019) study improved 

on that effort in several ways. First, a generalized 

additive model was used that accounted for 

geographic variations in performance to combine 

predictions from three models (neural network, 

random forest, and gradient boosting) to make the 

final optimal PM2.5 predictions. Second, the 

datasets were updated that were used in model 

training and included additional variables such as 

12-km CMAQ modeling as predictors. Finally, more 

recent years were included in the Di et al. (2019) 

study. 41The HA2020 field is based on the V4.NA.03 

product available at: https://sites.wustl.edu/acag/ 

datasets/surface-pm2-5/. The name ‘‘HA2020’’ 

comes from the references for this product (Hammer 

et al., 2020; van Donkelaar et al., 2019). 

sites. At one background site in Illinois 

with long-term data available, the 

annual average particle number 

concentration declined between 2000 

and 2019, closely matching the 

reductions in annual PM2.5 mass over 

that same period (U.S. EPA, 2022b, 

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, 2022b, section 

2.3.2.6). However, the relationship 

between changes in ambient PM2.5 and 

UFPs cannot be comprehensively 

characterized due to the high variability 

and limited monitoring of UFPs (U.S. 

EPA, 2022b, section 2.3.2.6). 

5. Characterizing Ambient PM2.5

Concentrations for Exposure 

Epidemiologic studies use various 

methods to characterize exposure to 

ambient PM2.5. The methods used to 

estimate PM2.5 concentrations can vary 

from traditional methods using 

monitoring data from ground-based 

monitors to newer methods using more 

complex hybrid modeling approaches. 

Studies using hybrid modeling 

approaches aim to broaden the spatial 

coverage, as well as estimate more 

spatially-resolved ambient PM2.5

concentrations, by expanding beyond 

just those areas with monitors and 

providing estimates in areas that do not 

have ground-based monitors (i.e., areas 

that are generally less densely 

populated and tend to have lower PM2.5

concentrations) and at finer spatial 

resolutions (e.g., 1 km x 1 km grid cells). 

Ground-based PM2.5 monitors are 

generally sited in areas of expected 

maximum concentration. As such, the 

hybrid modeling approaches tend to 

broaden the areas captured in the 

exposure assessment, and in doing so, 

the studies that utilize these methods 

tend to report lower mean PM2.5

concentrations than monitor-based 

approaches. Further, other aspects of the 

approaches applied in the various 

epidemiologic studies to estimate PM2.5

exposure and/or to calculate the related 

study-reported mean concentration (i.e., 

population weighting, trim mean 

approaches) can affect those data values. 

More detail related to hybrid modeling 

methods, performance of the methods, 

and how the reported mean 

concentrations compare across 

approaches is provided in section 

2.3.3.2 of the 2022 PA (U.S. EPA, 

2022b). The subsections below discuss 

the characterization of PM2.5

concentrations based on monitoring 

data (I.D.5.a) and using hybrid modeling 

approaches (I.D.5.b). 

a. Predicted Ambient PM2.5 and 

Exposure Based on Monitored Data 

Ambient concentrations of PM2.5 are 

often characterized using measurements 

from national monitoring networks due 

to the accuracy and precision of the 

measurements and the public 

availability of data. For applications 

requiring PM2.5 characterizations across 

large areas or provide complete coverage 

from the site measurements, data 

interpolation and averaging techniques 

(such as Average Nearest Neighbor 

tools, and area-wide or populationweighted averaging of monitors) are 

sometimes used (U.S. EPA, 2019a, 

chapter 3). 

For an area to meet the NAAQS, all 

valid design values 39 in that area, 

including the highest annual and 24- 

hour design values, must be at or below 

the levels of the standards. Because the 

monitoring network siting requirements 

are specified to capture the high PM2.5

concentrations (U.S. EPA, 2022b, 

section 2.2.3), areas meeting an annual 

PM2.5 standard with a particular level 

would be expected to have long-term 

average monitored PM2.5 concentrations 

(i.e., averaged across space and over 

time in the area) somewhat below that 

standard level. This means that the 

PM2.5 design value in an area is 

associated with a distribution of PM2.5

concentrations in that area, and, based 

on monitoring siting requirements, 

should represent the highest 

concentration location applicable to be 

monitored under the PM2.5 NAAQS. 

Analyses in the 2022 PA indicate that, 

based on recent air quality in U.S. 

CBSAs, maximum annual PM2.5 design 

values are often 10% to 20% higher 

than annual average concentrations (i.e., 

averaged across multiple monitors in 

the same CBSA) (U.S. EPA, 2022b, 

section 2.3.3.1, Figures 2–28 and 2–29). 

This difference between the maximum 

annual design value and the average 

concentration in an area can vary, 

depending on factors such as the 

number of monitors, monitor siting 

characteristics, and the distribution of 

ambient PM2.5 concentrations. Given 

that higher PM2.5 concentrations have 

been reported at some near-road 

monitoring sites relative to the 

surrounding area (U.S. EPA, 2022b, 

section 2.3.2.2.2), recent requirements 

for PM2.5 monitoring at near-road 

locations in large urban areas (U.S. EPA, 

2022b, section 2.2.3.3) may increase the 

ratios of maximum design values to 

average annual design values in some 

areas. Such ratios may also depend on 

how the averages are calculated (i.e., 

averaged across monitors versus across 

modeled grid cells, as described below 

in section I.5.b). Compared to annual 

design values, the analysis in the 2022 

PA indicates a more variable 

relationship between maximum 24-hour 

PM2.5 design values and annual average 

concentrations (U.S. EPA, 2022b, 

section 2.3.3.1, Figure 2–29). 

b. Comparison of PM2.5 Hybrid 

Modeling Approaches in Estimating 

Exposure and Relative to Design Values 

Two types of hybrid approaches that 

have been utilized in several key PM2.5

epidemiologic studies in the 2019 ISA 

and ISA Supplement include neural 

network approaches and a satellitebased method with regression of 

residual PM2.5 with land-use and other 

variables to improve estimates of PM2.5

concentration in the U.S. As such, the 

2022 PA further compares these two 

types of approaches across various 

scales (e.g., CBSA versus nationwide), 

taking into account population 

weighting approaches utilized in 

epidemiologic studies when estimating 

PM2.5 exposure (U.S. EPA, 2022b, 

section 2.3.3.2.4). Additionally, the 

2022 PA assesses how average PM2.5

concentrations computed in 

epidemiologic studies using these 

hybrid surfaces compare to the 

maximum design values measured at 

ground-based monitors. For this 

assessment, the 2022 PA evaluates the 

DI2019 40 and HA2020 41 hybrid 

surfaces, surfaces that are used in 

several of the key epidemiologic studies 

in the 2022 PA. This analysis is 

intended to help inform how the 

magnitude of the overall study-reported 

mean PM2.5 concentrations in 

epidemiologic studies may be 


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42For the national scale, 3-year averages of the 

average annual PM2.5 concentrations generally range 

from about 5.3 mg/m3 to 8.1 mg/m3, compared to the 

CBSA scale, which ranges from 5.7 mg/m3 to 8.7 mg/ 

m3. (U.S. EPA, 2022b, section 2.3.3.2.4, Table 2–6). 

43For this analysis, the 2022 PA includes CBSAs 

with three or more valid design values for the 3- 

year period. The regulatory design values for the 

CBSAs were calculated for each 3-year period for 

the CBSAs with 3 or more design values in each of 

the 3-year periods. Using the maximum design 

value for each CBSA and by each 3-year period, the 

ratio of maximum design values to modeled average 

annual PM2.5 concentrations were calculated, for 

each 3-year period. More details about the 

analytical methods used for this analysis are 

described in section A.6 of Appendix A in the 2022 

PA (U.S. EPA, 2022b). 

44Sources 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, 

2022b, 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, 2019a, section 2.3.3). 

45 In addition to the 2020 review’s opening ‘‘call 

for information’’ (79 FR 71764, December 3, 2014), 

the 2019 ISA identified and evaluated studies and 

reports that have undergone scientific peer review 

and were published or accepted for publication 

between January 1, 2009, through approximately 

January 2018 (U.S. EPA, 2019a, p. ES–2). References 

that are cited in the 2019 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. 

46Short-term exposures are defined as those 

exposures occurring over hours up to 1 month, 

influenced by the approach used to 

compute that mean and how that value 

might compare to monitor reported 

concentrations. The PM2.5 standards are 

expected to achieve a pattern of air 

quality through the attainment of a 

specific design value at each monitor in 

the monitoring network. As a result, it 

is important to be able to assess the 

relationship between monitor 

concentrations and patterns of air 

quality evaluated in the epidemiologic 

studies. 

In estimating exposure, some studies 

focus on estimating concentrations in 

urban areas, while others examine the 

entire U.S. or large portions of the 

country. In general, the areas that are 

not included in the CBSA-only analysis 

tend to be more rural or less densely 

populated areas, tend to have lower 

PM2.5 concentrations, and likely 

correspond to those locations where 

monitoring data availability is limited or 

nonexistent (U.S. EPA, 2022b, section 

2.3.3.2.4, Figure 2–37). To evaluate the 

differences in mean PM2.5

concentrations across different spatial 

scales, the 2022 PA analysis compares 

the DI2019 and HA2020 surfaces. At the 

national scale, the two surfaces 

generally produce similar average 

annual PM2.5 concentrations, with the 

DI2019 surface being slightly higher 

compared to the HA2020 surface. The 

average annual PM2.5 concentrations are 

also slightly higher using the DI2019 

surface compared to the HA2020 surface 

when the analyses are conducted for 

CBSAs. Also, regardless of which 

surface is used, the average annual and 

3-year average of the average annual 

PM2.5 concentrations for the CBSA-only 

analyses are somewhat higher than for 

the nationwide analyses (4–8% higher) 

(U.S. EPA, 2022b, section 2.3.3.2.4, 

Table 2–5).42 Overall, these analyses 

suggest that there are only slight 

differences in the average PM2.5

concentrations depending on the hybrid 

modeling method employed, though 

including other hybrid modeling 

methods in this comparison could result 

in larger differences. 

The 2022 PA next evaluates how the 

averages of the hybrid model surfaces 

compare to regulatory design values 

using both the DI2019 and HA2020 

surfaces and how population weighting 

influences the mean PM2.5

concentration.43 As presented in the 

2022 PA, the results using the DI2019 

and HA2020 surfaces are similar for the 

average annual PM2.5 concentrations, for 

each 3-year period. When population 

weighting is not applied, the average 

annual PM2.5 concentrations generally 

range from 7.0 to 8.6 mg/m3. When 

population weighting is applied, the 

average annual PM2.5 concentrations are 

slightly higher, ranging from 8.2 to 10.2 

mg/m3. As with CBSAs versus the 

national comparison above, population 

weighting results in a higher average 

PM2.5 concentration than when 

population weighting is not applied 

(U.S. EPA, 2022b, section 2.3.3.2.4, 

Table 2–7). For the CBSAs included in 

the population weighted analyses, the 

average maximum annual design values 

generally range from 9.5 to 11.7 mg/m3. 

The results are similar for both the 

DI2019 and HA2020 surfaces and the 

maximum annual PM2.5 design values 

measured at the monitors are often 40% 

to 50% higher than average annual 

PM2.5 concentrations predicted by 

hybrid modeling methods when 

population weighting is not applied. 

However, when population weighting is 

applied, the ratio of the maximum 

annual PM2.5 design values to the 

predicted average annual PM2.5

concentrations are lower than when 

population weighting is not applied, 

with monitored design values generally 

15% to 18% higher than populationweighted hybrid modeling average 

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

2022b, section 2.3.3.2.4, Table 2–7). 

6. Background PM 

In this reconsideration, 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 2022 PA 

(U.S. EPA, 2022b, section 2.4). At 

annual and national scales, estimated 

background PM concentrations in the 

U.S. are small compared to 

contributions from domestic 

anthropogenic sources.44 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 2012 review. Contributions to 

background PM in the U.S. result 

mainly from sources within North 

America. Contributions from 

intercontinental events have also been 

documented (e.g., transport from dust 

storms occurring in deserts in North 

Africa and Asia), but these events are 

less frequent and represent a relatively 

small fraction of background PM in 

most of the U.S. (U.S. EPA, 2022b, 

section 2.4). 

II. Rationale for Decisions on the 

Primary PM2.5 Standards 

This section presents the rationale for 

the Administrator’s decision to revise 

the primary annual PM2.5 standard 

down to a level of 9 mg/m3 and retain 

the primary 24-hour PM2.5 standard. 

This rationale is based on a thorough 

review of the scientific evidence 

generally published through January 

2018,45 as evaluated in the 2019 ISA 

(U.S. EPA, 2019a), on the human health 

effects of PM2.5 associated with longand short-term exposures 46 to PM2.5 in 


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whereas long-term exposures are defined as those 

exposures occurring over 1 month to years (U.S. 

EPA, 2019a, section P.3.1). 

47The ISA Supplement represents an evaluation 

of recent studies that are of greatest policy 

relevance to the reconsideration of the 2020 final 

decision on the PM NAAQS. Specifically, the ISA 

Supplement focuses on studies of health effects for 

which the evidence in the 2019 ISA supported a 

‘‘causal relationship’’ (i.e., short- and long-term 

PM2.5 exposure and mortality and cardiovascular 

effects) because those were the health effects that 

were most useful in informing conclusions in the 

2020 PA. The ISA Supplement does not include an 

evaluation of studies for other PM2.5-related health 

effects (U.S. EPA, 2022a). 

48As noted in section I.A above, the legislative 

history describes such protection for the sensitive 

group of individuals and not for a single person in 

the sensitive group (see S. Rep. No. 91–1196, 91st 

Cong, 2d Sess. 10 [1970]); see also Am. Lung Ass’n 

v. EPA, 134 F.3d 388, 389 (D.C. Cir. 1998). 

the ambient air. Additionally, this 

rationale is based on a thorough 

evaluation of some studies that became 

available after the literature cutoff date 

of the 2019 ISA, as evaluated in the ISA 

Supplement, that could either further 

inform the adequacy of the current PM 

NAAQS or address key scientific topics 

that have evolved since the literature 

cutoff date for the 2019 ISA, generally 

through March 2021 (U.S. EPA, 

2022a).47 The Administrator’s rationale 

also takes into account: (1) The 2022 PA 

evaluation of the policy-relevant 

information in the 2019 ISA and ISA 

Supplement and presentation of 

quantitative analyses of air quality and 

health risks; (2) CASAC advice and 

recommendations; and (3) public 

comments received during the 

development of these documents. 

In presenting the rationale for the 

Administrator’s decisions and its 

foundations, section II.A provides 

background on the general approach for 

this reconsideration 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 CASAC advice and the 

basis for the proposed conclusions, 

addresses public comments received on 

the proposal and presents the 

Administrator’s conclusions on the 

adequacy of the current standards, 

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 

The general approach for this 

reconsideration of the 2020 final 

decision on the primary PM2.5 standards 

is fundamentally based on using the 

EPA’s assessment of the current 

scientific evidence and associated 

quantitative analyses to inform the 

Administrator’s judgment regarding 

primary PM2.5 standards that protect 

public health with an adequate margin 

of safety. The EPA’s assessments are 

primarily documented in the 2019 ISA, 

ISA Supplement, and 2022 PA, all of 

which have received CASAC review and 

public comment (83 FR 53471, October 

23, 2018; 83 FR 55529, November 6, 

2018; 85 FR 4655, January 27, 2020; 86 

FR 52673, September 22, 2021; 86 FR 

54186, September 30, 2021; 86 FR 

56263, October 8, 2021; 87 FR 958, 

January 7, 2022; 87 FR 22207, April 14, 

2022; 87 FR 31965, May 26, 2022). In 

bridging the gap between the scientific 

assessments of the 2019 ISA and ISA 

Supplement and the judgments required 

of the Administrator in determining 

whether the current standards provide 

the requisite public health protection, 

the 2022 PA evaluates policy 

implications of the evaluation of the 

current evidence in the 2019 ISA and 

ISA Supplement, and the risk 

information documented in the 2022 

PA. In evaluating the public health 

protection afforded by the current 

standards, the four basic elements of the 

NAAQS (i.e., indicator, averaging time, 

level, and form) are considered 

collectively. 

The final decision on the adequacy of 

the current primary PM2.5 standards is a 

public health policy judgment to be 

made by the Administrator. In reaching 

conclusions with regard to the 

standards, the decision will draw on the 

scientific information and analyses 

about health effects and population 

risks, as well as judgments about how to 

consider the range and magnitude of 

uncertainties that are inherent in the 

scientific evidence and analyses. This 

approach 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 of the Clean Air Act and with 

how the EPA and the courts have 

historically interpreted the Act 

(summarized in section I.A above). 

These provisions require the 

Administrator to establish primary 

standards that, in the judgment of the 

Administrator, 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 

(also referred to as ‘‘at-risk’’) groups.48

1. Background on the Current Standards 

The current primary PM2.5 standards 

were retained in 2020 based on the 

scientific evidence and quantitative risk 

information available at that time, as 

well as the then-Administrator’s 

judgments regarding the available health 

effects evidence and the appropriate 

degree of public health protection 

afforded by the existing standards (85 

FR 82718, December 18, 2020). With the 

2020 decision, the then-Administrator 

retained the primary annual PM2.5

standard with its level of 12.0 mg/m3

and retained the primary 24-hour PM2.5

standard with its level of 35 mg/m3. The 

key considerations and the thenAdministrator’s conclusions regarding 

the primary PM2.5 standards in the 2020 

review are summarized below. 

The health effects evidence base 

available in the 2020 review included 

extensive evidence from previous 

reviews as well as the evidence that had 

emerged since the prior review had been 

completed in 2012. This evidence base, 

spanning several decades, documents 

the relationship between short- and 

long-term PM2.5 exposure and mortality 

or serious morbidity effects. The 

evidence available in the 2019 ISA 

reaffirmed, and in some cases 

strengthened, the conclusions from the 

2009 ISA regarding the health effects of 

PM2.5 exposures (U.S. EPA, 2019a). 

Much of the evidence came from 

epidemiologic studies conducted in 

North America, Europe, or Asia 

examining short-term and long-term 

exposures that demonstrated generally 

positive, and often statistically 

significant, PM2.5 health effect 

associations with a range of outcomes 

including non- accidental, 

cardiovascular, or respiratory mortality; 

cardiovascular- or respiratory-related 

hospitalizations or emergency 

department visits; and other mortality/ 

morbidity outcomes (e.g., lung cancer 

mortality or incidence, asthma 

development). Experimental evidence, 

as well as evidence from panel studies, 

strengthened support for potential 

biological pathways through which 

PM2.5 exposures could lead to health 

effects reported in many populationbased epidemiologic studies, including 

support for pathways that could lead to 

cardiovascular, respiratory, nervous 

system, and cancer-related effects. 


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Based on this evidence, the 2019 ISA 

concluded there to be a causal 

relationship between long- and shortterm PM2.5 exposure and mortality and 

cardiovascular effects, as well as likely 

to be causal relationships between longand short-term PM2.5 exposure and 

respiratory effects, and between longterm PM2.5 exposure and cancer and 

nervous system effects (U.S. EPA, 

2019a, section 1.7). 

Epidemiologic studies reported PM2.5

health effect associations with mortality 

and/or morbidity across multiple U.S. 

cities and in diverse populations, 

including in studies examining 

populations and lifestages that may be 

at increased risk of experiencing a 

PM2.5-related health effect (e.g., older 

adults, children). The 2019 ISA cited 

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, 2019a, p. 12–1), including 

children and older adults, people with 

pre-existing respiratory or 

cardiovascular disease, minority 

populations, and low socioeconomic 

status (SES) populations. 

The risk information available in the 

2020 review included risk estimates for 

air quality conditions just meeting the 

existing primary PM2.5 standards, and 

also for air quality conditions just 

meeting potential alternative standards. 

The general approach to estimating 

PM2.5-associated health risks combined 

concentration-response (C–R) functions 

from epidemiologic studies with modelbased PM2.5 air quality surfaces, 

baseline health incidence data, and 

population demographics for 47 urban 

areas (U.S. EPA, 2020b, section 3.3, 

Figure 3–10, Appendix C). The risk 

assessment estimated that the existing 

primary PM2.5 standards could allow a 

substantial number of PM2.5-associated 

deaths in the U.S. 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 C–R relationship with 

mortality at low ambient PM2.5

concentrations, the potential for 

confounding and/or exposure 

measurement error, and the methods 

used to adjust PM2.5 air quality. 

Consistent with the general approach 

routinely employed in NAAQS reviews, 

the initial consideration in the 2020 

review of the primary PM2.5 standards 

was with regard to the adequacy of the 

protection provided by the existing 

standards. 

As an initial matter, the thenAdministrator considered the range of 

scientific evidence evaluating these 

effects, including studies of at-risk 

populations, to inform his review of the 

primary PM2.5 standards, placing the 

greatest weight on evidence of effects for 

which the 2019 ISA determined there to 

be a causal or likely to be causal 

relationship with long- and short-term 

PM2.5 exposures (85 FR 82714–82715, 

December 18, 2020). 

With regard to indicator, the thenAdministrator recognized that, 

consistent with the evidence available 

in prior reviews, the scientific evidence 

continued to provide strong support for 

health effects following short- and longterm PM2.5 exposures. He noted the 

2020 PA conclusions that the 

information continued to support the 

PM2.5 mass-based indicator and 

remained 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. Thus, 

the then-Administrator concluded that 

it was appropriate to retain PM2.5 as the 

indicator for the primary standards for 

fine particles (85 FR 82715, December 

18, 2020). 

With respect to averaging time and 

form, the then-Administrator noted that 

the scientific evidence continued 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, consistent with the conclusions 

in the 2020 PA. In the 2019 ISA, 

epidemiologic and controlled human 

exposure studies examined a variety of 

PM2.5 exposure durations. 

Epidemiologic studies continued 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 noted 

that associations with subdaily 

estimates are less consistent and, in 

some cases, smaller in magnitude (U.S. 

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

2020b, section 3.5.2.2). In addition, 

controlled human exposure and panelbased studies of subdaily exposures 

typically examined subclinical effects, 

rather than the more serious populationlevel effects that have been reported to 

be associated with 24-hour exposures 

(e.g., mortality, hospitalizations). Taken 

together, the 2019 ISA concluded that 

epidemiologic studies did not indicate 

that subdaily averaging periods were 

more closely associated with health 

effects than the 24-hour average 

exposure metric (U.S. EPA, 2019a, 

section 1.5.2.1). Additionally, while 

controlled human exposure studies 

provided consistent evidence for 

cardiovascular effects following PM2.5

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

<30 minutes to 5 hours), exposure 

concentrations in the studies were wellabove the ambient concentrations 

typically measured in locations meeting 

the existing standards (U.S. EPA, 2020b, 

section 3.2.3.1). Thus, these studies also 

did not suggest the need for additional 

protection against subdaily PM2.5

exposures (U.S. EPA, 2020b, section 

3.5.2.2). Therefore, the thenAdministrator judged that the 24-hour 

averaging time remained appropriate (85 

FR 82715, December 18, 2020). 

With regard to the form of the 24-hour 

standard (98th percentile, averaged over 

three years), the then-Administrator 

noted that epidemiologic studies 

continued to provide strong support for 

health effect associations with shortterm (e.g., mostly 24-hour) PM2.5

exposures (U.S. EPA, 2020b, section 

3.5.2.3) and that controlled human 

exposure studies provided evidence for 

health effects following single shortterm ‘‘peak’’ PM2.5 exposures. Thus, the 

evidence supported retaining a standard 

focused on providing supplemental 

protection against short-term peak 

exposures and supported a 98th 

percentile form for a 24-hour standard. 

The then-Administrator further noted 

that this form also provided 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, 2020b, section 3.5.2.3). As 

such, the then-Administrator concluded 

that the available information supported 

retaining 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) (85 FR 82715, 

December 18, 2020). 

With regard to the level of the 

standards, in reaching his final decision, 

the then-Administrator considered the 

large body of evidence presented and 

assessed in the 2019 ISA (U.S. EPA, 

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

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

2020b), advice from the CASAC, and 

public comments. In particular, in 

considering the 2019 ISA and 2020 PA, 

he considered key epidemiologic 

studies that evaluated 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 the 

information (85 FR 82715, December 18, 

2020). 


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As an initial matter, the thenAdministrator considered the protection 

afforded by both the annual and 24-hour 

standards together against long- and 

short-term PM2.5 exposures and health 

effects. The Administrator recognized 

that the annual standard was most 

effective in controlling ‘‘typical’’ PM2.5

concentrations near the middle of the 

air quality distribution (i.e., around the 

mean of the distribution), but also 

provided some control over short-term 

peak PM2.5 concentrations. On the other 

hand, the 24-hour standard, with its 

98th percentile form, was most effective 

at limiting peak 24-hour PM2.5

concentrations, but in doing so also had 

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 were 

not expected to be equally effective at 

limiting both types of exposures. Thus, 

consistent with previous reviews, the 

then-Administrator’s consideration of 

the public health protection provided by 

the existing primary PM2.5 standards 

was based on his consideration of the 

combination of the annual and 24-hour 

standards. Specifically, he recognized 

that the annual standard was more 

likely to appropriately limit the 

‘‘typical’’ daily and annual exposures 

that are most strongly associated with 

the health effects observed in 

epidemiologic studies. The thenAdministrator concluded that an annual 

standard (as the arithmetic mean, 

averaged over three years) remained 

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) 

was more directly tied to short-term 

peak PM2.5 concentrations, and more 

likely to appropriately limit exposures 

to such concentrations, the thenAdministrator concluded that the 

current 24-hour standard (with its 98th 

percentile form, averaged over three 

years) remained 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 then-Administrator 

recognized 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 then-Administrator 

further recognized 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 (85 

FR 82715–82716, December 18, 2020). 

Thus, in considering the adequacy of 

the 24-hour standard, the thenAdministrator noted the importance of 

considering whether additional 

protection was needed against shortterm exposures to peak PM2.5

concentrations. In examining the 

scientific evidence, he noted the limited 

utility of the animal toxicological 

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. The thenAdministrator noted that controlled 

human exposure studies provided 

evidence for health effects following 

single, short-term PM2.5 exposures that 

corresponded best to 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 

examined exposure concentrations 

considerably higher than are typically 

measured in areas meeting the standards 

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

particular, controlled human exposure 

studies often reported 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 2020 PA (U.S. EPA, 

2020b, p. 3–49) noted that 2-hour 

ambient concentrations of PM2.5 at 

monitoring sites meeting the current 

standards almost never exceeded 32 mg/ 

m3. In fact, even the extreme upper end 

of the distribution of 2-hour PM2.5

concentrations at sites meeting the 

primary PM2.5 standards remained wellbelow 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, 

the experimental evidence did not 

indicate the need for additional 

protection against exposures to peak 

PM2.5 concentrations, beyond the 

protection provided by the combination 

of the 24-hour and the annual standards 

(U.S. EPA, 2020b, section 3.2.3.1; 85 FR 

82716, December 18, 2020). 

With respect to the epidemiologic 

evidence, the then-Administrator noted 

that the studies did not indicate that 

associations in those studies were 

strongly influenced by exposures to 

peak concentrations in the air quality 

distribution and thus did not indicate 

the need for additional protection 

against short-term exposures to peak 

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

section 3.5.1). The then-Administrator 

noted that this was consistent with 

CASAC consensus support for retaining 

the current 24-hour standard. Thus, the 

then-Administrator concluded that the 

24-hour standard with its level of 35 mg/ 

m3 was adequate to provide 

supplemental protection (i.e., beyond 

that provided by the annual standard 

alone) against short-term exposures to 

peak PM2.5 concentrations (85 FR 82716, 

December 18, 2020). 

With regard to the level of the annual 

standard, the then-Administrator 

recognized that the annual standard, 

with its form based on the arithmetic 

mean concentration, was most 

appropriately meant to limit the 

‘‘typical’’ daily and annual exposures 

that were most strongly associated with 

the health effects observed in 

epidemiologic studies. However, the 

then-Administrator also noted that 

while epidemiologic studies examined 

associations between distributions of 

PM2.5 air quality and health outcomes, 

they did not identify particular PM2.5

exposures that cause effects and thus, 

they could not alone identify a specific 

level at which the standard should be 

set, as such a determination necessarily 

required the then-Administrator’s 

judgment. Thus, consistent with the 

approaches in previous NAAQS 

reviews, the then-Administrator 

recognized that any approach that used 

epidemiologic information in reaching 

decisions on what standards are 

appropriate necessarily required 

judgments about how to translate the 

information from the epidemiologic 

studies into a basis for appropriate 

standards. This approach included 

consideration of the uncertainties in the 

reported associations between daily or 

annual average PM2.5 exposures and 

mortality or morbidity in the 

epidemiologic 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 Clean Air Act (CAA) (85 FR 

82716, December 18, 2020). 

The then-Administrator emphasized 

uncertainties and limitations that were 

present in epidemiologic studies in 

previous reviews and persisted in the 

2020 review. These uncertainties 


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49The median of the study-reported mean (or 

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

was also above the level of the existing standard. 

included 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 (85 FR 82716, 

December 18, 2020). The thenAdministrator also noted the advice 

given by the CASAC on this matter. As 

described in section I.C.5 above, the 

CASAC did not reach consensus on the 

adequacy of the primary annual PM2.5

standard. ‘‘Some CASAC members’’ 

expressed support for retaining the 

primary annual PM2.5 standard while 

‘‘other members’’ expressed support for 

revising that standard in order to 

increase public health protection (Cox, 

2019b, p. 1 of consensus letter). The 

CASAC members who supported 

retaining the annual standard expressed 

their concerns with the epidemiologic 

studies, asserting that these studies did 

not provide a sufficient basis for 

revising the existing standards. They 

also identified several key concerns 

regarding the associations reported in 

epidemiologic studies and concluded 

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, 2019b, p. 8 consensus 

responses). 

Taking into consideration the views 

expressed by the CASAC members who 

supported retaining the annual 

standard, the then-Administrator 

recognized that epidemiologic studies 

examined associations between 

distributions of PM2.5 air quality and 

health outcomes, and they did not 

identify particular PM2.5 exposures that 

cause effects (U.S. EPA, 2020b, section 

3.1.2). While the Administrator 

remained concerned about placing too 

much weight on epidemiologic studies 

to inform conclusions on the adequacy 

of the primary standards, he noted the 

approach to considering such studies in 

the 2012 review. In the 2012 review, it 

was noted that the evidence of an 

association in any epidemiologic study 

was ‘‘strongest at and around the longterm average where the data in the study 

are most concentrated’’ (78 FR 3140, 

January 15, 2013). In considering the 

characterization of epidemiologic 

studies, the then-Administrator viewed 

that when assessing the mean 

concentrations of the key short-term and 

long-term epidemiologic studies in the 

U.S. that used ground-based monitoring 

(i.e., those studies where the mean is 

most directly comparable to the current 

annual standard), the majority of studies 

had mean concentrations at or above the 

level of the existing annual standard, 

with the mean of the study-reported 

means or medians equal to 13.5 mg/m3, 

a concentration level above the existing 

level of the primary annual standard of 

12 mg/m3. The then-Administrator 

further noted his caution in directly 

comparing the reported study mean 

values to the standard level given that 

study-reported mean concentrations, by 

design, are generally lower than the 

design value of the highest monitor in 

an area, which determines compliance. 

In the 2020 PA, analyses of recent air 

quality in U.S. CBSAs indicated that 

maximum annual PM2.5 design values 

for a given three-year period were often 

10% to 20% higher than average 

monitored concentrations (i.e., averaged 

across multiple monitors in the same 

CBSA) (U.S. EPA, 2020b, Appendix B, 

section B.7). He further noted his 

concern in placing too much weight on 

any one epidemiologic study but instead 

judged that it was more appropriate to 

focus on the body of studies together 

and therefore noted the calculation of 

the mean of study-reported means (or 

medians). Thus, while the thenAdministrator was cautious in placing 

too much weight on the epidemiologic 

evidence alone, he noted that: (1) The 

reported mean concentration in the 

majority of the key U.S. epidemiologic 

studies using ground-based monitoring 

data were above the level of the existing 

annual standard; (2) the mean of the 

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

13.5 mg/m3) was above the level of the 

current standard; 49 (3) air quality 

analyses showed the study means to be 

lower than their corresponding design 

values by 10–20%; and (4) these 

analyses must be considered in light of 

uncertainties inherent in the 

epidemiologic evidence. When taken 

together, the then-Administrator judged 

that, even if it were appropriate to place 

more weight on the epidemiologic 

evidence, this information did not call 

into question the adequacy of the 

current standards (85 FR 82716–17, 

December 18, 2020). 

In addition to the evidence, the thenAdministrator also considered the 

potential implications of the risk 

assessment. He noted that all risk 

assessments have limitations and that 

he remained concerned about the 

uncertainties in the underlying 

epidemiologic data used in the risk 

assessment. The then-Administrator 

also noted 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 included 

uncertainty in the shapes of C–R 

functions, particularly at low 

concentrations; uncertainties in the 

methods used to adjust air quality; and 

uncertainty in estimating risks for 

populations, locations and air quality 

distributions different from those 

examined in the underlying 

epidemiologic study (U.S. EPA, 2020b, 

section 3.3.2.4). Additionally, the thenAdministrator noted similar concern 

expressed by some members of the 

CASAC who support retaining the 

existing standards; they highlighted 

similar uncertainties and limitations in 

the risk assessment (Cox, 2019b). In 

light of all of this, the thenAdministrator judged 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 (85 

FR 82717, December 18, 2020). 

The then-Administrator additionally 

considered an emerging body of 

evidence from accountability studies 

that examined past reductions in 

ambient PM2.5 and the degree to which 

those reductions resulted in public 

health improvements. While the thenAdministrator agreed with public 

commenters that well-designed and 

conducted accountability studies can be 

informative, he viewed the 

interpretation of such studies in the 

context of the primary PM2.5 standards 

as complicated by the fact that some of 

the available studies had 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 had not been able to 

disentangle health impacts of the 

interventions from background trends in 

health (U.S. EPA, 2020b, section 3.5.1). 

He further recognized that the small 

number of available studies that did 

report public health improvements 

following past declines in ambient PM2.5

had not examined air quality meeting 

the existing standards (U.S. EPA, 2020b, 

Table 3–3). This included U.S. studies 

that reported increased life expectancy, 

decreased mortality, and decreased 

respiratory effects following past 

declines in ambient PM2.5

concentrations. Such studies examined 

‘‘starting’’ annual average PM2.5

concentrations (i.e., prior to the 

reductions being evaluated) ranging 

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

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

of available accountability studies 


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reporting public health improvements 

attributable to reductions in ambient 

PM2.5 in locations meeting the existing 

standards, together with his broader 

concerns regarding the lack of 

experimental studies examining PM2.5

exposures typical of areas meeting the 

existing standards, the thenAdministrator judged 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 existing 

primary PM2.5 standards (85 FR 82717, 

December 18, 2020). 

When the above considerations were 

taken together, the then-Administrator 

concluded that the scientific evidence 

assessed in the 2019 ISA, together with 

the analyses in the 2020 PA based on 

that evidence and consideration of 

CASAC advice and public comments, 

did not call into question the adequacy 

of the public health protection provided 

by the existing annual and 24-hour 

PM2.5 standards. In particular, the thenAdministrator judged that there was 

considerable uncertainty in the 

potential for additional public health 

improvements from reducing ambient 

PM2.5 concentrations below the 

concentrations achieved under the 

existing primary standards and that, 

therefore, standards more stringent than 

the existing standards (e.g., with lower 

levels) were not supported. That is, he 

judged that more stringent standards 

would be more than requisite to protect 

the public health with an adequate 

margin of safety. This judgment 

reflected the Administrator’s 

consideration of the uncertainties in the 

potential implications of the lower end 

of the air quality distributions from the 

epidemiologic studies due in part to the 

lack of supporting evidence from 

experimental studies and retrospective 

accountability studies conducted at 

PM2.5 concentrations meeting the 

existing standards (85 FR 82717, 

December 18, 2020). 

In reaching this conclusion in the 

2020 review, the then-Administrator 

judged that the existing standards 

provided an adequate margin of safety. 

With respect to the annual standard, the 

level of 12 mg/m3 was below the lowest 

‘‘starting’’ concentration (i.e., 13.2 mg/ 

m3) in the available accountability 

studies that showed public health 

improvements attributable to reductions 

in ambient PM2.5. In addition, while the 

then-Administrator placed less weight 

on the epidemiologic evidence for 

selecting a standard, he noted that the 

level of the annual standard was below 

the reported mean (and median) 

concentrations in the majority of the key 

U.S. epidemiologic studies using 

ground-based monitoring data (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 then-Administrator 

recognized that concentrations in areas 

meeting the existing 24-hour and annual 

standards remained well-below the 

PM2.5 exposure concentrations 

consistently shown to elicit effects in 

human exposure studies (85 FR 82717– 

82718, December 18, 2020). 

In addition, based on the thenAdministrator’s review of the science in 

the 2020 review, including controlled 

human exposure studies examining 

effects following short-term PM2.5

exposures, the epidemiologic studies, 

and accountability studies conducted at 

levels just above the existing annual 

standard, he judged that the degree of 

public health protection provided by the 

existing annual 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 then- Administrator to 

conclude that standards less stringent 

than the existing standards (e.g., with 

higher levels) were also not supported 

(85 FR 82718, December 18, 2020). 

In reaching his final decision in the 

2020 review, the then-Administrator 

concluded that the scientific evidence 

and technical information continued to 

support the existing annual and 24-hour 

PM2.5 standards. This conclusion 

reflected the then-Administrator’s view 

that there were important limitations 

and uncertainties that remained in the 

evidence. The then-Administrator 

concluded that these limitations 

contributed to considerable uncertainty 

regarding the potential public health 

implications of revising the existing 

primary PM2.5 standards. Given this 

uncertainty, and noting the advice from 

some CASAC members, he concluded 

that the primary PM2.5 standards, 

including the 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, remained requisite to 

protect the public health. Therefore, in 

the 2020 review, the Administrator 

reached the conclusion that the primary 

24-hour and annual PM2.5 standards, 

together, were requisite to protect public 

health from fine particles with an 

adequate margin of safety, including the 

health of at-risk populations, and 

retained the standards, without revision 

(85 FR 82718, December 18, 2020). 

2. Overview of the Health Effects 

Evidence 

The information summarized here 

and further detailed in section II.B of 

the proposal (88 FR 5580, January 27, 

2023), is an overview of the policyrelevant aspects of the health effects 

evidence available in this 

reconsideration; the assessment of this 

evidence is documented in the 2019 ISA 

(U.S. EPA, 2019a) and ISA Supplement 

(U.S. EPA, 2022a) and its policy 

implications are further discussed in the 

2022 PA (U.S. EPA, 2022b). While the 

2019 ISA provides the broad scientific 

foundation for this reconsideration, 

additional literature has become 

available since the cutoff date of the 

2019 ISA that expands the body of 

evidence related to mortality and 

cardiovascular effects for both shortand long-term PM2.5 exposure, which 

can inform the Administrator’s 

judgment on the adequacy of the current 

primary PM2.5 standards. As such, the 

ISA Supplement builds on the 

information presented within the 2019 

ISA with a targeted identification and 

evaluation of new scientific information 

(U.S. EPA, 2022a, section 1.2). The ISA 

Supplement focuses on PM2.5 health 

effects evidence where the 2019 ISA 

concludes a ‘‘causal relationship,’’ 

because such health effects are given the 

most weight in an Administrator’s 

decisions in a NAAQS review. As such, 

in selecting the health effects to evaluate 

within the ISA Supplement (i.e., newly 

available evidence related to short- and 

long-term PM2.5 exposure and mortality 

and cardiovascular effects), the primary 

rationale is based on the causality 

determinations for health effect 

categories presented in the 2019 PM 

ISA, and the subsequent use of the 

health effects evidence in the 2020 PM 

PA. Specifically, U.S. and Canadian 

epidemiologic studies for mortality and 

cardiovascular effects, along with 

controlled human exposure studies 

associated with cardiovascular effects at 

near ambient concentrations, were 

considered to be of greatest utility in 

informing the Administrator’s 

conclusions on the adequacy of the 

current primary PM2.5 standards. 

Additionally, studies examining 

associations outside the U.S. or Canada 

reflect air quality and exposure patterns 

that may be less typical of the U.S., and 

thus less likely to be informative for 

purposes of reviewing the NAAQS (U.S. 

EPA, 2022b, p.1–3). While the ISA 

Supplement does not include 

information for health effects other than 

mortality and cardiovascular effects, the 


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50As noted in the ISA Supplement (U.S. EPA, 

2022a, p. 1–3): ‘‘In the peer-reviewed literature, 

these epidemiologic studies are often referred to as 

causal inference studies or studies that used causal 

modeling methods. For the purposes of this 

Supplement, this terminology is not used to prevent 

confusion with the main scientific conclusions (i.e., 

the causality determinations) presented within an 

ISA. In addition, as is consistent with the weightof-evidence framework used within ISAs and 

discussed in the Preamble to the Integrated Science 

Assessments, an individual study on its own cannot 

inform causality, but instead represents a piece of 

the overall body of evidence.’’ 

51As with the epidemiologic studies for long- and 

short-term PM2.5 exposure and mortality and 

cardiovascular effects, epidemiologic studies of 

exposure or risk disparities and SARS–CoV–2 

infection and/or COVID–19 death were limited to 

those conducted in the U.S. and Canada. 

52 In this reconsideration 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 2019 

ISA to demonstrate a ‘‘causal’’ or ‘‘likely to be 

causal’’ relationship with PM2.5 exposures. 

53The majority of these studies examined nonaccidental mortality outcomes, though some 

Medicare studies lack cause-specific death 

information and, therefore, examine total mortality. 

scientific evidence for other health 

effect categories is evaluated in the 2019 

ISA, which in combination with the ISA 

Supplement represents the complete 

scientific record for the reconsideration 

of the 2020 final decision. 

The ISA Supplement also assessed 

accountability studies because these 

types of epidemiologic studies were part 

of the body of evidence that was a focus 

of the 2020 review. Accountability 

studies inform our understanding of the 

potential for public health 

improvements as ambient PM2.5

concentrations have declined over time. 

Further, the ISA Supplement considered 

studies that employed statistical 

approaches that attempt to more 

extensively account for confounders and 

are more robust to model 

misspecification (i.e., used alternative 

methods for confounder control),50

given that such studies were highlighted 

by the CASAC and identified in public 

comments in the 2020 review. Since the 

literature cutoff date for the 2019 ISA, 

multiple accountability studies and 

studies that employ alternative methods 

for confounder control have become 

available for consideration in the ISA 

Supplement and, subsequently, in this 

reconsideration. 

The ISA Supplement also considered 

recent health effects evidence that 

addresses key scientific issues where 

the literature has expanded since the 

completion of the 2019 ISA.51 The 2019 

ISA evaluated a couple of controlled 

human exposure studies that 

investigated the effect of exposure to 

near-ambient concentrations of PM2.5

(U.S. EPA, 2019a, section 6.1.10 and 

6.1.13). The ISA Supplement adds to 

this limited evidence, including a recent 

study conducted in young healthy 

individuals exposed to near-ambient 

PM2.5 concentrations (U.S. EPA, 2022a, 

section 3.3.1). Given the importance of 

identifying populations at increased risk 

of PM2.5-related effects, the ISA 

Supplement also included 

epidemiologic or exposure studies that 

examined whether there is evidence of 

exposure or risk disparities by race/ 

ethnicity or SES. These types of studies 

provide additional information related 

to factors that may increase risk of 

PM2.5-related health effects and provide 

additional evidence for consideration by 

the Administrator in reaching 

conclusions regarding the adequacy of 

the current standards. In addition, the 

ISA Supplement evaluated studies that 

examined the relationship between 

short- and long-term PM2.5 exposures 

and SARS-CoV–2 infection and/or 

COVID–19 death, as these studies are a 

new area of research and were raised by 

a number of public commenters in the 

2020 review. 

The evidence presented within the 

2019 ISA, along with the targeted 

identification and evaluation of new 

scientific information in the ISA 

Supplement, provides the scientific 

basis for the reconsideration of the 2020 

final decision on the primary PM2.5

standards. The subsections below 

briefly summarize the nature of PM2.5- 

related health effects (II.A.2.a), with a 

focus on those health effects for which 

the 2019 ISA concluded a ‘‘causal’’ or 

‘‘likely to be causal’’ relationship, the 

potential public health implications and 

populations at risk (II.A.2.b), and PM2.5

concentrations in key studies reporting 

health effects (II.A.2.c). 

a. Nature of Effects 

The evidence base available in the 

reconsideration includes decades of 

research on PM2.5-related health effects 

(U.S. EPA, 2004b; U.S. EPA, 2009a; U.S. 

EPA, 2019a), including the full body of 

evidence evaluated in the 2019 ISA 

(U.S. EPA, 2019a), along with the 

targeted evaluation of recent evidence in 

the ISA Supplement (U.S. EPA, 2022a). 

In considering the available scientific 

evidence, the sections below, and in 

more detail in section II.B.1 of the 

proposal (88 FR 5580, January 27, 2023), 

summarize the relationships between 

long-and short-term 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 2019 ISA concluded that the 

evidence supports either a ‘‘causal’’ or 

a ‘‘likely to be causal’’ relationship.52

i. Mortality 

Long-Term PM2.5 Exposures 

In the 2012 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, 2009a, p. 7–96). The strongest 

evidence supporting this conclusion 

was provided by epidemiologic studies, 

particularly those examining two 

seminal cohorts, the American Cancer 

Society (ACS) cohort and the Harvard 

Six Cities cohort. Analyses of the 

Harvard Six Cities cohort included 

evidence indicating that reductions in 

ambient PM2.5 concentrations are 

associated with reduced mortality risk 

(Laden et al., 2006) and 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

exposure and mortality (U.S. EPA, 

2019a). 

Cohort studies, which have become 

available since the completion of the 

2009 ISA and evaluated in the 2019 ISA, 

continue to provide consistent evidence 

of positive associations between longterm PM2.5 exposures and mortality. 

These studies add support for 

associations with all-cause and total 

(non-accidental) mortality,53 as well as 

with specific causes of mortality, 

including cardiovascular disease and 

respiratory disease (U.S. EPA, 2019a, 

section 11.2.2). Several of these studies 

conducted analyses over longer study 

durations and periods of follow-up than 

examined in the original ACS and 

Harvard Six Cities cohort studies and 

continue to report positive associations 

between long-term exposure to PM2.5

and mortality (U.S. EPA, 2019a, section 

11.2.2.1; Figures 11–18 and 11–19). In 

addition to studies focusing on the ACS 

and Harvard Six Cities cohorts, 

additional studies examining other 

cohorts also provide evidence of 

consistent, positive associations 

between long-term PM2.5 exposure and 

mortality across a wide range of 

demographic groups (e.g., age, sex, 

occupation), spatial and temporal 

extents, exposure assessment metrics, 

and statistical techniques (U.S. EPA, 

2019a, sections 11.2.2.1, 11.2.5; U.S. 

EPA, 2022a, Table 11–8). This includes 

some of the largest cohort studies 

conducted to date, such as analyses of 

the U.S. Medicare cohort that includes 


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nearly 61 million enrollees and studies 

that control for a range of individual 

and ecological covariates, including 

race, age, SES, smoking status, body 

mass index, and annual weather 

variables (e.g., temperature, humidity) 

(U.S. EPA, 2019a). 

In addition to those cohort studies 

evaluated in the 2019 ISA, recent North 

American cohort studies evaluated in 

the ISA Supplement continue to 

examine the relationship between longterm PM2.5 exposure and mortality and 

report consistent, positive, and 

statistically significant associations. 

These recent studies also utilize large 

and demographically diverse cohorts 

that are generally representative of the 

national populations in both the U.S. 

and Canada. These ‘‘studies published 

since the 2019 ISA support and extend 

the evidence base that contributed to the 

conclusion of a causal relationship 

between long-term PM2.5 exposure and 

mortality’’ (U.S. EPA, 2022a, section 

3.2.2.2.1, Figure 3–19, Figure 3–20). 

Furthermore, studies evaluated in the 

2019 ISA and the ISA Supplement that 

examined cause-specific mortality 

expand upon previous research that 

found consistent, positive associations 

between PM2.5 exposure and specific 

mortality outcomes, which include 

cardiovascular and respiratory 

mortality, as well as other mortality 

outcomes. For cardiovascular-related 

mortality, the evidence evaluated in the 

ISA Supplement is consistent with the 

evidence evaluated in the 2019 ISA with 

recent studies reporting positive 

associations with long-term PM2.5

exposure. When evaluating causespecific cardiovascular mortality, recent 

studies reported positive associations 

for a number of outcomes, such as 

ischemic heart disease (IHD) and stroke 

mortality (U.S. EPA, 2022a, Figure 3– 

23). Moreover, recent studies also 

provide some initial evidence that 

individuals with pre-existing health 

conditions, such as heart failure and 

diabetes, are at an increased risk of 

PM2.5-related health effects (U.S. EPA, 

2022a, section 3.2.2.4) and that these 

individuals have a higher risk of 

mortality overall, which was previously 

only examined in studies that used 

stratified analyses rather than a cohort 

of people with an underlying health 

condition (U.S. EPA, 2022a, section 

3.2.2.4). With regard to respiratory 

mortality, epidemiologic studies 

evaluated in the 2019 ISA and ISA 

Supplement continue to provide 

support for associations between longterm PM2.5 exposure and respiratory 

mortality (U.S. EPA, 2019a, section 

5.2.10; U.S. EPA, 2022a, Table 3–2). 

A series of epidemiologic studies 

evaluated in the 2019 ISA tested the 

hypothesis that past reductions in 

ambient PM2.5 concentrations are 

associated with increased life 

expectancy or a decreased mortality rate 

and report that reductions in ambient 

PM2.5 are associated with improvements 

in longevity (U.S. EPA, 2022a, 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, a time period 

with lower ambient PM2.5

concentrations and found a decrease in 

long-term PM2.5 concentration 

continued to be associated with an 

increase in life expectancy, though the 

magnitude of the increase was smaller 

than during the earlier time period (i.e., 

a 10 mg/m3 decrease in long-term PM2.5

concentration was associated with a 

0.35-year increase in life expectancy) 

(Correia et al., 2013). 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, 

2022a, section 11.2.2.5). 

Since the literature cutoff date for the 

2019 ISA, a few epidemiologic studies 

were published that examined the 

relationship between long-term PM2.5

exposure and life-expectancy (U.S. EPA, 

2022a, section 3.2.1.3) and report results 

that are consistent with and expand 

upon the body of evidence from the 

2019 ISA. For example, Bennett et al. 

(2019) reported that PM2.5

concentrations above the lowest 

observed concentration (2.8 mg/m3) were 

associated with a 0.15 year decrease in 

national life expectancy for women and 

0.13 year decrease in national life 

expectancy for men (U.S. EPA, 2022a, 

section 3.2.2.2.4, Figure 3–25). Another 

study compared participants living in 

areas with PM2.5 concentrations >12 mg/ 

m3 to participants living in areas with 

PM2.5 concentrations <12 mg/m3 and 

reported that the number of years of life 

lost due to living in areas with higher 

PM2.5 concentrations was 0.84 years 

over a 5-year period (Ward-Caviness et 

al., 2020; U.S. EPA, 2022a, section 

3.2.2.2.4). 

Additionally, a number of 

accountability studies, which are 

epidemiologic studies that evaluate 

whether an environmental policy or air 

quality intervention resulted in 

reductions in ambient air pollution 

concentrations and subsequent 

reductions in mortality or morbidity, 

have emerged and were evaluated in the 

ISA Supplement (U.S. EPA, 2022a, 

section 3.2.2.3). For example, Sanders et 

al. (2020a) examined whether policy 

actions (i.e., the first annual PM2.5

NAAQS implementation rule in 2005 

for the 1997 annual PM2.5 standard with 

a 3-year annual average of 15.0 mg/m3) 

reduced PM2.5 concentrations and 

mortality rates in Medicare beneficiaries 

between 2000–2013, and found that 

following implementation of the annual 

PM2.5 NAAQS, annual PM2.5

concentrations decreased by 1.59 mg/m3

(95% CI: 1.39, 1.80) which 

corresponded to a 0.93% reduction in 

mortality rates among individuals 65 

years and older ([95% CI: 0.10%, 

1.77%) in non-attainment counties 

relative to attainment counties. 

The 2019 ISA also evaluated a small 

number of studies that used alternative 

methods for confounder control to 

further assess relationship between 

long-term PM2.5 exposure and mortality 

(U.S. EPA, 2019a, section 11.2.2.4). In 

addition, multiple epidemiologic 

studies that implemented alternative 

methods for confounder control and 

were published since the literature 

cutoff date of the 2019 ISA were 

evaluated in the ISA Supplement (U.S. 

EPA, 2022a, section 3.2.2.3). These 

studies used a variety of statistical 

methods including generalized 

propensity score (GPS), inverse 

probability weighting (IPW), and 

difference-in-difference (DID) to reduce 

uncertainties related to confounding 

bias in the association between longterm PM2.5 exposure and mortality. 

These studies reported consistent 

positive associations between long-term 

PM2.5 exposure and total mortality (U.S. 

EPA, 2022a, section 3.2.2.3), and 

provided further support for the 

associations reported in the cohort 

studies referenced above. 

The 2019 ISA and ISA Supplement 

also evaluated the degree to which 

recent studies examining the 

relationship between long-term PM2.5

exposure and mortality addressed key 

policy-relevant issues and/or previously 

identified data gaps in the scientific 

evidence, including methods to estimate 

exposure, methods to control for 

confounding (e.g., co-pollutant 

confounding), the shape of the C–R 

relationship, as well as examining 

whether a threshold exists below which 

mortality effects do not occur. With 

respect to exposure assessment, based 

on its evaluation of the evidence, the 

2019 ISA concludes that positive 

associations between long-term PM2.5

exposures and mortality are robust 


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54 In public comments on the 2019 draft PA, the 

authors of the Pun et al. (2017) study further note 

that ‘‘the presence of unmeasured 

confounding. . .was expected given that we did not 

control for several potential confounders that may 

impact PM2.5-mortality associations, such as 

smoking, socio-economic status (SES), gaseous 

pollutants, PM2.5 components, and long-term time 

trends in PM2.5’’ and that ‘‘spatial confounding may 

bias mortality risks both towards and away from the 

null’’ (Docket ID EPA–HQ–OAR–2015–0072–0065; 

accessible in https://www.regulations.gov/). 

across recent analyses using various 

approaches to estimate PM2.5 exposures 

(e.g., based on monitors, models, 

satellite-based methods, or hybrid 

methods that combine information from 

multiple sources) (U.S. EPA, 2019a, 

section 11.2.5.1). Hart et al. (2015) 

report that correction for bias due to 

exposure measurement error increases 

the magnitude of the hazard ratios 

(confidence intervals widen but the 

association remains statistically 

significant), suggesting that failure to 

correct for exposure measurement error 

could result in attenuation or 

underestimation of risk estimates. 

The 2019 ISA additionally concludes 

that positive associations between longterm PM2.5 exposures and mortality are 

robust across statistical models that use 

different approaches to control for 

confounders or different sets of 

confounders (U.S. EPA, 2019a, sections 

11.2.3 and 11.2.5), across diverse 

geographic regions and populations, and 

across a range of temporal periods 

including periods of declining PM 

concentrations (U.S. EPA, 2019a, 

sections 11.2.2.5 and 11.2.5.3). 

Additional evidence further 

demonstrates that associations with 

mortality remain robust in copollutants 

analyses (U.S. EPA, 2019a, section 

11.2.3), and that associations persist in 

analyses restricted to long-term 

exposures (annual average PM2.5

concentrations) below 12 mg/m3 (Di et 

al., 2017b) or 10 mg/m3 (Shi et al., 2016), 

indicating that risks are not 

disproportionately driven by the upper 

portions of the air quality distribution. 

Recent studies evaluated in the ISA 

Supplement further assess potential 

copollutant confounding and indicate 

that while there is some evidence of 

potential confounding of the PM2.5- 

mortality association by copollutants in 

some of the studies (i.e., those studies of 

the Mortality Air Pollution Associations 

in Low Exposure Environments 

(MAPLE) cohort), this result is 

inconsistent with other recent studies 

evaluated in the 2019 ISA that were 

conducted in the U.S. and Canada that 

found associations in both single and 

copollutant models (U.S. EPA, 2019a; 

U.S. EPA, 2022a, section 3.2.2.4) 

Additionally, a few studies use 

statistical techniques to reduce 

uncertainties related to potential 

confounding to further inform 

conclusions on causality for long-term 

PM2.5 exposure and mortality, as further 

detailed in section II.B.1.a.i of the 

proposal (88 FR 5582, January 27, 2023), 

studies by Greven et al. (2011), Pun et 

al. (2017), and Eum et al. (2018) 

completed sensitivity analyses as part of 

their Medicare cohort study in which 

they decompose ambient PM2.5 into 

‘‘spatial’’ and ‘‘spatiotemporal’’ 

components in order to evaluate the 

potential for bias due to unmeasured 

spatial confounding. Pun et al. (2017) 

observed positive associations for the 

‘‘temporal’’ variation model and 

approximately null associations for the 

‘‘spatiotemporal’’ variation model for all 

causes of death except for COPD 

mortality. The difference in the results 

of these two models for most causes of 

death suggests the presence of 

unmeasured confounding, though the 

authors do not indicate anything about 

the direction or magnitude of this bias. 

It is important to note that the 

‘‘temporal’’ and ‘‘spatiotemporal’’ 

coefficients are not directly comparable 

to the results of other epidemiologic 

studies when examined individually 

and can only be used in comparison 

with one another to evaluate the 

potential for unmeasured confounding 

bias. Eum et al. (2018) and Wu et al. 

(2020) also attempted to address longterm trends and meteorological 

variables as potential confounders and 

found that not adjusting for temporal 

trends could overestimate the 

association, while effect estimates in 

analyses that excluded meteorological 

variables remained unchanged 

compared to the main analyses. While 

results of these analyses suggest the 

presence of some unmeasured 

confounding, they do not indicate the 

direction or magnitude of the bias.54

An additional important 

consideration in characterizing the 

public health impacts associated with 

PM2.5 exposure is whether C–R 

relationships are linear across the range 

of concentrations or if nonlinear 

relationships exist along any part of this 

range. Studies evaluated in the 2019 ISA 

and the ISA Supplement examine this 

issue, and continue to provide evidence 

of linear, no-threshold relationships 

between long-term PM2.5 exposures and 

all-cause and cause-specific mortality 

(U.S. EPA, 2019a, section 11.2.4; U.S. 

EPA, 2022a, section 3.2.2.2.7, Table 3– 

6). Across the studies evaluated in the 

2019 ISA and the ISA Supplement, a 

variety of statistical methods have been 

used to assess whether there is evidence 

of deviations in linearity (U.S. EPA, 

2019a, Table 11–7; U.S. EPA, 2022a, 

section 2.2.3.2). Studies have also 

conducted cut-point analyses that focus 

on examining risk at specific ambient 

PM2.5 concentrations. Generally, the 

evidence remains consistent in 

supporting a no-threshold relationship, 

and in supporting a linear relationship 

for PM2.5 concentrations >8 mg/m3. 

However, uncertainties remain about 

the shape of the C–R curve at PM2.5

concentrations <8 mg/m3, with some 

recent studies providing evidence for 

either a sublinear, linear, or supralinear 

relationship at these lower 

concentrations (U.S. EPA, 2019a, 

section 11.2.4; U.S. EPA, 2022a, section 

2.2.3.2). There was also some limited 

evidence indicating that the slope of the 

C–R function may be steeper 

(supralinear) at lower concentrations for 

cardiovascular mortality (U.S. EPA, 

2022a, section 3.1.1.2.6). 

The biological plausibility of PM2.5- 

attributable mortality is supported by 

the coherence of effects across scientific 

disciplines (i.e., animal toxicological, 

controlled human exposure studies, and 

epidemiologic) when evaluating 

respiratory and cardiovascular 

morbidity effects, which are some of the 

largest contributors to total 

(nonaccidental) mortality. The 2019 ISA 

outlines the available evidence for 

biologically plausible pathways by 

which inhalation exposure to PM2.5

could progress from initial events (e.g., 

pulmonary inflammation, autonomic 

nervous system activation) to endpoints 

relevant to population outcomes, 

particularly those related to 

cardiovascular diseases such as 

ischemic heart disease, stroke and 

atherosclerosis (U.S. EPA, 2019a, 

section 6.2.1), and to metabolic effects, 

including diabetes (U.S. EPA, 2019a, 

section 7.3.1). The 2019 ISA notes 

‘‘more limited evidence from respiratory 

morbidity’’ (U.S. EPA, 2019a, p. 11–101) 

such as development of chronic 

obstructive pulmonary disease (COPD) 

(U.S. EPA, 2019a, section 5.2.1) to 

support the biological plausibility of 

mortality due to long-term PM2.5

exposures (U.S. EPA, 2019a, section 

11.2.1). 

Taken together, epidemiologic 

studies, including those evaluated in the 

2019 ISA and more recent studies 

evaluated in the ISA Supplement, 

consistently report positive associations 

between long-term PM2.5 exposure and 

mortality across different geographic 

locations, populations, and analytic 

approaches (U.S. EPA, 2019a; U.S. EPA, 

2022a, section 3.2.2.4). As such, these 

studies reduce key uncertainties 

identified in previous reviews, 

including those related to potential 


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55As detailed in the Preface to the ISA, risk 

estimates are for a 10 mg/m3 increase in 24-hour avg 

PM2.5 concentrations, unless otherwise noted (U.S. 

EPA, 2019a). 

copollutant confounding, and provide 

additional information on the shape of 

the C–R curve. As evaluated in the 2019 

ISA, experimental and epidemiologic 

evidence for cardiovascular effects, and 

respiratory effects to a more limited 

degree, supports the plausibility of 

mortality due to long-term PM2.5

exposures. Overall, studies evaluated in 

the 2019 ISA support the conclusion of 

a causal relationship between long-term 

PM2.5 exposure and mortality, which is 

supported and extended by evidence 

from recent epidemiologic studies 

evaluated in the ISA Supplement (U.S. 

EPA, 2022a, section 3.2.2.4). 

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, 2009a). This conclusion was 

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

that consistently reported positive 

associations between short-term PM2.5

exposure and non-accidental mortality. 

These associations were strongest, in 

terms of magnitude and precision, 

primarily at lags of 0 to 1 days. 

Examination of the potential 

confounding effects of gaseous 

copollutants was limited, though 

evidence from single-city studies 

indicated that gaseous copollutants have 

minimal effect on the PM2.5-mortality 

relationship (i.e., associations remain 

robust to inclusion of other pollutants in 

copollutant models). The evaluation of 

cause-specific mortality found that 

effect estimates were larger in 

magnitude, but also had larger 

confidence intervals, for respiratory 

mortality compared to cardiovascular 

mortality. Although the largest mortality 

risk estimates were for respiratory 

mortality, the interpretation of the 

results was complicated by the limited 

coherence from studies of respiratory 

morbidity. However, the evidence from 

studies of cardiovascular morbidity 

provided both coherence and biological 

plausibility for the relationship between 

short-term PM2.5 exposure and 

cardiovascular mortality. 

Multicity studies evaluated in the 

2019 ISA and the ISA Supplement 

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) 55 at lags of 0 to 1 

days in single-pollutant models. 

Whereas many studies assign exposures 

using data from ambient monitors, other 

studies employ hybrid modeling 

approaches, which estimate PM2.5

concentrations using data from a variety 

of sources (i.e., from satellites, land use 

information, and modeling, in addition 

to monitors) and enable the inclusion of 

less urban and more rural locations in 

analyses (e.g., Kloog et al., 2013, Lee et 

al., 2015, Shi et al., 2016). 

Some studies have expanded the 

examination of potential confounders 

including long-term temporal trends, 

weather, and co-occurring pollutants. 

Mortality associations were found to 

remain positive, although in some cases 

were attenuated, when using different 

approaches to account for temporal 

trends or weather covariates (e.g., U.S. 

EPA, 2019a, section 11.1.5.1). For 

example, Sacks et al. (2012) examined 

the influence of model specification 

using the approaches for confounder 

adjustment from models employed in 

several multicity studies within the 

context of a common data set (U.S. EPA, 

2019a, section 11.1.5.1). These models 

use different approaches to control for 

long-term temporal trends and the 

potential confounding effects of 

weather. The authors report that 

associations between daily PM2.5 and 

cardiovascular mortality were similar 

across models, with the percent increase 

in mortality ranging from 1.5–2.0% 

(U.S. EPA, 2019a, Figure 11–4). Thus, 

alternative approaches to controlling for 

long-term temporal trends and for the 

potential confounding effects of weather 

may influence the magnitude of the 

association between PM2.5 exposures 

and mortality but have not been found 

to influence the direction of the 

observed association (U.S. EPA, 2019a, 

section 11.1.5.1). Taken together, the 

2019 ISA and the ISA Supplement 

conclude that recent multicity studies 

conducted in the U.S., Canada, Europe, 

and Asia continue to provide consistent 

evidence of positive associations 

between short-term PM2.5 exposures and 

total mortality across studies that use 

different approaches to control for the 

potential confounding effects of weather 

(e.g., temperature) (U.S. EPA, 2019a, 

section 1.4.1.5.1; U.S. EPA, 2022a, 

section 3.2.1.2). 

With regard to copollutants, studies 

evaluated in the 2019 ISA provide 

additional evidence that 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, 2019a, 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, 2019a, section 

11.1.4). Consistent with the studies 

evaluated in the 2019 ISA, studies 

evaluated in the ISA Supplement that 

used data from more recent years also 

indicate that associations between shortterm PM2.5 exposure and mortality 

remain unchanged in copollutant 

models. However, the evidence 

indicates that the association could be 

larger in magnitude in the presence of 

some copollutants such as oxidant gases 

(Lavigne et al., 2018; Shin et al., 2021). 

The generally positive associations 

reported with mortality are supported 

by a small group of studies employing 

alternative methods for confounder 

control or quasi-experimental statistical 

approaches (U.S. EPA, 2019a, section 

11.1.2.1). For example, two studies by 

Schwartz et al. report associations 

between PM2.5 instrumental variables 

and mortality (U.S. EPA, 2019a, Table 

11–2), including in an analysis limited 

to days with 24-hour average PM2.5

concentrations <30 mg/m3 (Schwartz et 

al., 2015; Schwartz et al., 2017). In 

addition to the main analyses, these 

studies conducted Granger-like 

causality tests as sensitivity analyses to 

examine whether there was evidence of 

an association between mortality and 

PM2.5 after the day of death, which 

would support the possibility that 

unmeasured confounders were not 

accounted for in the statistical model. 

Neither study reports evidence of an 

association with PM2.5 after death (i.e., 

they do not indicate unmeasured 

confounding). Yorifuji et al. (2016) 

conducted a quasi-experimental study 

to examine 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 reported 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. In another study, 

Schwartz et al. (2018) utilized three 

statistical methods including 

instrumental variable analysis, a 

negative exposure control, and marginal 

structural models to estimate the 

association between PM2.5 and daily 

mortality (Schwartz et al., 2018). Results 

from this study continue to support a 

relationship between short-term PM2.5

exposure and mortality. Additional 

epidemiologic studies evaluated in the 

ISA Supplement that employed 

alternative methods for confounder 

control to examine the association 

between short-term PM2.5 exposure and 


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56Lee et al. (2015) restrict exposures below 35 mg/ 

m3 only in areas with annual average 

concentrations <12 mg/m3. Additionally, Lee et al. 

(2015) also report that positive and statistically 

significant associations between short-term PM2.5

exposures and mortality persist in analyses 

restricted to areas with long-term concentrations 

below 12 mg/m3. 

mortality also report consistent positive 

associations in studies that examine 

effects across multiple cities in the U.S. 

(U.S. EPA, 2022a). 

The positive associations for total 

mortality reported across the majority of 

studies evaluated are further supported 

by cause-specific mortality analyses, 

which generally report consistent, 

positive associations with both 

cardiovascular and respiratory mortality 

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

Recent multicity studies evaluated in 

the ISA Supplement add to the body of 

evidence indicating a relationship 

between short-term PM2.5 exposure and 

cause-specific mortality, with more 

variability in the magnitude and 

precision of associations for respiratory 

mortality (U.S. EPA, 2022a; Figure 3– 

14). For both cardiovascular and 

respiratory mortality, there has been 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, which further 

supports the copollutant analyses 

conducted for total mortality. The strong 

evidence for ischemic events and heart 

failure, as detailed in the assessment of 

cardiovascular morbidity (U.S. EPA, 

2019a, Chapter 6), provides biological 

plausibility for PM2.5-related 

cardiovascular mortality, which 

comprises the largest percentage of total 

mortality (i.e., ∼33%) (NHLBI, 2017). 

Although there is evidence for 

exacerbations of COPD and asthma, the 

collective body of respiratory morbidity 

evidence provides limited biological 

plausibility for PM2.5-related respiratory 

mortality (U.S. EPA, 2019a, 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. Studies 

evaluated in the 2019 ISA examine both 

city-specific as well as regional 

characteristics to identify the 

underlying contextual factors that could 

contribute to this heterogeneity (U.S. 

EPA, 2019a, section 11.1.6.3). Analyses 

focusing on effect modification of the 

PM2.5 mortality relationship by PM2.5

components, regional patterns in PM2.5

components and city specific 

differences in composition and sources 

indicate some differences in the PM2.5

composition and sources across cities 

and regions, but these differences do not 

fully explain the observed 

heterogeneity. Additional studies find 

that factors related to potential exposure 

differences, such as housing stock and 

commuting, as well as city specific 

factors (e.g., land use, port volume, and 

traffic information), may also explain 

some of the observed heterogeneity 

(U.S. EPA, 2019a, section 11.1.6.3). 

Collectively, studies evaluated in the 

2019 ISA and the ISA Supplement 

indicate that the heterogeneity in PM2.5

mortality risk estimates cannot be 

attributed to one factor, but instead a 

combination of factors including, but 

not limited to, PM composition and 

sources as well as community 

characteristics that could influence 

exposures (U.S. EPA, 2019a, section 

11.1.12; U.S. EPA, 2022a, section 

3.2.1.2.1). 

A number of studies evaluated in the 

2019 ISA and ISA Supplement 

conducted systematic evaluations of the 

lag structure of associations for the 

PM2.5-mortality relationship by 

examining either a series of single day 

or multiday lags and these studies 

continue to support an immediate effect 

(i.e., lag 0 to 1 days) of short-term PM2.5

exposures on mortality (U.S. EPA, 

2019a, section 11.1.8.1; U.S. EPA, 

2022a, section 3.2.1.1). Recent studies 

also conducted analyses comparing the 

traditional 24-hour average exposure 

metric with a subdaily metric (i.e., 1- 

hour max) and 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. 

Multicity studies indicate that 

positive and statistically significant 

associations with mortality persist in 

analyses restricted to short-term (24- 

hour average PM2.5 concentrations) 

PM2.5 exposures below 35 mg/m3 (Lee et 

al., 2015),56 below 30 mg/m3 (Shi et al., 

2016), and below 25 mg/m3 (Di et al., 

2017a), indicating that risks associated 

with short-term PM2.5 exposures are not 

disproportionately driven by the peaks 

of the air quality distribution. 

Additional studies examined the shape 

of the C–R relationship for short-term 

PM2.5 exposure and mortality and 

whether a threshold exists below which 

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

2019a, section 11.1.10). These studies 

used various statistical approaches and 

consistently demonstrate linear C–R 

relationships with no evidence of a 

threshold. 

Moreover, recent studies evaluated in 

the ISA Supplement provide additional 

support for a linear, no-threshold C–R 

relationship between short-term PM2.5

exposure and mortality, with confidence 

in the shape decreasing at 

concentrations below 5 mg/m3 (Shi et al., 

2016; Lavigne et al., 2018). Recent 

analyses provide initial evidence 

indicating that PM2.5-mortality 

associations persist and may be stronger 

(i.e., a steeper slope) at lower 

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

Figure 11–12 in U.S. EPA, 2019). 

However, given the limited data 

available at the lower end of the 

distribution of ambient PM2.5

concentrations, the shape of the C–R 

curve remains uncertain at these low 

concentrations. Although difficulties 

remain in assessing the shape of the 

short-term PM2.5-mortality C–R 

relationship, to date, studies have not 

conducted systematic evaluations of 

alternatives to linearity and recent 

studies evaluated in the ISA 

Supplement continue to provide 

evidence of a no-threshold linear 

relationship, with less confidence at 

concentrations lower than 5 mg/m3. 

Overall, epidemiologic studies 

evaluated in the 2019 ISA and the ISA 

Supplement 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, provide 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 examining 

the C–R relationship. Overall, studies 

evaluated in the 2019 ISA support the 

conclusion of a causal relationship 

between short-term PM2.5 exposure and 

mortality, which is further supported by 

evidence from recent epidemiologic 

studies evaluated in the ISA 

Supplement (U.S. EPA, 2022a, section 

3.2.1.4, p. 3–69). 

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, 2009a). The strongest 

line of evidence comprised findings 

from several large epidemiologic studies 

of U.S. and Canadian cohorts that 

reported consistent positive associations 

between long-term PM2.5 exposure and 

cardiovascular mortality (Pope et al., 

2004; Krewski et al., 2009; Miller et al., 


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57As noted above for mortality, uncertainty in the 

shape of the C–R relationship increases near the 

upper and lower ends of the distribution due to 

limited data. 

2007; Laden et al., 2006). Studies of 

long-term PM2.5 exposure and 

cardiovascular morbidity were limited 

in number. Biological plausibility and 

coherence with the epidemiologic 

findings were provided by studies using 

genetic mouse models of atherosclerosis 

demonstrating enhanced atherosclerotic 

plaque development and inflammation, 

as well as changes in measures of 

impaired heart function, following 4- to 

6-month exposures to PM2.5

concentrated ambient particles (CAPs), 

and by a limited number of studies 

reporting CAPs-induced effects on 

coagulation factors, vascular reactivity, 

and worsening of experimentally 

induced hypertension in mice (U.S. 

EPA, 2009a). 

Consistent with the evidence assessed 

in the 2009 ISA, the 2019 ISA concludes 

that recent studies, together with the 

evidence available in previous reviews, 

support a causal relationship between 

long-term exposure to PM2.5 and 

cardiovascular effects. Additionally, 

recent epidemiologic studies published 

since the completion of the 2019 ISA 

and evaluated in the ISA Supplement 

expands the body of evidence and 

further supports such a conclusion (U.S. 

EPA, 2022a). As discussed above 

(section II.A.2.a.i), results from U.S. and 

Canadian cohort studies evaluated in 

the 2019 ISA conducted at varying 

spatial and temporal scales and 

employing a variety of exposure 

assessment and statistical methods 

consistently report positive associations 

between long-term PM2.5 exposure and 

cardiovascular mortality (U.S. EPA, 

2019, Figure 6–19, 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 C–R relationship 

between long-term PM2.5 exposures and 

cardiovascular mortality support a 

linear relationship and do not identify 

a threshold below which mortality 

effects do not occur (U.S. EPA, 2019a, 

section 6.2.16, Table 6–52). 

The body of literature 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 evaluated in the 2019 

ISA (U.S. EPA, 2019a, section 6.2). 

Though results for cardiovascular 

morbidity are less consistent than those 

for cardiovascular mortality (U.S. EPA, 

2019a, section 6.2), studies in the 2019 

ISA and the ISA Supplement provide 

some evidence for associations between 

long-term PM2.5 exposures and the 

progression of cardiovascular disease. 

Positive associations with 

cardiovascular morbidity (e.g., coronary 

heart disease, stroke, arrhythmias, 

myocardial infarction (MI), 

atherosclerosis progression) are 

observed in several epidemiologic 

studies (U.S. EPA, 2019a, sections 6.2.2 

to 6.2.9; U.S. EPA, 2022a, section 

3.1.2.2). Additionally, studies evaluated 

in the ISA Supplement report positive 

associations among those with preexisting conditions, among patients 

followed after a cardiac event 

procedure, and among those with a first 

hospital admission for heart attacks 

among older adults enrolled in 

Medicare (U.S. EPA, 2022a, sections 

3.1.1 and 3.1.2). 

Recent studies published since the 

literature cutoff date of the 2019 ISA 

and evaluated in the ISA Supplement 

further assessed the relationship 

between long-term PM2.5 exposure and 

cardiovascular effects by conducting 

accountability analyses or by using 

alternative methods for confounder 

control in evaluating the association 

between long-term PM2.5 exposure and 

cardiovascular hospital admissions 

(U.S. EPA, 2022a, section 3.1.2.3). 

Studies that apply alternative methods 

for confounder control increase 

confidence in the relationship between 

long-term PM2.5 exposure and 

cardiovascular effects by using methods 

that reduce uncertainties related to 

potential confounding through 

statistical and/or study design 

approaches. For example, to control for 

potential confounding Wei et al. (2021) 

used a doubly robust additive model 

(DRAM) and found an association 

between long-term exposure to PM2.5

and cardiovascular effects, including 

MI, stoke, and atrial fibrillation, among 

the Medicare population. For example, 

an accountability study by Henneman et 

al. (2019) utilized a difference-indifference (DID) approach to determine 

the relationship between coal-fueled 

power plant emissions and 

cardiovascular effects and found that 

reductions in PM2.5 concentrations 

resulted in reductions of cardiovascularrelated hospital admissions. 

Furthermore, several recent 

epidemiologic studies evaluated in the 

ISA Supplement reported that the 

association between long-term PM2.5

exposure with stroke persisted after 

adjustment for NO2 but was attenuated 

in the model with O3 and oxidant gases 

represented by the redox weighted 

average of NO2 and O3 (U.S. EPA, 2022a, 

section 3.1.2.2.8). Overall, these studies 

report consistent findings that long-term 

PM2.5 exposure is related to increased 

hospital admissions for a variety of 

cardiovascular disease outcomes among 

large nationally representative cohorts 

and provide additional support for a 

relationship between long-term PM2.5

exposure and cardiovascular effects. 

Positive associations reported in 

epidemiologic studies are supported by 

toxicological evidence evaluated in the 

2019 ISA. The positive associations 

reported in epidemiologic studies are 

supported by toxicological evidence for 

increased plaque progression in mice 

following long-term exposure to PM2.5

collected from multiple locations across 

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

6.2.4.2). A small number of 

epidemiologic studies also report 

positive associations between long-term 

PM2.5 exposure and heart failure, 

changes in blood pressure, and 

hypertension (U.S. EPA, 2019a, sections 

6.2.5 and 6.2.7). Associations with heart 

failure are supported by animal 

toxicological studies demonstrating 

decreased cardiac contractility and 

function, and increased coronary artery 

wall thickness following long-term 

PM2.5 exposure (U.S. EPA, 2019a, 

section 6.2.5.2). Similarly, a limited 

number of animal toxicological studies 

demonstrating a relationship between 

long-term PM2.5 exposure and consistent 

increases in blood pressure in rats and 

mice are coherent with epidemiologic 

studies reporting positive associations 

between long-term exposure to PM2.5

and hypertension. 

Additionally, a number of studies 

evaluated in the ISA Supplement 

focusing on morbidity outcomes, 

including those that focused on 

incidence of MI, atrial fibrillation (AF), 

stroke, and congestive heart failure 

(CHF), expand the evidence pertaining 

to the shape of the C–R relationship 

between long-term PM2.5 exposure and 

cardiovascular effects. These studies use 

statistical techniques that allow for 

departures from linearity (U.S. EPA, 

2022a, Table 3–3), and generally 

support the evidence characterized in 

the 2019 ISA showing linear, nothreshold C–R relationship for most 

cardiovascular disease (CVD) outcomes. 

However, there is evidence for a 

sublinear or supralinear C–R 

relationship for some outcomes (U.S. 

EPA, 2022a, section 3.1.2.2.9).57

Longitudinal epidemiologic analyses 

also report positive associations with 

markers of systemic inflammation (U.S. 

EPA, 2019a, section 6.2.11), coagulation 

(U.S. EPA, 2019a, section 6.2.12), and 


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58Some 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 fine particle 

component of the mixture. 

endothelial dysfunction (U.S. EPA, 

2019a, section 6.2.13). These results are 

coherent with animal toxicological 

studies generally reporting increased 

markers of systemic inflammation, 

oxidative stress, and endothelial 

dysfunction (U.S. EPA, 2019a, section 

6.2.12.2 and 6.2.14). 

In summary, the 2019 ISA concludes 

that there is consistent evidence from 

multiple epidemiologic studies 

illustrating that long-term exposure to 

PM2.5 is associated with mortality from 

cardiovascular causes. Epidemiologic 

studies evaluated in the ISA 

Supplement provide additional 

evidence of positive associations 

between long-term PM2.5 exposure and 

cardiovascular morbidity (U.S. EPA, 

2022a, section 3.1.2.2). Associations 

with coronary heart disease (CHD), 

stroke and atherosclerosis progression 

were observed in several additional 

epidemiologic studies, providing 

coherence with the mortality findings. 

Results from copollutant models 

generally support an independent effect 

of PM2.5 exposure on mortality. 

Additional evidence of the independent 

effect of PM2.5 on the cardiovascular 

system is provided by experimental 

studies in animals, which support the 

biological plausibility of pathways by 

which long-term exposure to PM2.5

could potentially result in outcomes 

such as CHD, stroke, CHF, and 

cardiovascular mortality. Overall, 

studies evaluated in the 2019 ISA 

support the conclusion of a causal 

relationship between long-term PM2.5

exposure and cardiovascular effects, 

which is supported and extended by 

evidence from recent epidemiologic 

studies evaluated in the ISA 

Supplement (U.S. EPA, 2022a, section 

3.1.2.2). 

Short-Term PM2.5 Exposures 

The 2009 ISA concluded that ‘‘a 

causal relationship exists between shortterm exposure to PM2.5 and 

cardiovascular effects’’ (U.S. EPA, 

2009a). The strongest evidence in the 

2009 ISA was from epidemiologic 

studies of emergency department (ED) 

visits and hospital admissions for IHD 

and heart failure (HF), with supporting 

evidence from epidemiologic studies of 

cardiovascular mortality (U.S. EPA, 

2009a). Animal toxicological studies 

provided coherence and biological 

plausibility for the positive associations 

reported with MI, ED visits, and 

hospital admissions. These included 

studies reporting reduced myocardial 

blood flow during ischemia and studies 

indicating altered vascular reactivity. In 

addition, effects of PM2.5 exposure on a 

potential indicator of ischemia (i.e., ST 

segment depression on an 

electrocardiogram) were reported in 

both animal toxicological and 

epidemiologic panel studies.58 Key 

uncertainties from the last review 

resulted from inconsistent results across 

disciplines with respect to the 

relationship between short-term 

exposure to PM2.5 and changes in blood 

pressure, blood coagulation markers, 

and markers of systemic inflammation. 

In addition, while the 2009 ISA 

identified a growing body of evidence 

from controlled human exposure and 

animal toxicological studies, 

uncertainties remained with respect to 

biological plausibility. 

Studies evaluated in the 2019 ISA 

provide additional support for a causal 

relationship between short-term PM2.5

exposure and cardiovascular effects. 

This includes generally positive 

associations observed in multicity 

epidemiologic studies of emergency 

department visits and hospital 

admissions for IHD, heart failure (HF), 

and combined cardiovascular-related 

endpoints. In particular, nationwide 

studies of older adults (65 years and 

older) using Medicare records report 

positive associations between PM2.5

exposures and hospital admissions for 

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

Moreover, recent multicity studies, 

published after the literature cutoff date 

of the 2019 ISA and evaluated in the 

ISA Supplement, are consistent with 

studies evaluated in the 2019 ISA that 

report positive association between 

short-term PM2.5 exposure and ED visits 

and hospital admission for IHD, heart 

attacks, and HF (U.S. EPA, 2022a, 

section 3.1). Epidemiologic studies 

conducted in single cities contribute 

some support to the causality 

determination, though associations 

reported in single-city studies are less 

consistently positive than in multicity 

studies, and include a number of studies 

reporting null associations (U.S. EPA, 

2019a, sections 6.1.2 and 6.1.3). As a 

whole, though, the recent body of IHD 

and HF epidemiologic evidence 

supports the evidence from previous 

ISAs reporting mainly positive 

associations between short-term PM2.5

concentrations and emergency 

department visits and hospital 

admissions. 

Consistent with the evidence assessed 

in the 2019 ISA, some studies evaluated 

in the ISA Supplement report no 

evidence of an association with stroke, 

regardless of stroke subtype. 

Additionally, as in the 2019 ISA, 

evidence evaluated in the ISA 

Supplement continues to indicate an 

immediate effect of PM2.5 on 

cardiovascular-related outcomes 

primarily within the first few days after 

exposure, and that associations 

generally persisted in models adjusted 

for copollutants (U.S. EPA, 2022a, 

section 3.1.1.2). 

The ISA Supplement includes 

additional epidemiologic studies, 

published since the literature cutoff date 

for the 2019 ISA, including 

accountability analyses and 

epidemiologic studies that employ 

alternative methods for confounder 

control to evaluate the association 

between short-term PM2.5 exposure and 

cardiovascular-related effects (U.S. EPA, 

2022a, section 3.1.1.3). These studies 

employ a number of statistical 

approaches and report positive 

associations, providing additional 

support for a relationship between 

short-term PM2.5 exposure and 

cardiovascular effects, while also 

reducing uncertainties related to 

potential confounder bias. 

A number of controlled human 

exposure, animal toxicological, and 

epidemiologic 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, 2019a, 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. 

Multiple controlled human exposure 

studies that examined the potential for 

endothelial dysfunction report an effect 

of PM2.5 exposure on measures of blood 

flow (U.S. EPA, 2019a, section 6.1.13.2). 

However, these studies report variable 

results regarding the timing of the effect 

and the mechanism by which reduced 

blood flow occurs (i.e., availability vs 

sensitivity to nitric oxide). In addition, 

some controlled human exposure 

studies using CAPs report evidence for 

small increases in blood pressure (U.S. 

EPA, 2019a, section 6.1.6.3). Although 

not entirely consistent, there is also 

some evidence across controlled human 

exposure studies for conduction 

abnormalities/arrhythmia (U.S. EPA, 

2019a, section 6.1.4.3), changes in heart 

rate variability (HRV) (U.S. EPA, 2019a, 

section 6.1.10.2), changes in hemostasis 

that could promote clot formation (U.S. 

EPA, 2019a, section 6.1.12.2), and 

increases in inflammatory cells and 

markers (U.S. EPA, 2019a, section 

6.1.11.2). A recent study by Wyatt et al. 


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(2020), evaluated in the ISA 

Supplement, adds to the limited 

evidence base of controlled human 

exposure studies conducted at near 

ambient PM2.5 concentrations. The 

study, completed in healthy young 

adults subject to intermittent exercise, 

found some significant cardiovascular 

effects (e.g., systematic inflammation 

markers, including C-reactive protein 

(CRP), and cardiac repolarization). 

Thus, when taken as a whole, controlled 

human exposure studies are coherent 

with epidemiologic studies in that they 

demonstrate that short-term exposures 

to PM2.5 may result in the types of 

cardiovascular endpoints that could 

lead to emergency department visits and 

hospital admissions for IHD or HF, as 

well as mortality in some people. 

Animal toxicological studies 

published since the 2009 ISA and 

evaluated in the 2019 ISA also support 

a relationship between short-term PM2.5

exposure and cardiovascular effects. A 

study demonstrating decreased cardiac 

contractility and left ventricular 

pressure in mice is coherent with the 

results of epidemiologic studies that 

report associations between short-term 

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

EPA, 2019a, section 6.1.3.3). In 

addition, and as with controlled human 

exposure studies, there is generally 

consistent evidence in animal 

toxicological studies for indicators of 

endothelial dysfunction (U.S. EPA, 

2019a, section 6.1.13.3). Some studies in 

animals also provide evidence for 

changes in a number of other 

cardiovascular endpoints following 

short-term PM2.5 exposure including 

conduction abnormalities and 

arrhythmia (U.S. EPA, 2019a, section 

6.1.4.4), changes in HRV (U.S. EPA, 

2019a, section 6.1.10.3), changes in 

blood pressure (U.S. EPA, 2019a, 

section 6.1.6.4), and evidence for 

systemic inflammation and oxidative 

stress (U.S. EPA, 2019a, section 

6.1.11.3). 

In summary, evidence evaluated in 

the 2019 ISA extends the consistency 

and coherence of the evidence base 

evaluated in the 2009 ISA and prior 

assessments. Epidemiologic 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). For some cardiovascular 

effects, there are inconsistencies in 

results across some animal 

toxicological, controlled human 

exposure, and epidemiologic panel 

studies, though this may be due to 

substantial differences in study design 

and/or study populations. Overall, the 

results from epidemiologic panel, 

controlled human exposure, and animal 

toxicological studies, in particular those 

related to endothelial dysfunction, 

impaired cardiac function, ST segment 

depression, thrombosis, conduction 

abnormalities, and changes in blood 

pressure provide coherence and 

biological plausibility for the consistent 

results from epidemiologic studies 

observing positive associations between 

short-term PM2.5 exposures and IHD and 

HF, and ultimately cardiovascular 

mortality. Overall, studies evaluated in 

the 2019 ISA support the conclusion of 

a causal relationship between short-term 

PM2.5 exposure and cardiovascular 

effects, which is supported and 

extended by evidence from recent 

epidemiologic studies evaluated in the 

ISA Supplement (U.S. EPA, 2022a, 

section 3.1.1.4). 

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, 2009a). 

This conclusion was based mainly on 

epidemiologic evidence demonstrating 

associations between long-term PM2.5

exposure and changes in lung function 

or lung function growth in children. 

Biological plausibility was provided by 

a single animal toxicological study 

examining pre- and post-natal exposure 

to PM2.5 CAPs, which found impaired 

lung development. Epidemiologic 

evidence for associations between longterm PM2.5 exposure and other 

respiratory outcomes, such as the 

development of asthma, allergic disease, 

and COPD; respiratory infection; and 

the severity of disease was limited, both 

in the number of studies available and 

the consistency of the results. 

Experimental evidence for other 

outcomes was also limited, with one 

animal toxicological study reporting 

that long-term exposure to PM2.5 CAPs 

results in morphological changes in 

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. 

Cohort studies evaluated in the 2019 

ISA provided 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, 

2019a, section 5.2.13). This relationship 

was further supported by a retrospective 

study that reports an association 

between declining PM2.5 concentrations 

and improvements in lung function 

growth in children (U.S. EPA, 2019a, 

section 5.2.11). Epidemiologic studies 

also examine asthma development in 

children (U.S. EPA, 2019a, section 

5.2.3), with 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, 2019a, section 5.2.13). 

Additionally, the 2019 ISA includes an 

animal toxicological study showing the 

development of an allergic phenotype 

and an increase in a marker of airway 

responsiveness supports the biological 

plausibility of the development of 

allergic asthma (U.S. EPA, 2019a, 

section 5.2.13). Other epidemiologic 

studies report a PM2.5-related 

acceleration of lung function decline in 

adults, while improvement in lung 

function was observed with declining 

PM2.5 concentrations (U.S. EPA, 2019a, 

section 5.2.11). A 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, 2019a, 

section 5.2.11). A common uncertainty 

across the epidemiologic evidence is the 

lack of examination of copollutants to 

assess the potential for confounding. 

While there is some evidence that 

associations remain robust in models 

with gaseous pollutants, a number of 

these studies examining copollutant 

confounding were conducted in Asia, 

and thus have limited generalizability 

due to high annual pollutant 

concentrations. 

When taken together, the 2019 ISA 

concludes that the epidemiologic 

evidence strongly supports a 

relationship with decrements in lung 

function growth asthma development in 

children, as well as increased bronchitis 

symptoms in children with asthma. 

Additionally, the epidemiologic 


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evidence strongly supports a 

relationship with an acceleration of lung 

function decline in adults, and with 

respiratory mortality and cause-specific 

respiratory mortality for COPD and 

respiratory infection (U.S. EPA, 2019a, 

p. 1–34). In support of the biological 

plausibility of associations reported in 

epidemiologic studies associated with 

respiratory health effects, animal 

toxicological studies evaluated in the 

2019 ISA continue to provide direct 

evidence that long-term exposure to 

PM2.5 results in a variety of respiratory 

effects, including pulmonary oxidative 

stress, inflammation, and morphologic 

changes in the upper (nasal) and lower 

airways. Other results show that 

changes are consistent with the 

development of allergy and asthma, and 

with impaired lung development. 

Overall, the 2019 ISA concludes that 

‘‘the collective evidence is sufficient to 

conclude that a causal relationship is 

likely to exist between long-term PM2.5

exposure and respiratory effects’’ (U.S. 

EPA, 2019a, section 5.2.13). 

Short-Term PM2.5 Exposures 

The 2009 ISA (U.S. EPA, 2009a) 

concluded that a ‘‘causal relationship is 

likely to exist’’ between short-term 

PM2.5 exposure and respiratory effects. 

This conclusion was based mainly on 

the epidemiologic evidence 

demonstrating positive associations 

with various respiratory effects. 

Specifically, the 2009 ISA described 

epidemiologic evidence as consistently 

showing PM2.5-associated increases in 

hospital admissions and ED 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 epidemiologic evidence was 

inconsistent for hospital admissions or 

emergency department visits for asthma. 

Studies examining copollutant models 

showed that PM2.5 associations with 

respiratory effects were robust to 

inclusion of CO or SO2 in the model, but 

often were attenuated (though still 

positive) with inclusion of O3 or NO2. In 

addition to the copollutant models, 

evidence supporting an independent 

effect of PM2.5 exposure on the 

respiratory system was provided by 

animal toxicological studies of PM2.5

CAPs demonstrating changes in some 

pulmonary function parameters, as well 

as inflammation, oxidative stress, 

injury, enhanced allergic responses, and 

reduced host defenses. Many of these 

effects have been implicated in the 

pathophysiology for asthma 

exacerbation, COPD exacerbation, or 

respiratory infection. In the few 

controlled human exposure studies 

conducted in individuals with asthma 

or COPD, PM2.5 exposure mostly had no 

effect on respiratory symptoms, lung 

function, or pulmonary inflammation. 

Available studies in healthy people also 

did not clearly demonstrate respiratory 

effects following short-term PM2.5

exposures. 

Epidemiologic studies evaluated in 

the 2019 ISA continue to provide strong 

evidence for a relationship between 

short-term PM2.5 exposure and several 

respiratory-related endpoints, including 

asthma exacerbation (U.S. EPA, 2019a, 

section 5.1.2.1), COPD exacerbation 

(U.S. EPA, 2019a, section 5.1.4.1), and 

combined respiratory-related diseases 

(U.S. EPA, 2019a, section 5.1.6), 

particularly from studies examining ED 

visits and hospital admissions. The 

generally positive associations between 

short-term PM2.5 exposure and asthma 

and COPD as well as ED visits and 

hospital admissions are supported by 

epidemiologic studies demonstrating 

associations with other respiratoryrelated effects such as symptoms and 

medication use that are indicative of 

asthma and COPD exacerbations (U.S. 

EPA, 2019a, sections 5.1.2.2 and 

5.4.1.2). The collective body of 

epidemiologic evidence for asthma 

exacerbation is more consistent in 

children than in adults. Additionally, 

epidemiologic studies examining the 

relationship between short-term PM2.5

exposure and respiratory mortality 

provide evidence of consistent positive 

associations, demonstrating a 

continuum of effects (U.S. EPA, 2019a, 

section 5.1.9). 

Epidemiologic studies evaluated in 

the 2019 ISA expand the assessment of 

potential copollutant confounding 

evaluated in the 2009 ISA. 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 including O3, NO2, 

SO2, and with more limited evidence for 

CO, as well as other particle sizes (i.e., 

PM10–2.5) (U.S. EPA, 2019a, section 

5.1.10.1). 

Insight into whether there is an 

independent effect of PM2.5 on 

respiratory health is also partially 

addressed by findings from animal 

toxicological studies evaluated in the 

2019 ISA. Specifically, short-term 

exposure to PM2.5 enhanced asthmarelated responses in an animal model of 

allergic airways disease and enhanced 

lung injury and inflammation in an 

animal model of COPD (U.S. EPA, 

2019a, 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. However, 

animal toxicological evidence for other 

respiratory effects is inconsistent. A 

recent study evaluated in the ISA 

supplement by Wyatt et al. (2020) and 

conducted at near ambient PM2.5

concentrations, adds to the limited 

evidence base of controlled human 

exposure studies. The study, completed 

in healthy young adults subject to 

intermittent exercise, found some 

significant respiratory effects (including 

decrease in lung function), however 

these findings were inconsistent with 

the controlled human exposure studies 

evaluated in the 2019 ISA (U.S. EPA, 

2019a, section 5.1.7.2, 5.1.2.3, and 

6.1.11.2.1). 

The 2019 ISA concludes that ‘‘[t]he 

strongest evidence of an effect of shortterm PM2.5 exposure on respiratory 

effects is provided by epidemiologic 

studies of asthma and COPD 

exacerbation. While animal 

toxicological studies provide biological 

plausibility for these findings, some 

uncertainty remains with respect to the 

independence of PM2.5 effects’’ (U.S. 

EPA, 2019a, p. 5–155). When taken 

together, the 2019 ISA concludes that 

this evidence ‘‘is sufficient to conclude 

that a causal relationship is likely to 

exist between short-term PM2.5 exposure 

and respiratory effects’’ (U.S. EPA, 

2019a, 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, 2009a). This 

conclusion was based primarily on 

positive associations observed in a 

limited number of epidemiologic 

studies of lung cancer mortality. The 

few epidemiologic studies that had 

evaluated PM2.5 exposure and lung 

cancer incidence or cancers of other 

organs and systems generally did not 

show evidence of an association. 

Toxicological studies did not focus on 

exposures to specific PM size fractions, 

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 


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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 completion of 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, 2019a, section 

10.2.5). Re-analyses 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, 2019a, Figure 10–3). 

Additional support for positive 

associations with lung cancer mortality 

is provided by recent epidemiologic 

studies using individual level data to 

control for smoking status, as well as by 

studies of people who have never 

smoked (though such studies generally 

report wide confidence intervals due to 

the small number of lung cancer 

mortality cases within this population), 

and in additional analyses of cohorts 

that relied upon proxy measures to 

account for smoking status (U.S. EPA, 

2019a, 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, studies in the 2019 ISA 

generally report positive associations 

with long-term PM2.5 exposures (U.S. 

EPA, 2019a, section 10.2.5.1.2). A subset 

of the studies focusing on lung cancer 

incidence also examined histological 

subtype, providing some evidence of 

positive associations for 

adenocarcinomas, the predominate 

subtype of lung cancer observed in 

people who have never smoked (U.S. 

EPA, 2019a, 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, 2019a, 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. A 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, 2019a, 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, 2019a, section 

10.2.5.1.3). 

Compared to total (non-accidental) 

mortality (U.S. EPA, 2019a, section 

10.2.4.1.4), fewer studies have examined 

the shape of the C–R curve for causespecific mortality outcomes, including 

lung cancer. Several studies of lung 

cancer mortality and incidence have 

reported no evidence of deviations from 

linearity in the shape of the C–R 

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, 

2019a, section 10.2.5.1.4). 

In support of the biological 

plausibility of an independent effect of 

PM2.5 on lung cancer, the 2019 ISA 

notes evidence from experimental and 

epidemiologic 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, 

2019a, section 10.2.7). For example, 

both in vitro and in vivo toxicological 

studies have shown that PM2.5 exposure 

can result in DNA damage (U.S. EPA, 

2019a, section 10.2.2). Although such 

effects do not necessarily equate to 

carcinogenicity, the evidence that PM 

exposure can damage DNA, and elicit 

mutations, provides support for the 

plausibility of epidemiologic 

associations exhibited with lung cancer 

mortality and incidence. Additional 

supporting studies indicate the 

occurrence of micronuclei formation 

and chromosomal abnormalities (U.S. 

EPA, 2019a, section 10.2.2.3), and 

differential expression of genes that may 

be relevant to cancer pathogenesis, 

following PM2.5 exposures. 

Experimental and epidemiologic studies 

that examine epigenetic effects indicate 

changes in DNA methylation, providing 

some support that PM2.5 exposure 

contributes to genomic instability (U.S. 

EPA, 2019a, section 10.2.3). Overall, 

there is limited evidence that long-term 

PM2.5 exposure is associated with 

cancers in other organ systems, though 

there is some evidence that PM2.5

exposure may reduce survival in 

individuals with cancer (U.S. EPA, 

2019a, section 10.2.7; U.S. EPA, 2022a, 

section 2.1.1.4.1). 

Epidemiologic evidence for 

associations between PM2.5 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 that a causal relationship is 

likely to exist between long-term PM2.5

exposure and cancer’’ (U.S. EPA, 2019, 

section 10.2.7). 

v. Nervous System Effects 

Reflecting the very limited evidence 

available in the 2012 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 2012 

review, this body of evidence has grown 

substantially (U.S. EPA, 2019, section 

8.2). Animal toxicological studies 

assessed in in the 2019 ISA report that 

long-term PM2.5 exposures can lead to 

morphologic changes in the 

hippocampus and to impaired learning 

and memory. This evidence is 

consistent with epidemiologic studies 

reporting that long-term PM2.5 exposure 

is associated with reduced cognitive 

function (U.S. EPA, 2019a, section 

8.2.5). Further, while the evidence is 

limited, the presence of early markers of 

Alzheimer’s disease pathology has been 

demonstrated in rodents following longterm exposure to PM2.5 CAPs. These 

findings support reported associations 

with neurodegenerative changes in the 

brain (i.e., decreased brain volume), allcause dementia, or hospitalization for 

Alzheimer’s disease in a small number 

of epidemiologic studies (U.S. EPA, 

2019a, section 8.2.6). Additionally, loss 

of dopaminergic neurons in the 

substantia nigra, a hallmark of 

Parkinson disease, has been reported in 

mice (U.S. EPA, 2019a, section 8.2.4), 

though epidemiologic studies provide 

only limited support for associations 

with Parkinson’s disease (U.S. EPA, 

2019a, section 8.2.6). Overall, the lack of 

consideration of copollutant 

confounding introduces some 

uncertainty in the interpretation of 

epidemiologic studies of nervous system 

effects, but this uncertainty is partly 

addressed by the evidence for an 

independent effect of PM2.5 exposures 

provided by experimental animal 

studies. 

While the findings described above 

are most relevant to older adults, several 

studies of neurodevelopmental effects in 

children have also been conducted. 

Epidemiologic 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 


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59While there is no exact corollary within the 

2019 ISA for these types of studies, the 2019 ISA 

presented evidence that evaluates the potential 

relationship between short- and long-term PM2.5

exposure and respiratory infection (U.S. EPA, 

2022a, section 5.1.5 and 5.2.6). Studies assessed in 

the 2019 ISA report some evidence of positive 

associations between short-term PM2.5 and hospital 

admissions and ED visits for respiratory infections, 

however the interpretation of these studies is 

complicated by the variability in the type of 

respiratory infection outcome examined (U.S. EPA, 

2022a, Figure 5–7). In the 2019 ISA, studies of longterm PM2.5 exposure were limited and while there 

were some positive associations reported, there was 

minimal overlap in respiratory infection outcomes 

examined across studies. Exposure to PM2.5 has 

been shown to impair host defense, specifically 

altering macrophage function, providing a 

biological pathway by which PM2.5 exposure could 

lead to respiratory infection (U.S. EPA, 2022a, 

sections 5.1.1 and 5.1.5.) There is some additional 

evidence that PM2.5 exposure can lead to decreases 

in an individual’s immune response, which can 

subsequently facilitate replication of respiratory 

viruses (Bourdrel et al., 2021). 

exposure to PM2.5 during the prenatal 

period and autism spectrum disorder 

(ASD) (U.S. EPA, 2019, section 8.2.7.2), 

the interpretation of these 

epidemiologic studies is limited due to 

the small number of studies, their lack 

of control for potential confounding by 

copollutants, and uncertainty related to 

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 studies indicate longterm PM2.5 exposures can lead to effects 

on the brain associated with 

neurodegeneration (i.e., 

neuroinflammation and reductions in 

brain volume), as well as cognitive 

effects in older adults (U.S. EPA, 2019a, 

Table 1–2). Animal toxicological studies 

provide evidence for a range of nervous 

system effects in adult animals, 

including neuroinflammation and 

oxidative stress, neurodegeneration, 

cognitive effects, and effects on 

neurodevelopment in young animals. 

The epidemiologic evidence is more 

limited, but studies generally support 

associations between long-term PM2.5

exposure and changes in brain 

morphology, cognitive decrements and 

dementia. There is also initial, and 

limited, evidence for 

neurodevelopmental effects, particularly 

ASD. The consistency and coherence of 

the evidence supports the 2019 ISA’s 

conclusion that ‘‘the collective evidence 

is sufficient to conclude that a causal 

relationship is likely to exist between 

long-term PM2.5 exposure and nervous 

system effects’’ (U.S. EPA, 2019a, 

section 8.2.9). 

vi. Other Effects 

For other health effect categories that 

were evaluated for their relationship 

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

PM2.5 exposure and nervous system 

effects and short- and long-term PM2.5

exposure and metabolic effects, 

reproduction and fertility, and 

pregnancy and birth outcomes (U.S. 

EPA, 2022a, Table ES–1), 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, 2019a, 

sections 7.14, 7.2.10, 8.1.6, and 9.1.5). 

The causality determination for shortterm PM2.5 exposure and nervous 

system effects in the 2019 ISA reflects 

a revision to the causality determination 

in the 2009 ISA from ‘‘inadequate to 

infer a causal relationship,’’ while this 

is the first-time assessments of causality 

were conducted for long-term PM2.5

exposure and nervous system effects, as 

well as short- and long-term PM2.5

exposure and metabolic effects reflect. 

Recent studies evaluated in the 2019 

ISA also further explored the 

relationship between short-and longterm UFP exposure and health effects. 

(i.e., cardiovascular effects and shortterm UFP exposures; respiratory effects 

and short-term UFP exposures; and 

nervous system effects and long- and 

short-term exposures (U.S. EPA, 2022a, 

Table ES–1). The currently available 

evidence is ‘‘suggestive of, but not 

sufficient to infer, a causal relationship’’ 

for short-term UFP exposure and 

cardiovascular and respiratory effects 

and for short- and long-term UFP 

exposure and nervous system effects, 

primarily due to uncertainties and 

limitations in the evidence, specifically, 

variability across studies in the 

definition of UFPs and the exposure 

metric used (U.S. EPA, 2019a, P.3.1; 

U.S. EPA, 2022a, section 3.3.1.6.3). The 

causality determinations for the other 

health effect categories evaluated in the 

2019 ISA are ‘‘inadequate to infer a 

causal relationship.’’ Additionally, this 

is the first time assessments of causality 

were conducted for short- and long-term 

UFP exposure and metabolic effects and 

long-term UFP exposure and nervous 

system effects (U.S. EPA, 2022a, Table 

ES–1). 

With the advent of the global COVID– 

19 pandemic, a number of recent studies 

evaluated in the ISA Supplement 

examined the relationship between 

ambient air pollution, specifically PM2.5, 

and SARS–CoV–2 infections and 

COVID–19 deaths, including a few 

studies within the U.S. and Canada 

(U.S. EPA, 2022a, section 3.3.2).59 Some 

studies examined whether daily changes 

in PM2.5 can influence SARS–CoV–2 

infection and COVID–19 death (U.S. 

EPA, 2022a, section 3.3.2.1). 

Additionally, several studies evaluated 

whether long-term PM2.5 exposure 

increases the risk of SARS–CoV–2 

infection and COVID–19 death in North 

America (U.S. EPA, 2022a, section 

3.3.2.2). While there is initial evidence 

of positive associations with SARS– 

CoV–2 infection and COVID–19 death, 

uncertainties remain due to 

methodological issues that may 

influence the results, including: (1) The 

use of ecological study design; (2) 

studies were conducted during the 

ongoing pandemic when the etiology of 

COVID–19 was still not well understood 

(e.g., specifically, there are important 

differences in COVID–19-related 

outcomes by a variety of factors such as 

race and SES); and (3) studies did not 

account for crucial factors that could 

influence results (e.g., stay-at-home 

orders, social distancing, use of masks, 

and testing capacity) (U.S. EPA, 2022a, 

chapter 5). Taken together, while there 

is initial evidence of positive 

associations with SARS–CoV–2 

infection and COVID–19 death, 

uncertainties remain due to 

methodological issues. 

b. Public Health Implications and AtRisk Populations 

The public health implications of the 

evidence regarding PM2.5-related health 

effects, as for other effects, are 

dependent on the type and severity of 

the effects, as well as the size of the 

population affected. Such factors are 

discussed below in the context of our 

consideration of the health effects 

evidence related to PM2.5 in ambient air. 

This section also summarizes the 

current information on population 

groups at increased risk of the effects of 

PM2.5 in ambient air. 

The information available in this 

reconsideration has not altered our 

understanding of human populations at 

risk of health effects from PM2.5

exposures. As recognized in the 2020 

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, 

2019a, p. 12–1). Factors that may 

contribute to increased risk of PM2.5- 

related health effects include lifestage 

(children and older adults), pre-existing 

diseases (cardiovascular disease and 


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60As described in the 2019 ISA, other factors that 

have the potential to contribute to increased risk 

include obesity, diabetes, genetic factors, smoking 

status, sex, diet, and residential location (U.S. EPA, 

2019, chapter 12). 

61Children, as used throughout this document, 

generally refers to those younger than 18 years old. 

respiratory disease), race/ethnicity, and 

SES.60

Children make up a substantial 

fraction of the U.S. population, and 

often have unique factors that contribute 

to their increased risk of experiencing a 

health effect due to exposures to 

ambient air pollutants because of their 

continuous growth and development.61

Children may be particularly at risk for 

health effects related to ambient PM2.5

exposures compared with adults 

because they have (1) a developing 

respiratory system, (2) increased 

ventilation rates relative to body mass 

compared with adults, and (3) an 

increased proportion of oral breathing, 

particularly in boys, relative to adults 

(U.S. EPA, 2019a, section 12.5.1.1). 

There is strong evidence that 

demonstrates PM2.5 associated health 

effects in children, particularly from 

epidemiologic studies of long-term 

PM2.5 exposure and impaired lung 

function growth, decrements in lung 

function, and asthma development. 

However, there is limited evidence from 

stratified analyses that children are at 

increased risk of PM2.5-related health 

effects compared to adults. 

Additionally, there is some evidence 

that indicates that children receive 

higher PM2.5 exposures than adults, and 

dosimetric differences in children 

compared to adults can contribute to 

higher doses (U.S. EPA, 2019a, section 

12.5.1.1). 

In the U.S., older adults, often defined 

as adults 65 years of age and older, 

represent an increasing portion of the 

population and often have pre-existing 

diseases or conditions that may 

compromise biological function. While 

there is limited evidence to indicate that 

older adults have higher exposures than 

younger adults, older adults may receive 

higher doses of PM2.5 due to dosimetric 

differences. There is consistent evidence 

from studies of older adults 

demonstrating generally consistent 

positive associations in studies 

examining health effects from short- and 

long-term PM2.5 exposure and 

cardiovascular or respiratory hospital 

admissions, emergency department 

visits, or mortality (U.S. EPA, 2019a, 

sections 6.1, 6.2, 11.1, 11.2, 12.5.1.2). 

Additionally, several animal 

toxicological, controlled human 

exposure, and epidemiologic studies did 

not stratify results by lifestage, but 

instead focused the analyses on older 

individuals, and can provide coherence 

and biological plausibility for the 

occurrence among this lifestage (U.S. 

EPA, 2019a, section 12.5.1.2). 

Individuals with pre-existing disease 

may be considered at greater risk of an 

air pollution-related health effect than 

those without disease because they are 

likely in a compromised biological state 

that can vary depending on the disease 

and severity. With regard to 

cardiovascular disease, we first note that 

cardiovascular disease is the leading 

cause of death in the U.S., accounting 

for one in four deaths, and 

approximately 12% of the adult 

population in the U.S. has a 

cardiovascular disease (U.S. EPA, 

2019a, section 12.3.1). Strong evidence 

demonstrates that there is a causal 

relationship between cardiovascular 

effects and long- and short-term 

exposures to PM2.5. Some of the 

evidence supporting this conclusion is 

from studies of panels or cohorts with 

pre-existing cardiovascular disease, 

which provide supporting evidence but 

do not directly demonstrate an 

increased risk (U.S. EPA, 2019a, section 

12.3.1). Epidemiologic evidence 

indicates that individuals with preexisting cardiovascular disease may be 

at increased risk for PM2.5-associated 

health effects compared to those 

without pre-existing cardiovascular 

disease. While the evidence does not 

consistently support increased risk for 

all pre-existing cardiovascular diseases, 

there is evidence that certain preexisting cardiovascular diseases (e.g., 

hypertension) may be a factor that 

increases PM2.5-related risk. 

Furthermore, there is strong evidence 

supporting a causal relationship for 

long- and short-term PM2.5 exposure and 

cardiovascular effects, particularly for 

IHD (U.S. EPA, 2019a, chapter 6, section 

12.3.1). 

With regard to respiratory disease, we 

first note that the most chronic 

respiratory diseases in the U.S. are 

asthma and COPD. Asthma affects a 

substantial fraction of the U.S. 

population and is the leading chronic 

disease among children. COPD 

primarily affects older adults and 

contributes to compromised respiratory 

function and underlying pulmonary 

inflammation. The body of evidence 

indicates that individuals with preexisting respiratory diseases, 

particularly asthma and COPD, may be 

at increased risk for PM2.5-related health 

effects compared to those without preexisting respiratory diseases (U.S. EPA, 

2019a, section 12.3.5). There is strong 

evidence indicating PM2.5-associated 

respiratory effects among those with 

asthma, which forms the primary 

evidence base for the likely to be causal 

relationship between short-term 

exposures to PM2.5 and respiratory 

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

12.3.5). For asthma, epidemiologic 

evidence demonstrates associations 

between short-term PM2.5 exposures and 

respiratory effects, particularly evidence 

for asthma exacerbation, and controlled 

human exposure and animal 

toxicological studies demonstrate 

support for the biological plausibility 

for asthma exacerbation with PM2.5

exposures (U.S. EPA, 2019a, section 

12.3.5.1). For COPD, epidemiologic 

studies report positive associations 

between short-term PM2.5 exposures and 

hospital admissions and emergency 

department visits for COPD, with 

supporting evidence from panel studies 

demonstration COPD exacerbation. 

Epidemiologic evidence is supported by 

some experimental evidence of COPDrelated effects, which provides support 

for the biological plausibility for COPD 

in response to PM2.5 exposures (U.S. 

EPA, 2019a, section 12.3.5.2). 

There is strong evidence for racial and 

ethnic disparities in PM2.5 exposures 

and PM2.5-related health risk, as 

assessed in the 2019 ISA and with even 

more evidence available since the 

literature cutoff date for the 2019 ISA 

and evaluated in the ISA Supplement. 

There is strong evidence demonstrating 

that Black and Hispanic populations, in 

particular, have higher PM2.5 exposures 

than non-Hispanic White populations 

(U.S. EPA, 2019a, Figure 12–2; U.S. 

EPA, 2022a, Figure 3–38). Black 

populations or individuals that live in 

predominantly Black neighborhoods 

experience higher PM2.5 exposures, in 

comparison to non-Hispanic White 

populations. There is also consistent 

evidence across multiple studies that 

demonstrate increased risk of PM2.5- 

related health effects, with the strongest 

evidence for health risk disparities for 

mortality (U.S. EPA, 2019a, section 

12.5.4). There is also evidence of health 

risk disparities for both Hispanic and 

non-Hispanic Black populations 

compared to non-Hispanic White 

populations for cause-specific mortality 

and incident hypertension (U.S. EPA, 

2022a, section 3.3.3.2). 

Socioeconomic status (SES) is a 

composite measure that includes 

metrics such as income, occupation, or 

education, and can play a role in access 

to healthy environments as well as 

access to healthcare. SES may be a 

factor that contributes to differential risk 

from PM2.5-related health effects. 

Studies assessed in the 2019 ISA and 

ISA Supplement provide evidence that 

lower SES communities are exposed to 

higher concentrations of PM2.5


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62As described in more detail in section 5 of the 

Preamble to the ISAs, judgments regarding causality 

take into consideration a number of aspects when 

evaluating the available scientific evidence (U.S. 

EPA, 2015, Table I). In reaching conclusions 

regarding causality, ‘‘evidence is evaluated for 

major outcome categories or groups of related 

endpoints (e.g., respiratory effects, vegetation 

growth), integrating evidence from across 

disciplines, and evaluating the coherence of 

evidence across a spectrum of related endpoints’’ 

(U.S. EPA, 2015, p. 24). Furthermore, ‘‘[i]n drawing 

judgments regarding causality for the criteria air 

pollutants, the ISA focuses on evidence of effects 

in the range of relevant pollutant exposures or 

doses and not on determination of causality at any 

dose. Emphasis is placed on evidence of effects at 

doses (e.g., blood Pb concentration) or exposures 

(e.g., air concentrations) that are relevant to, or 

somewhat above, those currently experienced by 

the population. The extent to which studies of 

higher concentrations are considered varies by 

pollutant and major outcome category, but generally 

includes those with doses or exposures in the range 

of one to two orders of magnitude above current or 

ambient conditions to account for intra-species 

variability and toxicokinetic or toxicodynamic 

differences between experimental animals and 

humans. Studies that use higher doses or exposures 

may also be considered to the extent that they 

provide useful information to inform understanding 

of mode of action, inter-species differences, or 

factors that may increase risk of effects for a 

population and if biological mechanisms have not 

been demonstrated to differ based on exposure 

concentration. Thus, a causality determination is 

based on weight-of-evidence evaluation for health 

or welfare effects, focusing on the evidence from 

exposures or doses generally ranging from recent 

ambient concentrations to one or two orders of 

magnitude above recent ambient concentrations’’ 

(U.S. EPA, 2015, p. 24). 

compared to higher SES communities 

(U.S. EPA, 2019a, section 12.5.3; U.S. 

EPA, 2022a, section 3.3.3.1.1). Studies 

using composite measures of 

neighborhood SES consistently 

demonstrated a disparity in both PM2.5

exposure and the risk of PM2.5-related 

health outcomes. There is some 

evidence that supports associations 

larger in magnitude between mortality 

and long-term PM2.5 exposures for those 

with low income or living in lower 

income areas compared to those with 

higher income or living in higher 

income neighborhoods (U.S. EPA, 

2019a, section 12.5.3; U.S. EPA, 2022a, 

section 3.3.3.1.1). Additionally, 

evidence supports conclusions that 

lower SES is associated with causespecific mortality and certain health 

endpoints (i.e., HI and CHF), but less so 

for all-cause or total (non-accidental) 

mortality (U.S. EPA, 2022a, section 

3.3.3.1). 

The magnitude and characterization 

of a public health impact is dependent 

upon the size and characteristics of the 

populations affected, as well as the type 

or severity of the effects. As summarized 

above, lifestage (children and older 

adults), race/ethnicity and SES are 

factors that increase the risk of PM2.5- 

related health effects. The American 

Community Survey (ACS) for 2019 

estimates that approximately 22% and 

16% of the U.S. population are children 

(age<18) and older adults (age 65+), 

respectively. For all ages, non-Hispanic 

Black and Hispanic populations 

comprise approximately 12% and 18% 

of the overall U.S. population in 2019. 

Currently available information that 

helps to characterize key features of 

these population is included in the 2022 

PA (U.S. EPA, 2022b, Table 3–2). 

As noted above, individuals with preexisting cardiovascular disease and preexisting respiratory disease may also be 

at increased risk of PM2.5-related health 

effects. Currently available information 

that helps to characterize key features of 

populations with cardiovascular or 

respiratory diseases or conditions is 

included in the 2022 PA (U.S. EPA, 

2022b, Table 3–3). The National Center 

for Health Statistics data for 2018 

indicate that, for adult populations, 

older adults (e.g., those 65 years and 

older) have a higher prevalence of 

cardiovascular diseases compared to 

younger adults (e.g., those 64 years and 

younger). For respiratory diseases, older 

adults also have a higher prevalence of 

emphysema than younger adults, and 

adults 44 years or older have a higher 

prevalence of chronic bronchitis. 

However, the prevalence for asthma is 

generally similar across all adult age 

groups. 

With respect to race, American 

Indians or Alaskan Native populations 

have the highest prevalence of all heart 

disease and coronary heart disease, 

while Black populations have the 

highest prevalence of hypertension and 

stroke. Hypertension has the highest 

prevalence across all racial groups 

compared to other cardiovascular 

diseases or conditions, ranging from 

approximately 22% to 32% of each 

racial group. Overall, the prevalence of 

cardiovascular diseases or conditions is 

lowest for Asians compared to Whites, 

Blacks, and American Indians or 

Alaskan Natives. Asthma prevalence is 

highest among Black and American 

Indian or Alaska Native populations, 

while the prevalence of chronic 

bronchitis and emphysema is generally 

similar across racial groups. Overall, the 

prevalence of respiratory diseases is 

lowest for Asians compared to Whites, 

Blacks, and American Indians or 

Alaskan Natives. With regard to 

ethnicity, cardiovascular and respiratory 

disease prevalence across all diseases or 

conditions is generally similar between 

Hispanic and non-Hispanic populations, 

although non-Hispanics have a slightly 

higher prevalence compared to 

Hispanics. 

Taken together, this information 

indicates that the groups at increased 

risk of PM2.5-related health effects 

represent a substantial portion of the 

total U.S. population. In evaluating the 

primary PM2.5 standards, an important 

consideration is the potential PM2.5- 

related public health impacts in these 

populations. 

c. PM2.5 Concentrations in Key Studies 

Reporting Health Effects 

To inform conclusions on the 

adequacy of the public health protection 

provided by the current primary PM2.5

standards, the sections below 

summarize the 2022 PA’s evaluation of 

the PM2.5 exposures, specifically the 

concentrations that have been examined 

in controlled human exposure studies, 

animal toxicological studies, and 

epidemiologic studies. The 2022 PA 

places the greatest emphasis on the 

health outcomes for which the 2019 ISA 

concludes that the evidence supports a 

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

relationship with short- or long-term 

PM2.5 exposures (U.S. EPA, 2022b, 

section 3.3.3). As described in greater 

detail in section II.A.2 above, this 

includes short- or long-term PM2.5

exposures and mortality, cardiovascular 

effects, and respiratory effects and longterm PM2.5 exposures and cancer and 

nervous system effects. While the 

causality determinations in the 2019 

ISA are informed by studies evaluating 

a wide range of PM2.5 concentrations,62

the sections below summarize the 

considerations in the 2022 PA regarding 

the degree to which the evidence 

assessed in the 2019 ISA and ISA 

Supplement supports the occurrence of 

PM-related health effects at 

concentrations relevant to informing 

conclusions on the primary PM2.5

standards. In so doing, the 2022 PA 

focuses on the available studies that are 

most directly informative to reaching 

conclusions regarding the adequacy of 

the current primary PM2.5 standards 

(e.g., epidemiologic studies with annual 

mean PM2.5 concentrations near or 

below the level of the standard; and 

controlled human exposure studies at 

PM2.5 exposures that elicit consistent 

effects, as well as examining PM2.5

exposures at concentrations that are at 

or near the level of the standard). 

i. PM2.5 Exposure Concentrations 

Evaluated in Experimental Studies 

Evidence for a particular PM2.5-related 

health outcome is strengthened when 

results from experimental studies 

demonstrate biologically plausible 

mechanisms through which adverse 

human health outcomes could occur 

(U.S. EPA, 2015, p. 20). Two types of 

experimental studies are of particular 

importance in understanding the effects 


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63 In contrast, controlled human exposure studies 

provide little evidence for respiratory effects 

following short-term PM2.5 exposures (U.S. EPA, 

2019a, section 5.1, Table 5–18). Therefore, this 

section focuses on cardiovascular effects evaluated 

in controlled human exposure studies of PM2.5

exposure. 

64The ATS/ERS described its 2017 statement as 

one ‘‘intended to provide guidance to policymakers, 

clinicians and public health professionals, as well 

as others who interpret the scientific evidence on 

the health effects of air pollution for risk 

management purposes’’ and further notes that 

‘‘considerations as to what constitutes an adverse 

health effect, in order to provide guidance to 

researchers and policymakers when new health 

effects markers or health outcome associations 

might be reported in future.’’ The most recent 

policy statement by the ATS, which once again 

broadens its discussion of effects, responses and 

biomarkers to reflect the expansion of scientific 

research in these areas, reiterates that concept, 

conveying that it does not offer ‘‘strict rules or 

numerical criteria, but rather proposes 

considerations to be weighed in setting boundaries 

between adverse and nonadverse health effects,’’ 

providing a general framework for interpreting 

evidence that proposes a ‘‘set of considerations that 

can be applied in forming judgments’’ for this 

context (Thurston et al., 2017). 

of PM exposures: controlled human 

exposure and animal toxicological 

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 

toxicological studies can provide 

information on the health effects of 

experimentally administered pollutant 

exposures under highly 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,63 and the most consistent 

evidence from these studies is for 

impaired vascular function (U.S. EPA, 

2019a, section 6.1.13.2). In addition, 

although less consistent, the 2019 ISA 

notes that studies examining PM2.5

exposures also provide evidence for 

increased blood pressure (U.S. EPA, 

2019a, section 6.1.6.3), conduction 

abnormalities/arrhythmia (U.S. EPA, 

2019a, section 6.1.4.3), changes in heart 

rate variability (U.S. EPA, 2019a, section 

6.1.10.2), changes in hemostasis that 

could promote clot formation (U.S. EPA, 

2019a, section 6.1.12.2), and increases 

in inflammatory cells and markers (U.S. 

EPA, 2019a, section 6.1.11.2). The 2019 

ISA concludes that, when taken as a 

whole, controlled human exposure 

studies demonstrate that short-term 

exposure to PM2.5 may impact 

cardiovascular function in ways that 

could lead to more serious outcomes 

(U.S. EPA, 2019a, section 6.1.16). Thus, 

such studies can provide insight into 

the potential for specific PM2.5

exposures to result in physiological 

changes that could increase the risk of 

more serious effects. Table 3–4 in the 

2022 PA summarizes information from 

the 2019 ISA and 2022 ISA supplement 

on available controlled human exposure 

studies that evaluate effects on markers 

of cardiovascular function following 

exposure to PM2.5 (U.S. EPA, 2022b). 

Most of the controlled human exposure 

studies in Table 3–4 of the 2022 PA 

have evaluated average PM2.5

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 concentrations lower than 120 mg/m3. 

Impaired vascular function, the effect 

identified in the 2019 ISA as the most 

consistent across studies (U.S. EPA, 

2019a, section 6.1.13.2) is shown 

following 2-hour exposures to PM2.5

concentrations at and above 149 mg/m3. 

Mixed results are reported in the studies 

that evaluated longer exposure 

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

lower (i.e., near-ambient) PM2.5

concentrations (U.S. EPA, 2022b, 

section 3.3.3.1). For example, significant 

effects for some outcomes were reported 

following 5-hour exposures to 24 mg/m3

in Hemmingsen et al. (2015b), but not 

for other outcomes following 5-hour 

exposures to 24 mg/m3 in Hemmingsen 

et al. (2015a) and not following 24-hour 

exposures to 10.5 mg/m3 in Bra ̈uner et 

al. (2008). Additionally, Wyatt et al. 

(2020) found significant effects for some 

cardiovascular (e.g., systematic 

inflammation markers, cardiac 

repolarization, and decreased 

pulmonary function) effects following 4- 

hour exposures to 37.8 mg/m3 in healthy 

young participants (18–35 years, n=21) 

who were subject to intermittent 

moderate exercise. The higher 

ventilation rate and longer exposure 

duration in this study compared to most 

controlled human exposure studies is 

roughly equivalent to a 2-hour exposure 

of 75–100 mg/m3 of PM2.5. Therefore, 

dosimetric considerations may explain 

the observed changes in inflammation 

in young healthy individuals. Though 

this study provides evidence of some 

effects at lower PM2.5 concentrations, 

overall, there is inconsistent evidence 

for inflammation in other controlled 

human exposure studies evaluated in 

the 2019 ISA (U.S. EPA, 2019a, sections 

5.1.7., 5.1.2.3.3, and 6.1.11.2.1; U.S. 

EPA, 2022a, section 3.3.1). 

While controlled human exposure 

studies are important in establishing 

biological plausibility, it is unclear how 

the results from these studies alone and 

the importance of the effects observed in 

these studies, should be interpreted 

with respect to adversity to public 

health. More specifically, impaired 

vascular function can signal an 

intermediate effect along the potential 

biological pathways for cardiovascular 

effects following short-term exposure to 

PM2.5 and show a role for exposure to 

PM2.5 leading to potential worsening of 

IHD and heart failure followed 

potentially by ED visits, hospital 

admissions, or mortality (U.S. EPA, 

2019a, section 6.1 and Figure 6–1). 

However, just observing the occurrence 

of impaired vascular function alone 

does not clearly suggest an adverse 

health outcome. Additionally, 

associated judgments regarding 

adversity or health significance of 

measurable physiological responses to 

air pollutants have been informed by 

guidance, criteria or interpretative 

statements developed within the public 

health community, including the 

American Thoracic Society (ATS) and 

the European Respiratory Society (ERS), 

which cooperatively updated the ATS 

2000 statement What Constitutes an 

Adverse Health Effect of Air Pollution 

(ATS, 2000) with new scientific 

findings, including the evidence related 

to air pollution and the cardiovascular 

system (Thurston et al., 2017).64 With 

regard to vascular function, the ATS/ 

ERS statement considers the adversity of 

both chronic and acute reductions in 

endothelial function. While the ATS/ 

ERS statement concluded that chronic 

endothelial and vascular dysfunction 

can be judged to be a biomarker of an 

adverse health effect from air pollution, 

they also conclude that ‘‘the health 

relevance of acute reductions in 

endothelial function induced by air 

pollution is less certain’’ (Thurston et 

al., 2017). This is particularly 

informative to our consideration of the 

controlled human exposure studies 

which are short-term in nature (i.e., 

generally ranging from 2- to 5-hours), 

including those studies that are 

conducted at near-ambient PM2.5

concentrations. 

The 2022 PA also notes that it is 

important to recognize that controlled 

human exposure studies include a small 

number of individuals compared to 

epidemiologic studies. Additionally, 

these studies tend to include generally 

healthy adult individuals, who are at a 

lower risk of experiencing health effects. 


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65Similar analyses of 4-hour and 5-hour PM2.5

concentrations are presented in Appendix A, Figure 

A–2 and Figure A–3, respectively of the 2022 PA 

(U.S. EPA, 2022b). 

These studies, therefore, often do not 

include children, older adults, or 

individuals with pre-existing 

conditions. As such, these studies are 

somewhat limited in their ability to 

inform at what concentrations effects 

may be elicited in at-risk populations. 

Nonetheless, to provide some insight 

into what these controlled human 

exposure studies may indicate regarding 

short-term exposure to peak PM2.5

concentrations and how concentrations 

relate to ambient PM2.5 concentrations, 

analyses in the 2022 PA (U.S. EPA, 

2022b, Figure 2–19) examine monitored 

2-hour PM2.5 concentrations (the 

exposure window most often utilized in 

the controlled human exposure studies) 

at sites meeting the current primary 

PM2.5 standards to evaluate the degree to 

which 2-hour ambient PM2.5

concentrations at such locations are 

likely to exceed the 2-hour exposure 

concentrations in the controlled human 

exposure studies at which statistically 

significant effects are reported in 

multiple studies for one or more 

indicators of cardiovascular function. At 

sites meeting the current primary PM2.5

standards, most 2-hour concentrations 

are below 10 mg/m3, and almost never 

exceed 30 mg/m3. The extreme upper 

end of the distribution of 2-hour PM2.5

concentrations is shifted higher during 

the warmer months (April to 

September), generally corresponding to 

the period of peak wildfire frequency in 

the U.S. At sites meeting the current 

primary PM2.5 standards, the highest 2- 

hour concentrations measured tend to 

occur during the period of peak wildfire 

frequency (i.e., 99.9th percentile of 2- 

hour concentrations is 62 mg/m3 during 

the warm season considered as a 

whole). Most of the sites measuring 

these very high concentrations are in the 

northwestern U.S. and California (U.S. 

EPA, 2022b, Appendix A, Figure A–1), 

where wildfires have been relatively 

common in recent years. When the 

typical fire season is excluded from the 

analysis, the extreme upper end of the 

distribution is reduced (i.e., 99.9th 

percentile of 2-hour concentrations is 55 

mg/m3).65 Given these results, the 2022 

PA concludes that PM2.5 exposure 

concentrations evaluated in most of 

these controlled human exposure 

studies are well-above the 2-hour 

ambient PM2.5 concentrations typically 

measured in locations meeting the 

current primary standards. 

With respect to animal toxicological 

studies, the 2019 ISA relies on animal 

toxicological studies to support the 

plausibility of a wide range of PM2.5- 

related health effects. While animal 

toxicological 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. The 2022 

PA considers these uncertainties when 

evaluating what the available animal 

toxicological studies may indicate with 

regard to the current primary PM2.5

standards. 

As with controlled human exposure 

studies, most animal toxicological 

studies evaluated in the 2019 ISA have 

examined effects following exposure to 

PM2.5 well above the concentrations 

likely to be allowed by the current PM2.5

standards. Such studies have generally 

examined short-term exposures to PM2.5

concentrations ranging 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, 2019a, Table 1–2). 

Two exceptions are animal toxicological 

studies 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 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 

similar to the ambient concentrations 

reported in some PM2.5 epidemiologic 

studies (U.S. EPA, 2019a, Table 1–2), 

though still above the ambient 

concentrations likely to occur in areas 

meeting the current primary PM2.5

standards. However, noting uncertainty 

in extrapolating the effects seen in 

animals, and the PM2.5 exposures and 

doses that cause those effects to human 

populations, animal toxicological 

studies are of limited utility in 

informing decisions on the public 

health protection provided by the 

current or alternative primary PM2.5

standards. Therefore, the animal 

toxicological studies are most useful in 

providing further evidence to support 

the biological mechanisms and 

plausibility of various adverse effects. 

ii. Ambient PM2.5 Concentrations in 

Locations of Epidemiologic Studies 

As summarized in section II.A.2.a 

above, epidemiologic studies examining 

associations between daily or annual 

average PM2.5 exposures and mortality 

or morbidity represent a large part of the 

evidence base supporting several of the 

2019 ISA’s ‘‘causal’’ and ‘‘likely to be 

causal’’ determinations. The 2022 PA 

considers the ambient PM2.5

concentrations present in areas where 

epidemiologic studies have evaluated 

associations with mortality or 

morbidity, and what such 

concentrations may indicate regarding 

the adequacy of the primary PM2.5

standards. The use of information from 

epidemiologic 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, and do not 

identify the specific exposures that can 

lead to the reported effects. Rather, 

health effects can occur over the entire 

distribution of ambient PM2.5

concentrations evaluated, and 

epidemiologic studies conducted to date 

do not identify a population-level 

threshold below which it can be 

concluded with confidence that PM2.5- 

associated health effects do not occur. 

Therefore, the 2022 PA evaluates the 

PM2.5 air quality distributions over 

which epidemiologic studies support 

health effect associations (U.S. EPA, 

2022b, section 3.3.3.2). In the absence of 

discernible thresholds, the 2022 PA 

considers the study-reported ambient 

PM2.5 concentrations reflecting 

estimated exposure with a focus around 

the middle portion of the PM2.5 air 

quality distribution, where the bulk of 

the observed data reside and which 

provides the strongest support for 

reported health effect associations. The 

section below, as well as in more detail 

in section II.B.3.b.i of the proposal (88 

FR 5594, January 27, 2023), describes 

the consideration of the key 

epidemiologic studies and observations 

from these studies, as evaluated in the 

2022 PA (U.S. EPA, 2022b, section 

3.3.3.2). 

As an initial matter, in considering 

the PM2.5 air quality distributions 

associated with mortality or morbidity 

in the key epidemiologic studies, the 

2022 PA recognizes that in previous 

reviews, the decision framework used to 

judge adequacy of the existing PM2.5

standards, and what levels of any 

potential alternative standards should 

be considered, placed significant weight 

on epidemiologic studies that assessed 

associations between PM2.5 exposure 

and health outcomes that were most 

strongly supported by the body of 

scientific evidence. In doing so, the 

decision framework recognized that 

while there is no specific point in the 

air quality distribution of any 


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66As detailed in the 2011 PA, we note the 

interrelatedness of the distributional statistics and 

a range of one standard deviation around the mean 

which represents approximately 68% of normally 

distributed data, and in that one standard deviation 

below the mean falls between the 25th and 10th 

percentiles (U.S. EPA, 2011, p. 2–71; U.S. EPA, 

2005, p. 5–22). 

epidemiologic study that represents a 

‘‘bright line’’ at and above which effects 

have been observed and below which 

effects have not been observed, there is 

significantly greater confidence in the 

magnitude and significance of observed 

associations for the part of the air 

quality distribution corresponding to 

where the bulk of the health events in 

each study have been observed, 

generally at or around the mean 

concentration. This is the case both for 

studies of daily PM2.5 exposures and for 

studies of annual average PM2.5

exposures (U.S. EPA, 2022b, section 

3.3.3.2.1). 

As discussed further in the 2022 PA, 

studies of daily PM2.5 exposures 

examine associations between day-today variation in PM2.5 concentrations 

and health outcomes, often over several 

years (U.S. EPA, 2022b, section 

3.3.3.2.1). 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 

health events occur at estimated 

exposures that reflect annual average 

PM2.5 concentrations around the middle 

of the air quality distributions 

examined. In both cases, epidemiologic 

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, as 

noted in the 2022 PA (U.S. EPA, 2022b, 

section 3.3.1.1), several epidemiologic 

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 epidemiologic studies, 

consistent with approaches in the 2012 

and 2020 reviews (78 FR 3161, January 

15, 2013; U.S. EPA, 2011, sections 2.1.3 

and 2.3.4.1; 85 FR 82716–82717, 

December 18, 2020; U.S. EPA, 2020b, 

sections 3.1.2 and 3.2.3), the 2022 PA 

evaluates study-reported means (or 

medians) of daily and annual average 

PM2.5 concentrations as indicators for 

the middle portions of the air quality 

distributions, over which studies 

generally provide strong support for 

reported associations and for which 

confidence in the magnitude and 

significance of associations observed in 

the epidemiologic studies is greatest (78 

FR 3101, January 15, 2013). In addition 

to the overall study means, the 2022 PA 

also focuses on concentrations 

somewhat below the means (e.g., 25th 

and 10th percentiles), when such 

information is available from the 

epidemiologic studies, which again is 

consistent with approaches used in 

previous reviews. In so doing, the 2022 

PA notes, as in previous reviews, that a 

relatively small portion of the health 

events are observed in the lower part of 

the air quality distribution and 

confidence in the magnitude and 

significance of the associations begins to 

decrease in the lower part of the air 

quality distribution. Furthermore, 

consistent with past reviews, there is no 

single percentile value within a given 

air quality distribution that is most 

appropriate or ‘‘correct’’ to use to 

characterize where our confidence in 

associations becomes appreciably lower. 

However, and as detailed further in the 

2022 PA, the range from the 25th to 10th 

percentiles is a reasonable range to 

consider as a region where there is 

appreciably less confidence in the 

associations observed in epidemiologic 

studies compared to the means (U.S. 

EPA, 2022b, p. 3–69).66

In evaluating the overall studyreported means, and concentrations 

somewhat below the means from 

epidemiologic studies, the 2022 PA 

focuses on the form, averaging time and 

level of the current primary annual 

PM2.5 standard. Consistent with the 

approaches used in the 2012 and 2020 

reviews (78 FR 3161–3162, January 15, 

2013; 85 FR 82716–82717, December 18, 

2020), the annual standard has been 

utilized as the primary means of 

providing public health protection 

against the bulk of the distribution of 

short- and long-term PM2.5 exposures. 

Thus, the evaluation of the studyreported mean concentrations from key 

epidemiologic studies lends itself best 

to evaluating the adequacy of the annual 

PM2.5 standard (rather than the 24-hour 

standard with its 98th percentile form). 

This is true for the study-reported 

means from both long-term and shortterm exposure epidemiologic studies, 

recognizing that the overall mean PM2.5

concentrations reported in studies of 

short-term (24-hour) exposures reflect 

averages across the study population 

and over the years of the study. Thus, 

mean concentrations from short-term 

exposure studies reflect long-term 

averages of 24-hour PM2.5 exposure 

estimates. In this manner, the 

examination of study-reported means in 

key epidemiologic studies in the 2022 

PA aims to evaluate the protection 

provided by the annual PM2.5 standard 

against the exposures where confidence 

is greatest for associations with 

mortality and morbidity. In addition, 

the protection provided by the annual 

standard is evaluated in conjunction 

with that provided by the 24-hour 

standard, with its 98th percentile form, 

which aims to provide supplemental 

protection against the short-term 

exposures to peak PM2.5 concentrations 

that can occur in areas with strong 

contributions from local or seasonal 

sources, even when overall ambient 

mean PM2.5 concentrations in an area 

remain relatively low. 

In focusing on the annual standard, 

and in evaluating the range of studyreported exposure concentrations for 

which the strongest support for adverse 

health effects exists, the 2022 PA 

examines exposure concentrations in 

key epidemiologic studies to determine 

whether the current primary annual 

PM2.5 standard provides adequate 

protection against these exposure 

concentrations. This means, as in past 

reviews, application of a decision 

framework based on assessing means 

reported in key epidemiologic studies 

must also consider how the study means 

were computed and how these values 

compare to the annual standard metric 

(including the level, averaging time and 

form) and the use of the monitor with 

the highest PM2.5 design value in an area 

for compliance. In the 2012 review, it 

was recognized that the key 

epidemiologic studies computed the 

study mean using an average across 

monitor-based PM2.5 concentrations. As 

such, the Agency noted that this 

decision framework applied an 

approach of using maximum monitor 

concentrations to determine compliance 

with the standard, while selecting the 

standard level based on consideration of 

composite monitor concentrations. 

Further, the Agency included analyses 

(Hassett-Sipple et al., 2010; Frank, 2012) 

that examined the differences in these 

two metrics (i.e., maximum monitor 

concentrations and composite monitor 

concentrations) across the U.S. and in 

areas included in the key epidemiologic 

studies and found that the maximum 

design value in an area was generally 

higher than the monitor average across 

that area, with the difference varying 


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67 In setting a standard level that would require 

the design value monitor to meet a level equal to 

the study-reported mean PM2.5 concentrations 

would generally result in lower concentrations of 

PM2.5 across the entire area, such that even those 

people living near an area design value monitor 

(where PM concentrations are generally highest) 

will be exposed to PM2.5 concentrations below the 

air quality conditions reported in the epidemiologic 

studies. 

68More detailed information about hybrid model 

methods and performance is described in section 

2.3.3.2 of the 2022 PA (U.S. EPA, 2022b). 

69 In those studies that use ground-based monitors 

alone to estimate long- or short-term PM2.5

concentrations, approaches include: (1) PM2.5

concentrations from a single monitor within a city/ 

county; (2) average of PM2.5 concentrations across 

all monitors within a city/county or other defined 

study area (e.g., CBSA); or (3) population-weighted 

averages of exposures. Once the study location 

average PM2.5 concentration is calculated, the 

study-reported long-term average is derived by 

averaging daily/annual PM2.5 concentrations across 

all study locations over the entire study period. 

70 Detailed information on the methods by which 

mean PM2.5 concentrations are calculated in key 

monitor- and hybrid model-based U.S. and 

Canadian epidemiologic studies are presented in 

Tables 3–6 through 3–9 in the 2022 PA (U.S. EPA, 

2022b). 

based on location and concentration. 

This information was taken into account 

in the Administrator’s final decision in 

selecting a level for the primary annual 

PM2.5 standard the 2012 review and 

discussed more specifically in her 

considerations on adequate margin of 

safety. 

Consistent with the approach taken in 

2012, in assessing how the overall mean 

(or median) PM2.5 concentrations 

reported in key epidemiologic studies 

can inform conclusions on the primary 

annual PM2.5 standard, the 2022 PA 

notes that the relationship between 

mean PM2.5 concentrations and the area 

design value continues to be an 

important consideration in evaluating 

the adequacy of the current or potential 

alternative annual PM2.5 standard levels 

in this reconsideration. In a given area, 

the area design value is based on the 

monitor in an area with the highest 

PM2.5 concentrations and is used to 

determine compliance with the 

standard. The highest PM2.5

concentrations spatially distributed in 

the area would generally occur at or 

near the area design value monitor and 

the distribution of PM2.5 concentrations 

would generally be lower in other 

locations and at monitors in that area. 

As such, when an area is meeting a 

specific annual standard level, the 

annual average exposures in that area 

are expected to be at concentrations 

lower than that level and the average of 

the annual average exposures across that 

area are expected (i.e., a metric similar 

to the study-reported mean values) to be 

lower than that level.67

Another important consideration is 

that there are a substantial number of 

different types of epidemiologic studies 

available since the 2012 review, 

included in both the 2019 ISA and the 

ISA Supplement, that make 

understanding the relationship between 

the mean PM2.5 concentrations and the 

area design value even more important 

(U.S. EPA, 2019a; U.S. EPA, 2022a). 

While the key epidemiologic studies in 

the 2012 review were all monitor-based 

studies, the newer studies include 

hybrid modeling approaches, which 

have emerged in the epidemiologic 

literature as an alternative to approaches 

that only use ground-based monitors to 

estimate exposure. As assessed in the 

2019 ISA and ISA Supplement, a 

substantial number of epidemiologic 

studies used hybrid model-based 

methods in evaluating associations 

between PM2.5 exposure and health 

effects (U.S. EPA, 2019a; U.S. EPA, 

2022a). Hybrid model-based studies 

employ various fusion techniques that 

combine ground-based monitored data 

with air quality modeled estimates and/ 

or information from satellites to 

estimate PM2.5 exposures.68

Additionally, hybrid modeling 

approaches tend to broaden the areas 

captured in the exposure assessment, 

and in so doing, tend to report lower 

mean PM2.5 concentrations than 

monitor-based approaches because they 

include more suburban and rural areas 

where concentrations are lower. While 

these studies provide a broader 

estimation of PM2.5 exposures compared 

to monitor-based studies (i.e., PM2.5

concentrations are estimated in areas 

without monitors), the hybrid modeling 

approaches result in study-reported 

means that are more difficult to relate to 

the annual standard metric and to the 

use of maximum monitor design values 

to assess compliance. In addition, and to 

further complicate the comparison, 

when looking across these studies, 

variations exist in how exposure is 

estimated between such studies, which 

in turn affects how the study means are 

calculated. Two important variations 

across studies include: (1) Variability in 

spatial scale used (i.e., averages 

computed across the nation (or large 

portions of the country) versus a focus 

on only CBSAs) and (2) variability in 

exposure assignment methods (i.e., 

averaging across all grid cells [nonpopulation weighting], averaging across 

a scaled-up area like a ZIP code [aspects 

of population weighting applied], and/ 

or applying population weighting). To 

elaborate further on the variability in 

exposure assignment methods, studies 

that use hybrid modeling approaches 

can estimate PM2.5 concentrations at 

different spatial resolutions, including 

at 1 km x 1 km grid cells, at 12 km x 

12 km grid cells, or at the census tract 

level. Mean reported PM2.5

concentrations can then be estimated 

either by averaging up to a larger spatial 

resolution that corresponds to the 

spatial resolution for which health data 

exists (e.g., ZIP code level) and therefore 

apply aspects of population weighting. 

These values are then averaged across 

all study locations at the larger spatial 

resolution (e.g., averaged across all ZIP 

codes in the study) over the study 

period, resulting in the study-reported 

mean 24-hour average or average annual 

PM2.5 concentration. Other studies that 

use hybrid modeling methods to 

estimate PM2.5 concentrations may use 

each grid cell to calculate the studyreported mean 24-hour average or 

average annual PM2.5 concentration. As 

such, these types of studies do not apply 

population weighting in their mean 

concentrations. In studies that use each 

grid cell to report a mean PM2.5

concentration and do not apply aspects 

of population weighting, the study mean 

may not reflect the exposure 

concentrations used in the 

epidemiologic study to assess the 

reported association. The impact of the 

differences in methods is an important 

consideration when comparing mean 

concentrations across studies (U.S. EPA, 

2022b, section 3.3.3.2.1). Thus, the 2022 

PA also considers the methods used to 

estimate PM2.5 concentrations, which 

vary from traditional methods using 

monitoring data from ground-based 

monitors 69 to those using more complex 

hybrid modeling approaches and how 

these methods calculate the studyreported mean PM2.5 concentration.70

Given the emergence of the hybrid 

model-based epidemiologic studies 

since the 2012 review, the 2022 PA 

explores the relationship between the 

approaches used in these studies to 

estimate PM2.5 concentrations and the 

impact that the different methods have 

on the study-reported mean PM2.5

concentrations. The 2022 PA further 

seeks to understand how the approaches 

and resulting mean concentrations 

compare across studies, as well as what 

the resulting mean values represent 

relative to the annual standard. In so 

doing, the 2022 PA presents analyses 

that compare the area annual design 

values, composite monitor PM2.5

concentrations, and mean 

concentrations from two hybrid 

modeling approaches, including 

evaluation of the means when 

population weighting is applied and 

when population weighting is not 


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71More details on the evaluation of the two 

hybrid modeling approaches is provided in section 

2.3.3.2.4 of the 2022 PA (U.S. EPA, 2022b). 

72The annual PM2.5 concentrations for the 

population-weighted averages ranged from 8.2–10.2 

mg/m3, while those that do not apply population 

weighting ranged from 7.0–8.6 mg/m3. Average 

maximum annual design values ranged from 9.5 to 

11.7 mg/m3. 

applied (U.S. EPA, 2022b, section 

2.3.3.1). 

In the air quality analyses comparing 

composite monitored PM2.5

concentrations with annual PM2.5 design 

values in U.S. CBSAs, maximum annual 

PM2.5 design values were approximately 

10% to 20% higher than annual average 

composite monitor concentrations (i.e., 

averaged across multiple monitors in 

the same CBSA) (sections I.D.5.a above 

and U.S. EPA, 2022b, section 2.3.3.1, 

Figure 2–28 and Table 2–3). The 

difference between the maximum 

annual design value and average 

concentration in an area can be smaller 

or larger than this range (10–20%), 

depending on a variety of factors such 

as the number of monitors, monitor 

siting characteristics, the distribution of 

ambient PM2.5 concentrations, and how 

the average concentrations are 

calculated (i.e., averaged across 

monitors versus across modeled grid 

cells). Results of this analysis suggest 

that there will be a distribution of 

concentrations across an area and the 

maximum annual average monitored 

concentration in an area (at the design 

value monitor, used for compliance 

with the standard), will generally be 10– 

20% higher than the average PM2.5

concentration across the other monitors 

in the area. Thus, in considering how 

the annual standard levels would relate 

to the study-reported means from key 

monitor-based epidemiologic studies, 

the 2022 PA generally concludes that an 

annual standard level that is no more 

than 10–20% higher than monitor-based 

study-reported mean PM2.5

concentrations would generally 

maintain air quality exposures to be 

below those associated with the studyreported mean PM2.5 concentrations, 

exposures for which the strongest 

support for adverse health effects 

occurring is available. 

The 2022 PA also evaluates data from 

two hybrid modeling approaches 

(DI2019 and HA2020) that have been 

used in several recent epidemiologic 

studies (U.S. EPA, 2022b, section 

2.3.3.2.4).71 The analysis shows that the 

means differ when PM2.5 concentrations 

are estimated in urban areas only 

(CBSAs) versus when the averages were 

calculated with all or most grid cells 

nationwide, likely because areas 

included outside of CBSAs tend to be 

more rural and have lower estimated 

PM2.5 concentrations. The 2022 PA 

recognizes the importance of this 

variability in the means since the study 

areas included in the calculation of the 

mean, and more specifically whether a 

study is focused on nationwide, 

regional, or urban areas, will affect the 

calculation of the study mean based on 

how many rural areas, with lower 

estimated PM2.5 concentrations, are 

included in the study area. While the 

determination of what spatial scale to 

use to estimate PM2.5 concentrations 

does not inherently affect the quality of 

the epidemiologic study, the spatial 

scale can influence the calculated 

reported long-term mean concentration 

across the study area and period. The 

results of the analysis show that, 

regardless of the hybrid modeling 

approach assessed, the annual average 

PM2.5 concentrations in CBSA-only 

analyses are 4–8% higher than for 

nationwide analyses, likely as a result of 

higher PM2.5 concentrations in more 

densely populated areas, and exclusion 

of more rural areas (U.S. EPA, 2022b, 

Table 2–4). When evaluating 

comparisons between surfaces that 

estimate exposure using aspects of 

population weighting versus surfaces 

that do not calculate means using 

population weighting, surfaces that 

calculate long-term mean PM2.5

concentrations with populationweighted averages have higher average 

annual PM2.5 concentrations, compared 

to annual PM2.5 concentrations in 

analyses that do not apply population 

weighting.72 Analyses show that average 

maximum annual design values are 40 

to 50% higher when compared to 

annual average PM2.5 concentrations 

estimated without population weighting 

versus 15% to 18% higher when 

compared to average annual PM2.5

concentrations estimated with 

population weighting applied (similar to 

the differences observed for the 

composite monitor comparison values 

for the monitor-based epidemiologic 

studies) (U.S. EPA, 2022b, section 

2.3.3.2.4). Given these results, it is 

worth noting that for the studies using 

the hybrid modeling approaches, the 

choice of methodology employed in 

calculating the study-reported means 

(i.e., using population weighting or not), 

and not a difference in estimates of 

exposure in the study itself, can 

produce substantially different studyreported mean values, where 

approaches that do not apply 

population weighting leading to much 

lower estimated mean PM2.5

concentrations. 

Based on these results, and similar to 

conclusions for the monitor-based 

studies, the 2022 PA generally 

concludes that study-reported mean 

concentrations in the studies that 

employ hybrid modeling approaches 

and calculate a population-weighted 

mean are associated with air quality 

conditions that would be achieved by 

meeting annual standard levels that are 

15–18% higher than study-reported 

means. Therefore, an annual standard 

level that is no more than 15–18% 

higher than the study-reported means 

would generally maintain air quality 

exposures to be below those associated 

with the study-reported mean PM2.5

concentrations, exposures for which we 

have the strongest support for adverse 

health effects occurring. For the studies 

that utilize hybrid modeling approaches 

but do not incorporate population 

weighting in calculating the mean, the 

annual design values associated with 

these air quality conditions are expected 

to be much higher (i.e., 40–50% higher) 

and this larger difference makes it more 

difficult to consider how these studies 

can be used to determine the adequacy 

of the protection afforded by the current 

or potential alternative annual 

standards. Additionally, as noted above 

in studies that utilize hybrid modeling 

approaches and that do not incorporate 

population weighting in calculating the 

mean (e.g., use each grid cell to 

calculate a mean PM2.5 concentration), 

the study mean does not reflect the 

exposure concentrations used in the 

epidemiologic study to assess the 

reported association. 

The 2022 PA notes that while these 

analyses can be useful to informing the 

understanding of the relationship 

between study-reported mean 

concentrations and the level of the 

annual standard, some limitations of 

this analysis must be recognized (U.S. 

EPA, 2022a, section 3.3.3.2.1). First, the 

comparisons used only two hybrid 

modeling approaches. Although these 

two hybrid modeling surfaces have been 

used in a number of recent 

epidemiologic studies, they represent 

just two of the many hybrid modeling 

approaches that have been used in 

epidemiologic studies to estimate PM2.5

concentrations. These methods continue 

to evolve, with further development and 

improvement to prediction models that 

estimate PM2.5 concentrations in 

epidemiologic studies. In addition to 

differences in hybrid modeling 

approaches, epidemiologic studies also 

use different methods to assign a 

population weighted average PM2.5

concentration to their study population, 

and the assessment presented in the 


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73As described 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 may be 

informative; however, it 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. Traditionally, 

statistical significance is used to a larger extent to 

evaluate the findings of controlled human exposure 

and animal toxicological studies. Understanding 

that statistical inferences may result in both false 

positives and false negatives, consideration is given 

to both trends in data and reproducibility of results. 

Thus, in drawing judgments regarding causality, the 

U.S. EPA emphasizes statistically significant 

findings from experimental studies, but does not 

limit its focus or consideration to statistically 

significant results in epidemiologic studies.’’ 

74This emphasis on studies conducted in the U.S. 

or Canada is consistent with the approach in the 

2012 and 2020 reviews of the PM NAAQS (U.S. 

EPA, 2011, section 2.1.3; U.S. EPA, 2020b, section 

3.2.3.2.1) and with approaches taken in other 

NAAQS reviews. However, the importance of 

studies in the U.S., Canada, and other countries in 

informing an ISA’s considerations of the weight of 

the evidence that informs causality determinations 

is recognized. 

75The cohorts examined in the studies included 

in Figure 3–4 to Figure 3–7 of the 2022 PA include 

large numbers of individuals in the general 

population, and often also include those 

populations identified as at-risk (i.e., children, 

older adults, minority populations, and individuals 

with pre-existing cardiovascular and respiratory 

disease). 76For some studies of long-term PM2.5 exposures, 

exposure is estimated from air quality data 

corresponding to only part of the study period, 

often including only the later years of the health 

data, and are not likely to reflect the full ranges of 

ambient PM2.5 concentrations that contributed to 

reported associations. While this approach can be 

reasonable in the context of an epidemiologic study 

that is evaluating health effect associations with 

long-term PM2.5 exposures, under the assumption 

that spatial patterns in PM2.5 concentrations are not 

appreciably different during time periods for which 

air quality information is not available (e.g., Chen 

et al., 2016), the 2022 PA focuses on the 

distribution of ambient PM2.5 concentrations that 

could have contributed to reported health 

outcomes. Therefore, the 2022 PA identifies studies 

as key epidemiologic studies when the years of air 

quality data and health data overlap in their 

entirety. 

77Such studies are identified as those that use 

hybrid modeling approaches for which recent 

methods and models were used (e.g., recent 

versions and configurations of the air quality 

models); studies that are fused with PM2.5 data from 

national monitoring networks (i.e., FRM/FEM data); 

and studies that reported a thorough model 

performance evaluation for core years of the study. 

2022 PA does not evaluate all of the 

potential methods that could be used. 

Additionally, while some of these 

epidemiologic studies also provide 

information on the broader distributions 

of exposure estimates and/or health 

events and the PM2.5 concentrations 

corresponding to the lower percentiles 

of those data (e.g., 25th and/or 10th), the 

air quality analysis in the 2022 PA 

focuses on mean PM2.5 concentrations 

and a similar comparison for lower 

percentiles of data was not assessed. 

Therefore, any direct comparison of 

study-reported PM2.5 concentrations 

corresponding to lower percentiles and 

annual design values is more uncertain 

than such comparisons with the mean. 

Finally, air quality analysis presented in 

the 2022 PA and detailed above in 

section I.D.5 included two hybrid 

modeling-based approaches that used 

U.S.-based air quality information for 

estimating PM2.5 concentrations. As 

such, the analyses are most relevant to 

interpreting the study-reported mean 

concentrations from U.S. epidemiologic 

studies and do not provide additional 

information about how the mean 

exposures concentrations reported in 

epidemiologic studies in other countries 

would compare to annual design values 

observed in the U.S. In addition, while 

information from Canadian studies can 

be useful in assessing the adequacy of 

the annual standard, differences in the 

exposure environments and population 

characteristics between the U.S. and 

other countries can affect the studyreported mean value and its relationship 

with the annual standard level. Sources 

and pollutant mixtures, as well as PM2.5

concentration gradients, may be 

different between countries, and the 

exposure environments in other 

countries may differ from those 

observed in the U.S. Furthermore, 

differences in population characteristics 

and population densities can also make 

it challenging to directly compare 

studies from countries outside of the 

U.S. to a design value in the U.S. 

As with the experimental studies 

discussed above, the 2022 PA focuses 

on epidemiologic studies assessed in the 

2019 ISA and ISA Supplement that have 

the potential to be most informative in 

reaching decisions on the adequacy of 

the primary PM2.5 standards. The 2022 

PA focuses on epidemiologic studies 

that provide strong support for ‘‘causal’’ 

or ‘‘likely to be causal’’ relationships 

with PM2.5 exposures in the 2019 ISA. 

Further, the 2022 PA also focuses on the 

health effect associations that are 

determined in the 2019 ISA and ISA 

Supplement to be consistent across 

studies, coherent with the broader body 

of evidence (e.g., including animal and 

controlled human exposure studies), 

and robust to potential confounding by 

co-occurring pollutants and other 

factors.73 In particular the 2022 PA 

considers the U.S. and Canadian 

epidemiologic studies to be more useful 

for reaching conclusions on the current 

standards than studies conducted in 

other countries, given that the results of 

the U.S. and Canadian studies are more 

directly applicable for quantitative 

considerations, whereas studies 

conducted in other countries reflect 

different populations, exposure 

characteristics, and air pollution 

mixtures. Additionally, epidemiologic 

studies outside of the U.S. and Canada 

generally reflect higher PM2.5

concentrations in ambient air than are 

currently found in the U.S., and are less 

relevant to informing questions about 

adequacy of the current standards.74

However, and as noted above, the 2022 

PA also recognizes that while 

information from Canadian studies can 

be useful in assessing the adequacy of 

the annual standard, there are still 

important differences between the 

exposure environments in the U.S. and 

Canada and interpreting the data (e.g., 

mean concentrations) from the Canadian 

studies in the context of a U.S.-based 

standard may present challenges in 

directly and quantitatively informing 

questions regarding the adequacy of the 

current or potential alternative the 

levels of the annual standard. Lastly, the 

2022 PA emphasizes multicity/ 

multistate studies that examine health 

effect associations, as such studies are 

more encompassing of the diverse 

atmospheric conditions and population 

demographics in the U.S. than studies 

that focus on a single city or State. 

Figures 3–4 through 3–7 in the 2022 PA 

summarize the study details for the key 

U.S. and Canadian epidemiologic 

studies (U.S. EPA, 2022b, section 

3.3.3.2.1).75

The key epidemiologic studies 

identified in the 2022 PA indicate 

generally positive and statistically 

significant associations between 

estimated PM2.5 exposures (short- or 

long-term) and mortality or morbidity 

across a range of ambient PM2.5

concentrations (U.S. EPA, 2022b, 

section 3.3.3.2.1), report overall mean 

(or median) PM2.5 concentrations, and 

include those for which the years of 

PM2.5 air quality data used to estimate 

exposures overlap entirely with the 

years during which health events are 

reported.76 Additionally, for studies that 

estimate PM2.5 exposure using hybrid 

modeling approaches, the 2022 PA also 

considers the approach used to estimate 

PM2.5 concentrations and the approach 

used to validate hybrid model 

predictions when evaluating those 

studies as key epidemiologic studies 77

and focuses on those studies that use 

recent methods based on surfaces that 

are with fused with monitored PM2.5


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78Canadian studies that use ground-based 

monitors estimate long- or short-term PM2.5

exposures are found in Figure 3–9 of the 2022 PA, 

including concentrations corresponding to the 25th 

and 10th percentiles of estimated exposures or 

health events, when available (U.S. EPA, 2022b). 

79That is, 25% of the total health events occurred 

in study locations with mean PM2.5 concentrations 

(i.e., averaged over the study period) below the 25th 

percentiles identified in Figure 3–8 of the 2022 PA 

and 10% of the total health events occurred in 

study locations with mean PM2.5 concentrations 

below the 10th percentiles identified. 80For most studies in Figure 2 below (Figure 3– 

14 in the 2022 PA), 25th percentiles of exposure 

estimates are presented. The exception is Di et al. 

(2017b), for which Figure 2 (U.S. EPA, 2022b, 

Figure 3–14) presents the short-term PM2.5 exposure 

estimates corresponding to the 25th and 10th 

percentiles of deaths in the study population (i.e., 

25% and 10% of deaths occurred at concentrations 

below these concentrations). In addition, the 

authors of Di et al. (2017b) provided populationweighted exposure values. The 10th and 25th 

percentiles of these population-weighted exposure 

estimates are 7.9 and 9.5 mg/m3, respectively. 

81Overall mean (or median) PM2.5 concentrations 

reported in key Canadian studies that use modelbased approaches to estimate long- or short-term 

PM2.5 concentrations and the concentrations 

corresponding to the 25th and 10th percentiles of 

estimated exposures or health events, when 

available are found in Figure 3–9 of the 2022 PA 

(U.S. EPA, 2022b). 

concentration data (U.S. EPA, 2022b, 

section 3.3.3.2.1). 

Figure 1 below (U.S. EPA, 2022b, 

Figure 3–8) highlights the overall mean 

(or median) PM2.5 concentrations 

reported in key U.S. studies that use 

ground-based monitors alone to estimate 

long- or short-term PM2.5 exposure.78

For the small subset of studies with 

available information on the broader 

distributions of underlying data, Figure 

1 below also identifies the study-period 

PM2.5 concentrations corresponding to 

the 25th and 10th percentiles of health 

events 79 (see Appendix B, Section B.2 

of the 2022 PA for more information). 

Figure 2 (U.S. EPA, 2022a, Figure 3–14) 

presents overall means of predicted 

PM2.5 concentrations for key U.S. 

model-based epidemiologic studies that 

apply aspects of population-weighting, 

and the concentrations corresponding to 

the 25th and 10th percentiles of 

estimated exposures or health events 80

when available (see Appendix B, section 

B.3 for additional information).81


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Figure 1. Monitor-based PM2.s Concentrations in Key U.S. Epidemiologic Studies. (Asterisks denote studies in

Supplement) 

ST Exposure Franklin 2007 (US: 27 Cities) 

& Mortality Franklin 2008 (US: 25 Cities) 

Klemm 2003 (US: Harvard 6 City) 

Krall 2013 (US: 72 Cities) 

Dai 2014 (US: 75 Cities) 

zanobettl and Schwartz 2009 (us: 112 Cltles) 

Liu 2019 (US: 107 cities; ST Exposure)• 

ST exposure ostro 2016 (us: 8 California counties) 

& Morbidity Zanobetti 2009 {US: 26cities) 

Bell 2014 (US: 4 Counties in MA & CT) 

Dominici 2006 (US: 204 Urban Counties) 

Bell 2008 (US: 202 Counties) 

Bravo 2017 (US: 418 Counties) 

Bell 2015 (us: 70 urban counties) 

Peng 2009 (US: 119 Urban Counties) 

LT exposure Zeger 2008 (us: 421 Eastern Region counties) 

& Mortality Zeger 2008 {US: 62 Western Region Counties) 

Hart 2015 (US: Nationwide) 

Eum 2018 (us: Eastern Geographic Region; LT Exposure)• 

Kioumourtzogtou 2016 (us: 207 Cities) 

Eum 2018 {US: 3 Geographic Regions overall; LT Exposure)• 

Eum 2018 {US: Western Geographic Region; LT Exposure)• 

0 

0' 

Zeger 2008 (US: 185 Central Region counties) i 

Eum 2018 (US: Central Geographic Region; LT Exposure)• I 

---------··---·----------·-·····--· .. --.-·---~-~------·-·-f-~~-- '-·--·- • 

LT Exposure Mcconnell 2010 {us: 13 California communities) • • • 

& Morbidity Gharibvand 2016 (US: Nationwide) 

• 

• 

• 

• 

• 

-~ ' . ",- 

10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 1

Overall PM2. 5 Concentration for the Study Period (μg/m3} 
82This is generally consistent with, but slightly 

below, the lowest study-reported mean PM2.5

concentration from monitor-based studies available 

Continued 

BILLING CODE 6560–50–C 

Based on its evaluation of studyestimated PM

significant associations between 

generally positive and statistically 

conducted in the U.S. or Canada report 

PA notes that key epidemiologic studies 

reported mean concentrations, the 2022 

2.5 exposures (short- or 

long-term) and mortality or morbidity 

across a wide range of ambient PM2.5

the study-reported PM

number of observations with regard to 

section 3.3.3.2.1). The 2022 PA makes a 

concentrations (U.S. EPA, 2022b, 

2.5 concentrations 

epidemiologic studies. 

in the key U.S. and Canadian 

The 2022 PA first considers the PM2.5

concentrations from the key U.S. 

use monitors to estimate PM

epidemiologic studies. For studies that 

2.5

exposures, overall mean PM2.5

concentrations range between 9.9 mg/ 

m3 82 to 16.5 mg/m3 (Figure 1 above and 


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Figure 2. Hybrid Model-Predicted PM2.s Concentrations in Key U.S. Epidemiologic Studies that Ap

Weighting. (Asterisks denote studies included in the ISA Supplement) 

-------------------------------~---------------------

ST Exposure & Mortality Di 2011a (US: Natioowide; STExposure) 0 

Lee 2015b (US: 3 SE Statl!S; ST Bq>osute} 

ST expc:>sure & Morbidity deSouza2921 (us: Nationwide; sr ExpoS1Jre)" 

wvatt 202oc (us:sao us counties; ST ExP9Sum)* 

LT eJ<posure & Mortality Thurstlll!Wl!l (us: !>states and 2 MSAs; LT Elq)osure) 

Hart_ 2015. {US: Nationwide; LT Exposure) 

DI 2017b (US: Nationwide; LT Exposure) 

Dominici 2019 (us: Nationwide; LT Exposure}'" 

Wang 2017 (US: 7 SE States; LT Exposure) Wang 2020 (us: Nationwide; LT Exposure)* 

wu 2020a (us: Nationwide; LT Bq>osure)* 

• 

0 • 

• 

• 

5 6 7 8 9 10 

Overal I PM2 .,; Concentration for the Study Per
16246 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

in the 2020 PA, which was 10.7 mg/m3 (U.S. EPA, 

2020a, Figure 3–7). 

83 In the one study that reports 25th percentile 

exposure estimates of 4.6 mg/m3 (Shi et al., 2016), 

the authors report that most deaths occurred at or 

above the 75th percentile of annual exposure 

estimates (i.e., 10 mg/m3). The short-term exposure 

estimates accounting for most deaths are not 

presented in the published study. 

U.S. EPA, 2022b, Figure 3–8). For key 

U.S. epidemiologic studies that use 

hybrid model-predicted exposures and 

apply aspects of population-weighting, 

mean PM2.5 concentrations range from 

9.3 mg/m3 to just above 12.2 mg/m3

(Figure 2 above and U.S. EPA, 2022b, 

Figure 3–14). In studies that average up 

from the grid cell level to the ZIP code, 

postal code, or census tract level, mean 

PM2.5 concentrations range from 9.8 mg/ 

m3 to 12.2 mg/m3. The one study that 

population-weighted the grid cell prior 

to averaging up to the ZIP code or 

census tract level reported mean PM2.5

concentrations of 9.3 mg/m3. Based on 

air quality analyses noted above, these 

hybrid modelled epidemiologic studies 

are expected to report means similar to 

those from monitor-based studies. 

Other key U.S. epidemiologic studies 

that use hybrid modeling approaches 

estimate mean PM2.5 exposure by 

averaging each grid cell across the entire 

study area, whether that be the nation 

or a region of the country. These studies 

do not weight the estimated exposure 

concentrations based on population 

density or location of health events. As 

such, the study mean reported in these 

studies may not reflect the exposure 

concentrations used in the 

epidemiologic study to assess the 

reported association. As a result, these 

reported mean concentrations are the 

most different (and much lower) than 

the means reported in monitor-based 

studies. Due to the methodology 

employed in calculating the studyreported means and not necessarily a 

difference in estimates of exposure, 

these epidemiologic studies are 

expected to report some of the lowest 

mean values. For these studies, the 

reported mean PM2.5 concentrations 

range from 8.1 mg/m3 to 11.9 mg/m3 (U.S. 

EPA, 2022b, Figure 3–14). As noted 

above, for studies that utilize hybrid 

modeling approaches but do not 

incorporate population weighting into 

the reported mean calculation, the 

associated annual design values would 

be expected to be much higher (i.e., 40– 

50% higher) than the study-reported 

means. This larger difference between 

design values and study-reported mean 

concentrations makes it more difficult to 

consider how these studies can be used 

to determine the adequacy of the 

protection afforded by the current or 

potential alternative annual standards 

(U.S. EPA, 2022b, section 3.3.3.2.1). 

In addition to the mean PM2.5

concentrations, a subset of the key U.S. 

epidemiologic studies report PM2.5

concentrations corresponding to the 

25th and 10th percentiles of health data 

or exposure estimates to provide insight 

into the concentrations that comprise 

the lower quartile of the air quality 

distributions. In studies that use 

monitors to estimate PM2.5 exposures, 

25th percentiles of health events 

correspond to PM2.5 concentrations (i.e., 

averaged over the study period for each 

study city) at or above 11.5 mg/m3 and 

10th percentiles of health events 

correspond to PM2.5 concentrations at or 

above 9.8 mg/m3 (i.e., 25% and 10% of 

health events, respectively, occur in 

study locations with PM2.5

concentrations below these values) 

(Figure 1 above and U.S. EPA, 2022b, 

Figure 3–8). Of the key U.S. 

epidemiologic studies that use hybrid 

modeling approaches and apply 

population-weighting to estimate longterm PM2.5 exposures, the ambient PM2.5

concentrations corresponding to 25th 

percentiles of estimated exposures are 

9.1 mg/m3 (Figure 2 and U.S. EPA, 

2022b, Figure 3–14). In key U.S. 

epidemiologic studies that use hybrid 

modeling approaches and apply 

population-weighting to estimate shortterm PM2.5 exposures, the ambient 

concentrations corresponding to 25th 

percentiles of estimated exposures, or 

health events, are 6.7 mg/m3 (Figure 2 

and U.S. EPA, 2022b, Figure 3–14). In 

key U.S. epidemiologic studies that use 

hybrid modeling approaches and do not 

apply population-weighting to estimate 

PM2.5 exposures, the ambient 

concentrations corresponding to 25th 

percentiles of estimated exposures, or 

health events, range from 4.6 to 9.2 mg/ 

m3 (U.S. EPA, 2022b, Figure 3–14).83 In 

the key epidemiologic studies that apply 

hybrid modeling approaches with 

population-weighting and with 

information available on the 10th 

percentile of health events, the ambient 

PM2.5 concentration corresponding to 

that 10th percentile range from 4.7 mg/ 

m3 to 7.3 mg/m3 (Figure 2 and U.S. EPA, 

2022b, Figure 3–14). 

The 2022 PA next considers the PM2.5

concentrations from the key Canadian 

epidemiologic studies. Generally, the 

study-reported mean concentrations in 

Canadian studies are lower than those 

reported in the U.S. studies for both 

monitor-based and hybrid model 

methods. For the majority of key 

Canadian epidemiologic studies that use 

monitor-based exposure, mean PM2.5

concentrations generally ranged from 

7.0 mg/m3 to 9.0 mg/m3 (U.S. EPA, 

2022b, Figure 3–9). For these studies, 

25th percentiles of health events 

correspond to PM2.5 concentrations at or 

above 6.5 mg/m3 and 10th percentiles of 

health events correspond to PM2.5

concentrations at or above 6.4 mg/m3

(U.S. EPA, 2022b, Figure 3–9). For the 

key Canadian epidemiologic studies that 

use hybrid model-predicted exposure, 

the mean PM2.5 concentrations are 

generally lower than in U.S. modelbased studies (U.S. EPA, 2022b, Figure 

3–10), ranging from approximately 6.0 

mg/m3 to just below 10.0 mg/m3 (U.S. 

EPA, 2022b, Figure 3–11). The majority 

of the key Canadian epidemiologic 

studies that used hybrid modeling were 

completed at the nationwide scale, 

while four studies were completed at 

the regional geographic spatial scale. In 

addition, all the key Canadian 

epidemiologic studies apply aspects of 

population weighting, where all grid 

cells within a postal code are averaged, 

individuals are assigned exposure at the 

postal code resolution, and study mean 

PM2.5 concentrations are based on the 

average of individual exposures. The 

majority of studies estimating exposure 

nationwide range between just below 

6.0 mg/m3 to 8.0 mg/m3 (U.S. EPA, 

2022b, Figure 3–11). One study by 

Erickson et al. (2020) presents an 

analysis related immigrant status and 

length of residence in Canada versus 

non-immigrant populations, which 

accounts for the four highest mean PM2.5

concentrations which range between 9.0 

mg/m3 and 10.0 mg/m3 (U.S. EPA, 2022b, 

Figure 3–11). The four studies that 

estimate exposure at the regional scale 

report mean PM2.5 concentrations that 

range from 7.8 mg/m3 to 9.8 mg/m3 (U.S. 

EPA, 2022b, Figure 3–11). Three key 

Canadian epidemiologic studies report 

information on the 25th percentile of 

health events. In these studies, the 

ambient PM2.5 concentration 

corresponding to the 25th percentile is 

approximately 8.0 mg/m3 in two studies, 

and 4.3 mg/m3 in a third study (U.S. 

EPA, 2022b, Figure 3–11). 

In addition to the expanded body of 

evidence from the key U.S. 

epidemiologic studies discussed above, 

there are also a subset of epidemiologic 

studies that have emerged that further 

inform an understanding of the 

relationship between PM2.5 exposure 

and health effects, including studies 

with the highest exposures excluded 

(restricted analyses), epidemiologic 

studies that employed statistical 

approaches that attempt to more 

extensively account for confounders and 

are more robust to model 

misspecification (i.e., used alternative 


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84As noted in the ISA Supplement (U.S. EPA, 

2022a, p. 1–3): ‘‘In the peer-reviewed literature, 

these epidemiologic studies are often referred to as 

alternative methods for confounder control. For the 

purposes of this Supplement, this terminology is 

not used to prevent confusion with the main 

scientific conclusions (i.e., the causality 

determinations) presented within an ISA. In 

addition, as is consistent with the weight-ofevidence framework used within ISAs and 

discussed in the Preamble to the Integrated Science 

Assessments, an individual study on its own cannot 

inform causality, but instead represents a piece of 

the overall body of evidence.’’ 

85 Given the nature of these studies, the majority 

tend to focus on time periods in the past during 

which ambient PM2.5 concentrations were 

substantially higher than those measured more 

recently (e.g., see U.S. EPA, 2022b, Figure 2–16). 

methods for confounder control),84 and 

accountability studies (U.S. EPA, 2019a, 

U.S. EPA, 2021a, U.S. EPA, 2022a). 

Restricted analyses are studies that 

examine health effect associations in 

analyses with the highest exposures 

excluded, restricting analyses to daily 

exposures less than the 24-hour primary 

PM2.5 standard and annual exposures 

less than the annual PM2.5 standard. The 

2022 PA presents a summary of 

restricted analyses evaluated in the 2019 

ISA and ISA Supplement (U.S. EPA, 

2022b, Table 3–10). The restricted 

analyses can be informative in assessing 

the nature of the association between 

long-term exposures (e.g., annual 

average concentrations <12.0 mg/m3) or 

short-term exposures (e.g., daily 

concentrations <35 mg/m3) when 

looking only at exposures to lower 

concentrations, including whether the 

association persists in such restricted 

analyses compared to the same analyses 

for all exposures, as well as whether the 

association is stronger, in terms of 

magnitude and precision, than when 

completing the same analysis for all 

exposures. While these studies are 

useful in supporting the confidence and 

strength of associations at lower 

concentrations, these studies also have 

inherent uncertainties and limitations, 

including uncertainty in how studies 

exclude concentrations (e.g., are they 

excluded at the modeled grid cell level, 

the ZIP code level) and in how 

concentrations in studies that restrict air 

quality data relate to design values for 

the annual and 24-hour standards. 

Further, these studies often do not 

report descriptive statistics (e.g., mean 

PM2.5 concentrations, or concentrations 

at other percentiles) that allow for 

additional consideration of this 

information. As such, while these 

studies can provide additional 

supporting evidence for associations at 

lower concentrations, the 2022 PA notes 

that there are also limitations in how to 

interpret these studies when evaluating 

the adequacy of the current or potential 

alternative standards. 

Restricted analyses provide additional 

information on the nature of the 

association between long- or short-term 

exposures when analyses are restricted 

to lower PM2.5 concentrations and 

indicate that effect estimates are 

generally greater in magnitude in the 

restricted analyses for long- and shortterm PM2.5 exposure compared to the 

main analyses. In two U.S. studies that 

report mean PM2.5 concentrations in 

restricted analyses and that estimate 

effects associated with long-term 

exposure to PM2.5, the effect estimates 

are greater in the restricted analyses 

than in the main analyses. Di et al. 

(2017a) and Dominici et al. (2019) report 

positive and statistically significant 

associations in analyses restricted to 

concentrations less than 12.0 mg/m3 for 

all-cause mortality and effect estimates 

are greater in the restricted analyses 

than effect estimates reported in main 

analyses. In addition, both studies 

report mean PM2.5 concentrations of 9.6 

mg/m3. While none of the U.S. studies of 

short-term exposure present mean PM2.5

concentrations for the restricted 

analyses, these studies generally have 

mean 24-hour average PM2.5

concentrations in the main analyses 

below 12.0 mg/m3, and report increases 

in the effect estimates in the restricted 

analyses compared to the main analyses. 

Additionally, in the one Canadian study 

of long-term PM2.5 exposure, Zhang et 

al. (2021) conducted analyses where 

annual PM2.5 concentrations were 

restricted to concentrations below 10.0 

mg/m3 and 8.8 mg/m3, which presumably 

have lower mean concentrations than 

the mean of 7.8 mg/m3 reported in the 

main analyses, though restricted 

analysis mean PM2.5 concentrations are 

not reported. Effect estimates for nonaccidental mortality are greater in 

analyses restricted to PM2.5

concentrations less than 10.0 mg/m3, but 

less in analyses restricted to <8.8 mg/m3. 

The second type of studies that have 

recently emerged and further inform the 

consideration of the relationship 

between PM2.5 exposure and health 

effects in the 2022 PA are those that 

employ alternative methods for 

confounder control. Alternative 

methods for confounder control seek to 

mimic randomized experiments through 

the use of study design and statistical 

methods to more extensively account for 

confounders and are more robust to 

model misspecification. The 2022 PA 

presents a summary of the studies that 

employ alternative methods for 

confounder control, and employ a 

variety of statistical methods, which are 

evaluated in the 2019 ISA and ISA 

Supplement (U.S. EPA, 2022b, Table 3– 

11). These studies reported consistent 

results among large study populations 

across the U.S. and can further inform 

the relationship between long- and 

short-term PM2.5 exposure and total 

mortality. Studies that employ 

alternative methods for confounder 

control to assess the association 

between long-term exposure to PM2.5

and mortality reduce uncertainties 

related to confounding and provide 

additional support for the associations 

reported in the broader body of cohort 

studies that examined long-term PM2.5

exposure and mortality. 

Lastly, there is a subset of 

epidemiologic studies that assess 

whether long-term reductions in 

ambient PM2.5 concentrations result in 

corresponding reductions in health 

outcomes. These include studies that 

evaluate the potential for improvements 

in public health, including reductions 

in mortality rates, increases in life 

expectancy, and reductions in 

respiratory disease as ambient PM2.5

concentrations have declined over time. 

Some of these studies, accountability 

studies, provide insight on whether the 

implementation of environmental 

policies or air quality interventions 

result in changes/reductions in air 

pollution concentrations and the 

corresponding effect on health 

outcomes.85 The 2022 PA presents a 

summary of these studies, which are 

assessed in the 2019 ISA and ISA 

Supplement (U.S. EPA, 2022b, Table 3– 

12). These studies lend support for the 

conclusion that improvements in air 

quality are associated with 

improvements in public health. 

More specifically, of the 

accountability studies that account for 

changes in PM2.5 concentrations due to 

a policy or the implementation of an 

intervention and whether there was 

evidence of changes in associations with 

mortality or cardiovascular effects as a 

result of changes in annual PM2.5

concentrations, Corrigan et al. (2018), 

Henneman et al. (2019) and Sanders et 

al. (2020a) present analyses with 

starting PM2.5 concentrations (or 

concentrations prior to the policy or 

intervention) below 12.0 mg/m3. 

Henneman et al. (2019) explored 

changes in modeled PM2.5

concentrations following the retirement 

of coal fired power plants in the U.S., 

and found that reductions from mean 

annual PM2.5 concentrations of 10.0 mg/ 

m3 in 2005 to mean annual PM2.5

concentrations of 7.2 mg/m3 in 2012 

from coal-fueled power plants resulted 

in corresponding reductions in the 

number of cardiovascular-related 


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86For the annual PM2.5 standard, design values 

are calculated as the annual arithmetic mean PM2.5

concentration, averaged over 3 years. For the 24- 

hour standard, design values are calculated as the 

98th percentile of the annual distribution of 24- 

hour PM2.5 concentrations, averaged over three 

years (Appendix N of 40 CFR part 50). 

hospital admissions, including for all 

cardiovascular disease, acute MI, stroke, 

heart failure, and ischemic heart disease 

in those aged 65 and older. Corrigan et 

al. (2018) examined whether there was 

a change in the cardiovascular mortality 

rate before (2000–2004) and after (2005– 

2010) implementation of the first annual 

PM2.5 NAAQS implementation based on 

mortality data from the National Center 

for Health Statistics and reported 1.10 

(95% confidence interval (CI): 0.37, 

1.82) fewer cardiovascular deaths per 

year per 100,000 people for each 1 mg/ 

m3 reduction in annual PM2.5

concentrations. When comparing 

whether counties met the annual PM2.5

standard (attainment counties), there 

were 1.96 (95% CI: 0.77, 3.15) fewer 

cardiovascular deaths for each 1 mg/m3

reduction in annual PM2.5

concentrations between the two periods 

for attainment counties, whereas in nonattainment counties (e.g., counties that 

did not meet the annual PM2.5 standard), 

there were 0.59 (95% CI: ¥ 0.54, 1.71) 

fewer cardiovascular deaths between the 

two periods. And lastly, Sanders et al. 

(2020a) examined whether policy 

actions (i.e., the first annual PM2.5

NAAQS implementation rule in 2005 

for the 1997 annual PM2.5 standard with 

a 3-year annual average of 15 mg/m3) 

reduced PM2.5 concentrations and 

mortality rates in Medicare beneficiaries 

between 2000–2013. They report 

evidence of changes in associations with 

mortality (a decreased mortality rate of 

∼0.5 per 1,000 in attainment and nonattainment areas) due to changes in 

annual PM2.5 concentrations in both 

attainment and non-attainment areas. 

Additionally, attainment areas had 

starting concentrations below 12.0 mg/ 

m3 prior to implementation of the 

annual PM2.5 NAAQS in 2005. In 

addition, following implementation of 

the annual PM2.5 NAAQS, annual PM2.5

concentrations decreased by 1.59 mg/m3

(95% CI: 1.39, 1.80) which 

corresponded to a reduction in mortality 

rates among individuals 65 years and 

older (0.93% [95% CI: 0.10%, 1.77%]) 

in non-attainment counties relative to 

attainment counties. In a life expectancy 

study, Bennett et al. (2019) reports 

increases in life expectancy in all but 14 

counties (1325 of 1339 counties) that 

have exhibited reductions in PM2.5

concentrations from 1999 to 2015. These 

studies provide support for 

improvements in public health 

following the implementation of 

policies, including in areas with PM2.5

concentrations below the level of the 

current annual standard, as well as 

increases in life expectancy in areas 

with reductions in PM2.5 concentrations. 

d. Uncertainties in the Health Effects 

Evidence 

The 2022 PA recognizes that there are 

a number of uncertainties and 

limitations associated with the available 

health effects evidence. Although the 

epidemiologic studies clearly 

demonstrate associations between longand short-term PM2.5 exposures and 

health outcomes, several uncertainties 

and limitations in the health effects 

evidence remain. Epidemiologic studies 

evaluating short-term PM2.5 exposure 

and health effects have reported 

heterogeneity in associations between 

cities and geographic regions within the 

U.S. Heterogeneity in the associations 

observed across epidemiologic 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, 

and a limited understanding of factors 

including exposure error related to 

measurement-related issues, variability 

in PM2.5 composition regionally, and 

factors that result in differential 

exposures (e.g., topography, the built 

environment, housing characteristics, 

personal activity patterns). 

Heterogeneity is expected when the 

methods or the underlying distribution 

of covariates vary across studies (U.S. 

EPA, 2019a, p. 6–221). Studies assessed 

in the 2019 ISA and ISA Supplement 

have advanced the state of exposure 

science by presenting innovative 

methodologies to estimate PM exposure, 

detailing new and existing measurement 

and modeling methods, and further 

informing our understanding of the 

influence of exposure measurement 

error due to exposure estimation 

methods on the associations between 

PM2.5 and health effects reported in 

epidemiologic studies (U.S. EPA, 2019a, 

section 1.2.2; U.S. EPA, 2022a). Data 

from PM2.5 monitors continue to be 

commonly used in health studies as a 

surrogate for PM2.5 exposure, and often 

provide a reasonable representation of 

exposures throughout a study area (U.S. 

EPA, 2019a, section 3.4.2.2; U.S. EPA, 

2022a, section 3.2.2.2.2). However, an 

increasing number of studies employ 

hybrid modeling methods to estimate 

PM2.5 exposure using data from several 

sources, often including satellites and 

models, in addition to ground-based 

monitors. These hybrid models typically 

have good cross-validation, especially 

for PM2.5, and have the potential to 

reduce exposure measurement error and 

uncertainty in the health effect 

estimates from epidemiologic models of 

long-term exposure (U.S. EPA, 2019a, 

section 3.5; U.S. EPA, 2022a, section 

2.3.3). 

While studies using hybrid modeling 

methods have reduced exposure 

measurement error and uncertainty in 

the health effect estimates, these studies 

use a variety of approaches to estimate 

PM2.5 concentrations and to assign 

exposure to assess the association 

between health outcomes and PM2.5

exposure. This variability in 

methodology has inherent limitations 

and uncertainties, as described in more 

detail in section 2.3.3.1.5 of the 2022 

PA, and the performance of the 

modeling approaches depends on the 

availability of monitoring data which 

varies by location. Factors that likely 

contribute to poorer model performance 

often coincide with relatively low 

ambient PM2.5 concentrations, in areas 

where predicted exposures are at a 

greater distance to monitors, and under 

conditions where the reliability and 

availability of key datasets (e.g., air 

quality modeling) are limited. Thus, 

uncertainty in hybrid model predictions 

becomes an increasingly important 

consideration as lower predicted 

concentrations are considered. 

Regardless of whether a study uses 

monitoring data or a hybrid modeling 

approach when estimating PM2.5

exposures, one key limitation that 

persists is associated with the 

interpretation of the study-reported 

mean PM2.5 concentrations and how 

they compare to design values, the 

metric that describes the air quality 

status of a given area relative to the 

NAAQS.86 As discussed above in 

section II.B.3.b, the overall mean PM2.5

concentrations reported by key 

epidemiologic studies reflect averaging 

of short- or long-term PM2.5 exposure 

estimates across location (i.e., across 

multiple monitors or across modeled 

grid cells) and over time (i.e., over 

several years). For monitor-based 

studies, the comparison is somewhat 

more straightforward than for studies 

that use hybrid modeling methods, as 

the monitors used to estimate exposure 

in the epidemiologic studies are 

generally the same monitors that are 

used to calculate design values for a 

given area. It is expected that areas 

meeting a PM2.5 standard with a 

particular level would be expected to 

have average PM2.5 concentrations (i.e., 

averaged across space and over time in 

the area) somewhat below that standard 

level., but the difference between the 

maximum annual design value and 


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average concentration in an area can be 

smaller or larger than analyses 

presented above in section I.D.5.a, likely 

depending on factors such as the 

number of monitors, monitor siting 

characteristics, and the distribution of 

ambient PM2.5 concentrations. For 

studies that use hybrid modeling 

methods to estimate PM2.5

concentrations, the comparison between 

study-reported mean PM2.5

concentrations and design values is 

more complicated given the variability 

in the modeling methods, temporal 

scales (i.e., daily versus annual), and 

spatial scales (i.e., nationwide versus 

urban) across studies. Analyses above in 

section I.D.5.b and detailed more in the 

2022 PA (U.S. EPA, 2022b, section 

2.3.3.2.4) present a comparison between 

two hybrid modeling surfaces, which 

explored the impact of these factors on 

the resulting mean PM2.5 concentrations 

and provided additional information 

about the relationship between mean 

concentrations from studies using 

hybrid modeling methods and design 

values. However, the results of those 

analyses only reflect two surfaces and 

two types of approaches, so uncertainty 

remains in understanding the 

relationship between estimated modeled 

PM2.5 concentrations and design values 

more broadly across hybrid modeling 

studies. Moreover, this analysis was 

completed using two hybrid modeling 

methods that estimate PM2.5

concentrations in the U.S., thus an 

additional uncertainty includes 

understanding the relationship between 

modeled PM2.5 concentrations and 

design values reported in Canada. 

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, 2019a, 

section 1.5.1; U.S. EPA, 2022a, section 

2.2.1). Studies evaluated in the 2019 

ISA and ISA Supplement further 

examined the potential confounding 

effects of both gaseous and particulate 

copollutants on the relationship 

between long- and short-term PM2.5

exposure and health effects. As noted in 

the Appendix to the 2019 ISA (U.S. 

EPA, 2019a, Table A–1), copollutant 

models are not without their limitations, 

such as instances for which correlations 

are high between pollutants resulting in 

greater copollutant confounding bias in 

results. However, the studies continue 

to provide evidence indicating that 

associations with PM2.5 are relatively 

unchanged in copollutants models (U.S. 

EPA, 2019a, section 1.5.1; U.S. EPA, 

2022a, section 2.2.1). 

Another area of uncertainty is 

associated with other potential 

confounders, beyond copollutants. 

Some studies have expanded the 

examination of potential confounders to 

not only include copollutants, but also 

systematic evaluations of the potential 

impact of inadequate control from longterm temporal trends and weather (U.S. 

EPA, 2019a, section 11.1.5.1). Analyses 

examining these covariates further 

confirm that the relationship between 

PM2.5 exposure and mortality is unlikely 

to be biased by these factors. Other 

studies have explored the use of 

alternative methods for confounder 

control to more extensively account for 

confounders and are more robust to 

model misspecification that can further 

inform the causality determination for 

long-term and short-term PM2.5 and 

mortality and cardiovascular effects 

(U.S. EPA, 2019a, section 11.2.2.4; U.S. 

EPA, 2022a, sections 3.1.1.3, 3.1.2.3, 

3.2.1.2, and 3.2.2.3). These studies 

indicate that bias from unmeasured 

confounders can occur in either 

direction, although controlling for these 

confounders did not result in the 

elimination of the association, but 

instead provided additional support for 

associations between long-term PM2.5

exposure and mortality when 

accounting for additional confounders 

(U.S. EPA, 2022a, section 3.2.2.2.6). 

Another important limitation 

associated with the evidence is that, 

while epidemiologic studies indicate 

associations between PM2.5 and health 

effects, the currently available evidence 

does not identify particular PM2.5

concentrations that do not elicit health 

effects. Rather, health effects can occur 

over the entire distribution of ambient 

PM2.5 concentrations evaluated, and 

epidemiologic studies conducted to date 

do not identify a population-level 

threshold below which it can be 

concluded with confidence that PM2.5- 

related effects do not occur. 

Overall, evidence assessed in the 2019 

ISA and ISA Supplement continues to 

indicate a linear, no-threshold C–R 

relationship for PM2.5 concentrations >8 

mg/m3. However, uncertainties remain 

about the shape of the C–R curve at 

PM2.5 concentrations <8 mg/m3, with 

some recent studies providing evidence 

for either a sublinear, linear, or 

supralinear relationship at these lower 

concentrations (U.S. EPA, 2019a, 

section 11.2.4; U.S. EPA, 2022a, section 

2.2.3.2). 

There are also a number of 

uncertainties and limitations associated 

with the experimental evidence (i.e., 

controlled human exposure studies and 

animal toxicological studies). With 

respect to controlled human exposure 

studies, the PA recognizes that these 

studies include a small number of 

individuals compared to epidemiologic 

studies. Additionally, these studies tend 

to include generally healthy adult 

individuals, who are at a lower risk of 

experiencing health effects. These 

studies, therefore, often do not include 

populations that are at increased risk of 

PM2.5-related health effects, including 

children, older adults, or individuals 

with pre-existing conditions. As such, 

these studies are somewhat limited in 

their ability to inform at what 

concentrations effects may be elicited in 

at-risk populations. With respect to 

animal toxicological studies, while 

these studies often examine more severe 

health outcomes and longer exposure 

durations and higher exposure 

concentrations 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. 

Consideration of health effects are 

informed by the epidemiologic, 

controlled human exposure, and animal 

toxicological studies. The evaluation 

and integration of the scientific 

evidence in the ISA focuses on 

evaluating the findings from the body of 

evidence across disciplines, including 

evaluating the strengths and weaknesses 

in the overall collection of studies 

across disciplines. Integrating evidence 

across disciplines can strengthen causal 

inference, such that a weak inference 

from one line of evidence can be 

addressed by other lines of evidence, 

and coherence of these lines of evidence 

can add support to a cause-effect 

interpretation of the association. 

Evaluation and integration of the 

evidence also includes consideration of 

uncertainties that are inherent in the 

scientific findings (U.S. EPA, 2015, pp. 

13–15), some of which are described 

above. 

3. Summary of Exposure and Risk 

Estimates 

Beyond the consideration of the 

scientific evidence, discussed above in 

section II.B, 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. 

Additionally, the 2022 PA includes an 

at-risk analysis that assesses PM2.5- 

attributable risk associated with PM2.5

air quality that has been adjusted to 

simulate air quality scenarios of policy 


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87Additional detail regarding the selection of 

epidemiologic studies and specification of C–R 

functions is provided in the 2022 PA (U.S. EPA, 

2022b, Appendix C, section C.1.1). 

88While the 2019 ISA also found that evidence 

supports the determination of a ‘‘causal 

relationship’’ between long- and short-term PM2.5

exposures and cardiovascular effects, 

cardiovascular mortality was not included as a 

health outcome as it will be captured in the 

estimates of all-cause mortality. 

89For these areas, the annual standard is the 

‘‘controlling standard’’ because when air quality is 

adjusted to simulate just meeting the current or 

potential alternative annual standards, that air 

quality also would meet the 24-hour standard being 

evaluated. 

90For these areas, the 24-hour standard is the 

controlling standard because when air quality is 

adjusted to simulate just meeting the current or 

potential alternative 24-hour standards, that air 

quality also would meet the annual standard being 

evaluated. Some areas classified as being controlled 

by the 24-hour standard also violate the annual 

standard. 

91 In these 6 areas, the controlling standard 

depended on the air quality adjustment method 

used and/or the standard scenarios evaluated. 

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

or potential alternative standards). 

Drawing on the summary in section II.C 

of the proposal, the sections below 

provide a brief overview of key aspects 

of the assessment design (II.A.3.a), key 

limitations and uncertainties (II.A.3.b), 

and exposure/risk estimates (II.A.3.c). 

a. Key Design Aspects 

Risk assessments combine data from 

multiple sources and involve various 

assumptions and uncertainties. Input 

data for these analyses includes C–R 

functions from epidemiologic studies 

for each health outcome and ambient 

annual or 24-hour PM2.5 concentrations 

for the study areas utilized in the risk 

assessment (U.S. EPA, 2022b, section 

3.4.1). Additionally, quantitative and 

qualitative methods were used to 

characterize variability and uncertainty 

in the risk estimates (U.S. EPA, 2022b, 

section 3.4.1.7). 

Concentration-response functions 

used in the risk assessment are from 

large, multicity U.S. epidemiologic 

studies that evaluate the relationship 

between PM2.5 exposures and mortality. 

Epidemiologic studies and 

concentration-response studies that 

were used in the risk assessment to 

estimate risk were identified using 

criteria that take into account factors 

such as study design, geographic 

coverage, demographic populations, and 

health endpoints (U.S. EPA, 2022b, 

section 3.4.1.1).87 The risk assessment 

focuses on all-cause or nonaccidental 

mortality associated with long-term and 

short-term PM2.5 exposures, for which 

the 2019 ISA concluded that the 

evidence provides support for a ‘‘causal 

relationship’’ (U.S. EPA, 2022b, section 

3.4.1.2).88

As described in more detail in the 

2022 PA, the risk assessment first 

estimated health risks associated with 

air quality for 2015 adjusted to simulate 

‘‘just meeting’’ the current primary 

PM2.5 standards (i.e., the annual 

standard with its level of 12.0 mg/m3

and the 24-hour standard with its level 

of 35 mg/m3). Air quality modeling was 

then used to simulate air quality just 

meeting an alternative standard with a 

level of 10.0 mg/m3 (annual) and 30 mg/ 

m3 (24-hour). In addition to the modelbased approach, for the subset of 30 

areas controlled by the annual standard 

linear interpolation and extrapolation 

were employed to simulate just meeting 

alternative annual standards with levels 

of 11.0 (interpolated between 12.0 and 

10.0 mg/m3), 9.0 mg/m3, and 8.0 mg/m3

(both extrapolated from 12.0 and 10.0 

mg/m3) (U.S. EPA, 2022b, section 

3.4.1.3). The 2022 PA notes that there is 

greater uncertainty regarding whether a 

revised 24-hour standard (i.e., with a 

lower level) is needed to further limit 

‘‘peak’’ PM2.5 concentration exposure 

and whether a lower 24-hour standard 

level would most effectively reduce 

PM2.5-associated health risks associated 

with ‘‘typical’’ daily exposures. The risk 

assessment estimates health risks 

associated with air quality adjusted to 

meet a revised 24-hour standard with a 

level of 30 mg/m3, in conjunction with 

estimating the health risks associated 

with meeting a revised annual standard 

with a level of 10.0 mg/m3 (U.S. EPA, 

2022b, section 3.4.1.3). More details on 

the air quality adjustment approaches 

used in the risk assessment are 

described in section 3.4.1.4 and 

Appendix C of the 2022 PA (U.S. EPA, 

2022b). 

When selecting U.S. study areas for 

inclusion in the risk assessment, the 

available ambient monitors, geographic 

diversity, and ambient PM2.5 air quality 

concentrations were taken into 

consideration (U.S. EPA, 2022b, section 

3.4.1.4). When these factors were 

applied, 47 urban study areas were 

identified, which include nearly 60 

million people aged 30–99, or 

approximately 30% of the U.S 

population in this age range (U.S. EPA, 

2022b, section 3.4.1.5, Appendix C, 

section C.1.3). Of the 47 study areas, 

there were 30 study areas where just 

meeting the current standards is 

controlled by the annual standard,89 11 

study areas where just meeting the 

current standards is controlled by the 

daily standard,90 and 6 study areas 

where the controlling standard differed 

depending on the air quality adjustment 

approach (U.S. EPA, 2022b, section 

3.4.1.5).91

In addition to the overall risk 

assessment, the 2022 PA also includes 

an at-risk analysis and estimates 

exposures and health risks of specific 

populations identified as at-risk that 

would be allowed under the current and 

potential alternative standards to further 

inform the Administrator’s conclusions 

regarding the adequacy of the public 

health protection provided by the 

current primary PM2.5 standards. In so 

doing, the 2022 PA evaluates exposure 

and PM2.5 mortality risk for older adults 

(e.g., 65 years and older), stratified for 

White, Black, Asian, Native American, 

Non-Hispanic, and Hispanic individuals 

residing in the same study areas 

included in the overall risk assessment. 

This analysis utilizes a recent 

epidemiologic study that provides raceand ethnicity-specific risk coefficients 

(Di et al., 2017b). 

b. Key Limitations and Uncertainties 

Uncertainty in risk estimates (e.g., in 

the size of risk estimates) can result 

from a number of factors, including the 

assumptions about the shape of the C– 

R function with mortality at low 

ambient PM concentrations, the 

potential for confounding and/or 

exposure measurement error in the 

underlying epidemiologic studies, and 

the methods used to adjust PM2.5 air 

quality. More specifically, the use of air 

quality modeling to adjust PM2.5

concentrations are limited as they rely 

on model predictions, are based on 

emission changes scaled by fixed 

percentages, and use only two of the full 

set of possible emission scenarios and 

linear interpolation/extrapolation to 

adjust air quality that may not fully 

capture potential non-linearities 

associated with real-world changes in 

air quality. Additionally, the selection 

of case study areas is limited to urban 

areas predominantly located CA and in 

the Eastern U.S. that are controlled by 

the annual standard. While the risk 

assessment does not report quantitative 

uncertainty in the risk estimates as 

exposure concentrations are reduced, it 

does provide information on the 

distribution of concentrations associated 

with the risk estimates when evaluating 

progressively lower alternative annual 

standards. Based on these data, as lower 

alternative annual standards are 

evaluated, larger proportions of the 

distributions in risk occur at or below 

10 mg/m3 (at concentrations below or 

near most of the study-reported means 

from the key U.S. epidemiologic 

studies) and at or below 8 mg/m3 (the 

concentration at which the ISA reports 

increasing uncertainty in the shape of 

the C–R curve based on the body of 

epidemiologic evidence). 


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Similarly, the at-risk analysis is also 

subject to many of these same 

uncertainties noted above. Additionally, 

the at-risk analysis included C–R 

functions from only one study (Di et al., 

2017b), which reported associations 

between long-term PM2.5 exposures and 

mortality, stratified by race/ethnicity, in 

populations age 65 and older, as 

opposed to the multiple studies used in 

the overall risk assessment to convey 

risk estimate variability. These and 

other sources of uncertainty in the 

overall risk assessment and the at-risk 

analyses are characterized in more 

depth in the 2022 PA (U.S. EPA, 2022b, 

section 3.4.1.7, section 3.4.1.8, 

Appendix C, section C.3). 

c. Summary of Risk Estimates 

Although limitations in the 

underlying data and approaches lead to 

some uncertainty regarding estimates of 

PM2.5-associated risk, 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 up to 45,100 deaths in 2015 

are attributable to long-term PM2.5

exposures associated with just meeting 

the current annual and 24-hour PM2.5

standards (U.S. EPA, 2022b, section 

3.4.2.1). Additionally, as described in 

more detail in the 2022 PA, the at-risk 

analysis suggests that a lower annual 

standard level (i.e., below 12 mg/m3 and 

down as low as 8 mg/m3) will help to 

reduce PM2.5 exposure and may also 

help to mitigate exposure and risk 

disparities in populations identified as 

particularly at-risk for adverse effects 

from PM exposures (i.e., minority 

populations). 

Compared to the current annual 

standard, meeting a revised annual 

standard with a lower level is estimated 

to reduce PM2.5-associated health risks 

in the 30 study areas controlled by the 

annual standard by about 7–9% for a 

level of 11.0 mg/m3, 15–19% for a level 

of 10.0 mg/m3, 22–28% for a level of 9.0 

mg/m3, and 30–37% for a level of 8.0 mg/ 

m3) (U.S. EPA, 2022b, Table 3–17). 

Meeting a revised annual standard with 

a lower level may also help to mitigate 

exposure and risk disparities in 

populations identified as particularly atrisk for adverse effects from PM 

exposures (i.e., minority populations) in 

simulated scenarios just meeting 

alternative annual standards. However, 

though reduced, disparities by race and 

ethnicity persist even at an alternative 

annual standard level of 8 mg/m3, the 

lowest alternative annual standard 

included in the risk assessment (U.S. 

EPA, 2022b, section 3.4.2.4). 

Revising the level of the 24-hour 

standard to 30 mg/m3 is estimated to 

lower PM2.5-associated risks across a 

more limited population and number of 

areas than revising the annual standard 

(U.S. EPA, 2022, section 3.4.2.4). Risk 

reduction predictions are largely 

confined to areas located in the western 

U.S., several of which are also likely to 

experience risk reductions upon 

meeting a revised annual standard. In 

the 11 areas controlled by the 24-hour 

standard, when air quality is simulated 

to just meet the current 24-hour 

standard, PM2.5 exposures are estimated 

to be associated with as many as 2,570 

deaths annual. Compared to just 

meeting the current standard, air quality 

just meeting an alternative 24-hour 

standard level of 30 mg/m3 is associated 

with reductions in estimated risk of 9– 

13% (U.S. EPA, 2022b, section 3.4.2.3). 

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 2012 and 2020 reviews and reflects 

upon the body of evidence and 

information newly available in this 

reconsideration. In so doing, the 

Administrator has taken into account 

both evidence-based and risk-based 

considerations, as well as advice from 

the CASAC and public comments. 

Evidence-based considerations draw 

upon the EPA’s integrated assessment of 

the scientific evidence of health effects 

related to PM2.5 exposure presented in 

the 2019 ISA and ISA Supplement 

(summarized in the proposal in sections 

II.B (88 FR 5580, January 27, 2023) and 

II.D.2.a (88 FR 5609, January 27, 2023), 

and also in section II.A.2 above) to 

address key policy-relevant questions in 

the reconsideration. Similarly, the riskbased considerations draw upon the 

assessment of population exposure and 

risk (summarized in the proposal in 

sections II.C (88 FR 5605, January 27, 

2023) and II.D.2.b (88 FR 5614, January 

27, 2023), and also in section II.A.3 

above) in addressing policy-relevant 

questions focused on the potential for 

PM2.5 exposures associated with 

mortality under air quality conditions 

just meeting the current and potential 

alternative standards. 

The approach to reviewing the 

primary standards is consistent with 

requirements of the provisions of the 

CAA related to the review of the 

NAAQS and with how the EPA and the 

courts have historically interpreted the 

CAA. As discussed in section I.A above, 

these provisions require the 

Administrator to establish primary 

standards that, in the Administrator’s 

judgment, are requisite (i.e., neither 

more nor less stringent than necessary) 

to protect public health with an 

adequate margin of safety. Consistent 

with the Agency’s approach across all 

NAAQS reviews, the EPA’s approach to 

informing these judgments is based on 

a recognition that the available health 

effects evidence generally reflects a 

continuum that includes ambient air 

exposures for which scientists generally 

agree that health effects are likely to 

occur through lower levels at which the 

likelihood and magnitude of response 

become increasingly uncertain. The 

CAA does not require the Administrator 

to establish a primary standard at a zerorisk level or at background 

concentration levels, but rather at a 

level that reduces risk sufficiently so as 

to protect public health, including the 

health of sensitive groups, with an 

adequate margin of safety. 

The decisions on the adequacy of the 

current primary PM2.5 standards 

described below is a public health 

policy judgment by the Administrator 

that draws on the scientific evidence for 

health effects, quantitative analyses of 

population exposures and/or health 

risks, and judgments about how to 

consider the uncertainties and 

limitations that are inherent in the 

scientific evidence and quantitative 

analyses. The four basic elements of the 

NAAQS (i.e., indicator, averaging time, 

form, and level) have been considered 

collectively in evaluating the public 

health protection afforded by the 

current standards. 

Section II.B.2 below briefly 

summarizes the basis for the 

Administrator’s proposed decision, 

drawing from section II.D.3 of the 

proposal (88 FR 5617, January 27, 2023). 

The advice and recommendations of the 

CASAC and public comments on the 

proposed decision are addressed below 

in sections II.B.1 and II.B.3, 

respectively. The Administrator’s final 

conclusions in this reconsideration 

regarding the adequacy of the current 

primary PM2.5 standards and whether 

any revisions are appropriate are 

described in section II.B.4. 

1. CASAC Advice 

As part of its review of the 2019 draft 

PA, the CASAC provided advice on the 

adequacy of the public health protection 

afforded by the current primary PM2.5

standards. Its advice is documented in 


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a letter sent to the EPA Administrator 

(Cox, 2019b). 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, 

2019b, 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, 2019b, 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, 2019b, p. 8 of consensus 

responses). They noted that associations 

do not necessarily reflect causal effects, 

and they contended that recent 

epidemiologic studies assessed in the 

2019 ISA that report 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, 2019b, p. 

10 of consensus responses), 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, 2019b, 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, 

2019b, p. 8 of consensus responses). 

These members of the CASAC further 

concluded that the quantitative risk 

assessment included in the 2019 draft 

PA 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, 2019b, 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, 2019b, p. 11 of 

consensus responses). 

Drawing from their evaluation of the 

evidence and the risk assessment in the 

2019 draft PA, 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 . . . 2012 PM2.5 annual 

standard’’ (Cox, 2019b, 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, 2019b, 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 epidemiologic studies, but also 

includes support for biological 

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, 2019b, 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, 2019b, 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, 2019b, p. 11 of 

consensus responses). These CASAC 

members concluded that the available 

evidence 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, 2019b). 

As a part of this reconsideration, the 

CASAC reviewed the 2021 draft PA 

(developed to support the 

reconsideration as described in section 

I.C.5 above). As a part of their review of 

the 2021 draft PA, the CASAC provided 

advice on the adequacy of the current 

primary PM2.5 standards. The range of 

views summarized here generally 

reflects differing judgments as to the 

relative weight to place on various types 

of evidence, the risk-based information, 

and the associated uncertainties, as well 

as differing judgments about the 

importance of various PM2.5-related 

health effects from a public health 

perspective. 

In its comments on the 2021 draft PA, 

the CASAC stated that: ‘‘[o]verall the 

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written and appropriate for helping to 

‘bridge the gap’ between the agency’s 

scientific assessments and quantitative 

technical analyses, and the judgments 

required of the Administrator in 

determining whether it is appropriate to 

retain or revise the National Ambient 

Air Quality Standards (NAAQS)’’ 

(Sheppard, 2022a, p. 1 of consensus 

letter). The CASAC also stated that the 

‘‘[d]raft PA adequately captures and 

appropriately characterizes the key 

aspects of the evidence assessed and 

integrated in the 2019 ISA and Draft ISA 

Supplement of PM2.5-related health 

effects’’ (Sheppard, 2022b, p. 2 of 

consensus letter). The CASAC also 

stated that ‘‘[t]he interpretation of the 

risk assessment for the purpose of 

evaluating the adequacy of the current 

primary PM2.5 annual standard is 

appropriate given the scientific findings 

presented’’ (Sheppard, 2022a, p. 2 of 

consensus letter). 

With regard to the adequacy of the 

current primary annual PM2.5 standard, 

‘‘all CASAC members agree that the 

current level of the annual standard is 

not sufficiently protective of public 

health and should be lowered’’ 

(Sheppard, 2022a, p. 2 of consensus 

letter). Additionally, ‘‘the CASAC 

reached consensus that the indicator, 

form, and averaging time should be 

retained, without revision’’ (Sheppard, 

2022a, p. 2 of consensus letter). With 

regard to the level of the primary annual 

PM2.5 standard, the CASAC had 

differing recommendations for the 

appropriate range for an alternative 

level. The majority of the CASAC 

‘‘judge[d] that an annual average in the 

range of 8–10 mg/m3’’ was most 

appropriate, while the minority of the 

CASAC members stated that ‘‘the range 

of the alternative standard of 10–11 mg/ 

m3 is more appropriate’’ (Sheppard, 

2022a, p. 16 of consensus responses). 

The CASAC did highlight, however, that 

‘‘the alternative standard level of 10 mg/ 

m3 is within the range of acceptable 

alternative standards recommended by 

all CASAC members, and that an annual 

standard below 12 mg/m3 is supported 

by a larger and coherent body of 

evidence’’ (Sheppard, 2022a, p. 16 of 

consensus responses). 

In reaching conclusions on a 

recommended range of 8–10 mg/m3 for 

the primary annual PM2.5 standard, the 

majority of the CASAC placed weight on 

various aspects of the available 

scientific evidence and quantitative risk 

assessment information discussed in the 

2021 draft PA (Sheppard, 2022a, p. 16 

of consensus responses). In particular, 

these members cited recent U.S.- and 

Canadian-based epidemiologic studies 

that show positive associations between 

PM2.5 exposure and mortality with 

study-reported mean concentrations 

below 10 mg/m3. Further, these members 

also noted that the lower portions of the 

air quality distribution (i.e., 

concentrations below the mean) provide 

additional information to support 

associations between health effects and 

PM2.5 concentrations lower than the 

reported long-term mean concentration. 

In addition, the CASAC members 

recognized that the available evidence 

has not identified a threshold 

concentration, below which an 

association no longer remains, pointing 

to the conclusion in the draft ISA 

Supplement that the ‘‘evidence remains 

clear and consistent in supporting a nothreshold relationship, and in 

supporting a linear relationship for 

PM2.5 concentrations >8 mg/m3’’ 

(Sheppard, 2022a, p. 16 of consensus 

responses). Finally, these CASAC 

members placed weight on the at-risk 

analysis as providing support for 

protection of at-risk demographic 

groups, including minority populations. 

In recommending a range of 10–11 mg/ 

m3 for the primary annual PM2.5

standard, the minority of the CASAC 

emphasized that there were few key 

epidemiologic studies that reported 

positive and statistically significant 

health effects associations for PM2.5 air 

quality distributions with overall mean 

concentrations below 9.6 mg/m3

(Sheppard, 2022a, p. 17 of consensus 

responses). In so doing, the minority of 

the CASAC specifically noted the 

variability in the relationship between 

study-reported means and area annual 

design values based on the methods 

utilized in the studies, noting that 

design values are generally higher than 

area average exposure levels. Further, 

the minority of the CASAC stated that 

‘‘uncertainties related to copollutants 

and confounders make it difficult to 

justify a recommendation below 10–11 

mg/m3’’ (Sheppard, 2022a, p. 17 of 

consensus responses). Finally, the 

minority of the CASAC placed less 

weight on the risk assessment results, 

noting large uncertainties, including the 

approaches used for adjusting air 

quality to simulate just meeting the 

current and alternative standards. 

With regard to the current primary 24- 

hour PM2.5 standard, in their review of 

the 2021 draft PA, the CASAC did not 

reach consensus regarding the adequacy 

of the public health protection provided 

by the current standard. As described 

further below, the majority of the 

CASAC members concluded ‘‘that the 

available evidence calls into question 

the adequacy of the current 24-hour 

standard’’ (Sheppard, 2022a, p. 3 of 

consensus letter), while the minority of 

the CASAC members agreed with ‘‘the 

EPA’s preliminary conclusion [in the 

draft PA] to retain the current 24-hour 

PM2.5 standard without revision’’ 

(Sheppard, 2022a, p. 4 of consensus 

letter). The CASAC recommended that 

in future reviews, the EPA should also 

consider alternative forms for the 

primary 24-hour PM2.5 standard. 

Specifically, the CASAC ‘‘suggests 

considering a rolling 24-hour average 

and examining alternatives to the 98th 

percentile of the 3-year average,’’ 

pointing to concerns that computing 24- 

hour average PM2.5 concentrations using 

the current midnight-to-midnight 

timeframe could potentially 

underestimate the effects of high 24- 

hour exposures, especially in areas with 

wood-burning stoves and wintertime 

stagnation (Sheppard, 2022a, p. 18 of 

consensus responses). 

As noted above, the majority of the 

CASAC favored revising the level of the 

primary 24-hour PM2.5 standard, 

suggesting that a range of 25–30 mg/m3

would be adequately protective. In so 

doing, the majority of the CASAC 

placed weight on the available 

epidemiologic evidence, including 

epidemiologic studies that restricted 

analyses to 24-hour PM2.5

concentrations below 25 mg/m3. These 

members also placed weight on results 

of controlled human exposure studies 

with exposures close to the current 

standard, which they note provide 

support for the epidemiologic evidence 

to lower the standard. These members 

noted the limitations in using controlled 

human exposure studies alone in 

considering the adequacy of the 24-hour 

standard, recognizing that controlled 

human exposure studies preferentially 

recruit less susceptible individuals and 

have a typical exposure duration shorter 

than 24 hours. These members also 

placed ‘‘greater weight on the scientific 

evidence than on the values estimated 

by the risk assessment,’’ citing their 

concerns that the risk assessment ‘‘may 

not adequately capture areas with 

wintertime stagnation and residential 

wood-burning where the annual 

standard is less likely to be protective’’ 

(Sheppard, 2022a, p. 17 of consensus 

responses). Furthermore, these CASAC 

members ‘‘also are less confident that 

the annual standard could adequately 

protect against health effects of shortterm exposures’’ (Sheppard, 2022a, p. 

17 of consensus responses). 

The minority of the CASAC agreed 

with the EPA’s preliminary conclusion 

in the 2021 draft PA to retain the 

current primary 24-hour PM2.5 standard. 

In so doing, the minority of the CASAC 

placed greater weight on the risk 

assessment, noting that the risk 


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assessment accounts for both the level 

and the form of the current standard and 

the manner by which attainment with 

the standard is determined. Further, the 

minority of the CASAC stated that the 

‘‘risk assessment indicates that the 

annual standard is the controlling 

standard across most of the urban study 

areas evaluated and revising the level of 

the 24-hour standard is estimated to 

have minimal impact on the PM2.5- 

associated risks’’ and therefore, ‘‘the 

annual standard can be used to limit 

both long- and short-term PM2.5

concentrations’’ (Sheppard, 2022a, p. 18 

of consensus responses). Further, the 

minority of the CASAC placed more 

weight on the controlled human 

exposure studies, which show ‘‘effects 

at PM2.5 concentrations well above those 

typically measured in areas meeting the 

current standards’’ and which suggest 

that ‘‘the current standards are 

providing adequate protection against 

these exposures’’ (Sheppard, 2022a, p. 

18 of consensus responses). 

While the CASAC members expressed 

differing opinions on the appropriate 

revisions to the current standards, they 

did ‘‘find that both primary standards, 

24-hour and annual, are critical to 

protect public health given the evidence 

on detrimental health outcomes at both 

short-term and long-term exposures 

including peak events’’ (Sheppard, 

2022a, p. 13 of consensus responses). 

The comments from the CASAC also 

took note of uncertainties that remain in 

this reconsideration of the primary 

PM2.5 standards and they identified a 

number of additional areas for future 

research and data gathering and 

dissemination that would inform future 

reviews of the primary PM2.5 NAAQS 

(Sheppard, 2022a, pp. 14–15 of 

consensus responses). 

2. Basis for the Proposed Decision 

In reaching his proposed decisions to 

revise the level of the primary annual 

PM2.5 standard from its current level of 

12.0 mg/m3 to within the range of 9.0 to 

10.0 mg/m3, and to retain the current 

primary 24-hour PM2.5 standard (88 FR 

5558, January 27, 2023), the 

Administrator carefully considered the 

assessment of the current evidence and 

conclusions reached in the 2019 ISA 

and ISA Supplement; the currently 

available exposure and risk information, 

including associated limitations and 

uncertainties, described in detail in the 

2022 PA; the considerations and staff 

conclusions and associated rationales 

presented in the 2022 PA; the advice 

and recommendations from the CASAC; 

and public comments that had been 

offered up to that point (88 FR 5558, 

January 27, 2023). 

In reaching his proposed conclusions 

on whether the currently available 

scientific evidence and quantitative 

risk-based information support or call 

into question the adequacy of the public 

health protection afforded by the 

current primary PM2.5 standards, and as 

is the case with NAAQS reviews in 

general, the extent to which the current 

primary PM2.5 standards are judged to 

be adequate will depend on a variety of 

factors, including science policy and 

public health policy judgments to be 

made by the Administrator on the 

strength and uncertainties of the 

scientific evidence. The factors relevant 

to judging the adequacy of the standards 

also include the interpretation of, and 

decisions as to the weight to place on, 

different aspects of the results of the risk 

assessment for the study areas included 

and the associated uncertainties. Thus, 

in reaching proposed conclusions of the 

current standards, the Administrator 

recognized that such a determination 

depends in part on judgments regarding 

aspects of the evidence and risk 

estimates, and judgments about the 

degree of protection that is requisite to 

protect public health with an adequate 

margin of safety. 

The Administrator’s full rationale for 

his proposed conclusions is presented 

in section II.D.3 of proposal (88 FR 

5658, January 27, 2023), but is also 

briefly summarized here. In reaching the 

proposed decision to revise the annual 

standard level to 9–10 mg/m3, the 

Administrator placed weight on the full 

body of scientific information. He noted 

that the 2019 ISA finds that exposure to 

PM2.5 causes mortality and 

cardiovascular effects and is likely to 

cause respiratory effects, cancer, and 

nervous system effects as detailed 

further in section II.B.1 of the proposal. 

As detailed further in section II.B.4 of 

the proposal, he additionally noted that 

the 2019 ISA identifies at-risk 

populations at greater risk of health 

effects from exposure to PM2.5, 

including children, older adults, people 

with pre-existing respiratory or 

cardiovascular disease, minority 

populations, and low socioeconomic 

status (SES) populations. 

The Administrator also recognized 

that epidemiologic studies provide the 

strongest scientific evidence when 

evaluating the adequacy of the level of 

the annual standard. He noted that there 

is no specific point in the air quality 

distribution of any epidemiologic study 

that represents a ‘bright line’ at and 

above which effects have been observed 

and below which effects have not been 

observed. In his proposed decision, he 

noted previous decision-making 

frameworks, which placed weight on 

values at or near the study-reported 

mean PM2.5 concentrations, which is 

where the most confidence in the 

reported association of the 

epidemiologic study exists. He further 

noted that there are a number of 

epidemiologic studies available in this 

reconsideration that use new PM2.5

exposure estimation techniques (e.g., 

hybrid modeling) that were not used in 

epidemiologic studies that were 

available in previous reviews. These 

recent epidemiologic studies that use 

new exposure estimation techniques 

report long-term mean PM2.5

concentrations that are well below 

corresponding design values, which is 

an important consideration in reaching 

decisions on the level of the annual 

PM2.5 standard. 

In reaching his proposed decision, the 

Administrator noted that a level of 9–10 

mg/m3 would near or below the reported 

25th percentiles in key U.S. based 

epidemiologic studies, while also 

recognizing that he has less confidence 

in the magnitude and significance of the 

association at even lower percentiles 

(e.g., 10th percentile), where even fewer 

health events are observed. The 

Administrator also noted that a 

proposed level of 9–10 mg/m3 would be 

near the mean PM2.5 reported in 

Canadian based studies, though he also 

recognized that there are a number of 

factors associated with the studies in 

Canada (e.g., exposure environments) 

that make it more difficult to compare 

mean concnetrations from Canadian 

studies to design values, which 

determine compliance with the standard 

in the U.S. 

The Administrator took note of 

additional pieces of scientific evidence, 

which were not available in previous 

reviews, including restricted analyses, 

which support that the association seen 

in epidemiologic studies does not just 

occur from the peaks of the exposure 

distribution. Additionally, he notes that 

a level of 9–10 mg/m3 would be below 

the starting concentration in newly 

available accountability studies, though 

he did note that it is more difficult to 

interpret these studies in the context of 

selecting the level of the annual PM2.5

standard. 

Further, the Administrator took into 

consideration the advice of the CASAC, 

noting that all members included 10 mg/ 

m3 in their recommended range, and 

that the proposed range of 9–10 mg/m3

for the level of the primary annual PM2.5

standard was within the range 

recommended by the majority of the 

CASAC. 

In reaching the proposed conclusion 

of a range between 9–10 mg/m3, the 

Administrator noted that a level as high 


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as 11 mg/m3 might not provide an 

adequate margin of safety, given that 11 

mg/m3 was well above many of the 

epidemiologic study-reported mean 

PM2.5 concentrations. Additionally, the 

Administrator noted the uncertainties 

associated with the scientific and 

quantitative information supporting a 

level as low as 8 mg/m3, which call into 

question the potential public health 

improvements of a standard below 9 mg/ 

m3. The Administrator specifically 

noted the lack of key U.S. studies with 

mean concentrations below 9.3 mg/m3

and he further noted that the risk 

assessment suggests that the risk 

remaining under a standard of 8 mg/m3

would occur at very low concentrations 

(e.g., mainly 7 mg/m3 and below). 

As such, the Administrator’s 

proposed decision noted that the 

current PM2.5 annual standard did not 

adequately provide requisite protection 

against exposures to PM2.5 and that a 

proposed range of 9–10 mg/m3 would 

provide an adequate margin of safety. 

In his proposed decision to retain the 

current primary 24-hour PM2.5 standard 

with a level of 35 mg/m3, the 

Administrator first considered the 

scientific information related to shortterm exposures to PM2.5 and health 

effects. He noted that the controlled 

human exposure studies are the 

strongest line of evidence for informing 

his conclusions regarding the adequacy 

of the current 24-hour standard. In so 

doing, the Administrator recognized 

that controlled human exposure studies 

are conducted with healthy adult 

volunteers and that these studies do not 

include individuals who may be at 

increased risk of PM2.5-related health 

effects (i.e., children, older adults, 

people with pre-existing diseases). He 

also noted that the effects observed in 

the controlled human exposure studies 

(e.g., changes in vascular function) are 

not effects that are judged to be clearly 

adverse. He recognized the most 

consistent evidence of effects in these 

studies occurs at higher concentrations 

(e.g., >120 mg/m3) following 1–5 hour 

exposures, and that one study observed 

effects at concentrations as low as 38 mg/ 

m3 following 4-hour exposures. 

However, the Administrator reiterated 

that these studies do not tell us at 

exactly what concentrations an adverse 

effect might occur, especially for at-risk 

populations. As noted above in section 

II.A.2.c, controlled human exposure 

studies tend to include generally 

healthy adult individuals who are at a 

lower risk of experiencing health effects, 

and often do not include at-risk 

populations (e.g., children, older adults, 

or individuals with pre-existing 

conditions). As such, the Administrator 

recognized that these studies are 

somewhat limited in their ability to 

inform at what concentrations effects 

may be elicited in in at-risk populations. 

The Administrator also considered air 

quality analyses in the 2022 PA that 

demonstrate that there will be very few, 

if any, days with PM2.5 concentrations at 

levels evaluated in controlled human 

exposure studies that are associated 

with effects in areas that meet the 

current primary 24-hour PM2.5 standard. 

The Administrator also noted that as, 

in previous PM NAAQS reviews, the 

protection provided by the suite of 

standards (e.g., annual and 24-hour 

standards) is evaluated together. He 

noted that the annual standard is the 

controlling standard in most areas of the 

country. He also considered air quality 

analyses in the 2022 PA that suggest 

that revision of the annual standard to 

a level between 9–10 mg/m3 would also 

control 24-hour PM2.5 concentrations in 

most areas to, or below, 30 mg/m3. 

Finally, the Administrator noted the 

agreement with the advice from the 

minority of CASAC and additionally 

noted the limited rationale and evidence 

provided by the majority CASAC’s 

recommendation to support revision of 

the 24-hour standard. As such, the 

Administrator proposed to retain the 

current 24-hour standard with its level 

of 35 mg/m3. 

Additionally, the Administrator 

proposed to conclude that it is 

appropriate to retain all other elements 

(i.e., indicator, averaging time, and 

form) of the annual and 24-hour 

standards. 

3. Comments on the Proposed Decision 

With respect to the adequacy of the 

primary annual PM2.5 standard, a 

number of commenters, primarily those 

from industry and industry groups, nongovernmental organizations, and some 

State and local governments, disagree 

with the EPA’s proposed decision to 

revise the level of the primary annual 

PM2.5 standard. These commenters 

generally expressed the view that the 

current standards provide the requisite 

degree of public health protection and 

should be retained, consistent with the 

2020 final decision. In supporting their 

view, these commenters assert that the 

scientific evidence available in this 

reconsideration is essentially 

unchanged since the 2020 final decision 

and that the additional scientific 

evidence and quantitative risk 

information available for the 

reconsideration does not support 

strengthening the primary annual PM2.5

standard. These commenters also assert 

that uncertainties associated with the 

available scientific evidence have not 

changed since the 2020 final decision, 

and they note that these uncertainties 

were essential factors in the thenAdministrator’s decision to retain the 

primary annual PM2.5 standard. These 

commenters argue that, while the 

current Administrator acknowledges 

these uncertainties, he does not place 

enough weight on them in reaching his 

conclusions regarding the current 

standard. The commenters specifically 

highlight uncertainties related to 

exposure misclassification, 

confounding, and other sources of 

potential bias, which they claim 

supports retaining the current level of 

the annual standard. These commenters 

also note that these uncertainties were 

emphasized by the minority of the 

CASAC in their review of the 2021 draft 

PA, and the commenters further suggest 

that the lack of consensus from the 

CASAC on the appropriate level for the 

primary annual PM2.5 standard show 

that the research is unclear. The 

commenters contend that there is not 

support in this reconsideration for 

deviating from the then-Administrator’s 

decision in 2020. 

In contrast, other commenters, 

primarily from public health and 

environmental organizations, some State 

and local elected representatives, and 

some State and local government 

agencies agree with the EPA’s proposed 

decision that the primary annual PM2.5

standard is not adequate. These 

commenters support revising the level 

of the primary annual PM2.5 standard 

and emphasize that the available 

scientific evidence, in particular 

epidemiologic studies, along with the 

CASAC’s advice in their review for the 

2021 draft PA, provide strong support 

for the proposed decision. In particular, 

these commenters agree with the EPA’s 

conclusions about the strength of the 

scientific evidence, including 

uncertainties, and they emphasize that 

the CASAC reached consensus in their 

review of the 2021 draft PA that the 

current primary annual PM2.5 standard 

is not adequate. Some of these 

commenters also note that a revised 

primary annual PM2.5 standard would 

result in significant public health 

benefits by reducing morbidity and 

mortality associated with PM2.5

exposure, especially for at-risk 

populations. 

The EPA agrees with commenters that 

the primary annual PM2.5 standard is 

not adequate. The EPA recognizes the 

longstanding body of health evidence 

supporting relationships between PM2.5

exposures (short- and long-term) and 

both mortality and serious morbidity 

effects. The evidence available in this 

reconsideration (i.e., the studies 


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assessed in the 2019 ISA and ISA 

Supplement summarized above in 

section II.A.2.a) reaffirms, and in some 

cases strengthens, the conclusions from 

the 2009 ISA regarding the health effects 

of PM2.5 exposures. As noted above, 

epidemiologic studies demonstrate 

generally positive and often statistically 

significant associations between PM2.5

exposures and health effects. Such 

studies report associations between 

estimated PM2.5 exposures and nonaccidental, cardiovascular, or 

respiratory mortality; cardiovascular or 

respiratory hospitalizations or 

emergency room visits; and other 

mortality/morbidity outcomes (e.g., lung 

cancer mortality or incidence, asthma 

development). Recent experimental 

evidence, as well as evidence from 

epidemiologic panel studies, 

strengthens support for potential 

biological pathways through which 

PM2.5 exposures could lead to the 

serious effects reported in many 

population-level epidemiologic studies, 

including support for pathways that 

could lead to cardiovascular, 

respiratory, nervous system, and cancerrelated effects. Moreover, these recent 

epidemiologic studies strengthen 

support for health effect associations at 

PM2.5 concentrations lower than in 

those evaluated in epidemiologic 

studies available at the time of previous 

reviews. 

Additionally, as discussed in more 

detail in section I.C.5.b above, the ISA 

Supplement focused on studies that 

were most likely to inform decisions on 

the appropriate standard, but not to 

reassess areas that, based on the 

assessment of available science 

published since the cutoff date of the 

2019 ISA and through 2021, were 

judged unlikely to have new 

information that would be useful for the 

Administrator’s decision making. The 

ISA Supplement included U.S. and 

Canadian epidemiologic studies for 

health effect categories where the 2019 

ISA concluded a causal relationship 

(i.e., short- and long-term PM2.5

exposure and cardiovascular effects and 

mortality), as well as U.S. and Canadian 

epidemiologic studies that employed 

alternative methods for confounder 

control or conducted accountability 

analyses (i.e., studies that examined the 

effect of a policy on reducing PM2.5

concentrations). These studies, 

summarized in section II.A.2.a above, 

examine both short- and long-term PM2.5

exposure and cardiovascular effects and 

mortality. Additionally, studies that 

employ alternative methods for 

confounder control, as described in 

II.A.2.a above and in Table 3–11 and of 

the 2022 PA (U.S. EPA, 2022b), use a 

variety of statistical methods to control 

for confounding bias. These studies 

consistently report positive associations, 

which further supports the broader body 

of epidemiologic evidence for both 

cardiovascular effects and mortality. 

In addition, there are epidemiologic 

studies that provide supplemental 

information for consideration in 

reaching conclusions that the current 

suite of PM2.5 standards is not adequate. 

These studies include analyses that 

restrict annual average PM2.5

concentrations to concentrations below 

12 mg/m3 and provide support for 

positive and statistically significant 

associations with mortality and 

cardiovascular morbidity at mean PM2.5

concentrations below the current level 

of the primary annual PM2.5 standard 

(described above in section II.A.2.c.ii 

and in Table 3–10 of the 2022 PA (U.S. 

EPA, 2022b)). Recent accountability 

studies that have starting annual PM2.5

concentrations at or below 12 mg/m3

suggest public health improvements 

may occur at concentrations below 12 

mg/m3. These studies indicate positive 

and statistically significant associations 

with mortality and morbidity (e.g., 

cardiovascular hospital admissions) and 

reductions in PM2.5 concentrations in 

ambient air (described above in section 

II.A.2.c.ii and in Table 3–12 of the 2022 

PA (U.S. EPA, 2022b)). 

Thus, in considering the available 

scientific evidence to inform 

conclusions on the adequacy of the 

primary PM2.5 standards, the 

Administrator recognizes that the 2019 

ISA and the ISA Supplement together 

provides a strong scientific foundation 

for concluding that the current primary 

PM2.5 standards are not adequate. 

In addition to the scientific evidence 

above, the risk assessment estimates that 

the current primary annual PM2.5

standard could allow a substantial 

number of deaths in the U.S. Although 

the Administrator recognizes that while 

the risk estimates can help to place the 

evidence for specific health effects into 

a broader public health context, they 

should be considered along with the 

inherent uncertainties and limitations of 

such analyses when informing 

judgments about the potential for 

additional public health protection 

associated with PM2.5 exposures and 

related health effects. The Administrator 

takes into consideration these 

uncertainties, which are described in 

more detail in section II.A.3.b above, but 

notes that the general magnitude of risk 

estimates supports the potential for 

significant public health impacts, 

particularly for lower alternative annual 

standard levels. 

In the CASAC’s review of the 2019 

draft PA, the CASAC did not reach 

consensus on whether the current 

annual standard is adequate, with the 

majority of the CASAC recommending 

that the annual standard be retained and 

the minority of the CASAC 

recommending that the standard be 

revised. In their review of the 2021 draft 

PA, the CASAC unanimously 

recommended that the current annual 

standard is not sufficiently protective of 

public health (Sheppard, 2022a, p. 2 of 

consensus letter). 

The EPA disagrees with the 

commenters who state that the available 

scientific and quantitative information 

available in this reconsideration does 

not provide support for the current 

Administrator to reach a different 

decision than the then-Administrator 

reached in the 2020 final action. The 

EPA agrees with these commenters that 

there are uncertainties associated with 

the currently available scientific 

evidence. The EPA has considered these 

uncertainties extensively both in 

reaching conclusions in the 2022 PA 

(U.S. EPA, 2022b, sections 3.4.3, 3.6.1, 

and 4.6.3) and in the proposal (88 FR 

5604, 5609, January 27, 2023), and the 

EPA addresses more detailed public 

comments about these uncertainties, 

including those related to copollutant 

confounding, unmeasured confounding, 

and temporal and spatiotemporal 

confounding, in the Response to 

Comments document. However, we 

disagree with the commenters that the 

evidence does not provide support for 

the Administrator’s conclusion that the 

current primary annual PM2.5 standard 

is not adequate to protect public health 

with an adequate margin of safety, and 

should be revised. As described above, 

epidemiologic studies in the 2019 ISA 

and the ISA Supplement support and 

extend the evidence evaluated in the 

2009 ISA, through studies conducted in 

diverse populations and geographic 

locations, using various statistical 

models and approaches to control for 

potential confounders, and using a 

variety of exposure assessment 

methodologies. Therefore, the 

consistent, positive associations 

reported across studies (U.S. EPA, 

2019a, Figures 11–1 and 11–18; U.S. 

EPA, 2022a) are unlikely to be to be the 

result of unmeasured confounding and 

other biases are unlikely to account for 

the consistent positive associations 

observed across epidemiologic studies. 

Additionally, this reconsideration 

includes epidemiologic studies that 

were not before the then-Administrator 

for consideration in reaching his final 

decisions at the time of the 2020 

decision and that specifically evaluate 


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92As noted in the ISA Supplement: ‘‘In the peerreviewed literature, these epidemiologic studies are 

often referred to as causal inference studies or 

studies that used causal modeling methods. For the 

purposes of this Supplement, this terminology is 

not used to prevent confusion with the main 

scientific conclusions (i.e., the causality 

determinations) presented within an ISA. In 

addition, as is consistent with the weight-ofevidence framework used within ISAs and 

discussed in the Preamble to the Integrated Science 

Assessments, an individual study on its own cannot 

inform causality, but instead represents a piece of 

the overall body of evidence’’ (U.S. EPA, 2022a, p. 

1–3). 

93The EPA notes that, in considering the 

additional scientific evidence available in this 

reconsideration, one member of the CASAC who 

reviewed both the 2019 draft PA and the 2021 draft 

PA found that the available scientific and 

quantitative information available in this 

reconsideration supported revising the level of the 

primary annual PM2.5 standard, whereas he 

recommended retaining the standard during the 

review of the 2019 draft PA. 

confounding using alternative methods 

for confounder control). These recent 

epidemiologic studies provide support 

for the current Administrator’s 

conclusion that the suite of primary 

PM2.5 standards are not adequate. While 

confounding was an uncertainty noted 

by the then-Administrator in the 2020 

decision, he recognized ‘‘that 

methodological study designs to address 

confounding, such as causal inference 

methods, are an emerging field of 

study’’ (85 FR 82710, December 18, 

2020). The ISA Supplement considered 

studies that employed statistical 

approaches that attempt to more 

extensively account for confounders and 

are more robust to model 

misspecification (i.e., used alternative 

methods for confounder control),92

given that such studies were highlighted 

by the CASAC in their review of the 

2019 draft PA and identified in public 

comments on the 2020 proposal. Since 

the literature cutoff date for the 2019 

ISA, multiple studies that employ 

alternative methods for confounder 

control have become available for 

consideration in the ISA Supplement 

and, subsequently, in this 

reconsideration. For example, one study 

before the Administrator in this 

reconsideration that was not available in 

the 2019 ISA is Schwartz et al. (2021), 

which used a causal modeling approach 

focused on exposure changes and 

controls for measured confounders by 

design in order to evaluate the 

association between long-term PM2.5

exposure and mortality in the Medicare 

population. The study authors found 

significant associations of PM2.5 with 

increased mortality rates using a causal 

modeling approach robust to omitted 

confounding. The results of this study 

and other studies in the ISA 

Supplement that employ alternative 

methods to control for confounders lend 

support to the robustness of positive 

associations between PM2.5 exposure 

and multiple morbidity and mortality 

endpoints exhibited across 

epidemiologic studies, and also indicate 

that unmeasured confounding and other 

biases are unlikely to account for the 

consistent positive associations 

observed across epidemiologic studies 

(U.S. EPA, 2022b, sections 3.1.1.3, 

3.1.2.3, 3.2.1.3, and 3.2.2.3). 

Further, the EPA disagrees with the 

commenters who argue that the 

Administrator did not appropriately 

consider the strengths and limitations of 

the health evidence in reaching his 

decision to revise the current primary 

annual PM2.5 standard in this 

reconsideration. In reaching his 

proposed decision, the Administrator 

considered the entire body of evidence 

and how to appropriately weigh the 

uncertainties associated with the health 

evidence (88 FR 5617, January 27, 

2023). Such an approach is consistent 

with setting standards that are neither 

more nor less stringent than necessary, 

recognizing that ‘‘Congress provided 

that the Administrator is to use his 

judgment in setting air quality standards 

precisely to permit him to act in the face 

of uncertainty,’’ the Administrator must 

set standards on ‘‘the frontiers of 

scientific and medical knowledge’’ and 

‘‘Congress directed the Administrator to 

err on the side of caution in making the 

necessary decisions.’’ Lead Indus. Ass’n, 

Inc. v. EPA, 647 F.2d 1130, 1155 & n.50 

(D.C. Cir. 1980) (quoting H.R. Rep. No. 

95–294, at 50). As such, a determination 

of identifying a specific level at which 

the standard should be set necessarily 

requires the Administrator’s judgement 

(e.g., weighing the uncertainties and 

margin of safety). 

Additionally, the EPA disagrees with 

the commenters that contend that there 

is no basis in this reconsideration for 

deviating from the previous 

Administrator’s decision in 2020. It is 

well-established that in CAA section 

109 Congress specifically left the 

determination of the requisite NAAQS 

to the judgment of the Administrator 

and, moreover, that ‘‘decisions about the 

appropriate NAAQS level must 

‘necessarily . . . rest largely on policy 

judgments.’ ’’ Mississippi v. EPA, 744 

F.3d 1344, 1357 (D.C. Cir. 2013) 

(quoting Lead Industries Ass’n v. EPA, 

647 F.2d 1130, 1147 (D.C. Cir. 1980)). 

As the Court of Appeals for the D.C. 

Circuit has noted, ‘‘Every time EPA 

reviews a NAAQS, it (presumably) does 

so against contemporary policy 

judgments and the existing corpus of 

scientific knowledge.’’ Id., at 1343. 

In this reconsideration, both the 

existing corpus of scientific knowledge 

as well as the Administrator’s policy 

judgments about how to interpret and 

weigh that evidence to protect public 

health with an adequate margin of safety 

have changed. The expansion of the air 

quality criteria to encompass additional 

studies, information and analyses in the 

ISA Supplement and 2022 PA, as well 

as the additional consideration of the 

scientific record by the CASAC and the 

public provided the Administrator with 

significant additional information on 

which to base his decision.93 In 

addition, in this reconsideration, the 

Administrator is reaching different 

judgments about how to weigh the 

epidemiologic evidence, including the 

uncertainties in the scientific evidence, 

and how to ensure an adequate margin 

of safety to protect against uncertain 

harms, compared to the approach in the 

2020 final decision. For example, as 

discussed in greater detail above in 

section II.A.1 and in the 2020 notice of 

final rulemaking (85 FR 82717, 

December 18, 2020), in considering the 

epidemiologic evidence as part of his 

decision to retain the current primary 

annual PM2.5 standard in the 2020 

decision, the then-Administrator placed 

weight on the mean of the studyreported means (or medians) (i.e., 13.5 

mg/m3) from key U.S. epidemiologic 

studies that are monitor-based being 

above the level of the current primary 

annual PM2.5 standard of 12.0 mg/m3. By 

contrast, in this reconsideration, the 

current Administrator has taken an 

approach more similar to how the EPA 

has considered study-reported mean 

PM2.5 concentrations relative to the level 

of the primary annual PM2.5 standard in 

other recent PM NAAQS reviews. In so 

doing, in reaching his decision to revise 

the level of the primary annual PM2.5

standard to 9.0 mg/m3, he is using an 

approach that places weight on selecting 

a level for the standard that is below the 

study-reported mean PM2.5

concentrations reported in key U.S. 

epidemiologic studies, including recent 

epidemiologic studies that use hybrid 

model-based methods, as well as being 

near or below the 25th percentile PM2.5

concentrations in those key U.S. 

epidemiologic studies that report these 

concentrations. 

As such and further detailed in 

section II.B.4 below, in considering the 

adequacy of the current primary PM 

standards in this reconsideration, the 

Administrator has carefully considered 

the: (1) Policy-relevant evidence and 

conclusions contained in the 2019 ISA 

and 2022 ISA Supplement; (2) the 

quantitative information presented and 


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94 In providing advice on the 2019 draft PA, the 

CASAC did not weigh in specifically on the 

averaging time of the primary 24-hour PM2.5

standard but did recommend that the standard be 

retained because the available evidence does not 

call into question its adequacy (Cox, 2019b, p. 3 of 

consensus letter). 

assessed in the 2022 PA; (3) the 

evaluation of this evidence, the 

quantitative information, and the 

rationale and conclusions presented in 

the 2022 PA; (4) the advice and 

recommendations from the CASAC; and 

(5) public comments. The Administrator 

concludes that the current suite of 

primary PM2.5 standards are not 

adequate to protect public health with 

an adequate margin of safety. 

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. The 

EPA received relatively few comments 

on the averaging time and form for the 

primary PM2.5 standards, but those who 

did provide comments on these 

elements were primarily from public 

health and environmental organizations, 

State and local elected representatives, 

and State and local government 

agencies. Some commenters assert that 

the current 24-hour averaging time for 

the primary 24-hour PM2.5 standard 

does not adequately protect against 

short-term peaks. These commenters 

further state that the 24-hour averaging 

time protects against chronic exposures 

but does not adequately protect against 

serious acute risks from certain sources 

such as prescribed burning. Also, a few 

commenters explicitly recommend that 

a subdaily averaging time would be 

more appropriate, although none of the 

commenters recommended a specific 

averaging time for consideration. 

Additionally, some commenters cite to 

the CASAC’s advice in their review of 

the 2021 draft PA that future reviews of 

the PM NAAQS should include 

evaluation of alternative forms and 

averaging times of the current primary 

24-hour PM2.5 standard. 

The EPA disagrees with commenters 

that the current primary 24-hour PM2.5

standard, with its 24-hour averaging 

time, does not adequately protect 

against short-term peaks and disagrees 

that that there is sufficient information 

to conclude that a subdaily averaging 

time would be more appropriate than a 

24-hour averaging time. The EPA has 

reviewed the currently available 

scientific evidence and finds that it does 

not indicate that alternative averaging 

times would be more appropriate for the 

primary PM2.5 standards. Accordingly, 

the EPA concludes that it is appropriate 

to retain both the annual and 24-hour 

averaging times for standards meant to 

protect against long- and short-term 

PM2.5. 

As noted in the proposal, the 2019 

ISA and ISA Supplement found that the 

scientific evidence continues to provide 

strong support for health effect 

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

annual or multi-year) and short-term 

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

PM2.5. Epidemiologic 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 we note that 

subdaily effect estimates are less 

consistent and, in some cases, smaller in 

magnitude (88 FR 5618, January 27, 

2023). Controlled human exposure and 

panel-based studies of subdaily 

exposures typically examine subclinical 

effects rather than the more serious 

population-level effects that have been 

reported to be associated with 24-hour 

exposures (e.g., mortality, 

hospitalizations). Collectively, the 2019 

ISA concludes that epidemiologic 

studies do not indicate that subdaily 

averaging periods are more closely 

associated with health effects than the 

24-hour average exposure metric (U.S. 

EPA, 2019a, section 1.5.2.1). 

Additionally, the EPA notes that 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, 2022a, 

section 3.3.3.1). Therefore, this 

information does not indicate that a 

revision to the averaging time is needed 

to provide additional protection against 

subdaily PM2.5 exposures, beyond that 

provided by the current primary 

standards. This conclusion is also 

supported by the advice given to EPA by 

the CASAC in their review of the 2021 

draft PA, which reached consensus that 

averaging times for the standards should 

be retained, without revision (Sheppard, 

2022a, p. 2 of consensus letter).94 For all 

of these reasons, the Administrator 

concludes that the currently available 

evidence does not support considering 

alternatives to the annual and 24-hour 

averaging times for standards meant to 

protect against long- and short-term 

PM2.5 exposures. 

Multiple commenters, primarily from 

public health and environmental 

organizations, recommend revising the 

form of the primary 24-hour PM2.5

standard to a 99th percentile to provide 

increased public health protection 

against peak PM2.5 exposures, 

particularly for at-risk populations. 

These commenters express concern that 

the current 98th percentile form allows 

7 exceedances per year and contend that 

a 99th percentile form that would allow 

half that number is more appropriate. 

Commenters also cite to the CASAC’s 

advice in their review of the 2021 draft 

PA, which recommended that the EPA 

consider alternative percentiles for the 

form of the primary 24-hour PM2.5

standard in the future. 

The EPA disagrees that the current 

98th percentile form does not provide 

the requisite public health protection 

against peak PM2.5 exposures and 

concludes that the 98th percentile, 

averaged over three years, remains 

appropriate for the primary 24-hour 

PM2.5 standard. As noted in previous 

reviews and in the proposal, the EPA 

has set both an annual standard and a 

24-hour standard to provide protection 

from health effects associated with both 

long- and short-term exposures to PM2.5

(62 FR 38667, July 18, 1997; 88 FR 5620, 

January 27, 2023). With respect to the 

form of the 24-hour standard, as 

described just above, the epidemiologic 

studies continue to provide strong 

support for health effect associations 

with short-term (e.g., mostly 24-hour) 

PM2.5 exposures and controlled human 

exposure studies provide evidence for 

health effects following single shortterm ‘‘peak’’ PM2.5 exposures (88 FR 

5619, January 27, 2023). Both the 98th 

and the 99th percentile form provide a 

very high degree of control of peak 

concentrations. As the commenters 

point out, a 99th percentile would 

reduce the number of allowable 

exceedances to four days per year. The 

EPA anticipates, however, that such a 

revision to the form would make the 

attainment status of an area more 

subject to change from unpredictable 

nonanthropogenic factors, such as 

meteorological events. The EPA has 

often noted that frequent shifts in 

attainment status that are unrelated to 

long-term air quality trends is 

inconsistent with providing a stable 

target for air quality planning and risk 

management programs, which in turn 

provides for the most effective public 

health protection in the long run (78 FR 

3127, January 15, 2013; 80 FR 65351, 

October 26, 2015). Thus, the EPA’s 

interest in an appropriate degree of 

stability is to ensure that the State air 

quality programs are effective in 

controlling pollution and that the public 

health protections of the standard are 

achieved. As discussed above, while 

recent controlled human exposure 

studies provide consistent evidence for 

cardiovascular effects following PM2.5


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95The CASAC did not provide advice or 

recommendations regarding the forms of the 

primary PM2.5 standards in their review of the 2019 

draft PA (Cox, 2019b). 

96The Administrator notes that, in their review of 

the 2021 draft PA, a majority of members of the 

CASAC noted that there are some limitations for 

this approach ‘‘for the purpose of informing the 

adequacy of the standards’’ (Sheppard, 2022a, p. 8 

of consensus responses) and advised that future 

reviews should include evaluation of other metrics, 

including the distribution of concentrations 

reported in epidemiologic studies and in analyses 

restricting concentrations to below the current 

standard level. The Administrator also notes that, 

in their review of the 2019 draft PA, the CASAC 

lacked consensus on the inferences to be drawn 

from the epidemiologic evidence, with a majority of 

CASAC having concerns about confounding, error 

and bias and concluding that newer studies did not 

provide a basis for revising the current standards, 

while a minority concluded that the evidence, 

including more recent studies showing associations 

in areas with average long-term PM2.5

concentrations below the current annual standard, 

supported their conclusion that the current 

standards are inadequate (Cox, 2019b, pp. 8–9 of 

consensus responses). 

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, 2022a, 

section 3.3.3.1), and the 98th percentile 

form is very effective at limiting 

occurrences of exposures of concern. 

Taking into consideration the available 

scientific information and quantitative 

information, the EPA therefore 

concludes that the 98th percentile form 

provides an appropriate balance 

between limiting the occurrence of peak 

24-hour PM2.5 concentrations and 

identifying a stable target for risk 

management programs. This conclusion 

is also supported by the advice given to 

the EPA by the CASAC in their review 

of the 2021 draft PA, where they 

reached consensus that the form for the 

standards should be retained, without 

revision (Sheppard, 2022a, p. 2 of 

consensus letter).95

Additionally, the EPA recognizes the 

CASAC’s advice in their review of the 

2021 draft PA, where they 

recommended ‘‘that in future reviews, 

the EPA provide a more comprehensive 

assessment of the 24-hour standard that 

includes the form as well as the level’’ 

(Sheppard, 2022a, p. 4 of consensus 

letter). This advice is reflected in the 

proposal by the EPA, which noted ‘‘that 

it would be appropriate to gather 

additional air quality and scientific 

information and further consider these 

issues in future reviews’’ (88 FR 5619, 

January 27, 2023). The EPA will 

consider the information provided by 

the commenters regarding the form of 

the 24-hour PM2.5 standard in the next 

review of the PM NAAQS. 

A number of commenters who 

support revising the level of the primary 

annual PM2.5 standard, particularly 

those who support a revised level of 8 

mg/m3, disagree with how the EPA has 

emphasized the mean PM2.5

concentrations reported in key 

epidemiologic studies to inform 

conclusions on the level of the primary 

PM2.5 standard. These commenters 

argue that, in this reconsideration, the 

EPA is arbitrarily emphasizing 

uncertainties in key epidemiologic 

studies in the focus on mean 

concentrations. Many of these 

commenters recommend that the EPA 

consider the full distribution of PM2.5

concentrations from the key 

epidemiologic studies in reaching 

conclusions on the appropriate level for 

the primary annual PM2.5 standards, in 

particular concentrations below the 

mean, such as the 25th percentile. In 

supporting this view, commenters point 

to the CASAC’s advice in their review 

of the 2021 draft PA, where the majority 

of the CASAC stated that the ‘‘use of the 

mean to define where the data provide 

the most evidence is conservative since 

robust data clearly indicate effects 

below the mean in concentrationresponse functions’’ (Sheppard, 2022a, 

p. 16 of consensus responses), and that 

‘‘[e]pidemiologic studies require 

consideration of distribution around the 

mean of exposure to identify effects and 

thus lower levels than the mean must be 

considered as part of the range where 

the data provide higher confidence’’ 

(Sheppard, 2022a, p. 13 of consensus 

responses). 

As an initial matter, consistent with 

some previous approaches and as 

detailed by the Administrator in 

reaching conclusions on the level of the 

primary annual PM2.5 standard in 

section II.B.4 below, the EPA considers 

the long-term study-reported mean 

PM2.5 concentrations from key 

epidemiologic studies and sets the level 

of the standard to somewhat below the 

lowest long-term mean PM2.5

concentration. Additionally, as 

discussed further below, the EPA also 

considers the available information from 

a subset of epidemiologic studies that 

report exposure estimates or health 

events at the 25th and 10th percentiles 

of PM2.5 concentrations. The 

Administrator gives some weight to the 

25th percentile data, although he 

recognizes that his confidence in the 

magnitude and significance in the 

reported concentrations, and their 

ability to inform decisions on the 

appropriate level of the annual 

standard, decreases with reduced data 

(below the mean) and diminishes 

further at percentiles that are even 

further below the mean and the 25th 

percentile. Therefore, the Administrator 

places weight on the reported 25th 

percentiles concentrations, rather than 

the reported 10th percentile 

concentrations, for the subset of studies 

that report lower percentile PM2.5

concentrations in reaching his 

conclusions regarding the appropriate 

level for the primary annual PM2.5

standard. 

In considering the available scientific 

evidence to reach decisions on the 

adequacy of the suite of primary PM2.5

standards, the EPA notes that in 

previous PM NAAQS reviews 

(including the 1997, 2006 and 2012 

reviews), evidence-based approaches 

were used that focused on identifying 

standard levels near or somewhat below 

long-term mean concentrations reported 

in key epidemiologic studies. These 

approaches were supported by the 

CASAC in previous reviews and were 

supported in this reconsideration by the 

CASAC in their review of the 2021 draft 

PA.96

In considering the available scientific 

evidence, the EPA notes the strength of 

the epidemiologic evidence which 

includes multiple studies that 

consistently report positive associations 

for short- and long-term PM2.5 exposures 

and mortality and cardiovascular 

effects. Some available studies also use 

a variety of statistical methods to 

control for confounding bias and report 

similar associations, which further 

supports the broader body of 

epidemiologic evidence for both 

mortality and cardiovascular effects. 

Additionally, the EPA notes that recent 

epidemiologic studies strengthen 

support for health effect associations at 

PM2.5 concentrations lower than in 

those evaluated in epidemiologic 

studies available at the time of previous 

reviews. 

While these epidemiologic studies 

evaluate associations between 

distributions of ambient PM2.5

concentrations and health outcomes, 

they do not identify the specific 

exposures that led to the reported 

effects. As such, there is no specific 

point in the air quality distribution of 

any epidemiologic study that represents 

a ‘‘bright line’’ at and above which 

effects have been observed and below 

which effects have not been observed. 

Studies of daily PM2.5 exposures 

examine associations between day-today 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 


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97The Wang et al. (2017) study only reports the 

25th percentile of the estimated PM2.5

concentrations, not the 10th percentile. 

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 health events 

occur at estimated exposures that reflect 

annual average PM2.5 concentrations 

around the middle of the air quality 

distributions examined. In both cases, 

epidemiologic 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. Therefore, in the absence 

of discernible thresholds, long-term 

study-reported means—that is, the 

study-reported ambient PM2.5

concentrations in the epidemiologic 

studies that reflect estimated exposures 

with a focus around the middle portion 

of the PM2.5 air quality distribution 

where the bulk of the observed data 

reside—provide the strongest support 

for reported health effect associations in 

epidemiologic studies. 

Based on the air quality criteria for 

this reconsideration, as described in the 

2019 ISA, ISA Supplement, 2022 PA 

and the proposal, the EPA believes it is 

appropriate to continue to use the mean 

PM2.5 concentrations from the key 

epidemiologic studies to inform 

conclusions regarding the appropriate 

level for the primary annual PM2.5

standard. 

There are a large number of key 

epidemiologic studies available in this 

reconsideration to inform conclusions 

regarding the level of the primary 

annual PM2.5 standard. For the key U.S. 

epidemiologic studies, the studyreported mean PM2.5 concentrations 

range from 9.9–16.5 mg/m3 for monitorbased studies (Figure 1 above) and range 

from 9.3–12.2 mg/m3 for hybrid 

modeling-based studies (Figure 2 

above). 

In addition to the study-reported 

mean PM2.5 concentrations, the EPA 

agrees with the CASAC’s advice in their 

review of the 2021 draft PA and public 

comments that information on other 

percentiles below the mean can also be 

informative, and the EPA notes that the 

CASAC advised that for the purpose of 

informing the adequacy of the 

standards, future reviews should 

include an evaluation of other metrics, 

including the distribution of 

concentrations reported in 

epidemiologic studies (Sheppard, 

2022a, p. 9 of consensus responses). As 

such, in reaching conclusions in this 

reconsideration, the EPA takes note of 

the additional study-reported PM2.5

concentrations below the means (e.g., 

25th and 10th percentiles) that are 

available from a limited subset of key 

U.S. epidemiologic studies. As shown in 

Figures 1 and 2 above, six key U.S. 

epidemiologic studies report 

information on other percentiles (e.g., 

10th and 25th percentiles of PM2.5

concentrations or 10th and 25th 

percentiles of PM2.5 concentrations 

associated with health events) that are 

below the mean.97 Three of the studies 

are monitor-based and three are hybrid 

model-based. 

The key U.S. epidemiologic studies 

that report percentiles below the mean 

that are monitor based are older studies. 

These studies included smaller numbers 

of people than the newer hybrid modelbased studies. For the three older, 

monitor-based studies, because the 

cohorts were smaller in size, a relatively 

smaller portion of the health events 

were observed in the lower part of the 

air quality distribution. As such, our 

confidence in the magnitude and 

significance of the associations begins to 

decrease in the lower part of the air 

quality distribution of those older, 

monitor-based studies. 

The three newer, hybrid model-based 

studies have larger cohort sizes than the 

older, monitor-based studies and, as 

noted by commenters, have more health 

events in the lower part of the air 

quality distribution. For these reasons, 

the EPA notes that we have more 

confidence in the reported association at 

concentrations lower than the reported 

mean in these more recent hybrid 

model-based studies, particularly at the 

25th percentile compared to the 10th 

percentile. While the cohort sizes in the 

more recent, hybrid model-based 

studies are larger than the older, 

monitor-based studies, the EPA notes 

that the 10th percentiles are well below 

the middle portion of the air quality 

distribution for which we have the 

greatest confidence, and as noted above, 

our confidence in the magnitude and 

significance of associations in the lower 

parts of the air quality distribution 

begins to decrease. While we have more 

confidence in the lower percentiles 

because of the larger cohort sizes in the 

more recent hybrid model-based 

studies, we also have more confidence 

in the 25th percentiles than in the 10th 

percentiles, which are further from the 

means and closer to the lower end of the 

air quality distribution. 

In considering how the six studies 

that report percentiles lower than the 

mean can be used to inform conclusions 

regarding the level of the primary 

annual PM2.5 standard, the EPA first 

notes that the three monitor-based 

epidemiologic studies (Bell et al., 2008; 

Franklin et al., 2007; Zanobetti and 

Schwartz, 2009) report 25th percentile 

concentrations that are at or above 11.5 

mg/m3. For two of the more recent 

hybrid model-based studies (Di et al., 

2017b; Wang et al., 2017), the 25th 

percentile of estimated PM2.5

concentrations are just above 9 mg/m3, 

while one study (Di et al., 2017a) reports 

a PM2.5 concentrations corresponding to 

25th percentiles of health events of just 

below 7 mg/m3. For the Di et al. (2017a) 

study, the 25th percentile PM2.5

concentration (6.7 mg/m3) is based on 

the PM2.5 concentration at which the 

25th percentile of deaths occur in the 

study, while the reported mean (11.6 mg/ 

m3) is based on estimated PM2.5

exposure concentrations. Additionally, 

the 25th percentiles of the other two 

recently available hybrid model-based 

studies (Di et al., 2017b; Wang et al., 

2017) are based on estimated PM2.5

concentrations. As such, the PM2.5

concentration at which the 25th 

percentile of health events occur may be 

different from the estimated 25th 

percentile PM2.5 concentration in this 

study (Di et al., 2017a), creating an 

uncertain basis for comparison with the 

studies by Di et al. (2017b) and Wang et 

al. (2017). The 25th percentiles from 

these studies, in particular those that are 

more recently available, help to inform 

the Administrator’s judgments regarding 

the appropriate level for the primary 

annual PM2.5 standard. 

Some commenters disagree with the 

EPA’s consideration of the relationship 

between mean PM2.5 concentrations 

reported in the key epidemiologic 

studies and design values to inform 

conclusions on the appropriate level for 

the primary annual PM2.5 standards. 

Commenters contend that setting the 

level of the primary annual standard 

below the design values in the 

epidemiologic studies, rather than 

below the study-reported mean 

concentrations, might keep overall mean 

PM2.5 concentrations throughout an area 

below the study-reported means but 

allow PM2.5 concentrations in some 

parts of the area, including near the 

‘‘design value monitor’’ to remain above 

the study-reported mean PM2.5

concentrations, which are the 

concentrations where the evidence of 

health effects is strongest. Commenters 

contend that such a decision framework 

would not result in a standard that 

would provide requisite protection with 

an adequate margin of safety, 


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particularly for at-risk populations. 

These commenters further support this 

view by citing the CASAC’s advice in 

their review of the 2021 draft PA, where 

the majority of CASAC stated that ‘‘even 

if a design value is somewhat higher 

than the area average, it reflects actual 

exposure levels and thus any portion of 

the population living near the design 

value monitor does experience 

exposures at that level and consequent 

health effects of exposure to that higher 

concentration’’ (Sheppard, 2022a, p. 14 

of consensus responses). Additionally, 

these commenters suggest that the EPA 

should not deviate from the approach 

taken in the 2012 review, which was to 

set the standard at a level ‘‘somewhat 

below’’ the lowest mean PM2.5

concentration in the key epidemiologic 

studies. 

To the extent that commenters are 

suggesting that the EPA is setting the 

level of the primary annual PM2.5

standard below the design values in the 

epidemiologic studies, rather than 

below the study-reported mean PM2.5

concentrations, we disagree with the 

commenters. In reaching conclusions on 

the level of the primary annual PM2.5

standard, the EPA considers the longterm study-reported mean PM2.5

concentrations from key epidemiologic 

studies and sets the level of the standard 

to somewhat below the lowest long-term 

mean PM2.5 concentration, not below 

the design values in the epidemiologic 

studies. Additionally, the EPA also 

considers the available information from 

a subset of epidemiologic studies that 

report exposure estimates or health 

events at the 25th and 10th percentiles 

of PM2.5 concentrations. The EPA 

particularly considers the 25th 

percentile data, while recognizing that 

our confidence in the magnitude and 

significance in the reported 

concentrations, and the ability of the 

lower percentile PM2.5 concentrations to 

inform decisions on the appropriate 

level of the annual standard, decreases 

with reduced data (below the mean) and 

diminishes further at percentiles that 

are even further below the mean and the 

25th percentile. 

However, the EPA notes that it is 

important to understand, and to not 

ignore, the relationship between the 

study-reported mean PM2.5

concentrations reported in key 

epidemiologic studies and the area 

design value. As an initial matter, the 

NAAQS consists of all four elements of 

the standard (indicator, averaging time, 

form, and level) and setting a standard 

that is requisite to protect public health 

includes consideration of all four 

elements together. Following 

implementation of the NAAQS, the 

design value is the metric used to 

determine compliance with the standard 

and is the statistic that describes the air 

quality status of a given location relative 

to the level of the primary annual PM2.5

NAAQS. The design value is different 

from the study-reported mean PM2.5

concentrations. This is because the 

study-reported mean PM2.5

concentrations are an annual average 

PM2.5 concentration, similar to the level 

of the standard, but the epidemiologic 

studies do not report statistics that take 

into account the other elements of the 

standard (i.e., averaging time and form). 

Therefore, when considering the 

appropriate revisions to the annual 

PM2.5 standard, the EPA must consider 

the protection provided by a revised 

standard taking into account all of the 

elements of the standard, not just the 

annual average PM2.5 concentration 

alone. 

In considering the annual standard, 

and in assessing the range of studyreported exposure concentrations for 

which we have the strongest support for 

adverse health effects observed in 

epidemiologic studies, the EPA focuses 

on whether the current primary annual 

PM2.5 standard provides adequate 

protection against these exposure 

concentrations or if the level of the 

standard should be revised to provide 

the appropriate public health 

protection. This means that, as in some 

previous reviews, it is important to 

consider how the study means were 

computed and how these concentrations 

compare to the annual standard metric 

(including the level, averaging time and 

form) which must be met at the monitor 

with the highest PM2.5 design value in 

an area for compliance with the 

NAAQS. This approach is based on the 

application of a decision framework 

based on assessing means (as well as the 

lower distribution of reported PM2.5

concentration, as noted above) reported 

in key epidemiologic studies. In the 

2012 review, the available key 

epidemiologic studies computed the 

mean PM2.5 concentrations using an 

average across monitor-based PM2.5

concentrations. As such, at that time, 

the decision framework used an 

approach based on maximum monitor 

concentrations to determine compliance 

with the standard, while selecting the 

standard level based on consideration of 

composite monitor concentrations (i.e., 

selecting the standard level of 12.0 mg/ 

m3 was just below the long-term studyreported mean PM2.5 concentrations in 

key epidemiologic studies). Further, the 

EPA conducted analyses that examined 

the differences in these two metrics (i.e., 

maximum monitor concentrations, 

which is how compliance with the 

standard is assessed and composite 

monitor concentrations, which is how 

key epidemiologic studies report their 

mean concentrations) across the U.S. 

and in areas included in the key 

epidemiologic studies and found that 

the maximum design value in an area 

was generally higher than the monitor 

average across that area, with the 

amount of difference between the two 

metrics varying based on location and 

concentration (Hassett-Sipple et al., 

2010; Frank, 2012). This information 

was taken into account by the thenAdministrator’s final decision in 

selecting a level of 12.0 mg/m3 for the 

primary annual PM2.5 standard in the 

2012 review and discussed more 

specifically in her considerations on 

adequate margin of safety. 

The relationship between the mean 

PM2.5 concentrations and the area 

design value continues to be an 

important consideration in evaluating 

the adequacy of the current or potential 

alternative annual standard levels in 

this reconsideration. Again, in a given 

area, the area design value is based on 

the monitor in an area with the highest 

PM2.5 concentrations and is used to 

determine compliance with the 

standard, including the averaging time 

and form of the standard (i.e., an annual 

average over 3-years must not exceed 

the level of the of the annual PM2.5

standard). The highest PM2.5

concentrations spatially distributed in 

the area would generally occur at or 

near the area design value monitor and 

the distribution of PM2.5 concentrations 

would generally be lower in other 

locations and at monitors in that area. 

As such, when an area is meeting a 

specific annual standard level (e.g., 9.0 

mg/m3), we would expect the annual 

average exposures (i.e., a metric similar 

to the study-reported mean values) in 

that area to be at concentrations lower 

than that level (e.g., lower than 9.0 mg/ 

m3). 

However, as described in section 

II.A.2.c.ii, we note that there are a 

substantial number of different types of 

epidemiologic studies available since 

the 2012 review, as assessed in both the 

2019 ISA and the ISA Supplement, that 

make understanding the relationship 

between the mean PM2.5 concentrations 

and the area design value an even more 

important consideration in this 

reconsideration (U.S. EPA, 2019a; U.S. 

EPA, 2022a). While the key 

epidemiologic studies in the 2012 

review were all monitor-based studies, 

the recent epidemiologic studies in this 

reconsideration include hybrid 

modeling approaches that have emerged 

in the epidemiologic literature as an 


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alternative to approaches that only use 

ground-based monitors to estimate PM2.5

exposure. As assessed in the 2019 ISA 

and ISA Supplement, a substantial 

number of epidemiologic studies used 

hybrid model-based methods in 

evaluating associations between PM2.5

exposure and health effects. Hybrid 

model-based studies employ various 

fusion techniques that combine groundbased monitored data with air quality 

modeled estimates and/or information 

from satellites to estimate PM2.5

exposures. While these studies provide 

a broader estimation of PM2.5 exposures 

compared to monitor-based studies (i.e., 

PM2.5 concentrations are estimated in 

areas without monitors), the hybrid 

modeling approaches result in studyreported means that are more difficult to 

relate to the annual standard metric and 

to the maximum monitor design values 

used to assess compliance. In addition, 

to further complicate the comparison, 

when looking across these studies, we 

find variations in how exposure is 

estimated between such studies, and 

thus, how the study means are 

calculated. Two important variations 

across studies include: (1) Variability in 

spatial scale used (i.e., averages 

computed across the national (or large 

portions of the country) versus a focus 

on only CBSAs); and (2) variability in 

exposure assignment methods (i.e., 

averaging across all grid cells, averaging 

across a scaled-up area like a ZIP code, 

and population weighting). The 

differences in these approaches can 

result in studies reporting different 

study means, even though the 

association between PM2.5 exposure and 

health effects outcomes are similar. 

To emphasize the importance of the 

differences between the studies, we 

revisit the simplified example in the 

State of Georgia from the 2022 PA that 

evaluates monitors and hybrid modeling 

approaches, noting that this example is 

useful to exhibit how the differences in 

the methods used to estimate exposure 

can lead to differences in the reported 

mean concentrations (U.S. EPA, 2022b, 

p. 3–71). In this example, for all 

monitors within the Atlanta-Sandy 

Springs-Roswell CBSA, the average 

PM2.5 concentration is 9.3 mg/m3, while 

the area design value (based on the 

highest monitored PM2.5 concentration 

in the area) is 10.4 mg/m3. This 

comparison helps to illustrate the fact 

that composite monitor values tend to 

be somewhat lower than the highest 

area monitor values, consistent with the 

key points made in the 2012 review. 

This example also illustrates how 

monitors are sited to represent the 

higher concentrations within the area 

and that the area’s annual design value, 

which is used for compliance with the 

standard, is calculated based on the 

highest monitor in the area. Next, in this 

example, mean PM2.5 concentrations 

were calculated using similar 

approaches to those used in hybrid 

modeling-based epidemiologic studies 

to compute study-reported means, 

including (1) the average concentration 

across the entire State of Georgia; (2) the 

population-weighted average across the 

entire State; (3) the average 

concentration across the Atlanta-Sandy 

Springs-Roswell CBSA; and (4) the 

population-weighted average across the 

Atlanta-Sandy Springs-Roswell CBSA. 

At the urban level (e.g., Atlanta-Sandy 

Springs-Roswell CBSA), the average 

PM2.5 concentration when taking the 

mean of all grid cells is 9.2 mg/m3, 

whereas the population-weighted mean 

is 9.6 mg/m3. Across Georgia, the average 

PM2.5 concentration using the hybrid 

approach and averaged across each grid 

cell is 8.3 mg/m3, which is lower than 

the population-weighted statewide 

average of 9.1 mg/m3. While this is a 

simple example completed in one State 

and one CBSA, it suggests that the 

lowest mean values tend to result from 

the approaches that use concentrations 

from all or most grid cells (e.g., did not 

apply population weighting), both urban 

and rural, across the study area to 

compute the mean. Higher mean values 

are observed when the approach focuses 

on the urban areas alone or when the 

approach incorporates population 

weighting. Overall, this example 

suggests that the means from studies 

using hybrid modeling approaches are 

generally lower than the means from 

monitor-based approaches, and means 

from both approaches are lower than the 

annual design values for the same area. 

Population weighting tends to increase 

the calculated mean concentration, 

likely because more densely populated 

areas also tend to have higher PM2.5

concentrations. In other words, this 

simplified example exhibits how not all 

reported mean PM2.5 concentrations 

from key epidemiologic studies are the 

same; some reported means are from 

monitored studies and some reported 

means are from hybrid modeling 

studies, while some reported means 

include only urban areas, and other 

reported means include both urban and 

rural areas, and some reported means 

include aspects of population weighting 

while others do not. 

As detailed above in section I.D.5, in 

the air quality analyses comparing 

composite monitored PM2.5

concentrations with annual PM2.5 design 

values in U.S. CBSAs, maximum annual 

PM2.5 design values were approximately 

10% to 20% higher than annual average 

composite monitor concentrations (i.e., 

averaged across multiple monitors in 

the same CBSA). Based on these results, 

this analysis suggests that there will be 

a distribution of concentrations and the 

maximum annual average monitored 

concentration in an area (at the design 

value monitor, used for compliance 

with the standard), will generally be 10– 

20% higher than the average across the 

other monitors in the area. Thus, in 

considering how the annual standard 

levels would relate to the study-reported 

means from monitor-based studies, we 

can generally conclude that an annual 

standard level that is no more than 10– 

20% higher than monitor-based studyreported mean PM2.5 concentrations 

would generally maintain air quality 

exposures to be below those associated 

with the study-reported mean PM2.5

concentrations, exposures for which we 

have the strongest support for adverse 

health effects occurring. 

Air quality analyses described in 

section I.D.5 above also consider 

information from the epidemiologic 

studies that utilized the hybrid 

modeling approaches. Analyses show 

that average maximum annual design 

values are 40–50% higher when 

compared to annual average PM2.5

concentrations estimated without 

population weighting and are 15–18% 

higher when compared to average 

annual PM2.5 concentrations with 

population weighting applied. Given 

these results, it is worth noting that for 

the studies using the hybrid modeling 

approaches, the choice of methodology 

employed in calculating the studyreported means (i.e., using population 

weighting versus not applying aspects of 

population weighting), and not a 

difference in estimates of exposure in 

the study itself, can produce 

substantially different study-reported 

mean values, with the approach that 

does not employ population weighting 

producing a much lower reported mean 

PM2.5 concentration. Therefore, the 

impact of the differences in methods is 

an important consideration when 

comparing mean concentrations across 

studies. 

Because of the differences in the 

methods employed by the key 

epidemiologic studies, and as 

demonstrated by the example and air 

quality analyses above, the application 

of any decision framework that 

considers the study-reported mean 

PM2.5 concentrations, and evaluates 

whether the current annual standard 

provides adequate protection against 

these reported exposure concentrations, 

is more complicated than the 


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approaches used in past reviews. As 

such, the EPA disagrees with 

commenters who argue that the EPA’s 

consideration of the relationship 

between mean PM2.5 concentrations 

reported in key epidemiologic studies 

and design values is not appropriate and 

should be ignored. 

In considering the information from 

the epidemiologic studies, while the 

EPA does not dispute the reported 

associations of epidemiologic studies in 

hybrid modeling studies that report 

long-term mean concentrations and do 

not apply aspects of population 

weighting, using the reported long-term 

mean concentration from these studies 

in informing an appropriate level of the 

annual PM2.5 standard is more 

uncertain. Given this, hybrid modeling 

studies that do not apply aspects of 

population weighting provide less 

information on conclusions regarding 

the appropriate level of the primary 

annual PM2.5 standard. In support of 

this, some commenters also noted this 

consideration and suggested that the 

Administrator place lower weight on 

U.S. studies that did not use population 

weighting. 

In considering the relationship 

between study-reported mean PM2.5

concentrations and the design values, 

the EPA agrees with commenters that 

setting the level of the primary annual 

standard below the design values, rather 

than below the study-reported mean 

concentrations, might allow PM2.5

concentrations in some part of the area 

near the design value monitor to remain 

above the study-reported mean 

PM2.5concentration, where evidence of 

health effects is strongest. As discussed 

in the proposal and in section II.B.4 

below, the Administrator specifically 

notes that that the highest PM2.5

concentrations spatially distributed in 

the area would generally occur at or 

near the area design value monitor and 

that PM2.5 concentrations will be equal 

to or lower at other monitors in the area. 

Furthermore, since monitoring strategies 

aim to site monitors in areas with higher 

PM2.5 concentrations, monitored areas 

will generally have higher 

concentrations compared to areas 

without monitors. Therefore, by setting 

the level of the standard to 9.0 mg/m3

and just below the lowest studyreported mean PM2.5 concentration (e.g., 

9.3 mg/m3), the highest possible design 

value in a given area would be just 

below the study-reported mean PM2.5

concentration, the concentration where 

we have the most confidence in the 

reported health effect association, and 

we anticipate that, based on our 

assessment of air quality data, the 

distribution of PM2.5 concentrations 

would decrease even further with 

distance from the highest monitor (i.e., 

the ‘‘design value monitor’’) (see, for 

example, U.S. EPA, 2022a, section 

2.3.3.2.4 and pp. 3–71 to 3–77). The 

Administrator further notes that when 

an epidemiologic study reports a mean 

PM2.5 concentration that reflects the 

average of annual average monitor-based 

concentrations across an area, the area 

design value will generally be higher 

than the study-reported mean. 

Similarly, he observes that when a study 

reports a mean that reflects the average 

of annual average concentrations 

estimated at across an area using a 

hybrid modeling approach, the area 

design value will generally be higher. 

As such, by evaluating the difference 

between the study-reported mean PM2.5

concentrations and design values, the 

Administrator seeks to set the level of 

the standard below the lowest studyreported mean, while ensuring that the 

primary annual PM2.5 standard, 

including its averaging time and form, 

provides protection against the 

exposures associated with health effects 

observed in the key epidemiologic 

studies. 

Additionally, the EPA disagrees with 

commenters who contend that the 

approach taken may allow PM2.5 near 

the design value monitor to remain 

above the study-reported mean PM2.5

concentrations. In following this 

approach of setting the annual standard 

level somewhat below the lowest 

reported mean PM2.5 concentration, 

setting a standard level that requires the 

design value monitor (which is the 

highest monitor in an area) to be just 

below the lowest study-reported mean 

across key studies will generally result 

in distributions of even lower 

concentrations of PM2.5 across the entire 

area, such that even those people living 

near an area design value monitor 

(where PM2.5 concentrations are 

generally highest) will be exposed to 

PM2.5 concentrations below the PM2.5

concentrations reported in the 

epidemiologic studies where there is the 

highest confidence of an association. In 

their review of the 2021 draft PA, the 

majority of the CASAC had some 

concerns about the approach for 

comparing study means and design 

values, questioning whether such an 

approach would provide adequate 

protection for people who live in areas 

with higher concentrations, such as 

those living in areas with higher 

concentrations (e.g., near the design 

value monitor) (Sheppard, 2022a, p. 8 of 

consensus responses). The minority of 

the CASAC, in considering the 

relationship between the study-reported 

mean PM2.5 concentration and design 

values, stated that ‘‘the form of the 

standard and the way attainment with 

the standard is determined (i.e., highest 

design value in the CBSA) are important 

factors when determining the 

appropriate level for the standard’’ and 

noted that that design values are 

generally higher than area average 

exposure levels (Sheppard, 2022a, p. 17 

of consensus responses). For all of the 

reasons discussed above, and consistent 

with the minority of the CASAC’s 

advice in their review of the 2021 draft 

PA, we disagree with the commenters 

that areas near the design value 

monitors would be expected to 

experience PM2.5 concentrations above 

the study-reported mean concentrations. 

Several commenters assert that 

epidemiologic studies that restrict PM2.5

concentration to below 12 mg/m3

provide additional support for revising 

the level of the primary annual PM2.5

standard to 8 mg/m3. Some commenters 

disagree with the EPA’s assertion that 

the studies that employ restricted 

analyses do not provide enough 

information to understand how the 

studies were restricted to certain PM2.5

concentrations, with commenters 

providing additional information on the 

methods for restricted analyses. The 

commenters state that for the long-term 

studies at issue here, the study authors 

simply examined their database that 

linked subjects to long-term PM2.5

concentrations above 12 mg/m3, 

removed those data from the analysis, 

and reran the analysis. Additionally, 

one commenter provided an explanation 

of how the restricted analyses were 

conducted in studies for which he was 

an author. The commenter notes that for 

each year a subject was in the study, 

annual PM2.5 concentrations were 

assigned at the ZIP code level. If they 

moved, they were assigned the ZIP code 

level PM2.5 concentration for the new 

ZIP code. The commenter notes that 

these restricted analyses only included 

subjects whose annual PM2.5 exposure 

never exceeded that restricted 

concentration for any year of follow-up 

in the study. The commenter suggested 

that the EPA may be concerned as to 

how PM2.5 concentrations in restricted 

analyses related to a design value since 

these are exposures for individuals who 

may have relocated during the study but 

argue that that is not the point. The 

commenters assert that while the 

analyses were restricted to people never 

exposed above certain concentrations 

over longer periods of time, the actual 

PM2.5 exposure was one year of 

exposure in most of these studies. 

Commenters also suggest that, since the 


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EPA has deviated from its approach 

from the 2012 review for considering 

study-reported mean PM2.5

concentrations, the EPA should dismiss 

its concerns regarding being able to 

relate the mean PM2.5 concentrations 

from these studies to design values. 

First, the EPA agrees with 

commenters that studies that employ 

restricted analyses can be used for 

informing conclusions regarding the 

appropriate level of the primary annual 

PM2.5 standard. However, the EPA 

disagrees that the information provided 

by the commenters provides a sufficient 

basis for an annual standard level of 8 

mg/m3. Restricted analyses provide 

additional support for effects at lower 

concentrations, exhibiting associations 

for mean concentrations presumably 

below the mean concentrations for the 

main analyses. However, even though 

commenters note that any individual 

with exposures over the restricted 

analyses is excluded from restricted 

analyses, uncertainties remain with 

regard to how the mean PM2.5

concentrations in restricted analyses 

compare to design values, particularly 

in light of the removal of entire ZIP 

codes from analyses. Design values are 

calculated based on all measured PM2.5

concentrations. When an analysis is 

restricted below a certain level, some 

parts of the air quality distribution are 

removed, but comparing the restricted 

mean to a design value is not possible 

because these are two different metrics. 

For example, in a study that restricts 

concentrations below 12 mg/m3, that 

represents only part of the air quality 

distribution, whereas a design value for 

that study area would include all PM2.5

concentrations, not just the ones below 

12 mg/m3. Therefore, in contrast to 

means from the main (unrestricted) 

analysis, it is not possible to compare 

mean concentrations from restricted 

analyses to design values. Further, it is 

unclear how one could evaluate such a 

relationship between design values and 

mean PM2.5 concentrations from studies 

that use restricted analyses because the 

standard is set based on all of its 

elements (indicator, averaging time, 

form, and level) and removing PM2.5

concentrations from the calculation of 

the design value for such a comparison 

would result in a metric that is no 

longer a design value that would 

provide the intended protection of the 

standard. This leads to greater 

uncertainty in how to use the mean 

PM2.5 concentrations from these studies 

that use restricted analyses in a similar 

decision framework as the 

epidemiologic studies that report longterm mean PM2.5 concentrations for 

health effect associations for the full 

distribution of PM2.5 concentrations. 

As described in reaching his 

conclusions in the section below, the 

Administrator judges that, despite these 

uncertainties and limitations, studies 

that use restricted analyses can provide 

supplemental information for 

consideration in reaching conclusions 

regarding both the adequacy and level of 

the standard. He notes two studies (Di 

et al., 2017b and Dominici et al., 2019) 

are available in this reconsideration that 

report means in their restricted analyses 

(restricting annual average PM2.5

exposure below 12 mg/m3) and used 

population-weighted approaches to 

estimate PM2.5 exposures and these 

studies report mean PM2.5

concentrations of 9.6 mg/m3. He 

recognizes that these studies are just one 

line of evidence for consideration and 

that along with the broader evidence 

base, including the key epidemiologic 

studies, these studies provide support 

that the level of the primary annual 

PM2.5 standard should be set below 10 

mg/m3. 

We disagree with the commenters that 

concerns about relating the mean PM2.5

concentrations from restricted analyses 

to design values are not valid. As an 

initial matter, restricted analyses were 

not available and did not inform the 

2012 decision to revise the annual PM2.5

standard level to 12.0 mg/m3. The 

approach in 2012 in revising the annual 

standard was to set the level to 

somewhat below the mean of key 

epidemiologic studies. As noted above, 

while the EPA believes that restricted 

analyses can help inform conclusions 

regarding the adequacy and the level of 

the primary annual PM2.5 standard, in 

the context of placing the studies in a 

decision framework to inform the 

appropriate level of the annual PM2.5

standard, the EPA has not deviated from 

its approach from the 2012 review. 

Given that restricted analyses are new 

since the 2012 review, the EPA 

disagrees with commenters that 

uncertainties associated with these 

studies should not be considered, and 

that these studies should be used in a 

similar manner to their main analyses in 

taking an approach to set a level of the 

standard somewhat below the lowest 

long-term reported mean PM2.5

concentration. Specifically, as detailed 

above there are uncertainties and 

limitations associated with relating the 

mean PM2.5 concentrations from these 

studies to design values for studies that 

use restricted analyses, and many of 

these studies did not expressly report a 

mean PM2.5 concentration for the 

restricted analysis which makes it 

impossible to make such a comparison. 

Several commenters contend that in 

considering the accountability studies, 

the EPA inappropriately reached 

conclusions regarding the level of the 

primary annual PM2.5 standard based on 

the starting PM2.5 concentrations of 

these studies, rather than the ending 

concentrations (i.e., concentrations after 

a policy was implemented). The 

commenters assert that these studies 

provide support for revising the level of 

the primary annual PM2.5 standard to 

below the proposed range of 9–10 mg/m3

to protect public health with an 

adequate margin of safety. 

Accountability studies examine the 

effect of a policy on reducing PM2.5

concentrations in ambient air and 

evaluate whether such reductions were 

observed to also lead to reductions in 

PM2.5- associated health outcomes (e.g., 

mortality). Additionally, accountability 

studies can reduce uncertainties related 

to residual confounding of temporal and 

spatial factors (U.S. EPA, 2022a, p. 3– 

25). Prior to implementation of the 

policies, three accountability studies 

newly available in this reconsideration 

and assessed in the ISA Supplement, 

report mean PM2.5 concentrations below 

the level of the current annual standard 

level (12.0 mg/m3) and ranged from 10.0 

mg/m3 to 11.1 mg/m3 (Sanders et al., 

2020b; Corrigan et al., 2018; and 

Henneman et al., 2019). These studies 

suggest that public health improvements 

may occur following the 

implementation of a policy that reduces 

annual average PM2.5 concentrations 

below the level of the current standard 

of 12.0 mg/m3, and potentially below the 

lowest ‘‘starting’’ concentrations in 

these studies of 10.0 mg/m3. However, 

while the small number of studies may 

provide limited information related to 

informing the adequacy and level of the 

annual PM2.5 standard, we note that 

accountability studies are only one line 

of evidence, and that these studies 

provide supplemental information for 

consideration in addition to the full 

body of evidence. Further, the EPA does 

not believe it would be appropriate to 

determine the level of the standard by 

reference to ending concentrations in 

accountability studies. Accountability 

studies are most informative in 

demonstrating that public health 

improvements may occur following the 

implementation of a policy that reduces 

annual average PM2.5 concentrations 

below the level of the current standard 

of 12.0 mg/m3, and potentially below the 

lowest ‘‘starting’’ concentrations in 

these studies of 10.0 mg/m3. However, 

the EPA finds the available information 

from accountability studies is too 

limited to support a conclusion that the 


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98All of the key U.S. epidemiologic studies 

considered in this reconsideration focus on all or 

subsections of the continental U.S. 

appropriate level at which to set the 

primary annual PM2.5 standard would 

be equal to the ending concentrations of 

those studies, as the commenters 

suggest. These studies demonstrate that 

there are reductions in health outcomes 

when PM2.5 concentrations are reduced 

in these studies from the starting 

concentration to the ending 

concentration, but do not provide 

support for health effect associations at 

or below the ending concentrations that 

would warrant a more stringent 

standard. 

Commenters disagree with the 

Administrator placing less weight on 

the epidemiologic studies conducted in 

Canada when reaching conclusions 

regarding the level of the primary 

annual PM2.5 standard. These 

commenters argue that the Canadian 

epidemiologic studies provide support 

for setting the level at the lowest end of 

the proposed range (i.e., 8 mg/m3) 

because they report mean PM2.5

concentrations, in some cases, below 8 

mg/m3. Commenters disagree with the 

EPA’s reasoning for placing less weight 

on the Canadian epidemiologic studies, 

suggesting it conflicts with the 

approaches in previous PM NAAQS 

reviews and arguing that the findings of 

the Canadian epidemiologic studies can 

be directly translated into a primary 

annual PM2.5 standard. Additionally, 

while the commenters disagree with the 

EPA’s approach for considering the 

study-reported mean PM2.5

concentrations and design values in 

general, they note that the CASAC, in 

their review of the 2021 PA, noted that 

‘‘while there may be no design value in 

Canada, there are data that indicate 

what a U.S. design value would be if an 

area average like that found in the 

Canadian studies were to occur in the 

U.S.’’ (Sheppard, 2022a, p. 13 of 

consensus responses). The commenters 

contend that the EPA failed to 

acknowledge this advice from the 

CASAC, specifically noting that the 

majority of the CASAC highlighted 

Canadian epidemiologic studies as a 

part of their rationale for revising the 

level of the primary annual PM2.5

standard to within the range of 8–10 mg/ 

m3. 

In considering the information from 

the epidemiologic studies in reaching 

his conclusions, the Administrator 

considered the full body of evidence, 

including studies conducted in the U.S. 

and Canada. However, as described in 

the proposal and in section II.B.4 below, 

the Administrator also recognizes that 

the exposure environments in the U.S. 

are different from those in Canada. In 

particular, the U.S. population density 

is approximately 43 people per square 

kilometer in the contiguous U.S.98

compared to Canada, which has one of 

the lowest population densities on the 

Earth with 4.2 people per square 

kilometer (Statistics Canada, 2023). This 

difference in population density 

between the U.S. and Canada was not as 

apparent, and did not need to be 

highlighted, in the 2012 review given 

that the available Canadian 

epidemiologic studies used populationweighting and focused on urban areas 

where monitors were available and 

population densities were more 

comparable with those in the U.S. Given 

this, the study-reported mean 

concentrations from U.S. and Canadian 

studies in the 2012 review were very 

similar. The recent epidemiologic 

evidence available in this 

reconsideration, however, includes 

studies that utilize approaches that 

highlight the importance of considering 

the differences between the two 

exposure environments in the U.S. 

versus Canada. When focusing on the 

recently available Canadian monitorbased epidemiologic studies in this 

reconsideration, the information 

indicates that these studies, unlike the 

studies available in the 2012 review, do 

not apply population weighting (e.g., 

Lavigne et al., 2018; Liu et al., 2019). As 

noted in responding to other public 

comments above, the absence of 

population weighting is an important 

consideration that limits the utility of 

these studies in informing the 

appropriate level of the primary annual 

PM2.5 standard. In addition, there are 

recently available studies in the 2019 

ISA and ISA Supplement that expand 

the geographical extent of the 

epidemiologic study areas by estimating 

exposure concentrations in areas where 

there are no monitors. To do this, these 

studies use either a statistical 

extrapolation of monitored values or use 

air quality modeling and other forms of 

data (e.g., hybrid model-based 

approaches). For these Canadian 

studies, the EPA notes two important 

considerations in using the information 

to directly translate to policy decisions 

regarding the level of the annual 

standard in the U.S. The first is that in 

incorporating a larger portion of Canada 

into these recent studies, more rural 

areas are included, and as such, the 

population densities and exposure 

environment differences become more 

important. The second is that in 

analyses that evaluate and validate 

hybrid models, there is less certainty in 

PM2.5 exposure estimates in more rural 

areas, which are further from air quality 

monitors and where PM2.5

concentrations in the ambient air tend 

to be lower (U.S. EPA, 2022b, pp. 2–51 

and 2–63). Additionally, it is unclear 

what portion of the PM2.5 concentrations 

from rural areas are contributing to the 

study reported mean. Given this, studies 

that incorporate more rural areas into 

the epidemiologic studies highlight the 

importance of considering the 

differences between the population 

exposures in the studies themselves and 

in the U.S. versus Canadian study areas, 

as well as the influence these 

differences have on the interpretation of 

the epidemiologic study results. For 

these reasons, while the Canadian 

epidemiologic studies provide 

additional support for associations 

between PM2.5 concentrations and 

health effects, the long-term means from 

Canadian epidemiologic studies are a 

less certain basis for informing the 

EPA’s selection of the annual standard 

level, given that it is a U.S.-based 

standard. 

With respect to the CASAC’s advice 

in their review of the 2021 draft PA, the 

EPA recognizes that the majority of the 

CASAC pointed to the Canadian studies 

as supporting their recommendation to 

revise the annual standard level to 

within the range of 8–10 mg/m3. 

However, the EPA also notes that the 

CASAC did not advise the EPA to revise 

the annual standard to a level that was 

below the study-reported means in the 

key Canadian epidemiologic studies. 

Indeed, the CASAC noted that some of 

the Canadian studies showed 

associations below 8 mg/m3, but did not 

recommend that the Administrator 

consider levels below 8 mg/m3 for the 

annual standard. Further, based on the 

CASAC’s advice, the Administrator is 

not excluding Canadian studies from his 

consideration in this reconsideration, 

but he is considering them in light of 

the limitations and challenges presented 

and in the context of the full body of 

available scientific evidence. 

Lastly, the EPA disagrees with 

commenters that the findings of the 

Canadian epidemiologic studies can be 

directly translated into a primary annual 

PM2.5 standard based on the evaluation 

of the relationship between U.S. studyreported mean PM2.5 concentrations and 

U.S. design values. It is unclear whether 

the relationship between U.S. studyreported mean PM2.5 concentrations and 

U.S. design values (which, in the case 

of U.S. hybrid model-based studies, 

indicates that design values are 15–18% 

greater than area mean PM2.5

concentrations) would apply to the 

Canadian epidemiologic studies and 

their reported mean PM2.5


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99The 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); 

see also, e.g., Am. Lung Ass’n v. EPA, 134 F.3d 388, 

389 (D.C. Cir. 1998) (‘‘If a pollutant adversely affects 

the health of these sensitive individuals, EPA must 

strengthen the entire national standard’’). 

100Additional information on all available at-risk 

epidemiologic studies in this reconsideration are 

available in section 3.4 and Appendix C of the 2022 

PA (U.S. EPA, 2022b, section 3.4, Figure 3–17, and 

Appendix C, section C.3.2). 

concentrations, given that these studies 

generally report lower PM2.5

concentrations than the U.S.-based 

studies. As such, interpreting the studyreported mean concentrations from the 

Canadian studies in the context of a 

U.S.-based standard may present 

challenges in directly and quantitatively 

informing decisions regarding potential 

alternative levels of the annual 

standard, particularly noting the 

different in exposure relationships in 

the U.S. versus Canada given the large 

difference in population densities 

between the two countries. Further, as 

mentioned above, while the CASAC 

advised the EPA to consider the 

Canadian studies as relevant evidence 

and found that placing weight on the 

Canadian studies supported their 

recommendation to revise the annual 

standard level to within the range of 8– 

10 mg/m3, the lower end of their 

recommended range for the level of the 

annual standard did not extend below 

the lower study-reported means from 

those studies. 

Commenters who supported retaining 

and revising the primary annual PM2.5

standard both raised concerns regarding 

how the EPA used the scientific 

evidence and quantitative risk 

assessment related to disparities in 

PM2.5 exposure and risk in informing 

conclusions on the standard. 

Commenters who supported retaining 

the standard assert that the available 

scientific evidence that demonstrates 

disparities for minority populations do 

not support revising the standard, 

noting that these studies are in areas 

that tend to have large minority 

populations and more sources of PM. 

These commenters contend that because 

the studies conclude that minority 

populations experience more effects 

than others living in the same area that 

something other than PM2.5

concentrations in ambient air is causing 

the disproportionate impact on minority 

populations, providing proximity to a 

source as an example. The commenters 

note that it is unclear how a national 

standard will reduce exposure 

disparities for population groups living 

in the same area, and further assert that 

studies of exposure disparities among 

minority populations were considered 

in reaching the 2020 final decision to 

retain the standards. 

Conversely, commenters who support 

revising the standard assert that the atrisk analyses conducted in the 2022 PA 

provide support for revising the primary 

annual PM2.5 standard to a level of 8 mg/ 

m3. In particular, these commenters 

state that the at-risk analysis 

demonstrated that while disparities in 

mortality risk remain at a standard level 

of 9.0 mg/m3, disparities in exposure are 

significantly reduced for an alternative 

standard level of 8.0 mg/m3 (U.S. EPA, 

2022b, p. 3–162). 

As discussed in section I above, the 

primary (health-based) NAAQS are 

established at a level that is requisite to 

protect public health, including the 

health of sensitive or at-risk groups, 

with an adequate margin of safety.99 In 

so doing, decisions on the NAAQS are 

based on an explicit and comprehensive 

assessment of the current scientific 

evidence and associated risk analyses. 

More specifically, the EPA expressly 

considers the available information 

regarding health effects among at-risk 

populations in decisions on the primary 

NAAQS. Where populations with 

disparities in exposure and risk are 

among the at-risk populations, the 

decision on the standards is based on 

providing requisite protection for these 

and other at-risk populations and 

lifestages. 

The Administrator expressly 

considered the available information 

regarding health effects among at-risk 

populations in reaching the proposed 

decisions that the current primary 

annual PM2.5 standard is not requisite to 

protect public health with an adequate 

margin of safety, and should be revised. 

The 2019 ISA and ISA Supplement 

identified children, older adults, people 

with pre-existing diseases 

(cardiovascular disease and respiratory 

disease), minority populations, and low 

SES populations as at-risk populations. 

The Administrator is thus, in his final 

decision, establishing primary PM2.5

standards which, in his judgment, will 

provide protection for these at-risk 

populations, including minority 

populations, with an adequate margin of 

safety. 

With respect to the risk assessment, 

while the EPA notes that the analyses 

support the conclusion that the primary 

PM2.5 standards are not adequate, as 

detailed further in the proposal and 

above in section II.A.3, the EPA also 

cautions against an over-interpretation 

of the absolute results. The quantitative 

risk assessment provides estimates of 

PM2.5-attributable mortality based on 

input data that include C–R functions 

from epidemiologic studies that do not 

quantitatively account for uncertainties 

in associations between PM2.5 exposure 

and health effects at lower 

concentrations and are based on an air 

quality adjustment approach that 

incorporates proportional decreases in 

PM2.5 concentrations to meet lower 

alternative standard levels. As a result, 

simulated air quality improvements 

used in the risk assessment will always 

lead to proportional decreases in risk 

(i.e., each additional mg/m3 reduction 

produces additional benefits with no 

clear stopping point), without 

considering the substantially greater 

uncertainties associated with the 

relationship between PM2.5 exposures 

and health effects at lower 

concentrations. 

The same is true for the new at-risk 

analysis in the risk assessment 

presented in the 2022 PA that is based 

on a recent epidemiologic study that is 

available in this reconsideration that 

provides mortality risk coefficients for 

older adults (i.e., 65 years and older) 

based on PM2.5 exposure and stratified 

by racial and ethnic demographics. 

Generally, the results of at-risk analyses 

can vary greatly depending on the 

inputs to the analyses, including the 

representativeness of the populations 

and demographics captured by the 

study areas that are a part of the 

analyses, as well as the available C–R 

functions from epidemiologic studies 

that stratify by race and ethnicity and 

the air quality adjustment approaches 

that are used to simulate air quality at 

different standard levels. In fact, for this 

at-risk analysis, the results are even 

more uncertain than similar estimates 

from the overall risk assessment due to 

additional sources of uncertainty 

specific to the at-risk analysis, such as 

using C–R functions derived from 

smaller epidemiologic sample sizes 

along with the sources of uncertainty 

that apply to the overall risk assessment 

(U.S. EPA, 2022b, section 3.4.1.8). 

Additionally, in characterizing at-risk 

populations, the at-risk analysis only 

used one of the air quality adjustment 

approaches used in the overall risk 

assessment, which decreases the 

potential representativeness of the PM2.5

concentrations across the study areas 

(U.S. EPA, 2022b, section 3.4.1.8). 

Lastly, this at-risk analysis relies on the 

stratified risk coefficients from only one 

epidemiologic study.100 For these 

reasons, the Administrator places little 


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weight on the absolute results of the risk 

assessment, including the at-risk 

analysis, for purposes of selecting the 

level of the annual standard that is 

requisite. 

While there are substantial 

uncertainties in the absolute results of 

the quantitative risk assessment, the 

EPA also notes that recent scientific 

evidence evaluated in the ISA 

Supplement, which built upon the 2019 

PM ISA conclusions, found that the 

evidence ‘‘[c]ontinue[s] to support 

disparities in PM2.5 exposure and health 

risks by race and ethnicity’’ while 

studies of SES ‘‘provide additional 

support indicating there may be 

disparities in PM2.5 exposure and health 

risk by SES’’ (U.S. EPA, 2022a, p. 5–4). 

Thus, in light of the statutory 

requirement to provide protection for atrisk populations, it is not surprising that 

the stratified population results of the 

risk assessment suggest that meeting a 

revised standard would result in higher 

risk reductions for minority and low 

SES populations. 

In conclusion, the EPA recognizes 

that the at-risk analysis was based on 

one epidemiologic study that stratified 

by race/ethnicity for older adults (e.g., 

65+ years old) and that there is 

increasing uncertainty in quantitative 

estimates of stratified risk estimates at 

the lower end of the range of standard 

levels assessed. Moreover, the EPA finds 

that the goal of the NAAQS is to provide 

the requisite protection to at-risk 

groups, and where minority populations 

are included among the at-risk groups, 

providing requisite protection to 

minority populations will also result in 

protecting the public health of other 

populations. Thus, in setting the 

NAAQS to protect the health of at-risk 

groups with an adequate margin of 

safety, the Administrator is selecting the 

standard that will provide requisite 

protection, including for minority 

populations and other at-risk 

populations, which also generally 

results in protecting the public health of 

other populations and reducing risk 

disparities. 

A number of commenters, primarily 

from industries and industry groups and 

some States, support the EPA’s 

proposed decision to retain the primary 

24-hour PM2.5 standard. Many of these 

commenters contend that the available 

scientific evidence and quantitative 

information has not significantly 

changed since the 2020 final decision 

and note that important uncertainties 

remain. The commenters agree with the 

EPA’s conclusions regarding the 

controlled human exposure studies and 

their relationship to short-term peak 

PM2.5 concentrations in ambient air. 

These commenters also noted the 

primary annual and 24-hour PM2.5

standards work together to provide 

public health protection, with the 98th 

percentile form of the 24-hour standard 

effectively limiting peak daily 

concentrations. The commenters agree 

with the EPA that the current suite of 

standards maintain subdaily 

concentrations below the higher 

concentrations in controlled human 

exposure studies where more consistent 

health effects are observed. Commenters 

also agree with the EPA’s conclusions 

that the epidemiologic studies are not 

useful for informing decisions on the 

level of the primary 24-hour PM2.5

standard because the standard focuses 

on reducing peak exposures with its 

98th percentile form, while the 

epidemiologic studies often focus on the 

mean or median as the percentile for 

which associations with short-term 

exposures are observed. These 

commenters also agree with the EPA’s 

focus on U.S.-based studies because of 

differences compared to Canadian 

studies. The commenters also generally 

agree with the Administrator’s judgment 

that it was appropriate to place less 

weight on the risk assessment, noting 

that the annual standard is controlling 

in most areas of the country and 

revising the annual standard would 

have the most potential to reduce risk 

related to PM2.5 exposures and would 

reduce both average (annual) and peak 

(daily) PM2.5 concentrations. Finally, 

these commenters note that the CASAC 

did not reach consensus on whether the 

current primary 24-hour PM2.5 standard 

should be revised, and they agree with 

the minority of the CASAC’s 

recommendation in their review of the 

2021 draft PA that the primary 24-hour 

primary PM2.5 standard should be 

retained. These commenters also note 

the CASAC’s support in their review of 

the 2019 draft PA for retaining the 

primary 24-hour PM2.5 standard. 

A number of commenters, primarily 

from public health and environmental 

organizations and some States, oppose 

the EPA’s proposed decision to retain 

the primary 24-hour PM2.5 standard. 

These commenters support revising the 

level of the primary 24-hour PM2.5

standard, contending that a more 

stringent standard is necessary to 

provide requisite public health 

protection with an adequate margin of 

safety, particularly for at-risk groups. In 

so doing, these commenters place 

weight on the same aspects of the 

available scientific evidence as the 

majority of the CASAC in their review 

of the 2021 draft PA, and generally 

advocate for revising the level of the 

standard to within the range of 25–30 

mg/m3 as recommended by the majority 

of the CASAC. Some of these 

commenters support a level no higher 

than 25 mg/m3 and others support a 

level of 20 mg/m3. These commenters 

generally cite to the available scientific 

evidence, including evidence of 

disproportionate exposures and risks for 

certain at-risk groups, and the CASAC’s 

advice in support for their 

recommendation. Some of these 

commenters also suggest that decisions 

regarding the primary 24-hour PM2.5

standard should not be related to 

decisions on the primary annual PM2.5

standard. 

As an initial matter, the EPA disagrees 

with commenters who suggest that 

decisions regarding the primary 24-hour 

PM2.5 standard should not be related to 

decisions on the primary annual PM2.5

standard. In reviewing the adequacy of 

the public health protection afforded by 

the primary PM2.5 standards, the 

Administrator’s consistent past practice 

has been to evaluate the combination of 

the annual and 24-hour standards 

together. In 2012, the thenAdministrator concluded that the most 

effective and efficient way to reduce 

total population risk associated with 

both long- and short-term PM2.5

exposures was to set a generally 

controlling annual standard, and to 

provide supplemental protection by 

means of a 24-hour standard set at the 

appropriate level. In so doing, the thenAdministrator explicitly recognized that 

potential air quality changes associated 

with meeting a revised annual standard 

(with a level of 12 mg/m3) would result 

in lowering risks associated with both 

long- and short-term PM2.5 exposures by 

lowering the overall distribution of air 

quality concentrations, and that 

retaining a 24-hour standard at the 

appropriate level would ensure an 

adequate margin of safety against shortterm effects in areas with high peak-tomean ratios (78 FR 3163, January 15, 

2013). In this reconsideration, also, the 

Administrator considers it appropriate 

to rely on the annual standard 

(arithmetic mean, averaged over three 

years) for targeting protection against 

both long- and short-term PM2.5

exposures, noting that the annual 

standard is typically controlling, while 

the 24-hour standard (98th percentile, 

averaged over three years) can provide 

supplemental protection against the 

occurrence of peak 24-hour PM2.5

concentrations (U.S. EPA, 2022b, 

section 3.6.3). Further, the 

Administrator notes that, as in the 2012 

review, changes in PM2.5 air quality to 

meet a revised annual standard would 


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101Similarly, 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. However, as noted in 

2012, an approach that relied on setting the level 

of the 24-hour standard such that the 24-hour 

standard was generally controlling would be less 

effective and result in less uniform protection 

across the U.S. than an approach that focuses on 

setting a generally controlling annual standard (78 

FR 3163, January 15, 2013). 

102 Judgments regarding adversity or health 

significance of measurable physiological responses 

to air pollutants have been informed by guidance, 

criteria or interpretative statements developed 

within the public health community, including the 

American Thoracic Society (ATS) and the European 

Respiratory Society (ERS), which cooperatively 

updated the ATS 2000 statement What Constitutes 

an Adverse Health Effect of Air Pollution (ATS, 

2000) with new scientific findings, including the 

evidence related to air pollution and the 

cardiovascular system (Thurston et al., 2017). 

103The ATS/ERS described its 2017 statement as 

one ‘‘intended to provide guidance to policymakers, 

clinicians and public health professionals, as well 

as others who interpret the scientific evidence on 

the health effects of air pollution for risk 

management purposes’’ and further notes that 

‘‘considerations as to what constitutes an adverse 

health effect, in order to provide guidance to 

researchers and policymakers when new health 

effects markers or health outcome associations 

might be reported in future.’’ The most recent 

policy statement by the ATS, which once again 

broadens its discussion of effects, responses and 

biomarkers to reflect the expansion of scientific 

research in these areas, reiterates that concept, 

conveying that it does not offer ‘‘strict rules or 

numerical criteria, but rather proposes 

considerations to be weighed in setting boundaries 

between adverse and nonadverse health effects,’’ 

providing a general framework for interpreting 

evidence that proposes a ‘‘set of considerations that 

can be applied in forming judgments’’ for this 

context (Thurston et al., 2017). 

104 In their review of the 2021 draft PA, the 

majority of the CASAC advised that ‘‘evidence of 

effects from controlled human exposure studies 

with exposures close to the current standard 

support epidemiologic evidence for lowering the 

standard’’ (Sheppard, 2022a, p. 4 of consensus 

letter). The minority of the CASAC also advised that 

it was appropriate to place ‘‘more emphasis on the 

controlled human exposure studies, showing effects 

at PM2.5 concentrations well above those typically 

measured in areas meeting the current standards’’ 

(Sheppard, 2022a, p. 4 of consensus letter), in 

evaluating adequacy of the 24-hour standard. 

affect the entire distribution of long- and 

short-term concentrations, thus likely 

resulting 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.101 Thus, the 

Administrator continues to conclude it 

is appropriate to consider whether the 

annual and 24-hour standards together 

provide requisite protection of public 

health, rather than considering each 

standard in isolation. 

Regarding the appropriate basis for 

determining the level of the 24-hour 

standard, a number of commenters who 

support revising the primary 24-hour 

PM2.5 standard to a lower level contend 

that the EPA should not rely on the 

controlled human exposure studies in 

evaluating the adequacy of the public 

health protection afforded by the 

primary 24-hour PM2.5 standard. These 

commenters support this view by citing 

the CASAC comments in their review of 

the 2019 draft PA which advised that 

controlled human exposure studies have 

limitations that may impact their ability 

to inform conclusions on the adequacy 

of the public health protection afforded 

by the primary 24-hour PM2.5 standard. 

Commenters noted that these studies do 

not include the most vulnerable 

populations and often involve exposure 

to only one pollutant to elicit a 

response, and therefore are not 

representative of real-world exposures. 

Other commenters support the EPA’s 

use of the controlled human exposure 

studies to inform the adequacy of the 

public health protection and note that 

the 24-hour standard must at least 

provide protection against the health 

effects observed in controlled human 

exposure studies. Some of the 

commenters cite the Wyatt et al. (2020) 

study that demonstrated cardiovascular 

effects following 2-hour exposures to 

120 mg/m3 and 4-hour exposures to 37.8 

mg/m3. Some of these commenters 

contend that the current primary 24- 

hour PM2.5 standard allows PM2.5

exposures comparable to those observed 

to elicit effects in the controlled human 

exposure studies, and therefore, the EPA 

must revise the level of the current 

standard to protect public health. To 

support this view, some commenters 

submitted an analysis of monitoring 

data from 2017–2020, which compares 

the number of days per year where 

maximum daily PM2.5 concentrations 

exceed 120 mg/m3 and 37.8 mg/m3. 

Additionally, other commenters assert 

that the EPA should focus less on peak 

PM2.5 concentrations ‘‘typically 

measured’’ in areas meeting the current 

primary PM2.5 standards even if they do 

not exceed the concentrations in the 

controlled human exposure studies 

because, in their view, the standard 

needs to protect against atypical 

exposures to atypical peak PM2.5

concentrations. These commenters 

conclude that, when considered 

together, the controlled human exposure 

studies and the epidemiologic studies 

warrant strengthening the level of the 

primary 24-hour PM2.5 standard. 

The EPA generally disagrees with 

commenters who contend that it is 

inappropriate to rely on the controlled 

human exposures studies in evaluating 

the adequacy of the public health 

protection afforded by the primary 24- 

hour PM2.5 standard. The Agency 

considers these studies informative both 

for establishing biological plausibility 

and for determining an appropriate level 

for the 24-hour standard. When looking 

to the experimental studies, the EPA 

finds that the 2019 ISA and ISA 

Supplement included controlled human 

exposure studies that 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

(and at and above 149 mg/m3 for 

vascular impairment, the effect shown 

to be most consistent across studies). As 

noted in the 2019 ISA, these studies are 

important in establishing biological 

plausibility for PM2.5 exposures causing 

more serious health effects, such as 

those seen in short-term exposure 

epidemiologic studies, and they provide 

support that more adverse effects may 

be experienced following longer 

exposure durations and/or exposure to 

higher concentrations. Additionally, one 

controlled human exposure study 

assessed in the ISA Supplement reports 

evidence of some effects for 

cardiovascular markers at lower PM2.5

concentrations, 4-hour exposures to 37.8 

mg/m3 (Wyatt et al., 2020). However, 

there is inconsistent evidence for 

inflammation in other controlled human 

exposure studies evaluated in the 2019 

ISA. The EPA notes that although the 

controlled human exposure studies do 

not provide a threshold below which no 

effects occur, the observed effects in 

these controlled human exposures 

studies are ones that signal an 

intermediate effect in the body, likely 

due to short-term exposure to PM2.5, and 

typically would not, by themselves, be 

judged as adverse (88 FR 5620, January 

27, 2023) 102 103

The EPA notes that the majority of the 

CASAC, in their review of the 2021 draft 

PA, commented that these controlled 

human exposure studies generally do 

not include populations with 

substantially increased risk from 

exposure to PM2.5, such as children, 

older adults, or those with more severe 

underlying illness, and often involve 

exposure to only one pollutant to elicit 

a response. However, both the majority 

and the minority of the CASAC 

explained that, even taking into 

consideration their limitations, the 

controlled human exposure studies 

provide some support for assessing the 

adequacy of the 24-hour standard.104

The EPA agrees with the CASAC that 

the controlled human exposure studies 

generally do not include populations 

with substantially increased risk from 

exposure to PM2.5, like children, older 

adults, or those with pre-existing severe 

illness, like cardiovascular effects. As 

such, and as an initial note, these 


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105 Jones et al. (2023). Comparison of Occurrence 

of Scientifically Relevant Air Quality Observations 

Between Design Value Groups. Memorandum to the 

Rulemaking Docket for the Review of the National 

Ambient Air Quality Standards for Particulate 

Matter (EPA–HQ–OAR–2015–0072). Available at: 

https://www.regulations.gov/docket/EPA-HQ-OAR2015-0072. 

studies are therefore somewhat limited 

in their ability to inform at what 

concentrations effects may be elicited in 

at-risk populations. In spite of this 

limitation, the EPA also agrees with the 

CASAC, that even taking into 

consideration the limitations of the 

controlled human exposure studies, 

these studies can provide some support 

for evaluating the adequacy of the 24- 

hour standard. However, the EPA 

further notes that while the controlled 

human exposure studies are important 

in establishing biological plausibility, 

the health outcomes observed in these 

controlled human exposure studies are 

often ‘‘intermediate’’ outcomes (i.e., not 

always clearly adverse) and therefore it 

is unclear how the importance of the 

effects observed in the studies should be 

interpreted with respect to adversity to 

public health. The EPA finds that it is 

appropriate to consider these study 

limitations in assessing the information 

provided by controlled human exposure 

studies in evaluating the adequacy of 

the primary 24-hour PM2.5 standard. 

The EPA agrees with commenters that 

the primary 24-hour PM2.5 standard 

must at least provide protection against 

the health effects consistently observed 

in controlled human exposure studies. 

As discussed in the proposal, the EPA 

looks at whether the exposures that 

elicit a response following exposure to 

PM2.5 in the controlled human exposure 

studies occur under recent air quality 

conditions in areas meeting the current 

standards. Based on these air quality 

analyses, the EPA concludes that these 

types of exposures very rarely occur 

when the current standards are being 

met. 

The EPA did receive multiple 

comments questioning these results and 

the approach in the EPA’s analyses. For 

example, some commenters submitted 

an analysis of monitoring data from 

2017–2020, which compares the number 

of days per year where maximum daily 

PM2.5 concentrations exceed 120 mg/m3

and 37.8 mg/m3 and evaluate the number 

of days subset by groups of monitors 

with 4-year average PM2.5

concentrations close to the levels of 

combinations of current and proposed 

annual (+/¥ 0.2 mg/m3) and 24-hour (+/ 

¥2 mg/m3) PM2.5 standards. To support 

their view that the primary PM2.5

standards should be revised, the 

commenters describe decreases in days 

per monitor per year with 2-hour 

maximum concentrations greater than 

120 mg/m3 and 4-hour maximum 

concentrations greater than 37.8 mg/m3

when comparing monitors that achieve 

close to 10 and 30 mg/m3 versus 

monitors that meet close to 8 mg/m3 and 

25 mg/m3. The commenters noted 

decreases in the number of days per 

monitor per year with 2-hour maximum 

concentrations over 120 mg/m3 and 4- 

hour max concentration over 37.8 mg/m3

were also seen when comparing 

monitors close to achieving 24-hour 

standards with levels of 35 mg/m3 versus 

25 mg/m3. 

First, the EPA notes that this analysis 

submitted by commenters was limited 

to a very small number of monitors and 

did not include a national perspective. 

Second, the EPA notes that this analysis 

focused on number of days (rather than 

the number of times) where there was a 

2-hour maximum concentration over 

120 mg/m3 or a 4-hour max 

concentration over 37.8 mg/m3. In order 

to evaluate the protection provided by 

the current 24-hour standard against 

peak exposures, including exposures 

with 2-hour concentrations greater than 

120 mg/m3 and 4-hour concentrations 

greater than 37.8 mg/m3, the EPA 

considers it more informative and 

appropriate from a public health 

perspective to assess the number of 

times a subdaily exposure of concern 

occurs in a year, rather than the number 

of days on which they occur because the 

former identifies more potential 

exposures of concern and provides more 

information about the scale and scope of 

the occurrences of those exposures. 

Lastly, the analyses allowed monitors 

somewhat above the standards to be 

included. Therefore, it is unclear 

whether the exceedances of the 2-hour 

or 4-hour benchmarks would still have 

occurred if the area had actually been 

meeting the current primary PM2.5

standards. However, in considering the 

analyses submitted by the commenters, 

the EPA conducted new analyses 105

that looked at all individual monitors 

across the U.S. and evaluated the 

percentage of times the monitors 

experienced a 2-hour maximum 

concentration over 120 mg/m3 or a 4- 

hour max concentration over 37.8 mg/m3

when that monitor was meeting the 

current standards. Further, given that 

the Administrator concludes that the 

level of the current primary annual 

PM2.5 is not adequate and that it should 

be revised to 9.0 mg/m3, the new 

analysis evaluates the percentage of 

times during a recent 3-year period (i.e. 

2019–2021) that individual monitors 

experienced a 2-hour maximum 

concentration over 120 mg/m3 or a 4- 

hour max concentration over 37.8 mg/m3

when that monitor was meeting the 

current primary 24-hour PM2.5 standard 

with its level of 35 mg/m3 and a revised 

primary annual PM2.5 standard of 9.0 

mg/m3. 

In evaluating the results from the new 

analyses, it is important to keep in mind 

that the 2019 ISA and ISA Supplement 

concluded that the most consistent 

evidence from the controlled human 

exposures studies is for impaired 

vascular function 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 

concentrations lower than 120 mg/m3. 

The new analyses show that across all 

monitors, on average, only 0.029 percent 

of 2-hour observations reach PM2.5

concentrations higher than 120 mg/m3 in 

areas meeting the current 24-hour 

standard and a revised annual standard 

of 9.0 mg/m3. Further, recognizing that 

one purpose of the 24-hour standard is 

to protect against exposure in areas with 

high peak-to-mean ratios, when 

assessing the monitors individually 

across the U.S. under these same 

conditions, the monitors reporting the 

highest PM2.5 concentrations have only 

0.47 percent of 2-hour observations 

reach PM2.5 concentrations higher than 

120 mg/m3. 

Additionally, the analyses also 

evaluated the frequency of reporting a 4- 

hour maximum concentration over 37.8 

mg/m3 when monitors were meeting the 

current 24-hour standard and a revised 

annual standard of 9.0 mg/m3. For this 

part of the analysis, the EPA finds that 

across all monitors, on average, only 

0.41 percent of 4-hour observations 

reach PM2.5 concentrations higher than 

37.8 mg/m3 in areas meeting the current 

24-hour standard and a revised annual 

standard of 9.0 mg/m3. Further, when 

assessing the monitors individually 

across the U.S. under these same 

conditions, the monitors reporting the 

highest PM2.5 concentrations have only 

2.6 percent of 4-hour observations reach 

PM2.5 concentrations higher than 37.8 

mg/m3. Thus, the EPA disagrees with 

commenters that the current primary 24- 

hour PM2.5 standard typically allows 

PM2.5 exposures at or above those 

observed to cause health effects in 

controlled human exposure studies. 

Furthermore, the EPA notes that in light 

of the small number of occurrences and 

the intermediate nature of the effects 

observed in Wyatt et al. (2020) at 

concentrations of 37.8 mg/m3 (i.e., 

effects that typically would not, by 

themselves, be judged as adverse), there 

is substantial basis to doubt whether 

further improvements in public health 


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would be achieved by further reducing 

these exposures. In drawing this 

conclusion, the EPA notes the lack of 

evidence of effects from controlled 

human exposure studies at levels below 

the current 24-hour standard and the 

fact that the results of Wyatt et al. (2020) 

are inconsistent with other currently 

available studies, and this study only 

observes intermediate effects. 

In response to commenters that cited 

the majority of the CASAC’s view that, 

in general, ‘‘[t]here is . . . less 

confidence that the annual standard 

could adequately protect against health 

effects of short-term exposures’’ 

(Sheppard, 2022a, p. 4 of consensus 

letter), the EPA disagrees with the 

majority of CASAC, noting that the 

results of the EPA’s analysis suggest that 

high peak concentrations are extremely 

infrequent in areas meeting an annual 

standard of 9.0 mg/m3, occurring less 

than 0.029–0.41 percent of the time (for 

2-hour concentrations >120 mg/m3 and 

4-hour concentrations >37.8 mg/m3, 

respectively). This suggests that in most 

locations, even the upper tail of the 

distribution would be controlled quite 

well under a revised annual standard. 

With regard to the likelihood that the 

current standards would allow peak 

concentrations that are clearly of 

concern from a health perspective, 

therefore, the EPA concludes that such 

occurrences are extremely infrequent— 

and will be even less frequent under the 

improved air quality conditions 

associated with meeting a revised 

annual PM2.5 standard of 9.0 mg/m3. 

A number of commenters who 

support revising the primary 24-hour 

PM2.5 standard to a lower level suggest 

that the available epidemiologic 

evidence provides support for such a 

revision. To support their view, the 

commenters note that the currently 

available evidence, including a number 

of epidemiologic studies that 

demonstrate associations between shortterm PM2.5 exposures and health effects, 

provides support for causal 

relationships for short-term PM2.5

exposures and health effects as 

described in the 2019 ISA and ISA 

Supplement. The commenters further 

note that the available epidemiologic 

studies include diverse populations that 

are broadly representative of the U.S. 

population, including at-risk 

populations, which they assert is an 

advantage over the controlled human 

exposure studies and the risk 

assessment, which are not as broadly 

representative. 

These commenters highlight a number 

of specific epidemiologic studies that 

they suggest provide support for 

revising the level of the 24-hour 

standard. Additionally, commenters 

contend that there are epidemiologic 

studies using restricted analyses that 

show that positive and statistically 

significant associations between shortterm PM2.5 exposure and mortality 

persist at daily mean concentrations 

below 25 mg/m3. The commenters also 

cite several studies that provide no 

evidence of a threshold. These 

commenters also point to the CASAC 

advice in their review of the 2021 draft 

PA, where the majority of the CASAC 

cited epidemiologic studies using 

restricted analyses as offering support 

for revision. The commenters argue that 

the EPA cannot base discretion on 

uncertainties related to the methods 

used in restricted analyses in the 

epidemiologic studies. In so doing, 

these commenters disagree with the 

EPA that it is important to take into 

consideration that these studies do not 

consider the form or averaging time of 

the 24-hour standard. Finally, the 

commenters claim that while the EPA 

stated that the study-reported means 

from epidemiologic studies that use 

restricted analyses are more useful for 

identifying impacts from typical 24- 

hour exposures than for peak 24-hour 

exposures, the commenters assert that 

the studies also indicate that there are 

health risks at relatively high 

concentrations below the current level 

of the primary 24-hour PM2.5 standard 

that must be addressed. 

As noted by the commenters, 

epidemiologic studies that show 

positive and statistically significant 

associations between short-term PM2.5

exposure and mortality provide support 

for the causal determination in the 2019 

ISA. The EPA also agrees that the 

available epidemiologic studies include 

diverse populations that are broadly 

representative of the U.S. population, 

including at-risk populations. Further, 

the EPA agrees that studies evaluated in 

the 2019 ISA and the ISA Supplement 

continue to provide evidence of linear, 

no-threshold concentration-response 

relationships, but with less certainty in 

the shape of the curve at lower 

concentrations (i.e., below about 8 mg/ 

m3), with some recent studies providing 

evidence for either a sublinear, linear, or 

supralinear relationship at these lower 

concentrations (U.S. EPA, 2019a, 

section 11.2.4; U.S. EPA, 2022a, section 

2.2.3.2). 

However, findings of positive, 

significant associations in short-term 

epidemiologic studies do not directly 

indicate that short-term effects would 

occur in areas meeting the 24-hour 

standard and therefore, do not directly 

address the question of whether the 

current 24-hour standard is adequate. 

While short-term epidemiologic studies 

evaluate associations between 

distributions of ambient PM2.5 and 

health outcomes, they do not identify 

the specific exposures (i.e., a specific 

24-hour concentration) that can lead to 

the reported effects. Short-term 

epidemiologic studies evaluate the 

association between day-to-day 

variation in daily (24-hour) PM2.5

exposure and health endpoints (e.g., 

mortality) to understand how these 

changes in air pollution concentrations 

are associated with changes in health 

outcomes. But these studies do not 

report daily concentrations; rather, they 

report the long-term mean concentration 

of the 24-hour PM2.5 concentrations over 

the entire multi-year period of the 

study, and typically report their results 

as a relative risk (e.g., for each 10 mg/ 

m3 increase in PM2.5, the risk of 

mortality or cardiovascular hospital 

admissions increases by a certain 

percentage, across the full range of the 

24-hour PM2.5 concentrations in the 

study). This means that there is no 

specific point in the air quality 

distribution of any epidemiologic study 

that represents a ‘‘bright line’’ at and 

above which effects have been observed 

and below which effects have not been 

observed. Nor, as noted above, do these 

studies allow for any direct inferences 

about health impacts associated with 

the short-term ‘‘peak’’ exposures that 

the primary 24-hour standard is 

designed to protect against. While there 

can be considerable variability in daily 

exposures over a multi-year study 

period, most of the estimated exposures 

in these epidemiologic studies reflect 

days with ambient PM2.5 concentrations 

around the mean or middle of the air 

quality distributions examined (i.e., 

‘‘typical’’ days rather than days with 

extremely high or extremely low 

concentrations). This is true of longterm epidemiologic studies as well. The 

difference between epidemiologic 

studies examining associations with 

long-term exposures and short-term 

exposures is comparing different levels 

of exposure over different exposure 

durations (i.e., long-term studies 

exposures are defined as those that are 

annual or multi-year, while short-term 

exposures are defined as those that are 

mostly 24-hour) (U.S. EPA, 2019a, 

section P.3.1). Thus, in both cases, and 

in the absence of a discernible 

threshold, epidemiologic studies of 

short-term and long-term exposures 

provide the strongest support and 

confidence for reported health effect 

associations around the middle portion 

of the PM2.5 air quality distribution (e.g., 

the study-reported mean PM2.5


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106 In the 2022 PA, the EPA has identified a 

number of key areas for additional research and 

data collection for PM2.5, based on the uncertainties 

and limitations that remain in the scientific 

evidence and technical information. In addition to 

research and data collection, the EPA specifically 

highlights additional information that could be 

reported in the epidemiologic studies that may help 

inform future reviews of the primary PM2.5

standards, including additional descriptive 

statistics in the upper percentiles of the air quality 

distribution (i.e., from the 95th to the 99th 

percentile), as well as the number of days of 

concentrations and/or health events within each of 

these percentiles (U.S. EPA, 2022a, section 3.7). 

concentration), which corresponds to 

the bulk of the underlying data, rather 

than at the extreme upper or lower ends 

of the distribution. However, the 

difference between the annual standard 

and the 24-hour standard, aside from 

averaging times, is that the form of the 

annual standard is a mean PM2.5

concentration, which is based on the 

bulk of the air quality data, while the 

form of the 24-hour standard is a 98th 

percentile form, which is based on peak 

concentrations. Both long-term and 

short-term epidemiologic studies are 

informative for determining the 

appropriate level of the annual PM2.5

standard, which is designed to control 

‘‘typical’’ daily exposures and risks, 

because these studies most often report 

long-term mean (or median) PM2.5

concentrations that are representative of 

‘‘typical’’ exposures that are associated 

with health effects. In contrast, while 

the short-term epidemiologic studies 

examine health effects associated with 

shorter exposure durations (e.g., mostly 

24-hour exposures), these studies are 

less informative for determining the 

appropriate level of the 24-hour PM2.5

standard because these studies do not 

report the 98th percentile PM2.5

concentrations,106 which is more 

directly comparable to the form of the 

24-hour standard. Additionally, if the 

98th percentile of data were reported, 

the EPA would consider the peak 

concentrations observed in these studies 

(which by definition rarely occur) in 

conjunction with other supporting 

evidence. However, as already noted, 

there is an absence of new information 

in this reconsideration (either from 

controlled human exposure studies or 

epidemiologic studies) suggesting that 

peak concentrations just below the level 

of the current 24-hour standard (with its 

level of 35 mg/m3) are associated with 

adverse effects. Instead, the evidence 

links risk to more typical daily 

exposures near the middle of the air 

quality distribution—exposures most 

effectively controlled through a 

strengthening of the annual standard. As 

noted in the 2012 final rule, ‘‘reducing 

the annual standard is the most efficient 

way to reduce the risks from short-term 

exposures . . . as the bulk of the risk 

comes from the large number of days 

across the bulk of the air quality 

distribution, not the relatively small 

number of days with peak 

concentrations’’ (78 FR 3156, January 

15, 2013). 

As noted above, in evaluating the 

adequacy of the current standards, the 

EPA has consistently considered the 

annual standard (based on arithmetic 

mean concentrations) and 24-hour 

standard (based on 98th percentile 

concentrations) together in evaluating 

the public health protection provided by 

the standards against the full 

distribution of short- and long-term 

PM2.5 exposures. Moreover, the EPA has 

previously noted that the annual 

standard is generally controlling in most 

parts of the country, providing an 

effective and efficient way to reduce 

total population risk to both long- and 

short-term PM2.5 exposures, while the 

24-hour standard, with its 98th 

percentile form, provides supplemental 

protection, particularly for areas with 

high peak-to-mean ratios of 24-hour 

PM2.5 concentrations (78 FR 3158, 

January 15, 2013). In such areas, annual 

average PM2.5 concentrations could be 

quite low, and the 24-hour standard 

provides a means of ensuring control of 

episodic peaks possibly associated with 

strong local or seasonal sources, or 

PM2.5-related effects that may be 

associated with shorter-than daily 

exposure periods. The approach taken 

in evaluating the adequacy and 

alternative levels of the annual standard 

has been to evaluate the long-term mean 

PM2.5 concentrations of both long-term 

and short-term key epidemiologic 

studies, where we have the most 

confidence in the reported health effects 

association, while also giving some 

consideration to lower percentiles of the 

air quality distribution (e.g., 25th 

percentiles). However, using a similar 

approach to evaluate the adequacy of 

the current and any potential alternative 

levels of the 24-hour standard with 

short-term epidemiologic studies, as the 

majority of CASAC and some 

commenters are suggesting, presents 

challenges. 

Short-term epidemiologic studies, 

including those that use restricted 

analyses, often report metrics that 

include mean PM2.5 concentrations, 

with some studies also reporting lower 

percentiles, such as the 25th percentile. 

As previously noted above, for studies 

of daily PM2.5 exposure, which examine 

associations between day-to-day 

variation in PM2.5 concentrations and 

health outcomes, often over several 

years, most of the estimated exposures 

reflect days with ambient PM2.5

concentrations around the middle of the 

air quality distributions examined (i.e., 

the mean or median). However, there is 

not a metric or statistic reported in 

short-term epidemiologic studies that 

allows for a direct comparison to the 

current 24-hour PM2.5 standard and its 

98th percentile form. While a 98th 

percentile of PM2.5 concentrations is a 

metric that might be more closely 

compared to the 24-hour standard level, 

98th percentile PM2.5 concentrations 

were not reported in key epidemiologic 

studies. Consistent with the 

Administrator’s final decision in 2012, 

the EPA notes that even if 98th 

percentile values were reported, it 

would be inappropriate to focus on 

these concentrations without also 

considering the impact of a revised 

annual standard on short-term 

concentrations, since many areas would 

be expected to experience decreasing 

short- and long-term PM2.5

concentrations in response to a revised 

annual standard (78 FR 3156, January 

15, 2013). Furthermore, in light of the 

scarcity of days at the very upper end 

of the distribution, and to avoid placing 

undue reliance on the peak 

concentrations observed in these studies 

(which by definition rarely occur), the 

EPA finds that such values would need 

to be considered in conjunction with 

other supporting evidence. In addition, 

as described above, the other lines of 

evidence available for consideration by 

the EPA do not indicate that the current 

primary 24-hour standard requires 

revision to protect public health with an 

adequate margin of safety. The EPA 

notes again the lack of corroborating 

evidence from controlled human 

exposure studies. While the EPA agrees 

with the CASAC that the controlled 

human exposure studies are limited in 

their ability to speak to the 

concentrations at which effects may be 

elicited in at-risk populations, as 

discussed above the lowest 

concentration associated with effects is 

37.8 mg/m3 and the effects observed 

were ‘‘intermediate’’ outcomes that are 

not by themselves considered adverse. 

We also note that, as detailed in section 

II.A.2.a above, the study that observed 

intermediate effects at concentrations of 

37.8 mg/m3 was evaluated in the ISA 

Supplement and the results of this study 

were inconsistent with the controlled 

human exposure studies evaluated in 

the 2019 ISA. Additionally, as noted 

above, the EPA finds that across all 

monitors, on average, only 0.41 percent 

of 4-hour observations reach PM2.5

concentrations higher than 38 mg/m3 in 

areas meeting the current 24-hour 


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standard and a revised annual standard 

of 9.0 mg/m3. Given the rarity of these 

occurrences and the fact that the effects 

associated with exposures to this PM2.5

concentration have not been found to be 

adverse in and of themselves, the EPA 

finds it reasonable to conclude that this 

pattern of air quality will protect at-risk 

populations, even though such 

populations were not in the study 

groups. The EPA concludes that further 

evidence would be needed at specific 

short-term (i.e., hourly or daily) 

concentrations below the level of the 

current 24-hour standard to support any 

revision to the current 24-hour standard. 

With regard to the data that are 

available from the short-term 

epidemiologic studies (which, as noted, 

do not include 98th percentile values), 

the EPA considers it inappropriate to 

utilize the study-reported means from 

the short-term epidemiologic evidence 

to assess the adequacy of the 24-hour 

standard, with its 98th percentile form, 

considering that the study-reported 

mean concentrations do not provide 

meaningful insight regarding the 

frequency or health significance of peak 

concentrations occurring during the 

study period. As indicated in the 2022 

PA, the study-reported means of shortterm epidemiologic studies do not serve 

a purpose in determining a level at 

which we can confidently attribute 

effects to the impact of ‘‘peak’’ 

exposures. The 24-hour standard is 

intended to provide supplemental 

protection against short-term peak 

exposures and while there is a general 

relationship between mean 

concentrations and 98th percentile 

concentrations in individual locations, 

such relationships vary by location and 

there is not an established relationship 

that can be relied upon to predict 98th 

percentile concentrations based on 

mean PM2.5 concentrations reported in 

multi-city epidemiologic studies. 

Instead, mean concentrations from 

short-term epidemiologic studies are 

more useful in addressing questions 

regarding the effects of ‘‘typical’’ or 

average 24-hour exposures, which are 

addressed through the annual standard. 

For this reason, the EPA does consider 

the mean concentrations of short-term 

studies (as well as the means from the 

long-term studies) in evaluating the 

level of the annual standard, which the 

EPA recognizes as the generally 

controlling standard for both long- and 

short-term exposures. However, the EPA 

does not agree with commenters that it 

is appropriate to use means from shortterm epidemiologic studies as the basis 

for a decision-making framework to 

determine the adequacy of the current 

24-hour standard, with its 98th 

percentile form. 

As described in the proposal (88 FR 

5613, January 27, 2023), the 2022 PA 

also noted the epidemiologic studies 

that restrict 24-hour average PM2.5

concentrations to values of less than 35 

mg/m3, and in some cases less than 25 

mg/m3, and annual average PM2.5

concentrations less than 12 mg/m3. 

Restricted analyses use a subset of data 

from their main analyses and conduct 

an epidemiologic study with health 

events that occur at concentrations 

below a certain concentration (e.g., 25 

mg/m3). While some of these studies do 

not report the mean PM2.5 concentration 

for the restricted analysis, the mean of 

the restricted analysis is presumably 

less than the mean PM2.5 concentration 

in the main analysis. Restricted analyses 

from long-term and short-term exposure 

epidemiologic studies are informative in 

providing support that the health effects 

associations are not driven by just the 

upper peaks of the PM2.5 air quality 

distributions and provide support for 

revision to the level of the annual PM2.5

standard. Short-term restricted analyses 

also report positive associations 

between short-term PM2.5 exposure and 

morbidity and mortality. As an example, 

in a restricted analysis evaluating the 

association between short-term 

exposures and PM2.5 concentrations less 

than 25 mg/m3, Di et al. (2017a) removed 

6.3 percent of the data from their main 

analyses, (i.e., all PM2.5 concentrations 

greater than 25 mg/m3), and still found 

a positive and significant association 

between short-term PM2.5 exposure and 

mortality. This study provides 

additional support that the association 

between short-term exposure to PM2.5

and mortality in the main epidemiologic 

analysis is not driven by the upper 

peaks of the PM2.5 air quality 

distribution, which in turn supports the 

conclusion that lowering the entire 

distribution of air quality concentrations 

through a revised annual standard is an 

appropriate means of protecting against 

adverse effects from short-term 

exposure, as discussed further below. 

In their review of the 2021 draft PA, 

the majority of the CASAC highlighted 

three U.S.-based epidemiologic studies 

that restricted 24-hour average PM2.5

concentrations below 25 mg/m3 as a part 

of their rationale for recommending that 

the EPA revise the level of the primary 

24-hour PM2.5 standard. Similarly, in 

evaluating positive associations in 

restricted analyses, some commenters 

also suggest that because an association 

exists at 24-hour concentrations below 

25 mg/m3, the 24-hour standard level 

should be set at the concentration at 

which the analysis was restricted (e.g., 

25 mg/m3). However, the EPA notes that 

neither the CASAC nor public 

commenters provided any detail 

regarding, how, in their view, these 

studies demonstrate that the level of the 

current 24-hour standard is not 

adequate, and/or how these studies 

demonstrate what revised level of the 

24-hour standard would provide 

requisite public health protection with 

an adequate margin of safety. The EPA 

considers that such an approach would 

have several important limitations. 

First, the approach assumes that a 

specific point on the air quality 

distribution (e.g., the point at which the 

analysis was restricted) is where health 

effects are exhibited and where we have 

the most confidence in the reported 

association. However, in addition to the 

limitations associated with the shortterm epidemiologic studies outlined 

above, the EPA does not agree that it 

would be appropriate to identify the 

requisite level of the primary 24-hour 

PM2.5 standard based on the specific 

concentration at which the analyses 

restrict their studies. The choice to 

restrict the data at a particular 

concentration is in effect arbitrary, and 

does not establish that any particular 

effects are attributable to that 

concentration as opposed to other 

concentrations within the restricted 

analysis. 

Further, these restricted analyses do 

not report the PM2.5 concentration at the 

98th percentile of data or other metrics 

relating to the upper end of the 

distribution that could provide 

information about health risks 

associated with peak exposures. For 

example, the CASAC does not provide 

a discussion of what the comparable 

98th percentile concentration is in the 

distribution of remaining 24-hour PM2.5

concentrations of restricted analyses 

(because such data is not reported by 

the study authors) and what degree of 

confidence the Administrator should 

place on those upper percentile values 

(e.g., 98th percentile values). In order to 

identify a level of the 24-hour standard 

based on associations between the 

‘‘upper end’’ of exposures, either in the 

unrestricted or the restricted analyses, 

and adverse health effects, it would be 

necessary to have both greater detail on 

the distribution of air quality in the 

study and greater confidence in the 

reported association at the peak 

concentrations such as the 98th 

percentile—in other words, a better 

understanding of how specific 24-hour 

concentrations correspond to the 

frequency and total number of observed 

health events in the study. 

Further, the EPA notes that when 

resulting analyses based on the 


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restricted dataset continue to find 

positive associations between the 

remaining air quality distribution and 

health effects, it suggests that the 

relationship was in fact not driven 

primarily by the upper tail (now 

removed from the dataset) but rather by 

lower portions of the distribution of air 

quality. In other words, we have no 

confidence that the remaining upper 

end of the air quality distribution is 

driving the remaining associations 

reported in the restricted analyses, as 

opposed to the vast array of health 

events at and around the mean PM2.5

concentration. In fact, it is reasonable to 

conclude that to effectively address the 

health effects observed in the study, it 

is necessary to control not just the peak 

concentrations but to reduce the bulk of 

the exposures (occurring near the 

mean), a task more effectively achieved, 

as noted above through a tightening of 

the annual standard, which has the 

effect of shifting the entire distribution 

of PM2.5 concentrations downward (both 

peaks and means). Therefore, while the 

EPA agrees that both short- and longterm epidemiologic studies that 

completed restricted analyses and 

reported the resulting study means 

could be used to inform conclusions 

regarding the adequacy of the annual 

standard, given that the resulting study 

means (when reported) could be 

evaluated in the context of the decision 

framework described above for 

informing decisions on the level of the 

annual standard, the EPA considers that 

current short-term epidemiologic 

studies that restrict analyses are subject 

to the same limitations outlined above 

for current short-term epidemiologic 

studies in how they can be used in a 

decision-making framework to inform 

the adequacy and alternative level of the 

primary 24-hour PM2.5 standard. As 

such, while the available short-term 

epidemiologic studies that restrict their 

analyses are useful for informing 

conclusions regarding the strength of 

the associations for health outcomes, 

they are not, as currently designed, as 

useful for informing conclusions 

regarding the adequacy of the current 

primary 24-hour PM2.5 standard. In 

reaching this conclusion, the EPA notes 

that the majority of the CASAC did not 

address the limitations of these studies 

outlined in the 2021 draft PA, 

particularly in the context of the 24- 

hour standard with its 98th percentile 

form. Among the future research needs 

identified by the EPA in the 2022 final 

PA, the Agency noted a number of gaps 

in the currently available information 

reported in the epidemiologic studies of 

short-term exposure, including 

‘‘descriptive statistics of PM2.5

concentrations at individual percentiles 

from the 95th percentile to the 99th 

percentile, as well as the number of 

days of concentrations and/or health 

events within each of these percentiles’’ 

and other descriptive statistics and 

details regarding analytical design in 

studies employing restricted analyses 

(U.S. EPA, 2022b, pp. 3–225 to 3–226). 

Such information could significantly 

improve the EPA’s ability to draw 

conclusions from these studies with 

regard to the adequacy of the current 

primary 24-hour PM2.5 standard. 

Due to the limitations and 

uncertainties outlined above, in 

reaching his decision on the primary 24- 

hour PM2.5 standard, the Administrator 

judges that the information from 

currently available short-term 

epidemiologic studies, including those 

that use restricted analyses, is 

inadequate to inform decisions 

regarding the adequacy of the current 

24-hour standard. Additionally, 

consistent with the final decision in 

2012, the EPA continues to view an 

approach that focuses on setting a 

generally controlling annual standard as 

the most effective and efficient way to 

reduce total population risk associated 

with both long- and short-term PM2.5

exposures. Potential air quality changes 

associated with meeting an annual 

standard level of 9.0 mg/m3 will result 

in lowering risk associated with both 

long- and short-term PM2.5 exposure by 

lowering the overall air quality 

distribution. As discussed above, 

reducing the annual standard is the 

most efficient way to reduce the risks 

from short-term exposures identified in 

the epidemiologic studies, as the 

available evidence suggests the bulk of 

the risk comes from the large number of 

days across the bulk of the air quality 

distribution, not the relatively small 

number of days with peak 

concentrations. However, as in the 2012 

review, the Administrator recognizes 

that an annual standard alone would not 

be expected to offer sufficient protection 

with an adequate margin of safety 

against the effects of short-term PM2.5

exposures in all parts of the country, 

particularly in areas with high peak-tomean ratios, and concludes that it is 

appropriate to continue to provide 

supplemental protection by means of a 

24-hour standard. In so doing, the 

Administrator concludes that retaining 

the level of the primary 24-hour PM2.5

standard of 35 mg/m3 will provide 

requisite protection against short-term 

peak PM2.5 concentrations, in 

conjunction with a revised annual 

standard level of 9.0 mg/m3. 

4. Administrator’s Conclusions 

This section summarizes the 

Administrator’s considerations and 

conclusions related to the adequacy of 

the current primary PM2.5 standards and 

presents his decision to revise the 

primary annual PM2.5 standard to a level 

of 9.0 mg/m3 and retain the primary 24- 

hour PM2.5 standard. 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 

reconsideration 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. 

The initial issue to be addressed in 

the reconsideration of the primary PM2.5

standards is whether, in view of the 

advances in scientific knowledge and 

other information reflected in the 2019 

ISA, ISA Supplement, and 2022 PA, the 

current standards are requisite to protect 

public health with an adequate margin 

of safety. In considering the adequacy of 

the current suite of primary PM2.5

standards, the Administrator has 

considered the large body of evidence 

presented and assessed in the 2019 ISA 

and ISA Supplement, the conclusions 

presented in the 2022 PA, the views 

expressed by the 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. The 

Administrator has additionally 


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considered the associated public health 

policy judgments and judgments about 

the uncertainties inherent in the 

scientific evidence and quantitative 

analyses that are integral to the 

conclusions on the adequacy of the 

current primary PM2.5 standards. 

In evaluating the adequacy of the 

current standards, the Administrator 

first recognizes the longstanding body of 

health evidence supporting 

relationships between PM2.5 exposures 

(short- and long-term) and mortality and 

serious morbidity effects. The evidence 

available in this reconsideration (i.e., 

that assessed in the 2019 ISA and ISA 

Supplement) and summarized above in 

section II.A.2.a reaffirms, and in some 

cases strengthens, the conclusions from 

the 2009 ISA regarding the health effects 

of PM2.5 exposures. Recent 

epidemiologic studies demonstrate 

generally positive and often statistically 

significant associations between PM2.5

exposures and a number of health 

effects, including non-accidental, 

cardiovascular, or respiratory mortality; 

cardiovascular or respiratory 

hospitalizations or emergency room 

visits; and other mortality/morbidity 

outcomes (e.g., lung cancer mortality or 

incidence, asthma development). Recent 

controlled human exposure and animal 

toxicological studies, as well as 

evidence from epidemiologic panel 

studies, strengthens support for 

potential biological pathways through 

which PM2.5 exposures could lead to the 

serious effects reported in many 

population-level epidemiologic studies, 

including support for pathways that 

could lead to cardiovascular, 

respiratory, nervous system, and cancerrelated effects. In considering the 

available scientific evidence, and 

consistent with approaches employed in 

past NAAQS reviews, the Administrator 

places the most weight on evidence 

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

causal’’ relationship with long or shortterm PM2.5 exposures. In addition, the 

Administrator also takes note of those 

populations identified to be at greater 

risk of PM2.5-related health effects, as 

characterized in the 2019 ISA and ISA 

Supplement, and the potential public 

health implications. 

In evaluating what existing or revised 

standards may be requisite to protect 

public health, as described above in 

section II.A.2, the Administrator’s 

approach recognizes that the current 

annual standard (based on arithmetic 

mean concentrations) and 24-hour 

standard (based on 98th percentile 

concentrations), together, are intended 

to provide public health protection 

against the full distribution of short- and 

long-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. 

Further, consistent with the approach 

adopted in 2012, the Administrator 

concludes that the most effective and 

efficient way to reduce total population 

risk associated with both long- and 

short-term PM2.5 exposures is to set a 

generally controlling annual standard, 

and to provide supplemental protection 

against the occurrence of peak 24-hour 

PM2.5 concentrations by means of a 24- 

hour standard set at the appropriate 

level. In reaching this conclusion, the 

Administrator explicitly recognizes that 

air quality changes associated with 

meeting a revised annual standard 

would result in lowering risks 

associated with both long- and shortterm PM2.5 exposures by lowering the 

overall distribution of air quality 

concentrations, leading to 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. However, 

as noted in 2012, he also recognizes that 

an approach that relies on setting the 

level of the 24-hour standard such that 

the 24-hour standard is generally 

controlling would be less effective and 

result in less uniform protection across 

the U.S. than an approach that focuses 

on setting a generally controlling annual 

standard. Thus, he concludes that 

relying on a revised annual standard as 

the controlling standard will reduce 

aggregate risks associated with both 

long- and short-term exposures more 

consistently than a generally controlling 

24-hour standard. He further concludes 

that retaining a 24-hour standard at the 

appropriate level will ensure an 

adequate margin of safety against shortterm effects in areas with high peak-tomean ratios. 

In light of his focus on the annual 

standard as the generally controlling 

standard, in considering whether the 

primary PM2.5 standards are adequate, 

the Administrator first considers 

information available to inform his final 

conclusions regarding the primary 

annual PM2.5 standard. In so doing, he 

notes that in this reconsideration, a 

large number of key U.S. epidemiologic 

studies report positive and statistically 

significant associations for air quality 

distributions with overall mean PM2.5

concentrations that are well below the 

current level of the annual standard of 

12.0 mg/m3. He further recognizes that 

there is additional scientific evidence 

assessed in the 2019 ISA and newly 

assessed in this reconsideration in the 

ISA Supplement that can provide 

supplemental information to inform his 

decisions. In addition to the key U.S. 

epidemiologic studies, the 

Administrator also recognizes that key 

Canadian epidemiologic studies also 

demonstrate positive and statistically 

significant associations at 

concentrations below 12 mg/m3. He also 

recognizes that epidemiologic studies 

that restrict annual average PM2.5

concentrations to below 12 mg/m3 also 

provide support for positive and 

statistically significant associations at 

lower mean PM2.5 concentrations, as do 

accountability studies that also suggest 

public health improvements may occur 

at concentrations below 12 mg/m3. 

With regard to the available scientific 

evidence to inform his final decisions 

on the adequacy of the current 24-hour 

standard, the Administrator finds that 

there is less information available to 

support decisions on the 24-hour 

standard than that summarized above 

for the annual standard. The 

Administrator first notes that controlled 

human exposure studies, including 

those newly available in this 

reconsideration, demonstrate effects 

following short-term PM2.5 exposures at 

concentrations higher than the current 

24-hour standard. The Administrator 

also considers air quality analyses 

conducted in the 2022 PA and in 

responding to public comments, as 

described above in section II.B.3, that 

evaluate PM2.5 concentrations in 

ambient air for similar durations to the 

controlled human exposure studies. As 

noted above, these air quality analyses 

indicate that the current 24-hour 

standard, particularly in conjunction 

with the revised level of the annual 

standard, provides a high degree of 

protection against subdaily PM2.5

concentrations that have been shown to 

elicit effects in controlled human 

exposure studies. The Administrator 

considers a limited number of available 

epidemiologic studies that report 

associations with health effects when 

the analyses are restricted to daily PM2.5

concentrations below 35 mg/m3. As 

described above, although these studies 

are useful in demonstrating that health 

effects are associated with exposure to 

daily PM2.5 concentrations in the lower 

part of the air quality distribution, they 

do not provide information about health 

effects associated with the short-term 


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‘‘peak’’ exposures that the 24-hour 

standard is designed to protect against. 

Accordingly, these studies have limited 

relevance in informing a decision about 

the appropriate level of the 24-hour 

standard. 

In addition to the scientific evidence, 

the Administrator also considers the 

information from the risk assessment. In 

so doing, he notes that the risk 

assessment estimates that the current 

primary annual PM2.5 standard could 

allow a substantial number of deaths in 

the U.S. With respect to the 24-hour 

standard, the Administrator recognizes 

that there are only a small number of 

study areas where the 24-hour standard 

is controlling and changes in the 24- 

hour standard level are estimated to 

have a much smaller impact on public 

health. The Administrator recognizes 

that while the risk estimates can help to 

place the evidence for specific health 

effects into a broader public health 

context, they should be considered 

along with the inherent uncertainties 

and limitations of such analyses when 

informing judgments about the potential 

for additional public health protection 

associated with PM2.5 exposure and 

related health effects. While the 

Administrator recognizes that these 

uncertainties are important, he also 

notes that the general magnitude of the 

risk estimates provide support for 

significant public health impacts, 

particularly for lower alternative annual 

standard levels. 

In reaching his final conclusions 

regarding the adequacy of the primary 

PM2.5 standards, the Administrator also 

considers the CASAC’s advice and 

recommendations, as well as public 

comments. With respect to the CASAC’s 

advice, the Administrator recognizes 

that, in their review of the 2021 draft 

PA, the CASAC reached consensus that 

the current primary annual PM2.5

standard is not adequate and that it is 

not sufficiently protective of public 

health. The Administrator also takes 

note of the CASAC’s advice in their 

review of the 2019 draft PA, where the 

CASAC did not reach consensus on the 

adequacy of the primary annual PM2.5

standard, with the minority 

recommending revision and the 

majority recommending the standard be 

retained. Furthermore, he recognizes 

that in reviewing the 2019 draft PA, the 

CASAC reached consensus regarding 

the adequacy of the primary 24-hour 

PM2.5 standard, concluding that the 

standard should be retained. 

Conversely, in their review of the 2021 

draft PA, the majority of the CASAC 

advised that the current primary 24- 

hour PM2.5 standard is not adequate and 

recommended revising the level of the 

standard, while the minority of the 

CASAC concluded that the standard 

was adequate and should be retained. 

However, in considering the advice of 

the CASAC collectively in the context of 

this reconsideration, the Administrator 

recognizes that the 2021 draft PA 

included scientific evidence and 

quantitative risk information that was 

not available in the 2019 draft PA, and 

therefore, the advice and 

recommendations of the CASAC in their 

review of the 2021 draft PA are based 

on consideration of the full body of 

scientific evidence available in this 

reconsideration, including the evidence 

evaluated in the 2019 ISA and the ISA 

Supplement. 

The Administrator recognizes that 

much of the scientific evidence 

available in this reconsideration was 

also available in the 2019 ISA and was 

considered by the then-Administrator 

when he decided that the current 

primary PM2.5 standards are requisite to 

protect public health with an adequate 

margin of safety. However, as described 

in section I.C.5.b above, in reaching his 

decision to reconsider the 2020 final 

decision, the Administrator also 

recognized that there were a number of 

studies published since the literature 

cutoff date of the 2019 ISA that were 

raised by some members of the CASAC 

in their review of the 2019 draft PA, in 

public comments on the 2020 proposal, 

and in the petitions for reconsideration. 

As such, the expansion of the air quality 

criteria in this reconsideration to 

encompass both the 2019 ISA and the 

additional scientific evidence evaluated 

in the ISA Supplement, along with 

evidence and updated quantitative 

analyses in the 2022 PA also provided 

an expanded record for the CASAC’s 

review and public comments as a part 

of this reconsideration. Taken together, 

the 2019 ISA, ISA Supplement, and 

2022 PA, along with the CASAC’s 

advice and recommendations and 

public comments, provide the 

Administrator with additional 

information for consideration in 

reaching his final conclusions in this 

reconsideration. As a result, the record 

before him notably expands upon and 

strengthens the basis for the conclusions 

of the 2019 ISA while reducing some 

uncertainties that were identified in the 

2020 final action. 

In considering the available 

information in this reconsideration, the 

current Administrator reached different 

conclusions regarding the appropriate 

weight to place on certain aspects of the 

evidence than the then-Administrator in 

the 2020 final decision. For example, in 

reaching his conclusions on the primary 

annual PM2.5 standard in 2020, the thenAdministrator concluded that it was 

appropriate to place more weight on 

epidemiologic studies that used groundbased monitors and to place less weight 

on the studies that used hybrid modelbased approaches, citing to increased 

uncertainties associated with this new 

and emerging approach to estimating 

exposure. In placing more weight on the 

key U.S. monitor-based studies, the 

then-Administrator noted that the 

majority of these studies had mean 

concentrations at or above the level of 

the annual standard (12.0 mg/m3). 

However, unlike the approach for 

considering such studies in the 2012 

review, the then-Administrator 

concluded that it was appropriate to 

consider the study-reported means 

collectively, and in so doing, he placed 

weight on the average of the studyreported means (or medians) across the 

U.S. monitor-based studies of 13.5 mg/ 

m3, and noted that this concentration 

was above the level of the standard (85 

FR 82717, December 18, 2020). In 

contrast, in this reconsideration, the 

current Administrator judges that it is 

appropriate to consider the individual 

study-reported mean PM2.5

concentrations from not only the U.S. 

monitor-based epidemiologic studies, 

but also the U.S. hybrid model-based 

epidemiologic studies, which are an 

advancement in the available science 

since the completion of the 2009 ISA. 

The current Administrator also adopts 

an approach similar to some previous 

approaches for the PM NAAQS in 

which he judges it most appropriate to 

set the level of the standard to 

somewhat below the lowest long-term 

study-reported mean PM2.5

concentration reported in key U.S. 

epidemiologic studies, which is 9.3 mg/ 

m3. The study that reports the long-term 

mean PM2.5 concentration of 9.3 mg/m3

is newly available in this 

reconsideration and is evaluated in the 

ISA Supplement. In the 2019 ISA, the 

lowest long-term study-reported mean 

PM2.5 concentrations for U.S.-based 

studies that use ground-based monitors 

and hybrid model-based approaches are 

9.9 mg/m3 and 10.7 mg/m3, respectively. 

In judging that it is appropriate to 

consider both monitor- and hybrid 

model-based epidemiologic studies and 

that it is appropriate to adopt an 

approach to set the level of the standard 

to somewhat below the lowest long-term 

mean PM2.5 concentration, the current 

Administrator judges that the available 

scientific evidence—evaluated in both 

the 2019 ISA and in the ISA 

Supplement—provide support for his 

conclusion that that current primary 


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107The EPA notes that, in considering the 

additional scientific evidence available in this 

reconsideration, one member of the CASAC who 

reviewed both the 2019 draft PA and the 2021 draft 

PA found that the available scientific and 

quantitative information available in this 

reconsideration supported revising the level of the 

primary annual PM2.5 standard to within the range 

of 10–11 mg/m3, whereas he recommended retaining 

the standard during the review of the 2019 draft PA. 

PM2.5 standard is not adequate and 

should be revised. 

In addition to adopting a different 

approach than the previous 

Administrator for considering the longterm mean PM2.5 concentrations from 

key U.S. epidemiologic studies (one 

more consistent with the approach of 

the EPA in other prior reviews), the 

current Administrator both has 

information newly available in this 

reconsideration before him and is 

reaching different conclusions about 

how to weigh the evidence before him 

in reaching his final conclusions. For 

example, in reaching his final decision 

in 2020, the then-Administrator was 

concerned about placing too much 

weight on epidemiologic studies to 

inform his conclusions on the adequacy 

of the primary PM2.5 standards, noting 

that the epidemiologic studies do not 

identify particular PM2.5 concentrations 

that cause effects and cannot alone 

identify a specific level at which to set 

the standard. In so doing, the thenAdministrator placed greater weight on 

the uncertainties and limitations 

associated with the epidemiologic 

studies, including exposure 

measurement error, potential 

confounding by copollutants, increased 

uncertainty of associations at lower 

PM2.5 concentrations, and heterogeneity 

of effects across different cities or 

regions (85 FR 82716, December 18, 

2020). The Administrator recognizes 

that in reaching these judgments, the 

then-Administrator took into 

consideration the views of some 

members of the CASAC, who, in their 

advice on the 2019 draft PA, expressed 

the view that the current PM NAAQS 

should be retained because reported 

associations between short- and longterm PM2.5 exposures and adverse 

health outcomes ‘‘can reasonably be 

explained in light of uncontrolled 

confounding and other potential sources 

of error and bias’’ (Cox, 2019b, p. 8 of 

consensus responses). 

In this reconsideration, the current 

Administrator notes that the ISA 

Supplement evaluates additional 

studies that employed statistical 

approaches that attempted to more 

extensively account for confounders and 

are more robust to model 

misspecification (i.e., used alternative 

methods for confounder control, which 

are sometimes referred to as causal 

modeling or causal inference methods) 

that build upon those studies available 

and evaluated in the 2019 ISA (U.S. 

EPA, 2019, sections 11.1.2.1 and 

11.2.2.4). These studies report 

consistent positive associations between 

long-term and short-term PM2.5

exposure and total mortality and 

cardiovascular effects (U.S. EPA, 2022a, 

section 3.2.2.3). In considering the 

epidemiologic evidence evaluated in the 

2019 ISA, along with the newly 

available studies evaluated in the ISA 

Supplement, the current Administrator 

also recognizes that there are 

uncertainties and limitations associated 

with the epidemiologic studies, but 

judges that it is appropriate to place less 

weight on these uncertainties than the 

then-Administrator placed on them in 

reaching his final decision in 2020, 

given the strength of the longstanding 

large body of epidemiologic evidence, 

employing a variety of study designs, 

that demonstrates associations between 

long- and short-term PM2.5 exposures 

and health effects across multiple U.S. 

cities and in diverse populations, 

including in studies examining 

populations and lifestages that may be 

at comparatively higher risk of 

experiencing a PM2.5-related health 

effect (e.g., older adults, children). 

In reaching this final decision, the 

Administrator recognizes he is differing 

not only with the prior Administrator 

but also with the advice some members 

of the CASAC provided during their 

review of the 2019 draft PA. 

Specifically, taking into consideration 

the strength of the evidence providing 

support for causality determinations, 

the advice of other members of the 

CASAC and the need to protect public 

health with an adequate margin of 

safety, the current Administrator 

disagrees with these members of CASAC 

regarding the weight to be given to 

epidemiologic evidence ‘‘based on its 

methodological limitations’’ (Cox, 

2019b, p. 8 of consensus responses), 

such as the possibility ‘‘that such 

associations could reasonably be 

explained by uncontrolled confounding 

and other potential sources of error and 

bias’’ (Cox, 2019b, p. 8 of consensus 

responses). 

As another example of information 

that was not available to the CASAC in 

providing advice to the Administrator in 

reaching his final decision in 2020, the 

then-Administrator noted in his final 

decision that, while some members of 

the CASAC and public commenters 

highlighted a number of accountability 

studies that examined past reductions in 

ambient PM2.5 concentrations and the 

degree to which those reductions have 

resulted in public health improvements, 

the small number of available 

accountability studies did not examine 

air quality with starting concentrations 

meeting the primary annual PM2.5

standard of 12.0 mg/m3

. The thenAdministrator took into consideration 

the absence of such accountability 

studies, as part of his consideration of 

the full body of scientific evidence, in 

reaching his judgment 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 existing 

primary PM2.5 NAAQS (85 FR 82717, 

December 18, 2020). However, there are 

several accountability studies available 

since the literature cutoff date of the 

2019 ISA and evaluated in the ISA 

Supplement in this reconsideration that 

have starting concentrations (or 

concentrations prior to the policy or 

intervention) below 12.0 mg/m3

(Corrigan et al, 2018; Henneman et al., 

2019; Sanders et al., 2020a). The current 

Administrator concludes that, while the 

number of available accountability 

studies is limited, he recognizes that 

these studies provide supplemental 

information for consideration for 

informing decisions on the appropriate 

level of the primary annual PM2.5

standard along with the full body of 

evidence. 

As EPA has frequently noted 

throughout this document, the extent to 

which the current primary PM2.5

standards are judged to be adequate 

depends in part on science policy and 

public health policy judgments to be 

made by the Administrator on the 

strength and uncertainties of the 

scientific evidence, such as how to 

consider epidemiologic evidence and 

the need for an adequate margin of 

safety in setting the standards. Thus, it 

would be pure speculation to guess 

whether the then-Administrator would 

have reached the same or different 

conclusions in the 2020 final decision 

had the record before him included the 

newly available information in this 

reconsideration.107 However, the 

current Administrator concludes that, 

for the reasons explained herein that, in 

his judgment, based on the record before 

him in this reconsideration, it is 

necessary and appropriate to revise the 

primary annual PM2.5 NAAQS to 

provide requisite protection of public 

health with an adequate margin of 

safety. 

Based on the available scientific 

evidence and quantitative information, 

as well as consideration of the CASAC’s 

advice and public comments, the 

Administrator concludes that the 


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108The CASAC did not provide advice or 

recommendations regarding the indicator of the 

primary PM2.5 standards in their review of the 2019 

draft PA (Cox, 2019b). 

109The CASAC did not provide advice or 

recommendations regarding the averaging times of 

the primary PM2.5 standards in their review of the 

2019 draft PA (Cox, 2019b). 

110The CASAC did not provide advice or 

recommendations regarding the forms of the 

primary PM2.5 standards in their review of the 2019 

draft PA (Cox, 2019b). 

111The CASAC did not provide advice or 

recommendations regarding the forms of the 

primary PM2.5 standards in their review of the 2019 

draft PA (Cox, 2019b). 

current primary annual PM2.5 standard 

is not adequate to protect public health 

with an adequate margin of safety. In 

addition, he finds the available 

information insufficient to call into 

question the adequacy of the public 

health protection afforded by the 

current primary 24-hour PM2.5 standard. 

In considering how to revise the 

current suite of primary PM2.5 standards 

in order to achieve the requisite 

protection for public health, with an 

adequate margin of safety, against longand short-term PM2.5 exposures the 

Administrator considers the four basic 

elements of the NAAQS (indicator, 

averaging time, form, and level) 

collectively. With respect to indicator, 

the Administrator recognizes that the 

scientific evidence in this 

reconsideration, as in previous reviews, 

continues to provide strong support for 

health effects associated with PM2.5

mass. He notes the 2022 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 of PM (U.S. EPA, 2022b, section 

3.6.3.2.1). In its advice on the adequacy 

of the current primary PM2.5 standards 

in their review of the 2021 draft PA, the 

CASAC reached consensus that the 

PM2.5 mass-based indicator should be 

retained, without revision (Sheppard, 

2022a, p. 2 of consensus letter).108

Additionally, there was no information 

in the public comments that provided a 

rationale for an alternative indicator. 

For all of these reasons, the 

Administrator concludes that it is 

appropriate to retain PM2.5 mass as the 

indicator for the primary standards for 

fine particles. 

Consistent with his proposed 

conclusions regarding averaging time, 

the Administrator notes that the 

scientific evidence continues to provide 

strong support for health effect 

associations with both long- and shortterm PM2.5 exposures (88 FR 5618, 

January 27, 2023). Epidemiologic 

studies continue to provide strong 

support for health effects associated 

with short-term PM2.5 exposures based 

on 24-hour averaging periods, and 

associations in epidemiologic studies 

with subdaily estimates are less 

consistent and, in some cases, smaller in 

magnitude (88 FR 5618, January 27, 

2023). Taken together, the 2019 ISA 

concludes that epidemiologic studies do 

not indicate that subdaily averaging 

periods are more closely associated with 

health effects than the 24-hour average 

exposure metric (U.S. EPA, 2019a, 

section 1.5.2.1). In addition, controlled 

human exposure and panel-based 

studies of subdaily exposures typically 

examine subclinical effects rather than 

the more serious population-level 

effects that have been reported to be 

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

mortality, hospitalizations). 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), air quality 

analyses have shown that the current 

averaging times can effectively protect 

against the exposure concentrations in 

these studies. This information does not 

indicate that a revision to the averaging 

time is necessary to provide additional 

protection against subdaily PM2.5

exposures, beyond that provided by the 

current primary annual and 24-hour 

PM2.5 standards. The Administrator also 

notes that this conclusion is also 

support by the CASAC’s advice in their 

review of the 2021 draft PA where they 

reached consensus that averaging times 

for the primary PM2.5 standards should 

be retained, without revision (Sheppard, 

2022a, p. 2 of consensus letter).109 The 

Administrator also considers the 

relatively few public comments received 

that support a subdaily averaging time, 

but concludes that the currently 

available information does not provide 

support for an alternate averaging time. 

Consistent with his proposed decision, 

the Administrator concludes that it is 

appropriate to retain the annual and 24- 

hour averaging times for the primary 

PM2.5 standards to protect against longand short-term PM2.5 exposures. 

With regard to form, the 

Administrator first notes that the EPA 

has set both an annual standard and a 

24-hour standard to provide protection 

from health effects associated with both 

long- and short-term exposures to PM2.5

(62 FR 38667, July 18, 1997; 88 FR 5620, 

January 27, 2023). With regard to the 

form of the annual standard, the 

Administrator recognizes that a large 

majority of the recently available 

epidemiologic studies continue to report 

associations between health effects and 

annual average PM2.5 concentrations. 

These studies of annual average PM2.5

concentrations provide support for 

retaining the current form of the annual 

standard to provide protection against 

long- and short-term PM2.5 exposures. In 

its review of the 2021 draft PA, the 

CASAC reached consensus that the form 

of the annual standard (i.e., annual 

mean, averaged over 3 years) should be 

retained, without revision (Sheppard, 

2022a, p. 2 of consensus letter).110 The 

Administrator also notes that there were 

no public comments that recommended 

an alternative form for the primary 

annual PM2.5 standard. 

With regard to the form of the 24-hour 

standard (98th percentile, averaged over 

three years), epidemiologic studies 

continue to provide strong support for 

health effect associations with shortterm (e.g., mostly 24-hour) PM2.5

exposures, and controlled human 

exposure studies provide evidence for 

health effects following single shortterm ‘‘peak’’ PM2.5 exposures (88 FR 

5618, January 27, 2023). Therefore, the 

Administrator concludes that 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, 

in combination with a primary annual 

PM2.5 standard with its annual mean 

averaged over three years form. As 

described in the proposal and in 

responding to comments in section 

II.B.3 above, the Administrator further 

notes that the 98th percentile, averaged 

over three years, 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, 2022b, section 3.6.3.2.3). 

Furthermore, the Administrator notes 

that the multi-year percentile form (i.e., 

averaged over three 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. This conclusion is also 

supported by the CASAC’s advice in 

their review of the 2021 draft PA, where 

they reached consensus that the form for 

the primary PM2.5 standards should be 

retained, without revision (Sheppard, 

2022a, p. 2 of consensus letter).111

The Administrator also recognizes 

that the CASAC recommended that in 

future reviews, the EPA also consider 

alternative forms for the primary 24- 

hour PM2.5 standard (Sheppard, 2022a, 


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p. 18 of consensus responses). Based on 

the CASAC’s advice, the proposal 

solicited comment on alternatives to the 

current form for consideration in future 

reviews (88 FR 5619, January 27, 2023). 

The Administrator recognizes that there 

were a limited number of public 

comments related to the form of the 

primary PM2.5 standards as discussed in 

section II.D.3 above and in the Response 

to Comments document, and notes that, 

the EPA will consider the information 

provided by the commenters regarding 

the form of the 24-hour PM2.5 standard 

in the next review of the PM NAAQS. 

Consistent with his proposed decision, 

in considering the information 

summarized above, the Administrator 

concludes that it is appropriate to retain 

the forms of the current annual and 24- 

hour PM2.5 standards. 

In considering how to revise the 

current suite of PM2.5 standards to 

provide the requisite public health 

protection with an adequate margin of 

safety, the Administrator next evaluates 

the appropriate levels of the primary 

PM2.5 standards, beginning with the 

annual PM2.5 standard. In having 

carefully considered public comments 

related to the primary annual PM2.5

standard, the Administrator believes 

that the fundamental conclusions 

regarding the scientific evidence and 

quantitative information that supported 

his proposed conclusions (as described 

in the 2019 ISA, ISA Supplement, 2022 

PA, and the proposal) remain valid. In 

considering the level at which the 

primary annual PM2.5 standard should 

be set, the Administrator considers the 

entire body of evidence and 

information, giving appropriate weight 

to each part of that body of evidence 

and information. He continues to place 

the greatest weight in this 

reconsideration on the available 

scientific evidence that provides 

support for associations between health 

effects and long- and short-term PM2.5

exposures. In conjunction with his 

decisions to retain the current indicator, 

averaging time, and form as described 

above, the Administrator is revising the 

level of the primary annual PM2.5

standard to 9.0 mg/m3. In so doing, he 

is selecting a primary annual PM2.5

standard that, together with the primary 

24-hour PM2.5 standard, provides 

requisite public health protection with 

an adequate margin of safety, based on 

his judgments about and interpretation 

of the scientific evidence and 

quantitative risk information. 

The Administrator’s decision to revise 

the level of the primary annual PM2.5

standard to 9.0 mg/m3 builds upon his 

conclusion that the overall body of 

scientific evidence and quantitative risk 

information calls into question the 

adequacy of public health protection 

afforded by the current standard, 

particularly for at-risk populations. 

Consistent with his consideration of the 

available information in reaching his 

proposed decisions, the Administrator’s 

final decision on the level of the 

primary annual PM2.5 standard places 

the greatest emphasis on key U.S. 

epidemiologic studies that report 

associations between long- and shortterm PM2.5 exposures and mortality and 

morbidity. As in the proposal, and as 

discussed further below, he views 

additional epidemiologic studies (i.e., 

studies that employ alternative methods 

for confounding control, studies that 

employ restricted analyses, and 

accountability studies), the controlled 

human exposure studies, and the risk 

assessment as providing supplemental 

information in support of his decision to 

revise the current annual standard, but 

recognizes that some of these lines of 

evidence and information provide a 

more limited basis for selecting a 

particular standard level among a range 

of options. See Mississippi, 744 F. 3d at 

1351–52 (studies can legitimately 

support a decision to revise the 

standard, but not provide sufficient 

information to justify their use in setting 

the level of a revised standard). 

Given his consideration of the 

scientific evidence, quantitative risk 

information, advice from the CASAC, 

and public comments, the 

Administrator judges that a primary 

annual PM2.5 standard with a level of 

9.0 mg/m3 is requisite to protect public 

health with an adequate margin of 

safety. He notes that the determination 

of what constitutes an adequate margin 

of safety is expressly left to the 

judgment of the EPA Administrator. See 

Lead Industries Association v. EPA, 647 

F.2d at 1161–62; Mississippi, 744 F.3d 

at 1353. He further notes that in 

evaluating how particular standards 

address the requirement to provide an 

adequate margin of safety, it is 

appropriate to consider such factors as 

the nature and severity of the health 

effects, the size of the at-risk 

populations, and the kind and degree of 

the uncertainties present. In considering 

the need for an adequate margin of 

safety, the Administrator notes that a 

primary annual PM2.5 standard with a 

level of 9.0 mg/m3 would be expected to 

provide substantial improvements in 

public health compared to the current 

annual standard, including for at-risk 

groups such as children, older adults, 

people with preexisting conditions, 

minority populations, and low SES 

populations. 

Consistent with his conclusions on 

the need for revision of the current 

annual standard, in reaching a decision 

on level, the Administrator places the 

most weight on information from 

epidemiologic studies. In so doing, the 

Administrator notes that these studies 

provide consistent evidence of positive 

and statistically significant associations 

between long- and short-term exposure 

to PM2.5 and mortality and morbidity 

(88 FR 5624, January 27, 2023). The 

Administrator recognizes that placing 

weight on the information from the 

epidemiologic studies allows for 

examination of the entire population, 

including those that may be at 

comparatively higher risk of 

experiencing a PM2.5-related health 

effects (e.g., children, older adults, 

minority populations) (88 FR 5624, 

January 27, 2023). The Administrator 

also recognizes that recent 

epidemiologic studies continue to 

support a no-threshold relationship, 

meaning that there is no ‘‘bright line’’ 

below which no effects have been 

found. These studies also support a 

linear relationship between health 

effects and PM2.5 exposures at PM2.5

concentrations greater than 8 mg/m3, 

though uncertainties remain about the 

shape of the C–R curve at PM2.5

concentrations less than 8 mg/m3, with 

some recent studies providing evidence 

for either a sublinear, linear, or 

supralinear relationship at these lower 

concentrations (U.S. EPA, 2019a, 

section 11.2.4; U.S. EPA, 2022a, section 

2.2.3.2; 88 FR 5625, January 27, 2023). 

As at the time of proposal, the 

Administrator notes that some recent 

epidemiologic studies have adopted a 

broad range of approaches to examine 

confounding and the results of those 

examinations support the robustness of 

reported associations seen in 

epidemiologic studies. These include 

studies that employ alternative methods 

for confounder control and studies that 

evaluate the uncertainty related to 

exposure measurement error, both of 

which continue to support associations 

between PM2.5 exposures and health 

effects while taking approaches to 

address uncertainties. 

In considering the epidemiologic 

evidence, the Administrator judges that, 

in reaching his decision on an 

appropriate level for the annual 

standard that will protect public health 

with an adequate margin of safety, in 

the absence of any discernible 

population-level thresholds, and in 

recognizing the need to weigh 

uncertainties associated with the 

epidemiologic evidence, it is most 

appropriate to examine where the 

evidence of associations observed in the 


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112 ‘‘Long-term’’ represents PM2.5 exposures and 

concentrations that are annual or multi-year. 

113As described in section II.A.2.c above, key 

epidemiologic studies are those that report overall 

mean (or median) PM2.5 concentrations and for 

which the years of PM2.5 air quality data used to 

estimate exposures overlap entirely with the years 

during which health events are reported. 

114Reported mean PM2.5 concentrations in 

monitor-based studies are averaged across monitors 

in each study area with multiple monitors, referred 

to as a composite monitor concentration, in contrast 

to the highest concentration monitored in the study 

area, referred to as a maximum monitor 

concentration (i.e., the ‘‘design value’’ 

concentration), which is used to determine whether 

an area meets a given standard. 

115Studies that use hybrid modeling approaches 

employ methods to estimate ambient PM2.5

concentrations across large geographical areas, 

including areas without monitors, and thus, when 

compared to monitor-based studies, require 

additional information to inform the relationship 

between the estimated PM2.5 concentrations across 

an area and the maximum monitor design values 

used to assess compliance. 

epidemiologic studies is strongest and, 

conversely, to place less weight where 

he has less confidence in the 

associations observed in the 

epidemiologic studies. As at the time of 

proposal, the Administrator notes that 

in previous reviews, evidence-based 

approaches noted that the evidence of 

an association in any epidemiologic 

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). Given this, 

these approaches focused on identifying 

standard levels near or somewhat below 

long-term mean concentrations reported 

in key epidemiologic studies. These 

approaches were supported by previous 

CASAC advice as well as the CASAC’s 

advice in their review of the 2021 draft 

PA as a part of this reconsideration. 

Additionally, the Administrator 

acknowledges that in the 2020 final 

action, the then-Administrator decided 

to retain the standard based in part on 

concerns about placing reliance on the 

epidemiologic studies and his judgment 

that even if he did rely on them, the 

majority of the studies had means or 

medians, as well as the mean of all of 

the key study-reported means or 

medians, above the level of the current 

annual standard. However, after 

considering the evidence, the advice of 

CASAC, and public comments the 

Administrator judges that this approach 

is insufficient to protect public health 

with an adequate margin of safety. The 

Administrator’s decision to reach a 

different judgment about the 

appropriate level of the annual standard 

reflects the updated and expanded 

scientific record available to the 

Administrator in this reconsideration, as 

well as the additional advice from the 

CASAC and the public comments based 

on this newly available information. In 

addition, the Administrator observes the 

decision in this action to place weight 

on the epidemiologic studies, and to 

revise the annual primary standard to a 

level below the lowest long-term mean 

in the U.S.-based epidemiologic studies, 

is consistent with the EPA’s past 

practice in PM NAAQS reviews. 

In this reconsideration, the 

Administrator is considering the 

scientific record which has been 

expanded and updated since the 2020 

final action, as well as the additional 

advice from the CASAC and the public 

comments that are based on the newly 

available information that expands upon 

the information previously available. In 

addition, the Administrator is exercising 

his judgment about how to interpret and 

weigh the expanded evidence in a way 

that is more consistent with the 

approaches used in prior PM NAAQS 

reviews. As a result, the Administrator 

has concluded on reconsideration that 

the level of the primary annual standard 

is not adequate and should be revised to 

protect public health with an adequate 

margin of safety. 

Consistent with his proposed 

decisions, in reaching conclusions on 

the level of the primary annual PM2.5

standard, the Administrator considers 

the long-term 112 study-reported mean 

PM2.5 concentrations from key long- and 

short-term epidemiologic studies and 

sets the level of the standard to 

somewhat below the lowest long-term 

mean PM2.5 concentration.113 He notes 

that in previous PM NAAQS reviews 

(including the 1997, 2006 and 2012 

reviews), evidence-based approaches 

focused on identifying standard levels 

near or somewhat below long-term 

mean concentrations reported in key 

long- and short-term epidemiologic 

studies. These approaches were 

supported by the CASAC in previous 

reviews and were supported in this 

reconsideration by the CASAC in their 

review of the 2021 draft PA. In 

considering the available scientific 

evidence to inform such an approach, 

the Administrator notes the strength of 

the epidemiologic evidence which 

includes multiple studies that 

consistently report positive associations 

for short- and long-term PM2.5 exposure 

and mortality and cardiovascular 

effects. Some available studies also use 

a variety of statistical methods to 

control for confounding bias and report 

similar associations, which further 

supports the broader body of 

epidemiologic evidence for both 

mortality and cardiovascular effects. 

Additionally, he notes that recent 

epidemiologic studies available for 

consideration in reaching his final 

decision strengthen support for health 

effect associations at PM2.5

concentrations lower than in those 

evaluated in epidemiologic studies 

available at the time of previous 

reviews. The Administrator does 

recognize, however, that while these 

epidemiologic studies evaluate 

associations between distributions of 

ambient PM2.5 concentrations and 

health outcomes, they do not identify 

the specific exposures that led to the 

reported effects. As such, he notes that 

there is no specific point in the air 

quality distribution of any 

epidemiologic study that represents a 

‘‘bright line’’ at and above which effects 

have been observed and below which 

effects have not been observed. The 

Administrator further notes that the 

epidemiologic 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, and concludes that the 

long-term study-reported means from 

both long- and short-term studies 

provide the strongest support for 

reported health effect associations in 

epidemiologic studies. For these 

reasons, as described in the proposal 

and in responding to public comments 

in section II.B.3 above, the 

Administrator concludes that it is 

appropriate to continue to employ an 

approach that focuses on the mean 

PM2.5 concentrations from the key 

epidemiologic studies to inform his 

conclusions regarding the appropriate 

level for the primary annual PM2.5

standard. 

In adopting such an approach, the 

Administrator considers the long-term 

mean concentrations reported in two 

types of key epidemiologic studies: (1) 

Monitor-based studies 114

(epidemiologic studies that used 

ground-based monitors to estimate 

exposure, similar to approaches used in 

past reviews), and (2) hybrid modelingbased studies 115 (epidemiologic studies 

that used hybrid modeling approaches 

and apply aspects of population 

weighting to estimate exposures). In 

reaching conclusions regarding the level 

of a standard that would provide 

requisite protection with an adequate 

margin of safety, the Administrator 

recognizes that he must use his 

judgment regarding the appropriate 

weight to place on the available 

evidence and technical information, 

including uncertainties. As shown in 

Figures 1 and 2 above, for the key U.S. 

monitor-based epidemiologic studies, 


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116Analyses in the 2022 PA suggest that the 

highest monitored value would be expected to be 

greater than the study-reported mean values by 10– 

20% for monitor-based studies and 15–18% for 

hybrid modeling studies that apply aspects of 

population weighting. 

117The Wang et al. (2017) study only reports the 

25th percentile of the estimated PM2.5

concentrations, not the 10th percentile. 

118There is a third hybrid model-based study, as 

described in the 2022 PA and in section II.B.3 above 

in responding to public comments, but it is not 

referenced here because it reports a 25th percentile 

PM2.5 concentration based on the 25th percentile of 

health events that occur in the study (Di et al., 

2017a) rather than report the 25th percentile based 

on air quality concentrations. 

the study-reported mean concentrations 

range from 9.9–16.5 mg/m3, and for the 

key U.S. hybrid modeling-based 

epidemiologic studies, the mean 

concentrations range from 9.3–12.2 mg/ 

m3. The Administrator also recognizes 

that, in their review of the 2021 draft 

PA, both the majority and minority of 

the CASAC emphasized the 

epidemiologic studies in support of 

their recommendations for the level of 

the annual standard, but they weighed 

the studies in different ways (Sheppard, 

2022a, p. 16–17 of consensus 

responses). 

Based on this information, and in 

considering the CASAC’s advice in their 

review of the 2021 draft PA, the 

Administrator judges that it is 

appropriate to set the level of the 

primary PM2.5 standard at least as low 

as the lowest mean PM2.5 concentration 

from these key U.S.-based 

epidemiologic studies, which is 9.3 mg/ 

m3. The Administrator additionally 

notes that setting the annual standard 

level at 9.0 mg/m3, which is below the 

lowest study-reported mean PM2.5

concentration of 9.3 mg/m3, would be 

expected to shift the distribution of 

PM2.5 concentrations in an area such 

that the area’s highest monitor would 

generally be at or below 9.0 mg/m3

annually, when meeting the annual 

standard. In this situation, the resulting 

average or mean PM2.5 concentration for 

the entire area (measured across a 

number of monitors) would be even 

further below the study-reported 

means,116 and will provide adequate 

protection not only in areas where the 

highest allowable concentrations would 

be expected (i.e., near design value 

monitors) but also in other parts of the 

area where PM2.5 concentrations would 

be expected to be maintained even 

lower. 

As noted above, however, the 

Administrator must exercise his 

judgment regarding the appropriate 

weight to place on the available 

scientific evidence and quantitative 

information, including uncertainties, in 

determining what level of the annual 

standard is sufficient to protect public 

health with an adequate margin of 

safety. In so doing, he considers other 

information available in this 

reconsideration to inform his 

judgments, including study-reported 

PM2.5 concentrations at lower 

percentiles in key epidemiologic 

studies, supplemental information from 

other types of epidemiologic studies, 

study-reported PM2.5 concentrations 

from key Canadian epidemiologic 

studies, and the results from the 

quantitative risk assessment. 

In weighing the evidence in 

considering the requisite level of the 

annual standard, the Administrator also 

takes into account additional 

information from the key long- and 

short-term U.S. epidemiologic studies 

available that provide study-reported 

PM2.5 concentrations below the mean 

and, in particular, the subset of 

epidemiologic studies that report 25th 

and 10th percentile concentrations. 

Consistent with his proposed 

conclusions, as well as the CASAC’s 

advice in their review of the 2021 draft 

PA and public comments, the 

Administrator judges that it is 

appropriate to place some weight on 

these lower percentiles in reaching his 

conclusions on the level of the primary 

annual standard. There are six key U.S. 

epidemiologic studies that report 

information on other percentiles (e.g., 

10th and 25th percentiles of PM2.5

concentrations or 10th and 25th 

percentiles of PM2.5 concentrations 

associated with health events) that are 

below the mean.117 In considering the 

information from these studies, the 

Administrator first notes that the three 

older, monitor-based studies that report 

lower percentiles of PM2.5

concentrations have smaller cohort sizes 

than the three hybrid model-based 

studies. Thus, the Administrator 

recognizes that the older, monitor-based 

studies had a relatively smaller portion 

of the health events that were observed 

in the lower part of the air quality 

distribution because of the generally 

smaller size of the cohorts. He further 

notes that the recent hybrid modelbased studies have larger cohort sizes 

than the older, monitor-based studies, 

and therefore, have more health events 

in the lower part of the air quality 

distribution. Because of the larger 

cohort sizes and having a larger portion 

of health events that are observed across 

the air quality distribution, the 

Administrator has more confidence in 

the magnitude and significance of the 

associations in the lower parts of the air 

quality distribution for the recent, 

hybrid model-based studies compared 

to the older, monitor-based studies. 

Given this, the Administrator judges 

that it is appropriate to place weight on 

the 25th percentile concentrations 

reported in the recently available hybrid 

model-based studies in reaching his 

conclusions regarding the appropriate 

level for the primary annual PM2.5

standard. However, the Administrator 

also recognizes that his confidence in 

the magnitude and significance in the 

reported concentrations, and their 

ability to inform decisions on the 

appropriate level of the annual 

standard, starts to diminish at 

percentiles that are even further below 

the mean and the 25th percentile. For 

these reasons, the Administrator places 

weight on the reported 25th percentiles 

concentrations in the recent hybrid 

model-based studies, rather than the 

reported 10th percentile concentrations, 

in reaching his conclusions regarding 

the appropriate level for the primary 

annual PM2.5 standard. 

In considering the information from 

these studies, as described in section 

II.A.2.c and in responding to public 

comments in section II.B.3 above, the 

Administrator notes that there are two 

hybrid model-based studies with large 

cohort sizes that apply population 

weighting and report lower percentile 

values. These studies are Di et al. 

(2017b) and Wang et al. (2017) and the 

reported 25th percentile concentration 

is 9.1 mg/m3 for both studies.118 In 

considering these studies, the 

Administrator concludes that it is 

appropriate to place weight on the 25th 

percentile concentrations of these newer 

hybrid model-based studies (of 9.1 mg/ 

m3) such that setting the level of the 

standard near these 25th percentile 

concentrations would provide requisite 

protection. The Administrator observes 

that an annual standard level of 9.0 mg/ 

m3 would be near the reported 25th 

percentile concentrations in these 

studies. 

As at the time of proposal, the 

Administrator also takes note of the 

study-reported long-term mean PM2.5

concentrations in long- and short-term 

Canadian epidemiologic studies, which 

ranged from 6.9 to 13.3 mg/m3 for 

monitor-based studies and 5.9 to 9.8 mg/ 

m3 for hybrid model-based studies. 

While the Administrator notes that 

these studies provide additional support 

for associations between PM2.5

concentrations and health effects, he is 

also mindful that there are important 

differences between the exposure 

environments in the U.S. and Canada 

and that interpreting the data (e.g., 

study-reported mean concentrations) 


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from the Canadian studies in the context 

of a U.S.-based standard may present 

challenges in directly and quantitatively 

informing decisions regarding potential 

alternative levels of the annual 

standard. For example, in terms of 

people per square kilometer, the U.S. 

population density is nearly 10 times in 

the contiguous U.S. compared to 

Canada. As described in more detail in 

responding to public comments in 

section II.B.3 above, in this 

reconsideration, the Administrator 

recognizes that this difference in 

population density between the U.S. 

and Canada is more apparent than in 

previous reviews because the studies 

available in this reconsideration use 

different approaches than those 

previously available. In the 2012 review, 

the available Canadian epidemiologic 

studies used population-weighting and 

focused on urban areas where monitors 

were available and population densities 

were more comparable with those in the 

U.S., and at that time, the U.S. and 

Canadian studies reported similar mean 

PM2.5 concentrations. However, in this 

reconsideration, the Administrator takes 

note that for the new Canadian 

epidemiologic studies: (1) The Canadian 

monitor-based studies available in this 

reconsideration do not apply population 

weighting as the previously available 

studies did; and (2) some of the studies 

now use hybrid modeling approaches 

for estimating exposure. The 

Administrator recognizes that these 

differences are important to consider in 

reaching conclusions on how these 

Canadian epidemiologic studies should 

be interpreted regarding decisions on 

the requisite level of the primary annual 

PM2.5 standard. Specifically, the 

Administrator notes that the more 

recent Canadian studies that use hybrid 

modeling incorporate larger portions of 

the country, and therefore include more 

rural areas. The more rural areas that are 

included in the study using the hybrid 

modeling approaches, the more 

important it is to consider how the 

population densities and exposure 

environments differ between the U.S. 

and Canada. Additionally, the 

Administrator notes that for hybrid 

modeling-based studies there is less 

certainty in PM2.5 exposure estimates in 

more rural areas, which are further from 

air quality monitors and where PM2.5

concentrations in the ambient air tend 

to be lower. For these hybrid modelbased studies, the portion of the rural 

areas that are contributing to the studyreported mean PM2.5 concentrations in 

these studies is unclear. For these 

reasons, the Administrator concludes 

that it is important to consider the 

differences between the population 

exposures in the U.S. and Canadian 

study areas and how these differences 

influence the interpretation of the 

epidemiologic study results. 

Thus, the Administrator considers the 

Canadian studies to inform his 

judgments on what level for the annual 

standard is requisite in light of the 

limitations and challenges presented. 

The Administrator also recognizes that 

the majority of the CASAC in their 

review of the 2021 draft PA, as well as 

a number of public commenters, place 

weight on the Canadian epidemiologic 

studies in recommending that the level 

of the primary annual PM2.5 standard be 

revised to 8–10 mg/m3. The 

Administrator further notes while the 

majority of the CASAC advised the EPA 

to consider the Canadian studies in 

revising the annual standard level to 

within the range of 8.0–10.0 mg/m3, they 

did not advise the EPA to set the annual 

standard level below the study-reported 

means from those studies. Given these 

considerations, the Administrator 

judges that it is appropriate to set the 

level of annual standard within the 

range of 8–10 mg/m3 to be consistent 

with the majority of the CASAC’s advice 

in their consideration of these studies. 

The Administrator also recognizes 

that information from epidemiologic 

studies that included analyses that 

restrict annual average PM2.5

concentrations to concentrations below 

the level of the current annual standard 

can be useful for informing conclusions 

regarding the appropriate level of the 

primary annual PM2.5 standard. In so 

doing, he particularly notes the two key 

U.S. epidemiologic studies (Di et al., 

2017b and Dominici et al., 2019) that 

restrict annual average PM2.5

concentrations to less than 12 mg/m3

and report positive and statistically 

significant associations with all-cause 

mortality and mean PM2.5

concentrations of 9.6 mg/m3. He also 

considers these results along with the 

uncertainties and limitations associated 

with studies that restricted analyses 

below certain PM2.5 concentrations. As 

described in responding to comments in 

section II.B.3 above, uncertainties 

associated with how the studies exclude 

PM2.5 concentrations from the analyses 

(e.g., at what spatial resolution are 

concentrations being excluded), make it 

difficult to understand how to interpret 

the results of the restricted analyses in 

the context of the approach employed in 

this reconsideration, which takes into 

consideration the relationship between 

mean PM2.5 concentrations and design 

values. 

The Administrator also recognizes 

that, in their review of the 2021 draft 

PA, the CASAC noted that 

epidemiologic studies that restrict 

analyses below certain PM2.5

concentrations represent one area for 

which the evidence has expanded in 

this reconsideration, stating that these 

studies provide support for mortality 

effects at concentrations below the 

current PM NAAQS (Sheppard, 2022a, 

p. 5 of consensus responses). In their 

recommendations on alternative levels 

for the primary annual PM2.5 standard, 

the majority of the CASAC cited to 

studies that restrict PM2.5 concentrations 

to below 12 mg/m3 as a part of their 

rationale for supporting a level within 

the range of 8–10 mg/m3 (Sheppard, 

2022a p. 16 of consensus responses). 

Additionally, the Administrator notes 

that some members of the CASAC, in 

their review of the 2019 draft PA, 

concluded that the epidemiologic 

studies that restrict analyses below 12 

mg/m3 and show positive associations 

with health effects, along with other 

aspects of the scientific evidence, 

provide support for their conclusion 

that the primary annual PM2.5 standard 

is not adequate (Cox, 2019b, p. 9 of 

consensus responses). Furthermore, the 

Administrator takes note of public 

commenters who also noted that the 

epidemiologic studies that restrict PM2.5

concentrations to below the current 

standard provide support, along with 

the other available information, for 

lowering the level of the primary annual 

PM2.5 standard. In considering the 

studies that include restricted analyses, 

along with the CASAC’s advice and 

public comments on these types of 

studies, the Administrator concludes 

that, although there are inherent 

uncertainties associated with this 

limited body of evidence, these studies 

that apply restricted analyses provide 

support for serious effects (e.g., 

mortality) at concentrations below 10.0 

mg/m3. Given this, the Administrator 

concludes that it is appropriate to place 

some weight on these studies, and in 

doing so, notes that a standard level of 

9.0 mg/m3 would be below the reported 

mean PM2.5 concentrations of 9.6 mg/m3

in these studies and would, thus, be 

expected to provide protection against 

exposures related to these reported 

mean concentrations. 

The Administrator also takes into 

consideration recent U.S. accountability 

studies, which assess the health effects 

associated with actions that improve air 

quality (e.g., air quality policies or 

implementation of an intervention). 

These types of studies can also reduce 

uncertainties related to residual 

confounding of temporal and spatial 

factors (U.S. EPA, 2022a, p. 3–25). The 


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Administrator notes that in the 2020 

review, the available accountability 

studies had ‘‘starting’’ annual average 

PM2.5 concentrations (i.e., mean 

concentration prior to reductions being 

evaluated) from 13.2–31.5 mg/m3, and 

the then-Administrator cited the lack of 

accountability studies in areas where 

the ‘‘starting’’ concentration met the 

current primary PM2.5 standards as part 

of his rationale for retaining the 

standards. As at the time of proposal, 

the current Administrator notes that in 

three studies newly available in this 

reconsideration and assessed in the ISA 

Supplement, prior to implementation of 

the policies, mean PM2.5 concentrations 

in these studies were below the level of 

the current annual standard level (12.0 

mg/m3) and ranged from 10.0 mg/m3 to 

11.1 mg/m3. These studies report 

positive and significant associations 

between mortality and cardiovascular 

morbidity and reductions in ambient 

PM2.5 following the implementation of a 

policy (Henneman et al., 2019; Corrigan 

et al., 2018; Sanders et al., 2020a; 88 FR 

5627, January 27, 2023). These studies 

suggest that public health improvements 

may occur following the 

implementation of a policy that reduces 

annual average PM2.5 concentrations 

below the level of the current standard 

of 12.0 mg/m3. The Administrator 

recognizes that in their review of the 

2021 draft PA, the CASAC noted that 

the availability of recent accountability 

studies was one area where the evidence 

had been strengthened and that the 

studies assessed in the ISA Supplement 

provide evidence of mortality effects at 

annual average PM2.5 concentrations 

below the current NAAQS (Sheppard, 

2022a, p. 5 of consensus responses). The 

Administrator recognizes that the 

CASAC also concluded that, along with 

other lines of evidence, the 

accountability studies with starting 

concentrations below the levels of the 

current standards are appropriate to 

consider for informing conclusions on 

alternative standard levels (Sheppard, 

2022a, p. 13 of consensus responses). 

The Administrator also notes the advice 

of the CASAC in their review of the 

2019 draft ISA, where they suggested 

that accountability studies be taken into 

account and such studies provide 

potentially crucial information about 

whether and how much decreasing 

PM2.5 causes decreases in future health 

effects, which reflects the primary 

purpose of the NAAQS (Cox, 2019b, p. 

8 and 10 of consensus responses). The 

Administrator also notes that in their 

review of the 2019 draft ISA, some 

members of the CASAC cautioned 

against placing more weight on the data 

from accountability studies based on the 

methodological limitations of the 

studies (Cox, 2019b, p. 8 of consensus 

responses). The Administrator notes 

that the CASAC did not explicitly cite 

to accountability studies in their 

reviews of the 2019 draft PA or 2021 

draft PA as support for their 

recommendations on the adequacy of 

the primary annual PM2.5 standard or 

potential alternative standard levels. A 

number of public commenters who 

support revising the level of the 

standard to 8 mg/m3 cite these 

accountability studies, along with the 

broader evidence base, as support for a 

more protective standard. The 

Administrator, in considering the 

evidence, the advice from the CASAC, 

and public comment, first recognizes 

that accountability studies are just one 

line of evidence to be considered in the 

broader evaluations of the information 

available to inform conclusions on the 

level of the standard. In so doing, he 

notes that public health improvements 

may occur following the 

implementation of a policy that reduces 

annual average PM2.5 concentrations 

below the level of the current standard 

of 12.0 mg/m3, and potentially below the 

lowest ‘‘starting’’ concentrations in 

these studies of 10.0 mg/m3. However, 

the Administrator concludes that the 

limited number of accountability 

studies provide limited information for 

informing decisions on the appropriate 

level of the primary annual PM2.5

standard but recognizes that these 

studies provide supplemental 

information for consideration along 

with the full body of evidence. Taken 

together, the Administrator notes a 

revised annual standard level of 9.0 mg/ 

m3 is at or below the lowest starting 

concentration of these accountability 

studies (i.e., 10.0 mg/m3), and judges 

that it is appropriate to place some 

weight on these studies, particularly for 

informing his public policy judgments 

regarding an adequate margin of safety. 

In addition to his consideration of and 

conclusions regarding the available 

scientific evidence, the Administrator 

also considers the results of the 

quantitative risk assessment to inform 

his conclusions regarding the 

appropriate level for the primary annual 

PM2.5 standard. The Administrator 

recognizes that the risk estimates can 

help to place the evidence for specific 

health effects into a broader public 

health context, but should be 

considered along with the inherent 

uncertainties and limitations of such 

analyses when informing judgments 

about the potential for additional public 

health protection associated with PM2.5

exposure and related health effects. The 

Administrator recognizes that the 

overall risk assessment estimates 

suggest that the current primary annual 

PM2.5 standard could allow a substantial 

number of PM2.5-associated deaths in 

the U.S. The Administrator also 

recognizes that the CASAC concurred 

with the 2021 draft PA’s assessment that 

meaningful risk reductions will result 

from lowering the annual PM2.5

standard (Sheppard, 2022a, p. 16 of 

consensus responses). 

Additionally, with respect to the 

results of the quantitative risk 

assessment, the Administrator 

recognizes that the 2022 PA also 

provides information on the distribution 

of concentrations associated with the 

estimated mortality risk at each 

alternative standard level assessed (U.S. 

EPA, 2022b, sections 3.4.2.2 and 3.6.2.2, 

Figure 3–18 and 3–19). When meeting 

an annual standard of 9.0 mg/m3 at the 

design value monitor, the exposure 

concentrations within an area are 

estimated to be below 9 mg/m3, with the 

majority of those exposures being at 

concentrations of below 8 mg/m3. The 

Administrator notes that this range of 

concentrations is below the lowest 

means in the key long- and short-term 

epidemiologic studies (concentrations at 

which the evidence is the strongest in 

supporting an association between 

exposure to PM2.5 and adverse health 

effects observed in the key 

epidemiologic studies available in this 

reconsideration). Thus, the 

Administrator concludes that the results 

of the quantitative risk assessment 

suggest that a revised annual standard 

level of 9.0 mg/m3 is estimated to reduce 

PM2.5 exposures to fall within the range 

of concentrations in which there is the 

most confidence in the associations and 

thus, confidence that estimated risk 

reductions will actually occur. 

The Administrator also notes the 

information provided by the 

quantitative risk assessment on the 

distribution of concentrations associated 

with the estimated mortality risk for a 

higher annual standard level of 10.0 mg/ 

m3 and a lower standard level of 8.0 mg/ 

m3 (U.S. EPA, 2022b, sections 3.4.2.2 

and 3.6.2.2, Figure 3–18 and 3–19). The 

Administrator finds that, for an annual 

standard level of 10.0 mg/m3, the 

quantitative risk assessment estimates 

that the standard would allow multiple 

exposures at concentrations above the 

lowest means in the key epidemiologic 

studies, and therefore, calls into 

question whether a standard level of 

10.0 mg/m3 would provide enough 

public health protection. Additionally, 

the Administrator also finds that, for a 

lower annual standard level of 8.0 mg/ 


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119Analyses in the 2022 PA suggest that the 

highest monitored value would be expected to be 

greater than the study-reported mean values by 10– 

20% for monitor-based studies and 15–18% for 

hybrid modeling studies that apply aspects of 

population weighting (U.S. EPA, 2022b, section 

2.3.3.2.4). 

120The risk assessment in the 2022 PA used air 

quality adjustments to simulate just meeting the 

current primary PM2.5 standards, as well as 

alternative standard levels (U.S. EPA, 2022b, 

section 3.4.1.4 and Appendix C, section C.1.4). 

m3, the quantitative risk assessment 

estimates the exposure concentrations to 

be below 8 mg/m3, with the majority of 

those exposures being at concentrations 

of below 7 mg/m3. The Administrator 

observes that the majority of exposure 

concentrations under this air quality 

scenario are estimated to fall outside of 

the range of concentrations in which he 

has the most confidence in the 

associations and that the additional risk 

reductions will actually occur. 

Recognizing and building upon the 

above considerations and judgments, 

and with consideration of advice from 

the CASAC and public comment, the 

Administrator concludes that the 

current body of scientific evidence and 

quantitative risk assessment support his 

judgment that the level of the primary 

annual PM2.5 standard should be revised 

to a level of 9.0 mg/m3. Revising the 

level of the primary annual PM2.5

standard will, in the Administrator’s 

judgment, provide requisite public 

health protection with an adequate 

margin of safety. 

The Administrator recognizes that 

placing weight on the information from 

the epidemiologic studies allows for 

examination of the entire population, 

including those that may be at 

comparatively higher risk of 

experiencing a PM2.5-related health 

effects (e.g., children, older adults, 

minority populations) (88 FR 5624, 

January 27, 2023). In considering the 

epidemiologic evidence, the 

Administrator judges that, in reaching 

his decision on an appropriate level for 

the annual standard that will protect 

public health with an adequate margin 

of safety, in the absence of any 

discernible population-level thresholds, 

and in recognizing the need to weigh 

uncertainties associated with the 

epidemiologic evidence, it is most 

appropriate to examine where the 

evidence of associations observed in the 

epidemiologic studies is strongest and, 

conversely, to place less weight where 

he has less confidence in the 

associations observed in the 

epidemiologic studies. The 

Administrator notes that in previous 

reviews, evidence-based approaches 

noted that the evidence of an 

association in any epidemiologic study 

is ‘‘strongest at and around the longterm average where the data in the study 

are most concentrated’’ (78 FR 3140, 

January 15, 2013). These approaches 

were supported by previous CASAC 

advice as well as the CASAC’s advice in 

their review of the 2021 draft PA as a 

part of this reconsideration. Given this, 

the Administrator notes that in revising 

the annual PM2.5 standard to a level of 

9.0 mg/m3, he is setting the standard at 

a level below the long-term mean PM2.5

concentrations in the key long- and 

short-term epidemiologic studies, 

including the lowest study reported 

mean of 9.3 mg/m3, following an 

approach that is consistent with 

previous PM NAAQS reviews. The 

Administrator additionally notes that air 

quality analyses in the 2022 PA 

demonstrate that areas meeting a revised 

annual standard of 9.0 mg/m3 would be 

expected to shift the distribution of 

PM2.5 exposure concentrations in an 

area such that the area’s highest monitor 

would generally be at or below 9.0 mg/ 

m3 annually, and most of the resulting 

PM2.5 concentrations across the area 

would be even further below the studyreported means.119 120 Thus, a standard 

level of 9.0 mg/m3 is expected to provide 

sufficient protection not only in areas 

where the highest allowable 

concentration would be located (i.e., 

near design value monitors) but also in 

other parts of the area where PM2.5

concentrations would be expected to be 

maintained even lower. 

Furthermore, the Administrator 

recognizes the CASAC’s advice in their 

review of the 2021 draft PA, as well as 

public comments, that weight should be 

placed on study-reported PM2.5

concentrations that are somewhat below 

the mean, particularly for some of the 

newer epidemiologic studies with larger 

cohort sizes. In weighing uncertainties 

associated with using these data to 

inform a revised annual standard level, 

as well as noting the limited studies for 

which this information is available, the 

Administrator judges that some weight 

should be placed on these data, but they 

should not receive the same weight as 

the study-reported mean concentrations. 

Thus, the Administrator concludes that 

it would be appropriate to set the 

annual standard level near the 25th 

percentile PM2.5 concentrations in the 

two newer key epidemiologic studies for 

which these values were reported. In 

doing so, the Administrator notes that a 

decision to revise the annual standard to 

9.0 mg/m3 would set a level of the 

standard near and somewhat below the 

reported 25th percentile PM2.5

concentrations of 9.1 mg/m3 in these two 

more recent hybrid model-based 

studies. 

The Administrator also takes note of 

the study-reported long-term mean 

PM2.5 concentrations in the key 

Canadian epidemiologic studies. While 

the Administrator notes that these 

studies provide additional support for 

associations between PM2.5

concentrations and health effects, he is 

also mindful that there are important 

differences between the exposure 

environments in the U.S. and Canada 

that affect interpretation of the data in 

the context of informing decisions 

regarding potential alternative levels of 

the annual standard. The Administrator 

also recognizes that the majority of the 

CASAC in their review of the 2021 draft 

PA, as well as a number of public 

commenters, placed weight on the 

Canadian epidemiologic studies in 

recommending that the level of the 

primary annual PM2.5 standard be 

revised to 8–10 mg/m3. The 

Administrator notes that a decision to 

revise the annual standard to 9.0 mg/m3

would set the level of the standard 

within the range of levels recommended 

by the majority of CASAC in their 

consideration of these studies. 

Additionally, the Administrator also 

considers the information provided by 

epidemiologic studies that use restricted 

analyses, as well as accountability 

studies. With respect to the restricted 

analyses, the Administrator, in 

considering the CASAC’s advice in their 

review of the 2021 draft PA and many 

public comments on these types of 

studies, concludes that, although there 

are inherent uncertainties associated 

with this limited body of evidence, the 

studies that apply restricted analyses 

provide support for serious effects (e.g., 

mortality) at concentrations below 10.0 

mg/m3. Additionally, in considering 

accountability studies, the 

Administrator concludes that while the 

small number of these studies provide 

limited information for informing 

decisions on the appropriate level of the 

primary annual PM2.5 standard, these 

studies provide supplemental 

information for consideration along 

with the full body of evidence. The 

Administrator further notes that these 

studies suggest that public health 

improvements may occur following the 

implementation of a policy that reduces 

annual average PM2.5 concentrations 

below the level of the current standard 

of 12.0 mg/m3, and potentially below the 

lowest ‘‘starting’’ concentrations in 

these studies of 10.0 mg/m3. Taken 

together, the Administrator judges that 

it is appropriate to place some weight 

on these types of studies, particularly 

for informing his public policy 

judgments regarding an adequate margin 


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of safety, and notes that a revised 

annual standard level of 9.0 mg/m3 is 

below the lowest starting concentration 

of the accountability studies (i.e., 10.0 

mg/m3), and below the concentration at 

which studies that apply restricted 

analyses provide support for serious 

effects (i.e., 9.6 mg/m3). 

The Administrator also judges that the 

results of the quantitative risk 

assessment provide support for a 

primary annual PM2.5 standard with a 

level of 9.0 mg/m3. The results of the risk 

assessment suggest that when meeting 

an annual standard of 9.0 mg/m3, PM2.5

exposures are maintained below 9 mg/ 

m3 at the design value monitor, with the 

majority of those exposures being at 

concentrations below 8 mg/m3. Thus, the 

Administrator notes that an annual 

standard level of 9.0 mg/m3 would be 

expected to provide protection from 

exposures where he has the greatest 

confidence in the associations between 

health effects and PM2.5 exposures (i.e. 

the long-term mean PM2.5

concentrations in the key U.S. 

epidemiologic studies, of which the 

lowest is 9.3 mg/m3) and would provide 

an adequate margin of safety by 

maintaining most PM2.5 exposures even 

further below 9.0 mg/m3. 

When considering adequate margin of 

safety, the Administrator notes that in 

his decision to revise the annual 

standard level to 9.0 mg/m3, he is 

placing weight on the information from 

the epidemiologic studies which allows 

for examination of the entire 

population, including those that may be 

at comparatively higher risk of 

experiencing a PM2.5-related health 

effects (e.g., children, older adults, 

minority populations). Additionally, as 

discussed above, the Administrator also 

recognizes that setting the annual 

standard level at 9.0 mg/m3, which is 

below concentrations at which the 

evidence is the strongest in supporting 

an association between exposure to 

PM2.5 and adverse health effects 

observed in the key epidemiologic 

studies available in this reconsideration, 

would be expected to shift the 

distribution of PM2.5 exposure 

concentrations in an area such that the 

area’s highest monitor would generally 

be at or below 9.0 mg/m3 annually, and 

most of the resulting PM2.5

concentrations across the area would be 

even lower. In considering these air 

quality relationships, the Administrator 

judges that a revised annual standard 

level of 9.0 mg/m3 would provide 

requisite protection with adequate 

margin of safety, for all populations, 

including those most at-risk. 

In reaching this conclusion, the 

Administrator recognizes that in 

establishing primary standards under 

the Act that are requisite to protect 

public health with an adequate margin 

of safety, he is seeking 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 or to protect the most sensitive 

individual, but rather at a level that 

avoids unacceptable risks to public 

health. In this context, the 

Administrator’s conclusion is that 

revised primary annual standard, in 

conjunction with the 24-hour standard, 

provides the appropriate degree of 

protection, and that more or less 

stringent standards would not be 

requisite. 

In considering the requirement for an 

adequate margin of safety, the 

Administrator notes that the 

determination of what constitutes an 

adequate margin of safety is expressly 

left to the judgment of the EPA 

Administrator. See Lead Industries 

Association v. EPA, 647 F.2d at 1161– 

62; Mississippi, 744 F.3d at 1353. He 

further notes that in evaluating how 

particular standards address the 

requirement to provide an adequate 

margin of safety, it is appropriate to 

consider such factors as the nature and 

severity of the health effects, the size of 

sensitive population(s) at risk, and the 

kind and degree of the uncertainties 

present. Consistent with past practice 

and long-standing judicial precedent, 

and as described in this section, the 

Administrator takes the need for an 

adequate margin of safety into account 

as an integral part of his decision 

making on a standard. See, e.g., NRDC 

v. EPA, 902 F. 2d 962, 973–74 (D.C. Cir. 

1990). 

Given all of the evidence and 

information discussed above, the 

Administrator judges that a standard 

with a level of 9.0 mg/m3 is requisite to 

protect public health with an adequate 

margin of safety. In so doing, he first 

recognizes that a less stringent standard 

would allow the occurrence of higher 

long- and short-term PM2.5

concentrations at a level at or above the 

mean PM2.5 concentrations in key U.S. 

epidemiologic studies. That is, a less 

stringent standard would be expected to 

allow more PM2.5 exposures at 

concentrations at or above which the 

key U.S. epidemiologic studies have 

reported associations between mean 

PM2.5 concentrations and serious health 

effects and would deviate from some 

past approaches for selecting the 

appropriate level of the annual 

standard. A less stringent standard 

would also not provide requisite 

protection with an adequate margin of 

safety against PM2.5 exposures in the 

lower percentiles of the air quality 

distribution (i.e., 25th percentile) for 

which associations with health effects 

have been observed in a limited number 

of epidemiologic studies. Furthermore, 

the Administrator notes that the primary 

annual and 24-hour PM2.5 standards, 

together, are intended to provide public 

health protection against the full 

distribution of long- and short-term 

PM2.5 exposures. As noted above, the 

Administrator recognizes that the 

changes in PM2.5 air quality designed to 

meet a less stringent annual standard 

would likely result in higher exposures 

across the distribution of air quality, 

including both higher average (or 

typical) concentrations as well as higher 

short-term peak PM2.5 concentrations. 

Taking into consideration both the full 

evidence base for associations of PM2.5

with mortality and other adverse health 

effects, including the reported mean 

PM2.5 concentrations from key long- and 

short-term U.S. epidemiologic studies, 

information from epidemiologic studies 

that report 25th percentile PM2.5

concentrations, supplemental 

information from other epidemiologic 

studies (i.e., epidemiologic studies that 

use restricted analyses, accountability 

studies, and Canadian epidemiologic 

studies), and the results of the risk 

assessment, as well as the advice from 

the CASAC and public comments, the 

Administrator concludes that a less 

stringent standard would allow risks of 

mortality and other adverse health 

effects that are too great, and thus would 

not provide sufficient protection for 

public health as required by the CAA. 

Additionally, in considering a less 

stringent standard, the Administrator 

recognizes that through its control of 

long- and short-term PM2.5

concentrations, the annual standard 

provides a margin of safety for less wellstudied exposure levels and population 

groups for which the evidence is limited 

or lacking. In so doing, he recognizes 

that our understanding of the 

relationships between the presence of a 

pollutant in ambient air and associated 

health effects is based on a broad body 

of information encompassing not only 

more established aspects of the 

evidence, such as the conclusion that 

long- and short-term exposures to PM2.5

are causally related to mortality and 

cardiovascular effects and likely to be 

causally related to respiratory effects, 

but also aspects with which there may 

be substantial uncertainty. In particular, 

the Administrator notes that there are 

other categories of effects with causality 

determinations that are suggestive of, 

but not sufficient to infer, a causal 


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relationship between PM2.5 exposure 

and health outcomes. These include, but 

are not limited t,o short-term exposure 

and nervous system effects, as well as 

long- and short-term exposure and 

pregnancy and birth outcomes, where 

the evidence is less certain but which 

represent potentially substantial 

additional risk to public health from 

exposure to PM2.5. He recognizes the 

CAA requirement that requires primary 

standards to provide an adequate 

margin of safety was intended to 

address uncertainties associated with 

inconclusive scientific and technical 

information as well as to provide a 

reasonable degree of protection against 

hazards that research has not yet 

identified and in his judgment, the 

primary NAAQS must be set at a level 

that is adequately protective against 

these and other effects which research 

has not yet identified. Thus, even if the 

Administrator had somewhat greater 

concerns about the possibility of 

confounding, error and bias in the 

epidemiologic studies, which reduced 

his confidence in finding that PM2.5 is 

causally related to mortality and 

cardiovascular effects, he would still 

find it appropriate to set the primary 

NAAQS below the means of key U.S. 

epidemiologic studies given the strength 

of the evidence providing support for 

the association, as well as additional 

evidence linking PM2.5 to other 

endpoints of substantial public health 

concern, and the need to protect public 

health with an adequate margin of 

safety. In considering the uncertainties 

in both the epidemiologic evidence and 

the controlled human exposures studies, 

the Administrator recognizes that 

collectively, the 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. In light of these uncertainties, 

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 and technical 

information, as well as to provide a 

reasonable degree of protection against 

hazards that research has not yet 

identified. The Administrator has taken 

the need to provide for an adequate 

margin of safety into account as an 

integral part of his decision-making on 

the appropriate standards in setting the 

standard at a level below the level 

where available epidemiologic studies, 

which include diverse populations that 

are broadly representative of the U.S. 

population including at-risk 

populations, have provided the 

strongest evidence supporting effects, 

and in other ways as well. For example, 

consideration of a margin of safety is 

reflected in the approach of setting the 

level of the annual standard near and 

somewhat below the 25th percentile 

PM2.5 concentrations from key U.S. 

epidemiologic studies (i.e., 9.1 mg/m3), 

as well as recognition that attaining a 

design value will generally result in 

significantly broader and greater 

improvements of air quality across an 

area (including but certainly not limited 

to areas near the design value monitor) 

(U.S. EPA, 2022a, sections 2.3.3.2.4 and 

3.3.3.2.1, Table 3–5). Based on all of the 

considerations noted here, and 

considering the current body of 

evidence, including the associated 

limitations and uncertainties, in 

combination with the exposure/risk 

information, the Administrator 

concludes that a less stringent standard 

than the current standard would not 

provide the requisite protection of 

public health, including an adequate 

margin of safety. 

Having concluded that a less stringent 

standard would not provide the 

requisite protection of public health, the 

Administrator next considers whether a 

more stringent standard would be 

appropriate. In so doing, he notes that 

a decision to set the level of the annual 

standard to below 9.0 mg/m3 would 

place a large amount of the emphasis on 

potential public health importance of 

further reducing the occurrence of PM2.5

concentrations of concern, though the 

exposures about which he is most 

concerned are well controlled with an 

annual standard level of 9.0 mg/m3, as 

demonstrated by the quantitative risk 

assessment. Such a decision would also 

place greater weight on (1) further 

reducing ambient PM2.5 concentrations 

relative to those observed in long-and 

short-term epidemiologic studies, 

including those that he had judged to 

have significant uncertainties, including 

Canadian studies, studies using 

restricted analyses, and accountability 

studies; (2) shifting the air quality 

distribution in areas such that the 

highest exposure concentrations are 

reduced to below PM2.5 concentrations 

observed in epidemiologic studies to be 

in the 25th or lower percentile, for 

which the evidence is limited; and (3) 

further shifting exposure concentrations 

to those shown at the lower end of the 

distribution in the quantitative risk 

assessment, despite the important 

uncertainties in the overall risk 

assessment. As discussed in this section 

and in responses to significant 

comments above and in the Response to 

Comments document, the Administrator 

has concluded that placing a large 

emphasis on these factors and revising 

the standard to a level below 9.0 mg/m3

would result in a standard that is more 

stringent than the evidence indicates to 

be sufficient to protect public health 

with an adequate margin of safety. 

Compared to a primary annual PM2.5

standard set at a level of 9.0 mg/m3, the 

Administrator concludes that the extent 

to which lower standard levels could 

result in further public health 

improvements becomes notably less 

certain. 

Thus, having carefully considered the 

scientific evidence, quantitative 

information, CASAC advice, and public 

comments relevant to his decision on 

the level of the primary annual PM2.5

standard, as discussed above and in the 

Response to Comments document, the 

Administrator is revising the level of the 

primary annual PM2.5 standard to 9.0 

mg/m3. In the Administrator’s judgment, 

based on the currently available 

evidence and information, an annual 

standard set at this level and using the 

specified indicator, averaging time, and 

form, in conjunction with the other 

primary PM standards, would be 

requisite to protect public health with 

an adequate margin of safety. The 

Administrator judges that such a 

standard would protect, with an 

adequate margin of safety, the health of 

at-risk populations, including children, 

older adults, those with pre-existing 

cardiovascular and respiratory diseases, 

minority populations, and low SES 

populations. The Administrator believes 

that a standard set at 9.0 mg/m3 would 

be sufficient to protect public health 

with a margin of safety, and believes 

that a lower standard would be more 

than what is necessary to provide this 

degree of protection. This judgment by 

the Administrator appropriately 

considers the degree of protection that 

is neither more nor less stringent than 

necessary for this purpose and 

recognizes that the CAA does not 

require that primary standards be set at 

a zero-risk level, but rather at a level 

that reduces risk sufficiently so as to 

protect public health with an adequate 

margin of safety. 

In reaching his conclusions on 

adequacy of the current suite of primary 

PM2.5 standards, based on consideration 

of the available scientific evidence and 

quantitative information, the CASAC’s 

advice and public comments, the 

Administrator finds that the available 

information is insufficient to call into 

question the adequacy of the public 


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health protection afforded by the 

current primary 24-hour PM2.5 standard. 

As described earlier in this section, the 

Administrator concludes that it is 

appropriate to retain the current 

indicator (PM2.5), averaging time (24- 

hour), and form (98th percentile, 

averaged over three years) for the 

primary 24-hour PM2.5 standard and 

below explains the basis for his final 

decision that is also appropriate to 

retain the current level of the primary 

24-hour PM2.5 standard. 

In reaching his conclusion to retain 

the current primary 24-hour PM2.5

standard the Administrator does so in 

light of the conclusion that the 

epidemiologic evidence supports 

associations between short- and longterm PM2.5 exposures and adverse 

health effects, but that the 

epidemiologic evidence does not 

identify specific concentrations at 

which those effects occur and the 

Administrator has greatest confidence in 

effects where the bulk of the data is 

reported (i.e., the mean PM2.5

concentration, with some consideration 

for the 25th percentile of the air quality 

distribution). Thus, in considering the 

epidemiologic evidence, the 

Administrator concludes it is 

appropriate to focus on setting a 

generally controlling annual standard as 

the most effective and efficient way to 

reduce total population risk associated 

with both long- and short-term PM2.5

exposures, and that it is appropriate to 

revise the level of the annual standard 

level to 9.0 mg/m3. In addition to the 

epidemiologic evidence, the 

Administrator also considers the 

available controlled human exposure 

studies, which provide evidence for 

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

that typically correspond to upper end 

of the PM2.5 air quality distribution in 

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

so doing, the Administrator notes that 

these studies 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

and at and above 149 mg/m3 for vascular 

impairment, the effect shown to be most 

consistent across studies. In particular, 

the Administrator notes that a single 

study is assessed in the ISA Supplement 

that reports effects following 4-hour 

exposures at 37.8 mg/m3, although the 

results of this study are inconsistent 

with the results of the controlled human 

exposure studies assessed in the 2019 

ISA. Along with the inconsistent results 

from the controlled human exposure 

studies, the Administrator also 

recognizes that effects observed in these 

studies are intermediate effects which 

are not typically considered adverse and 

that the study participants were healthy 

individuals. Taking into consideration 

the available scientific evidence, 

including the uncertainties and 

limitations, along with the CASAC’s 

advice, the Administrator concludes 

that it is appropriate to maintain a 

primary 24-hour PM2.5 standard to 

protect against peak exposures. 

Thus, the Administrator considers 

what primary 24-hour PM2.5 standard is 

requisite to provide supplemental 

protection against peak exposures. 

While having confidence that the 

revised annual standard will result in 

lowering risk associated with both longand short-term PM2.5 exposure by 

lowering the overall air quality 

distribution, as in the 2012 review, the 

Administrator recognizes that an annual 

standard alone would not be expected to 

offer sufficient protection with an 

adequate margin of safety against the 

effects of short-term PM2.5 exposures in 

all parts of the country. Therefore, he 

continues to conclude that it is 

appropriate to continue to provide 

supplemental protection by means of a 

24-hour standard, in conjunction with a 

revised annual standard level of 9.0 mg/ 

m3. 

In considering the available scientific 

evidence assessed in the 2019 ISA and 

ISA Supplement, the Administrator first 

considers the controlled human 

exposure studies for informing his 

decisions on the primary 24-hour PM2.5

standard. In so doing, he notes that in 

their review of the 2021 draft PA, the 

majority of CASAC members expressed 

the view that controlled human 

exposure studies are not the best 

evidence to use for justifying retaining 

the 24-hour standard without revision, 

in part because these studies 

preferentially recruit less susceptible 

individuals and have a typical exposure 

duration much shorter than 24 hours. 

Thus, in the view of the majority, ‘‘the 

evidence of effects from controlled 

human exposure studies with exposures 

close to the current 24-hour standard 

supports epidemiological evidence for 

lowering the standard’’ (Sheppard, 

2022a, p. 3–4 of consensus letter). In 

reviewing the controlled human 

exposure studies, the Administrator 

agrees with the majority of CASAC that 

these controlled human exposure 

studies generally do not include 

populations with substantially 

increased risk from exposure to PM2.5, 

such as children, older adults, or those 

with more severe underlying illness. 

However, he disagrees with any 

conclusion that they should not be used 

to inform a decision about the adequacy 

of the current standard. The 

Administrator finds the information 

available from these studies to be useful, 

noting that the recently available 

controlled human exposure studies 

provide evidence for health effects 

following single, short-term exposures 

to PM2.5 concentrations that are greater 

than those allowed under the current 

standard. The results of the controlled 

human exposure studies are 

inconsistent, particularly at lower PM2.5

concentrations, but some studies do 

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 (and at and above 149 mg/m3 for 

vascular impairment, the effect shown 

to be most consistent across studies). 

Additionally, one controlled human 

exposure study assessed in the ISA 

Supplement reports evidence of some 

effects for cardiovascular markers 

following 4-hour exposures to 37.8 mg/ 

m3 (Wyatt et al., 2020). However, there 

is inconsistent evidence for 

inflammation in other controlled human 

exposure studies evaluated in the 2019 

ISA. The Administrator finds these 

studies are important in establishing 

biological plausibility for PM2.5

exposures causing more serious health 

effects, such as those seen in short-term 

exposure epidemiologic studies, and 

they provide support that more adverse 

effects may be experienced following 

longer exposure durations and/or 

exposure to higher concentrations. As 

described in more detail in responding 

to public comments in section II.B.3 

above, he notes that although the 

controlled human exposure studies do 

not provide a threshold below which no 

effects occur, the observed effects in 

these controlled human exposures 

studies are ones that signal an 

intermediate effect in the body, likely 

due to short-term exposure to PM2.5, and 

typically would not, by themselves, be 

judged as adverse. As noted in sections 

II.A.2 and II.B.3 above, associated 

judgments regarding adversity or health 

significance of measurable physiological 

responses to air pollutants in previous 

NAAQS reviews have been informed by 

guidance, criteria or interpretative 

statements developed within the public 

health community. This type of 

information on adversity of effects is 

particularly informative to the 

Administrator’s judgments regarding the 

adversity of the effects observed in the 

controlled human exposure studies 

which are short-term in nature (i.e., 

generally ranging from 2- to 5-hours), 

including those studies that are 


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121 Jones et al. (2023). Comparison of Occurrence 

of Scientifically Relevant Air Quality Observations 

Between Design Value Groups. Memorandum to the 

Rulemaking Docket for the Review of the National 

Ambient Air Quality Standards for Particulate 

Matter (EPA–HQ–OAR–2015–0072). Available at: 

https://www.regulations.gov/docket/EPA-HQ-OAR2015-0072. 

122 Jones et al. (2023). Comparison of Occurrence 

of Scientifically Relevant Air Quality Observations 

Between Design Value Groups. Memorandum to the 

Rulemaking Docket for the Review of the National 

Ambient Air Quality Standards for Particulate 

Matter (EPA–HQ–OAR–2015–0072). Available at: 

https://www.regulations.gov/docket/EPA-HQ-OAR2015-0072. 

conducted at near-ambient PM2.5

concentrations. Based on the 

observation that the effects observed in 

Wyatt et al. (2020) are not by themselves 

adverse, and the fact that the findings of 

this study are inconsistent with other 

currently available evidence regarding 

the level at which effects are observed, 

the Administrator disagrees with the 

view expressed by the majority of 

CASAC that this study supports 

epidemiologic evidence for lowering the 

24-hour standard. 

Consistent with his approach in 

reaching his proposed decision and 

taking into consideration these points as 

well as balancing these limitations (i.e., 

that the health outcomes observed in 

these controlled human exposure 

studies are not clearly adverse and that 

the studies generally do not include 

those at increased risk from PM2.5

exposure), the Administrator still 

considers it appropriate to ensure that 

the 24-hour PM2.5 standard provides 

protection against health effects 

consistently observed in the controlled 

human exposure studies. He next 

examines the air quality analyses, 

described in more detail in section 

II.A.c.i above, to assess whether during 

recent air quality conditions, areas 

meeting the current standards would 

experience PM2.5 concentrations 

reported in these controlled human 

exposure studies. He observes that air 

quality analyses demonstrate that the 

PM2.5 exposures shown to cause 

consistent effects in the controlled 

human exposure studies are well above 

the ambient concentrations typically 

measured in locations meeting the 

current primary standards, and therefore 

suggest that the current primary PM2.5

standards provide protection against 

these ‘‘peak’’ concentrations. In fact, at 

air quality monitoring sites meeting the 

current primary PM2.5 standards (i.e., 

the 24-hour standard of 35 mg/m3 and 

the annual standard of 12 mg/m3), the 2- 

hour concentrations generally remain 

below 10 mg/m3, and rarely exceed 30 

mg/m3. Though two-hour concentrations 

are higher at monitoring sites violating 

the current standards, they generally 

remain below 16 mg/m3 and rarely 

exceed 80 mg/m3, still below 

concentrations in CHE studies where 

consistent effects are observed (e.g., 

greater than 120 mg/m3) (U.S. EPA, 

2022b, section 2.3.2.2.3, Figure 2–19, 

and section 3.3.3.1). Additionally, and 

in response to public comments, the 

Administrator notes additional air 

quality analyses conducted by the 

EPA,121 that provide a more refined 

analysis of whether areas that meet the 

current standards experience peak 

concentrations reported in controlled 

human exposure studies. He notes that 

2-hour observations greater than 120 mg/ 

m3 and 4-hour observations greater than 

38 mg/m3 rarely occur (e.g., 0.025% of 

rolling 2-hour observations are greater 

than 120 mg/m3 and 0.78% of rolling 4- 

hour observations greater than 38 mg/ 

m3). Based on this information, the 

Administrator finds that the current 

suite of standards maintains subdaily 

concentrations of PM2.5 in ambient air 

far below the exposure concentrations 

in controlled human exposure studies 

where consistent effects have been 

observed, and notes that while these 

studies generally do not include the 

most at-risk individuals, the exposure 

concentrations in these studies also do 

not elicit adverse effects. 

Further, in light of the 

Administrator’s emphasis on the annual 

standard as the controlling standard, 

with the 24-hour standard providing 

supplemental protection against peak 

concentrations, he next considers the 

potential impact of a revised annual 

standard of 9.0 mg/m3 on the occurrence 

of peak sub-daily PM2.5 concentrations. 

Specifically, the Administrator takes 

note of the new air quality analyses 122

where he observes that lower 

percentages of concentrations greater 

than 120 mg/m3 and 38 mg/m3 occur in 

areas meeting an annual standard of 9.0 

mg/m3 and a 24-hour standard of 35 mg/ 

m3, versus an annual standard of 12.0 

mg/m3 and a 24-hour standard of 35 mg/ 

m3. Thus, he concludes that an annual 

standard that is controlling across most 

areas of the country will continue to 

effectively limit peak daily 

concentrations in conjunction with the 

existing 24-hour standard, with its level 

of 35 mg/m3 and 98th percentile form, 

which continues to provide 

supplemental protection against peak 

concentrations. 

In addition, the Administrator also 

notes that the majority of the CASAC in 

their review of the 2021 draft PA, as 

well as a number of public commenters, 

support their recommendation to revise 

the current 24-hour standard by 

pointing to ‘‘substantial epidemiologic 

evidence from both morbidity and 

mortality studies’’ which ‘‘includes 

three U.S. air pollution studies with 

analyses restricted to 24-hour 

concentrations below 25 mg/m3’’ 

(Sheppard, 2022a, p. 17 consensus 

responses). The Administrator notes 

that the epidemiologic evidence 

available in this reconsideration, 

including the studies that restrict shortterm PM2.5 exposures (i.e., 24-hour 

PM2.5 concentrations) to levels below 25 

mg/m3, provides support for positive and 

statistically significant associations 

between short-term exposure to PM2.5

and all-cause mortality (Di et al., 2017a) 

and CVD hospital admissions (deSouza 

et al., 2021; Di et al., 2017a). He agrees 

that these studies help to provide 

additional support for reaching 

conclusions on causality in the 2019 

ISA. He further agrees that the available 

epidemiologic studies provide 

important information that it is 

appropriate to consider in this 

reconsideration, including information 

on associations between health effects 

and PM2.5 exposures in diverse 

populations that are broadly 

representative of the U.S. population, 

and include populations identified as 

at-risk (e.g., older adults, minority 

populations), as well as evidence of 

linear, no-threshold concentrationresponse relationships in those 

associations, although with less 

certainty in the shape of the curve at 

long-term average concentrations below 

about 8 mg/m3. 

However, the Administrator also 

notes significant limitations in the 

currently available epidemiologic 

information that limit his ability to draw 

conclusions from the key short-term 

studies, including those that employ 

restricted analyses, to inform his 

decision regarding the level of the 24- 

hour PM2.5 standard. As a result of these 

limitations, the Administrator does not 

find that the short-term epidemiologic 

studies, or the other evidence such as 

the controlled human exposure studies 

or the risk assessment, provide a 

sufficient justification for revising the 

24-hour standard. 

First, he notes that short-term 

epidemiologic studies examine 

associations between day-to-day 

variations in PM2.5 concentrations and 

health outcomes, often over multi-year 

study periods. As such, these studies 

report long-term mean 24-hour PM2.5

concentrations (e.g., mean 24-hour 

PM2.5 concentrations over multi-year 

study periods), rather than at specific 

points in the distribution (i.e., 90th or 

98th percentile 24-hour concentrations) 

at which effects occur. Further, he notes 


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123The annual mean is calculated by averaging 

daily values in a calendar quarter and then 

averaging calendar quarters. See 40 CFR part 50 

Appendix N, section 4.4. 

124These studies do not report information about 

the distribution of the health events and PM2.5

concentrations (e.g., means, medians, other 

percentiles) in the restricted analyses. 

that while there can be considerable 

variability in daily exposures over a 

multi-year study period, the bulk of the 

observations reflect days with ambient 

PM2.5 concentrations in the middle of 

the air quality distribution (i.e., 

‘‘typical’’ days rather than days with 

extremely low or extremely high 

concentrations). As a result, the results 

of these studies are more directly 

applicable to decisions regarding the 

annual standard (which is based on the 

long-term mean of both short- and longterm epidemiologic studies), and the 

fact that they do not report other air 

quality statistics, such as the 98th 

percentile concentrations which might 

be more directly compared to the level 

of the 24-hour standard, makes them 

less useful for informing decisions on 

the 24-hour standard. As discussed in 

responding to comments above, the 

form of the annual standard is based on 

the annual mean PM2.5 concentration 

averaged over three years,123 which 

makes it better suited as a basis for 

controlling air quality to avoid effects 

observed in both long-term and shortterm epidemiologic studies. By contrast, 

the form of the 24-hour standard is the 

98th percentile averaged over three 

years, which makes it appropriate for 

controlling short-term peak 

concentrations. However, based on the 

available air quality information, 

including distribution statistics of PM2.5

concentrations and health events 

reported in the short-term 

epidemiologic studies, these studies are 

too limited in their ability to identify 

health effects attributable to specific 

short-term peak concentrations that are 

necessary to evaluate whether the 24- 

hour standard with its 98th percentile 

form should be revised (e.g., restricted 

epidemiologic studies do not report the 

number or the percentile of health 

events or the percentile of PM2.5

concentrations across the highest part of 

the restricted air quality distribution, 

including the 98th percentile). Thus, the 

Administrator does not consider it 

appropriate to use the reported means 

from short-term studies to determine the 

appropriate level for a 24-hour standard 

with a 98th percentile form. 

Similarly, the Administrator does not 

consider the results of the restricted 

analyses to be well suited to informing 

the choice of level for a 24-hour 

standard. Restricted analyses use a 

subset of data from their main analyses 

to evaluate health events that occur at 

concentrations below a certain 

concentration (e.g., 25 mg/m3). The 

Administrator notes that the 

associations between the health effects 

(e.g., mortality and cardiovascular 

morbidity) and PM2.5 concentrations 

remain even after excluding higher 

concentrations in the restricted 

analyses, and he also recognizes that the 

magnitude of the effect is generally 

greater in the restricted analyses 

compared to the associations reported in 

the main analysis. He considers such 

analyses to be informative in indicating 

that the health effects association 

reported in the main (unrestricted) 

analysis are not driven only by the 

upper peaks of the PM2.5 air quality 

distribution, but rather persist at lower 

portions of the distribution (consistent 

with his emphasis on the annual 

standard, which is focused on exposures 

near the mean concentration, where the 

bulk of the exposure distribution is 

concentrated). Indeed, he notes that if 

peak concentrations were the principal 

driver of health effects associated with 

PM2.5 exposure, one might expect the 

associations to become weaker as the 

upper portion of the data is excluded in 

the restricted analyses, which is not 

what is reported by the analyses (e.g., 

the restricted analyses generally report 

associations that are greater in 

magnitude compared to the main 

analyses). However, he disagrees with 

the assertion by the CASAC in their 

review of the 2021 draft PA and some 

public commenters that it would be 

appropriate to focus on the specific 

PM2.5 concentration (e.g., 25 or 30 mg/ 

m3) at which the analysis was restricted 

as the basis for choosing a 24-hour 

standard level. The Administrator 

recognizes that in restricted analyses, 

while an association continues to persist 

across the full range of the air quality 

distribution, and that the cutpoint 

concentration at which the analysis was 

restricted (e.g., 25 or 30 mg/m3) becomes 

the maximum PM2.5 concentration in 

the distribution, he also notes that these 

studies do not provide information 

related to the distribution of health 

events and PM2.5 concentrations, and as 

such, he is more uncertain where the 

bulk of the data are and where he has 

confidence in the reported 

association.124 He notes that no 

evidence exists to support a conclusion 

that the PM2.5 concentration chosen as 

the cutpoint in a restricted analysis has 

any bearing on the concentration at 

which effects are likely to occur (or not 

occur). He notes that, as with long-term 

studies, the evidence does not suggest 

there is a specific point in the air quality 

distribution of these short-term studies 

that represents a ‘‘bright line’’ at and 

above which effects have been observed 

and below which effects have not been 

observed. In order to identify a level of 

the 24-hour standard based on 

associations between the ‘‘upper end’’ of 

exposures, either in the unrestricted or 

the restricted analyses, and adverse 

health effects, it would be necessary to 

have a better understanding of how 

specific 24-hour concentrations 

correspond to the frequency and total 

number of observed health events in the 

study. Currently, such information, 

including 98th percentile statistics, are 

not reported in the key short-term 

epidemiologic studies (and if they were 

reported, the Administrator would have 

to carefully consider how to weigh the 

data). As such, in reaching his decision 

on the primary 24-hour PM2.5 standard, 

the Administrator judges that the 

currently available information from 

short-term epidemiologic studies, 

including those that employ restricted 

analyses, does not provide a sufficient 

basis to revise the current 24-hour 

standard, given that the 24-hour 

standard focuses on reducing ‘‘peak’’ 

exposures (with its 98th percentile 

form), but rather that such information 

supports his judgment that it is 

appropriate to focus on revising the 

annual standard for purposes of 

reducing all exposures, across the entire 

distribution of air quality, to increase 

public health protection. 

In reaching final decisions regarding 

the adequacy of the primary 24-hour 

PM2.5 standard, the Administrator 

continues to view an approach that 

focuses on setting a generally 

controlling annual standard as the most 

effective and efficient way to reduce 

total population risk associated with 

both long- and short-term PM2.5

exposures. Additionally, he emphasizes 

that improvements in air quality 

associated with meeting an annual 

standard level of 9.0 mg/m3 will result 

in lowering risk associated with both 

long- and short-term PM2.5 exposure by 

lowering the overall air quality 

distribution. The Administrator 

concludes that reducing the annual 

standard is the most efficient way to 

reduce the risks from short-term 

exposures identified in the 

epidemiologic studies, as the available 

evidence suggests the bulk of the risk 

comes from the large number of days 

across the bulk of the air quality 

distribution, not the relatively small 

number of days with peak 

concentrations. However, as in the 2012 


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125 Jones et al. (2023). Comparison of Occurrence 

of Scientifically Relevant Air Quality Observations 

Between Design Value Groups. Memorandum to the 

Rulemaking Docket for the Review of the National 

Ambient Air Quality Standards for Particulate 

Matter (EPA–HQ–OAR–2015–0072). Available at: 

https://www.regulations.gov/docket/EPA-HQ-OAR2015-0072. 

review, the Administrator recognizes 

that an annual standard alone would not 

be expected to offer sufficient protection 

with an adequate margin of safety 

against the effects of short-term PM2.5

exposures in all parts of the country and 

concludes that, in conjunction with a 

revised annual standard level of 9.0 mg/ 

m3, it is appropriate to continue to 

provide supplemental protection by 

means of a 24-hour standard, 

particularly for areas with high peak-tomean ratios possibly associated with 

strong local or seasonal sources. 

In selecting the level of a 24-hour 

standard designed to provide 

supplemental protection against peak 

exposures (in conjunction with a 

revised annual standard of 9.0 mg/m3), 

the Administrator considers the 

information from the controlled human 

exposure studies and the EPA’s analysis 

of peak concentrations observed in areas 

meeting the current standard of 35 mg/ 

m3 in conjunction with a revised 

standard of 9.0 mg/m3 to be of particular 

relevance. He notes the controlled 

human exposure evidence includes 

studies reporting effects on one or more 

indicators of cardiovascular function 

following 2-hour exposures at and above 

120 mg/m3, including effects reported at 

and above 149 mg/m3 for vascular 

impairment, the effect shown to be most 

consistent across studies, and less 

consistent effects at lower 

concentrations, including a single study 

at near ambient concentrations (Wyatt et 

al., 2020) reporting effects following 4- 

hour exposures at 37.8 mg/m3. He 

recognizes that the effects observed (in 

those studies that observed effects) are 

ones that signal an intermediate effect in 

the body, likely due to short-term 

exposure to PM2.5, and typically would 

not, by themselves, be judged as 

adverse, and the study participants were 

healthy individuals. 

He notes in particular that, in the 

EPA’s analysis, in areas meeting the 

current 24-hour standard and the 

revised annual standard 0.029 percent 

of 2-hour observations and 0.41 percent 

of 4-hour observations reach PM2.5

concentrations higher than 120 mg/m3

and 37.8 mg/m3, respectively. He also 

notes the lack of evidence of effects 

from controlled human exposure studies 

at levels below the current 24-hour 

standard and the fact that the results of 

Wyatt et al. (2020) are inconsistent with 

other available studies, as well as the 

intermediate nature of effects observed 

in this study. In his judgment, the small 

number of occurrences of peak 

exposures indicate that, in conjunction 

with a revised annual standard of 9.0 

mg/m3, the current 24-hour standard of 

35 mg/m3 remains requisite to protect 

public health with an adequate margin 

of safety, and that there is substantial 

basis to doubt whether further 

improvements in public health would 

be achieved by further reducing these 

exposures. Furthermore, the 

Administrator concludes that due to the 

limitations and uncertainties outlined 

above, the information from recent 

short-term epidemiologic studies, 

including those that use restricted 

analyses, is inadequate to inform 

decisions regarding the adequacy of the 

current 24-hour standard. Thus, in 

reaching his decision on the primary 24- 

hour PM2.5 standard, the Administrator 

concludes that currently available 

evidence does not call into question the 

adequacy of the current standard. 

In addition to the scientific evidence, 

the Administrator also considers the 

risk assessment in evaluating the 

appropriate level of the 24-hour PM2.5

standard. The risk assessment indicates 

that the annual standard is the 

controlling standard across most of the 

urban study areas evaluated (i.e., when 

air quality related to the annual average 

PM2.5 concentrations decrease, daily 

average PM2.5 concentrations are also 

expected to decrease). When air quality 

is adjusted to just meet an alternative 

24-hour standard level of 30 mg/m3 in 

the areas where the 24-hour standard is 

controlling, the risk assessment 

estimates reductions in PM2.5-associated 

risks across a more limited population 

and number of areas compared to when 

air quality is adjusted to simulate 

alternative levels for the annual 

standard (i.e., where the annual 

standard is controlling), and these 

predictions are largely confined to areas 

located in the western U.S., several of 

which are also likely to experience risk 

reductions upon meeting a revised 

annual standard. With respect to the 

CASAC’s advice in their review of the 

2021 draft PA, the Administrator notes 

that the minority of CASAC advised that 

these results suggest that the annual 

standard can be used to limit both longand short-term PM2.5 concentrations and 

views these risk assessment results as 

supporting the conclusion that the 

current 24-hour standard is adequate 

(Sheppard, 2022a, p. 4 of consensus 

letter). In contrast, the majority of 

CASAC members in their review of the 

2021 draft PA, as well as a number of 

public commenters that support 

revision of the 24-hour standard, placed 

greater weight on the evidence-based 

considerations (e.g. scientific evidence, 

like the restricted analyses) than on the 

values estimated by the risk assessment, 

noting the potential for uncertainties in 

how the risk assessment was able to 

‘‘capture areas with wintertime 

stagnation and residential wood-burning 

where the annual standard is less likely 

to be protective’’ (Sheppard, 2022a, p. 4 

of consensus letter). 

In considering the application of the 

risk assessment to judgments about the 

adequacy of the current primary 24-hour 

PM2.5 standard, the Administrator again 

notes that the risk assessment analyses 

of PM2.5-attributable mortality use input 

data that include C–R functions from 

epidemiologic studies that have no 

threshold and a linear C–R relationship 

down to zero, as well an air quality 

adjustment approach that incorporates 

proportional decreases in PM2.5

concentrations to meet lower standard 

levels. As such, the Administrator notes 

that this quantitative approach does not 

incorporate any elements of uncertainty 

in associations of health effects at lower 

concentrations and that simulated air 

quality improvements will always lead 

to proportional decreases in risk (i.e., 

each additional mg/m3 reduction 

produces additional benefits with no 

clear stopping point at any PM2.5

concentration). Therefore, the 

Administrator recognizes that while the 

risk estimates can help to place the 

evidence for specific health effects into 

a broader public health context, the 

results should be considered along with 

the inherent uncertainties and 

limitations of such analyses when 

informing judgments about the potential 

for additional public health protection 

associated with PM2.5 exposure and 

related health effects. Further, the 

Administrator notes additionally that air 

quality analyses have also been 

considered in looking at the adequacy of 

the 24-hour standard in controlling peak 

PM2.5 concentrations of potential 

concern,125 and that those analyses 

included monitoring information from 

across the entire U.S., specifically 

highlighting areas with higher peak 

concentrations and including areas 

impacted by wintertime stagnation and 

residential wood-burning. Thus, while 

the risk assessment may have focused 

on a subset of areas across the U.S. 

based on the study area selection 

criteria, the Administrator is 

considering a broader set of information 

in reaching his conclusions regarding 

the appropriateness of the current 24- 


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hour standard to control peak 

concentrations. 

The Administrator also considers the 

advice from the CASAC in their reviews 

of the 2019 draft PA and 2021 draft PA. 

In their review of the 2019 draft PA, the 

CASAC ‘‘agrees with the EPA and finds 

that the available evidence does not call 

into question the adequacy of public 

health protection afforded by the 

current 24-hour PM2.5 standard and 

concurs that it be retained’’ (Cox, 2019b, 

p. 3 of letter). He also notes that in their 

review of the 2021 draft PA, the CASAC 

did not reach consensus on whether the 

current 24-hour standard is adequate, 

with the majority of the CASAC 

recommending that the 24-hour 

standard be revised and the minority of 

the CASAC recommending that the 

standard be retained. The majority of 

the CASAC members further stated that 

‘‘[t]here is also less confidence that the 

annual standard could adequately 

protect against health effects of shortterm exposures. A range of 25–30 mg/m3

for the 24-hour PM2.5 standard would be 

adequately protective’’ (Sheppard, 

2022a, p. 4 of consensus letter). The 

Administrator also acknowledges that 

some public commenters agreed with 

the majority of the CASAC in 

supporting a revision to the level of the 

24-hour standard to a range between 25– 

30 mg/m3. These commenters cite a 

number of reasons, including: (1) 

Results from controlled human 

exposure studies at near ambient 

concentrations; (2) aspects of the 

scientific evidence, including restricted 

analyses that report positive and 

significant associations below 35 mg/m3; 

and (3) quantitative risk analyses that 

show decreasing risk with decreasing 

PM2.5 concentrations. In responding to 

these comments, the Administrator 

recognizes that some commenters have 

different interpretations of the evidence, 

air quality information, and quantitative 

results from the risk assessment in this 

review and would make different 

judgments about the weight to place on 

the relative strength and limitations of 

the currently available scientific 

evidence and information and how such 

information could be used in making 

public health policy decisions on the 

24-hour standard. However, as outlined 

above, the Administrator has carefully 

considered the information available 

from controlled human exposure studies 

and short-term epidemiologic studies, 

and weighed the strengths and 

limitations of this evidence in 

formulating his decisions. Furthermore, 

as discussed above the Administrator 

has noted significant uncertainties and 

limitations inherent in the risk 

estimates, as well as noting that very 

few areas were included. In addition, he 

has given careful consideration to the 

majority of the CASAC’s advice in their 

review of the 2021 draft PA, but has 

drawn different conclusions with 

respect to how currently available 

evidence and air quality information 

inform the selection of level for the 24- 

hour primary PM2.5 standard. 

In considering the advice of the 

majority of CASAC, the Administrator 

notes that a decision to set the level of 

the 24-hour standard to below 35 mg/m3

would place a large amount of emphasis 

on the potential public health 

importance of further reducing the 

occurrence of peak PM2.5

concentrations. However, the 

Administrator concludes that there is 

insufficient basis to conclude that a 

more stringent standard to further 

reduce peak concentrations is needed or 

would benefit public health. As 

discussed above, he judges that the 

PM2.5 exposures in controlled human 

exposure studies that correspond to 

peak concentrations will already be well 

controlled via the combination of the 

revised annual standard, with a level of 

9.0 mg/m3, and the 24-hour standard 

with its level 35 mg/m3 and its 98th 

percentile form. Taking into 

consideration the inconsistent results 

reported in controlled human exposure 

studies, the intermediate nature of the 

health effects observed in the controlled 

human exposure studies that are not 

typically considered adverse, the health 

status of the study participants, and 

how infrequently peak concentrations of 

potential concern are anticipated to 

occur in areas meeting the revised 

primary annual PM2.5 standard, he 

judges that the current 24-hour standard 

is requisite to protect against the effects 

reported in these studies with an 

adequate margin of safety. Likewise, he 

judges that neither the epidemiologic 

studies (including the studies that use 

restricted analyses) nor the risk 

assessment provide a sufficient basis for 

revising the 24-hour standard. As 

discussed above, the epidemiologic 

studies, including short-term studies 

and those with restricted analyses, are 

not well-suited for identifying a level for 

a 24-hour standard to address health 

effects associated with peak 

concentrations. The restricted analyses 

support the conclusion that the health 

effects associated with PM2.5 is not 

associated primarily with exposure to 

higher concentrations of the main 

analyses, but like other epidemiologic 

studies they typically report only longterm mean 24-hour concentrations (e.g., 

restricted epidemiologic studies do not 

report the number or the percentile of 

health events or the percentile of PM2.5

concentrations across the highest part of 

the restricted air quality distribution, 

including the 98th percentile) and do 

not identify any particular 

concentration within the air quality 

distribution above which effects have 

been observed and below which effects 

have not been observed. Similarly, the 

risk assessment highlights that the 

annual standard is controlling across 

much of the U.S. and is generally more 

effective at reducing risk than the 24- 

hour standard and, taking into account 

the limitations and assumptions of the 

risk assessment discussed above, does 

not provide a basis for revising the 24- 

hour standard. For the reasons 

discussed herein, the Administrator 

judges that the uncertainties as to 

whether there would be public health 

benefits from a more stringent 24-hour 

standard are too great to justify revising 

the standard. 

Thus, having carefully considered the 

scientific evidence, quantitative 

information, CASAC advice, and public 

comments, the Administrator is 

retaining the current primary 24-hour 

PM2.5 standard, with its level of to 35 

mg/m3 and its 98th percentile form. In 

the Administrator’s judgment, based on 

the currently available evidence and 

information, a 24-hour standard set at 

this level and using the specified 

indicator, averaging time, and form 

would be requisite to protect public 

health with an adequate margin of 

safety, in conjunction with the annual 

standard. As noted, in evaluating the 

adequacy of the current standards, the 

Administrator focuses on evaluating the 

public health protection afforded by the 

annual and 24-hour standards, taken 

together, against adverse health effects 

associated with long- or short-term 

PM2.5 exposures. A 24-hour standard set 

at a level of 35 mg/m3, in conjunction 

with a revised annual standard level of 

9.0 mg/m3, in the judgment of the 

Administrator, provides an appropriate 

level of public health protection, for 

both long- and short-term PM2.5

exposures. The Administrator believes 

that a 24-hour standard set at 35 mg/m3

would continue to be sufficient to 

protect public health with a margin of 

safety, and believes that a lower 

standard would be more than what is 

necessary to provide this degree of 

protection when considered in 

conjunction with a revised annual 

standard. The Administrator concludes 

the current 24-hour standard at a level 

of 35 mg/m3, in conjunction with a 

revised annual standard level of 9.0 mg/ 

m3, will provide appropriate protection 


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126 In addition to the review’s opening ‘‘call for 

information’’ (79 FR 71764, December 3, 2014), the 

2019 ISA identified and evaluated studies and 

reports that have undergone scientific peer review 

and were published or accepted for publication 

between January 1, 2009, through approximately 

January 2018 (U.S. EPA, 2019a, p. ES–2). References 

cited in the 2019 ISA, the references considered for 

inclusion but not cited, and electronic links to 

bibliographic information and abstracts can be 

found at: https://hero.epa.gov/hero/particulatematter. 

in areas in which the long-term mean 

concentrations are already relatively 

low (i.e., below 9 mg/m3) but where 

there may be elevated short-term peak 

PM2.5 concentrations, often associated 

with strong local or seasonal sources. 

This judgment by the Administrator 

appropriately considers the degree of 

protection that is neither more nor less 

stringent than necessary for this purpose 

and recognizes that the CAA does not 

require that primary standards be set at 

a zero-risk level, but rather at a level 

that reduces risk sufficiently so as to 

protect public health with an adequate 

margin of safety. 

In making this decision to retain the 

current level of the primary PM2.5 24- 

hour standard at 35 mg/m3 in 

conjunction with revising the annual 

standard level from 12.0 mg/m3 to 9.0 

mg/m3, given all of the evidence and 

information discussed above, the 

Administrator judges that the revised 

suite of primary PM2.5 standards and the 

rationale supporting these levels 

appropriately reflects consideration of 

the strength of the available evidence 

and other information and its associated 

uncertainties as well as the advice of 

CASAC and consideration of public 

comments. He additionally judges that 

this suite of primary PM2.5 standards is 

requisite to protect public health, 

including at-risk populations, with an 

adequate margin of safety from effects 

associated with long and short-term 

exposures to fine particles. This 

judgment by the Administrator 

appropriately considers the requirement 

for standards that are requisite to protect 

public health but are neither more nor 

less stringent than necessary. 

C. Decisions on the Primary PM2.5

Standards 

For the reasons discussed above, and 

taking into account the information and 

assessments presented in the 2019 ISA 

and ISA Supplement, the scientific and 

quantitative risk information in the 2022 

PA, the advice and recommendations of 

the CASAC, and public comments, the 

Administrator revises the current suite 

of primary PM2.5 standards. Specifically, 

the Administrator revises the level of 

the primary annual PM2.5 standard to 

9.0 mg/m3 while retaining its form, 

indicator and averaging time. In 

conjunction with revising the primary 

annual PM2.5 standard level to provide 

protection from effects associated with 

long- and short-term PM2.5 exposures, 

the Administrator retains the level of 35 

mg/m3 and the 98th percentile form, 

indicator and averaging time of the 

primary 24-hour PM2.5 standard to 

continue to provide supplemental 

protection for areas with high peak 

PM2.5 concentrations. The 

Administrator concludes that this suite 

of standards is requisite to protect 

public health with an adequate margin 

of safety against health effects 

potentially associated with long- and 

short-term PM2.5 exposures. 

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 January 

2018 126 and evaluated in the 2019 ISA, 

on human health effects associated with 

PM10–2.5 in ambient air. As described in 

section I above and in section 1.2 of the 

ISA Supplement, the scope of the 

updated scientific evaluation of the 

health effects evidence is based on those 

PM size fractions, exposure durations, 

and health effects category 

combinations where the 2019 ISA 

concluded a causal relationship exists 

(U.S. EPA, 2019a, U.S. EPA, 2022b).). 

Therefore, because the 2019 ISA did not 

conclude a causal relationship for 

PM10–2.5 for any exposure durations or 

health effect categories, the ISA 

Supplement does not include an 

evaluation of additional studies for 

PM10–2.5. As a result, the 2019 ISA 

continues to serve as the scientific 

foundation for assessing the adequacy of 

the primary PM10 standard in this 

reconsideration of the 2020 final 

decision (U.S. EPA, 2019a, section 1.7; 

U.S. EPA, 2022a). The Administrator’s 

decision also takes into account the 

2022 PA evaluation of the policyrelevant information in the 2019 ISA, 

CASAC advice and recommendations, 

and public comments. 

In presenting the rationale for the 

Administrator’s final decision and its 

foundations, Section III.A provides 

background on the 2020 final decision 

to retain the primary PM10 and 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 conclusions 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 2019 ISA and the 2022 PA, advice 

from the CASAC, and comments from 

the public. Section III.C summarizes the 

Administrator’s decision on the primary 

PM10 standard. 

A. Introduction 

The general approach for this 

reconsideration of the 2020 final 

decision on the primary PM10 standard 

relies on the scientific information 

available for this review, as well as the 

Administrator’s judgments regarding the 

available public health effects evidence, 

and the appropriate degree of public 

health protection for the existing 

standards. With the 2020 decision, the 

then-Administrator 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 short-term 

exposures to PM10–2.5 (85 FR 82725, 

December 18, 2020). 

1. Background on the Current Standard 

Consistent with the 2009 ISA, the 

2019 ISA concluded that the available 

epidemiologic, controlled human 

exposure, and animal toxicological 

studies, including uncertainties, 

provided support for the causality 

determinations of ‘‘suggestive of, but not 

sufficient to infer, a causal relationship’’ 

between short-term exposures to 

PM10–2.5 and cardiovascular effects, 

respiratory effects, and mortality (U.S. 

EPA, 2019a, section 1.4.2). The 2019 

ISA also reached the conclusion that the 

evidence supports a ‘‘suggestive of, but 

not sufficient to infer, a causal 

relationship’’ between short-term 

PM10–2.5 exposures and metabolic 

effects, an endpoint that was not 

evaluated in the 2009 ISA (U.S. EPA, 

2019a, section 1.4.2). 

Compared to the 2009 ISA, the 2019 

ISA includes expanded evidence for the 

relationships between long-term 

exposures and cardiovascular effects, 

metabolic effects, nervous system 

effects, cancer, and mortality. The 2019 

ISA concluded that the small number of 

epidemiologic and experimental 

studies, including uncertainties, 

contribute to the determination that, 

‘‘the evidence is suggestive of, but not 

sufficient to infer, a causal relationship 

between long-term PM10–2.5 exposure 

and cardiovascular effects, metabolic 

effects, nervous system effects, cancer, 

and mortality and cancer (U.S. EPA, 

2019a, p. 10–87). For long-term 

exposures and cardiovascular effects, 


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127As discussed further below, methods 

employed by the epidemiologic studies to estimate 

ambient PM10–2.5 concentrations include: (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 co-located, and (3) direct measurement 

of PM10–2.5 using a dichotomous sampler (U.S. EPA, 

2019a, section 1.4.2). 

128Non-inhalation exposure experiments (i.e., 

intratracheal [IT] instillation) are informative for 

size fractions (e.g., PM10–2.5) that cannot penetrate 

the airway of a study animal and may provide 

information relevant to biological plausibility and 

dosimetry (U.S. EPA, 2019a, section A–12). 

cardiovascular effects, and cancer, this 

is an upgrade from the ‘‘inadequate to 

infer the presence or absence of a causal 

relationship’’ conclusions in the 2009 

ISA (U.S. EPA, 2019a, section 1.4.2). 

This determination is also the first for 

long-term exposures and metabolic 

effects, as the 2009 ISA did not include 

metabolic effects as an endpoint (U.S. 

EPA, 2019a section 1.4.2). 

In considering the available body of 

evidence, it was noted in the 2020 

review there were considerable 

uncertainties and limitations associated 

with the experimental evidence for 

PM2.5 exposures and health effects, and 

as such more weight was placed on the 

available epidemiologic evidence. 

Therefore, the primary focus in the 2020 

review was on multi-city and single-city 

epidemiologic studies that evaluated 

associations between short-term 

PM10–2.5 and mortality, cardiovascular 

effects (hospital admissions and 

emergency department visits, as well as 

blood pressure and hypertension), and 

respiratory effects. Despite differences 

in the approaches 127 used to estimate 

ambient PM10–2.5 concentrations, the 

majority of the studies reported positive, 

though often not statistically significant, 

associations with short-term PM10–2.5

exposures. Most PM10–2.5 effect 

estimates remained positive in 

copollutant models that included either 

gaseous pollutants or other particulate 

matter size fractions (e.g., PM2.5). In U.S. 

study locations likely to have met the 

PM10 standard during the study period, 

a few studies reported positive 

associations between PM10–2.5 and 

mortality that were statistically 

significant and remained so in 

copollutant models (U.S. EPA, 2019a). 

In addition to the epidemiologic studies, 

there were a small number of controlled 

human exposure studies evaluated in 

the 2019 ISA that reported alterations in 

heart rate variability or increased 

pulmonary inflammation following 

short-term exposure to PM10–2.5, 

providing some support for the 

associations in the epidemiologic 

studies. Animal toxicological studies 

examined the effect of short-term 

PM10–2.5 exposures using non-inhalation 

(e.g., intratracheal instillation) route.128

Therefore, these studies provided 

limited evidence for the biological 

plausibility of PM10–2.5-induced effects 

(U.S. EPA, 2019a). Although the 

scientific evidence available in the 2019 

ISA expanded the understanding of 

health effects associated with PM10–2.5

exposures, a number of important 

uncertainties remained. These 

uncertainties, and their implications for 

interpreting the scientific evidence, 

include the following: 

• The potential for confounding by 

copollutants, notably PM2.5, was 

addressed with copollutant models in a 

relatively small number of PM10–2.5

epidemiologic studies (U.S. EPA, 

2019a). This was particularly important 

given the relatively small body of 

experimental evidence (i.e., controlled 

human exposure and animal 

toxicological studies) available to 

support the independent effect of 

PM10–2.5 on human health. This 

increases the uncertainty regarding the 

extent to which PM10–2.5 itself, rather 

than one or more copollutants, is 

responsible for the mortality and 

morbidity effects reported in 

epidemiologic studies. 

• There was greater spatial variability 

in PM10–2.5 concentrations than PM2.5

concentrations, resulting in the 

potential for increased exposure error 

for PM10–2.5 (U.S. EPA, 2019a). Available 

measurements did not provide sufficient 

information to adequately characterize 

the spatial distribution of PM10–2.5

concentrations (U.S. EPA, 2019a). The 

limitations in estimates of ambient 

PM10–2.5 concentrations ‘‘would tend to 

increase uncertainty and make it more 

difficult to detect effects of PM10–2.5 in 

epidemiologic studies’’ (U.S. EPA, 

2019a). 

• Estimation of PM10–2.5

concentrations over which reported 

health outcomes occur remain highly 

uncertain. When compared with PM2.5, 

there is uncertainty spanning all 

epidemiologic studies examining 

associations with PM10–2.5 including 

deficiencies in the existing monitoring 

networks, the lack of a systematic 

evaluation of the various methods used 

to estimate PM10–2.5 concentrations and 

the resulting uncertainty in the spatial 

as well as the temporal variability in 

PM10–2.5 concentration (U.S. EPA, 

2019a).). Given these limitations in 

routine monitoring, epidemiologic 

studies employed a number of different 

approaches for estimating PM10–2.5

concentrations, including (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 co-located, and (3) direct 

measurement of PM10–2.5 using a 

dichotomous sampler (U.S. EPA, 2019a, 

section 1.4.2). Given the relatively small 

number of PM10–2.5 monitoring sites, the 

relatively large spatial variability in 

ambient PM10–2.5 concentrations, the use 

of different approaches to estimating 

ambient PM10–2.5 concentrations across 

epidemiologic studies, and the 

limitations inherent in such estimates, 

the distributions of PM10–2.5

concentrations over which reported 

health outcomes occur remain highly 

uncertain (U.S. EPA, 2019a). 

There was relatively little information 

available to characterize potential 

exposure differences that may inform 

the apparent variability in associations 

between short-term PM10–2.5 exposures 

and health effects across study locations 

(U.S. EPA, 2019a). Specifically, the 

potential spatial and temporal 

variability in PM10–2.5 exposures 

complicates the interpretation of results 

between study locations as well as the 

relative lack of information on the 

chemical and biological composition of 

PM10–2.5 (U.S. EPA, 2009a U.S. EPA, 

2019a). 

In reaching his decision in 2020 to 

retain the existing 24-hour primary 

PM10 standard, the then-Administrator 

specifically noted that, while the health 

effects evidence was somewhat 

expanded since the prior reviews, the 

overall conclusions in the 2019 ISA, 

including uncertainties and limitations, 

were generally consistent with what was 

considered in the 2012 review (85 FR 

82725, December 18, 2020). In addition, 

the then-Administrator recognized that 

there were still a number of 

uncertainties and limitations associated 

with the available evidence. 

With regard to the evidence on 

PM10–2.5-related health effects, the thenAdministrator noted that epidemiologic 

studies continued to report positive 

associations with mortality and 

morbidity in cities across North 

America, Europe, and Asia, where 

PM10–2.5 sources and composition were 

expected to vary widely. While 

significant uncertainties remained in the 

2020 review, the then-Administrator 

recognized that this expanded body of 

evidence had broadened the range of 

effects that have been linked with 

PM10–2.5 exposures. The studies 

evaluated in the 2019 ISA expanded the 

scientific foundation presented in the 

2009 ISA and led to revised causality 

determinations (and new 

determinations) for long-term PM10–2.5


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exposures and mortality, cardiovascular 

effects, metabolic effects, nervous 

system effects, and cancer (85 FR 82726, 

December 18, 2020). Drawing from his 

consideration of this evidence, the thenAdministrator concluded that the 

scientific information available since 

the time of the last review supported 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 (85 FR 82726, 

December 18, 2020). With regard to 

uncertainties in the available evidence, 

the then-Administrator first noted that a 

number of limitations were identified in 

the 2012 review related to: (1) Estimates 

of ambient PM10–2.5 concentrations used 

in epidemiologic 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. Despite the 

expanded body of evidence for PM10–2.5

exposures and health effects, the thenAdministrator recognized that 

uncertainties in the 2020 review 

continued to include those associated 

with the exposure estimates used in 

epidemiologic studies, the 

independence of the PM10–2.5 health 

effect associations, and the biologically 

plausible pathways for PM10–2.5 health 

effects (85 FR 82726, December 18, 

2020). These uncertainties contributed 

to the 2019 ISA determinations that the 

evidence is at most ‘‘suggestive of, but 

not sufficient to infer’’ causal 

relationships (85 FR 82726, December 

18, 2020). In considering the available 

evidence in his basis for the decision, 

the then-Administrator emphasized 

evidence supporting ‘‘causal’’ and 

‘‘likely to be causal’’ relationships, and 

therefore, judged that the PM10–2.5- 

related health effects evidence provided 

an uncertain scientific foundation for 

making standard-setting decisions. He 

further judged limitations in the 

evidence raised questions as to whether 

additional public health improvements 

would be achieved by revising the 

existing PM10 standard (85 FR 24126, 

April 30, 2020). In the 2020 decision, for 

all of the reasons discussed above and 

recognizing the CASAC conclusion that 

the evidence provided support for 

retaining the current standard, the thenAdministrator concluded that it was 

appropriate to retain the existing 

primary PM10 standard, without 

revision. His decision was consistent 

with the CASAC advice related to the 

primary PM10 standard. Specifically, the 

CASAC agreed with the 2020 PA 

conclusions that, while these effects are 

important, the ‘‘evidence does not call 

into question the adequacy of the public 

health protection afforded by the 

current primary PM10 standard’’ and 

‘‘supports consideration of retaining the 

current standard in this review’’ (Cox, 

2019b, p. 3 of consensus letter). Thus, 

the then-Administrator concluded that 

the primary PM10 standard (in all of its 

elements (i.e., indicator, averaging time, 

form, and level)) was requisite to protect 

public health with an adequate margin 

of safety against effects that have been 

associated with PM10–2.5. In light of this 

conclusion, the EPA retained the 

existing PM10 standard. 

2. Overview of the Health Effects 

Evidence 

The information summarized here is 

based on the scientific assessment of the 

health effects evidence available in this 

reconsideration; this evaluation is 

documented in the 2019 ISA and its 

policy implications are discussed 

further in the 2022 PA. As noted above, 

the ISA Supplement does not include an 

evaluation of studies for PM10–2.5, and 

the 2019 ISA continues to serve as the 

scientific foundation for this 

reconsideration. 

a. Nature of Effects 

For the health effect categories and 

exposure duration combinations 

evaluated, the 2019 ISA concludes that 

the evidence supports causality 

determinations for PM10–2.5 that are at 

most ‘‘suggestive of, but not sufficient to 

infer, a causal relationship’’. While the 

evidence supporting the causal nature of 

relationships between exposure to 

PM10–2.5 has been strengthened for some 

health effect categories since the 

completion of the 2009 ISA, the 2019 

ISA concludes that overall ‘‘the 

uncertainties in the evidence identified 

in the 2009 ISA have, to date, still not 

been addressed’’ (U.S. EPA, 2019a, 

section 1.4.2, p. 1–41; U.S. EPA, 2022b, 

section 4.3.1). Specifically, 

epidemiologic studies available in the 

2012 review relied on various methods 

to estimate PM10–2.5 concentrations, and 

these methods had not been 

systematically compared to evaluate 

spatial and temporal correlations in 

PM10–2.5 concentrations. Methods 

included: (1) Calculating the difference 

between PM10 and PM2.5 concentrations 

at co-located monitors, (2) calculating 

the difference between county-wide 

averages of monitored PM10- and PM2.5- 

based on monitors that are not 

necessarily co-located, and (3) direct 

measurement of PM10–2.5 using a 

dichotomous sampler (U.S. EPA, 2019a, 

section 1.4.2). As described in the 2019 

ISA, there continues to be variability 

across epidemiologic studies in the 

approaches used to estimate PM10–2.5

concentrations. Additionally, some 

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. The various 

methods used to estimate PM10–2.5

concentrations have not been 

systematically evaluated (U.S. EPA, 

2019a, section 3.3.1.1), contributing to 

uncertainty regarding the spatial and 

temporal correlations in PM10–2.5

concentrations across methods and in 

the PM10–2.5 exposure estimates used in 

epidemiologic studies (U.S. EPA, 2019a, 

section 2.5.1.2.3). Given the greater 

spatial and temporal variability of 

PM10–2.5 and the lower number of 

PM10–2.5 monitoring sites, compared to 

PM2.5, this uncertainty is particularly 

important for the coarse size fraction. 

Beyond the uncertainty associated with 

PM10–2.5 exposure estimates in 

epidemiologic studies, the limited 

information on the potential for 

confounding by copollutants and the 

limited support available for the 

biological plausibility of health effects 

following PM10–2.5 exposures also 

continue to contribute to uncertainty in 

the PM10–2.5 health evidence. 

Uncertainty related to potential 

confounding stems from the relatively 

small number of epidemiologic studies 

that have evaluated PM10–2.5 health 

effect associations in copollutants 

models with both gaseous pollutants 

and other PM size fractions. On the 

other hand, uncertainty related to the 

biological plausibility of effects 

attributed to PM10–2.5 exposures results 

from the small number of controlled 

human exposure and animal 

toxicological studies that have evaluated 

the health effects of experimental 

PM10–2.5 inhalation exposures. The 

evidence supporting the 2019 ISA’s 

‘‘suggestive of, but not sufficient to 

infer, a causal relationship’’ causality 

determinations for PM10–2.5, including 

uncertainties in this evidence, is 

summarized below in sections III.B.1.a 

through III.B.1.f. 

i. Mortality 

Due to the dearth of studies 

examining the association between longterm PM10–2.5 exposure and mortality, 

the 2009 ISA concluded that the 

evidence was ‘‘inadequate to determine 

if a causal relationship exists’’ (U.S. 

EPA, 2009a). As reported in the 2019 

ISA, some cohort studies conducted in 

the U.S. and Europe report positive 

associations between long-term PM10–2.5

exposure and total (nonaccidental) 


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mortality, though results are 

inconsistent across studies (U.S. EPA, 

2019a, Table 11–11). The examination 

of copollutant models in these studies 

remains limited and, when included, 

PM10–2.5 effect estimates are often 

attenuated after adjusting for PM2.5 (U.S. 

EPA, 2019a, Table 11–11). Across 

studies, PM10–2.5 exposure 

concentrations are estimated using a 

variety of approaches, including direct 

measurements from dichotomous 

samplers, calculating the difference 

between PM10 and PM2.5 concentrations 

measured at collocated monitors, and 

calculating difference of area-wide 

concentrations of PM10 and PM2.5. As 

discussed above, temporal and spatial 

correlations between these approaches 

have not been evaluated, contributing to 

uncertainty regarding the potential for 

exposure measurement error (U.S. EPA, 

2019a, section 3.3.1.1 and Table 11–11). 

The 2019 ISA concludes that this 

uncertainty ‘‘reduces the confidence in 

the associations observed across 

studies’’ (U.S. EPA, 2019a, p. 11–125). 

The 2019 ISA additionally concludes 

that the evidence for long-term PM10–2.5

exposures and cardiovascular effects, 

respiratory morbidity, and metabolic 

disease provide limited biological 

plausibility for PM10–2.5-related 

mortality (U.S. EPA, 2019a, 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, 2019a, 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, 2009a). The 

2019 ISA included multicity 

epidemiologic studies conducted 

primarily in Europe and Asia that 

continue to provide consistent evidence 

of positive associations between shortterm PM10–2.5 exposure and total 

(nonaccidental) mortality (U.S. EPA, 

2019a, Table 11–9). Although these 

studies contribute to increasing 

confidence in the PM10–2.5-mortality 

relationship, the use of various 

approaches to estimate PM10–2.5

exposures continues to contribute 

uncertainty to the associations observed. 

Recent studies expand the assessment of 

potential copollutant confounding of the 

PM10–2.5-mortality relationship and 

provide evidence that PM10–2.5

associations generally remain positive 

in copollutant models, though 

associations are attenuated in some 

instances (U.S. EPA, 2019a, section 

11.3.4.1, Figure 11–28, Table 11–10). 

The 2019 ISA concludes that, overall, 

the assessment of potential copollutant 

confounding is limited due to the lack 

of information on the correlation 

between PM10–2.5 and gaseous pollutants 

and the small number of locations in 

which copollutant analyses have been 

conducted. Associations with causespecific mortality (i.e., cardiovascular 

and respiratory mortality) provide some 

support for associations with total 

(nonaccidental) mortality, though 

associations with respiratory mortality 

are more uncertain (i.e., wider 

confidence intervals) and less consistent 

(U.S. EPA, 2019a, section 11.3.7). The 

2019 ISA concludes that the evidence 

for PM10–2.5-related cardiovascular 

effects provides only limited support for 

the biological plausibility of a 

relationship between short-term 

PM10–2.5 exposure and cardiovascular 

mortality (U.S. EPA, 2019a, 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, 2019a, p. 11–120). 

ii. Cardiovascular Effects 

In the 2009 ISA, the evidence 

describing the relationship between 

long-term exposure to PM10–2.5 and 

cardiovascular effects was characterized 

as ‘‘inadequate to infer the presence or 

absence of a causal relationship.’’ The 

limited number of epidemiologic 

studies reported contradictory results 

and experimental evidence 

demonstrating an effect of PM10–2.5 on 

the cardiovascular system was lacking 

(U.S. EPA, 2019a, section 6.4). 

The evidence relating long-term 

PM10–2.5 exposures to cardiovascular 

mortality remains limited, with no 

consistent pattern of associations across 

studies and, as discussed above, 

uncertainty stemming from the use of 

various approaches to estimate PM10–2.5

concentrations (U.S. EPA, 2019a, Table 

6–70). The evidence for associations 

with cardiovascular morbidity has 

grown and, while results across studies 

are not entirely consistent, some 

epidemiologic studies report positive 

associations with ischemic heart disease 

(IHD) and MI (U.S. EPA, 2019a, Figure 

6–34); stroke (U.S. EPA, 2019a, Figure 

6–35); atherosclerosis (U.S. EPA, 2019a, 

section 6.4.5); venous thromboembolism 

(VTE) (U.S. EPA, 2019a, section 6.4.7); 

and blood pressure and hypertension 

(U.S. EPA, 2019a, Section 6.4.6). 

PM10–2.5 cardiovascular mortality effect 

estimates are often attenuated, but 

remain positive, in copollutants models 

that adjust for PM2.5. For morbidity 

outcomes, associations are inconsistent 

in copollutant models that adjust for 

PM2.5, NO2, and chronic noise pollution 

(U.S. EPA, 2019a, p. 6–276). The lack of 

toxicological evidence for long-term 

PM10–2.5 exposures represents a data gap 

(U.S. EPA, 2019a, section 6.4.10), 

resulting in the 2019 ISA conclusion 

that ‘‘evidence from experimental 

animal studies is of insufficient quantity 

to establish biological plausibility’’ (U.S. 

EPA, 2019a, p. 6–277). Based largely on 

the observation of positive associations 

in some epidemiologic studies, the 2019 

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, 2019a, p. 6–277). 

With regard to short-term PM10–2.5

exposures and cardiovascular effects, 

the 2009 ISA found that the available 

evidence for short-term PM10–2.5

exposure and cardiovascular effects was 

‘‘suggestive of a causal relationship.’’ 

This conclusion was based on several 

epidemiologic studies reporting 

associations between short-term 

PM10–2.5 exposure and cardiovascular 

effects, including IHD hospitalizations, 

supraventricular ectopy, and changes in 

heart rate variability (HRV). In addition, 

dust storm events resulting in high 

concentrations of crustal material were 

linked to increases in total 

cardiovascular disease emergency 

department visits and hospital 

admissions. However, the 2009 ISA 

noted the potential for exposure 

measurement error primarily due to the 

different methods used across studies to 

estimate PM10–2.5 concentrations and 

copollutant confounding in these 

epidemiologic studies. In addition, there 

was only limited evidence of 

cardiovascular effects from a small 

number of experimental studies (e.g. 

animal toxicological studies and 

controlled human exposure studies) that 

examined short-term PM10–2.5 exposures 

(U.S. EPA, 2009a, section 6.2.12.2). In 

the 2019 ISA, key uncertainties 

included the potential for exposure 

measurement error, copollutant 

confounding, and limited evidence of 

biological plausibility for cardiovascular 

effects following inhalation exposure 

(U.S. EPA, 2019a, section 6.3.13). 

The evidence for short-term PM10–2.5

exposure and cardiovascular outcomes 

has expanded since the 2009 ISA, 

though important uncertainties remain. 

The 2019 ISA notes that there are a 

small number of epidemiologic studies 

reporting positive associations between 

short-term exposure to PM10–2.5 and 

cardiovascular-related morbidity 


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outcomes. However, the 2019 ISA notes 

that there is limited evidence to support 

that these associations are biologically 

plausible, or independent of copollutant 

confounding. The 2019 ISA also 

concludes that it remains unclear how 

the approaches used to estimate PM10–2.5

concentrations in epidemiologic studies 

compare amongst one another and 

subsequently how exposure 

measurement error varies between each 

method. Specifically, it is unclear how 

well-correlated PM10–2.5 concentrations 

are both temporally and spatially across 

these methods and therefore whether 

exposure measurement error varies 

across these methods. Taken together, 

the 2019 ISA concludes that ‘‘the 

evidence is suggestive of, but not 

sufficient to infer, a causal relationship 

between short-term PM10–2.5 exposures 

and cardiovascular effects’’ (U.S. EPA, 

2019a, p. 6–254). 

iii. Respiratory Effects 

With regard to short-term PM10–2.5

exposures and respiratory effects, the 

2009 ISA (U.S. EPA, 2009a) concluded 

that the relationship between short-term 

exposure to PM10–2.5 and respiratory 

effects is ‘‘suggestive of a causal 

relationship’’ based on a small number 

of epidemiologic studies observing 

associations with some respiratory 

effects and limited evidence from 

experimental studies to support 

biological plausibility. Epidemiologic 

findings were consistent for respiratory 

infection and combined respiratoryrelated diseases, but not for COPD. 

Studies were characterized by overall 

uncertainty in the exposure assignment 

approach and limited information 

regarding potential copollutant 

confounding. Controlled human 

exposure studies of short-term PM10–2.5

exposures found no lung function 

decrements and inconsistent evidence 

for pulmonary inflammation. Animal 

toxicological studies were limited to 

those using non-inhalation (e.g., intratracheal instillation) routes of PM10–2.5

exposure. 

Recent epidemiologic findings 

consistently link PM10–2.5 exposure to 

asthma exacerbation and respiratory 

mortality, with some evidence that 

associations remain positive (though 

attenuated in some studies of mortality) 

in copollutant models that include 

PM2.5 or gaseous pollutants. 

Epidemiologic studies provide limited 

evidence for positive associations with 

other respiratory outcomes, including 

COPD exacerbation, respiratory 

infection, and combined respiratoryrelated diseases (U.S. EPA, 2019a, Table 

5–36). As noted above for other 

endpoints, an uncertainty in these 

epidemiologic studies is the lack of a 

systematic evaluation of the various 

methods used to estimate PM10–2.5

concentrations and the resulting 

uncertainty in the spatial and temporal 

variability in PM10–2.5 concentrations 

compared to PM2.5 (U.S. EPA, 2019a, 

sections 2.5.1.2.3 and 3.3.1.1). 

Specifically, the existing monitoring 

networks do not provide a good 

characterization of how well correlated 

concentrations are both spatially and 

temporally across the PM10–2.5

estimation methods and overall spatial 

and temporal patterns in PM10–2.5

concentrations. Taken together, the 2019 

ISA concludes that ‘‘the collective 

evidence is suggestive of, but not 

sufficient to infer, a causal relationship 

between short-term PM10–2.5 exposure 

and respiratory effects’’ (U.S. EPA, 

2019a, p. 5–270). 

iv. Cancer 

In the 2012 review, little information 

was available from studies of cancer 

following inhalation exposures to 

PM10–2.5. Thus, the 2009 ISA determined 

the evidence was ‘‘inadequate to 

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

(U.S. EPA, 2009a). The scientific 

information evaluated in the 2019 ISA 

of long-term PM10–2.5 exposure and 

cancer remains limited, with a few 

recent epidemiologic studies reporting 

positive, but imprecise, associations 

with lung cancer incidence (U.S. EPA, 

2019a). Moreover, uncertainty remains 

in these studies with respect to 

exposure measurement error due to the 

use of PM10–2.5 predictions that have not 

been validated by monitored PM10–2.5

concentrations (U.S. EPA, 2019a, 

sections 3.3.2.3 and 10.3.4). Relatively 

few experimental studies of PM10–2.5

have been conducted, though available 

studies indicate that PM10–2.5 exhibits 

two key characteristics of carcinogens: 

genotoxicity and oxidative stress. While 

limited, such experimental studies 

provide some evidence of biological 

plausibility for the findings in a small 

number of epidemiologic studies (U.S. 

EPA, 2019a, section 10.3.4). 

Taken together, the small number of 

epidemiologic and experimental 

studies, along with uncertainty with 

respect to exposure measurement error, 

contribute to the determination in the 

2019 ISA that, ‘‘the evidence is 

suggestive of, but not sufficient to infer, 

a causal relationship between long-term 

PM10–2.5 exposure and cancer’’ (U.S. 

EPA, 2019a, p. 10–87). 

v. Metabolic Effects 

The 2009 ISA did not make a 

causality determination for PM10–2.5- 

related metabolic effects. One 

epidemiologic study in the 2019 ISA 

reports an association between longterm PM10–2.5 exposure and incident 

diabetes, while additional crosssectional studies report associations 

with effects on glucose or insulin 

homeostasis (U.S. EPA, 2019a, section 

7.4). As discussed above for other 

outcomes, uncertainties with the 

epidemiologic evidence include the 

potential for copollutant confounding 

and exposure measurement error due to 

the different methods used across 

studies to estimate PM10–2.5

concentrations (U.S. EPA, 2019a, Tables 

7–14 and 7–15). The evidence base to 

support the biological plausibility of 

metabolic effects following PM10–2.5

exposures is limited, but a crosssectional study that investigated 

biomarkers of insulin resistance and 

systemic and peripheral inflammation 

may support a pathway leading to type 

2 diabetes (U.S. EPA, 2019a, sections 

7.4.1 and 7.4.3). Based on the expanded, 

though still limited evidence base, the 

2019 ISA concludes that, ‘‘[o]verall, the 

evidence is suggestive of, but not 

sufficient to infer, a causal relationship 

between [long]-term PM10–2.5 exposure 

and metabolic effects’’ (U.S. EPA, 2019a, 

p. 7–56). 

vi. Nervous System Effects 

The 2009 ISA did not make a 

causality determination for PM10–2.5- 

related nervous system effects. In the 

2019 ISA, available epidemiologic 

studies report associations between 

PM10–2.5 and impaired cognition and 

anxiety in adults in longitudinal 

analyses (U.S. EPA, 2019a, Table 8–25, 

section 8.4.5). Associations of long-term 

exposure with neurodevelopmental 

effects are not consistently reported in 

children (U.S. EPA, 2019a, sections 

8.4.4 and 8.4.5). Uncertainties in these 

studies include the potential for 

copollutant confounding, as no studies 

examined copollutants models (U.S. 

EPA, 2019a, section 8.4.5), and for 

exposure measurement error, given the 

use of various methods to estimate 

PM10–2.5 concentrations (U.S. EPA, 

2019a, Table 8–25). In addition, there is 

limited animal toxicological evidence 

supporting the biological plausibility of 

nervous system effects (U.S. EPA, 

2019a, sections 8.4.1 and 8.4.5). Overall, 

the 2019 ISA concludes that, ‘‘the 

evidence is suggestive of, but not 

sufficient to infer, a causal relationship’’ 

between long-term PM10–2.5 exposure 

and nervous system effects (U.S. EPA, 

2019a, p. 8–75). 


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129As described in section I.C.5.b above, the 

scope of the ISA Supplement did not include 

consideration of studies of health effects associated 

with exposure to PM10–2.5. Therefore, the 

information and conclusions presented in the 2022 

PA are very similar to those in the 2020 PA. 

130As described in section I.C.5.b above, the 

scope of the ISA Supplement did not include 

consideration of studies of health effects associated 

with exposure to PM10–2.5. Therefore, the 

information and conclusions presented in the 2022 

PA are very similar to those in the 2020 PA. 

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 2020 review and reflects upon the 

body of information and evidence 

available in this reconsideration. 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 integrated synthesis of 

the scientific evidence from animal 

toxicologic, controlled human exposure, 

and epidemiologic studies evaluating 

health effects related to exposures to 

PM10–2.5 as presented in the 2019 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 2022 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 in the Response to Comments 

document, can be found in the docket 

for this rulemaking (Docket No. EPA– 

HQ–OAR–2015–00072). This document 

is available for review in the docket for 

this rulemaking and through EPA’s 

NAAQS website (link). The 

Administrator’s conclusions in this 

reconsideration 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 

As described in section I.X, the EPA 

decided to prepare a revised PA for the 

reconsideration of the 2020 final 

decision. The CASAC’s advice on the 

2019 draft PA and the 2021 draft PA 

was documented in letters to the prior 

and current Administrators (Cox, 2019b; 

Sheppard, 2022a) and is summarized 

below. In reviewing both the 2019 draft 

PA and the 2021 draft PA, the CASAC 

agreed with the EPA’s preliminary 

conclusion that the available scientific 

evidence, including its uncertainties 

and limitations, does not call into 

question the adequacy of the current 

primary PM10 standard and that the 

standard should be retained, without 

revision. 

In its review of the 2019 draft PA, the 

CASAC concurred with the overall 

preliminary conclusion that it is 

appropriate to consider retaining the 

current primary PM10 standard, without 

revision. In their agreement with the 

conclusions in the 2019 draft PA, the 

CASAC stated that ‘‘that key 

uncertainties identified in the last 

review remain’’ (Cox, 2019b) and that 

‘‘none of the identified health outcomes 

linked to PM10–2.5’’ were judged to be 

causal or likely to be causal (Cox, 2019b, 

p. 12 of consensus responses). 

Moreover, to reduce these uncertainties 

in future reviews, the CASAC 

recommended improvements to PM10–2.5

exposure assessment, including a more 

extensive network for direct monitoring 

of the PM10–2.5 fraction (Cox, 2019b, p. 

13 of consensus responses). The CASAC 

also recommended additional controlled 

human exposure and animal 

toxicological studies of the PM10–2.5

fraction to improve the understanding of 

biological mechanisms and pathways 

(Cox, 2019b, p. 13 of consensus 

responses). Overall, the CASAC agreed 

with the EPA’s preliminary conclusion 

in the 2019 draft PA 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, 2019b, p. 3 of letter). 

In its review of the 2021 draft PA, the 

CASAC provided advice on the 

adequacy of the current primary PM10

standard in the context of its review of 

the revised PA for this reconsideration 

(Sheppard, 2022a) 129.) 130. In this 

context, the CASAC supported the 

preliminary conclusion in the 2021 draft 

PA that the evidence reviewed in the 

2019 ISA does not call into question the 

public health protection provided by the 

current primary PM10 standard against 

PM10–2.5 exposures and concurs with the 

2021 draft PA’s overall preliminary 

conclusion that it is appropriate to 

consider retaining the current primary 

PM10 standard (Sheppard, 2022a, p. 4 of 

consensus letter). Additionally, the 

CASAC concurred that ‘‘. . . at this 

time, PM10 is an appropriate choice as 

the indicator for PM10–2.5’’ and ‘‘that it 

is important to retain the level of 

protection afforded by the current PM10

standard’’ (Sheppard, 2022a, p. 4 of 

consensus letter). The CASAC also 

recognized uncertainties associated with 

the scientific evidence, including 

‘‘compared to PM2.5 studies, the more 

limited number of epidemiology studies 

with positive statistically significant 

findings, and the difficulty in extracting 

the sole contribution of coarse PM to 

observed adverse health effects’’ 

(Sheppard, 2022a, p. 19 of consensus 

responses). 

The CASAC recommended several 

areas for additional research to reduce 

uncertainties in the PM10–2.5 exposure 

estimates used in the epidemiologic 

studies, to evaluate the independence of 

PM10–2.5 health effect associations, to 

evaluate the biological plausibility of 

PM10–2.5-related effects, and to increase 

the number of studies examining 

PM10–2.5-related health effects in at-risk 

populations (Sheppard, 2022a, p. 20 of 

consensus responses). Furthermore, the 

CASAC ‘‘recognizes a need for, and 

supports investment in research and 

deployment of measurement systems to 

better characterize PM10–2.5’’ and to 

‘‘provide information that can improve 

public health’’ (Sheppard, 2022a, p. 20 

of consensus responses). 

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 2019 ISA, 

considerations and staff conclusions 

and associated rationales presented in 

the 2020 PA and 2022 PA, and advice 

and recommendations of the CASAC (88 

FR 5634, January 27, 2023). Consistent 

with previous reviews, the 

Administrator first considered the 

available scientific evidence for 

PM10–2.5-related exposures and health 

effects, as evaluated in the 2019 ISA. As 

an initial matter, the Administrator 

recognized that the scientific evidence 

for PM10–2.5-related effects available in 

this reconsideration is the same body of 

evidence that was available at the time 

of the 2020 review, as evaluated in the 

2019 ISA and summarized in section 

III.A.2 above. The 2019 ISA concludes 

that the evidence supports ‘‘suggestive 

of, but not sufficient to infer’’ causal 

relationships between short- and longterm exposures to PM10–2.5 and 

cardiovascular effects, cancer, and 

mortality and long-term PM10–2.5

exposures and metabolic effects and 

nervous system effects (U.S. EPA, 

2019a). The Administrator noted that 


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the evidence for several PM10–2.5-related 

health effects has expanded since the 

completion of the 2009 ISA, but 

important uncertainties remain. The 

uncertainties in the epidemiologic 

studies contribute to the determinations 

in the 2019 ISA that the evidence for 

short and long-term PM10–2.5 exposures 

and mortality, cardiovascular effects, 

metabolic effects, nervous system 

effects, and cancer is ‘‘suggestive of, but 

not sufficient to infer’’ causal 

relationships (U.S. EPA, 2019a; U.S. 

EPA, 2022b, section 4.3.1). Drawing 

from the evidence evaluated in the 2019 

ISA and consideration of the scientific 

evidence in the 2022 PA, the 

Administrator noted that, consistent 

with previous reviews, the 2019 ISA 

and the 2022 PA highlight a number of 

uncertainties associated with the 

evidence, including: (1) PM10–2.5

exposure estimates used in 

epidemiologic studies, (2) independence 

of PM10–2.5 health effect associations, 

and (3) biological plausibility of the 

PM10–2.5-related effects. These 

uncertainties contribute to the 

determinations in the 2019 ISA that the 

evidence for short-term PM10–2.5

exposures and key health effects is 

‘‘suggestive of, but not sufficient to 

infer’’ causal relationships. In 

considering the available scientific 

evidence, consistent with approaches 

employed in past NAAQS reviews, the 

Administrator placed the most weight 

on evidence supporting ‘‘causal’’ and 

‘‘likely to be causal’’ relationships. In so 

doing, he noted that the available 

evidence for short- and long-term 

PM10–2.5 exposures and health effects 

does not support causality 

determinations of a ‘‘causal 

relationship’’ or ‘‘likely to be causal 

relationship.’’ Furthermore, the 

Administrator recognized that, because 

of the uncertainties and limitations in 

the evidence base, the 2022 PA does not 

include a quantitative assessment of 

PM10–2.5 exposures and risk that might 

further inform decisions regarding the 

adequacy of the current 24-hour primary 

PM10 standard. Therefore, in light of the 

2019 ISA conclusions that the evidence 

supports ‘‘suggestive of, but not 

sufficient to infer’’ causal relationships. 

The Administrator judged that there are 

substantial uncertainties that raise 

questions regarding the degree to which 

additional public health improvements 

would be achieved by revising the 

existing PM10 standard. In considering 

the available evidence for long-term 

PM10–2.5 exposures, the Administrator 

noted that there is limited evidence that 

would support consideration of an 

annual standard to provide protection 

against such effects, in conjunction with 

the current primary 24-hour PM10

standard. He preliminarily concluded 

that the current primary 24-hour PM2.5

standard that reduces 24-hour exposures 

also likely reduces long-term average 

exposures, and therefore provides some 

margin of safety against the health 

effects associated with long-term 

PM10–2.5 exposures. 

In reaching his proposed decision on 

the adequacy of the current primary 24- 

hour PM10 standard, the Administrator 

also considered advice from the CASAC. 

As noted above in section III.B.1, the 

CASAC recognized uncertainties 

associated with the scientific evidence 

and agreed with the 2019 draft PA and 

2021 draft PA conclusions that the 

scientific evidence does not call into 

question the adequacy of the primary 

PM10 standard and supports 

consideration of retaining the current 

standard. 

When considering the above 

information together, 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. Additionally, he recognized 

that there are important uncertainties 

and limitations associated with the 

available evidence for PM10–2.5-related 

health effects, for both short and longterm exposure, as evaluated in the 2019 

ISA. Consistent with the decisions in 

the previous reviews, the Administrator 

proposed to conclude that these 

limitations lead to considerable 

uncertainty regarding the potential 

public health implications of revising 

the level of the current primary 24-hour 

PM10 standard. Thus, based on his 

consideration of the evidence and 

associated uncertainties and limitations 

for PM10–2.5-related health effects and 

his consideration of CASAC advice on 

the primary PM10 standard, the 

Administrator proposed to retain the 

current 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 

agree with the EPA’s proposed decision 

to retain the primary PM10 standard. In 

so doing, these commenters agree with 

the EPA’s rationale regarding the 

available scientific information, 

including uncertainties and limitations, 

for informing decisions on the standard. 

These commenters state that no new 

scientific evidence or quantitative 

information has emerged since the 2020 

decision to retain the current standard. 

Furthermore, these commenters note 

that the EPA did not evaluate any new 

scientific evidence related to PM10–2.5

exposures and health effects as a part of 

the 2022 ISA Supplement developed for 

this reconsideration, nor did the revised 

2022 PA consider any new or different 

information from the 2020 PA, and 

therefore, the EPA reached the same 

conclusion as is the 2020 PA that the 

current standard is adequate and should 

be retained. This group includes 

industries and industry groups, as well 

as some State and local governments. 

All of these commenters generally note 

their agreements with the rationale 

provided in the proposal and the 

CASAC concurrence with the 2021 draft 

PA conclusion that the available 

information does not call into question 

the adequacy of the current standard, 

and therefore, does not support revision 

and that the current standard should be 

retained. 

Some commenters, including those 

from environmental and public health 

organizations and groups, some states, 

and individuals, disagreed with the 

Administrator’s proposed decision to 

retain the current primary PM10

standard. These commenters 

recommend that the EPA revise the 

primary PM10 standard to a lower level 

to provide increased public health 

protection, citing to the available 

scientific evidence, as well as the 

proposed revision to the primary PM2.5

standard. 

Commenters who disagreed with the 

proposal to retain the current standard 

state that revision to the primary PM10

standard is necessary 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. Some of these 


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131PM10 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 

132PM2.5 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 

commenters assert that the level of the 

primary PM10 standard should be 

revised to 140 or 145 mg/m3, concurrent 

with a strengthened primary 24-hour 

PM2.5 standard, while other commenters 

recommend revising the level of the 

standard to within the range of 65–75 

mg/m3, to provide increased public 

health protection. 

We disagree with the commenters that 

the primary PM10 standard should be 

revised because of reductions in 

ambient concentrations of PM2.5. 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 2019,131 while PM2.5

concentrations in ambient air have 

declined by 43% during this same time 

period.132 As noted in the 2022 PA (p. 

2–41), the majority of PM10–2.5 sites have 

generally remained steady and do not 

exhibit a trend of increasing or 

decreasing concentrations during this 

time period, reflecting the relatively 

consistent level of dust emission across 

the U.S. from 2000 to 2019 (U.S. EPA, 

2022b). 

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 

reconsideration (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. 

The EPA recognizes that when the 

primary annual PM2.5 standard was 

revised from 15.0 mg/m3 to 12.0 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 (78 FR 3085, 

March 03, 2013). As described in the 

2019 ISA, PM10 has become 

considerably coarser across the U.S. 

compared to similar observations in the 

2009 ISA such that, in urban areas, the 

mass of the coarse fraction of PM is 

similar to or greater than the mass of the 

fine fraction of PM (U.S. EPA, 2019, 

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

However, in considering recent air 

quality data, the EPA notes that in most 

areas of the country PM2.5 and PM10

concentrations have declined and are 

well below their respective 24-hour 

standards. While the contribution of 

fine and coarse PM to PM10 mass 

concentrations may vary spatially and 

temporally, based on the trends in 

recent air quality data, the 

Administrator concludes that the 

current primary 24-hour PM10 standard 

is maintaining air quality at level that 

provides requisite protection against 

PM10–2.5. That is, recent air quality data 

does not suggest that PM10–2.5

concentrations have been increasing as 

PM2.5 concentrations have been 

decreasing. In considering the available 

PM10–2.5 health effects evidence in this 

reconsideration, there continue to be 

significant uncertainties and limitations, 

specifically with respect to the exposure 

assessment methods used to estimate 

PM10–2.5 concentrations, 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 5558, 

January 27, 2023), the uncertainties and 

limitations in the health effects 

evidence for PM10–2.5 contributed 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’’ or 

‘‘inadequate to infer the presence, or 

absence of a causal relationship’’ (U.S. 

EPA, 2019a). While the evidence base 

for PM10–2.5-related health effects has 

somewhat expanded since the 2009 ISA, 

the Administrator concludes that the 

evidence remains too limited to inform 

judgments regarding whether a more 

protective primary PM10 standard is 

warranted at this time. 

Beyond the uncertainties and 

limitations associated with the available 

scientific evidence, the EPA also notes 

that, while the NCore monitoring 

network has been expanded since the 

time of the last review, epidemiologic 

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 PM10–2.5

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, 2022b, 

section 4.6), and inform future reviews 

of the PM NAAQS. Taken together, as at 

the time of proposal, the Administrator 

concludes 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, particularly 

when considering such judgments along 

with his decision to retain the current 

primary 24-hour PM2.5 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 

reconsideration. 

Consistent with their comments on 

the 2020 proposal, 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 increased 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 highest to the 4th 

highest 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). 


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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 (i.e., indicator, 

averaging time, level) 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 reduce 

the public health protection provided 

against exposures to PM10–2.5 in ambient 

air, resulting in a less stringent primary 

PM10 standard than the current 

standard. 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 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. The EPA 

continues to update and develop 

documentation and tools to facilitate the 

implementation of the 2016 Exceptional 

Events Rule, including new PM2.5

implementation focused products under 

development that are intended to assist 

air agencies with the development of 

demonstrations for specific types of 

exceptional events. 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. 

4. Administrator’s Conclusions 

This section summarizes the 

Administrator’s considerations and 

conclusions related to the current 

primary PM10 standard. In establishing 

primary standards under the Act that 

are ‘‘requisite’’ to protect the public 

health with an adequate margin of 

safety, the Administrator is seeking to 

establish standards that are neither more 

nor less stringent than necessary for this 

purpose. In so doing, the Administrator 

notes that his final decision in this 

reconsideration 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 

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. 

Given these requirements, and 

consistent with the primary PM2.5

standards discussed above (section 

II.C.3), the Administrator’s final 

decision in this reconsideration of the 

current primary PM10 standard will be 

a public health policy judgment that 

draws upon the scientific information 

examining the health effects of PM10–2.5

exposures, including how to consider 

the range and magnitude of 

uncertainties inherent in that 

information. The Administrator’s final 

decision is based on an interpretation of 

the scientific evidence that neither 

overstates nor understates its strengths 

and limitations, nor the appropriate 

inferences to be drawn. 

Having carefully considered advice 

from the CASAC and public comments, 

as discussed above, the Administrator 

notes that the fundamental scientific 

conclusions on health effects of PM10–2.5

in ambient air that were reached in the 

2019 ISA and summarized in the 2020 

PA and 2022 PA remain valid. 

Additionally, the Administrator believes 

the judgments he proposed (85 FR 5558, 

January 27, 2023) with regard to the 

evidence remain appropriate. Further, 

in considering the adequacy of the 

current primary PM10 standard in this 

reconsideration, the Administrator has 

carefully considered the policy-relevant 

evidence and conclusions contained in 

the 2019 ISA; the rationale and 

conclusions presented in the 2020 PA 

and 2022 PA; the advice and 

recommendations from the CASAC in 

their reviews of the 2019 draft PA and 

2021 draft PA; and public comments, as 

addressed in section III.B.3 above and in 

the RTC document. In the discussion 

below, the Administrator gives weight 

to the conclusions in the 2020 PA and 

2022 PA, with which the CASAC has 

concurred, as summarized in section 

III.C of the proposal and takes note of 

the key aspects of the rationale for those 

conclusions that contribute to his 

decision in this review. In considering 

this information, the Administrator 

concludes that the preliminary 

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 

2020 PA and 2022 PA evaluations and 

conclusions, the Administrator notes 

that, while the health effects evidence is 

somewhat expanded since the 2009 ISA 

as described in section III.A.2 above, the 

overall conclusions are generally 

consistent with those reached in the 

2009 ISA (U.S. EPA, 2020b, section 4.4). 

In so doing, he additionally notes that 

the CASAC supported the preliminary 

conclusion in the 2019 draft PA and 

2021 draft PA that the evidence 

reviewed in the 2019 ISA does not call 

into question the public health 

protection provided by the current 

primary PM10 standard against PM10–2.5

exposures and concurs that it is 

appropriate to consider retaining the 

current primary PM10 standard (Cox, 


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2019b, p. 13 of consensus responses; 

Sheppard, 2022a, p. 4 of consensus 

letter). 

As noted below, the scientific 

evidence for PM10–2.5-related health 

effects has expanded somewhat since 

the 2012 review, in particular for longterm 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 5558, January 27, 2023) 

and below. With regard to the current 

evidence on PM10–2.5-related health 

effects, the Administrator takes note of 

recent epidemiologic 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 

2019 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, 

2019a; U.S. EPA, 2022b, section 4.2). 

Drawing from his consideration of this 

evidence, the Administrator concludes 

that the available scientific information 

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 2012 review related to: (1) Estimates 

of ambient PM10–2.5 concentrations used 

in epidemiologic 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. Despite the 

expanded body of evidence for PM10–2.5

exposures and health effects assessed in 

the 2019 ISA, the Administrator 

recognizes that uncertainties remain, 

similar to those in the 2012 review. As 

summarized in section III.A.2 above and 

in responding to public comments, 

uncertainties in the available scientific 

evidence continue to include those 

associated with the exposure estimates 

used in epidemiologic 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, 2022b, section 4.3). 

These uncertainties contribute to the 

2019 ISA determinations that the 

evidence is ‘‘suggestive of, but not 

sufficient to infer’’ causal relationships 

(U.S. EPA, 2019a). The Administrator 

recognizes that the NAAQS must allow 

for a margin of safety but also places 

emphasis on evidence supporting 

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

relationships (as described in sections 

II.A.2 and III.A.2 above). 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, 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. In light 

of these considerations and 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 particular, 

he notes limitations in the approaches 

used to estimate ambient PM10–2.5

concentrations in epidemiologic studies, 

limited examination of the potential for 

confounding by co-occurring pollutants, 

and limited support for the biological 

plausibility of the serious effects 

reported in many epidemiologic studies 

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 

primary PM10 standard for PM10–2.5

exposures and he does not judge that 

the available information and the 

associated uncertainties 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. In their comments, the 

CASAC noted that uncertainties that 

were identified in the 2012 review 

persist in the evidence for PM10–2.5- 

related health effects (Cox, 2019b, p. 13 

of consensus responses; Sheppard, 

2022a, p. 4 of consensus letter) In 

considering these comments, the 

Administrator takes note of the CASAC 

consideration of the evidence, and 

associated uncertainties, and its 

conclusion that the evidence reviewed 

in the 2019 ISA does not call into 

question the adequacy of the public 

health protection afforded by the 

current primary PM10 standard (Cox, 

2019b, p. 3 of letter; Sheppard, 2022a, 

p. 4 of consensus letter). The 

Administrator further notes the 

unanimous conclusions of the CASAC 

that evidence supports consideration of 

retaining the current primary PM10

standard (Cox, 2019b, p. 3 of consensus 

letter; Sheppard, 2022a, p. 4 of 

consensus letter). In addition to the 

CASAC’s advice, the Administrator also 

considers public comments, the 

majority of which supported retaining 

the primary PM10 standard, citing to and 

agreeing with the Administrator’s 

rationale for his proposed decision. The 

Administrator also recognizes that a few 

public commenters supported revising 

the primary PM10 standard in order to 

provide increased protection against 

PM10–2.5-related health effects. 

The Administrator also notes that the 

scientific record for his decision on the 

primary PM10 standard is the same as 

the record before the then-Administrator 

in 2020, as the scope of the ISA 

Supplement focused on health effect 

categories where the 2019 ISA 

concluded a causal relationship (i.e., 


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133See http://www.airnow.gov/. 

134 In 1976, the EPA established a nationally 

uniform air quality index, then called the Pollutant 

Standard Index (PSI), for use by State and local 

agencies on a voluntary basis (41 FR 37660, 

September 7, 1976; 52 FR 24634, July 1,1987). In 

August 1999, the EPA adopted revisions to this air 

quality index (64 FR 42530, August 4, 1999) and 

renamed the index the AQI. 

short- and long-term PM2.5 exposure and 

cardiovascular effects and mortality). 

Therefore, because no health outcome 

categories for short- or long-term 

PM10–2.5 exposure in the 2019 ISA were 

greater than ‘‘suggestive of, but not 

sufficient to infer, a causal 

relationship’’, the ISA Supplement did 

not evaluate studies published after the 

literature cutoff date of the 2019 ISA 

related to PM10–2.5 exposures and health 

effects. The Administrator further notes 

his decision is consistent with the 

decision of the prior Administrator in 

2020 to retain the primary PM10

standard. 

With regard to the indicator for the 

primary PM10 standard, the 

Administrator recognizes that the 2022 

PA notes that the evidence continues to 

support retaining the PM10 indicator to 

provide public health protection against 

PM10–2.5-related effects. He notes that, 

consistent with the approaches in 

previous reviews, a standard with a 

PM10 mass-based indicator, in 

conjunction with a PM2.5 mass-based 

standard, will result in controlling 

allowable concentrations of PM10–2.5. 

The Administrator also takes note of the 

2019 ISA comparison that showed that 

the relative contribution of PM2.5 and 

PM10–2.5 to PM10 concentrations can vary 

across the U.S. by region and season, 

with urban locations having a somewhat 

higher contribution of PM2.5

contributing to PM10 concentrations 

than PM10–2.5 (U.S. EPA, 2019a, section 

2.5.1.1.4, Table 2–7). In these urban 

locations, where PM2.5 concentrations 

are somewhat higher than in rural 

locations, the toxicity of the PM10 may 

be higher due to contaminating PM2.5. 

Further, although uncertainties with the 

evidence persist, the strongest health 

effects evidence associated with PM10–2.5

comes from epidemiologic studies 

conducted in urban areas. He also notes 

that the CASAC agreed with the EPA’s 

conclusions that a PM10 indicator 

remained appropriate (Cox, 2019b, p. 13 

of consensus responses; Sheppard, 

2022a, p. 4 of letter). In light of this 

information, the Administrator 

concludes that the PM10 indicator 

remains appropriate and 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 

in all of its elements, is requisite. As an 

initial matter, the Administrator notes 

that the current primary PM10 standard, 

with its level of 150 mg/m3, 24-hour 

averaging time, not to be exceeded more 

than once per year on average over three 

years, is intended to protect against 

short-term peak PM10–2.5 exposures. In 

so doing, while the Administrator notes 

that changes in PM2.5 concentrations in 

ambient air can influence the 

contribution of the fine and coarse 

fractions to PM10 mass, such that 

reductions in PM2.5 concentrations can 

lead to more allowable PM10–2.5 under 

the current primary PM10 standard, he 

recognizes that there is no new 

information available in this 

reconsideration to suggest that the 

public health protection provided by the 

current standard is not requisite or that 

a more stringent standard is warranted 

at this time. The Administrator 

concludes that, particularly in light of 

his decision to retain the primary 24- 

hour PM2.5 standard with its level of 35 

mg/m3 as described in section II.B.4 

above, the primary PM10 standard 

would be expected to maintain PM10–2.5

concentrations in ambient air below 

those that have been considered to be 

associated with serious health effects in 

past NAAQS reviews. The 

Administrator also notes that while the 

scientific evidence available in the 2019 

ISA has expanded since the completion 

of the 2009 ISA, he concludes that this 

information does not provide support 

for the causal or likely to be causal 

relationships upon which he places the 

greatest weight in considering the 

adequacy of the current standards. He 

further concludes that the uncertainties 

and limitations of the scientific 

evidence, along with the absence of 

information to inform a quantitative 

exposure or risk assessment, make it 

difficult to reach decisions regarding 

whether a more protective standard is 

warranted at this time. 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’s 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 

considering information and 

assessments presented in the 2019 ISA 

and the 2022 PA, the advice from the 

CASAC, and 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. Communication of Public Health 

A. Air Quality Index Overview 

Information about the public health 

implications of ambient concentrations 

of criteria pollutants is communicated 

to the public using the Air Quality 

Index (AQI) reported on the EPA’s 

AirNow website.133 The current AQI has 

been in use since its inception in 

1999.134 It provides useful, timely, and 

easily understandable information about 

the daily degree of pollution. The goal 

of the AQI is to establish a nationally 

uniform system of indexing pollution 

concentrations for ozone, carbon 

monoxide, nitrogen dioxide, PM, and 

sulfur dioxide. The AQI is recognized 

internationally as a proven tool to 

effectively communicate air quality 

information to the public as 

demonstrated by the fact that many 

countries have created similar indices 

based on the AQI. 

The AQI converts an individual 

pollutant concentration in a 

community’s air to a number on a scale 

from 0 to 500. Reported AQI values for 

specific pollutants enable the public to 

know whether air pollution levels in a 

particular location are characterized as 

good (0–50), moderate (51–100), 

unhealthy for sensitive groups (101– 

150), unhealthy (151–200), very 

unhealthy (201–300), or hazardous 

(301+). Across criteria pollutants, the 

AQI value of 100 typically corresponds 

to the level of the short-term (e.g., 24- 

hour, 8-hour, or 1-hour standard) 

NAAQS for each pollutant. Below an 

index value of 100, an intermediate 

value of 50 is defined either as the level 

of the annual standard if an annual 

standard has been established (e.g., 

PM2.5, nitrogen dioxide), a 


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135 In evaluating the scientific evidence available 

to inform decisions regarding the AQI breakpoints, 

the EPA considered studies that were included as 

a part of the 2019 ISA and ISA Supplement, but 

also considered other studies that were not 

included as a part of the review of the air quality 

criteria. The ISAs have specific criteria for study 

inclusion and consideration in reaching 

conclusions regarding causal relationships, and 

some studies that may not have met those criteria 

(e.g., epidemiologic studies that evaluate the health 

effects of wildfire smoke exposure that would have 

higher PM2.5 concentrations, which are outside of 

the scope of the ISA) were identified as studies that 

could be used to inform decisions on the AQI, 

particularly for the upper breakpoints. 

concentration equal to one-half the 

value of the 24-hour standard used to 

define an index value of 100 (e.g., 

carbon monoxide), or a concentration 

based directly on health effects evidence 

(e.g., ozone). An AQI value greater than 

100 means that a pollutant is in one of 

the unhealthy categories (i.e., unhealthy 

for sensitive groups, unhealthy, very 

unhealthy, or hazardous). An AQI value 

at or below 100 means that a pollutant 

concentration is in one of the 

satisfactory categories (i.e., moderate or 

good). The scientific evidence on 

pollutant-related health effects for each 

NAAQS review support decisions 

related to pollutant concentrations at 

which to set the various AQI 

breakpoints, which delineate the AQI 

categories for each individual pollutant 

(i.e., the pollutant concentrations 

corresponding to index values of 150, 

200, 300, and 500). The AQI is reported 

three ways by the EPA and State, local 

and Tribal agencies, all of which are 

useful and complementary. The daily 

AQI is reported for the previous day and 

used to observe trends in community air 

quality, the AQI forecast helps people 

plan their outdoor activities for the next 

day, and the near-real-time AQI, or 

NowCast AQI, tells people whether it is 

a good time for outdoor activity. 

Historically, State and local agencies 

have primarily used the AQI to provide 

general information to the public about 

air quality and its relationship to public 

health. For more than two decades, 

many State and local agencies, as well 

as the EPA and other Federal agencies, 

have been developing new and 

innovative programs and initiatives to 

provide more information related to air 

quality and health messaging to the 

public in a more timely way. These 

initiatives, including air quality 

forecasting, near real-time data reporting 

through the AirNow website, use of data 

from air quality sensors on the EPA and 

U.S. Forest Service’s (USFS) Fire and 

Smoke Map, and air quality action day 

programs, provide useful, up-to-date, 

and timely information to the public 

about air pollution and its health effects. 

Such information can help the public 

learn when their well-being may be 

compromised, so they can take actions 

to avoid or to reduce exposures to 

ambient air pollution at concentrations 

of concern. This information can also 

encourage the public to take actions that 

will reduce air pollution on days when 

concentrations are projected to be of 

concern to local communities (e.g., air 

quality action day programs can 

encourage individuals to drive less or 

carpool). 

B. Air Quality Index Category 

Breakpoints for PM2.5

Recognizing the scientific information 

available and current AQI reporting 

practices, the EPA proposed several 

revisions to the AQI PM2.5 breakpoints. 

EPA solicited and received comments 

on these proposed revisions. Upon 

reviewing the information in the 

proposal and considering the comments 

received EPA is making final revisions 

to the AQI category breakpoints for 

PM2.5. This section summarizes the 

proposed revisions, which can be read 

in full in the proposal (88 FR 5638, 

January 27, 2023), significant comments, 

and final revisions. 

1. Summary of Proposed Revisions 

One purpose of the AQI is to 

communicate to the public when air 

quality is poor and thus when they 

should consider taking actions to reduce 

their exposures. The higher the AQI 

value, the higher the level of air 

pollution and the greater the health 

concern. In recognition of the scientific 

information available that is informing 

the reconsideration of the 2020 final 

decision on the primary PM2.5

standards, including a number of new 

controlled human exposure and 

epidemiologic studies published since 

the completion of the 2009 ISA, as well 

as additional epidemiologic studies 

from other peer reviewed documents 

that evaluate the health effects of 

wildfire smoke exposure and that can 

inform the selection of AQI breakpoints 

at higher PM2.5 concentrations,135 the 

EPA proposed to make two sets of 

changes to the PM2.5 sub-index of the 

AQI. First, the EPA proposed to 

continue to use the approach used in 

the revisions to the AQI in 2012 (77 FR 

38890, June 29, 2012) of setting the 

lower breakpoints (50, 100 and 150) to 

be based on the levels of the primary 

PM2.5 annual and 24-hour standards and 

proposed to revise the lower 

breakpoints to be consistent with 

changes to the primary PM2.5 standards 

that are part of this reconsideration. 

Second, the EPA proposed to revise the 

upper AQI breakpoints (200 and above) 

and to replace the linear-relationship 

approach used in 1999 to set these 

breakpoints, with an approach that more 

fully considers the PM2.5 health effects 

evidence from controlled human 

exposure and epidemiologic studies that 

have become available in the last 20 

years (64 FR 42530, August 4, 1999). 

a. Air Quality Index Values of 50, 100 

and 150 

With respect to the lower AQI 

breakpoints in the proposal (88 FR 5638, 

January 27, 2023), the EPA proposed to 

conclude that it is appropriate to 

continue setting these breakpoints to be 

consistent with the primary annual and 

24-hour PM2.5 standard levels. The 

lowest AQI value of 50 provides the 

breakpoint between the ‘‘good’’ and 

‘‘moderate’’ categories. At and below 

this concentration, air quality is 

considered ‘‘good’’ for everyone. Above 

this concentration, in the ‘‘moderate’’ 

category, the AQI contains advisories for 

unusually sensitive individuals. The 

EPA has historically set this breakpoint 

at the level of the primary annual PM2.5

standard. In doing so, the EPA has 

recognized that: (1) The annual standard 

is set to provide protection to the 

public, including at-risk populations, 

from PM2.5 concentrations, which, when 

experienced on average for a year, have 

the potential to result in adverse health 

effects; and (2) the AQI exposure period 

represents a shorter exposure period 

(e.g., 24-hour (or less)) while focusing 

on the most sensitive individuals. The 

EPA saw no basis for deviating from this 

approach in this reconsideration. Thus, 

the EPA proposed to set the AQI value 

of 50 at a daily (i.e., 24-hour) average 

concentration equal to the level of the 

primary annual PM2.5 standard that is 

promulgated. 

The historical approach to setting an 

AQI value of 100, which is the 

breakpoint between the ‘‘moderate’’ and 

‘‘unhealthy for sensitive groups’’ 

categories, and above which advisories 

are generated for sensitive groups, is to 

set it at the same level as the primary 

24-hour PM2.5 standard. In so doing, the 

EPA has recognized that the primary 24- 

hour PM2.5 standard is set to provide 

protection to the public, including atrisk populations, from short-term 

exposures to PM2.5 concentrations that 

have the potential to result in adverse 

health effects. Given this, it is 

appropriate to generate advisories for 

sensitive groups at concentrations above 

this level. In the past, State, local, and 

Tribal air quality agencies have 

expressed strong support for this 

approach (78 FR 3086, January 15, 

2013). The EPA saw no basis to deviate 


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136The AQI breakpoint at 150 was originally set 

in 1999 to be linearly related to the concentrations 

at the 100 and 500 breakpoints but then revised in 

2012 to be proportional to the AQI breakpoint 

concentration at 100 (78 FR 3181, January 15, 2013). 

137 In this reconsideration, the controlled human 

exposure studies were evaluated in the 2019 ISA, 

whereas the epidemiologic studies of wildfire 

smoke exposures were included in the EPA 

Comparative Assessment of the Impacts of 

Prescribed Fire Versus Wildfire (CAIF): A Case 

Study in the Western U.S. (U.S. EPA 2021b). 

from this approach in this 

reconsideration. In the proposal (88 FR 

5638, January 27, 2023), the EPA 

proposed to retain the current primary 

24-hour PM2.5 standard with its level of 

35 mg/m3 but took comment on revising 

the level of that standard to 25 mg/m3

(section II.D.3.b). Thus, the EPA 

proposed to retain the AQI value of 100 

set at the level of the current primary 

24-hour PM2.5 standard concentration of 

35 mg/m3 (i.e., 24-hour average). 

With respect to an AQI value of 150, 

which is the breakpoint between the 

‘‘unhealthy for sensitive groups’’ and 

‘‘unhealthy categories,’’ this breakpoint 

concentration in this reconsideration is 

based upon the considering the same 

health effects information, as assessed 

in the 2019 ISA and ISA Supplement 

and described in section II above, that 

informs the proposed decisions on the 

level of the 24-hour standard and the 

AQI value of 100. Previously, the 

Agency has used a proportional 

adjustment in which the AQI value of 

150 was set proportionally to the AQI 

value of 100. This proportional 

adjustment inherently recognizes that 

the available epidemiologic studies 

provide no evidence of discernible 

thresholds, below which effects do not 

occur in either sensitive groups or in the 

general population, that could inform 

conclusions regarding concentrations at 

which to set this breakpoint. Given that 

the epidemiologic evidence continues to 

be the most relevant health effects 

evidence for informing this range of AQI 

values, the EPA saw no basis to deviate 

from this approach in this 

reconsideration. Therefore, the EPA 

proposed to set an AQI value of 150 

proportionally, depending on the 

breakpoint concentration of the AQI 

value of 100 (i.e., 55.4 for a 24-hour 

standard of 35 mg/m3). 

b. Air Quality Index Values of 200 and 

Above 

In the proposal (88 FR 5639, January 

27, 2023), the EPA summarized the 

history of setting the AQI values of 300 

and above in the 1999 rule (64 FR 

42530, August 4, 1999) and established 

breakpoints for PM2.5 in that range. In 

general, the AQI values between 100 

and 500 were based on PM2.5

concentrations that generally reflected a 

linear relationship between increasing 

index values and increasing PM2.5

concentrations.136 It was found that this 

linear relationship was generally 

consistent with the health effects 

evidence, which suggested that as PM2.5

concentrations increase, increasingly 

larger numbers of people are likely to 

experience serious health effects in this 

range of PM2.5 concentrations (64 FR 

42536, August 4, 1999). For the AQI 

breakpoint of 500, the concentration 

was based on the method used to 

establish a previously existing PM10

breakpoint that was informed by studies 

conducted in London using the British 

Smoke method, which uses a different 

particle size cutpoint as noted in the 

proposal (88 FR 5639, January 27, 2023). 

Due to limited ambient PM2.5

monitoring data available at that time, 

the decision on the 500 value 

concentration for PM2.5 was based on 

the stated assumption that PM 

concentrations measured by the British 

Smoke method were approximately 

equivalent to PM2.5 concentrations (64 

FR 42530, August 4, 1999). Given that 

the British Smoke method has a larger 

particle size cutpoint than the current 

PM2.5 monitoring method, which has a 

cutpoint of 2.5 microns, a concentration 

of 500 mg/m3 based on the British 

Smoke method would be equivalent to 

a lower PM2.5 concentration. With 

respect to the upper breakpoints of the 

AQI, the EPA has historically been 

concerned about establishing these 

upper breakpoints using evidence based 

on larger size fractions of PM, given that 

PM2.5 is the indicator for the AQI. While 

monitoring data for higher PM2.5

concentrations in ambient air has been 

available for many years, the health 

effects evidence has only recently 

become available for consideration in 

informing decisions on the upper 

breakpoints of the AQI. 

As part of this reconsideration, the 

EPA recognized that the health effects 

evidence associated with PM2.5

exposure has greatly expanded in recent 

years. Multiple controlled human 

exposure studies have become available 

that provide information about health 

effects across a range of concentrations. 

While many of the new studies 

evaluated in the 2019 ISA focused on 

examining health effects associated with 

exposure to lower PM2.5 concentrations, 

there are also several new controlled 

human exposure studies that provide 

information about the health effects 

observed in study participants at 

concentrations well above the standard 

levels. Additionally, there are also 

epidemiologic studies now available 

and evaluated in other Agency peerreviewed documents that can inform 

health effects associated with higher 

PM2.5 concentrations (U.S. EPA, 

2021b).137 Thus, the EPA concluded 

that it is appropriate to reevaluate the 

upper AQI breakpoints, taking into 

account the expanded body of scientific 

evidence, particularly given several new 

epidemiologic studies conducted during 

high pollution events like wildfires and 

multiple controlled human exposure 

studies. While it remains unclear the 

exact PM2.5 concentrations at which 

specific health effects occur, the more 

recent studies do provide more refined 

information about the concentration 

range in which these effects might occur 

in some populations. These studies 

provide support for coherence of effects 

across scientific disciplines and 

potentially biologically plausible 

pathways for the overt population-level 

health effects observed in epidemiologic 

studies. Therefore, taking into account 

the short exposure time period in these 

studies (e.g., 1–6 hours) and that the 

studies generally do not include at-risk 

(or sensitive) populations, but rather 

young, healthy adults, these studies, in 

conjunction with information from 

epidemiologic studies, the EPA 

preliminarily concluded it would be 

appropriate to be more cautionary and 

offer advisories to the public for 

reducing exposures at lower 

concentrations than recommended with 

the current AQI breakpoints. 

The AQI value of 200 is the 

breakpoint between the ‘‘unhealthy’’ 

and ‘‘very unhealthy’’ categories. At 

AQI values above 200, the AQI would 

be providing a health warning that the 

risk of anyone experiencing a health 

effect following short-term exposures to 

these PM2.5 concentrations has 

increased. To inform proposed 

decisions on this breakpoint, the EPA 

takes note of studies indicating the 

potential for respiratory or 

cardiovascular effects that are on their 

own representative of or are on the 

biologically plausible pathway to more 

serious health outcomes (e.g., 

emergency department visits, hospital 

admissions). The controlled human 

exposure studies evaluated in the 2009 

and 2019 ISAs provide evidence of 

inflammation as well as cardiovascular 

effects in healthy subjects at and above 

120 mg/m3. For example, Ramanathan et 

al. (2016) observed a transient reduction 

in antioxidant/anti-inflammatory 

function after exposing healthy young 

subjects to a mean concentration of 150 

mg/m3 of PM2.5 for 2 hours. Urch et al. 


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138Although participants in Lucking et al. (2011) 

were exposed to diesel exhaust (DE), the authors 

also conducted analyses using a particle trap, and 

as noted in the 2019 ISA, this type of study design 

allows for the assessment of the role of PM2.5 on the 

health effects observed by removing PM from the 

DE mixture. 

(2010) also reported increased markers 

of inflammation when exposing both 

asthmatic and non-asthmatic subjects to 

a mean concentration of 140 mg/m3 of 

PM2.5 for 3 hours. In studies specifically 

examining cardiovascular effects, Ghio 

et al. (2000) and Ghio et al. (2003) 

exposed healthy subjects to a mean 

concentration of 120 mg/m3 for 2 hours 

and reported significantly increased 

levels of fibrinogen, a marker of 

coagulation that increases during 

inflammation. Sivagangabalan et al. 

(2011) exposed healthy subjects to a 

mean concentration of 150 mg/m3 of 

PM2.5 for 2 hours and noted an 

increased QT interval (3.4 ± 1.4) 

indicating some evidence for 

conduction abnormalities, an indicator 

of possible arrhythmias. Lastly, Brook et 

al. (2009) reported a transient increase 

of 2.9 mm Hg in diastolic blood pressure 

in healthy subjects during the 2-hour 

exposure to a mean concentration of 148 

mg/m3 of PM2.5. 

In addition to epidemiologic studies 

evaluated in the 2019 ISA that analyzed 

exposures at ambient PM2.5

concentrations, there are a number of 

recent epidemiologic studies focusing 

on wildfire smoke that have become 

available that were evaluated in the 

EPA’s recently released peer-reviewed 

assessment on wildland fire (U.S. EPA, 

2021b). One of these studies, 

Hutchinson et al. (2018), conducted a 

bidirectional case-crossover analysis to 

examine associations between wildfirespecific PM2.5 exposure and respiratoryrelated healthcare encounters (i.e., ED 

visits, inpatient hospital admissions, 

and outpatient visits) prior and during 

the 2007 San Diego wildfires. This study 

found positive and significant 

associations to PM2.5 exposures and 

respiratory-related healthcare 

encounters. Further, during the initial 5- 

day period of the wildfire event, the 

study observed that there was evidence 

of increases in a number of respiratoryrelated outcomes particularly ED visits 

for asthma, upper respiratory infection, 

respiratory symptoms, acute bronchitis, 

and all respiratory-related visits 

(Hutchinson et al., 2018). When 

examining the air quality during the 

wildfire event, PM2.5 concentrations 

were highest during the initial five days 

of the wildfire, with 24-hour average 

PM2.5 concentrations of 89.1 mg/m3

across all zip codes and with the highest 

24-hour average of 160 mg/m3 on the 

first day (Hutchinson et al., 2018). 

When considering this collective body 

of evidence from controlled human 

exposure and epidemiologic studies, the 

Agency proposed to set an AQI value of 

200 at a daily (i.e., 24-hour average) 

concentration of PM2.5 of 125 mg/m3. As 

discussed above and in the proposal (88 

FR 5640, January 27, 2023), this 

concentration is at the lower end of the 

concentrations consistently shown to be 

associated with respiratory and 

cardiovascular effects in controlled 

human exposure studies following 

short-term exposures (e.g., 2–3 hours) 

and in young, healthy adults (Ghio et 

al., 2000; Ghio et al., 2003; Urch et al., 

2010; Ramanathan et al., 2016; 

Sivagangabalan et al., 2011; and Brook 

et al., 2009) and also within the range 

of 5-day average and maximum 

concentrations observed to be associated 

with respiratory-related outcomes 

following exposure to wildfire smoke 

(Hutchinson et al., 2018). 

The AQI value of 300 denotes the 

breakpoint between the ‘‘very 

unhealthy’’ and ‘‘hazardous’’ categories, 

and thus marks the beginning of the 

‘‘hazardous’’ AQI category. At AQI 

values above 300, the AQI provides a 

health warning that everyone is likely to 

experience effects following short-term 

exposures to these PM2.5 concentrations. 

To inform decisions on this AQI 

breakpoint, the EPA takes note of 

controlled human exposure studies that 

consistently show subclinical effects 

which are often associated with more 

severe cardiovascular outcomes. As 

discussed above, Brook et al. (2009) 

reported a transient increase of 2.9 mm 

Hg in diastolic blood pressure in 

healthy subjects during the 2-hour 

exposure to a mean concentration of 148 

mg/m3 of PM2.5. Bellavia et al. (2013) 

exposed healthy subjects to an average 

PM2.5 concentration of 242 mg/m3 for 2 

hours and reported increased systolic 

blood pressure (2.53 mm Hg). Tong et al. 

(2015) exposed healthy subjects to an 

average PM2.5 concentration of 253 mg/ 

m3 for 2 hours and observed a 

significant increase in diastolic blood 

pressure (2.1 mm Hg) and a 

nonsignificant increase in systolic blood 

pressure (2.5 mm Hg). Lucking et al. 

(2011) reported impaired vascular 

function and increased potential for 

coagulation when exposing healthy 

subjects to diesel exhaust (DE) with an 

average PM2.5 concentration of 320 mg/ 

m3 for a duration of 1 hour.138 These 

studies all provided evidence of 

impaired vascular function, including 

vasodilatation impairment and 

increased thrombus formation, with 

Tong et al. (2015), Bellavia et al. (2013), 

Brook et al. (2009) all reporting 

increases in blood pressure. 

Additionally, Behbod et al. (2013) 

reported increased inflammatory 

markers following a 2-hour exposure to 

an average PM2.5 concentration of 250 

mg/m3 in healthy subjects. 

In addition to the controlled human 

exposure studies discussed above, the 

epidemiologic study conducted by 

DeFlorio-Barker et al. (2019) examined 

the relationship between wildfire smoke 

and cardiopulmonary hospitalizations 

among adults 65 years of age and older 

from 2008–2010 in 692 U.S. counties. 

The authors reported a 2.22% increase 

in all-cause respiratory hospitalizations 

on wildfire smoke days for a 10 mg/m3

increase in 24-hour average PM2.5

concentrations (DeFlorio-Barker et al., 

2019). The maximum 24-hour average 

concentration in this study on wildfire 

smoke days was 212.5 mg/m3 (DeFlorioBarker et al., 2019). In considering this 

study, the EPA notes the increased 

probability that even healthy adults 

experience effects at this maximum 

exposure concentration, particularly 

given that this maximum concentration 

is near the exposure concentrations in 

controlled human exposure studies that 

consistently reported evidence of 

impaired vascular function and several 

that reported increases in blood 

pressure in healthy adults following 2- 

hour exposures. 

Based on the information discussed 

above and in the proposal (88 FR 5640, 

January 27, 2023), the EPA proposed to 

revise the 300 level of the AQI, which 

marks the beginning of the ‘‘hazardous’’ 

AQI category, to a concentration that is 

consistent with the PM2.5 concentrations 

associated with health effects as 

reported in the controlled human 

exposure (Brook et al., 2009; Bellavia et 

al., 2013; Tong et al., 2015; Behbod et 

al., 2013) and epidemiologic studies 

(DeFlorio-Barker et al. (2019). 

Specifically, the Agency proposed to set 

an AQI value of 300 at a daily (i.e., 24- 

hour average) PM2.5 concentration of 

225 mg/m3. This concentration falls 

between the 2-hour average 

concentrations reported in controlled 

human exposure studies found to be 

consistently associated, in healthy 

adults, with impaired vascular function 

and/or increases in blood pressure, 

which could both be a precursor to more 

severe cardiovascular effects following 

short-term (1- to 2-hour) exposures, and 

the maximum 24-hour average PM2.5

concentrations on wildfire smoke days 

reported in the epidemiologic study 

conducted by DeFlorio-Barker et al. 

(2019). 


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139These effects were attenuated when the DE 

was filtered, to reduce PM2.5 concentrations, 

indicating the effects were likely associated with 

PM2.5 exposure. 

140When applying a particle trap, PM2.5

concentrations were reduced, and effects associated 

with cardiovascular function including impaired 

vascular function, as measured by vasodilatation 

and thrombus formation were attenuated indicating 

associations with PM2.5. 

c. Air Quality Index Value of 500 

Lastly, the EPA also proposed 

revisions to the 500 value of the AQI. 

The 500 value of the AQI is within the 

‘‘hazardous’’ category but is specified 

and used to calculate the slope of the 

AQI values in the ‘‘hazardous category’’ 

above and below AQI values of 500. In 

the past, this breakpoint had a very 

prominent role in determining the 

current upper AQI values given that it 

was used as part of the linear 

relationship with the concentration at 

the AQI value of 100 to determine the 

AQI values of 200 and 300 in 1999 (64 

FR 42530, August 4, 1999). 

As discussed above and in the 

proposal (88 FR 5641, January 27, 2023), 

the current breakpoint concentration for 

the 500 value of the AQI was set in 1999 

at a 24-hour average PM2.5 concentration 

of 500 mg/m3 and was based on studies 

conducted in London using the British 

Smoke method, which used a different 

particle size cutpoint and likely 

overestimated the PM2.5 concentration. 

In looking to improve upon that 

approach, the EPA considered several 

recent controlled human exposure 

studies that observe health effects that 

are on the biologically plausible 

pathway to more severe cardiovascular 

outcomes and note that these seem to 

follow exposures to high PM2.5

concentrations that are well above those 

typically observed in ambient air. More 

specifically, in controlled human 

exposure studies, Vieira et al. (2016a) 

and Vieira et al. (2016b) exposed 

healthy subjects and subjects with heart 

failure to diesel exhaust (DE) with a 

mean PM2.5 concentration of 325 mg/m3

for 21 minutes and reported decreased 

stroke volume, and increased arterial 

stiffness (an indicator of endothelial 

dysfunction) in both healthy and heart 

failure subjects.139 Also as summarized 

above and discussed in the proposal (88 

FR 5641, January 27, 2023), Lucking et 

al. (2011) exposed healthy subjects to 

DE with a mean PM2.5 concentration of 

320 mg/m3 for 1 hour.140 Epidemiologic 

studies have linked the types of 

cardiovascular effects observed in these 

controlled human exposure studies with 

the exacerbation of ischemic heart 

disease (IHD) and heart failure as well 

as myocardial infarction (MI) and 

stroke. 

In addition to the controlled human 

exposure studies discussed in the 

proposal (88 FR 5641, January 27, 2023) 

and summarized above, recent 

epidemiologic studies examining the 

relationship between concentrations of 

PM2.5 during wildfires and respiratory 

health also informed the proposed 

decisions on the concentration for the 

AQI value of 500. As discussed in the 

proposal (88 FR 5641, January 27, 2023) 

and summarized earlier in this section, 

Hutchinson et al. (2018) reported 

increases in a number of respiratoryrelated ED visits for asthma, upper 

respiratory infection, respiratory 

symptoms, acute bronchitis, and all 

combined respiratory-related visits 

based on data from Medi-Cal claims for 

emergency department presentations, 

inpatient hospitalizations, and 

outpatient visits during the initial 5-day 

period of the 2007 San Diego fire. 

During the initial 5-day window, PM2.5

concentrations were found to be at their 

highest with the 95th percentile of 24- 

hour average concentrations of 333 mg/ 

m3. 

Although studies of short-term (i.e., 

daily) exposures to wildfire smoke are 

more informative in considering 

alternative level for the AQI value of 

500 since they mirror the 24-hour 

exposure timeframe, additional 

information from epidemiologic studies 

of longer-term exposures (i.e., over 

many weeks) during wildfire events can 

provide supporting information. As 

discussed in the proposal (88 FR 5641, 

January 27, 2023) and summarized here, 

Orr et al. (2020) conducted a 

longitudinal study that reported 

exposure to wildfire smoke from a 

multi-month fire resulted in reduced 

lung function in subsequent years and 

concluded that exposure to high PM2.5

concentrations during a multi-week fire 

event may lead to health consequences, 

such as declines in lung function. 

During the 2017 wildfire event (August 

1 to September 19, 2017), Orr et al. 

(2020) reported that many days during 

the multi-month fire had PM2.5

concentrations above 300 mg/m3, 

resulting in a daily average PM2.5

concentration of 220.9 mg/m3 with a 

maximum PM2.5 concentration of 638 

mg/m3. 

The controlled human exposure 

studies provide biological plausibility 

for results of epidemiologic studies that 

document increases in respiratoryrelated health care events during the 

wildfires. The collective evidence from 

controlled human exposure and 

epidemiologic studies, which includes 

decreases in stroke volume, increased 

arterial stiffness, impaired vascular 

function and respiratory-related 

healthcare encounters provide healthbased evidence that informed the 

proposed decisions on the level of the 

AQI value of 500. Given the 

concentrations observed in these 

studies, the Agency proposed to revise 

the AQI value of 500 to a level set at a 

daily (i.e., 24-hour average) PM2.5

concentration of 325 mg/m3. This 

concentration is at or below the lowest 

concentrations observed in the 

controlled human exposure studies 

associated with more severe effects 

discussed above and also at the low end 

of the daily concentrations observed in 

the epidemiologic studies conducted by 

Hutchinson et al. (2018) and Orr et al. 

(2020). 

Table 1 below summarizes the 

proposed breakpoints for the PM2.5 subindex. 

TABLE 1—PROPOSED BREAKPOINTS FOR PM2.5 SUB-INDEX

AQI category Index values 

Proposed 

breakpoints 

(μg/m3, 24-hour 

average) 

Good ................................................................................................................................................ 0–50 0.0–(9.0–10.0) 

Moderate .......................................................................................................................................... 51–100 (9.1–10.1)–35.4 

Unhealthy for Sensitive Groups ....................................................................................................... 101–150 35.5–55.4 

Unhealthy ......................................................................................................................................... 151–200 55.5–125.4 

Very Unhealthy ................................................................................................................................ 201–300 125.5–225.4 


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TABLE 1—PROPOSED BREAKPOINTS FOR PM2.5 SUB-INDEX—Continued 

AQI category Index values 

Proposed 

breakpoints 

(μg/m3, 24-hour 

average) 

Hazardous 1 ..................................................................................................................................... 301+ 225.5 

1AQI values between breakpoints are calculated using equation 1 in appendix G. For AQI values in the hazardous category, AQI values greater than 500 should be calculated using equation 1 and the PM2.5 concentration specified for the AQI value of 500. 

2. Summary of Significant Comments on 

Proposed Revisions 

The EPA received many comments on 

the proposed changes to the PM2.5 AQI 

breakpoints. Many commenters 

generally supported all the proposed 

revisions to the AQI breakpoints based 

on the revisions to the primary annual 

and daily PM2.5 standards and recent 

scientific evidence discussed in the 

proposal (88 FR 5558, January 27, 2023). 

However, we received specific 

comments on proposed revisions to the 

breakpoints in the lower end of the AQI, 

related to their linkage to the annual 

and daily PM2.5 standards, and proposed 

revisions to the breakpoints at the upper 

end of the AQI, based on EPA’s 

interpretation of available health effects 

evidence. 

a. Air Quality Index Values of 50, 100, 

and 150 

Some commenters agreed with using 

the historical approach of setting the 50, 

100 and 150 breakpoints of the AQI to 

be consistent with the primary PM2.5

standards. Some cited the reason that 

this approach creates consistent 

communication with respect to air 

quality and the standards, and this is 

how the other AQI sub-indices are set. 

A few commenters disagreed with the 

historical approach and suggested 

instead that the 50 breakpoint of the 

AQI should not be revised at all, or that 

the 50 and 100 breakpoints of the AQI 

should be supported directly by health 

data similar to the basis for the 

proposed 200, 300 and 500 breakpoints. 

The few commenters that disagreed 

with the historical approach of the 50 

breakpoint of the AQI noted that setting 

a short-term breakpoint to annual 

standard was not logical since it is a 

long-term standard and not meant to be 

interpreted for short-term messaging 

with the AQI, in particular when 

reported hourly via the NowCast. These 

commenters also noted that additional 

studies are needed to identify the health 

impacts of short-term exposures at low 

concentrations. They also noted that 

lowering the 50 breakpoint of the AQI 

in conjunction with the annual standard 

may cause confusion with the public 

because some State programs and policy 

decisions are connected to the AQI 

while others are based on PM 

concentrations, which could lead to 

inconsistent messaging reducing the 

public’s trust. These comments were 

supported by noting that revised 

breakpoints could lead to more 

moderate days than in the past, but the 

monitor values would be the same as 

before when the commenters considered 

it ‘‘healthy,’’ possibly eroding trust in 

air agencies’ messaging. Commenters 

also noted if the breakpoints are revised, 

the public will not visually be able to 

detect the difference between what was 

considered a good AQI day versus a 

now moderate AQI day. 

The EPA disagrees with these 

commenters. With respect to setting a 

short-term breakpoint to the level of a 

much longer-term (annual) standard, 

setting the lower AQI breakpoints at the 

level of the annual and daily PM2.5

standards for communication purposes 

was discussed in the proposed 

reconsideration (88 FR 5558, January 27, 

2023) and previously supported by State 

organizations in the 2012 PM Final Rule 

(77 FR 38890, June 29, 2012). Both the 

AQI and the Pollutant Standards Index, 

which came before it, have historically 

been normalized across pollutants by 

defining an index value of 50 and 100 

as the numerical level of the annual 

(when defined) and short-term (i.e., 

averaging time of 24-hours or less) 

primary NAAQS for each pollutant. 

This approach clearly communicates the 

air quality to the public. The EPA 

considers this approach to be 

appropriate given the available evidence 

and structure of the standard. As 

discussed in section II.B above and in 

the notice of final rulemaking for the 

2012 review (77 FR 38890, June 29, 

2012), the primary annual and 24-hour 

PM2.5 standards work together in 

concert to provide public health 

protection. The annual PM2.5 standard is 

generally viewed as the principal means 

of providing public health protection 

against ‘‘typical’’ daily and annual PM2.5

exposures, while the 24-hour PM2.5

standard is generally viewed as a means 

of providing protection against shortterm exposures to ‘‘peak’’ PM2.5

concentrations, such as can occur in 

areas with strong contributions from 

local or seasonal sources, even when 

annual average PM2.5 concentrations 

remain relatively low. Because the 

annual standard provides public health 

protection for typical daily PM2.5

exposures, the EPA thinks it is 

appropriate to use that level for the 50 

breakpoint of the AQI and describe 

daily air quality at and below the level 

of the annual standard ‘‘Good.’’ Since an 

annual standard allows for days with air 

quality above that level, it is appropriate 

to call days just above it ‘‘Moderate.’’ If 

the 50 breakpoint of the AQI was set at 

a level above the annual standard, it 

would be possible for the majority of 

days to be called ‘‘good’’ in a year when 

an area exceeds the annual standard. 

This could cause confusion with the 

public about air quality if the general 

perception is that local air quality is 

‘‘good,’’ but the area fails to meet the 

annual standard. In addition, the EPA 

continues to find it appropriate to use 

the NowCast with the PM2.5 AQI index 

to provide more real-time information to 

the public. As discussed in the AQI 

Technical Assistance Document, while 

the NowCast algorithm is approximating 

a 24-hour average exposure, it can 

reflect concentrations observed over 

shorter averaging times when air quality 

is changing rapidly (U.S. EPA, 2018a). 

The EPA continues to consider the use 

of the primary annual standard level 

suitable in the NowCast given the health 

evidence supporting the standard and 

given that the reported concentrations 

are an approximation of ‘‘typical’’ daily 

exposure. Additionally, the EPA reflects 

the nature of the NowCast in the 

associated health messaging. 

With regard to the commenter stating 

the public may not be able to visually 

detect a difference in the air quality, the 

EPA notes that the AQI is intended to 

be a communication tool for public 

awareness precisely because it is 

generally difficult for the public to 

visually judge air quality risks when air 

pollution is ‘‘moderate.’’ Moreover, 

since the establishment of the AQI, the 

EPA and State and local air agencies 

and organizations have developed 

experience in educating the public 

about changes in the standards and, 


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concurrently, related changes to AQI 

breakpoints and advisories. When the 

standards change, the EPA and State 

and local agencies have sought to help 

the public understand that air quality is 

not getting worse, it’s that the health 

evidence underlying the standards and 

the AQI has changed. The EPA’s Air 

Quality System (AQS), the primary 

repository for air quality monitoring 

data, is also adjusted to reflect the 

revised breakpoints. Specifically, all 

historical AQI values in AQS are 

recomputed with the revised 

breakpoints, so that all data queries and 

reports downstream of AQS will show 

appropriate trends in AQI values over 

time. If any State, local or Tribal air 

agency is concerned that people are or 

will be confused on a moderate AQI 

day, then they could use the 

communication information that has 

been developed with this rulemaking. 

Some commenters stated that the AQI 

should not necessarily be linked to the 

primary PM2.5 standards. One example 

is the comment that if the annual 

standard is not lowered to 8 mg/m3, the 

EPA should lower the 50 breakpoint of 

the AQI to that level to better inform the 

public of the need for behavioral 

modifications to reduce the harm to 

health from PM2.5 exposure. Similar to 

the reasons discussed above, the EPA 

concludes that setting the 50 breakpoint 

of the AQI at the level of the annual 

PM2.5 standard is appropriate from a 

health perspective and for 

communication purposes. The 

Administrator has judged the primary 

annual standard (in conjunction with 

the other primary standards) as revised 

in this final action to be requisite to 

protect public health with an adequate 

margin of safety, based on the health 

evidence discussed in section II.A.2. 

Setting the 50 breakpoint lower than the 

annual standard also has the potential to 

cause confusion with the public since it 

does not reflect the standards and the 

Administrator’s judgments about the 

standards as well. 

With regard to the 100 breakpoint of 

the AQI, several commenters expressed 

the view that the level of the 24-hour 

PM2.5 standard and an AQI value of 100 

should be set at 25 mg/m3 based on the 

body of evidence and lower end of the 

range recommended by CASAC. These 

commenters noted that if the current 24- 

hour standard and AQI value of 100 is 

retained at 35 mg/m3 then the public 

will not be able to make informed 

decisions about actions to take to 

protect their health. Many of these 

commenters further recommended that 

the AQI value of 100 should be lowered 

to 25 mg/m3 even if the standard is 

retained. Commenters expressed the 

view that this would more adequately 

allow the public to take healthprotective actions. 

The EPA disagrees with these 

commenters and notes that many State, 

Tribal and local air agencies have 

expressed strong support for aligning 

the 100 breakpoint of the AQI with the 

short-term 24-hour primary PM2.5

standards as discussed in the proposal 

(88 FR 5558, January 27, 2023). The EPA 

agrees with the view, expressed by 

State, local and Tribal entities, that 

aligning the lower breakpoints with the 

standards enables clear communication 

of the standards. This alignment 

approach is also utilized in the other 

AQI sub-indices lower breakpoints and 

taking a different approach with the 

PM2.5 AQI could cause confusion. 

Additionally, the Administrator has 

judged that it is appropriate to retain the 

24-hour standard at a level of 35 mg/m3

(in conjunction with the other primary 

standards) to protect public health with 

an adequate margin of safety, based on 

the health evidence discussed in section 

II.A.2. Thus, EPA disagrees that it is 

necessary or appropriate to set the 100 

breakpoint at a lower concentration to 

provide further information to the 

public. The 50 breakpoint, which is set 

at a level below 25 mg/m3, will continue 

to provide information to members of 

the public particularly concerned about 

exposures to PM2.5. As with the 50 

breakpoint, aligning the breakpoint with 

the standard both reflects the 

Administrator’s judgment about the 

health risks and eliminates the potential 

to cause confusion in the public about 

those risks. 

b. Air Quality Index Values of 200 and 

Above 

Some commenters supported the 

proposed revisions to the 200, 300 and 

500 breakpoints that recognize the 

expanded body of scientific evidence, 

particularly several new epidemiologic 

studies conducted during high pollution 

events such as wildfires and multiple 

controlled human exposure studies. A 

few commenters agreed with 

incorporating the expanded body of 

scientific evidence into the 200, 300 and 

500 breakpoints, but suggested a 

modified linear approach between 200 

(115 mg/m3) and 500 (312 mg/m3, setting 

the 300 breakpoint to 187 mg/m3) based 

on recent epidemiologic wildfire smoke 

studies. 

Other commenters disagreed with the 

proposed revisions and suggested the 

EPA should continue using the previous 

breakpoints that follow the 1999 linear 

approach (64 FR 42530, August 4, 1999), 

because not changing the breakpoints 

would simplify communications. A few 

commenters stated the proposed 

revisions to the AQI upper breakpoints 

are not justified because the scientific 

evidence supporting the revisions is 

inadequate. To support this view, the 

commenters suggest that only three 

epidemiologic studies were used in 

determining the upper breakpoints and 

none of them were representative of 

potential effects in the general public; of 

the 13 studies cited only three were near 

the proposed revised breakpoints; four 

of the studies involved exposure to PM 

from diesel and traffic pollution, which 

is different than PM from wildfire 

smoke; and the data supporting the 

revisions only indicated ‘‘mild’’ health 

effects that were mostly in sensitive 

populations. 

The EPA agrees with the majority of 

commenters that supported utilizing the 

expanded body of scientific evidence to 

revise the 200, 300 and 500 breakpoints 

of the AQI. The EPA appreciates the 

suggestion of using a revised linear 

approach from 200 to 500. But rather 

than using the available evidence to 

only set the breakpoint of 500, the EPA 

finds it appropriate to set the 

breakpoints for 200, 300 and 500 using 

an evidence-based approach, by relying 

on information presented in both 

controlled human exposure studies and 

epidemiologic studies that examine 

relationships between high PM2.5

exposure episodes (i.e., periods of 

wildfire smoke) and various health 

outcomes. Setting these breakpoints 

based directly on health effects 

evidence, which can be communicated, 

is more useful and appropriate than 

using a linear approach, because it can 

better describe the potential health 

effects and symptoms which also helps 

the public better understand why more 

health protective actions are needed. By 

its nature, a linear approach does not 

evaluate and identify associated health 

effects and risk factors. 

The EPA disagrees with the 

commenters that expressed the view 

that these upper breakpoints should not 

be revised based largely on the 

numerous peer-reviewed studies 

published since the 200, 300 and 500 

breakpoints were originally established 

in 1999 (64 FR 42530, August 4, 1999). 

As discussed in the proposal (88 FR 

5641, January 27, 2023), the rationale 

behind the proposed revisions is rooted 

in the fact the upper AQI breakpoints 

are based on outdated scientific 

evidence. Specifically, the traditional 

linear approach was predicated on the 

500 value of the AQI, which was 

estimated using health studies that used 

the British Smoke Method. The British 

Smoke Method is based on a particle 

size fraction (4.5 microns) that is larger 


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than PM2.5. Given that the British Smoke 

method has a larger particle size 

cutpoint than the current PM2.5

monitoring method, which has a 

cutpoint of 2.5 microns, a concentration 

of 500 mg/m3 based on the British 

Smoke method would be equivalent to 

a lower PM2.5 concentration (88 FR 

5641, January 27, 2023). The 

combination of a larger particle size 

fraction informing previous decisions 

around upper AQI breakpoints and 

more recent scientific evidence than the 

London Fog Episode, on the potential 

health consequences of what we 

currently consider to be high PM2.5

exposures, provides the underlying 

basis for revising the upper breakpoints 

to better inform the public about air 

quality to allow the public to take health 

protective actions as appropriate. 

Moreover, as discussed above, until 

recently there was limited information 

upon which to base the breakpoints 

between 150 and 500, so the linear 

approach was a reasonable substitute. 

While not changing the breakpoints may 

be easier because there is no change to 

communicate, using a health-based 

approach is more appropriate, because it 

helps the public better understand that 

more health protective actions are 

needed. 

The Agency disagrees that the 

scientific evidence discussed in the 

proposal is inadequate to revise the 200, 

300 and 500 breakpoints of the AQI (88 

FR 5640, January 27, 2023). The EPA 

disagrees that these studies should not 

be considered because they ‘‘indicated 

mild health effects in sensitive 

populations.’’ The EPA notes that many 

of the subclinical effects discussed in 

the proposal (88 FR 5640, January 27, 

2023) that informed the breakpoints are 

on the biologically plausible pathway 

(see 2019 ISA, section 6.1.1 and Figure 

6–1) to more severe cardiovascular 

outcomes, such as ED visits, hospital 

admissions, and death as depicted in 

the large number of epidemiologic 

studies evaluated in the 2019 ISA and 

ISA Supplement. From a public health 

perspective, the purpose of the AQI is 

to inform the public when air quality 

could adversely affect their health. The 

scientific evidence informed revisions 

to the breakpoints at the upper end of 

the AQI allow it to better reflect the risk 

of experiencing health effects at higher 

PM2.5 concentrations. In addition, the 

EPA disagrees with the commenter that 

the effects reported at these higher 

concentrations were observed only in 

sensitive populations as these effects 

were also reported in healthy 

populations (Ghio et al., 2000; Ghio et 

al., 2003; Urch et al., 2010; Ramanathan 

et al., 2016; Sivagangabalan et al., 2011; 

Brook et al., 2009; Bellavia et al. (2013); 

Tong et al. (2015); Behbod et al. (2013); 

Vieira et al. (2016a) Vieira et al. (2016b); 

and Lucking et al. (2011)). 

c. Other Comments 

The EPA received a few additional 

comments on elements of the PM2.5 AQI, 

including the averaging time. Some 

commenters expressed the view that the 

24-hour averaging time was not useful 

when informing the public how to 

protect their health, particularly during 

rapidly changing conditions such as 

wildfire smoke events. Instead, they 

suggested a subdaily averaging time of 

1–3 hours would be more effective 

because it more closely aligns with how 

people breathe. 

A few of these commenters suggested 

that instead of changing the AQI 

averaging time, which aligns with the 

short-term standard, the EPA could 

create a public health warning system 

for unhealthy PM2.5 levels. The 

commenters noted that aligning the AQI 

averaging time with the short-term 

standard could be useful for consistent 

communication with the standards and 

attainment but suggested that a subdaily 

warning system could better allow the 

public to take health protective actions. 

The EPA disagrees that a shorter 

averaging period for the PM2.5 AQI subindex would be better. The health 

effects evidence supporting a subdaily 

metric is limited and inconsistent. As 

part of its review of the health effects 

evidence, the 2019 ISA evaluated 

whether a subdaily metric would be 

more closely related to health effects. 

Most epidemiologic studies that 

examined the relationship between 

short-term PM2.5 exposures and health 

effects evaluated an exposure metric 

averaged over 24-hours. Some recent 

studies, focusing on respiratory and 

cardiovascular effects and mortality, 

have examined whether there is 

evidence that subdaily exposure metrics 

are more closely related to health effects 

than a traditional 24-hour average 

metric. After evaluating this limited 

newer evidence, the 2019 ISA 

concluded that ‘‘collectively, the 

available evidence does not indicate 

that subdaily averaging periods for 

PM2.5 are more closely associated with 

health effects than the 24-hour avg 

exposure metric,’’ (2019 ISA, chapter 1, 

section 1.5.2.1, pp. 146–147; U.S. EPA, 

2022a). 

In addition, there are communication 

benefits to aligning the averaging time of 

the AQI with the daily standard, as 

some of these commenters note, such as 

providing consistent messages about 

when it may be beneficial for people to 

take actions to reduce PM2.5 exposures. 

Furthermore, with regard to an 

additional warning system, the EPA is 

concerned that having two air quality 

communication systems operating at the 

same time would likely be confusing to 

the public and reduce the effectiveness 

of the systems. 

At the same time, the EPA recognizes 

that when air quality is rapidly 

changing, such as during wildfire smoke 

events, reporting information based on a 

24-hour metric may not be as useful for 

the public as reporting more frequently 

would be. The EPA has balanced 

concerns about being able to provide 

timely communication of air quality 

hazards when conditions are changing 

quickly with the goal of limiting the 

number of air quality communications 

systems and its judgment that the 

evidence supports a 24-hour-based 

metric linked to the daily standard by 

establishing the NowCast, which takes 

into consideration subdaily PM2.5

concentrations and provides a near realtime AQI value based on the AQI colors 

and scale. Specifically, the NowCast 

shows air quality conditions for the 

most current hour of PM2.5 data 

available by using a calculation that 

involves multiple hours of past data. As 

noted in the AQI Technical Assistance 

Document, the NowCast currently uses 

longer averages during periods of stable 

air quality and shorter averages (down 

to a 3-hour average) when air quality is 

changing rapidly, such as during a 

wildfire (U.S. EPA, 2018a). As discussed 

further in section IV.D.2 of this notice, 

the EPA uses the NowCast to 

approximate the complete daily AQI 

(24-hour average) during any given 

hour. This means the subdaily NowCast 

is approximating a 24-hour average 

exposure, which aligns with the health 

evidence and the existing AQI 

communications network, while also 

being capable of communicating rapidly 

changing conditions to the public. 

3. Summary of Final Revisions 

Upon reviewing and considering the 

comments on the proposed revisions 

(summarized above in Section IV.C) 

along with the scientific evidence 

outlined in the proposal (88 FR 5639, 

January 27, 2023) and summarized 

above in section IV.A, the EPA is 

finalizing the proposed changes to the 

AQI. 

Thus, as discussed in section IV of the 

preamble (88 FR 5639, January 27, 2023) 

to the proposed rule, the EPA is taking 

final action to revise the AQI value of 

50 to 9.0 mg/m3, 24-hour average, 

consistent with the final decision on the 

primary annual PM2.5 standard level as 

summarized in section II.C of the 


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preamble to the final rule; retain the 

AQI value of 100 at 35 mg/m3, 24-hour 

average, consistent with the final 

decision on the primary 24-hour PM2.5

standard level as summarized in section 

II.C of the preamble to the final rule; 

and retain the AQI value of 150 at 55 mg/ 

m3, 24-hour average. The EPA is also 

taking action to revise the AQI value of 

200 to 125 mg/m3, 24-hour average; 300 

to 225 mg/m3, 24-hour average; and 500 

to 325 mg/m3, 24-hour average, 

consistent with the rationale discussed 

above and the health evidence 

discussed in section IV of the preamble 

(88 FR 5639, January 27, 2023) to the 

proposed rule. The EPA has prepared 

communications materials to assist 

States with adjusting to the revised AQI 

and looks forward to working with, and 

learning from the experiences of, State, 

local, and Tribal governments in 

implementing these changes. 

C. Air Quality Index Category 

Breakpoints for PM10

The EPA proposed to retain the PM10

sub-index of the AQI consistent with the 

proposed decision to retain the primary 

PM10 standard, and consistent with the 

health effects information that supports 

this proposed decision, as discussed in 

section III.D of the proposal (88 FR 

5632, January 27, 2023). EPA did not 

receive comments on this and is taking 

final action to retain the PM10 sub-index 

of the AQI for the reasons stated in the 

preamble to the proposed rule (88 FR 

5642, January 27, 2023). 

D. Air Quality Index Reporting 

With respect to the reporting 

requirements for the AQI and as noted 

in the proposal (88 FR 5642, January 27, 

2023) there have been many 

technological advances in air quality 

monitoring and data reporting since the 

appendix G to 40 CFR part 58 was last 

revised in 1999. Federal, State, local, 

and Tribal agencies have used these 

changes to make health information and 

air quality data more readily available 

and easier to access. Given this, it is 

useful to update the reporting 

requirements and recommendations to 

match current practices and ensure the 

public has the most useful and timely 

information to take health-protective 

behaviors. 

1. Summary of Proposed Revisions 

Currently, appendix G defines daily 

reporting as five days per week. When 

this reporting requirement was 

originated in 1999 the technology 

available at that time was not sufficient 

to calculate and report the AQI more 

than five days per week without 

requiring additional staffing on the 

weekends. Since that time, advances in 

technology have allowed for reporting 

seven days per week automatically 

without expending additional resources 

on weekends. As a result, most State, 

local, and Tribal air agencies now report 

the AQI seven days per a week. Given 

these technological advances and noting 

that reporting agencies currently report 

the AQI seven days per week, the EPA 

proposed that State, local, and Tribal 

agencies that report the AQI be required 

to report it seven days a week, ensuring 

that the members of the public continue 

to have access to daily air quality and 

health information that they can use to 

take steps to protect their health. 

Improvements in monitoring 

networks and modeling capabilities 

have also enabled the ability to report 

the AQI in near real-time. This allows 

State, local, and Tribal air agencies to 

provide timely air quality information to 

the public for making health-protective 

decisions and to help satisfy AQI 

reporting requirements. The availability 

of near real-time AQI data also allows 

for more timely responses by the public 

when air quality conditions are 

changing rapidly, such as during 

wildfire smoke events. Subdaily 

reporting of the AQI can be critical 

when there are rapidly change 

conditions and/or high pollution events 

so that the public is able to make 

informed decisions to protect their 

health. Many State, local, and Tribal air 

agencies currently report the AQI hourly 

to ensure that the public has access to 

accurate and timely information. In 

recognition of these advances, and to 

continue to provide for near-real time 

AQI reporting that the public has come 

to rely on, the EPA proposed to 

recommend that State, local, and Tribal 

agencies report the AQI in near-real 

time. 

In lieu of or along with reporting the 

near-real-time AQI directly to the 

public, most State/local and Tribal 

agencies submit hourly air quality data 

to the EPA. The EPA and some State, 

local and Tribal air quality agencies use 

this near-real-time data to create 

products for use by the public, weather 

service providers and the media as 

discussed in the proposal (88 FR 5643, 

January 27, 2023). To continue to ensure 

the availability of the products that the 

public and many stakeholders rely 

upon, the EPA proposed to recommend 

that State, local, and Tribal air quality 

agencies submit hourly data to the 

EPA’s air quality database. Submitting 

hourly data to the EPA for use on the 

AirNow website and in other products 

also enables State, local, and Tribal air 

quality agencies to meet the 

recommendation to report the AQI in 

near-real-time. 

In addition to the proposed updates to 

the reporting requirements and 

recommendations for near-real-time 

reporting and data submission 

recommendations, the Agency also 

proposed reformatting the question-andanswer format used in appendix G to 

align with the current standard 

formatting used in the Code of Federal 

Regulations. In proposing to update the 

format, the EPA did not reopen the 

language that has merely been moved or 

rearranged as there are no substantive 

changes. 

Another change the EPA proposed to 

make to appendix G is with regard to 

Table 2—Breakpoints for the AQI for 

purposes of clarity. As discussed in the 

proposal (88 FR 5642, January 27, 2023) 

and summarized here, the EPA 

proposed to collapse the two rows 

presented for the Hazardous Category 

into one. The two rows in the current 

table specify pollutant concentrations 

for two AQI ranges within the 

Hazardous category (301–400 and 401– 

500), with an intermediate break at 400. 

The 400 breakpoint for all criteria 

pollutants in the current Table 2 is set 

at the proportional pollutant 

concentration approximately halfway 

between the Index values of 300 and 

500. In proposing updated AQI 

breakpoints for PM2.5, the EPA 

considered adjusting the 400 breakpoint 

similarly. However, the EPA concluded 

that collapsing the two rows into a 

single range (301–500) would provide a 

more transparent and easy-to-follow 

presentation of the pollutant 

concentrations corresponding to the 

AQI range for the Hazardous category. 

Moreover, collapsing the Hazardous 

category into a single row in Table 2 has 

no substantive effect on the Emergency 

Episode program in 40 CFR part 51, 

appendix L. Thus, the EPA proposed to 

remove the breakpoint of 400 from the 

table in appendix G but this change 

would not substantively affect the 

derivation of the AQI for any pollutant. 

In addition, the EPA proposed to 

move some information currently in 

appendix G into the Technical 

Assistance Document for the Reporting 

of Daily Air Quality, or TAD (U.S. EPA, 

2018a), so that it can be updated in a 

more timely manner to reflect current 

scientific and health effects evidence 

and current communication methods, 

thereby assisting State, local, and Tribal 

agencies in providing accurate and 

timely information to the public. 

Information that was proposed to be 

moved from appendix G to the TAD 

included the definitions of the sensitive 

(at-risk) populations for each pollutant. 


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141U.S. EPA. (2013). Transitioning to a New 

NowCast Method. Presentation available in the 

Rulemaking Docket for the Review of the National 

Ambient Air Quality Standards for Particulate 

Matter (EPA–HQ–OAR–2015–0072), at: https://

www.regulations.gov/docket/EPA-HQ-OAR-2015- 

0072. 

This definition is typically evaluated 

and updated, as warranted, in most 

NAAQS reviews, even if the standard is 

not revised. Generally, if the standard is 

not revised in a review of the NAAQS, 

then appendix G is also not revised. 

Moving the definitions of sensitive 

groups to the TAD allows them to be 

updated even when a NAAQS is not 

revised to be consistent with the 

definitions of the sensitive (at-risk) 

populations identified in the ISA for 

that NAAQS review. Also, the proposal 

(88 FR 5642, January 27, 2023) 

recognized that the ways that air quality 

and health information is supplied to 

the news media and public changes 

regularly and thus proposed that 

information about suggested approaches 

for public communication be taken out 

of appendix G and discussed in the 

TAD. 

2. Summary of Significant Comments on 

the Proposed Revisions 

The EPA received many comments on 

the proposed changes to AQI reporting, 

many of which supported the proposed 

revisions. EPA discusses several of the 

topics that received the most attention 

from commenters below. Discussion of 

other comments received on the 

proposed changes to the AQI can be 

found in section IV of the Responses to 

Significant Comments on the 2023 

Proposed Reconsideration of the 

National Ambient Air Quality Standards 

for Particulate Matter. 

Most commenters expressed support 

for revising the definition of ‘‘daily 

reporting’’ from five days a week to 

seven days a week. A commenter did 

not support this change and 

recommended the EPA maintain the 

definition of daily as five days per week, 

noting that State and local air agencies 

do not routinely work seven days per 

week and would not be available to 

perform quality control of this data and 

report it reliably on weekends. 

The EPA appreciates the support for 

this proposed revision and disagrees 

that the proposed change would require 

personnel to perform quality control of 

AQI data on weekends. 40 CFR part 58 

Appendix D defines continuous 

monitoring requirements for agencies 

participating in the State/Local Air 

Monitoring Stations (SLAMS) network, 

and Appendix G states that agencies ‘‘ 

. . . must use concentration data from 

State/Local Air Monitoring Stations 

(SLAMS) required by 40 CFR 58.10’’ 

when reporting the AQI. Therefore, as 

noted in Appendix D and G, Agencies 

are required to report the AQI using 

monitors within SLAMS, which are not 

subject to daily quality control/ 

validation. 

A few commenters noted that the 

proposal preamble language mentioned 

AQI is reported three ways (88 FR 5637, 

5638, January 27, 2023): ‘‘The AQI is 

reported three ways all of which are 

useful and complementary. The daily 

AQI is reported for the previous day and 

used to observe trends in community air 

quality, the AQI forecast helps people 

plan their outdoor activities for the next 

day, and the near-real-time AQI, or 

NowCast AQI, tells people whether it is 

a good time for outdoor activity.’’ These 

commenters suggested that the NowCast 

is being codified in 40 CFR part 58 

Appendix G as a method of calculating 

the AQI, which they oppose, saying that 

codifying its use is inappropriate given 

the shortest averaging period of the 

PM2.5 NAAQS remains at 24-hours. 

Some stated that NowCast values have 

no direct correlation to the AQI 

calculation methodology codified in 40 

CFR part 58 Appendix G. These 

commenters say that codifying the 

NowCast would impose a significant 

burden on States’ forecasting staff. 

However, some other commenters 

noted they appreciate the publicfriendly format and near real-time data 

the NowCast provides and use it in their 

clinical encounters with patients. One 

air agency recognized the importance of 

the NowCast near real-time AQI during 

high pollution events and suggested the 

EPA should provide more ‘‘concrete’’ 

health messaging for these short-term 

spikes. 

The EPA disagrees that the preamble 

language proposed to codify the 

NowCast or to impose a burden on 

reporting agencies. The preamble to the 

proposed rule references the AQI being 

reported in three ways and it does so 

because the EPA and many State, local 

and Tribal air quality agencies already 

report it these three ways. However, text 

included in the preamble is generally 

explanatory and does not alter 

regulatory provisions. Comments that 

State that EPA is codifying the NowCast 

into Appendix G are incorrect. Further, 

in proposed revisions to 40 CFR part 58 

Appendix G, the EPA recommended, 

but did not propose to require, the use 

of air quality forecasts and a subdaily 

AQI. Consistent with the proposal, the 

EPA is therefore not finalizing any 

additional requirement or burden on 

States’ forecasting staff relative to 

forecasts or a subdaily AQI. 

The EPA disagrees with the comment 

that the NowCast values have no direct 

correlation to the AQI calculation 

methodology codified in 40 CFR part 58 

Appendix G. As noted in the AQI 

Technical Assistance Document 

(Technical Assistance Document for the 

Reporting of Daily Air Quality—the Air 

Quality Index (AQI)), the NowCast 

algorithm is based on the AQI 

methodology but provides more realtime information to the public (U.S. 

EPA, 2018a). While the NowCast 

algorithm is approximating a 24-hour 

average exposure, it can reflect 

concentrations observed over shorter 

averaging times when air quality is 

changing rapidly (U.S. EPA, 2018a). The 

EPA reflects the nature of the NowCast 

in the health messaging provided there. 

As noted in the above discussion of 

the AQI, air quality can change quickly 

during the day. A central purpose of the 

AQI is to help the public know when it 

is prudent to take action to reduce their 

exposure to pollution. Accordingly, the 

EPA developed the NowCast to estimate 

the 24-hour AQI for the current hour to 

give people information and tools to 

reduce their exposures to protect their 

health, particularly when air quality 

may be changing. The NowCast gives 

people the knowledge and ability to take 

timely action. They can use this 

information to reduce their exposure— 

reducing exposures if PM2.5 is high only 

during a few hours a day will help 

reduce a person’s 24-hour exposure—or 

be active when air quality is better. 

The first NowCast method was 

developed in 2003 and was designed so 

‘‘current conditions’’ represent the 24- 

hour PM2.5 standard as closely as 

possible. This method proved to be slow 

to respond during rapid air quality 

changes. In 2013, the EPA developed an 

updated NowCast method for PM2.5 141

that responds more quickly to rapidly 

changing air quality conditions, such as 

those we see during wildfires, to make 

air quality alerts more timely. We 

analyzed millions of data points in 

developing this NowCast method and 

presented this information to State, 

local and Tribal air agencies. The 

updated NowCast, which is still in use, 

was launched August 1, 2013, on 

AirNow.gov. It was designed to 

represent a shorter average (target 3- 

hour) when air quality is changing 

rapidly, in part because 3-hour averages 

from some continuous monitors are 

more stable than 1-hour averages. The 

NowCast reflects a longer-term (12-hour) 

average when air quality is stable. 

After evaluating the 2013 NowCast 

method, the EPA concluded that it 

matched the desired characteristics. The 

NowCast method responds to rapid 

changes in air quality yet still reflects a 


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142Consistent with the 2016 Integrated Review 

Plan (U.S. EPA, 2016), other welfare effects of PM, 

including ecological effects, are being considered in 

the separate, on-going review of the secondary 

NAAQS for oxides of nitrogen, oxides of sulfur and 

PM. Accordingly, the public welfare protection 

provided by the secondary PM standards against 

ecological effects such as those related to deposition 

of nitrogen- and sulfur-containing compounds in 

vulnerable ecosystems is being considered in that 

separate review. Thus, the Administrator’s decision 

in this reconsideration will be focused only and 

specifically on the adequacy of public welfare 

protection provided by the secondary PM standards 

from effects related to visibility, climate, and 

materials and hereafter ‘‘welfare effects’’ refers to 

non-ecological welfare effects (i.e., visibility, 

climate, and materials effects). 

143 In addition to the 2020 review’s opening ‘‘call 

for information’’ (79 FR 71764, December 3, 2014), 

the 2019 ISA identified and evaluated studies and 

reports that have undergone scientific peer review 

and were published or accepted for publication 

between January 1, 2009 through approximately 

January 2018 (U.S. EPA, 2019a, p. ES–2). References 

that are cited in the 2019 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. 

144As described in more detail in the ISA 

Supplement, ‘‘the scope of this Supplement 

provides specific criteria for the types of studies 

considered for inclusion within the Supplement. 

Specifically, studies must be peer reviewed and 

published between approximately January 2018 and 

March 2021’’ (U.S. EPA, 2022a, section 1.2.2). 

145As described in section 1.2.1 of the ISA 

Supplement, ‘‘the selection of welfare effects to 

evaluate within this Supplement is based on the 

causality determinations reported in the 2019 PM 

ISA and the subsequent use of scientific evidence 

in the 2020 PM PA. The 2019 PM ISA concluded 

a causal relationship for each of the welfare effects 

categories evaluated (i.e., visibility, climate effects, 

and materials effects). While the 2020 PM PA 

considered the broader set of evidence for these 

effects, for climate effects and material effects, it 

concluded that there remained ‘substantial 

uncertainties with regard to the quantitative 

relationships with PM concentrations and 

concentration patterns that limit[ed] [the] ability to 

quantitatively assess the public welfare protection 

provided by the standards from these effects (U.S. 

EPA, 2020b). Given these uncertainties and 

limitations, the basis of the discussion on 

conclusions regarding the secondary standards in 

the 2020 PM PA primarily focused on visibility 

effects. Therefore, this Supplement focuses only on 

visibility effects in evaluating newly available 

scientific information and is limited to studies 

Continued 

longer-term average when air quality is 

stable; will work in any location with 

adequate air quality data and for any air 

quality situation; gives people the best 

possible estimate of a 24-hour exposure; 

allows the EPA to caution people in 

time for them to take protective action 

and reduce their 24-hour exposure; and 

ensures that AQI maps on AirNow more 

closely match what people see. 

The AQI is designed to allow people 

to reduce their exposure when pollution 

levels are higher and be active outdoors 

when pollution levels are lower. Since 

air quality almost always changes 

during the day, that level of granularity 

is not possible with a 24-hour forecast. 

If the public has only the 24-hour 

forecast, they may miss the times to be 

active outdoors when air quality is 

better and may be active outdoors when 

air quality is worse. 

Also as noted above, many entities 

appreciate the near real-time reporting 

of the AQI that the NowCast provides 

and suggested more specific messaging 

is needed. The EPA appreciates this 

insight and will continue to consider 

ways to communicate air quality 

information most effectively to the 

public. For example, in light of recent 

wildfire events, the EPA worked with 

the USFS to pilot the AirNow Fire and 

Smoke Map. 

3. Summary of Final Revisions 

Upon reviewing and considering the 

comments on the proposed revisions 

(summarized above in Section IV.C) 

along with the rationale outlined in the 

proposal (88 FR 5638, January 27, 2023) 

and summarized above in section IV.C, 

the EPA is finalizing the proposed 

changes to the AQI reporting 

requirements. Thus, as discussed in 

section IV of the preamble to the 

proposed rule, the EPA is taking final 

action to require the AQI be reported 

seven days a week; recommend that 

State, local, and Tribal agencies report 

the AQI in near-real time; recommend 

that State, local, and Tribal air quality 

agencies submit hourly data to the 

EPA’s air quality database; reformat 

appendix G to align with the current 

standard formatting used in the Code of 

Federal Regulations; collapse the two 

rows in Table 2 presented for the 

Hazardous Category into one by 

removing the 400 breakpoint; and move 

some information currently in appendix 

G into the Technical Assistance 

Document for the Reporting of Daily Air 

Quality, or TAD (U.S. EPA, 2018a) such 

as including the definitions of the 

sensitive (at-risk) populations for each 

pollutant and suggested approaches for 

public communication as stated in the 

revised Appendix G. 

Table 2 below summarizes the 

breakpoints for the PM2.5 sub-index. 

TABLE 2—BREAKPOINTS FOR PM2.5 SUB-INDEX

AQI category Index values Breakpoints 

(μg/m3, 24-hour average) 

Good ................................................................................................................................................ 0–50 0.0–9.0 

Moderate .......................................................................................................................................... 51–100 9.1–35.4 

Unhealthy for Sensitive Groups ....................................................................................................... 101–150 35.5–55.4 

Unhealthy ......................................................................................................................................... 151–200 55.5–125.4 

Very Unhealthy ................................................................................................................................ 201–300 125.5–225.4 

Hazardous 1 ..................................................................................................................................... 301+ 225.5 

1AQI values between breakpoints are calculated using equation 1 in appendix G. For AQI values in the hazardous category, AQI values greater than 500 should be calculated using equation 1 and the PM2.5 concentration specified for the AQI value of 500. 

V. Rationale for Decisions on the 

Secondary PM Standards 

This section presents the rationale for 

the Administrator’s decision that no 

change to the current secondary PM 

standards is required at this time to 

provide requisite protection against the 

public welfare effects of PM within the 

scope of this reconsideration (i.e., 

visibility, climate, and materials 

effects).142 This decision is based on a 

thorough review of the scientific 

evidence generally published through 

December 2017,143 as presented in the 

2019 ISA (U.S. EPA, 2019a), on the nonecological public welfare effects of PM 

pertaining to the presence of PM in 

ambient air, specifically visibility, 

climate, and materials effects. 

Additionally, this decision is based on 

a thorough evaluation of some studies 

that became available after the literature 

cutoff date of the 2019 ISA that could 

either further inform the adequacy of 

the current PM NAAQS or address key 

scientific topics that have evolved since 

the literature cutoff date for the 2019 

ISA, generally through March 2021, as 

presented in the ISA Supplement 144

(U.S. EPA, 2022a). The selection of 

welfare effects evaluated within the ISA 

Supplement was based on the causality 

determinations reported in the 2019 ISA 

and the subsequent use of scientific 

evidence in the 2020 PA.145


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conducted in the U.S. and Canada’’ (U.S. EPA, 

2022a, section 1.2.1). 

Specifically, for welfare effects, the 

focus within the ISA Supplement is on 

visibility effects. The ISA Supplement 

does not include an evaluation of 

studies on climate or materials effects. 

The Administrator’s decision also takes 

into account the 2022 PA evaluation of 

the policy-relevant information in the 

2019 ISA and ISA Supplement and 

presentation of quantitative analysis of 

air quality related to visibility 

impairment; CASAC advice and 

recommendations, as reflected in 

discussions of the drafts of the ISA 

Supplement and 2022 PA at public 

meetings and in the CASAC’s letters to 

the Administrator; and public 

comments received on the proposal. 

In presenting the rationale for the 

Administrator’s final decision and its 

foundations, section V.A provides 

background on the 2020 final decision 

to retain the secondary PM standards 

(section V.A.1), and also provides brief 

summaries of key aspects of the 

currently available welfare effects 

evidence (section V.A.2) and 

quantitative information (section V.A.3). 

Section V.B summarizes the CASAC’s 

advice (section V.B.1) and the proposed 

conclusions (section V.B.2), addresses 

public comments received on the 

proposal (section V.B.3), and presents 

the Administrator’s conclusions on the 

adequacy of the current standards 

(section V.B.4), drawing on 

consideration of the available scientific 

and quantitative information, advice 

from the CASAC, and comments from 

the public. Section V.C summarizes the 

Administrator’s decision on the 

secondary PM standards. 

A. Introduction 

The general approach for this 

reconsideration of the 2020 final 

decision on the secondary PM standards 

relies on the EPA’s assessments of the 

current scientific evidence and 

associated quantitative analyses to 

inform the Administrator’s judgments 

regarding secondary standards that are 

requisite to protect the public welfare 

from known or anticipated adverse 

effects associated with the pollutant’s 

presence in the ambient air. The EPA’s 

assessments are primarily documented 

in the 2019 ISA, ISA Supplement, and 

2022 PA, which builds on the 2020 PA, 

all of which have received CASAC 

review and public comment (83 FR 

53471, October 23, 2018; 83 FR 55529, 

November 6, 2018; 85 FR 4655, January 

27, 2020; 86 FR 52673, September 22, 

2021; 86 FR 54186, September 30, 2021; 

86 FR 56263, October 8, 2021; 87 FR 

958, January 7, 2022; 87 FR 22207, April 

14, 2022; 87 FR 31965, May 26, 2022). 

In bridging the gap between the 

scientific assessments of the 2019 ISA 

and ISA Supplement and the judgments 

required of the Administrator in 

determining whether the current 

standards provide the requisite public 

welfare protection, the 2022 PA 

evaluates policy implications of the 

evaluation of the current evidence in the 

2019 ISA and ISA Supplement, and the 

quantitative information documented in 

the 2022 PA. In evaluating the public 

welfare protection afforded by the 

current standards against PM-related 

effects within the scope of this 

reconsideration, the four basic elements 

of the NAAQS (indicator, averaging 

time, level, and form) are considered 

collectively. 

The final decision on the adequacy of 

the current secondary standards is a 

public welfare policy judgment to be 

made by the Administrator. In reaching 

conclusions with regard to the standard, 

the decision draws on the scientific 

information and analyses about welfare 

effects, and associated public welfare 

significance, as well as judgments about 

how to consider the range and 

magnitude of uncertainties that are 

inherent in the scientific evidence and 

analyses. This approach is based on the 

recognition that the available evidence 

generally reflects a continuum that 

includes ambient air exposures at which 

scientists agree that effects are likely to 

occur through lower levels at which the 

likelihood and magnitude of responses 

become increasingly uncertain. This 

approach is consistent with the 

requirements of the provisions of the 

Clean Air Act related to the review of 

NAAQS and with how the EPA and the 

courts have historically interpreted the 

Act. These provisions require the 

Administrator to establish secondary 

standards that, in the judgment of the 

Administrator, are requisite to protect 

public welfare from known or 

anticipated adverse effects associated 

with the presence of the pollutant in the 

ambient air. 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 

standards be set at a zero-risk level, but 

rather at a level that reduces risk 

sufficiently so as to protect the public 

welfare from known or anticipated 

adverse effects. 

1. Background on the Current Standards 

The current secondary PM standards 

were retained in 2020 based on the 

scientific and technical information 

available at that time, as well as the 

then-Administrator’s judgments 

regarding the available welfare effects 

evidence, the appropriate degree of 

public welfare protection for the 

existing standards, and available air 

quality information on visibility 

impairment that may be allowed by 

such a standard (85 FR 82684, December 

18, 2020). With the 2020 decision, the 

then-Administrator retained the 

secondary 24-hour PM2.5 standard, with 

its level of 35 mg/m3, the annual PM2.5

standard, with its level of 15.0 mg/m3, 

and the 24-hour PM10 standard, with its 

level of 150 mg/m3. The subsections 

below focus on the key considerations 

and the then-Administrator’s 

conclusions in the 2020 final decision 

for climate and materials effects (section 

V.A.1.a) and visibility effects (section 

V.A.2.b). 

a. Non-Visibility Effects 

In light of the robust evidence base, 

the 2019 ISA concluded there to be 

causal relationships between PM and 

climate effects and materials effects 

(U.S. EPA, 2019a, sections 13.3.9 and 

13.4.2). The 2020 final decision was 

based on a thorough review in the 2019 

ISA of the scientific information on PMinduced climate and materials effects. 

The decision also took into account: (1) 

Assessments in the 2020 PA of the most 

policy-relevant information in the 2019 

ISA regarding evidence of adverse 

effects of PM to climate and materials, 

(2) uncertainties in the available 

evidence to inform a quantitative 

assessment of PM-related climate and 

materials effects, (3) CASAC advice and 

recommendations, and (4) public 

comments received during the 

development of these documents and on 

the proposal document. 

In considering non-visibility welfare 

effects in the 2020 decision, the thenAdministrator concluded that, while it 

is important to maintain an appropriate 

degree of control of fine and coarse 

particles to address non-visibility 

welfare effects, ‘‘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’’ (85 

FR 82744, December 18, 2020). 

With regard to climate, the thenAdministrator recognized that there 

were a number of improvements and 

refinements to climate models since the 

2012 review. However, while the 

evidence continued to support a causal 

relationship between PM and climate 

effects, the then-Administrator noted 

that significant limitations continued to 

exist related to quantifying the 

contributions of direct and indirect 


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146Preference studies were available in four 

urban areas. 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 2022 PA (U.S. EPA, 

2022b). 

effects of PM and PM components on 

climate forcing (U.S. EPA, 2020b, 

sections 5.2.2.1.1 and 5.4). He also 

recognized that the models continued 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 led the then-Administrator 

to conclude in the 2020 decision that 

the available scientific information 

remained insufficient to quantify 

climate impacts associated with 

particular concentrations of PM in 

ambient air (U.S. EPA, 2020b, 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 was insufficient information 

available to base a national ambient 

standard on climate impacts (85 FR 

82744, December 18, 2020). 

With regard to materials effects, the 

then-Administrator noted that the 

evidence available in the 2019 ISA 

continued to support a causal 

relationship between materials effects 

and PM deposition (U.S. EPA, 2019a, 

section 13.4). He recognized 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 building materials, and the 

weakening of material components. 

While some new information was 

available in the 2019 ISA, the 

information was from studies primarily 

conducted outside of the U.S. in areas 

where PM concentrations in ambient air 

are higher than those observed in the 

U.S. (U.S. EPA, 2020b, section 13.4). 

Additionally, the information assessed 

in the 2019 ISA did not support 

quantitative analyses of PM-related 

materials effects in the 2020 PA (U.S. 

EPA, 2020b, section 5.2.2.2.2). Given the 

limited amount of information available 

and its inherent uncertainties and 

limitations, the Administrator 

concluded that he was 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 was insufficient information 

available to support a distinct national 

ambient standard based on materials 

effects (85 FR 82744, December 18, 

2020). 

In reviewing the 2019 draft PA, the 

CASAC agreed with staff conclusions 

that, while these effects are important, 

‘‘the available evidence does not call 

into question the protection afforded by 

the current secondary PM standards’’ 

and recommended that the secondary 

standards ‘‘should be retained’’ (Cox, 

2019b, p. 3 of letter). In reaching a final 

decision in 2020, for all of the reasons 

discussed above and recognizing the 

CASAC conclusion that the evidence 

provided support for retaining the 

current secondary PM standards, the 

then-Administrator concluded that it 

was appropriate to retain the existing 

secondary PM standards, without 

revision. For climate and materials 

effects, this conclusion reflected 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 was insufficient 

information to establish distinct 

secondary PM standards to address nonvisibility PM-related welfare effects (85 

FR 82744, December 18, 2020). 

b. Visibility Effects 

The 2019 ISA concluded that, ‘‘the 

evidence is sufficient to conclude that a 

causal relationship exists between PM 

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

2019a, section 13.2.6). The 2020 

decision on the adequacy of the 

secondary standards with regard to 

visibility effects was a public welfare 

policy judgment made by the thenAdministrator, which drew upon the 

available scientific evidence for PMrelated visibility effects and on analyses 

of visibility impairment, as well as 

judgments about the appropriate weight 

to place on the range of uncertainties 

inherent in the evidence and analyses. 

The 2020 final decision was based on a 

thorough review in the 2019 ISA of the 

scientific information on PM-related 

visibility effects. The decision also took 

into account: (1) Assessments in the 

2020 PA of the most policy-relevant 

information in the 2019 ISA regarding 

evidence of adverse effects of PM on 

visibility; (2) air quality analyses of the 

PM2.5 visibility index and design values 

based on the form and averaging time of 

the existing secondary 24-hour PM2.5

standard; (3) CASAC advice and 

recommendations; and (4) public 

comments received during the 

development of these documents and on 

the 2020 proposal document. 

In considering the visibility effects in 

the 2020 review, the then-Administrator 

noted the long-standing body of 

evidence for PM-related visibility 

impairment. This evidence, which is 

based on the fundamental relationship 

between light extinction and PM mass, 

demonstrated that ambient PM can 

impair visibility in both urban and 

remote areas, and had changed very 

little since the 2012 review (U.S. EPA, 

2019a, section 13.1; U.S. EPA, 2009a, 

section 9.2.5). The evidence related to 

public perception of visibility 

impairment was from studies from four 

areas in North America.146 These 

studies provided 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 

then-Administrator noted that no new 

visibility studies conducted in the U.S. 

were discussed in the 2019 ISA, and 

there was little newly available 

information with regard to acceptable 

levels of visibility impairment in the 

U.S. The Administrator recognized that 

visibility impairment can have 

implications for people’s enjoyment of 

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

degree to which the current secondary 

standards protect against PM-related 

visibility impairment. 

Consistent with the 2012 review, in 

the 2020 review, the then-Administrator 

first concluded 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 thenAdministrator considered the specific 

aspects of such an index, including the 

appropriate indicator, averaging time, 

form and level (78 FR 82742–82744, 

December 18, 2020). 

First, with regard to indicator, the 

then-Administrator noted that in the 

2012 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, which allows the estimation 

of the light extinction using routinely 

monitored components of PM2.5, 

PM10–2.5 mass, and estimates of relative 

humidity. The then-Administrator 

recognized that, while there have been 

some revisions to the IMPROVE 

algorithm since the time of the 2012 


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147 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. 

148For comparison, 20 dv, 25 dv, and 30 dv are 

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

respectively. 

review, our fundamental understanding 

of the relationship between PM in 

ambient air and light extinction had 

changed little and the various IMPROVE 

algorithms appropriately reflected this 

relationship across the U.S. In the 

absence of a monitoring network for 

direct measurement of light extinction, 

he concluded that a calculated light 

extinction indicator that utilizes the 

IMPROVE algorithms continued to 

provide a reasonable basis for defining 

a target level of protection against PMrelated visibility impairment (78 FR 

82742–82744, December 18, 2020). 

In further defining the characteristics 

of a visibility index, the thenAdministrator next considered the 

appropriate averaging time, form, and 

level of the index. Given the available 

scientific information the review, and in 

considering the CASAC’s advice and 

public comments, the thenAdministrator concluded that, 

consistent with the decision in the 2012 

review, a visibility index with a 24-hour 

averaging time and a form based on the 

3-year average of annual 90th percentile 

values remained reasonable. With 

regard to the averaging time and form of 

such an index, the Administrator noted 

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, 

2013), indicating that a 24-hour 

averaging time is an appropriate 

surrogate for the subdaily time periods 

of the perception of PM-related 

visibility impairment and the relevant 

exposure periods for segments of the 

viewing public. This decision in the 

2020 review 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 thenAdministrator recognized that there was 

no new information to support updated 

analyses of this nature, and therefore, he 

believed these analyses continued 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 noted that, consistent 

with the 2012 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 then-Administrator 

judged 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, remained appropriate (78 FR 

82742–82744, December 18, 2020). 

With regard to the level of a visibility 

index, consistent with the 2012 review, 

the then-Administrator judged that it 

was appropriate to establish a target 

level of protection of 30 deciviews 

(dv),147 148 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 (U.S. EPA, 2011, section 4.3.4). 

At the time of the 2012 review, the thenAdministrator 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, 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 in 2012, with their inherent 

uncertainties and limitations, the thenAdministrator 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 candidate protection 

levels (CPLs). 

Given that there were no new 

preference studies in the 2019 ISA, the 

then-Administrator’s judgments in 2020 

were based on the same studies, with 

the same range of levels, available in the 

2012 review. The 2020 PA (U.S. EPA, 

2020b, section 5.5), discussed a number 

of limitations and uncertainties 

associated with these studies. 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 thenAdministrator concluded that it was 

appropriate to again use a level of 30 dv 

for the visibility index (78 FR 82742– 

82744, December 18, 2020). 

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, was requisite to 

protect public welfare with regard to 

visual air quality, the Administrator 

next considered the degree of protection 

from visibility impairment afforded by 

the existing suite of secondary PM 

standards. 

In this context, the thenAdministrator considered the updated 

analyses of visibility impairment 

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

2020b, section 5.2.1.2), which reflected 

a number of improvements since the 

2012 review. Specifically, the updated 

analyses examined multiple versions of 

the IMPROVE equation, including the 

version incorporating revisions since 

the time of the 2012 review. These 

updated analyses provided a further 

understanding of how variation in the 

inputs to the algorithms affect the 

estimates of light extinction (U.S. EPA, 

2020b, Appendix D). Additionally, for a 

subset of monitoring sites with available 

PM10–2.5 data, the updated analyses 

better characterized the influence of 

coarse PM on light extinction than in 

the 2012 review (U.S. EPA, 2020b, 

section 5.2.1.2). 

The results of the updated analyses in 

the 2020 PA were consistent with those 

from the 2012 review. Regardless of 

which version of the IMPROVE equation 

was used, the analyses demonstrated 

that, based on 2015–2017 data, the 3- 

year visibility metric was 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, 2020b, section 5.2.1.2). 

While the inclusion of the coarse 

fraction had a relatively modest impact 

on the estimates of light extinction, the 

then-Administrator recognized 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 analyses 

in the 2020 PA due to a lack of available 

data (U.S. EPA, 2019a, section 13.2.4.1; 

U.S. EPA, 2020b, section 5.2.1.2). He 


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149All particles scatter light and, although a 

larger particle scatters more light than a similarly 

shaped smaller particle of the same composition, 

the light scattered per unit of mass is greatest for 

particles with diameters from ∼0.3–1.0 mm (U.S. 

EPA, 2009a, section 2.5.1; U.S. EPA, 2019a, section 

13.2.1). Particles with hygroscopic components 

(e.g., particulate sulfate and nitrate) contribute more 

to light extinction at higher relative humidity than 

at lower relative humidity because they change size 

in the atmosphere in response to relative humidity. 

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. 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 PM2.5 standard. Thus, the thenAdministrator judged that the 24-hour 

PM2.5 standard provided 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 judged appropriate (78 FR 

82742–82744, December 18, 2020). 

2. Overview of Welfare Effects Evidence 

The information summarized here is 

based on the scientific assessment of the 

welfare effects evidence available in this 

reconsideration; this assessment is 

documented in the 2019 ISA and ISA 

Supplement and its policy implications 

are further discussed in the 2022 PA. 

While the 2019 ISA provides the broad 

scientific foundation for this 

reconsideration, additional literature 

has become available since the cutoff 

date of the 2019 ISA that expands the 

body of evidence related to visibility 

effects that can inform the 

Administrator’s judgment on the 

adequacy of the current secondary PM 

standards. As such, the ISA Supplement 

builds on the information in the 2019 

ISA with a targeted identification and 

evaluation of new scientific information 

regarding visibility effects. As described 

in the ISA Supplement and the 2022 

PA, the selection of welfare effects to 

evaluate within the ISA Supplement 

were based on the causality 

determinations reported in the 2019 ISA 

and the subsequent use of scientific 

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

2019a, section 1.2; U.S. EPA, 2022a, 

section 1.4.2). The ISA Supplement 

focuses on U.S. and Canadian studies 

that provide new information on public 

preferences for visibility impairment 

and/or developed new methodologies or 

conducted quantitative analyses of light 

extinction (U.S. EPA, 2022a, section 

1.2). Such studies of visibility effects 

and quantitative relationships between 

visibility impairment and PM in 

ambient air were considered to be of 

greatest utility in informing the 

Administrator’s conclusions on the 

adequacy of the current secondary PM 

standards. The visibility effects 

evidence presented within the 2019 

ISA, along with the targeted 

identification and evaluation of new 

scientific information in the ISA 

Supplement, provides the scientific 

basis for the reconsideration of the 2020 

final decision on the secondary PM 

standards for visibility effects. For 

climate and materials effects, the 2020 

PA concluded that there were 

substantial uncertainties associated with 

the quantitative relationships with PM 

concentrations and the concentration 

patterns that limited the ability to 

quantitatively assess the public welfare 

protection provided by the standards 

from these effects. Therefore, the 

evaluation of the information related to 

these effects draws heavily from the 

2019 ISA and 2020 PA. The subsections 

below briefly summarize the nature of 

PM-related visibility (section V.B.1.a), 

climate (section V.B.1.b), and materials 

(section V.B.1.c) effects. 

a. Nature of Effects 

Visibility impairment can have 

implications for people’s enjoyment of 

daily activities and for their overall 

sense of well-being (U.S. EPA, 2009a, 

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, 2009a), which confirms a 

well-established ‘‘causal relationship 

exists between PM and visibility 

impairment’’ (U.S. EPA, 2009a, 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, 2009a, 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, 2009a, p. 

2–31). 

The evidence available in this 

reconsideration is consistent with the 

evidence available at the time of the 

2012 and 2020 reviews and supports the 

conclusions of causal relationships 

between PM and visibility, climate, and 

materials effects (U.S. EPA, 2019a, 

chapter 13). Evidence newly available in 

this reconsideration augments the 

previously available evidence of the 

relationship between PM and visibility 

impairment (U.S. EPA, 2019a, section 

13.2; U.S. EPA, 2022a, section 4), 

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

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

2019a, section 13.4). 

i. Visibility 

The fundamental relationship 

between light extinction and PM mass, 

and the EPA’s understanding of this 

relationship, has changed little since the 

2009 ISA (U.S. EPA, 2009a). 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.149 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, 2019a, 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, 2019a, section 13.2.3; Mie, 1908, 

Van de Hulst, 1981). Fine particles 

scatter more light than coarse particles 

on a per unit mass basis and include 

sulfates, nitrates, organics, lightabsorbing 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, 2019a, 

section 13.2.3). 

As at the time of the 2012 and 2020 

reviews, direct measurements of PM 

light extinction, scattering, and 

absorption continue to be considered 

more accurate for quantifying visibility 

than PM mass-based estimates because 

measurements do not depend on 

assumptions about particle 

characteristics (e.g., size, shape, density, 

component mixture, etc.) (U.S. EPA, 

2019a, 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., 


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150The 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). 

151Preference studies were available in four 

urban areas in the last review: Denver, Colorado 

(Ely et al., 1991), Vancouver, British Columbia, 

Canada (Pryor, 1996), Phoenix, Arizona (BBC 

Research & Consulting, 2003), and Washington, DC 

(Abt Associates, 2001; Smith and Howell, 2009). 

152The Grand Canyon study used a single scene 

looking west down the canyon with a small 

landscape feature of a 100-km-distant mountain 

(Mount Trumbull), along with other closer 

landscape features. The scenes presented in the 

previously available visibility preference studies are 

presented in more detail in Table D–9 in the 2022 

PA (U.S. EPA, 2022b, Appendix D). 

153The Grand Canyon study superimposed light 

extinction ranging from 3 dv to 20 dv on the image 

slides shown to participants compared to the 

previously available preference studies. In those 

studies, the visibility ranges presented were as low 

as 9 dv and as high as 45 dv. The visibility ranges 

presented in the previously available visibility 

preference studies are described in more detail in 

Table D–9 in the 2022 PA (U.S. EPA, 2022b, 

Appendix D). 

154 In the Grand Canyon study, the level of 

impairment that was determined to be ‘‘acceptable’’ 

by at least 50 percent of study participants was 7 

dv (Malm et al., 2019). 

transmissometers, integrating 

nephelometers, teleradiometers, 

telephotometers, and photography and 

photographic modeling), although each 

of these methods has its own strengths 

and limitations (U.S. EPA, 2019a, Table 

13–1). While some recent research 

confirms and adds to the body of 

knowledge regarding direct 

measurements as is described in the 

2019 ISA and ISA Supplement, no 

major new developments have been 

made with these measurement methods 

since prior reviews (U.S. EPA, 2019a, 

section 13.2.2.2; U.S. EPA, 2022a, 

section 4.2). 

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, 

2019a, Equation 13.5), can be used to 

estimate light extinction by combining 

mass scattering efficiencies of particles 

with particle concentrations (U.S. EPA, 

2019a, section 13.2.3; U.S. EPA, 2009a, 

sections 9.2.2.2 and 9.2.3.1). This 

estimation of light extinction is 

consistent with the method used in 

previous reviews. The algorithm used to 

estimate light extinction, known as the 

IMPROVE algorithm,150 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, 2019a, sections 

13.2.3.1 and 13.2.3.3). 

At the time of the 2012 review, two 

versions of the IMPROVE algorithm 

were available in the literature—the 

original IMPROVE algorithm 

(Lowenthal and Kumar, 2004, Malm and 

Hand, 2007, Ryan et al., 2005) and the 

revised IMPROVE algorithm (Pitchford 

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

the 2022 PA (U.S. EPA, 2022b, section 

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

2019a, section 13.2.3), the algorithm has 

been further evaluated and refined since 

the time of the 2012 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 

reconsideration. 

Consistent with the evidence 

available at the time of the 2012 and 

2020 reviews, our understanding of 

public perception of visibility 

impairment comes from visibility 

preference studies conducted in four 

areas in North America.151 The detailed 

methodology for these studies are 

described in the 2022 PA (U.S. EPA, 

2022b, section 5.3.1.1), the 2019 ISA 

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

(U.S. EPA, 2019a). 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, 2020b, 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). 

Since the completion of the 2009 and 

2019 ISAs, there has been only one 

public preference study that has become 

available in the U.S. This study uses 

images of the Grand Canyon, AZ, 

described in the ISA Supplement (U.S. 

EPA, 2022a). The Grand Canyon study, 

conducted by Malm et al. (2019), has a 

similar study design to that used in the 

public preference studies discussed 

above; however, there are several 

important differences that make it 

difficult to directly compare the results 

of the Malm et al. (2019) study with 

other public preference studies. As an 

initial matter, the Grand Canyon study 

was conducted in a Federal Class I area, 

as opposed to in an urban area, with a 

scene depicted in the photographs that 

did not include urban features.152 We 

recognize that public preferences with 

respect to visibility in Federal Class 1 

areas may well differ from visibility 

preferences in urban areas and other 

contexts, although there is currently a 

lack of information to on such 

questions. Further, the Malm et al. 

(2019) study also used a much lower 

range of superimposed ‘‘haze’’ than the 

preference studies discussed above.153 It 

is unclear whether the participant 

preferences are a function in part of the 

range of potential values presented, 

such that the participant preferences for 

the Grand Canyon were generally 

lower 154 than the other preference 

studies in part because of the lower 

range of superimposed ‘‘haze’’ for the 

images in that study, or if their 

preferences would vary if presented 

with images with a range of 

superimposed ‘‘haze’’ more comparable 

to the levels used in the other studies 

(i.e., more ‘‘haze’’ superimposed on the 

images). 

The Malm et al. (2019) study also 

explored alternate methods for 

evaluating ‘‘acceptable’’ levels of visual 

air quality from the preference studies, 

including the use of scene-specific 

visibility indices as potential indicators 

of visibility levels as perceived by the 

observer (Malm et al., 2019). In addition 

to measures of atmospheric haze, such 


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155Radiative 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, 

2019a, section 13.3.2.2). 

156As discussed in the 2019 ISA (U.S. EPA, 

2019a, 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, 2019a, 

p. 13–1), the assessment of the evidence in the 2019 

ISA considers the combined impacts. 

as atmospheric extinction, used in 

previously available preference studies, 

other indices for visual air quality 

include color and achromatic contrast of 

single landscape figures, average and 

equivalent contrast of an entire scene, 

edge detection algorithms such as the 

Sobel index, and just-noticeable 

difference or change indexes. The 

results reported by Malm et al. (2019) 

suggest that scene-dependent metrics, 

such as contrast, may be useful alternate 

predictors of preference levels 

compared to universal metrics like light 

extinction (U.S. EPA, 2022a, section 

4.2.1). This is because extinction alone 

is not a measure of ‘‘haze,’’ but of light 

attenuation per unit distance, and 

visible ‘‘haze’’ is dependent on both 

light extinction and distance to a 

landscape feature (U.S. EPA, 2022a, 

section 4.2.1). However, there are very 

few studies available that use scenedependent metrics (i.e., contrast) to 

evaluate public preference information, 

which makes it difficult to evaluate 

them as an alternative to the light 

extinction approach. 

ii. Climate 

The available evidence continues to 

support the conclusion of a causal 

relationship between PM and climate 

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

Since the 2012 review, climate impacts 

have been extensively studied and 

recent research reinforces and 

strengthens the evidence evaluated in 

the 2009 ISA. Recent evidence provides 

greater specificity about the details of 

radiative forcing effects 155 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 

completion of the 2009 ISA, 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 2009 ISA, the 2019 ISA 

draws substantially on the IPCC report 

to summarize climate effects. As 

discussed in more detail in the 2022 PA 

(U.S. EPA, 2022b, section 5.3.2.1.1), 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 2012 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 completion of the 

2009 ISA, the evidence base has 

expanded with respect to the 

mechanisms of climate responses and 

feedbacks to PM radiative forcing; 

however, the recently published 

literature assessed in the 2019 ISA does 

not reduce the considerable 

uncertainties that continue to exist 

related these mechanisms. 

As described in the proposal (88 FR 

5650, January 27, 2023), 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 variables with 

possible differences in the direction of 

the forcing in different regions or for 

different variables (U.S. EPA, 2019a, 

section 13.3.6). The complex climate 

system interactions lead to variation 

among climate models, which have 

suggested a range of factors that 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, 

2019a, section 13.3.7). As a result, there 

remains insufficient evidence to related 

climate effects to specific PM levels in 

ambient air or to establish a quantitative 

relationship between PM and climate 

effects, particularly at a regional scale. 

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 evidence assessed 

in the 2009 ISA, the available 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, 2019a, section 13.4.2). 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.156 There is a 

limited amount of recently available 

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 of PM in ambient air 

greater than those typically observed in 

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

Building on the evidence available in 

the 2009 ISA, and as described in detail 

in the proposal (88 FR 5650, January 27, 

2023) and in the 2019 ISA (U.S. EPA, 

2019a, 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 

completion of the 2009 ISA, 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 recent 

evidence assessed in the 2019 ISA is 

generally similar to the evidence 

available in the 2009 ISA, including 


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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 PMrelated effects on materials. 

3. Summary of Air Quality and 

Quantitative Information 

Beyond the consideration of the 

scientific evidence, as discussed in 

section V.A.2 above, quantitative 

analyses of PM air quality, when 

available, can also inform conclusions 

on the adequacy of the public welfare 

protection provided by the current 

secondary PM standards. 

a. Visibility Effects 

In the 2012 and 2020 reviews, 

quantitative analyses for PM-related 

visibility effects focused on daily 

visibility impairment, given the shortterm nature of PM-related visibility 

effects. The evidence and information 

available in this reconsideration 

continues to provide support for the 

short-term (i.e., hourly or daily) nature 

of PM-related visibility impairment. As 

such, the quantitative analyses 

presented in the 2022 PA continue to 

focus on daily visibility impairment and 

utilize a two-phase assessment approach 

for visibility impairment, consistent 

with the approaches taken in past 

reviews. First, the 2022 PA considers 

the appropriateness of the elements 

(indicator, averaging time, form, and 

level) of the visibility index for 

providing protection against PM-related 

visibility effects. Second, recent air 

quality was used to evaluate the 

relationship between the current 

secondary 24-hour PM2.5 standard and 

the visibility index. The information 

available since the 2012 review includes 

an updated equation for estimating light 

extinction, summarized in the 2022 PA 

(U.S. EPA, 2022b, section 5.3.1.1) and 

described in the 2019 ISA (U.S. EPA, 

2019a, section 13.2.3.3), as well as more 

recent air monitoring data, that together 

allow for development of an updated 

assessment of PM-related visibility 

impairment in study locations in the 

U.S. 

i. Target Level of Protection in Terms 

of a PM2.5 Visibility Index 

In evaluating the adequacy of the 

current secondary PM standards, the 

2022 PA first evaluates the 

appropriateness of the elements 

(indicator, averaging time, form, and 

level) identified for a visibility index to 

protect against visibility effects. In 

previous reviews, the visibility index as 

set at a level of 30 dv, with estimated 

light extinction as the indicator, a 24- 

hour averaging time, and a 90th 

percentile form, averaged over three 

years. 

With regard to an indicator for the 

visibility index, the 2022 PA recognizes 

the lack of availability of methods and 

an established network for directly 

measuring light extinction (U.S. EPA, 

2022b, section 5.3.1.1). Therefore, 

consistent with previous reviews, the 

2022 PA concludes that a visibility 

index based on estimates of light 

extinction by PM2.5 components derived 

from an adjusted version of the original 

IMPROVE algorithm to be the most 

appropriate indicator for the visibility 

index in this reconsideration. As 

described in section 5.3.1.1 of the 2022 

PA, the IMPROVE algorithm estimates 

light extinction using routinely 

monitored components of PM2.5 and 

PM10–2.5, along with estimates of relative 

humidity (U.S. EPA, 2022b, section 

5.3.1.1). 

With regard to averaging time, the 

2022 PA notes that the evidence 

continues to provide support for the 

short-term nature of PM-related 

visibility effects. Given that there is no 

new information available regarding the 

time periods during which visibility 

impairment occurs or public preferences 

related to specific time periods for 

visibility impairment, the 2022 PA 

concludes that it is appropriate to 

continue to focus on daily visibility 

impairment. In so doing, the 2022 PA 

relies on analyses that were conducted 

in the 2012 review that showed 

relatively strong correlations between 

24-hour and subdaily (i.e., 4-hour 

average) PM2.5 light extinction that 

indicated that a 24-hour averaging time 

is an appropriate surrogate for the 

subdaily time periods relevant for visual 

perception (U.S. EPA, 2011, Figures G– 

4 and G–5; Frank, 2012). These analyses 

continue to provide support for a 24- 

hour averaging time for the visibility 

index in this reconsideration. Consistent 

with previous reviews, the 2022 PA also 

notes that the 24-hour averaging time 

may be less influenced by atypical 

conditions and/or atypical instrument 

performance than a subdaily averaging 

time (85 FR 82740, December 18, 2020; 

78 FR 3226, January 15, 2013). 

With regard to the form for the 

visibility index, the available 

information continues to provide 

support for a 3-year average of annual 

90th percentile values. Given that there 

is no new information to inform 

selection of an alternate form, as in 

previous reviews, the 2022 PA notes 

that the 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 (85 

FR 82741, December 18, 2020; 78 FR 

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

p. 4–58). In so doing, the 2022 PA 

considers the evaluation in the 2010 

Urban-Focused Visibility Assessment 

(UFVA) of three different statistical 

forms: 90th, 95th, and 98th percentiles 

(U.S. EPA, 2010b, Chapter 4).). In 

considering this evaluation of statistical 

forms from the 2010 UFVA, consistent 

with the 2011 PA, the 2022 PA notes 

that the Regional Haze Program targets 

the 20 percent most impaired days for 

visibility improvements in visual air 

quality in Federal Class I areas and that 

the median of the distribution of these 

20 percent most impaired days would 

be the 90th percentile. The 2011 PA also 

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 visibility index 

would reasonably be expected to lead to 

improvements in visual air quality for 

the 20 percent most impaired days. 

Additionally, as in the 2011 PA, the 

2022 PA recognizes that the available 

public preference studies do not address 

frequency of occurrence of different 

levels of visibility (U.S. EPA, 2022b, 

section 5.3.1.2). Therefore, the analyses 

and consideration 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 

reconsideration. 

With regard to the level for the 

visibility index, the 2022 PA recognizes 

that there is an additional public 

preference study (Malm et al., 2019) 

available in this reconsideration. As 

noted above, however, this study differs 

from the previously available public 

preference studies in several ways, 

which makes it difficult to integrate this 

newly available study with the 

previously available studies. Most 

significantly, this study was evaluated 

public preferences for visibility in the 

Grand Canyon, perhaps the most 

notable Class I area in the country for 

visibility purposes. Therefore, the 2022 

PA concludes that the Grand Canyon 

study is not directly comparable to the 

other available preferences studies and 

public preferences of visibility 

impairment in the Malm et al. (2019) 

study are not appropriate to consider in 

identifying a range of levels for the 

target level of protection against 

visibility impairment for this 

reconsideration of the secondary PM 

NAAQS. 


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157As noted above, the available public 

preference studies include those conducted in 

Denver, Colorado (Ely et al., 1991), Vancouver, 

British Columbia, Canada (Pryor, 1996), Phoenix, 

Arizona (BBC Research & Consulting, 2003), and 

Washington, DC (Abt Associates, 2001; Smith and 

Howell, 2009). 

158The other preference studies did not include 

populations that were necessarily representative of 

the population in the area for which the images 

being judged. For example, in the Denver, CO, 

study, participants were from intact groups (i.e., 

those who were meeting for other reasons) and were 

asked to provide a period of time during a regularly 

scheduled meeting to participate in the study (Ely 

et al., 1991). As another example, in the British 

Columbia, Canada, study, participants were 

recruited from undergraduate and graduate students 

enrolled in classes at the University of British 

Columbia’s Department of Geography (Pryor, 1996). 

Therefore, the 2022 PA continues to 

rely on the same studies 157 and the 

range of 20 to 30 dv identified from 

those studies in previous reviews. With 

regard to selecting the appropriate target 

level of protection for visibility 

impairment within this range, the 2022 

PA notes that in previous reviews, a 

level at the upper end of the range (i.e., 

30 dv) was selected given the 

uncertainties and limitations associated 

with the public preference studies (U.S. 

EPA, 2022b, section 5.3.1.1). However, 

the 2022 PA also recognizes that (1) the 

degree of protection provided by a 

secondary PM NAAQS is not 

determined solely by any one element of 

the standard but by all elements (i.e., 

indicator, averaging time, form, and 

level) being considered together, and (2) 

decisions regarding the adequacy of the 

current secondary standards is a public 

welfare policy judgment to be made by 

the Administrator. As such, the 

Administrator may judge that a target 

level of protection below the upper end 

of the range (i.e., less than 30 dv) is 

appropriate, depending on his public 

welfare policy judgments, which draw 

upon the available scientific evidence 

for PM-related visibility effects and on 

analyses of visibility impairment, as 

well as judgments about the appropriate 

weight to place on the range of 

uncertainties inherent in the evidence 

and analyses. 

In considering the available public 

preference studies, consistent with past 

reviews, the 2022 PA concludes that it 

is reasonable to consider a range of 20 

to 30 dv for selecting a target level of 

protection, including a high value of 30 

dv, a midpoint value of 25 dv, and a low 

value of 20 dv. A target level of 

protection at or in the upper end of the 

range would focus on the Washington, 

DC, preference study results (Abt 

Associates, 2001; Smith and Howell, 

2009), which identified 30 dv as the 

level of impairment that was 

determined to be ‘‘acceptable’’ by at 

least 50 percent of study participants. 

The public preferences of visibility 

impairment in the Washington, DC, 

study are likely to be generally 

representative of urban areas that do not 

have valued scenic elements (e.g., 

mountains) in the distant background. 

This would be more representative of 

areas in the middle of the country and 

many areas in the eastern U.S., as well 

as possibly some areas in the western 

U.S. 

A target level of protection in the 

middle of the range would be most 

closely associated with the level of 

impairment that was determined to be 

‘‘acceptable’’ by at least 50 percent of 

study participants in the Phoenix, AZ, 

study (BBC Research & Consulting, 

2003), which was 24 dv. This study, 

while methodologically similar to the 

other public preference studies, 

included participants that were selected 

as a representative sample of the 

Phoenix area population 158 and used 

computer-generated images to depict 

specific uniform visibility impairment 

conditions. This study yielded the best 

results of the four public preference 

studies in terms of the least noisy 

preference results and the most 

representative selection of participants. 

Therefore, based on this study, the use 

of 25 dv to represent a midpoint within 

the range of target levels protection is 

well supported. 

A target level of protection at or just 

above the lower end of the range would 

focus on the Denver, CO, study, but may 

not be as strongly supported as higher 

levels within the range (Ely et al., 1991). 

Older studies, such as those conducted 

in Denver, CO (Ely et al., 1991), and 

British Columbia, Canada (Pryor, 1996), 

used photographs that were taken at 

different times of the day and on 

different days to capture a range of light 

extinction levels needed for the 

preference studies. Compared to studies 

that used computer-generated images 

(i.e., those in Phoenix, AZ, and 

Washington, DC) there was more 

variability in scene appearance in these 

older studies that could affect 

preference rating and includes 

uncertainties associated with using 

ambient measurements to represent 

sight path-averaged light extinction 

values rather than superimposing a 

computer-generated amount of haze 

onto the images. When using 

photographs, the intrinsic appearance of 

the scene can change due to 

meteorological conditions (i.e., shadow 

patterns and cloud conditions) and 

spatial variations in ambient air quality 

that can result in ambient light 

extinction measurement not being 

representative of the sight-path-averaged 

light extinction. Computer-generated 

images, such as those generated with 

WinHaze, do not introduce such 

uncertainties, as the same base 

photograph is used (i.e., there is no 

intrinsic change in scene appearance) 

and the modeled haze that is 

superimposed on the photograph is 

determined based on uniform light 

extinction throughout the scene. 

In addition to differences in 

preferences that may arise from 

photographs versus computer-generated 

images, urban visibility preference may 

differ by location, and such differences 

may arise from differences in the 

cityscape scene that is depicted in the 

images. These differences are related to 

the perceived value of objects and 

scenes that are included in the image, as 

objects at a greater distance have a 

greater sensitivity to perceived visibility 

changes as light extinction is changed 

compared to similar scenes with objects 

at shorter distances. For example, a 

person (regardless of their location) 

evaluating visibility in an image with 

more scenic elements such as 

mountains or natural views may value 

better visibility conditions in these 

images compared to the same level of 

visibility impairment in an image that 

only depicts urban features such as 

buildings and roads. That is, if a person 

was shown the same level of visibility 

impairment in two images depicting 

different scenes—one with mountains in 

the background and urban features in 

the foreground and one with no 

mountains in the background and 

nearby buildings in the image without 

mountains in the distance—may find 

the amount of haze to be unacceptable 

in the image with the mountains in the 

distance because of a greater perceived 

value of viewing the mountains, while 

finding the amount of haze to be 

acceptable in the image with the 

buildings because of a lesser value of 

viewing the cityscape or an expectation 

that such urban areas may generally 

have higher levels of haze in general. 

This is consistent when comparing the 

differences between the Denver, CO, 

study results (which found the 50% 

acceptance criteria occurred at the best 

visual air quality levels among the four 

cities) and the Washington, DC, results 

(which found the 50% acceptability 

criteria occurred at the worst visual air 

quality levels among the four cities). 

These results may occur because the 

most prominent and picturesque feature 

of the cityscape of Denver is the visible 

snow-covered mountains in the 

distance, while the prominent and 


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159The analyses presented in the 2022 PA focus 

on the visibility index and the current secondary 

24-hour PM2.5 standard with a level of 35 mg/m3. 

However, we recognize that all three secondary PM 

standards influence the PM concentrations 

associated with the air quality distribution. As 

noted in section V.A.1 above, the current secondary 

PM standards include the 24-hour PM2.5 standard, 

with its level of 35 mg/m3, the annual PM2.5

standard, with its level of 15.0 mg/m3, and the 24- 

hour PM10 standard, with its level of 150 mg/m3. 

With regard to the annual PM2.5 standard, we note 

that all 60 areas included in the analyses meet the 

current secondary annual PM standard (U.S. EPA, 

2022b, Table D–7). 160While 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 subdaily visibility 

impairment estimates. As detailed in the 2022 PA, 

there are uncertainties associated with the precision 

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

2022b, p. 2–18), as well as to the fractional 

uncertainty associated with 24-hour PM component 

measurements (U.S. EPA, 2022b, p. 2–21). Given 

the uncertainties present when evaluating data 

quality on a 24-hour basis, the uncertainty 

associated with subdaily 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 subdaily information. 

161A 3-year visibility metric with a level of 30 dv 

would be at the upper end of the range of levels 

identified from the public preference studies. 

162When light extinction is calculated using the 

original IMPROVE equation, all 60 sites have 3-year 

visibility metrics below 30 dv, 58 sites are at or 

below 25 dv, and 26 sites are at or below 20 dv (see 

U.S. EPA, 2022b, Appendix D, Table D–3). 163As described in more detail in the 2022 PA, 

the revised IMPROVE equation divides PM 

components into smaller and larger sizes of 

particles in PM2.5, with separate mass scattering 

efficiencies and hygroscopic growth functions for 

each size category (U.S. EPA, 2022b, section 

5.3.1.1). 164When light extinction is calculated using the 

revised IMPROVE equation, all 60 sites have 3-year 

visibility metrics below 30 dv, 56 sites are at or 

below 25 dv, and 26 sites are at or below 20 dv (see 

U.S. EPA, 2022b, Appendix D, Table D–3). 165When light extinction is calculated using the 

Lowenthal and Kumar IMPROVE equation, 59 sites 

have 3-year visibility metrics below 30 dv, 45 sites 

are at or below 25 dv, and 15 sites are at or below 

20 dv. The one site with a 3-year visibility metric 

of 32 dv exceeds the secondary 24-hour PM2.5

standard, with a design value of 56 mg/m3 (see U.S. 

EPA, 2022b, Appendix D, Table D–3). 

picturesque features of the Washington, 

DC, cityscape are buildings relatively 

nearby without prominent and/or 

valued scenic features that are more 

distant. Given these variabilities in 

preferences it is unclear to what extent, 

the available evidence provides strong 

support for a target level of protection 

at the lower end of the range. Future 

studies that reduce sources of noisiness 

and uncertainty in the results could 

provide more information that would 

support selection of a target level of 

protection at or just above the lower end 

of the range. 

Taken together, the 2022 PA 

concludes that available information 

continues to support a visibility index 

with estimated light extinction as the 

indicator, a 24-hour averaging time, and 

a 90th percentile form, averaged over 

three years, with a level within the 

range of 20 to 30 dv. 

ii. Relationship Between the PM2.5

Visibility Index and the Current 

Secondary 24-Hour PM2.5 Standard 

The 2022 PA presents quantitative 

analyses based on recent air quality that 

evaluate the relationship between recent 

air quality and calculated light 

extinction. As in previous reviews, 

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 2012 

and 2020 reviews (U.S. EPA, 2022b, 

section 5.3.1.2). As discussed in section 

V.C.1.a above, the 2022 PA concludes 

that the available evidence continues to 

support a visibility index with 

estimated light extinction as the 

indicator, a 24-hour averaging time, and 

a 90th percentile form, averaged over 

three years, with a level within the 

range of 20 to 30 dv. These analyses 

evaluate visibility impairment in the 

U.S. under recent air quality conditions, 

particularly those conditions that meet 

the current standards, and the relative 

influence of various factors on light 

extinction. Given the relationship of 

visibility with short-term PM, we focus 

particularly on the short-term PM 

standards.159 Compared to the 2012 

review, updated analyses incorporate 

several refinements, including (1) the 

evaluation of three versions of the 

IMPROVE equation to calculate light 

extinction (U.S. EPA, 2022b, Appendix 

D, Equations D–1 through D–3) in order 

to better understand the influence of 

variability in equation inputs; 160 (2) the 

use of 24-hour relative humidity data, 

rather than monthly average relative 

humidity as was used in the 2012 

review (U.S. EPA, 2022b, section 

5.3.1.2, Appendix D); and (3) the 

inclusion of the coarse fraction in the 

estimation of light extinction (U.S. EPA, 

2022b, section 5.3.1.2, Appendix D). 

The analyses in the reconsideration are 

updated from the 2012 and 2020 

reviews and include 60 monitoring sites 

that measure PM2.5 and PM10 and are 

geographically distributed across the 

U.S. in both urban and rural areas (U.S. 

EPA, 2022b, Appendix D, Figure D–1). 

When light extinction was calculated 

using the revised IMPROVE equation, in 

areas that meet the current 24-hour 

PM2.5 standard for the 2017–2019 time 

period, all sites have light extinction 

estimates at or below 26 dv (U.S. EPA, 

2022b, Figure 5–3). For the four 

locations that exceed the current 24- 

hour PM2.5 standard, light extinction 

estimates range from 22 dv to 27 dv 

(U.S. EPA, 2022b, Figure 5–3). These 

findings are consistent with the findings 

of the analyses using the same 

IMPROVE equation in the 2012 review 

with data from 102 sites with data from 

2008–2010 and in the 2020 review with 

data from 67 sites with data from 2015– 

2017. The analyses presented in the 

2022 PA indicate similar findings to 

those from the analyses in the 2012 and 

2020 reviews, i.e., the updated 

quantitative analysis shows that the 3- 

year visibility metric was no higher than 

30 dv 161 at sites meeting the current 

secondary PM standards, and at most 

such sites the 3-year visibility index 

values are much lower (e.g., an average 

of 20 dv across the 60 sites).162

When light extinction was calculated 

using the revised IMPROVE equation,163

the resulting 3-year visibility metrics are 

nearly identical to light extinction 

estimates calculated using the original 

IMPROVE equation (U.S. EPA, 2022b, 

Figure 5–4), but some sites are just 

slightly higher. Using the revised 

IMPROVE equation, for those sites that 

meet the current 24-hour PM2.5

standard, the 3-year visibility metric is 

at or below 26 dv. For the four locations 

that exceed the current 24-hour PM2.5

standard, light extinction estimates 

range from 22 dv to 29 dv (U.S. EPA, 

2022b, Figure 5–4). These results are 

similar to those for light extinction 

calculated using the original IMPROVE 

equation,164 and those from previous 

reviews. 

When light extinction was calculated 

using the refined equation from 

Lowenthal and Kumar (2016), the 

resulting 3-year visibility metrics are 

slightly higher at all sites compared to 

light extinction estimates calculated 

using the original IMPROVE equation 

(U.S. EPA, 2022b, Figure 5–5).165 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 nonremote areas, including the urban areas 

in the updated analyses in this 

reconsideration. 

Nevertheless, when light extinction is 

calculated using the Lowenthal and 


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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 28 dv. For those 

sites that exceed the current 24-hour 

PM2.5 standard, three of these sites have 

a 3-year visibility metric ranging 

between 26 dv and 30 dv, while one site 

in Fresno, California that exceeds the 

current 24-hour PM2.5 standard and has 

a 3-year visibility index value of 32 dv 

(compared to 29 dv when light 

extinction is calculated with the original 

IMPROVE equation) (see U.S. EPA, 

2022b, Appendix D, Table D–3). At this 

site, it is likely that the 3-year visibility 

metric using the Lowenthal and Kumar 

(2016) equation would be below 30 dv 

if PM2.5 concentrations were reduced 

such that the 24-hour PM2.5 level of 35 

mg/m3 was attained. 

In considering visibility impairment 

under recent air quality conditions, the 

2022 PA recognizes that the differences 

in the inputs to equations estimating 

light extinction can influence the 

resulting values. For example, given the 

varying chemical composition of 

emissions from different sources, the 2.1 

multiplier for converting OC to organic 

matter (OM) in the Lowenthal and 

Kumar (2016) equation may not be 

appropriate for all source types. At the 

time of the 2012 review, the EPA judged 

that a 1.6 multiplier was more 

appropriate, for the purposes of 

estimating visibility index at sites across 

the U.S., than the 1.4 or 1.8 multipliers 

used in the original and revised 

IMPROVE equations, respectively. A 

multiplier of 1.8 or 2.1 would account 

for the more aged and oxygenated 

organic PM that tends to be found in 

more remote regions than in urban 

regions, whereas a multiplier of 1.4 may 

underestimate the contribution of 

organic PM found in remote regions 

when estimating light extinction (78 FR 

3206, January 15, 2013; U.S. EPA, 2012, 

p. IV–5). The available scientific 

information and results of the air quality 

analyses indicate that it may be 

appropriate to select inputs to the 

IMPROVE equation (e.g., the multiplier 

for OC to OM) on a regional basis rather 

than a national basis when calculating 

light extinction. This is especially true 

when comparing sites with localized 

PM sources (such as sites in urban or 

industrial areas) to sites with PM 

derived largely from biogenic precursor 

emissions (that contribute to 

widespread secondary organic aerosol 

formation), such as those in the 

southeastern U.S. The 2022 PA notes, 

however, that conditions involving PM 

from such different sources have not 

been well studied in the context of 

applying a multiplier to estimate light 

extinction, contributing uncertainty to 

estimates of light extinction for such 

conditions. 

At the time of the 2012 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 2012 review suggested 

that, generally, PM10–2.5 was a minor 

contributor to visibility impairment 

most of the time (U.S. EPA, 2010b) 

although the coarse fraction may be a 

major contributor in some areas in the 

desert southwestern region of the U.S. 

Moreover, at the time of the 2012 

review, there were few data available 

from PM10–2.5 monitors to quantify the 

contribution of coarse PM to calculated 

light extinction. Since that time, an 

expansion in PM10–2.5 monitoring efforts 

has increased the availability of data for 

use in estimating light extinction with 

both PM2.5 and PM10–2.5 concentrations 

included as inputs in the equations. The 

analysis in the 2020 PA addressed light 

extinction at 20 of the 67 PM2.5 sites 

where collocated PM10–2.5 monitoring 

data were available. Since that time, 

PM10–2.5 monitoring data are available at 

more locations and the analyses 

presented in the 2022 PA include those 

for light extinction estimated with 

coarse and fine PM at all 60 sites. 

Generally, the contribution of the coarse 

fraction to light extinction at these sites 

is minimal, contributing less than 1 dv 

to the 3-year visibility metric (U.S. EPA, 

2020b, section 5.2.1.2). However, the 

2022 PA notes that in the updated 

quantitative analyses, only a few sites 

were in locations that would be 

expected to have high concentrations of 

coarse PM, such as the Southwest. 

These results are consistent with those 

in the analyses in the 2019 ISA, which 

found that mass scattering from 

PM10¥2.5 was relatively small (less than 

10%) in the eastern and northwestern 

U.S., whereas mass scattering was much 

larger in the Southwest (more than 20%) 

particularly in southern Arizona and 

New Mexico (U.S. EPA, 2019a, section 

13.2.4.1, p. 13–36). 

Overall, the findings of these updated 

quantitative analyses are generally 

consistent with those in the 2012 and 

2020 reviews. The 3-year visibility 

metric was generally below 26 dv in 

most areas that meet the current 24-hour 

PM2.5 standard. Small differences in the 

3-year visibility metric were observed 

between the variations of the IMPROVE 

equation, which may suggest that it may 

be more appropriate to use one version 

over another in different regions of the 

U.S. based on PM characteristics such as 

particle size and composition to more 

accurately estimate light extinction. 

b. Non-Visibility Effects 

Consistent with the evidence 

available at the time of the 2012 and 

2020 reviews, and as described in detail 

in the 2022 PA (U.S. EPA, 2022b, 

section 5.3.2.2), 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 (88 FR 5654, January 27, 

2023), our understanding of PM-related 

climate effects is still limited by 

significant key uncertainties. The 

recently 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, 

2022b, sections 5.3.2.1.1 and 5.5). 

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, 2022b, sections 

5.3.2.1.1 and 5.5). Additionally, while 

improvements have been made to 

climate models since the completion of 

the 2009 ISA, 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, 2022b, sections 5.3.2.1.1 and 5.5). 

While our understanding of climate 

forcing on a global scale is somewhat 

expanded since the 2012 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 recent research is available on 

climate forcing on a global scale, the 

remaining limitations and uncertainties 

are significant, and the recent 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 climaterelated effects in the U.S. that would be 

necessary to evaluate or consider a level 

of air quality to protect against such 

effects and for informing consideration 

of a national PM standard on climate in 

this reconsideration (U.S. EPA, 2022b, 

section 5.3.2.2.1; U.S. EPA, 2019a, 

section 13.3). 


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For PM-related materials effects, as 

explained in more detail in the 2022 PA 

(U.S. EPA, 2022b, section 5.3.2.2), 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 doseresponse 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, 2022b, section 5.3.2.1.2; 

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

evidence on materials effects 

characterized in the 2019 ISA 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, 2022b, 

section 5.3.2.1.2). While the available 

evidence assessed in the 2019 ISA is 

somewhat expanded since the time of 

the 2012 review, quantitative 

relationships have not been established 

for PM-related soiling and corrosion and 

frequency of cleaning or repair that 

further the understanding of the public 

welfare implications of materials effects 

(U.S. EPA, 2022b, section 5.3.2.2.2; U.S. 

EPA, 2019a, section 13.4). Therefore, 

there is insufficient information to 

inform quantitative analyses assessing 

materials effects to inform consideration 

of a national PM standard on materials 

in this reconsideration (U.S. EPA, 

2022b, section 5.3.2.2.2; U.S. EPA, 

2019a, section 13.4). 

B. Conclusions on the Secondary PM 

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 2020 decision and reflects upon the 

body of information and evidence 

available in this reconsideration. 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 

2022 PA (summarized in sections V.B 

and V.D.2 of the proposal, section V.A.2 

above). The quantitative informationbased considerations draw from the 

results of the quantitative analyses of 

visibility impairment presented in the 

2022 PA (as summarized in section V.C 

of the proposal and V.A.3 above) and 

consideration of these results in the 

2022 PA. 

Consideration of the scientific 

evidence and quantitative information 

in the 2022 PA and by the 

Administrator is framed by 

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

summarizes the rationale for the 

Administrators proposed decision, 

drawing from section V.D.3 of the 

proposal. The advice and 

recommendations of the CASAC and 

public comments on the proposed 

decision are addressed below in 

sections V.B.1 and V.B.3, respectively. 

The Administrator’s conclusions in this 

reconsideration regarding the adequacy 

of the secondary PM standards and 

whether any revisions are appropriate 

are described in section V.D.4. 

1. CASAC Advice 

In comments on the 2019 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, 2019b). 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, 2019b, p. 13 of consensus 

responses). Regarding climate, while the 

CASAC agreed that research on PMrelated effects has expanded since the 

2012 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, 

2019b, 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, 2019b, 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, 2019b, 

p. 14 of consensus responses). When 

considering the overall body of 

scientific evidence and technical 

information for PM-related effects on 

visibility, climate, and materials, the 

CASAC agreed with the EPA’s 

preliminary conclusions in the 2019 

draft PA, stating 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, 

2019b, p. 3 of letter). 

In this reconsideration, the CASAC 

provided its advice regarding the 

current secondary PM standards in the 

context of its review of the 2021 draft 

PA (Sheppard, 2022a). In its comments 

on the 2021 draft PA, the CASAC first 

recognized that the scientific evidence 

is sufficient to support a causal 

relationship between PM and visibility 

effects, climate effects and materials 

effects. 

With regard to visibility effects, the 

CASAC recognized that the 

identification of a target level of 

protection for the visibility index is 

based on a limited number of studies 

and suggested that ‘‘additional regionand view-specific visibility preference 

studies and data analyses are needed to 

support a more refined visibility target’’ 

(Sheppard, 2022a, p. 21 of consensus 

responses). While the CASAC did not 

recommend revising either the target 

level of protection for the visibility 

index or the level of the current 24-hour 

PM2.5 standard, they did state that a 

visibility index of 30 deciviews ‘‘needs 

to be justified’’ and ‘‘[i]f a value of 20– 

25 deciviews is deemed to be an 

appropriate visibility target level of 

protection, then a secondary 24-hour 

PM2.5 standard in the range of 25–35 

mg/m3 should be considered’’ 

(Sheppard, 2022a, p. 21 of consensus 

responses). 

The CASAC also recognized the 

limited availability of monitoring 

methods and networks for directly 

measuring light extinction. As such, 

they suggest that ‘‘[a] more extensive 

technical evaluation of the alternatives 

for visibility indicators and practical 

measurement methods (including the 

necessity for a visibility FRM) is need 

for future reviews’’ (Sheppard, 2022a, p. 

22 of consensus letter). The majority of 

the CASAC ‘‘recommend[ed] that an 

FRM for a directly measured PM2.5 light 

extinction indicator be developed’’ to 

inform the consideration of the 

protection afforded by the secondary 

PM standards against visibility 

impairment, the minority of the CASAC 

‘‘believe that a light extinction FRM is 

not necessary to set a secondary 

standard protective of visibility’’ 

(Sheppard, 2022a, p. 22 of consensus 

responses). 


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166The Regional Haze Program was established 

by Congress specifically to achieve ‘‘the prevention 

of any future, and the remedying of existing, 

impairment of visibility in mandatory Class I areas, 

which impairment results from man-made air 

pollution,’’ and that Congress established a longterm program to achieve that goal (CAA section 

169A). 

167 In adopting section 169A, Congress set a goal 

of eliminating anthropogenic visibility impairment 

at Class I areas, as well as a framework for achieving 

that goal which extends well beyond the planning 

process and timeframe for attaining secondary 

NAAQS. Thus, the Regional Haze Program will 

continue to contribute to reductions in visibility 

impairment in Class I areas. 

168As noted above, the Administrator viewed the 

Regional Haze Program as a complement to the 

secondary PM NAAQS, and thus took into 

consideration its approach to improving visibility 

in considering how to address visibility outside of 

Class I areas. 

With regard to climate, the CASAC 

noted that ‘‘there is a causal relationship 

between PM and climate change, but 

large uncertainties remain’’ and 

recommended additional research 

(Sheppard, 2022a, p. 22 of consensus 

responses). With respect to materials 

damage, the CASAC noted that 

‘‘[q]uantitative information on the 

relationship between PM and material 

damage is lacking’’ and suggested some 

additional studies and research 

approaches that could provide 

additional information on the effects of 

PM on materials and the quantitative 

assessment of the relationship between 

materials effects and PM in ambient air 

(Sheppard, 2022a, p. 23 of consensus 

responses). 

2. Basis for the Proposed Decision 

In reaching his proposed conclusions, 

the Administrator first recognized that, 

consistent with the scope of this 

reconsideration, his decision in this 

reconsideration will be focused only 

and specifically on the adequacy of 

public welfare protection provided by 

the secondary PM standards from effects 

related to visibility, climate, and 

materials. He then considered the 

assessment of the current evidence and 

conclusions reached in the 2019 ISA 

and ISA Supplement; the currently 

available quantitative information, 

including associated limitations and 

uncertainties, described in detail and 

characterized in the 2022 PA; 

considerations and staff conclusions 

and associated rationales presented in 

the 2022 PA; and the advice and 

recommendations from the CASAC (88 

FR 5655, January 27, 2023). 

With respect to visibility, the 

Administrator noted the longstanding 

body of evidence that demonstrates a 

causal relationship between ambient PM 

and effects on visibility (U.S. EPA, 

2019a, section 13.2). and that visibility 

impairment can have implications for 

people’s enjoyment of daily activities 

and for their overall sense of well-being. 

Therefore, as in previous reviews, he 

considered the degree to which the 

current secondary standards protect 

against PM-related visibility 

impairment. In so doing, and consistent 

with previous reviews, the 

Administrator considered the protection 

provided by the current secondary 

standards against PM-related visibility 

impairment in conjunction with the 

Regional Haze Program 166 for protecting 

visibility in Class I areas,167 which 

together would be expected to achieve 

appropriate visual air quality across all 

areas (88 FR 5658, January 27, 2023). 

The Administrator proposed to 

conclude that addressing visibility 

impairment in Class I areas is beyond 

the scope of the secondary PM NAAQS 

and that setting the secondary PM 

NAAQS at a level that would remedy 

visibility impairment in Class I areas 

would result in standards that are more 

stringent than is requisite. 

In further considering what standards 

are requisite to protect against adverse 

public welfare effects from visibility 

impairment, the Administrator adopted 

an approach consistent with the 

approach used in previous reviews (88 

FR 5645, January 27, 2023). That is, he 

first identified an appropriate target 

level of protection in terms of a PM 

visibility index that accounts for the 

factors that influence the relationship 

between particles in the ambient air and 

visibility (i.e., size fraction, species 

composition, and relative humidity). He 

then considered air quality analyses 

examining the relationship between this 

PM visibility index and the current 

secondary 24-hour PM2.5 standard in 

locations meeting the current 24-hour 

PM2.5 and PM10 standards (U.S. EPA, 

2022b, section 5.3.1.2; 88 FR 5650, 

January 27, 2023). 

To identify a target level of protection, 

the Administrator first considered the 

characteristics of the visibility index 

and defines its elements (indicator, 

averaging time, form, and level). With 

regard to the indicator for the visibility 

index, the Administrator recognized 

that there is a lack of availability of 

methods and an established network for 

directly measuring light extinction, 

consistent with the conclusions reached 

in the 2022 PA (U.S. EPA, 2022b, 

section 5.3.1.1) and with the CASAC’s 

recommendation for additional research 

on direct measurement methods for 

light extinction in their review of the 

2021 draft PA (Sheppard, 2022a, p. 22 

of consensus responses). Consistent 

with the approaches used in reaching 

decisions in 2012 and 2020, given the 

lack of such monitoring data, the 

Administrator preliminarily judged that 

estimated light extinction, as calculated 

using one or more versions of the 

IMPROVE algorithms, continues to be 

the most appropriate indicator for the 

visibility index in this reconsideration 

(88 FR 5659, January 27, 2023). 

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. With 

regard to the averaging time and form, 

the Administrator noted that in previous 

reviews, a 24-hour averaging time was 

selected and the form was defined as the 

3-year average of annual 90th percentile 

values. The Administrator recognized 

that the evidence available in this 

reconsideration and described in the 

2022 PA continue to provide support for 

the short-term nature of PM-related 

visibility effects. Considering the 

available analyses of 24-hour and 

subdaily PM2.5 light extinction, and 

noting that the CASAC did not provide 

advice or recommendations with regard 

to the averaging time of the visibility 

index, the Administrator preliminarily 

judged that the 24-hour averaging time 

continues to be appropriate for the 

visibility index (88 FR 5659, January 27, 

2023). 

With regard to the form of the 

visibility index, the Administrator noted 

that, consistent with the approach taken 

in other NAAQS, including the current 

secondary 24-hour PM2.5 NAAQS, a 

multi-year percentile form offers greater 

stability to the air quality management 

process by reducing the possibility that 

statistically unusual indicator values 

will lead to transient violations of the 

standard. Using a 3-year average 

provides stability from the occasional 

effects of inter-annual meteorological 

variability that can result in unusually 

high pollution levels for a particular 

year (88 FR 5659, January 27, 2023). In 

considering the percentile that would be 

appropriate with the 3-year average, the 

Administrator first noted that the 

Regional Haze Program targets the 20% 

most impaired days for improvements 

in visual air quality in Class I areas.168

Based on analyses examining 90th, 95th, 

and 98th percentile forms, the 

Administrator preliminarily judged that 

a focus similar to the Regional Haze 

Program focused on improving the 20% 

most impaired days suggest that the 

90th percentile, which represents the 

median of the 20% most impaired days, 

such that 90% of days have visual air 

quality that is at or below the target 

level of protection of the visibility 


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index, would be reasonably expected to 

lead to improvements in visual air 

quality for the 20% most impaired days 

(88 FR 5659, January 27, 2023). In the 

analyses of percentiles, the results 

suggest that a higher percentile value 

could have the effect of limiting the 

occurrence of days with peak PMrelated light extinction in areas outside 

of Federal Class I areas to a greater 

degree. However, the Administrator 

preliminarily concluded that it is 

appropriate to balance concerns about 

focusing on the group of most impaired 

days with concerns about focusing on 

the days with peak visibility 

impairment. Additionally, the 

Administrator noted that the CASAC 

did not provide advice or 

recommendations related to the form of 

the visibility index. Therefore, the 

Administrator preliminarily judged that 

it remains appropriate to define a 

visibility index in terms of a 24-hour 

averaging time and a form based on the 

3-year average of annual 90th percentile 

values (88 FR 5659, January 27, 2023). 

With regard to the level of the 

visibility index, the Administrator first 

noted that the scientific evidence that is 

available to inform the level of the 

visibility index is largely the same as in 

previous reviews, and continues to 

provide support for a level within the 

range of 20 to 30 dv (88 FR 5659–5660, 

January 27, 2023). The Administrator 

recognized that significant uncertainties 

and limitations remained, in particular 

those related to the public preference 

studies, including methodological 

differences between the studies, and 

that the available studies may not 

capture the full range of visibility 

preferences in the U.S. population (88 

FR 5659–5660, January 27, 2023). The 

Administrator also noted that, in their 

review of the 2021 draft PA, the CASAC 

recognized that a judgment regarding 

the appropriate target level of protection 

for the visibility index is based on a 

limited number of visibility preference 

studies, with studies conducted in the 

western U.S. reporting public 

preferences for visibility impairment 

associated with the lower end of the 

range of levels, while studies conducted 

in the eastern U.S. reporting public 

preferences associated with the upper 

end of the range (Sheppard, 2022a, p. 21 

of consensus responses). The 

Administrator noted that there have 

long been significant questions about 

how to set a national standard for 

visibility that is not overprotective for 

some areas of the U.S. In establishing 

the Regional Haze Program to improve 

visibility in Class I areas, Congress 

noted that ‘‘as a matter of equity, the 

national ambient air quality standards 

cannot be revised to adequately protect 

visibility in all areas of the country.’’ 

H.R. Rep. 95–294 at 205. Thus, in 

reaching his proposed conclusion, the 

Administrator recognized that there are 

substantial uncertainties and limitations 

in the public preference studies that 

should be considered when selecting a 

target level of protection for the 

visibility index and took the 

uncertainties and variability inherent in 

the public preference studies into 

account. In so doing, the Administrator 

first preliminarily judged that, 

consistent with similar judgments in 

past reviews, it is appropriate to 

recognize that the secondary 24-hour 

PM2.5 standard is intended to address 

visibility impairment across a wide 

range of regions and circumstances, and 

that the current standard works in 

conjunction with the Regional Haze 

Program to improve visibility, and 

therefore, it is appropriate to establish a 

target level of protection based on the 

upper end of the range of levels. In 

considering the information available in 

this reconsideration and the CASAC’s 

advice, the Administrator proposed to 

conclude that the protection provided 

by a visibility index based on estimated 

light extinction, a 24-hour averaging 

time, and a 90th percentile form, 

averaged over 3 years, set at a level of 

30 dv (the upper end of the range of 

levels) would be requisite to protect 

public welfare with regard to visibility 

impairment (88 FR 5660, January 27, 

2023). 

In preliminarily concluding that it 

remains appropriate in this 

reconsideration to define the target level 

of protection in terms of a visibility 

index based on estimated light 

extinction as described above (i.e., with 

a 24-hour averaging time; a 3-year, 90th 

percentile form; and a level of 30 dv), 

the Administrator next considered the 

degree of protection from visibility 

impairment afforded by the existing 

secondary standards. He considered the 

updated analyses of PM-related 

visibility impairment presented in the 

2022 PA (U.S. EPA, 2022b, section 

5.3.1.2), which reflect several 

improvements over the analyses 

conducted in the 2012 review. 

Specifically, the updated analyses 

examine multiple versions of the 

IMPROVE algorithm, including the 

version incorporating revisions since 

the 2012 review (section V.B.1.a), which 

provides an improved understanding of 

how variation in equation inputs 

impacts calculated light extinction (U.S. 

EPA, 2022b, Appendix D). In addition, 

unlike the analyses in the 2012 review 

and the 2020 PA, all of the sites 

included in the analyses had PM10–2.5

data available, which allows for better 

characterization of the influence of the 

coarse fraction on light extinction (U.S. 

EPA, 2022b, section 5.3.1.2). 

The Administrator noted that the 

results of these updated analyses are 

consistent with the results from the 

2012 and 2020 reviews (88 FR 5660, 

January 27, 2023). Regardless of the 

IMPROVE equation used, these analyses 

demonstrate that the 3-year visibility 

metric is at or below 28 dv in all areas 

meeting the current 24-hour PM2.5

standard (section V.C.1.b). Given the 

results of these analyses, the 

Administrator preliminarily concluded 

that the updated scientific evidence and 

technical information support the 

adequacy of the current secondary PM2.5

and PM10 standards to protect against 

PM-related visibility impairment. While 

the inclusion of the coarse fraction had 

a relatively modest impact on calculated 

light extinction in the analyses 

presented in the 2022 PA, he 

nevertheless recognized the continued 

importance of the PM10 standard given 

the potential for larger impacts in 

locations with higher coarse particle 

concentrations, such as in the 

southwestern U.S., for which only a few 

sites met the criteria for inclusion in the 

analyses in the 2022 PA (U.S. EPA, 

2019a, section 13.2.4.1; U.S. EPA, 

2022b, section 5.3.1.2). 

With regard to the adequacy of the 

secondary 24-hour PM2.5 standard, the 

Administrator noted that the CASAC 

stated that ‘‘[i]f a value of 20–25 

deciviews is deemed to be an 

appropriate visibility target level of 

protection, then a secondary 24-hour 

PM2.5 standard in the range of 25–35 mg/ 

m3 should be considered’’ (Sheppard, 

2022a, p. 21 of consensus responses). 

The Administrator recognized that the 

CASAC recommended that the 

Administrator provide additional 

justification for a visibility index target 

of 30 dv but did not specifically 

recommend that he choose an 

alternative level for the visibility index. 

The Administrator considered the 

CASAC’s advice, together with the 

available scientific evidence and 

quantitative information, in reaching his 

proposed conclusions. He recognized 

conclusions regarding the appropriate 

weight to place on the scientific and 

technical information examining PMrelated visibility impairment including 

how to consider the range and 

magnitude of uncertainties inherent in 

that information is a public welfare 

policy judgment left to the 

Administrator. As such, the 

Administrator noted his conclusion on 


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169As noted earlier, other welfare effects of PM, 

such as ecological effects, are being considered in 

the separate, on-going review of the secondary 

NAAQS for oxides of nitrogen, oxides of sulfur and 

PM. 

the appropriate visibility index (i.e., 

with a 24-hour averaging time; a 3-year, 

90th percentile form; and a level of 30 

dv) and his conclusions regarding the 

quantitative analyses of the relationship 

between the visibility index and the 

current secondary 24-hour PM2.5

standard. In so doing, he proposed to 

conclude that the current secondary 

standards provide requisite protection 

against PM-related visibility effects (88 

FR 5661, January 27, 2023). 

In reaching his proposed conclusions, 

the Administrator also recognized that 

the available evidence on visibility 

impairment generally reflects a 

continuum and that the public 

preference studies did not identify a 

specific level of visibility impairment 

that would be perceived as ‘‘acceptable’’ 

or ‘‘unacceptable’’ across the whole U.S. 

population. However, he noted that a 

judgment regarding the appropriate 

target level of protection would take 

into consideration the appropriate 

weight to place on the individual public 

preference studies. In so doing, he noted 

that placing more weight on the public 

preference study from Washington, DC, 

could provide support for a target level 

of protection at or near 30 dv, whereas 

placing more weight on the public 

preference study performed in the 

Phoenix, AZ, study could provide 

support for a target level of protection 

below 30 dv and down to 25 dv. While 

the Administrator noted that, in their 

review of the 2021 draft PA, the CASAC 

did not recommend revising the level of 

the current 24-hour PM2.5 standard, the 

Administrator recognized that they did 

recommend greater justification for a 

target level of protection of 30 dv, and 

noted that if a target level of protection 

of 20–25 dv was identified, then a 

secondary 24-hour PM2.5 standard in the 

range of 25–35 mg/m3 should be 

considered (Sheppard, 2022a, p. 21 of 

consensus responses). For these reasons, 

the Administrator solicited comment on 

his proposed decision to retain the 

current secondary 24-hour PM2.5

standard, as well as the appropriateness 

of a target level of protection for 

visibility below 30 dv and as low as 25 

dv, and on revising the level of the 

current secondary 24-hour PM2.5

standard to a level as low as 25 mg/m3. 

With respect to climate effects, the 

Administrator recognized that a number 

of improvements and refinements have 

been made to climate models since the 

time of the 2012 review. However, 

despite continuing research and the 

strong evidence supporting a causal 

relationship with climate effects (U.S. 

EPA, 2019a, section 13.3.9), the 

Administrator noted that there are still 

significant limitations in quantifying the 

contributions of the direct and indirect 

effects of PM and PM components on 

climate forcing (U.S. EPA, 2022b, 

sections 5.3.2.1.1 and 5.5). He also 

recognized that models continue to 

exhibit considerable variability in 

estimates of PM-related climate impacts 

at regional scales (e.g., ∼100 km), 

compared to simulations at the global 

scale (U.S. EPA, 2022b, sections 

5.3.2.1.1 and 5.5). As noted above, the 

CASAC recognized a causal relationship 

between PM and climate effects but also 

the large uncertainties associated with 

quantitatively assessing such effects, 

particularly on a national level in the 

context of a U.S.-based standard. These 

uncertainties led the Administrator to 

preliminarily conclude that the 

scientific information available in this 

reconsideration remains insufficient to 

quantify, with confidence, the impacts 

of ambient PM on climate in the U.S. 

(U.S. EPA, 2022b, section 5.3.2.2.1) and 

that there is insufficient information at 

this time to revise the current secondary 

PM standards or to promulgate a 

distinct secondary standard to address 

PM-related climate effects (88 FR 5661, 

January 27, 2023). 

With respect to materials effects, the 

Administrator noted that the available 

evidence continues to support the 

conclusion that there is a causal 

relationship with PM deposition (U.S. 

EPA, 2019a, section 13.4). He 

recognized that deposition of particles 

in the fine or coarse fractions can result 

in physical damage and/or impaired 

aesthetic qualities. Particles can 

contribute to materials damage by 

adding to the effects of natural 

weathering processes and by promoting 

the corrosion of metals, the degradation 

of painted surfaces, the deterioration of 

building materials, and the weakening 

of material components. While some 

recent evidence on materials effects of 

PM is available in the 2019 ISA, the 

Administrator noted that this evidence 

is primarily from studies conducted 

outside of the U.S. in areas where PM 

concentrations in ambient air are higher 

than those observed in the U.S. (U.S. 

EPA, 2019a, section 13.4). The CASAC 

also noted the lack of quantitative 

information relating PM and material 

effects. Given the limited amount of 

information on the quantitative 

relationships between PM and materials 

effects in the U.S., and uncertainties in 

the degree to which those effects could 

be adverse to the public welfare, the 

Administrator preliminarily judged that 

the scientific information available in 

this reconsideration remains insufficient 

to quantify, with confidence, the public 

welfare impacts of ambient PM on 

materials and that there is insufficient 

information at this time to revise the 

current secondary PM standards or to 

promulgate a distinct secondary 

standard to address PM-related 

materials effects (88 FR 5661, January 

27, 2023). 

Taken together, the Administrator 

proposed to conclude that the scientific 

and technical information for PMrelated visibility impairment, climate 

impacts, and materials effects, with its 

attendant uncertainties and limitations, 

supports the current level of protection 

provided by the secondary PM 

standards as being requisite to protect 

against known and anticipated adverse 

effects on public welfare. For visibility 

impairment, this proposed conclusion 

reflected his consideration of the 

evidence for PM-related light extinction, 

together with his consideration of 

updated analyses of the protection 

provided by the current secondary PM2.5

and PM10 standards. For climate and 

materials effects, this conclusion 

reflected his preliminary judgment that, 

although it remains important to 

maintain secondary PM2.5 and PM10

standards to provide some degree of 

control over long- and short-term 

concentrations of both fine and coarse 

particles, it is generally appropriate not 

to change the existing secondary 

standards at this time and that it is not 

appropriate to establish any distinct 

secondary PM standards to address PMrelated climate and materials effects at 

this time. As such, the Administrator 

recognized that current suite of 

secondary standards (i.e., the 24-hour 

PM2.5, 24-hour PM10, and annual PM2.5

standards) together provide such control 

for both fine and coarse particles and 

long- and short-term visibility and nonvisibility (e.g., climate and materials) 169

effects related to PM in ambient air. His 

proposed conclusions on the secondary 

standards were consistent with advice 

from the CASAC, which noted 

substantial uncertainties remain in the 

scientific evidence for climate and 

materials effects. Thus, based on his 

consideration of the evidence and 

analyses for PM-related welfare effects, 

as described above, and his 

consideration of CASAC advice on the 

secondary standards, the Administrator 

proposed not to change those standards 

(i.e., the current 24-hour and annual 

PM2.5 standards, 24-hour PM10 standard) 

at this time (88 FR 5662, January 27, 

2023). 


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170As noted earlier in section V, the 2019 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, 2019a, Appendix, p. A–3). 171As discussed in section I.D, the EPA has 

provisionally considered studies that were 

highlighted by commenters and that were published 

after the 2019 ISA. These studies are generally 

consistent with the evidence assessed in the 2019 

ISA, and they do not materially alter our 

understanding of the scientific evidence or the 

Agency’s conclusions based on that evidence. 

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). 

We first note that some commenters 

raise questions about the protection 

provided by the secondary PM 

standards for ecological effects (e.g., 

effects on ecosystems, ecosystem 

services, or species). However, 

consistent with the 2016 IRP and as 

described in the proposal (88 FR 5643, 

January 27, 2023), other welfare effects 

of PM, such as the ecological effects 

identified by commenters, are being 

considered as part of the separate, 

ongoing review of the secondary 

standards for oxides of sulfur, oxides of 

nitrogen and PM, and thus, those 

comments are beyond the scope of this 

action. 

Of the comments addressing the 

proposed decision for the secondary PM 

standards, 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, with a focus on the 

strength of the available scientific 

evidence for PM-related welfare effects. 

Most also recognize that the scientific 

evidence and quantitative information 

available in this reconsideration have 

not substantially altered our previous 

understanding of PM-related effects on 

non-ecological welfare effects (i.e., 

visibility, climate, and materials) and do 

not call into question the adequacy of 

the current secondary standards. They 

find the proposed decision not to 

change the standards at this time 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 

regarding the adequacy of the current 

secondary PM standards and the lack of 

support for revision of these standards 

at this time. 

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. 

Several commenters agree that the 

available scientific evidence provides 

support for the 2019 conclusion that 

there is a causal relationship between 

PM and climate effects, and the 

commenters also 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. 

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 

materials 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 EPA failed to explain in 

the proposal how the current standard 

is appropriate to protect materials from 

the effects of PM. These commenters 

disagree with the EPA’s conclusion that 

quantitative relationships have not been 

established for PM-related soiling and 

corrosion and frequency of cleaning or 

repair of materials, and cite to several 

studies conducted outside the U.S. that 

they contend that the EPA should 

consider since the same materials are 

present in the U.S. They further contend 

that, in discussing the available 

scientific evidence in the 2019 ISA for 

studies conducted outside of the U.S., 

the EPA did not provide references to 

these studies and, therefore, the public 

is unable to comment on these studies. 

They further State that EPA failed to 

consider the following information: (1) 

Recent work related to soiling of 

photovoltaic modules and other 

surfaces, and; (2) damage and 

degradation resulting from oxidant 

concentrations and solar radiation for a 

number of materials, including 

polymeric materials, plastic, paint, and 

rubber. These commenters further assert 

that the EPA failed to propose a 

standard that provides requisite 

protection against materials effects 

attributable to PM. 

As an initial matter, we note that the 

commenters submitted the same 

comments related to materials effects 

during the 2020 review. Consistent with 

our response in the 2020 notice of final 

rulemaking (85 FR 82737, December 18, 

2020), we disagree with the commenters 

that the EPA failed to consider the 

relevant scientific information about 

materials effects available in this 

reconsideration. The 2019 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.170 These 

include the Besson et al. (2017) study 

referenced by the commenters (U.S. 

EPA, 2019a, 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 for the 2019 ISA. However, the 

EPA provisionally considered new 

studies in responding to comments in 

the 2020 review, including the new 

studies highlighted by the commenters 

in their comments on the 2020 notice of 

proposed rulemaking, in the context of 

the findings of the 2019 ISA (see 

Appendix in U.S. EPA, 2020a).171 Based 

on the provisional consideration, the 

EPA concluded in the 2020 review that 

the new studies are not sufficient to 

alter the conclusions reached in the 

2019 ISA regarding PM and materials 

effects. For example, the Gr<ntoft et al. 

(2019) study was based on European air 

pollution which as the EPA has noted 

has higher concentrations (as well as 

diversity in sources, such as light duty 

diesel engines) compared to the U.S.. 

Thus, the EPA did not find it necessary 

or appropriate to reopen the air quality 

criteria to consider this study because it 

would not have been an adequate basis 

on which to set a NAAQS. As discussed 

in section I, when the EPA decided to 

reconsider the standards, it also decided 

to reopen the air quality criteria to a 

limited degree, based on its judgment 

that certain new studies were likely to 

be useful in reconsidering the standards. 


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Based on the provisional consideration 

in the 2020 review and the significant 

data gaps that existed at that time, the 

EPA did not include these studies 

within the scope of the 2022 ISA 

Supplement because, although these 

studies provide additional support for 

PM-related materials, the studies would 

not support quantitative analyses or 

alternative conclusions regarding these 

effects. As described in section I.C.5.b 

above, the ISA Supplement focuses on 

a thorough evaluation of some studies 

that became available after the literature 

cutoff date of the 2019 ISA that could 

either further inform the adequacy of 

the current PM NAAQS or address key 

scientific topics that have evolved since 

the literature cutoff date for the 2019 

ISA. In developing the ISA Supplement, 

the EPA focused on the non-ecological 

welfare effects for which the evidence 

supported a ‘‘causal relationship’’ and 

for which quantitative analyses could be 

supported by the evidence because 

those were the welfare effects that were 

most useful in informing conclusions in 

the 2020 PA. While the 2020 PA 

considered the broader set of evidence 

for materials effects, it concluded that 

there remained ‘substantial 

uncertainties with regard to the 

quantitative relationships with PM 

concentrations and concentration 

patterns that limit[ed] [the] ability to 

quantitatively assess the public welfare 

protection provided by the standards 

from these effects’ (U.S. EPA, 2020b).’’ 

Therefore, the ISA Supplement did not 

include an evaluation of scientific 

evidence for PM-related materials 

effects. However, the EPA has once 

again provisionally considered new 

studies in this reconsideration, 

including the studies highlighted by the 

commenters, in the context of the 2019 

ISA and concludes that, as in the 2020 

review, these studies are not sufficient 

to alter the conclusions reached in the 

2019 ISA regarding PM and materials 

effects or to provide sufficient 

information on which to base a 

secondary NAAQS. The EPA agrees 

there is a causal relationship between 

the presence of PM in the ambient air 

and materials effects, but to set a 

standard, the EPA needs not only to 

understand at what point materials 

effects become adverse to public welfare 

but to be able to relate specific 

concentrations of ambient PM to those 

levels of materials effects. Given the 

significant gaps in the evidence, 

particularly given that the majority of 

the recent evidence has been conducted 

outside of the U.S., establishing any 

quantitative relationships between 

particle size, concentration, chemical 

components, and specific measures of 

materials damage, such as frequency of 

painting or repair of materials, the EPA 

finds the evidence is insufficient to 

support a secondary NAAQS to protect 

against materials effects. 

With regard to studies conducted 

outside of the U.S., including those 

referenced by the commenters, as 

described in the proposal, in reaching 

his proposed conclusion, the 

Administrator recognized that while 

there was some newly available 

information related to materials effects 

of PM included in the 2019 ISA, ‘‘this 

evidence is primarily from studies 

conducted outside of the U.S. in areas 

where PM concentrations in ambient air 

are higher than those observed in the 

U.S. (U.S. EPA, 2019a, section 13.4)’’ 

(88 FR 5661, January 27, 2023). We 

disagree with the commenters that EPA 

did not provide references for these 

studies, nor that the lack of references 

inhibited the public’s ability to provide 

comment on this proposed conclusion. 

First, the reference to section 13.4 in the 

2019 ISA is a direct citation to the 

evaluation of newly available studies on 

PM-related materials effects, which 

includes citations for all materials 

effects evidence considered in the 2020 

review and in this reconsideration. 

Second, section 5.3.2.1.2 of the 2022 PA 

considers the available scientific 

evidence for PM-related materials 

effects—including citations to the 

studies newly available in the 2019 

ISA—and how that evidence informs 

conclusions regarding the adequacy of 

the standard (U.S. EPA, 2022b, section 

5.3.2.1.2). Therefore, the EPA disagrees 

that the proposal failed to provide the 

proper references to the studies 

conducted outside of the U.S., and that 

the public was not provided the 

opportunity to provide comment on 

these studies. 

Moreover, we disagree with the 

commenters that the EPA failed to 

consider quantitative information from 

studies available in this reconsideration. 

As detailed in sections 5.3.2.1.2 and 

5.3.2.2 of the 2022 PA, and consistent 

with the information available in the 

2020 review, 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 2012 review persist in 

the evidence in the 2020 review and in 

this reconsideration, 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 the 

2019 ISA, overall, the data are 

insufficient to conduct quantitative 

analyses for PM-related 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 in 

the U.S. (U.S. EPA, 2022b, section 

5.3.2.2.2; U.S. EPA, 2019a, 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, 2022b, 

section 5.3.2.2.2; U.S. EPA, 2019a, 

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 this 

reconsideration (U.S. EPA, 2022b, 

section 5.3.2.1.2). Additionally, we note 

the CASAC’s concurrence with 

conclusions in the 2020 PA (Cox, 2019b, 

p. 13 of consensus responses) and the 

2022 PA (Sheppard, 2022a, p. 23 of 

consensus responses) that uncertainties 

remain about the best way to evaluate 

materials effects of PM in ambient air. 

Further, no new studies are available in 

this reconsideration to link human 

perception of reduced aesthetic appeal 

of buildings 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, 2022b, section 5.6). 

With respect to the commenters’ 

assertion that the EPA failed to consider 

information related to materials damage 

and degradation from oxidant 

concentrations and solar radiation for a 

variety of materials, we first note that, 

even assuming these sources of 

materials damage are within the scope 

of this review of the PM NAAQS, the 

commenter did not provide any 

references to the scientific studies that 

they suggest that the EPA did not 

consider. Despite the lack of a list of 

specific references from the commenter, 

we note that the 2019 ISA considered a 

number of studies that examined the 


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relationships between PM and several of 

the materials listed by the commenters 

(e.g., paint, plastic, rubber). However, as 

described in the 2022 PA, these studies 

did not provide additional information 

regarding quantitative relationships 

between PM and materials that could 

inform quantitative analyses (U.S. EPA, 

2022b, sections 5.3.2.1.2 and 5.3.2.2.2), 

nor did they alter conclusions regarding 

the adequacy of the current standard 

(U.S. EPA, 2022b, section 5.5). 

As summarized above and in the 

proposal, the evidence in the 2020 

review and in this reconsideration for 

PM-related effects on materials is not 

substantively changed from that in the 

2012 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 the 2020 review and in this 

reconsideration continues to support a 

causal relationship between PM in 

ambient air and materials effects (U.S. 

EPA, 2019a, 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 (88 FR 5662, January 27, 

2023), 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, 

2022b, section 5.6) that may inform 

consideration of the materials effects of 

PM in ambient air in future reviews of 

the PM NAAQS. The EPA notes that one 

commenter objected to the 

Administrator’s proposed conclusion in 

the proposal (88 FR 5661, January 27, 

2023) that in light of the available 

evidence for PM-related impacts on 

climate and on materials that it is 

appropriate not to change the existing 

secondary standards at this time. The 

EPA has explained, in both the proposal 

and this final action, the basis for its 

conclusion that there is insufficient 

evidence to identify any particular 

secondary standard or standards that 

would provide requisite protection 

against climate effects or materials 

damage. The EPA acknowledges that, as 

a result, the adoption of any distinct 

secondary PM standards for those 

effects would be inconsistent with the 

requirements of the CAA. The EPA is 

clarifying that it is not basing its 

decisions on secondary standards in this 

reconsideration to address these welfare 

effects because it has concluded that the 

available scientific evidence is 

insufficient to allow the Administrator 

to make a reasoned judgment about 

what specific standard(s) would be 

requisite to protect against known or 

anticipated adverse effects to public 

welfare from PM-related materials 

damage or climate effects. 

Some commenters agree with the 

Administrator’s proposed conclusion 

that a target level of protection for 

visibility of 30 dv and the level of the 

secondary 24-hour PM2.5 standard of 35 

mg/m3 continues to be adequate to 

protect visibility, highlighting 

improvements in visibility in the U.S. 

Other commenters who disagree with 

the proposed decision indicated support 

for a more stringent standard for 

visibility impairment, although some of 

these commenters did not necessarily 

specify the alternative standard that 

would, in their judgment, address their 

concerns related to various aspects of 

the EPA’s proposal, including the 

available public preference studies, 

specific aspects of the visibility index, 

and the target level of protection 

identified by the Administrator. Rather, 

most commenters focused on particular 

aspects of the visibility metric 

underlying the current secondary 24- 

hour PM2.5 standard, including the form, 

averaging time, and target level of 

protection necessary to protect against 

visibility impairment. 

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 2012 review (78 

FR 3210, January 15, 2013). Given that 

the evidence related to public 

preferences has not substantially 

changed since the 2012 review, the EPA 

reiterated the limitations and 

uncertainties inherent in the evidence 

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

2020b, section 5.5), as well as in the 

2022 PA for this reconsideration (U.S. 

EPA, 2022b, section 5.6). The 2022 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, 2022b, 

section 5.6). Specifically, the EPA agrees 

with commenters that there are several 

areas where additional information 

would reduce uncertainty in our 

interpretation of the available 

information for purposes of 

characterizing visibility impairment. As 

described in more detail in the 2020 PA 

(U.S. EPA, 2020b, p. 5–41) and the 2022 

PA (U.S. EPA, 2022b, p. 5–53), briefly, 

these areas include: (1) Expanding the 

number and geographic coverage of 

preference studies in urban, rural, and 

Class I areas; (2) evaluating visibility 

preferences of the U.S. population 

today, given that the preference studies 

were conducted more than 15 years ago, 

during which time air quality in the 

U.S. has improved; (3) accounting for 

the influence of varying study methods 

may have on an individual’s response as 

to what level of visibility impairment is 

acceptable, and; (4) information on 

people’s judgments on acceptable 

visibility based on factors that can 

influence their perception of visibility 

(e.g., duration of impairment 

experiences, time of day, frequency of 

impairment). 

However, the EPA disagrees with the 

commenters that the current secondary 

PM standards are inadequate and 

should be made more stringent because 

of the limitations and uncertainties 

associated with the available public 

preference studies. The EPA does not 

view the limitations of the preference 

studies and other available evidence as 

so significant as to render the EPA 

unable to identify a secondary standard 

to protect against the adverse effects of 

PM on visibility, but the EPA also does 

not believe that the limitations 

themselves mean that the standards are 

inadequate. In fact, there is a limited 

amount of recently available scientific 

evidence to further inform our 

understanding of public preferences and 

visibility impairment is recognized by 

the Administrator in reaching his 

proposed decision not to change the 

current secondary PM standards at this 

time, given that the evidence base is 

largely the same as at the time of the 

2012 and 2020 reviews. 

These same commenters further 

contend that the EPA failed to use the 

latest science to develop a visibility 

index, stating that the EPA failed to 

consider the contrast of distance 

methodology employed in a recent 

meta-analysis of available preference 

studies (Malm et al., 2019). Commenters 

claim that the EPA draws conclusions 

from the Malm et al. (2019) study about 

how to relate contrast to acceptable 

visibility preferences in the 2022 ISA 

Supplement, yet ignores the findings of 

the study and fails to consider the 

‘‘contrast of distance’’ methodology in 

the 2022 PA and the proposal, thereby, 

in their view, departing from the 

CASAC’s advice to consider this 

evidence in setting the secondary 


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standard. Finally, the commenters assert 

that the EPA did not explain why the 

available public preference studies are 

adequate for analysis using a light 

extinction approach but not using the 

contrast of distance approach, and that 

such differential treatment is arbitrary. 

We disagree with the commenters that 

the EPA did not use the latest science 

in evaluating the visibility index, and 

that the EPA failed to consider the 

contrast of distance methodology used 

in Malm et al. (2019). As the 

commenters state, the Malm et al. (2019) 

study was included in the ISA 

Supplement (U.S. EPA, 2022a, section 

4.2.1). However, the EPA disagrees with 

the assertion that the ISA Supplement 

reached conclusions about how to relate 

contrast to acceptable visibility 

preferences. The ISA Supplement 

provided an overview of the Malm et al. 

(2019) study, stating that ‘‘[t]he main 

conclusion of this study was that the 

level of acceptable visual air quality is 

more consistent across studies using 

metrics that evaluate the distinction of 

an object from a background than using 

metrics that evaluate the greatest 

distance at which an object can be 

observed.’’ Furthermore, the statements 

that the commenters are referencing in 

support of this statement (i.e., U.S. EPA, 

2022b, pp. 4–5–4–6) are in fact the 

conclusions of the study itself, rather 

than conclusions of the EPA. For 

example, the ISA Supplement notes that 

‘‘Malm et al. (2019) suggested that 

scene-dependent metrics like contrast, 

which integrate the effects of bext along 

the sight paths between observers and 

landscape features, are better predictors 

of preference levels than universal 

metrics like light extinction.’’ The 

suggestion that the contrast of distance 

methodology is a better predictor than 

light extinction is one of the study 

authors, not the EPA. The EPA has not 

reached a conclusion on whether 

contrast of distance methodology would 

be a more appropriate indicator for a 

visibility index than estimated light 

extinction because the EPA finds that 

there is insufficient information in the 

record at this time to support that it is 

practical to evaluate, much less adopt, 

the contrast of distance methodology on 

a national basis. Specifically, the Malm 

et al. (2019) study does not provide as 

a part of their publication the specific 

input values to the equation to calculate 

the contrast of distance associated with 

the available public preference studies 

(e.g., sight paths from the images), nor 

do the preference studies present or 

make publicly available these data in 

their publications. In the absence of 

additional studies or publicly available 

data to further evaluate the contrast of 

distance methodology, the EPA is 

unable to consider contrast of distance 

as an alternative to estimated light 

extinction in this reconsideration, 

although we note that it may be 

appropriate to evaluate it more closely 

in future reviews. 

In reaching conclusions regarding the 

appropriate indicator for the visibility 

index, the 2022 PA specifically notes 

‘‘that limited new research is available 

on methods of characterizing visibility 

or on how visibility is valued by the 

public, such as visibility preference 

studies. Thus, while limited new 

research has further informed our 

understanding of the influence of 

atmospheric components of PM2.5 on 

light extinction, the available evidence 

to inform consideration of the public 

welfare implications of PM-related 

visibility impairment remains relatively 

unchanged’’ (U.S. EPA, 2022b, p. 5–50). 

The EPA again notes in the proposal 

that ‘‘there are very few studies 

available that use scene-dependent 

metrics (i.e., contrast) to evaluate public 

preference information, which makes it 

difficult to evaluate them as an 

alternative to the light extinction 

approach’’ (88 FR 5649–5650, January 

27, 2023). To further expand on this 

statement, the Malm et al. (2019) study 

does not provide enough information to 

replicate the results of their contrast of 

distance approach to allow for a 

comprehensive evaluation of the 

potential use of this methodology in 

considering the results of the public 

preference studies for determining the 

target level of protection for visibility. 

Some commenters suggests that the 

methodology could be approximated by 

simply ensuring that people could 

always see distant scenic elements, and 

that characterizing typical average and/ 

or maximal viewing distances cross 

different geographical areas and regions 

would be a straightforward 

Geographical Information Systems (GIS) 

exercise. The EPA disagrees that this 

assessment would be straightforward, 

given the lack of data establishing 

viewing distances in the available 

scientific record and the diversity of 

distance to scenic elements across 

different areas and regions of the U.S., 

and finds that this approach is also not 

practical to adopt in this 

reconsideration. Finally, while the 

Malm et al. (2019) study is using an 

alternative approach for evaluating 

public preferences and acceptability, we 

note that this study is evaluating the 

same public preference studies that 

have been available for the past several 

decades. For these reasons, the EPA 

disagrees with the commenters’ 

allegation that the EPA ignored the 

findings of the Malm et al. (2019) study 

and failed to consider the contrast of 

distance methodology in the 2022 PA 

and the proposal, and ignored the 

CASAC’s advice to consider this study. 

The ISA Supplement and the 2022 PA 

considered the Malm et al. (2019) study, 

along with the full body of available 

scientific evidence, and took into 

account the uncertainties and 

limitations associated with the evidence 

for visibility preferences, in reaching 

conclusions regarding the adequacy of 

the secondary 24-hour PM2.5 standard 

(U.S. EPA, 2022b, pp. 5–24–5.25, 5–50). 

Several comments in support of 

revising the secondary 24-hour PM2.5

standard to protect against visibility 

generally recommend revisions to the 

elements of the standard and visibility 

index (indicator, averaging time, form, 

and level) consistent with those 

supported by the CASAC and public 

comments in previous PM NAAQS 

reviews. Some commenters assert that 

the EPA’s approach in the 2022 PA and 

in the proposal for this reconsideration 

did not evaluate options for alternative 

secondary PM standards and thereby is 

flawed. We address comments on the 

elements of a visibility index and a 

revised standard for visibility effects 

below. 

As an initial matter, the EPA disagrees 

to the extent commenters are suggesting 

that the PA is legally required to analyze 

options for alternative standards. The 

PA is a document developed by the EPA 

in order to assist the Administrator and 

the CASAC in reaching conclusions 

regarding the adequacy of the current 

standards, and its scope is determined 

by the EPA. Moreover, the 2022 PA did 

assess a wide range of information 

relevant to the Administrator’s decision 

and considered a range of potential 

standards. 

First, in developing the 2022 PA and 

in responding to CASAC’s advice and 

recommendations during its review of 

the 2021 draft PA, the EPA expanded 

upon its discussion of determining the 

target level of protection for the 

visibility index and considered the 

extent to which the available scientific 

information would alter regarding the 

visibility index and the appropriate 

target level of protection against PMrelated visibility effects (U.S. EPA, 

2022b, pp. 5–27–5–29). This detailed 

discussion expands the consideration of 

the target level of protection for the 

visibility index presented in the 2020 

PA (U.S. EPA, 2020b) and the 2021 draft 

PA (U.S. EPA, 2021c), neither of which 

specifically considered the elements of 

the visibility index in determining the 

appropriate target level of protection. In 


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considering the available information in 

the 2022 PA, the EPA concluded that 

the available information continued to 

provide support for a visibility index 

with a level of 30 dv, with estimated 

light extinction as the indicator, a 24- 

hour averaging time, and a 90th 

percentile form, averaged over three 

years. 

Additionally, in summarizing the air 

quality and quantitative information in 

the proposal for this reconsideration, 

the EPA further expands upon the 

discussion added to the 2022 PA related 

to the target level of protection in terms 

of a PM2.5 visibility index. In so doing, 

the EPA considers even more 

extensively the available public 

preference studies and quantitative 

analyses (88 FR 5651–5652, January 27, 

2023). In particular, there is a more 

detailed discussion of the public 

preference studies, including the levels 

of impairment determined to be 

‘‘acceptable’’ by at least 50 percent of 

study participants and the 

methodologies used in the studies, 

including uncertainties and limitations 

associated with the methodologies (88 

FR 5652, January 27, 2023). In reaching 

a proposed decision regarding the 

adequacy of the secondary PM 

standards, as well as the appropriate 

target level of protection for the 

visibility index, the Administrator 

considered the available scientific 

evidence and quantitative analyses, as 

well as judgments about how to 

consider the range and magnitude of 

uncertainties that are inherent in the 

scientific evidence and analyses. In so 

doing, the Administrator proposed to 

conclude that the protection provided 

by a visibility index based on estimated 

light extinction, a 24-hour averaging 

time, and a 90th percentile form, 

averaged over 3 years, set at a level of 

30 dv would be requisite to protect 

public welfare with regard to visibility 

impairment (88 FR 5660, January 27, 

2023). 

Having provisionally concluded that 

it was appropriate to define the target 

level of protection in terms of a 

visibility index based on estimated light 

extinction as described above (i.e., with 

a 24-hour averaging time; a 3-year, 90th 

percentile form; and a level of 30 dv), 

the Administrator next considered the 

degree of protection from visibility 

afforded by the current secondary PM 

standards. In so doing, he considered 

the updated analyses of PM-related 

visibility impairment presented in the 

2022 PA (U.S. EPA, 2022b, section 

5.3.1.2) and described in more detail in 

the proposal (88 FR 5656, January 27, 

2023), which included estimating light 

extinction using multiple versions of the 

IMPROVE algorithm and inclusion of 

PM10-2.5 data at all sites to allow for 

better characterization of the influence 

of the coarse fraction of PM on light 

extinction. The Administrator noted 

that the results of the analyses in the 

2022 PA were consistent with those 

from the 2012 and 2020 reviews. He also 

recognized that, regardless of the 

IMPROVE equation that was used, the 

analyses demonstrated that the 3-year 

visibility metric is at or below 28 dv in 

all areas meting the current 24-hour 

PM2.5 standard (88 FR 5657, January 27, 

2023). The Administrator also noted 

that, in their review of the 2021 draft 

PA, the CASAC stated that ‘‘[i]f a value 

of 20–25 deciviews is deemed to be an 

appropriate visibility target level of 

protection, then a secondary 24-hour 

standard in the range of 25–35 mg/m3

should be considered (Sheppard, 2022a, 

p. 21 of consensus responses). The 

Administrator recognized that while the 

CASAC recommended that additional 

justification be provided for a visibility 

index target level of protection of 30 dv, 

they did not specifically recommend 

that he choose an alternative level for 

the visibility index. Therefore, the 

Administrator considered the available 

scientific evidence, quantitative 

information, and the CASAC’s advice in 

reaching his proposed conclusions. The 

Administrator recognized conclusions 

regarding the appropriate weight to 

place on the scientific and technical 

information, including how to consider 

the range and magnitude of 

uncertainties inherent in that 

information, is a public welfare policy 

judgment left to the Administrator. As 

such, the Administrator noted his 

preliminary conclusion on the 

appropriate visibility index (i.e., with a 

24-hour averaging time; a 3-year, 90th 

percentile form; and a level of 30 dv) 

and his preliminary conclusions 

regarding the quantitative analyses of 

the relationship between the visibility 

index and the current secondary 24- 

hour PM2.5 standard. In so doing, he 

proposed to conclude that the current 

secondary standards provide requisite 

protection against PM-related visibility 

effects (88 FR 5661, January 27, 2023). 

However, the Administrator 

additionally recognized that the 

available evidence on visibility 

impairment generally reflects a 

continuum and that the public 

preference studies did not identify a 

specific level of visibility impairment 

that would be perceived as ‘‘acceptable’’ 

or ‘‘unacceptable’’ across the whole U.S. 

population. He noted a judgment of a 

target level of protection, below 30 dv 

and down to 25 dv, could be supported 

if more weight was put on the public 

preference study performed in the 

Phoenix, AZ, study (BBC Research & 

Consulting, 2003). As described above, 

while the Administrator noted that the 

CASAC did not recommend revising the 

level of the current 24-hour PM2.5

standard in their review of the 2021 

draft PA, they did state that, should an 

alternative level be considered for the 

visibility index, revisions to the 

secondary 24-hour PM2.5 standard 

should also be considered (Sheppard, 

2022a, p. 21 of consensus responses). 

Thus, the Administrator solicited 

comment on the appropriateness of a 

target level of protection for visibility 

below 30 dv and down as low as 25 dv, 

and of revising the level of the current 

secondary 24-hour PM2.5 standard to a 

level as low as 25 mg/m3 (88 FR 5662, 

January 27, 2023), and the 

Administrator considered these public 

comments in reaching his final decision 

on the secondary standards. Thus, the 

EPA disagrees that the 2022 PA and the 

proposal did not adequately consider 

options for revising the secondary PM 

NAAQS. 

With regard to the elements of the 

visibility index, in considering the 

adequacy of the current secondary 24- 

hour PM2.5 standard to protect against 

visibility impairment, as described in 

the proposal (88 FR 5658–5660, January 

27, 2023), the Administrator first 

defined an appropriate target level of 

protection in terms of a PM visibility 

index. In considering the information 

available in this reconsideration and the 

CASAC’s advice, the Administrator 

proposed to conclude that the 

protection provided by a visibility index 

based on estimated light extinction, a 

24-hour averaging time, and 90th 

percentile form, averaged over 3 years, 

set at a level of 30 dv, would be 

requisite to protect public welfare with 

regard to visibility impairment (88 FR 

5660, January 27, 2023). 

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 extinctions, 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 2012 

and 2020 reviews. As described in the 

proposal (88 FR 5649, January 27, 2023) 

and above (section V.A.2), the 

IMPROVE algorithm estimates light 

extinction using routinely monitored 

components of PM2.5 and PM10–2.5, along 

with estimates of relative humidity. The 


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Administrator, while recognizing that 

some revisions to the IMRPOVE 

algorithm were newly available in the 

2020 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. 

(88 FR 5658–5659, January 27, 2023). As 

such, he judged that defining a target 

level of protection in terms of estimated 

light extinction continues to be a 

reasonable approach in this 

reconsideration. 

Some commenters who criticized the 

EPA’s interpretation and application of 

the Malm et al. (2019) study also 

contend that an indicator based on the 

contrast of distance would be a 

significant improvement over the 

current indicator for the visibility index 

and would more accurately evaluate 

public preferences. However, as 

described in the 2022 PA (U.S. EPA, 

2022b, section 5.3.1.1), while scenedependent metrics, such as contrast, 

may be useful alternative predictors of 

preferences compared to universal 

metrics like light extinction, there are a 

very limited number of studies that use 

such metrics to evaluate public 

preferences of visibility impairment and 

there is a lack of scientific evidence that 

supports one metric over another. 

Moreover, the EPA finds that even if the 

Administrator agreed that the contrast of 

distance methodology was an 

improvement over light extinction, there 

is insufficient information available to 

evaluate and adopt contrast of distance 

as an indicator for a national visibility 

target at this time. While, in its review 

of the 2021 draft PA the CASAC 

suggested that the EPA consider this 

method in developing the secondary PM 

standards, the CASAC also noted that 

‘‘more extensive technical evaluation of 

the alternatives for visibility indicators 

and practical measurement methods’’ is 

needed to inform future reviews of the 

secondary PM standards (Sheppard, 

2022a, p. 22 of consensus responses). 

The CASAC did not recommend using 

a different indicator for this 

reconsideration, with the majority of 

CASAC members reiterated past advice 

recommending development of a 

visibility FRM for a directly measured 

PM2.5 light extinction indicator 

(Sheppard, 2022a, p. 22 of consensus 

responses), a recommendation that was 

supported by other public commenters 

as well, and the minority of the CASAC 

suggested that such an FRM is not 

necessary. For these reasons, the EPA 

does not consider it feasible or 

appropriate to define the visibility index 

in terms of a contrast of distance 

indicator at this time. 

With regard to averaging time, some 

commenters suggested to the EPA that a 

secondary standard with a different 

form than the primary standard may be 

a more relevant for welfare effects. 

While they do not recommend a specific 

alternative form, the commenters point 

to CASAC advice in past reviews where 

the CASAC stated that a subdaily 

standard based on daylight hours better 

reflects visibility impairment. 

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 2012 review 

that showed relatively strong 

correlations between 24-hour and 

subdaily (i.e., 4-hour average) PM2.5

light extinction (88 FR 5659, January 27, 

2023; 85 FR 82740, December 18, 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 

a 24-hour averaging time for the 

visibility index in this reconsideration. 

The EPA also recognizes that the longer 

averaging time may be less influenced 

by atypical conditions and/or atypical 

instrument performance (88 FR 5659, 

January 27, 2023; 85 FR 82740, 

December 18, 2020; 78 FR 3226, January 

15, 2013). When taken together, the 

available scientific information and 

updated analyses of calculated light 

extinction available in this 

reconsideration continue to support that 

a 24-hour averaging time is appropriate 

when defining a target level of 

protection 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 

2012 review, the EPA recognized that 

hourly or subdaily (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 

the time of the 2012 review, 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 subdaily averaging time. These 

uncertainties, as described in the 2012 

review, included short-term variability 

in hourly data from available 

continuous monitoring methods, which 

would prohibit establishing a subdaily 

averaging time (78 FR 3209, January 15, 

2013). For all of these reasons, and 

consistent with the 2020 review, the 

EPA continues to believe that a subdaily 

averaging time is not supported by the 

information available in this 

reconsideration. 

With regard to the form of the 

visibility index, some commenters 

contend that the form used in evaluating 

visibility impairment is not appropriate. 

First, commenters contend that the EPA 

incorrectly stated that the CASAC did 

not provide advice on the 3-year, 90th 

percentile form of the visibility index 

and that the CASAC specifically 

recommended that the EPA further 

justify the metric and form, and by not 

doing so, the proposal arbitrarily 

departs from the CASAC’s 

recommendations. The commenters also 

contend that the EPA fails to explain 

how averaging the form over three years 

is protective given that the public does 

not perceive visibility in three-year 

averages. 

We disagree with the commenters that 

the EPA departed from the CASAC’s 

recommendations that ‘‘[t]he final PA 

should provide a robust justification for 

the daily light extinction percentile 

used in the analysis’’ (Sheppard, 2022a, 

p. 22 of consensus responses). In this 

statement, the CASAC did not make 

explicit recommendations for revisions 

to the form of the visibility index, as the 

commenters assert, but rather requested 

additional justification for the percentile 

selected for the visibility index in the 

2022 PA. In response to the CASAC’s 

recommendation after reviewing the 

2021 draft PA, the EPA included a new 

section in the 2022 PA that explicitly 

discusses the elements (i.e., indicator, 

averaging time, form, and level) of the 

visibility index, including additional 

justification for the conclusions 

regarding the appropriate elements for 

the index (U.S. EPA, 2022b, pp. 5–27– 

5–29). In so doing, the 2022 PA 

recognizes that there is no new 

information available in this 

reconsideration to inform selection of an 

alternative form of the visibility index, 

and therefore, relied on the analyses 

presented in the 2010 UFVA that 

evaluated the different statistical forms 

of the visibility index. The 2022 PA also 

discusses the approach to improving 

visual air quality in Federal Class I areas 

as a part of the Regional Haze Program 

(U.S. EPA, 2022b, p. 5–28). 

Furthermore, as reflected in responding 

to public comments below, and in 


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reaching his final conclusions in section 

V.B.4 below, the Administrator further 

considers the available scientific and 

quantitative information, the CASAC’s 

advice, and public comments in 

informing his final conclusions 

regarding the appropriate target level of 

protection for the visibility index. With 

regard to the commenters’ assertion that 

the EPA did not justify why averaging 

the form over three years is protective, 

we agree with the commenters that 

people do not perceive visibility 

impairment in three year averages. As 

described in the 2022 PA, visibilityrelated effects and perceived 

impairment are often associated with 

short-term PM concentrations, and 

therefore, the focus of the visibility 

analyses is centered on the adequacy of 

the 24-hour PM2.5 standard (U.S. EPA, 

2022b, p. 5–29). However, as described 

in the 2022 PA, the 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 (U.S. EPA, 2022b, p. 5–28). 

Occasional meteorological variability is 

of particular concern for the visibility 

index, which can be impacted by not 

only PM concentrations in ambient air 

but also relative humidity. The D.C. 

Circuit has previously recognized 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. See 

American Trucking Ass’ns v. Whitman, 

283 F.3d 355, 375–76 (D.C. Cir. 2002). 

The 2022 PA concluded that the 

available information continues to 

provide support for a 90th percentile 

form, averaged over three years, and the 

inclusion of additional justification for 

the elements of the visibility index 

responds to the CASAC’s 

recommendation (U.S. EPA, 2022b, 

section 5.3.1.2). 

Some commenters suggest that the 

90th percentile form is too low and 

would result in 36 days being excluded 

annually, presuming that the public 

only finds it objectionable when 

visibility is worse than the standard on 

37 or more days per year. The 

commenters also contend that the EPA’s 

approach of using a 90th percentile form 

for the visibility index is inconsistent 

with the goals of the Regional Haze 

Program. In so doing, the commenters 

note that the Regional Haze Rule focuses 

on improving conditions on the worst 

days, while they argue that a 90th 

percentile form for the visibility index 

would ignore the 36 worst visibility 

days, rather than identifying them and 

reducing pollution on those days. 

In reaching conclusions regarding the 

appropriate form of the visibility index, 

the EPA is following the same approach 

employed in past reviews of the 

secondary PM NAAQS, including those 

in the 2012 and 2020 rulemakings. In 

reaching conclusions regarding the 

appropriate form of the visibility index 

in the 2011 PA, the EPA considered the 

percentile forms of the visibility index 

assessed in the 2010 PA (i.e., 90th, 95th, 

98th) along with the approach for 

improving visual air quality under the 

Regional Haze Program. In so doing, the 

2011 PA notes that the Regional Haze 

Program targets the 20% most impaired 

days for improvements in visual air 

quality in Federal Class I areas (i.e., the 

days more impaired than the 80th 

percentile). The 2011 PA recognized 

that to increase the likelihood of 

improving visual air quality on the 

worst days, the form of the visibility 

index should be set well above the 80th 

percentile. The 2011 PA further 

concluded that a 90th percentile form 

would represent the median of the 

distribution of the 20% most impaired 

days, and meeting a visibility index 

with a 90th percentile form would mean 

that 90% of the days have visual air 

quality that is at or below the level of 

the visibility index and would 

reasonably expected to lead to 

improvements in visual air quality for 

the 20% most impaired days (U.S. EPA, 

2011, p. 4–59). The 2022 PA noted that 

there is no new information from public 

preference studies that would inform 

the Administrator’s consideration of the 

appropriate form for the visibility target 

index, and reached conclusions 

consistent with those of 2011 PA. 

However, as discussed below, the EPA 

disagrees that a focus on the 90th 

percentile ‘‘ignores’’ any days with 

worse visibility. It is possible to 

examine past patterns of air quality to 

judge the relationship between the 90th 

percentile and higher percentiles, and to 

assess whether achieving a 90th 

percentile visibility target will also 

result in air quality improvements, 

where necessary, at higher percentiles. 

Based on its assessment of past air 

quality and potential alternative 

percentiles for the form, the EPA judged 

that a 90th percentile would 

appropriately achieve improved air 

quality both above and below that 

percentile. 

Some commenters suggest that the 

analyses conducted in the 2010 UFVA 

are based on a different metric than the 

24-hour average being considered in the 

reconsideration, that the analyses are 

outdated and irrelevant. Therefore, the 

commenters assert that relying on the 

analyses in the 2010 UFVA is not a 

rational justification for the use of a 

90th percentile for the visibility index 

in this reconsideration. Moreover, these 

commenters state that, in past reviews, 

both the EPA and the CASAC have 

considered and recommended a 98th 

percentile form, but the proposal does 

not consider the 98th percentile. 


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172 Gantt, B., and Hagan, N. (2023). Analysis of 

Percentile Forms of the Visibility Index. 

Memorandum to the Rulemaking Docket for the 

Review of the National Ambient Air Quality 

Standards for Particulate Matter (EPA–HQ–OAR– 

2015–0072). Available at: https://

www.regulations.gov/docket/EPA-HQ-OAR-2015- 

0072. 

173 Gantt, B., and Hagan, N. (2023). Analysis of 

Percentile Forms of the Visibility Index. 

Memorandum to the Rulemaking Docket for the 

Review of the National Ambient Air Quality 

Standards for Particulate Matter (EPA–HQ–OAR– 

2015–0072). Available at: https://

www.regulations.gov/docket/EPA-HQ-OAR-2015- 

0072. 

174 Gantt, B., and Hagan, N. (2023). Analysis of 

Percentile Forms of the Visibility Index. 

Memorandum to the Rulemaking Docket for the 

Review of the National Ambient Air Quality 

Standards for Particulate Matter (EPA–HQ–OAR– 

2015–0072). Available at: https://

www.regulations.gov/docket/EPA-HQ-OAR-2015- 

0072. 

These commenters assert that the 

2010 UFVA was not considering the 

same metric under consideration here. 

However, the EPA was citing to the 

2010 UFVA for the conclusion that there 

are correlations between different 

statistical forms of the visibility index. 

To confirm whether these correlations 

occur under recent air quality, we 

conducted additional air quality 

analyses evaluating the visibility index 

using the current percentile form (i.e., 

90th) and two alternative forms (i.e., 

95th and 98th).172 While a higher 

percentile form would further limit the 

number of days with peak PM-related 

light extinction, the analyses confirm 

that a 90th percentile form is effective 

in limiting visibility impairment at 

higher percentiles. Based on these 

analyses, depending on which version 

of the IMPROVE equation is used to 

estimate light extinction, the differences 

in the 3-year averages of estimated light 

extinction for the 90th, 95th, and 98th 

percentile forms are small. For example, 

in areas that meet the current 24-hour 

PM2.5 standard, for light extinction 

estimated using the original IMPROVE 

equation, all sites have light extinction 

estimates for a 90th percentile form at 

or below 26 dv, for a 95th or 98th 

percentile form at or below 29 dv.173 In 

most locations, when estimating light 

extinction based on the original 

IMPROVE equation, the difference 

between a 95th or 98th percentile form 

and a 90th percentile form is generally 

less than 3 dv.174 As noted in previous 

reviews, a change of 1 to 2 dv in light 

extinction under many viewing 

conditions will be perceived as a small, 

but noticeable, change in the 

appearance of a scene, regardless of the 

initial amount of visibility impairment 

(88 FR 5657, January 27, 2023; U.S. 

EPA, 2004b; U.S. EPA, 2010b). Thus, 

differences between a 90th percentile 

form and a 95th or 98th percentile form 

remain small, and for any of these forms 

of the visibility index, the estimated 

light extinction based on the original 

IMPROVE equation in areas meeting the 

current secondary 24-hour PM2.5

standard is below the upper end of the 

range of the levels considered for the 

visibility index (i.e., below 30 dv). 

Some commenters disagree with the 

EPA’s proposed conclusion that a level 

of 30 dv is appropriate for the visibility 

index and support a lower level in order 

to provide increased protection against 

visibility impairment. Commenters who 

support a revised level for the visibility 

index state that a target level of 

protection of 30 dv would mean that 

less than 10% of participants in the 

public preference studies, other than the 

Washington, DC, study, would accept 

visibility conditions above 29 dv. These 

commenters further suggest that a 75% 

acceptability, rather than 50% 

acceptability, is requisite to protect 

visibility sources, which would be on 

average a level of 21 dv when using the 

light extinction method or 18 dv when 

using the contrast of distance method. 

These commenters argue that, based on 

the available information, a target level 

of protection for the visibility index of 

approximately 20 dv would be more 

appropriate, and therefore, the level of 

the secondary 24-hour PM2.5 standard 

should be strengthened to 25 mg/m3. 

Other commenters who support a 

revised level for the visibility index 

suggest that public preference studies 

with longer sight paths to distant 

landscape features or with lower target 

levels than those in the Washington, DC 

study, such as the Phoenix study, would 

support a lower level. These 

commenters support revising the target 

level of protection for the visibility 

index to a 25 dv, and revising the level 

of the secondary 24-hour PM2.5 standard 

to a level as low as 25 mg/m3, suggesting 

that in low relative humidity 

environments, 25 dv is consistent with 

PM2.5 concentrations of less than 25 mg/ 

m3. 

Some commenters state that EPA’s 

justification for setting a target level of 

protection at the upper end of the 20 to 

30 dv range is arbitrary. These 

commenters state that the EPA’s 

reliance on the standard operating in 

many regions and circumstances as 

support for the upper end of the range 

is irrational and illegal. Moreover, these 

commenters contend that EPA provided 

no rational connection between the 

Regional Haze Program and the 

proposed decision to set the target level 

of protection at the upper end of the 

range. They suggest that the EPA 

proposed to rely exclusively on the 

Regional Haze Program to protect 

visibility in Class I areas and to give 

visibility in these areas no weight in 

considering the secondary PM standard 

and that it is not rational to entirely 

ignore visibility in Class I areas when 

setting the secondary standard. These 

commenters assert that the Regional 

Haze Program provides no rational basis 

for a target level of protection at the 

upper end of the range, nor does the 

EPA identify one. 

Some commenters contend that the 

EPA failed to justify the adequacy of the 

current secondary annual PM2.5

standard, noting that the secondary 24- 

hour and annual PM2.5 standards work 

together to provide protection against 

short- and long-term effects of PM2.5. 

These commenters point to CASAC 

comments on the 2021 draft PA and the 

comments of an individual CASAC 

member’s support for strengthening the 

secondary annual PM2.5 standard to 

provide increased protection against 

climate and materials effects over time. 

They contend that EPA arbitrarily failed 

to discuss the secondary annual PM2.5

standard not only in the proposal, but 

also in the 2022 PA and in the 2020 

final decision. 

The EPA recognizes that the selection 

of the target level of protection for the 

visibility index is fundamentally a 

public welfare policy judgment for the 

Administrator. The Administrator is 

tasked by the CAA to judge when 

visibility impairment becomes an 

adverse effect on public welfare. It is 

clear that visibility impairment can 

become adverse to public welfare, but 

the Administrator does not consider that 

every deciview of impairment is adverse 

to public welfare. In considering the 

point at which visibility impairment 

becomes adverse to public welfare, such 

that the attainment of the secondary PM 

NAAQS would prevent the adverse 

effect, the Administrator gives weight to 

the public preference studies as to when 

visibility impairment is unacceptable. 

At the same time, the Administrator 

recognizes the limitations of these 

studies, which have been detailed in the 

proposal and the 2022 PA. Similarly, 

the EPA discussed the Regional Haze 

program in the proposal to highlight 

that there is a distinct program to 

protect against visibility impairment in 

Class I areas, and the existence of that 

program is relevant to the 

Administrator’s judgment about the 

level of visibility impairment that is 

adverse to public welfare under CAA 

109(d), because in determining what is 

requisite the Administrator is primarily 

considering visibility impairment 

outside of Class I areas. 


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In considering how to use the results 

of the public preference studies, the 

Administrator concludes that a 50th 

acceptability criterion is an appropriate 

tool. The Administrator’s task is to set 

standards that are neither more stringent 

nor less stringent than necessary, and a 

50% acceptability criterion seems most 

appropriate to use in judging when 

visibility impairments become adverse, 

because it should more closely represent 

when the median person would find the 

impairment to be adverse. The 

Administrator notes this conclusion is 

consistent with the approach adopted in 

the Denver study by Ely et al. (1991) 

where the 50% acceptability criterion 

for urban visibility was first presented. 

This study discussed the use of the 50% 

acceptability criteria as a reasonable 

basis for setting a standard to protect 

visibility in urban areas. In doing so, Ely 

et al. (1991) noted that the 50% 

acceptability criterion divided the slides 

into two groups—those judged 

acceptable and those judged 

unacceptable by a majority of people in 

the study—and therefore, was 

reasonable since it defines the point 

where the majority of the study 

participants began to judge levels of 

visibility impairment as unacceptable 

(Ely et al., 1991). 

In considering the appropriate target 

level of protection, we next look to the 

available public preference studies, 

noting that the selecting of the range of 

20 to 30 dv for the target level of 

protection for the visibility index is 

informed by the 50% acceptability 

values from these studies. The Denver, 

CO, (Ely et al., 1991) and British 

Columbia, Canada, (Pryor, 1996) studies 

met the 50% acceptability criteria at 20 

dv and 19–23 dv, respectively (U.S. 

EPA, 2022b, Table D–8). As described in 

the proposal, these studies used 

photographs that were taken at different 

times of the day and on different days 

to capture a range of light extinction 

levels needed for the preference studies 

(88 FR 5652, January 27, 2023). 

Compared to studies that used 

computer-generated images (i.e., those 

in Phoenix, AZ, and Washington, DC) 

there was more variability in scene 

appearance in these older studies that 

could affect preference rating and 

includes uncertainties associated with 

using ambient measurements to 

represent sight path-averaged light 

extinction values rather than 

superimposing a computer-generated 

amount of haze onto the images. When 

using photographs, the intrinsic 

appearance of the scene can change due 

to meteorological conditions (i.e., 

shadow patterns and cloud conditions) 

and spatial variations in ambient air 

quality that can result in ambient light 

extinction measurement not being 

representative of the sight-path-averaged 

light extinction. Computer-generated 

images, such as those generated with 

WinHaze, do not introduce such 

uncertainties, as the same base 

photograph is used (i.e., there is no 

intrinsic change in scene appearance) 

and the modeled haze that is 

superimposed on the photograph is 

determined based on uniform light 

extinction throughout the scene. 

Because of the uncertainties and 

limitations associated with the Denver, 

CO, and British Columbia, Canada, the 

EPA concludes that it is appropriate to 

place less weight on these studies, and 

to instead focus on the public 

preference studies that were designed to 

reduce these uncertainties and 

limitations. 

In so doing, we focus on the public 

preference studies that use computergenerated images (i.e., those in the 

Phoenix, AZ, and Washington, DC) 

studies. As described in the proposal, 

the use of computer-generated images 

have less variability in scene appears 

than in those studies that use 

photographs taken on different days and 

at different times of the days (i.e., those 

in the Denver, CO, study) that would be 

likely to influence preference rating and 

introduces uncertainties associated with 

using ambient measurements to present 

sight path-averaged light extinction 

values rather than superimposing a 

computer-generated amount of haze 

onto the images (88 FR 5652, January 

27, 2023). 

The Phoenix, AZ, public preference 

study (BBC Research & Consulting, 

2003) had several strengths compared to 

some of the other public preference 

studies. The Phoenix, AZ, study had the 

largest number of participants (385 in 27 

separate focus group sessions) of all of 

the public preference studies, with a 

sample group designed to be 

demographically representative of the 

Phoenix population at that time. The 

age range in the Phoenix study was also 

more inclusive (18–65+), with the 

distribution of the study participants 

corresponding reasonably well to the 

overall age distribution in the 2000 U.S. 

Census for the Phoenix area (BBC 

Research & Consulting, 2003). 

Furthermore, the 21 images used in the 

Phoenix, AZ, study were developed 

using the WinHaze software with visual 

air quality ranging from 15 to 35 dv, and 

the view was toward the southwest, 

including downtown Phoenix, with the 

Sierra Estrella Mountains in the 

background at a distance of 25 miles. 

This study had the least noisy 

preference results, perhaps because a 

larger, more representative group of 

participants combined with the use of 

computer-generated images resulted in 

the smoother distribution of responses 

of ‘‘acceptable’’ visual air quality. Based 

on the EPA’s evaluation of the public 

preference studies in the 2012 review, 

the 50% ‘‘acceptable’’ criteria was met 

at approximately 24 dv (U.S. EPA, 2010, 

Table 2–3). 

We also consider the public 

preferences for the Washington, DC, 

studies (Abt Associates, 2001; Smith 

and Howell, 2009). The 2001 

Washington, DC study included nine 

participants, and the 2009 Washington, 

DC, study replicated the 2001 study 

with 26 additional participants. Similar 

to the Phoenix study, the Washington, 

DC, studies also had the strength of 

having the 20 images included in the 

study generated using WinHaze with 

visual air quality ranging from 9 to 45 

dv. The study depicted a scene of a 

panoramic view of the Potomac River, 

the National Mall, and downtown 

Washington, DC. All of the distinct 

buildings in the scene were within four 

miles and the higher elevations in the 

background were less than 10 miles 

from where the image was taken from 

the Arlington National Cemetary in 

Virginia. The 50% ‘‘acceptable’’ criteria 

was met at approximately 29 dv (U.S. 

EPA, 2010, Table 2–3). 

As described in more detail in the 

proposal, visibility preferences can vary 

by location, and such differences may 

arise based on the differences in the 

cityscape scene that is depicted in the 

images (88 FR 5652, January 27, 2023). 

In considering the geographical 

differences between the public 

preference studies, we recognize that 

the methodological differences between 

the studies may influence the resulting 

‘‘acceptable’’ level of visibility 

impairment. In the Phoenix, AZ, study, 

the image depicted mountains in the 

background and urban features in the 

foreground, whereas the Washington, 

DC, study depicted nearby buildings in 

the image without mountains in the 

distance. As an initial matter, we note 

that the object of interest to the study 

participant could differ across the 

studies based on the scenes included in 

the images being evaluated—with the 

mountains being of greater interest in 

the images in the Phoenix, AZ, study, 

despite also depicting buildings that are 

similar to those shown and presumed to 

be of interest in the images in the 

Washington, DC, study (88 FR 5652, 

January 27, 2023). We also agree with 

the commenters that the distance 

between the object of interest and the 

camera is an important consideration in 


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evaluating the public preference studies. 

Objects at greater distances from the 

camera location (such as those in the 

Phoenix, AZ, study which had a 

maximum distance of 42 km (U.S. EPA, 

2022b, Table D–8)) have a greater 

sensitivity to light extinction, which 

alone could explain differences in 

preferences but coupled with an object 

of greater interest results in lower 

acceptable levels of visibility 

impairment. Conversely, objects at 

closer distances from the camera 

location (such as those in the 

Washington, DC, study which had a 

maximum distance of 8 km (U.S. EPA, 

2022b, Table D–8)) have less sensitivity 

to light extinction, which coupled with 

objects of interest (compared to the 

mountainous views in the Phoenix, AZ, 

study) result in higher acceptable levels 

of visibility impairment. These studies 

clearly demonstrate that there are 

differences in the public preferences 

across the studies depending on the 

images that are used, in particular the 

object of interest to the study participant 

depicted in the image and the distance 

of the sight path to the object, and that 

such differences can influence 

preference results. 

However, we note that these 

uncertainties and limitations have 

persisted from past reviews, and there is 

very little new information to inform 

conclusions regarding the interpretation 

of these results with regard to the target 

level of protection. In selecting a target 

level of protection, and in considering 

the CASAC’s advice in their review of 

the 2021 draft PA and public comments, 

we conclude that it is appropriate to 

consider the information from the 

public preference studies in 

Washington, DC, and Phoenix, AZ, and 

in so doing, that it is appropriate to 

place weight on both of these studies in 

reaching conclusions on the appropriate 

target level of protection. The EPA 

recognizes that the scenes depicted in 

these two studies are different and may 

influence public preferences of visibility 

impairment, but notes these studies can 

be considered together as providing 

information about different areas across 

the U.S. with variations in the scenes 

that people are likely to most commonly 

encounter. The scene depicted in the 

images used in the Washington, DC, 

study have a mix of buildings, 

landmarks, and open space. On the 

other hand, the scene depicted in the 

Phoenix, AZ, study included a mix of 

buildings in the foreground and with 

more distant mountains in the 

background. The Administrator 

considers it appropriate to consider 

these studies together because in 

combination, they provide a greater 

diversity of scenes, which is more likely 

to be representative of scenes people 

typically experience around the country 

(e.g., not only in eastern metropolitan 

statistical areas, but also in western 

areas with different vistas). In 

considering these two studies together, 

the EPA recognizes that, first, the 

‘‘object of interest’’ is a subjective 

judgment left to the participants of the 

public preference studies, and second, 

the images in these two studies may 

differ in terms of sensitivity to changes 

in light extinction because of the 

distance between the object of interest 

in the scene and the camera. As noted 

by the public commenters, the sight 

path for the images in the public 

preference studies is an important 

consideration in reaching conclusions 

regarding the appropriate target level of 

protection for the visibility index. In 

addition, the Administrator judges that 

giving weight to multiple studies is a 

more appropriate approach than 

focusing on a single study, particularly 

where the study design (including the 

representativeness of the participants 

and the scenes depicted in the images) 

may be important for interpreting the 

results of the public preference studies 

for informing conclusions regarding the 

visibility index. Given these 

considerations and taking into 

consideration public comments on the 

target level of protection for the 

visibility index, the Administrator 

recognizes that it is more appropriate to 

consider a broader range of public 

preferences, reflecting a broader range of 

scenes, by putting significant weight on 

both the Washington, DC, and Phoenix, 

AZ, studies. In so doing, he reaches the 

conclusion that it would be appropriate 

to identify secondary PM standards that 

generally limit visibility impairment to 

a level between the two studies. 

The Administrator next considers 

what target level of protection would be 

appropriate based on the available 

information from these public 

preference studies. He first recognizes 

that, in the 2012 and 2020 final 

decisions, the then-Administrators 

selected a target level of protection of 30 

dv, based on the upper end of the range. 

In so doing, the then-Administrators 

judged that it was appropriate to place 

more weight on the uncertainties 

associated with the public preference 

studies in reaching their conclusions. 

However, in this reconsideration, the 

current Administrator, while continuing 

to recognize that substantial 

uncertainties remain and that there is 

relatively limited new information 

regarding public preferences of visibility 

impairment, judges that it is important 

to balance the weight placed on 

uncertainties with the strength of the 

scientific evidence. As such, the 

Administrator concludes that it is 

appropriate to consider a target level of 

protection within the range of 20 to 30 

dv. He further concludes that in 

selecting a target level within that range 

it is appropriate to place weight on both 

the mid-point of the range, as supported 

by the study in Phoenix, AZ, as well as 

the upper end, as supported by the 

Washington, DC, study. The 

Administrator notes that these two 

studies both employ similar 

methodologies that are subject to fewer 

uncertainties than older public 

preference studies (including their use 

of WinHaze to reduce uncertainties in 

the preference solicitations) although he 

notes that the Phoenix, AZ, study 

yielded the best results of the four 

public preference studies in terms of the 

least noisy preference results and the 

most representative selection of 

participants. Furthermore, he notes the 

differences between the scenes used for 

each study and finds that consideration 

of these studies together is more 

appropriate in selecting a national target 

for visibility protection than considering 

either study alone. Thus, in considering 

this information, along with the 

uncertainties and limitations of the 

public preference studies, the 

Administrator judges that it would be 

appropriate to select a target level of 

protection based on placing equal 

weight on the upper end of the range 

(i.e., 30 dv) and the middle of the range 

(i.e., 24 dv based on the Phoenix, AZ, 

study) in order to identify a nationwide 

target for protection against visibility 

impairment. In so doing, the 

Administrator concludes that a visibility 

index with a target level of protection of 

27, defined in terms of estimated light 

extinction, with a 24-hour averaging 

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

would provide adequate protection 

against PM-related visibility effects on 

public welfare. Such a target level of 

protection balances the information 

from two key studies reflecting different 

participant preferences for different 

vistas in different parts of the country, 

appropriately weighting both near-field 

and more distant landscape features that 

may be of importance to public 

perceptions of visibility. 

The Administrator notes that the 

available evidence indicates that the 

relationship between PM and light 

extinction is complex, depending on 

factors such as PM composition, size 

fraction, and age of the particles in 

ambient air, as well as relative 


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175When light extinction is calculated using the 

revised IMPROVE equation, all 60 sites have 3-year 

visibility metrics below 28 dv, 56 sites are at or 

below 25 dv, and 26 sites are at or below 20 dv. 

When light extinction is calculated using the 

Lowenthal and Kumar IMPROVE equation, 59 sites 

have 3-year visibility metrics below 28 dv, 45 sites 

are at or below 25 dv, and 15 sites are at or below 

20 dv. The one site with a 3-year visibility metric 

of 32 dv exceeds the secondary 24-hour PM2.5

standard, with a design value of 56 mg/m3 (see U.S. 

EPA, 2022b, Appendix D, Table D–3). 

humidity. These factors can vary across 

the country based on differences in 

regional influences, as well as 

meteorological conditions that can vary 

spatially and temporally in different 

areas. The Administrator also recognizes 

that this variability, coupled with the 

age of the PM depending on the distance 

from the source to the monitor location, 

also complicates the selection of which 

IMPROVE equation is most appropriate 

in different areas, although he notes that 

different IMPROVE equations will yield 

similar, but not identical, results. In so 

doing, the Administrator takes note of 

the figures presented in the 2022 PA, 

which depict the comparisons using the 

original IMPROVE equation (Figure 5– 

3), the revised IMPROVE equation 

(Figure 5–4), and the Lowenthal & 

Kumar equation (Figure 5–6), as well as 

the estimated light extinction values for 

the three different equations presented 

in Table D–7. 

The Administrator notes that when 

light extinction is calculated using the 

original IMPROVE equation, all 60 sites 

have 3-year visibility metrics below 28 

dv, 58 sites are at or below 25 dv, 26 

sites are at or below 20 dv, and of the 

two sites above 25 dv one is at 26 dv 

and the other has a 24-hour PM2.5 design 

value of 56 mg/m3 (i.e., well above the 

current 24-hour standard). Results are 

similar for other IMPROVE equations.175

Based on these analyses, and consistent 

with the results of similar analyses in 

the 2012 review and the 2020 PA, the 

Administrator concludes that the 

current secondary 24-hour PM2.5

standard, with its level of 35 mg/m3, 

maintains the visibility index below 27 

dv, and in fact, the current standard 

maintains air quality such that many 

areas have visibility index values that 

range between 15 and 25 dv for all three 

IMPROVE equations. In the areas that 

meet the secondary 24-hour PM2.5

standard, all locations were below 27 dv 

when using the original and revised 

IMPROVE equation and all but three 

locations were at or below 27 dv when 

using the Lowenthal & Kumar IMPROVE 

equation. Three locations (two in 

California and one in Utah) had air 

quality that was at 28 dv when the 

Lowenthal & Kumar IMPROVE equation 

was used. As described in more detail 

in section V.A.1.3, we recognize that 

there are differences in the inputs for 

the three IMPROVE equations that can 

influence the resulting estimated light 

extinction values. The higher multiplier 

for converting OC to OM in the 

Lowenthal & Kumar IMPROVE equation 

(i.e., a multiplier of 2.1) may be more 

appropriate in more remote locations 

where there is more aged and 

oxygenated organic PM than in urban 

locations. The three locations with air 

quality at 28 dv are all in urban areas 

(downtown Los Angeles, CA; Rubidoux, 

CA; Salt Lake City, UT) and tend to have 

higher levels of nitrate and OC, 

especially during the wintertime when 

peak PM2.5 concentrations typically 

occur. In these locations, it may be more 

appropriate to use either the original or 

revised IMPROVE equation, which have 

multipliers of 1.4 and 1.8, respectively, 

in order to refine the inputs such that 

estimated light extinction in these 

locations is more accurately 

characterized based on site-specific 

characteristics. 

We also note that the four areas that 

exceed the secondary 24-hour PM2.5

standard also generally had air quality 

that was below 27 dv in terms of the 

visibility index, with only two locations 

experiencing a visibility index above 27 

dv. One location that exceeds the 

secondary 24-hour PM2.5 standard had a 

visibility index of 29 dv using the 

original IMPROVE equation, while two 

locations were 30 and 32 dv using the 

Lowenthal & Kumar IMPROVE 

equation. We believe attainment and 

maintenance of the secondary 24-hour 

PM2.5 standard will result in improved 

air quality in these areas, such that the 

visibility index values for these areas 

will decrease even further. 

The Administrator recognizes that in 

concluding that it is appropriate to 

identify secondary PM standards that 

generally limit visibility impairment to 

as low as 27 dv in terms of the visibility 

index, the current secondary PM 

standards continue to provide 

protection against visibility impairment 

associated with a visibility index as low 

as, or even lower than, 27 dv. In so 

doing, he notes that when meeting the 

current 24-hour PM2.5 standard, all sites 

have a visibility index at or below 27 dv 

with the original and revised IMPROVE 

equations, and all but three sites at or 

below 27 dv with the Lowenthal and 

Kumar IMPROVE equation. 

Furthermore, the Administrator notes 

that this conclusion is consistent with 

the CASAC’s advice who, in their 

review the 2021 draft PA, stated that 

‘‘[i]f a value of 20–25 deciviews is 

deemed to be an appropriate visibility 

target level of protection, then a 

secondary 24-hour PM2.5 standard in the 

range of 25–35 mg/m3 should be 

considered’’ (Sheppard, 2022a, p. 21 of 

consensus responses). 

Thus, the Administrator concludes 

that weight on both the upper end of the 

range of target levels of protection for 

the visibility index identified in 

previous reviews and the mid-point of 

the range, as presented by the Phoenix, 

AZ, public preference study, and 

focusing on a target level of protection 

of 27 dv, he still judges the current 

secondary 24-hour PM2.5 standard 

requisite to achieve that target because 

the standard generally maintains the 

visibility index at or below 27 dv such 

that more stringent standards are not 

warranted. 

The EPA agrees with the commenters 

that the secondary PM standards work 

together to provide protection against 

short- and long-term effects of both fine 

and coarse particles (U.S. EPA, 2022b, 

section 5.5; 88 FR 5661, January 27, 

2023). However, the EPA disagrees with 

commenters that we failed to discuss 

the secondary annual PM2.5 standard in 

the proposal, 2022 PA, and the 2020 

final notice and that we failed to justify 

the adequacy of the secondary annual 

PM2.5 standard. As described in the 

2022 PA and the proposal, we recognize 

that PM2.5 is the size fraction of PM 

responsible for most of the visibility 

impairment in urban areas (U.S. EPA, 

2022b, section 5.3.1.2; 88 FR 5654, 

January 27, 2023). Analyses in the 2019 

ISA found that mass scattering from 

PM10–2.5 was relatively small (less than 

10%) in the eastern and northwestern 

U.S., whereas mass scattering was much 

larger in the Southwest (more than 

20%), particularly in southern Arizona 

and New Mexico (U.S. EPA, 2019, 

section 13.2.4.1, p. 13–36). Given the 

relationship between visibility and 

PM2.5 along with the short-term nature 

of visibility effects, we focus more on 

the adequacy of the secondary 24-hour 

PM2.5 standard for providing protection 

against visibility impairment (U.S. EPA, 

2022b, section 5.3.1.2; 88 FR 5653, 

January 27, 2023). In reaching his 

proposed conclusions, the 

Administrator clearly states that he 

‘‘recognizes that the current suite of 

secondary standards (i.e., the 24-hour 

PM2.5, 24-hour PM10, and annual PM2.5

standards) together provide . . . control 

for both fine and coarse particulates and 

long- and short-term visibility and nonvisibility (e.g., climate and materials) 

effects related to PM in ambient air’’ (88 

FR 5661, January 27, 2023). Thus, by 

explaining how the secondary standards 

work together to provide protection 

from adverse effects, why we focus on 

the secondary 24-hour PM2.5 standard as 


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most relevant to visibility impairment, 

and how the Administrator selected the 

target level of protection for the 

visibility index, we have addressed the 

CASAC’s request to support the 

proposed decision to revise the 

secondary 24-hour PM2.5 standard while 

retaining the secondary annual PM2.5

standard. The commenters also cite to 

an individual CASAC member’s 

comments for the review of the 2021 

draft PA who stated ‘‘[f]or the limited 

scope of this reconsideration review, I 

see no reason to not simply set the 

Secondary equal to the Primary PM 

Standards, whatever they may be’’ 

(Sheppard, 2022a, p. A–3). This CASAC 

member did not provide a supporting 

rationale for revising the secondary 

standards to levels equal to the primary 

standards. Although areas across the 

country are required to attain both the 

primary and secondary PM2.5 standards 

so air quality is unaffected by the 

Administrator’s decision not to revise 

the secondary standards to be equal to 

the primary standards, as described in 

responding to comments above, the 

CAA provisions require the 

Administrator to establish secondary 

standards that, in the judgment of the 

Administrator, are requisite to protect 

public welfare from known or 

anticipated adverse effects associated 

with the presence of the pollutant in 

ambient air. 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 

standards be set at a zero-risk level, but 

rather at a level that reduces risk 

sufficiently so as to protect the public 

welfare from known or anticipated 

adverse effects. The final decision on 

the adequacy of the current secondary 

standards is a public welfare policy 

judgment to be made by the 

Administrator. In reaching his proposed 

and final decisions regarding the 

adequacy of the current secondary PM 

standards, the Administrator considered 

the available scientific information and 

analyses about welfare effects, and 

associated public welfare significance, 

as well as judgments about how to 

consider the range and magnitude of 

uncertainties that are inherent in the 

scientific evidence and analyses. In so 

doing, the Administrator concluded that 

the currently available scientific 

evidence and quantitative analyses, 

including uncertainties and limitations, 

do not call into question the adequacy 

of the current secondary PM standards 

and that the current secondary PM 

standards should be retained, without 

revision. The Administrator’s judgments 

and decisions on the primary and 

secondary standards are independent 

and consider different aspects of the 

available scientific evidence and 

information in reaching conclusions 

regarding the adequacy of the standards 

in protecting against PM-related health 

and welfare effects. 

4. Administrator’s Conclusions 

This section summarizes the 

Administrator’s considerations and 

conclusions related to the current 

secondary PM2.5 and PM10 standards 

and presents the rationale for his 

decision that no change is required for 

those standards at this time. The CAA 

provisions require the Administrator to 

establish secondary standards that, in 

the judgment of the Administrator, are 

requisite to protect public welfare from 

known or anticipated adverse effects 

associated with the presence of the 

pollutant in the ambient air. 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 

standards be set at a zero-risk level, but 

rather at a level that reduces risk 

sufficiently so as to protect the public 

welfare from known or anticipated 

adverse effects. The final decision on 

the adequacy of the current secondary 

standards is a public welfare policy 

judgment to be made by the 

Administrator. The decision should 

draw on the scientific information and 

analyses about welfare effects, and 

associated public welfare significance, 

as well as judgments about how to 

consider the range and magnitude of 

uncertainties that are inherent in the 

scientific evidence and analyses. This 

approach is based on the recognition 

that the available evidence generally 

reflects a continuum that includes 

ambient air exposures at which 

scientists agree that effects are likely to 

occur through lower levels at which the 

likelihood and magnitude of responses 

become increasingly uncertain. This 

approach is consistent with the 

requirements of the provisions of the 

Clean Air Act related to the review of 

NAAQS and with how the EPA and the 

courts have historically interpreted the 

Act. 

Given these requirements, the 

Administrator’s final decision in this 

reconsideration is a public welfare 

policy judgment that draws upon the 

scientific and technical information 

examining PM-related visibility 

impairment, climate effects and 

materials effects, including how to 

consider the range and magnitude of 

uncertainties inherent in that 

information. The Administrator 

recognizes that his final decision is 

based on an interpretation of the 

scientific evidence and technical 

analyses that neither overstates nor 

understates their strengths and 

limitations, or the appropriate 

inferences to be drawn. In particular, 

the Administrator notes that the 

assessment of when visibility 

impairment is adverse to public welfare 

requires a public welfare policy 

judgment informed by available 

scientific and quantitative information. 

In considering the adequacy of the 

current secondary PM standards in this 

reconsideration, the Administrator has 

carefully considered the: (1) Policyrelevant evidence and conclusions 

contained in the 2019 ISA and 2022 ISA 

Supplement; (2) the quantitative 

information presented and assessed in 

the 2022 PA; (3) the evaluation of this 

evidence, the quantitative information, 

and the rationale and conclusions 

presented in the 2022 PA; (4) the advice 

and recommendations from the CASAC; 

and (5) public comments. In the 

discussion below, the Administrator 

gives weight to the 2022 PA 

conclusions, with which the CASAC 

generally concurred during their review 

of the 2019 draft PA and 2021 draft PA, 

as summarized in section IV.B.1 of the 

2020 final notice and section V.D.1 of 

the 2022 proposal, and takes note of key 

aspects of the rationale for those 

conclusions that contribute to his 

decision in this reconsideration. After 

giving careful consideration to all of this 

information, the Administrator judges 

that no change is required for the 

secondary PM standards at this time. 

In considering the 2022 PA 

evaluations and conclusions, the 

Administrator takes note of the overall 

conclusions that the non-ecological 

welfare effects evidence and 

quantitative information are generally 

consistent with what was considered in 

the 2020 final decision and in the 2012 

review (U.S. EPA, 2022b, section 5.5). 

The scientific evidence for nonecological welfare effects in this 

reconsideration is largely the same as 

that available in the 2019 ISA and 2020 

PA. As described in section I.C.5.b 

above, the 2022 ISA Supplement 

included a limited number of newly 

available studies on PM-related 

visibility effects. This newly available 

evidence on visibility effects, along with 

the full body of non-ecological welfare 

effects evidence assessed in the 2019 

ISA, reaffirms conclusions on the 

visibility, climate, and materials effects 

recognized in the 2020 final decision 

and in the 2012 review, including key 

conclusions on which the standards are 

based. Further, as discussed in more 


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detail above, the updated quantitative 

analyses of visibility impairment for 

areas meeting the current standards in 

the 2022 PA support the adequacy of the 

current secondary PM standards to 

protect against PM-related visibility 

impairment. The Administrator also 

recognizes that uncertainties and 

limitations continue to be associated 

with the available scientific evidence 

and quantitative information. 

With regard to the current evidence 

on visibility effects, as summarized in 

the 2022 PA and discussed in detail in 

the 2019 ISA and ISA Supplement, the 

Administrator notes the long-standing 

body of evidence for PM-related 

visibility impairment. As in previous 

reviews, this evidence continues to 

demonstrate a causal relationship 

between PM in ambient air and effects 

on visibility (U.S. EPA, 2019a, section 

13.2). The Administrator recognizes that 

visibility impairment can have 

implications for people’s enjoyment of 

daily activities and for their overall 

sense of well-being. Therefore, as in 

previous reviews, he considers the 

degree to which the current secondary 

standards protect against PM-related 

visibility impairment and the degree to 

which PM-related visibility impairment 

is adverse to public welfare. In 

particular, in recognizing the short-term 

nature of visibility impairment along 

with the fact that PM2.5 is the size 

fraction that contributes most to light 

extinction, the Administrator especially 

focuses on the adequacy of the current 

secondary 24-hour PM2.5 standard in 

providing protection against PM-related 

visibility effects judged to be adverse. 

The Administrator also considers the 

protection provided by the current 

secondary 24-hour PM2.5 standard 

against PM-related visibility impairment 

in conjunction with the Regional Haze 

Program as a means of achieving 

appropriate levels of protection against 

PM-related visibility impairment in 

urban, suburban, rural, and Federal 

Class I areas across the U.S. Programs 

implemented to meet the secondary PM 

standards, along with the requirements 

of the Regional Haze Program 

established for protecting against 

visibility impairment in Class I areas, 

would be expected to improve visual air 

quality across all areas of the country. 

As described in the proposal (88 FR 

5658, January 27, 2023), the 

Administrator recognizes that the 

Regional Haze Program was established 

by Congress specifically to achieve ‘‘the 

prevention of any future, and the 

remedying of existing, impairment of 

visibility in mandatory Class I areas, 

which impairment results from manmade air pollution,’’ and that Congress 

established a long-term program to 

achieve that goal (CAA section 169A). In 

adopting section 169, Congress set a 

goal of eliminating anthropogenic 

visibility impairment at Class I areas, as 

well as a framework for achieving that 

goal which extends well beyond the 

planning process and timeframe for 

attaining the secondary PM NAAQS. 

Recognizing that the Regional Haze 

Program will continue to contribute to 

reductions in visibility impairment in 

Class I areas, consistent with his 

proposed conclusions, the 

Administrator concludes that 

addressing visibility impairment in 

Class I areas is largely beyond the scope 

of the secondary PM standards and that 

setting the secondary 24-hour PM2.5

standard at a level that would remedy 

visibility impairment in Class I areas 

would result in standards that are more 

stringent than is requisite. 

In further considering what standards 

are requisite to protect against adverse 

public welfare effects from visibility 

impairment, the Administrator 

concludes that it is appropriate to use 

an approach consistent with the 

approach used past reviews (88 FR 

5650, January 27, 2023). He first 

identifies an appropriate target level of 

protection in terms of a PM visibility 

index that takes into account the factors 

that influence the relationship between 

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

size fraction, species composition, and 

relative humidity). He then considers 

the air quality analyses conducted in the 

2022 PA that examine the relationship 

between the PM visibility index and the 

current secondary 24-hour PM2.5

standard in locations that meet the 

current 24-hour PM2.5 and PM10

standards (U.S. EPA, 2022b, section 

5.3.1.2). 

In reaching conclusions regarding the 

target level of protection, the 

Administrator first considers the 

characteristics of the visibility index 

and defines its elements (indicator, 

averaging time, form, and level). With 

regard to the indicator for the visibility 

index, the Administrator continues to 

recognize that, consistent with the 

conclusions of the 2022 PA and the 

CASAC’s advice in their review of the 

2021 draft PA, there is a lack of 

availability of methods and an 

established network for directly 

measuring light extinction. Therefore, 

the Administrator concludes that it 

continues to be appropriate to using an 

index based on estimates of light 

extinction by PM2.5 components based 

on the IMPROVE algorithm. In so doing, 

the Administrator recognizes that the 

fundamental understanding of the 

relationship between ambient PM and 

light extinction has generally changed 

very little over time; however, several 

versions of the IMPROVE equation have 

been developed and evaluated that 

could be used to estimate light 

extinction. As at the time of the 

proposal, the Administrator recognizes 

that the results of the quantitative 

analyses in the 2022 PA that examined 

three versions of the IMPROVE equation 

indicate that there are very small 

differences in estimates of light 

extinction between the equations, and 

that it is not always clear that one 

version of the IMPROVE equation is 

more appropriate for estimating light 

extinction across the U.S. than other 

versions of the IMPROVE algorithm (88 

FR 5659, January 27, 2023). He also 

recognizes that the selection of inputs to 

the IMPROVE equation (e.g., the 

multiplier for OC to OM) may be more 

appropriate on a regional basis rather 

than a national basis when calculating 

light extinction, and notes the CASAC’s 

advice that PM-visibility relationships 

are region specific (Sheppard, 2022a, p. 

21 of consensus responses). The 

Administrator further notes that neither 

the CASAC nor public commenters 

recommended a specific IMPROVE 

equation or an approach for using 

different IMPROVE equations across the 

U.S. Therefore, given the absence of a 

robust monitoring network to directly 

measure light extinction, the 

Administrator concludes that light 

estimated light extinction, as calculated 

using one or more versions of the 

IMPROVE algorithms, continues to be 

the most appropriate indicator for the 

visibility index. 

Having reached the conclusion that 

estimated light extinction is the 

appropriate indicator for the visibility 

index, the Administrator next considers 

the appropriate averaging time and form 

of the index. With regard to the 

averaging time and form, the 

Administrator notes that in previous 

reviews, a 24-hour averaging time was 

selected and the form was defined as the 

3-year average of annual 90th percentile 

values. As at the time of proposal, the 

Administrator recognizes that the 

available information continues to 

provide support for the short-term 

nature of visibility effects. He further 

recognizes that no new information is 

available in this reconsideration to 

inform his conclusions regarding 

averaging time, and therefore, he 

considers past analyses of 24-hour and 

subdaily PM2.5 light extinction to inform 

his conclusions on averaging time. As 

described in the proposal (88 FR 5659, 

January 27, 2023) and in responding to 

comments in section V.B.3 above, prior 


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176 Gantt, B., and Hagan, N. (2023). Analysis of 

Percentile Forms of the Visibility Index. 

Memorandum to the Rulemaking Docket for the 

Review of the National Ambient Air Quality 

Standards for Particulate Matter (EPA–HQ–OAR– 

2015–0072). Available at: https://

www.regulations.gov/docket/EPA-HQ-OAR-2015- 

0072. 

177 Gantt, B., and Hagan, N. (2023). Analysis of 

Percentile Forms of the Visibility Index. 

Memorandum to the Rulemaking Docket for the 

Review of the National Ambient Air Quality 

Standards for Particulate Matter (EPA–HQ–OAR– 

2015–0072). Available at: https://

www.regulations.gov/docket/EPA-HQ-OAR-2015- 

0072. 

178For reasons stated above and described in the 

2022 PA and proposal, the Administrator does not 

find it appropriate to use the most recent preference 

study based on the Grand Canyon study area (Malm 

et al., 2019) for purposes of identifying a target level 

of protection for the visibility index. 

analyses demonstrated that there are 

strong correlations between 24-hour and 

subdaily (i.e., 4-hour average) PM2.5

light extinction, indicating that a 24- 

hour averaging time is an appropriate 

surrogate for the subdaily time periods 

associated with when individuals 

experience visibility impairment and 

that a longer averaging time may also be 

less influenced by atypical conditions 

and/or atypical instrument performance. 

The Administrator also notes that the 

CASAC did not provide advice or 

recommendations with regard to the 

averaging time of the visibility index, 

although some public commenters 

referenced CASAC advice in past 

reviews that a subdaily standard based 

on daylight hours would better reflect 

the public welfare effects of public 

perceptions of visibility impairment 

than a 24-hour standard. However, in 

considering the available scientific and 

quantitative information, as well as the 

CASAC’s advice in their reviews of the 

2019 draft PA and 2021 draft PA, the 

Administrator concludes that the 24- 

hour averaging time continues to be 

appropriate for the visibility index 

because it is an appropriate surrogate for 

subdaily time periods and results in a 

more stable target. 

With regard to the form of the 

visibility index, the Administrator notes 

the approach in other NAAQS that a 

multi-year percentile form offers greater 

stability to the air quality management 

process by reducing the possibility that 

statistically unusual indicator values 

will lead to transient violations of the 

standard. He recognizes that using a 3- 

year average provides stability from the 

occasional effects of inter-annual 

meteorological variability (including 

relative humidity) that can result in 

unusually high pollution levels for a 

particular year (88 FR 5659, January 27, 

2023) and recognizes that a stable 

standard contributes to the benefits of 

the NAAQS by ensuring that attainment 

strategies are designed to address nontransient problems and achieve durable 

air quality improvements. For these 

reasons, he concludes that a 3-year 

average continues to be appropriate. 

In considering the percentile that 

would be appropriate with the 3-year 

average, the Administrator recognizes 

that there is very little new information 

available in this reconsideration to 

inform selection of an alternative form 

of the visibility index and that the 

appropriate form requires the exercise of 

public welfare policy judgment. In 

selecting the appropriate target level of 

protection for the visibility index, the 

Administrator is required to assess 

when visibility impairment becomes 

adverse to public welfare, weighing both 

the degree of visibility impairment (in 

dv) and the frequency of such 

impairment (through the form). As with 

the mass-based PM air quality standard, 

the target level of protection for the 

visibility index must be selected in 

conjunction with the form to determine 

the appropriate stringency. In so doing, 

consistent with approaches in past 

reviews, the Administrator first notes 

that the Regional Haze Program targets 

the 20% most impaired days for 

improvements in visual air quality in 

Class I areas, which are the days above 

the 80th percentile form of the visibility 

index. The Administrator concludes 

that a percentile form set at the 80th 

percentile would not be likely to 

sufficiently improve visual air quality 

on the worst days based on the visibility 

index. In considering the information 

available in past reviews regarding the 

form of the visibility index, as well as 

the analysis of alternative forms based 

on recent air quality discussed above, 

the Administrator notes that a 90th 

percentile form would represent the 

median of the distribution of the 20% 

most impaired days, and meeting a 

visibility index with a 90th percentile 

form would reasonably be expected to 

lead to improvements in visual air 

quality for days both above and below 

the 90th percentile (88 FR 5660, January 

27, 2023). In reaching his conclusion 

that a 90th percentile would 

appropriately achieve improved air 

quality both above and below that 

percentile, the Administrator took into 

consideration assessments of air quality 

data and potential alternative 

percentiles for the form. The 

Administrator further notes that, 

consistent with the conclusions in the 

2011 PA and 2020 PA, the 2022 PA 

concluded that there is no new 

information from public preference 

studies that would suggest that a 90th 

percentile form is not appropriate. The 

Administrator also considers air quality 

analyses described above in responding 

to public comments regarding the 

percentile form of the visibility index. 

In particular, the Administrator notes 

that while a higher percentile form (i.e., 

95th or 98th) would somewhat further 

limit the number of days with peak PMrelated light extinction, the differences 

in the 3-year averages of estimated light 

extinction for the 90th, 95th, and 98th 

percentile forms are small. For example, 

he notes that for the original IMPROVE 

equation, in areas that meet the current 

24-hour PM2.5 standard, all sites have 

light extinction estimates for a 90th 

percentile form at or below 26 dv, and 

for a 95th or 98th percentile form light 

extinction estimates are at or below 29 

dv.176 He further notes that, in most 

locations when estimating light 

extinction based on the original 

IMPROVE equation, the difference 

between a 95th or 98th percentile form 

and a 90th percentile form is generally 

less than 3 dv.177 Moreover, the 

Administrator concludes that a 90th 

percentile form achieves a very high 

degree of control but appropriately 

targets the group of worst days, rather 

than the few very worst days. Based on 

the available information and these 

analyses, the Administrator concludes 

that the information does not indicate 

that it would be appropriate to consider 

limiting the occurrence of days with 

peak PM-related light extinction to a 

greater degree, nor did the CASAC 

provide advice or recommendations 

related to the form of the visibility 

index. Therefore, the Administrator 

judges that it remains appropriate to 

define a visibility index in terms of a 24- 

hour averaging time and form based on 

the 3-year average of annual 90th 

percentile values. 

With regard to the level of the 

visibility index, as at the time of 

proposal, the Administrator continues 

to recognize that there is very little new 

information available to inform his 

judgment regarding the range of levels 

of visibility impairment judged to be 

acceptable by at least 50% of study 

participants in the visibility preference 

studies,178 and therefore, the range of 20 

to 30 dv identified in the 2022 PA 

remains appropriate for considering the 

level of the visibility index. The 

Administrator also recognizes that the 

uncertainties and limitations associated 

with the public preferences identified in 

the 2012 and 2020 reviews continue to 

persist, and that these limitations and 

uncertainties contributed to the 

decisions in 2012 and 2020 that a level 

at the upper end of the range (i.e., 30 dv) 

was selected. The Administrator 

specifically notes that, while the studies 


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are methodologically similar, there are a 

number of factors that can influence 

comparability across the studies and 

that the available studies may not 

capture the full range of visibility 

preferences in the U.S. population, as 

described in more detail in section 

V.D.3 of the 2022 proposal (88 FR 5659– 

5660, January 27, 2023). The 

Administrator also notes the CASAC’s 

advice in their review of the 2021 draft 

PA that there are a limited number of 

visibility preference studies available to 

inform the Administrator’s judgment 

regarding the appropriate target level of 

protection for the visibility index 

(Sheppard, 2022a, p. 21 of consensus 

responses). In considering the available 

information, including uncertainties 

and limitation, and the CASAC’s advice, 

the Administrator proposed to conclude 

that it is appropriate to consider a target 

level of protection for the visibility 

index within the range of 20 to 30 dv, 

and that establishing a target level of 

protection at the upper end of the range 

was appropriate. In so doing, the 

Administrator proposed to conclude 

that the protection provided by a 

visibility index based on estimated light 

extinction, a 24-hour averaging time, 

and a 90th percentile form, averaged 

over 3 years, set to a level of 30 dv 

would be requisite to protect public 

welfare with regard to visibility 

impairment. 

However, at the time of proposal, the 

Administrator recognized that the 

available evidence on visibility 

impairment generally reflects a 

continuum and that the public 

preference studies do not provide 

information about the specific level for 

which visibility impairment would be 

‘‘acceptable’’ or ‘‘unacceptable’’ across 

the country, and that alternative target 

levels of protection could be supported. 

At that time, in soliciting public 

comments, the Administrator 

recognized that other interpretations, 

assessments, and judgments based on 

the available welfare effects evidence for 

this reconsideration could be possible 

(88 FR 5662, January 27, 2023). 

With regard to the appropriate target 

level of protection for the visibility 

index, the Administrator first notes that 

while the public preference studies 

were conducted in several geographical 

areas across the U.S., and they provide 

insight into regional preferences for 

visibility impairment, none of the 

studies identify a specific level of 

visibility impairment that would be 

perceived as ‘‘acceptable’’ or 

‘‘unacceptable’’ across the whole U.S. 

population. He also noted that there 

have been significant questions about 

how to set a standard for visibility that 

is neither overprotective nor 

underprotective for some areas of the 

U.S. As described in the proposal (88 FR 

5660, January 27, 2023), in establishing 

the Regional Haze Program to improve 

visibility in Class I areas, Congress 

noted that ‘‘as a matter of equity, the 

national ambient air quality standards 

cannot be revised to adequately protect 

visibility in all areas of the country.’’ 

H.R. Rep. 95–294 at 205. For the reasons 

noted above, in reaching his proposed 

decision regarding visibility 

impairment, the Administrator 

recognized that he is not seeking to set 

a standard that would eliminate 

visibility impairment in Class I areas, 

but significant uncertainties remain 

regarding how to judge when visibility 

impairment becomes adverse to public 

welfare across the range of daily outdoor 

activities for Americans across the 

country. 

In reaching final conclusions 

regarding the available information, 

along with the CASAC’s advice and 

public comments, the Administrator 

again considers what constitutes an 

appropriate target level of protection, 

and in particular considers whether a 

target level of protection below 30 dv is 

warranted. In so doing, he first notes the 

variability in public preferences of 

visibility impairment as demonstrated 

by the available public preferences, 

which support a range of potential target 

levels of protection for the visibility 

index from 20 to 30 dv. He also notes 

that this range informed the 2012 and 

2020 then-Administrators final 

decisions that a target level of protection 

at the upper end of the range (i.e., 30 dv) 

would be most appropriate, given the 

uncertainties and limitations associated 

with the public preference studies. As 

described in in section V.B.3 above in 

responding to public comments, the 

Administrator recognizes that a number 

of factors can influence public 

preferences across studies, in particular 

due to the types of scenes depicted in 

the images as well as the distances at 

which the objects of interest are located 

from the camera. Furthermore, the 

Administrator recognizes the small 

number of public preference studies 

currently available makes precise 

interpretations of their results 

challenging for determining a nationally 

appropriate target level of visibility 

protection. The Administrator also 

recognizes that the CASAC, in their 

review of 2021 draft PA, reiterated that 

PM-visibility relationships are regionspecific based on aerosol composition, 

and that several public commenters 

emphasized the importance of the sight 

path distance in the images when 

considering how to interpret the public 

preference studies. 

In this reconsideration, the 

Administrator judges that in 

determining when visibility impairment 

becomes adverse to public welfare for 

purposes of the secondary NAAQS, 

while continuing to recognize that 

substantial uncertainties remain and 

that there is relatively limited new 

information regarding public 

preferences of visibility impairment, it 

is important to balance the weight 

placed on uncertainties with the 

strength of the scientific evidence. In so 

doing, the Administrator first concludes 

that, consistent with previous reviews 

and his proposed decision, it remains 

appropriate to consider a target level of 

protection within the range of 20 to 30 

dv. However, in further considering the 

available scientific and quantitative 

information, CASAC advice, and public 

comments, he further concludes that in 

selecting a target level within that range 

it is appropriate to place weight on both 

the middle of the range, as supported by 

the study in Phoenix, AZ, as well as the 

upper end, as supported by the 

Washington, DC, study. In so doing, he 

notes that the Washington, DC, and 

Phoenix, AZ, studies employ similar 

methodologies that are subject to fewer 

uncertainties than older public 

preference studies (including their use 

of WinHaze to reduce uncertainties in 

the preference solicitations) although he 

does note that the Phoenix, AZ, study 

yielded the best results of the four 

public preference studies in terms of the 

least noisy preference results and the 

most representative selection of 

participants. Further, the Administrator 

judges that this approach would take 

into account scenes that are similar to 

both the Washington, DC, study and 

Phoenix, AZ, study, which would be 

more representative of the ‘‘typical’’ 

scenes encountered across more areas of 

the U.S. than an approach that places 

weight on just one study or on studies 

conducted in certain geographical areas 

of the country. In considering this 

information, along with the 

uncertainties and limitations of the 

public preference studies, the 

Administrator judges that it would be 

appropriate to select a target level of 

protection based on placing equal 

weight on the upper end of the range 

(i.e., 30 dv) and the middle of the range 

(i.e., 24 dv based on the Phoenix, AZ, 

study) in order to provide protection 

against visibility impairment in 

different geographical areas of the U.S. 

For these reasons, the Administrator 

concludes that a visibility index with a 

target level of protection of 27 dv, 


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defined in terms of estimated light 

extinction, with a 24-hour averaging 

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

would provide adequate protection 

against PM-related visibility effects. In 

reaching this conclusion, the 

Administrator judges that such a target 

level of protection balances the 

information from these two key public 

preference studies in such a way 

appropriately weighs both near-field 

and more distant landscape features that 

may be of importance to public 

perceptions of visibility. 

In further considering the appropriate 

target level of protection for the 

visibility index, the Administrator again 

recognizes the complexity of the 

relationship between PM and light 

extinction which is dependent on a 

number of factors, including PM 

composition, size fraction, and age of 

the particles in ambient air, as well as 

relative humidity. As noted in 

responding to comments above, these 

factors can vary geographically across 

the U.S. and local or regional 

meteorological conditions can also vary 

spatially and temporally. These factors 

are critical inputs to the IMPROVE 

equation and can influence the resulting 

estimated light extinction such that it is 

not a straightforward comparison 

between estimated light extinction in 

one area of the country versus another. 

Moreover, the Administrator recognizes 

that there is variability in estimated 

light extinction depending on the 

version of the IMPROVE equation that is 

used. As described in more detail in the 

2022 PA and the proposal, and in 

reaching his decisions on the indicator 

of the visibility index above, the 

Administrator notes that the 2022 PA 

concluded that one version of the 

IMPROVE equation is not more accurate 

or precise in estimating light extinction, 

and that difference in locations may 

support the selection of inputs into the 

IMPROVE equation or of the appropriate 

IMPROVE equation to estimate light 

extinction on a regional basis rather 

than on a national basis. 

In considering the available 

information, including variations in 

both public preferences of visibility 

impairment and estimates of light 

extinction using one or more IMPROVE 

equation, as well as the CASAC’s advice 

in their review of the 2019 draft PA and 

2021 draft PA and public comments, the 

Administrator judges that a target level 

of protection of 27 dv would be 

appropriate. In so doing, he concludes 

that a target level of protection above 27 

dv would not provide adequate 

protection against PM-related visibility 

impairment based on the 50% 

acceptability values when both the 

Washington, DC, and Phoenix, AZ, 

studies are considered. However, he 

also notes that when considering the 

50% acceptability values from studies 

conducted in different areas of the U.S. 

and with different scenes and images 

depicted, the available public 

preference studies do not provide a 

‘‘bright line’’ at and above which 

visibility impairment is considered 

adverse to public welfare. He further 

recognizes that, as discussed just above, 

there are a number of region-specific 

factors that can influence light 

extinction, and thereby influence 

visibility impairment, as well as 

variations in public preferences of 

visibility impairment based on the 

available studies, that complicate 

selection of a single target level of 

protection that would be appropriate for 

a national visibility index. While the 

Administrator recognizes that the 

uncertainties and limitations associated 

with public preferences of visibility and 

estimating light extinction have 

persisted over the last several PM 

NAAQS reviews, he also recognizes that 

in reaching conclusions regarding the 

appropriate target level of protection for 

the visibility index also involves public 

welfare policy judgments regarding how 

to appropriately consider the particular 

uncertainties around identifying when 

visibility impairment becomes adverse 

to public welfare, and the limitations on 

relying on the public preference studies. 

The Administrator also places weight 

on the high degree of spatial and 

temporal variability in PM composition 

and relative humidity across the U.S. in 

considering a target level of protection. 

This approach of establishing a target 

level of protection that takes into 

account 50% acceptability values from 

both eastern and western sites is a more 

appropriate basis for determining the 

requisite level of protection against 

known or anticipated adverse effects on 

public welfare across diverse locations, 

i.e., a standard that is neither more nor 

less stringent than necessary 

nationwide. Specifically, the 

Administrator judges that a target level 

of protection for the visibility index 

focused on maintaining estimated light 

extinction between the upper end of the 

range of the target levels of protection 

(i.e., 30 dv based on the Washington, 

DC, study) and the middle of the range 

(i.e., 24 dv based on the Phoenix, AZ, 

study) to be more appropriate for a 

nationwide standard to protect against 

visibility impairment compared to a 

value derived from one location or one 

type of scene alone. For these reasons, 

in selecting a target level of protection, 

the Administrator concludes that a 

target level of protection somewhere 

between the upper end and middle of 

the range is appropriate because he 

judges that this approach, in 

conjunction with the Regional Haze 

program, is sufficient, but not more 

stringent than necessary, to protect 

against adverse effects on public 

welfare. Thus, he concludes a secondary 

24-hour PM2.5 NAAQS should be 

evaluated based on its ability to provide 

protection against visibility impairment 

associated with estimated light 

extinction of 27 dv based on estimated 

light extinction, a 24-hour averaging 

time, and a 90th percentile form, 

averaged over 3 years. 

Having concluded that it is 

appropriate to identify a target level of 

protection in terms of a visibility index 

based on estimated light extinction as 

described above, the Administrator next 

considers the degree of protection from 

visibility impairment afforded by the 

current secondary PM standards. He 

considers the updated analyses of PMrelated visibility impairment presented 

in the 2022 PA (U.S. EPA, 2022b, 

section 5.3.1.2) and described in section 

V.B.1.a of the proposal, and notes that 

the results of the analyses are consistent 

with the results from the 2012 and 2020 

reviews. 

Taking into consideration the full 

body of scientific evidence and 

technical information concerning the 

known and anticipated effects of PM on 

visibility impairment, the Administrator 

concludes that the current secondary 

PM2.5 and PM10 standards are requisite 

to protect against PM-related visibility 

impairment. While the inclusion of the 

coarse fraction had a relatively modest 

impact on calculated light extinction in 

the analyses presented in the 2022 PA, 

he recognizes the continued importance 

of the PM10 standard given the potential 

for larger impacts in locations with 

higher coarse particle concentrations, 

such as in the southwestern U.S., for 

which only a few sites met the criteria 

for inclusion in the analyses in the 2022 

PA (U.S. EPA, 2019a, section 13.2.4.1; 

U.S. EPA, 2022b, section 5.3.1.2). 

With regard to the adequacy of the 

secondary 24-hour PM2.5 standard, the 

Administrator notes that, in their review 

of the 2021 draft PA, the CASAC stated 

that ‘‘[i]f a value of 20–25 deciviews is 

deemed to be an appropriate visibility 

target level of protection, then a 

secondary 24-hour PM2.5 standard in the 

range of 25–35 mg/m3 should be 

considered’’ (Sheppard, 2022a, p. 21 of 

consensus responses). The 

Administrator recognizes that the 

CASAC recommended that the 

Administrator provide additional 

justification for a visibility index target 


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of 30 dv but did not specifically 

recommend that he choose an 

alternative level for the visibility index. 

The Administrator carefully considered 

the advice of CASAC and the public 

comments and concluded that a lower 

target level of visibility was appropriate 

in order to properly reflect both a 

broader set of studies and a broader 

range of vistas that were the subject of 

those studies. However, in their review 

of the 2021 draft PA, the CASAC 

recognized that even a visibility index 

target in the range of 20–25 dv could 

still warrant retention of the current 

secondary 24-hour PM2.5 standard. The 

Administrator also considers the advice 

from the CASAC in their review of the 

2019 draft PA, who ‘‘recogniz[ed] that 

uncertainties. . .remain about the best 

way to evaluate’’ PM-related visibility 

effects (Cox, 2019b, p. 13 consensus 

responses). The Administrator 

considered the CASAC’s advice, 

together with the available scientific 

evidence and quantitative information, 

in reaching his conclusions. 

The Administrator recognizes that 

conclusions regarding the appropriate 

weight to place on the scientific and 

technical information examining PMrelated visibility impairment, including 

how to consider the range and 

magnitude of uncertainties inherent in 

that information, is a public welfare 

policy judgment left to the 

Administrator. In reaching his final 

decision in 2020, the thenAdministrator noted that the available 

evidence regarding visibility effects had 

changed very little since the 2012 

review, specifically recognizing that, as 

evaluated in the 2019 ISA, there were 

no new visibility studies that were 

conducted in the U.S. and there was 

little new information available with 

regard to acceptable levels of visibility 

impairment in the U.S. (85 FR 82742, 

December 18, 2020). As such, the thenAdministrator concluded that the 

protection provided by a standard 

defined in terms of a PM2.5 visibility 

index, with a 24-hour averaging time, a 

90th percentiles form averaged over 

three years, set at a level of 30 dv, was 

requisite to protect public welfare 

against visibility impairment (85 FR 

82743, December 18, 2020). He also 

recognized that there was some new 

information to inform quantitative 

analyses of light extinction, but that the 

results of the analyses conducted in the 

2020 PA were consistent with those 

from the 2012 review. The thenAdministrator recognized that the 

analyses demonstrated that the 3-year 

visibility metric was at or below about 

30 dv in all areas that met the current 

secondary 24-hour PM2.5 standard, and 

was below 25 dv in most of those areas 

(85 FR 82743, December 18, 2020). 

Therefore, the Administrator judged that 

the secondary 24-hour PM2.5 standard 

provided sufficient protection for visual 

air quality of 30 dv, which he judged 

appropriate (88 FR 82744, December 18, 

2020). In this reconsideration, the ISA 

Supplement evaluated newly available 

studies on public preferences for 

visibility impairment and/or 

development methodologies or 

conducted quantitative analyses of light 

extinction. In considering the available 

scientific and quantitative information, 

including that newly available in this 

reconsideration, the current 

Administrator reached the same 

preliminary conclusions in the notice of 

proposed rulemaking regarding the 3- 

year visibility index and the current 

secondary PM standards as the thenAdministrator in the 2020 final 

decision. However, in light of public 

comments on the proposal, the 

Administrator has further considered 

the available scientific evidence and 

information, as well as the CASAC’s 

advice regarding visibility effects in 

their review of the 2021 draft PA. In so 

doing, the Administrator judges that it 

is appropriate to place more weight on 

certain aspects of the evidence that he 

had placed less weight on in reaching 

his proposed conclusions (i.e., he 

focused on the both the middle and the 

upper end of the range of the 50% 

acceptability values from the available 

public preference studies). As such, the 

Administrator notes his conclusion on 

the appropriate visibility index (i.e., 

with a 24-hour averaging time; a 3-year, 

90th percentile form; and a level of 27 

dv), which takes into account the 

regional variations in public preferences 

and equations for estimating light 

extinction, and his conclusions 

regarding the quantitative analyses of 

the relationship between the visibility 

index and the current secondary 24- 

hour PM2.5 standard. In so doing, the 

Administrator concludes that the 

current secondary standards provide 

requisite protection against PM-related 

visibility effects. 

With respect to climate effects, as at 

the time of proposal, the Administrator 

recognizes that a number of 

improvements and refinements have 

been made to climate models since the 

time of the 2012 review. However, 

despite continuing research and the 

strong evidence supporting a causal 

relationship with climate effects (U.S. 

EPA, 2019a, section 13.3.9), the 

Administrator notes that there are still 

significant limitations in quantifying the 

contributions of the direct and indirect 

effects of PM and PM components on 

climate forcing (U.S. EPA, 2022b, 

sections 5.3.2.1.1 and 5.5). He also 

recognizes that models continue to 

exhibit considerable variability in 

estimates of PM-related climate impacts 

at regional scales (e.g., ∼100 km), 

compared to simulations at the global 

scale (U.S. EPA, 2022b, sections 

5.3.2.1.1 and 5.5). Moreover, the effects 

of PM on climate are diverse as well as 

uncertain. Depending on the 

circumstances, the radiative forcing 

effects of PM in the atmosphere can 

vary, such that positive forcing could 

result in warming of the Earth’s surface, 

whereas a negative forcing could result 

in cooling (U.S. EPA, 2019a, section 

13.3.2.2). The resulting uncertainty 

leads the Administrator to conclude that 

the scientific information available in 

this reconsideration remains insufficient 

to quantify, with confidence, the 

impacts of ambient PM on climate in the 

U.S. (U.S. EPA, 2022b, section 5.3.2.2.1) 

and that there is not an adequate 

scientific basis to link attainment of any 

particular PM concentration in ambient 

air in the U.S. to specific climate effects. 

Consequently, the Administrator judges 

that there is insufficient information at 

this time to revise the current secondary 

PM standards or to promulgate a 

distinct secondary standard to address 

PM-related climate effects. 

With respect to materials effects, the 

Administrator notes that the available 

evidence continues to support the 

conclusion that there is a causal 

relationship with PM deposition (U.S. 

EPA, 2019a, section 13.4). He recognizes 

that deposition of particles in the fine or 

coarse fractions can result in physical 

damage and/or impaired aesthetic 

qualities. Particles can contribute to 

materials damage by adding to the 

effects of natural weathering processes 

and by promoting the corrosion of 

metals, the degradation of painted 

surfaces, the deterioration of building 

materials, and the weakening of material 

components. While some recent 

evidence on materials effects of PM is 

available in the 2019 ISA, the 

Administrator notes that this evidence 

is primarily from studies conducted 

outside of the U.S. in areas where PM 

concentrations in ambient air are higher 

than those observed in the U.S. (U.S. 

EPA, 2019a, section 13.4). Given the 

limited amount of information on the 

quantitative relationships between PM 

and materials effects in the U.S., and 

uncertainties in the degree to which 

those effects could be adverse to the 

public welfare, the Administrator judges 

that the available scientific information 


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179As noted earlier, other welfare effects of PM, 

such as ecological effects, are being considered in 

the separate, on-going review of the secondary 

NAAQS for oxides of nitrogen, oxides of sulfur and 

PM. 

remains insufficient to quantify, with 

confidence, the public welfare impacts 

of ambient PM on materials and that 

there is insufficient information at this 

time to revise the current secondary PM 

standards or to promulgate a distinct 

secondary standard to address PMrelated materials effects. 

Taken together, the Administrator 

concludes that the scientific and 

quantitative information for PM-related 

non-ecological welfare effects (i.e., 

visibility, climate, and materials),179

along with the uncertainties and 

limitations, supports the current level of 

protection provided by the secondary 

PM standards as being requisite to 

protect against known and anticipated 

adverse effects on public welfare. For 

visibility impairment, this conclusion 

reflects his consideration of the 

evidence for PM-related light extinction, 

together with his consideration of 

updated air quality analyses of the 

relationship between the visibility index 

and the current secondary 24-hour PM2.5

standard and the protection provided by 

the current secondary PM2.5 and PM10

standards. For climate and materials 

effects, this conclusion reflects his 

judgment that, although it remains 

important to maintain secondary PM2.5

and PM10 standards to provide some 

degree of control over long- and shortterm concentrations of both fine and 

coarse particles, it is appropriate not to 

change the existing secondary standards 

at this time and that it is not appropriate 

to establish any distinct secondary PM 

standards to address PM-related climate 

and materials effects at this time. As 

such, the Administrator recognizes that 

current suite of secondary standards 

(i.e., the 24-hour PM2.5, 24-hour PM10, 

and annual PM2.5 standards) together 

provide such control for both fine and 

coarse particles and long- and shortterm visibility and non-visibility (e.g., 

climate and materials) effects related to 

PM in ambient air. His conclusions on 

the secondary standards are consistent 

with advice from the CASAC, which 

noted substantial uncertainties remain 

in the scientific evidence for climate 

and materials effects, as well as the 

majority of public comments on the 

secondary PM standards. Thus, based 

on his consideration of the evidence and 

analyses for PM-related welfare effects, 

as described above, and his 

consideration of CASAC advice and 

public comments on the secondary 

standards, the Administrator concludes 

that it is appropriate not to change those 

standards (i.e., the current 24-hour and 

annual PM2.5 standards, 24-hour PM10

standard) at this time. 

C. Decision on the Secondary PM 

Standards 

For the reasons discussed above and 

taking into account information and 

assessments presented in the 2019 ISA, 

ISA Supplement, and 2022 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 not 

changing the standards at this time. 

VI. Interpretation of the NAAQS for PM 

The EPA is finalizing revisions on 

data calculations in appendix K for 

PM10 and appendix N for PM2.5. 

Revisions to appendix K make the PM10

data handling procedures for the 24- 

hour PM10 standards more consistent 

with those of other NAAQS pollutants 

and codify existing practices. Revisions 

to appendix N update references to the 

revision(s) of the standards and change 

data handling provisions related to 

combining data from nearby monitoring 

sites to codify existing practices that are 

currently being implemented as the EPA 

standard operating procedures. 

A. Amendments to Appendix K: 

Interpretation of the NAAQS for 

Particulate Matter 

The EPA proposed to modify its data 

handling procedures for the 24-hour 

PM10 standard in appendix K to part 50 

(88 FR 5662, January 27, 2023). The 

proposed modifications include: (1) 

Revising design value calculations to be 

on a site-level basis, (2) codifying site 

combinations to maintain a continuous 

data record, and (3) clarifying daily 

validity requirements for continuous 

monitors. The purpose of these 

modifications is to make the data 

handling procedures for the 24-hour 

PM10 standard more consistent with 

those of other NAAQS pollutants and 

codify existing practices that are 

currently being implemented as EPA 

standard operating procedures. 

The EPA received few comments on 

these proposed appendix K revisions, 

the majority of which were supportive. 

One commenter was not supportive of 

the proposed appendix K revision to 

site-level PM10 design values, asserting 

that it would amount to an imposition 

of a more stringent PM10 standard due 

to the potential high bias of FEMs. The 

EPA disagrees with this assertion 

because site-level design values would 

combine data from any high biased FEM 

with other monitors at the site rather 

than calculate a monitor-level design 

value with data solely from that highbiased FEM. The EPA tested the impact 

of calculating site-level PM10 design 

values for the 2019–2021 period by 

assigning the lowest parameter 

occurrence code as the primary monitor 

and calculating site-level design values. 

Most resulting site-level design values 

were either identical to or in-between 

the multiple monitor-level design values 

at the site. Combining data from two or 

more monitors also has the benefit of 

increasing the number of valid sample 

days at many sites. For the 2019–2021 

test period, approximately 10% of the 

sites with more than one monitor went 

from having multiple invalid design 

values to a single valid design value. 

One commenter was not supportive of 

a footnote in the preamble of the NPRM 

stating that in the absence of a 

designated primary monitor at a given 

site, the default primary monitor would 

be one with the most complete data 

record (88 FR 5662, January 27, 2023). 

Because the procedure for calculating 

PM10 design values on a site-level basis 

being finalized here will require 

monitoring agencies to designate a 

primary monitor for each site in their 

annual network plans (88 FR 5694, 

January 27, 2023; App. K, 1.0(b)), the 

EPA agrees with the commenter that 

this footnote was unnecessary. 

Therefore, the EPA is finalizing these 

appendix K revisions as proposed. 

B. Amendments to Appendix N: 

Interpretation of the NAAQS for PM2.5

The EPA proposed to modify its data 

handling procedures for the annual and 

24-hour PM2.5 standards in appendix N 

to part 50 (88 FR 5663, January 27, 

2023). These proposed revisions 

include: (1) Updating references to the 

revisions of the standards rather than 

stating the specific level, and (2) 

codifying site combinations to maintain 

a continuous data record. The purpose 

of both modifications is to codify 

existing practices that are currently 

being implemented as the EPA standard 

operating procedures. 

The EPA received few comments on 

these revisions in the proposed rule, 

with most supportive of the appendix N 

revisions. 

Although the EPA did not propose or 

request comment on this issue, one 

commenter suggested that appendix N 

be revised to only allow data from the 

primary monitor to be used in PM2.5

NAAQS designations asserting that it 

would add flexibility. The EPA 

disagrees with the commenter’s 

assertion that this would add flexibility 

because it could force agencies to run 


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180Mention of commercial names does not 

constitute EPA endorsement. 

their FRMs on a daily schedule or 

potentially lead to invalid design values 

if manual sampling interruptions or 

laboratory issues impact FRM data 

completeness. This change would also 

be undesirable because it could reduce 

by two-thirds the number of days used 

in calculations for the annual and 24- 

hour PM2.5 design values at many sites. 

Therefore, the EPA is finalizing these 

appendix N revisions as proposed. 

VII. Amendments to Ambient 

Monitoring and Quality Assurance 

Requirements 

The EPA is finalizing revisions to 

ambient air monitoring requirements for 

PM to improve the usefulness of and 

appropriateness of data used in 

regulatory decision making. These 

changes focus on ambient monitoring 

requirements found in 40 CFR parts 50 

(appendix L), 53, and 58 with associated 

appendices (A, B, C, D, and E). These 

changes include addressing updates in 

the approval of reference and equivalent 

methods, updates in quality assurance 

statistical calculations to account for 

lower concentration measurements, 

updates to support improvements in PM 

methods, a revision to the PM2.5

network design to account for at-risk 

populations, and updates to the Probe 

and Monitoring Path Siting Criteria for 

NAAQS pollutants. The EPA also took 

comment on how to incorporate data 

from next generation technologies into 

Agency efforts. A summary of the 

comments received is included in this 

section. 

A. Amendment to 40 CFR Part 50 

(Appendix L): Reference Method for the 

Determination of Fine Particulate Matter 

as PM2.5 in the Atmosphere—Addition 

of the Tisch Cyclone as an Approved 

Second Stage Separator 

The EPA proposed a change to the 

FRM for PM2.5 (40 CFR part 50, 

appendix L), the addition of an 

alternative PM2.5 particle size separator 

to that of the Well Impactor Ninety-Six 

(WINS) and the Very Shape Cut Cyclone 

(VSCC) size separators (88 FR 5663, 

January 27, 2023). The new separator is 

the TE–PM2.5C cyclone manufactured by 

Tisch Environmental Inc.,180 Cleves 

Ohio, which has been shown to have 

performance equivalent to that of the 

originally specified WINS impactor with 

regards to aerodynamic cutpoint and 

PM2.5 concentration measurement. In 

addition, the new TE–PM2.5C has a 

significantly longer service interval than 

the WINS and is comparable to that of 

the VSCC separator. Generally, the TE– 

PM2.5C is also physically 

interchangeable with the WINS and 

VSCC where both are manufactured for 

the same sampler. The proposed change 

would allow either the WINS, VSCC, or 

TE–PM2.5C to be used in a PM2.5 FRM 

sampler. As is the case for the WINS 

and VSCC, the TE–2.5C is now also an 

approved size separator for candidate 

PM2.5 FEMs. Currently, the EPA has 

designated one PM2.5 sampler 

configured with TE–PM2.5C separator as 

a Class II PM2.5 equivalent method and 

one as a PM10-2.5 equivalent method. 

Upon promulgation of this change to 

appendix L, these instruments would be 

redesignated as PM2.5 and PM10-2.5

FRMs, respectively. Owners of such 

samplers should contact the sampler 

manufacturer to receive a new reference 

method label for the samplers. 

The EPA received only one comment 

regarding this proposed change, which 

was supportive. Therefore, the EPA is 

finalizing this change to Appendix L as 

proposed. 

B. Issues Related to 40 CFR Part 53 

(Reference and Equivalent Methods) 

The EPA proposed to clarify the 

regulations associated with FRM and 

FEM applications for review by the EPA 

(88 FR 5664, January 27, 2023). 

Revisions were also proposed in 

instances where current regulatory 

specifications are no longer pertinent 

and require updating. In addition, the 

EPA proposed to correct a compiled a 

list of noted minor errors in the 

regulations associated with the testing 

requirements and acceptance criteria for 

FRMs and FEMs in part 53. These errors 

are typically not associated with the 

content of Federal Register documents 

but often relate to transcription errors 

and typographical errors in the 

electronic CFR (eCFR) and printed 

versions of the CFR. 

1. Update to Program Title and Delivery 

Address for FRM and FEM Applications 

The EPA proposed a change to 40 CFR 

53.4(a) to update the delivery address 

for FRM and FEM Applications and 

Modification Requests, as well as 

update the name of the program 

responsible for their review (88 FR 

5664, January 27, 2023). These revisions 

are due solely to organizational changes 

and do not affect the structure or role of 

the Reference and Equivalent Methods 

Designation Program in reviewing new 

FRM and FEM application requests and 

requests to modify existing designated 

instruments. The EPA received no 

comments on this revision and, 

therefore, the EPA is finalizing this 

revision as proposed. 

2. Requests for Delivery of a Candidate 

FRM or FEM Instrument 

The EPA proposed a change to 40 CFR 

53.4(d), which currently allows the EPA 

to request only candidate PM2.5 FRMs 

and Class II or Class III equivalent 

methods for testing purposes as part of 

the applicant review process (88 FR 

5664, January 27, 2023). The EPA 

proposed to revise this section to enable 

requesting any candidate FRM, FEM, or 

a designated FRM or FEM associated 

with a Modification Request, regardless 

of NAAQS pollutant type or metric. The 

EPA received no comments on these 

revisions; therefore, the EPA is 

finalizing this revision as proposed. 

3. Amendments to Requirements for 

Submission of Materials in 40 CFR 

53.4(b)(7) for Language and Format 

The EPA proposed a change to 40 CFR 

53.4(b)(7) to specify that all written 

FRM and FEM application materials 

must be submitted to the EPA in English 

in MS Word format and that submitted 

data must be submitted in MS Excel 

format (88 FR 5664, January 27, 2023). 

The EPA received no comments on 

these revisions; therefore, the EPA is 

finalizing this section as proposed. 

4. Amendment to Designation of 

Reference and Equivalent Methods 

The EPA proposed a change to 40 CFR 

53.8(a) to clarify the terms of new FRM 

and FEM methods to ensure that 

candidate samplers and analyzers are 

not publicly announced, marketed, or 

sold until the EPA’s approval has been 

formally announced in the Federal 

Register (88 FR 5664, January 27, 2023). 

The EPA received no comments on 

these revisions; therefore, the EPA is 

finalizing this section as proposed. 

5. Amendment to One Test Field 

Campaign Requirement for Class III 

PM2.5 FEMs 

The EPA proposed a change to 40 CFR 

53.35(b)(1)(ii)(D) that involves field 

comparability tests for candidate Class 

III PM2.5 FEMs, including the 

requirement that a total of five field 

campaigns must be conducted at four 

separate sites, A, B, C, and D (88 FR 

5664, January 27, 2023). The existing 

Site D specifications require that the site 

‘‘shall be in a large city east of the 

Mississippi River, having 

characteristically high sulfate 

concentrations and high humidity 

levels.’’ However, dramatic decreases in 

ambient sulfate concentration make it 

difficult for applicants to routinely meet 

the high sulfate concentration 

requirement. Therefore, the EPA 

proposed to revise the Site D 

specifications to read ‘‘shall be in a large 


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Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16345 

181Noah, G. (2023). Task 16 on PEP/NPAP Task 

Order: Bias and Precision DQOs for the PM2.5

Ambient Air Monitoring Network. Memorandum to 

the Rulemaking Docket for the Review of the 

National Ambient Air Quality Standards for 

Continued 

city east of the Mississippi River, having 

characteristically high humidity levels.’’ 

Only one comment was received on this 

proposed revision, which was 

supportive. Therefore, the EPA is 

finalizing the revision to 40 CFR 

53.35(b)(1)(ii)(D), as proposed. 

6. Amendment to Use of Monodisperse 

Aerosol Generator 

The EPA proposed a change to 40 CFR 

53.61(g), 53.62(e), and Table F–1 that 

involves the wind tunnel evaluation of 

candidate PM10 inlets and candidate 

PM2.5 fractionators under static 

conditions, which requires the 

generation and use of monodisperse 

calibration aerosols of specified 

aerodynamic sizes (88 FR 5664, January 

27, 2023). In the current regulations, the 

TSI Incorporated Vibrating Orifice 

Aerosol Generator (VOAG) is the only 

monodisperse generator that is 

approved for this purpose. However, 

TSI Incorporated no longer 

manufacturers nor supports the VOAG. 

Therefore, a commercially available 

monodisperse aerosol generator (Model 

1520 Fluidized Monodisperse Aerosol 

Generator, MSP Corporation, 

Shoreview, MN) has been added to list 

of approved generators for this purpose. 

No comments were received on this 

revision; therefore, the EPA is finalizing 

this revision as proposed. 

7. Corrections to 40 CFR Part 53 

(Reference and Equivalent Methods) 

Certain provisions of 40 CFR 53.14, 

Modification of a reference or 

equivalent method, incorrectly state an 

EPA response deadline of 30 days for 

receipt of modification materials in 

response to an EPA notice. Per a 2015 

amendment (80 FR 65460, 65416, Oct. 

26, 2015), all EPA response deadlines 

for modifications of reference or 

equivalent methods are 90 days from 

day of receipt. Thus, the EPA proposed 

a correction to specify the correct 90- 

day deadline (88 FR 5664, January 27, 

2023). 

Requirements for Reference and 

Equivalent Methods for Air Monitoring 

of Criteria Pollutants identifies the 

applicable 40 CFR part 50 appendices 

and 40 CFR part 53 subparts for each 

criteria pollutant. The four rows in the 

section for PM10–2.5 erroneously do not 

include the footnote instruction that the 

aforementioned pollutant alternative 

Class III requirements may be 

substituted in regard to Appendix O to 

Part 50—Reference Method for the 

Determination of Coarse Particulate 

Matter as PM10–2.5 in the Atmosphere. 

Table B–1 specifies that the 

interference equivalent for each 

interferent is ±0.005 ppm for both the 

standard-range and lower-range limits, 

with the exception of nitric oxide (NO) 

for the lower-range limit per note 4. 

When testing the lower range of SO2, the 

limit for NO is ±0.003 ppm, therefore, 

an incorrect lower limit (±0.0003) is 

currently stated in note 4 for this 

exception to the SO2 lower range limit. 

Thus, the EPA proposed a correction to 

Table B–1 to specify the correct limit in 

note 4 (88 FR 5664, January 27, 2023). 

After the EPA received an inquiry 

regarding the interaction of NO and O3, 

the EPA investigated the interferent 

testing requirements stated by 40 CFR 

part 53, subpart B. The EPA has 

determined that during the 2011 SO2

amendment and subsequent 2015 O3

amendment, several typographical 

errors were introduced into Table B–3, 

the most significant of which is the 

omission of note 3, which instructs the 

applicant to not mix the pollutant with 

the interferent. Thus, the EPA proposed 

revisions to Table B–3 to correct these 

errors (88 FR 5664, January 27, 2023). 

Additionally, appendix A to subpart B 

of part 53 provides figures depicting 

optional forms for reporting test results. 

Figure B–3 lists an incorrect formula: 

the lower detectible limit section is 

missing the proper operator in the LDL 

calculation formula and Figure B–5 lists 

an incorrect calculation metric, and 

there is a typesetting error in the 

calculation of the standard deviation. 

The EPA proposed to correct the 

typesetting errors and noted other errors 

to be corrected in several formulas 

provided throughout § 53.43 (88 FR 

5664, January 27, 2023). 

The EPA proposed a revision to 40 

CFR 53.43(a)(2)(xvi), 53.43(b)(2)(iv), and 

53.43(b)(2)(iv) to correct typographical 

errors in equations. 

The EPA proposed a revision to Table 

C–4 of part 53 Subpart C (88 FR 5700). 

This change is related to field 

comparability tests of candidate PM2.5, 

PM10–2.5, and PM10 FEMs, which 

requires testing at wide range of ambient 

concentrations. For this reason, Table 

C–4 specifies a minimum number of 

valid sample sets to be conducted at 

specified high concentrations. However, 

due to the dramatic decrease in ambient 

PM concentrations in the past two 

decades, these number of valid test days 

at high concentrations has been difficult 

to achieve. Accordingly, the EPA 

proposed to revise the testing 

specifications for high concentration 

events in Table C–4 to reflect current 

levels of ambient PM for all three PM 

metrics. In addition to the revision of 

the ambient PM concentration 

specifications to Table C–4, there are 

also several entry errors that required 

correction. 

The EPA received no comments on 

these proposed revisions; therefore, the 

EPA is finalizing the changes as 

proposed. 

C. Changes to 40 CFR Part 58 (Ambient 

Air Quality Surveillance) 

1. Quality Assurance Requirements for 

Monitors Used in Evaluations for 

National Ambient Air Quality Standards 

In the proposal, the EPA described 

how we evaluated the quality system as 

part of the PM NAAQS reconsideration 

(88 FR 5665, January 27, 2023). In this 

section, the EPA identified several areas 

for improvement in steadily declining 

average ambient PM2.5 concentrations 

across the country and the final decision 

to revise primary annual PM2.5 NAAQS 

described in section II above. We 

assessed PM2.5 concentration data across 

a range of values to determine if any 

changes to the statistical calculations 

used to evaluate the data quality in the 

PM2.5 network were warranted. This 

section describes the EPA’s assessment, 

comments received, and the EPA’s final 

decisions on the proposed changes. 

Other changes in this section include 

clarifications and other improvements 

that will facilitate consistency and the 

operation of quality assurance programs 

by State, local, and Tribal (SLT) 

agencies nationwide. 

a. Quality System Requirements 

The EPA reconsidered the appendix 

A, section 2.3.1.1 goal for acceptable 

measurement uncertainty (88 FR 5665, 

January 27, 2023) for automated and 

manual PM2.5 methods for total bias. 

The existing total bias goal is an upper 

90 percent confidence limit for the 

coefficient of variation (CV) of 10 

percent and ±10 percent for total bias. 

The intent of the proposal was to 

investigate if this bias goal is still 

realistic given updated precision and 

bias statistic. The EPA received one 

comment that bias reevaluation may be 

premature, since the final NAAQS 

standard had not yet been determined at 

the time of the proposal. The EPA 

acknowledges this comment but 

clarifies that the proposed new bias 

statistic was evaluated at a range of 

levels including the range of proposed 

PM2.5 standards in the technical 

memorandum, ‘‘Task 16 on PEP/NPAP 

Task Order: Bias and Precision DQOs 

for the PM2.5 Ambient Air Monitoring 

Network.’’ 181 Considering the 


ddrumheller on DSK120RN23PROD with RULES3
16346 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

Particulate Matter (EPA–HQ–OAR–2015–0072). 

Available at: https://www.regulations.gov/docket/ 

EPA-HQ-OAR-2015-0072. 

182See the EPA’s PM2.5 Data Quality Dashboard 

available at https://sti-r-shiny.shinyapps.io/QVA_

Dashboard/. 

183Noah, G. (2023). Task 16 on PEP/NPAP Task 

Order: Bias and Precision DQOs for the PM2.5

Ambient Air Monitoring Network. Memorandum to 

the Rulemaking Docket for the Review of the 

National Ambient Air Quality Standards for 

Particulate Matter (EPA–HQ–OAR–2015–0072). 

Available at: https://www.regulations.gov/docket/ 

EPA-HQ-OAR-2015-0072. 

justification in the technical 

memorandum and the lack of adverse 

comments regarding this part of the 

proposal, the EPA is retaining the 

appendix A, section 2.3.1.1, goal for 

acceptable measurement uncertainty for 

automated and manual PM2.5 methods 

for total bias. 

The EPA also proposed to update and 

clarify ambient air monitoring 

requirements found in 40 CFR part 58, 

appendix A, section 2.6.1 pertaining to 

EPA Protocol Gas standards used for 

ambient air monitoring and the Ambient 

Air Protocol Gas Verification Program 

(PGVP) (88 FR 5665, January 27, 2023). 

The EPA proposed to revise appendix A 

to clarify that in order to participate in 

the Ambient Air PGVP, producers of 

Protocol Gases must adhere to the 

requirements of 40 CFR 75.21(g), and 

only regulatory ambient air monitoring 

programs may submit cylinders for 

assay verification to the EPA Ambient 

Air PGVP. The EPA received mixed 

comments in support of and in 

opposition to this proposed revision. 

The sole commenter opposing the 

proposed revision indicated that the 

proposed PGVP requirements would be 

additional and is concerned with an 

increased resource burden. But the EPA 

responds that the PGVP requirements 

that were proposed to be added are 

consistent with the existing PGVP 

requirements in 40 CFR 75.21(g), and 

PGVP has been defined as a regulatory 

requirement since 2016 (81 FR 17263, 

March 28, 2016), so the proposed part 

58 changes are not ‘‘additional’’ to 

existing regulations. After consideration 

of the comments, the EPA is finalizing 

the update and clarification of ambient 

air monitoring requirements found in 

appendix A, section 2.6.1 pertaining to 

EPA Protocol Gas standards used for 

ambient air monitoring and the Ambient 

Air PGVP as proposed. 

b. Measurement Quality Check 

Requirements 

The EPA proposed to remove section 

3.1.2.2 from appendix A, which allows 

NO2 compressed gas standards to be 

used to generate audit standards (88 FR 

5665, January 27, 2023). The EPA 

received one comment supporting this 

change. As a result of the comment 

received and other general supportive 

comments regarding quality assurance, 

the EPA is finalizing the removal of 

section 3.1.2.2 from appendix A as 

proposed. 

The EPA proposed to revise the 

requirement in Appendix A, section 

3.1.3.3 changing the National 

Performance Audit Program (NPAP) 

requirement for annual verification of 

gaseous standards to the ORDrecommended certification periods 

identified in Table 2–3 of the EPA 

Traceability Protocol for Assay and 

Certification of Gaseous Calibration 

Standards (appendix A, section 6.0(4)) 

(88 FR 5665). The EPA received one 

comment supporting this change. As a 

result of the comment received and 

other general supportive comments 

regarding quality assurance, the EPA is 

finalizing the updated NPAP gaseous 

certification requirement in section 

3.1.3.3 as proposed. 

The EPA proposed to adjust the 

minimum value required by appendix 

A, section 3.2.4, to be considered valid 

sample pairs for the PM2.5 Performance 

Evaluation Program (PEP) from 3 mg/m3

to 2 mg/m3 (88 FR 5665, January 27, 

2023). The EPA received comments in 

support and against the change. In the 

only opposing comment, the commenter 

expressed concern that the method 

detection limit (MDL) for PM2.5 is 2 mg/ 

m3. The commenter also indicated that 

the MDL ‘‘typically has minimal value 

per the definition of the MDL.’’ 40 CFR 

part 50, appendix L states, ‘‘The lower 

detection limit of the mass 

concentration measurement range is 

estimated to be approximately 2 mg/m3, 

based on noted mass changes in field 

blanks in conjunction with the 24 m3

nominal total air sample volume 

specified for the 24-hour sample.’’ The 

EPA notes that field blanks currently 

average less than 10 mg nationally, and 

when divided by the 24 m3 nominal 

total air sample volume specified for a 

24-hour sample, the result is 0.4 mg/m3. 

The appendix L MDL referenced by the 

commenter was part of the 1997 PM 

NAAQS rulemaking (62 FR 38652, July 

18, 1997); current data shows that the 

MDL is substantially lower than the 

EPA’s original estimate. After review of 

the comments, and in consideration of 

the recently calculated detection limit 

for the PM2.5 FRM that is substantially 

lower than our original estimate,182 the 

EPA is finalizing the revised minimum 

value for valid sample pairs for the 

PM2.5 Performance Evaluation Program 

(PEP) from 3 mg/m3 to 2 mg/m3 in 

appendix A, section 3.2.4 as proposed. 

c. Calculations for Data Quality 

Assessments 

The EPA proposed to change 

Equations 6 and 7 of appendix A, 

section 4.2.1 that are used to calculate 

the Collocated Quality Control Sampler 

Precision Estimate for PM10, PM2.5 and 

Pb (88 FR 5666, January 27, 2023). The 

proposed new statistics are designed to 

address the high imprecision values that 

result from using these calculations to 

compare low concentrations that are 

now more routinely observed in the 

networks. The EPA received several 

comments in support of this change in 

general, but some commenters indicated 

that they believed there was an error in 

the new calculation that may result in 

high imprecision from the calculation of 

the equation. The EPA reviewed the 

technical memorandum and confirmed 

that a multiplier of 100 was 

unintentionally left in the proposed 

relative difference equation, Equation 6. 

Also, equation 6 was corrected from a 

normalized percent difference to a 

normalized relative percent difference 

that is appropriate for comparing 

collocated pairs at low concentrations. 

The technical memorandum titled 

‘‘Task 16 on PEP/NPAP Task Order: 

Bias and Precision DQOs for the PM2.5

Ambient Air Monitoring Network’’ has 

been amended to correct the error and 

is included in the docket for this 

action.183

Equation 6 as proposed at 88 FR 5666 

(January 27, 2023) was: 


ddrumheller on DSK120RN23PROD with RULES3
Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16347 

184Noah, G. (2023). Task 16 on PEP/NPAP Task 

Order: Bias and Precision DQOs for the PM2.5

Ambient Air Monitoring Network. Memorandum to 

the Rulemaking Docket for the Review of the 

National Ambient Air Quality Standards for 

Particulate Matter (EPA–HQ–OAR–2015–0072). 

Available at: https://www.regulations.gov/docket/ 

EPA-HQ-OAR-2015-0072. 

As a result of the positive comments 

received and the correction to the 

equation made in response to some 

comments, the EPA is finalizing the 

updated Equation 6 as described and is 

finalizing Equation 7 as proposed for the 

calculation of the Collocated Quality 

Control Sampler Precision Estimate for 

PM10, PM2.5, and Pb in section 4.2.1. 

The EPA proposed to update the 

appendix A, section 4.2.5, Equation 8, 

calculation for the Performance 

Evaluation Program Bias Estimate for 

PM2.5 (88 FR 5666–67, January 27, 

2023). Because average ambient PM 

concentrations across the nation have 

steadily declined since the 

promulgation of the PM2.5 standard, the 

EPA proposed to replace the current 

percent difference equation with a 

relative difference equation. The EPA 

received several comments in support of 

this change in general, but some 

commenters identified a potential error 

in the new calculation that resulted in 

an artificially high estimate, which they 

do not support. The EPA reviewed the 

technical memorandum and discovered 

that a multiplier of 100 was left in the 

new relative difference equation used in 

the bias equation. The technical 

memorandum, ‘‘Task 16 on PEP/NPAP 

Task Order: Bias and Precision DQOs 

for the PM2.5 Ambient Air Monitoring 

Network’’ has been amended to correct 

the error and is included in the 

docket.184 The proposed Equation 8 

proposed at 88 FR 5667 (January 27, 

2023) was: 

As a result of the supportive 

comments received and the correction 

to the equation in response to some 

comments, the EPA is updating and 

finalizing Equation 8 as described for 

the calculation for the Performance 

Evaluation Program Bias Estimate for 

PM2.5, in section 4.2.5. 

d. References 

The EPA proposed to update the 

references and hyperlinks in appendix 

A, section 6 (88 FR 5667, January 27, 

2023) to provide accuracy in identifying 

and locating essential supporting 

documentation and delete references to 

historical documents that do not 

represent current practices. The EPA 

received only favorable comments, and 

as a result, the EPA is finalizing the 

updated the references and hyperlinks 

in appendix A, section 6, as proposed. 

The EPA also proposed to add a 

footnote to Table A–1 of part 58, 

appendix A—Minimum Data 

Assessment Requirements for NAAQS 

Related Criteria Pollutant Monitors (88 

FR 5669, January 27, 2023). The 

proposed footnote clarifies the 

allowable time (i.e., every two weeks, 

once a month, once a quarter, once 

every six months, or distributed over all 

four quarters depending on the check) 

between checks and encourages 

monitoring organizations to perform 

data assessments at regular intervals. 

The EPA received two comments 

regarding this proposed footnote. One 

commenter indicated that this change is 

inconsistent with the QA Handbook for 

Air Pollution Measurement Systems: 

‘‘Volume II: Ambient Air Quality 

Monitoring Program QA Handbook.’’ 

The EPA agrees with the commenter; 

because the QA Handbook is guidance, 


ddrumheller on DSK120RN23PROD with RULES3ER06MR24.031</GPH> ER06MR24.032</GPH>

And the corrected Equation 6 is: 

meas- audit 

Si = -----;:==- X 100 

.Jaudit 

X--Y. t- = l l 

' J(Xi - Yi)/2 

Equation 7 is below and is unchanged. 

CV90NAAQS = 100 * kx_Ef=1 tf-O:f:.1 tJ2 X 

2k(k-1) 

k-1 

NAAQS Concentration*x:.1,k_1 

~!1 s- meas- audit 

100 * "'i=l I were h St= ----- -~ x 100 

n.JNAAQS concentration vaudit 

and the corrected Equation 8 is: 

Ln s meas - audit 100 x i=1 t where St = ----- n.JNAAQS concentration '1audit 
16348 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

185 In the proposal, in section VII.C.2 Quality 

Assurance Requirements for Prevention of 

Significant Deterioration (PSD) Air Monitoring (88 

FR 5667–69), the EPA inadvertently referred to 

‘‘appendix A’’ in the section rather than the correct 

‘‘appendix B.’’ The EPA’s intent to have proposed 

changes to appendix B on these pages is made clear 

by the section header, the Table of Contents on page 

5559, and the proposed regulatory text for appendix 

B on pages 5707–08. See, e.g., id. at p.5668 

(preamble erroneously states that the EPA proposed 

to change appendix A, section 2.6.1); id. at p.5668 

(preamble erroneously states that the EPA proposed 

to adjust the minimum value required by appendix 

A, section 3.2.4). 

186Noah, G. (2023). Task 16 on PEP/NPAP Task 

Order: Bias and Precision DQOs for the PM2.5

Ambient Air Monitoring Network. Memorandum to 

the Rulemaking Docket for the Review of the 

National Ambient Air Quality Standards for 

Particulate Matter (EPA–HQ–OAR–2015–0072). 

Available at: https://www.regulations.gov/docket/ 

EPA-HQ-OAR-2015-0072. 

the EPA will revise it after this action 

is finalized to be consistent with the 

updated CFR provision. Another 

commenter does not support the 

addition of the footnote due to concerns 

about limiting flexibility. In response, 

the EPA reiterates that the proposed 

revision is intended to clarify intent and 

does not make any changes to the 

required frequencies or acceptance 

criteria for data assessment. A ‘‘weight 

of evidence’’ narrative is still found in 

40 CFR part 58, appendix A, section 

1.2.3. As a result of the comments 

received and the rationale discussed 

above, the EPA is finalizing the addition 

of the new footnote to Table A–1 of part 

58, appendix A—Minimum Data 

Assessment Requirements for NAAQS 

Related Criteria Pollutant Monitors as 

proposed. 

2. Quality Assurance Requirements for 

Prevention of Significant Deterioration 

(PSD) Air Monitoring 

The EPA proposed to revise appendix 

B, Quality Assurance Requirements for 

Prevention of Significant Deterioration 

(PSD) Air Monitoring (88 FR 5667, 

January 27, 2023), in parallel to the 

proposal to revise appendix A. Thus, 

this section of the proposal included 

similar detail and proposed revisions 

related to evaluating quality system 

statistical calculations for PM2.5, 

clarifications and other improvements 

that would facilitate consistency and the 

operation of quality assurance programs 

for PSD by SLT agencies nationwide. 

a. Quality System Requirements 

The EPA reconsidered the goal in 

appendix B, section 2.3.1.1 for 

acceptable measurement uncertainty for 

automated and manual PM2.5 methods 

for total bias (88 FR 5668, January 27, 

2023).185 The current total bias goal is 

an upper 90 percent confidence limit for 

the coefficient of variation (CV) of 10 

percent and ±10 percent for total bias. 

The EPA’s intent was to investigate if 

this goal is still realistic given updated 

precision and bias statistics. The EPA 

received one comment that bias 

reevaluation may be premature, since 

the final NAAQS standard had not yet 

been determined at the time of the 

proposal. The EPA acknowledges this 

comment but clarifies that the proposed 

new bias statistic was evaluated at a 

range of levels including the proposed 

range of PM2.5 standards in the technical 

memorandum, ‘‘Task 16 on PEP/NPAP 

Task Order: Bias and Precision DQOs 

for the PM2.5 Ambient Air Monitoring 

Network.’’ 186 Considering the 

justification in the technical 

memorandum and the lack of adverse 

comments regarding the substantive 

proposal, the EPA is retaining the 

appendix B, section 2.3.1.1, goal for 

acceptable measurement uncertainty for 

automated and manual PM2.5 methods 

for total bias. 

The EPA also proposed to update and 

clarify ambient air monitoring 

requirements found in 40 CFR part 58, 

appendix B, section 2.6.1 pertaining to 

EPA Protocol Gas standards used for 

ambient air monitoring and the Ambient 

Air PGVP (88 FR 5668, January 27, 

2023). The EPA proposed to revise 

appendix B to clarify that in order to 

participate in the Ambient Air PGVP, 

producers of Protocol Gases must 

adhere to the requirements of 40 CFR 

75.21(g), and only regulatory ambient 

air monitoring programs may submit 

cylinders for assay verification to the 

EPA Ambient Air PGVP. The EPA 

received comments in support of and in 

opposition to this proposed revision. 

The commenter opposing the revision 

indicated that the proposed PGVP 

requirements would be additional and is 

concerned with an increased resource 

burden. However, the EPA disagrees 

with the commenter because that the 

proposed PGVP requirements are 

consistent with the existing PGVP 

requirements in 40 CFR 75.21(g). PGVP 

has been defined as a regulatory 

requirement since 2016 (81 FR 17263, 

March 28, 2016), so the proposed part 

58 changes are not ‘‘additional’’ to 

existing regulations. After consideration 

of the comments, the EPA is finalizing 

the update and clarification of ambient 

air monitoring requirements found in 

appendix B, section 2.6.1 pertaining to 

EPA Protocol Gas standards used for 

ambient air monitoring and the Ambient 

Air PGVP as proposed. 

b. Measurement Quality Check 

Requirements 

The EPA proposed to remove section 

3.1.2.2 from appendix B, which allows 

NO2 compressed gas standards to be 

used to generate audit standards (88 FR 

5668, January 27, 2023). The EPA 

received one comment supporting this 

change. As a result of the comment 

received and other general supportive 

comments regarding quality assurance, 

the EPA is finalizing the removal of 

section 3.1.2.2 from appendix B as 

proposed. 

The EPA proposed to revise the 

requirement in Appendix B, section 

3.1.3.3 changing the National 

Performance Audit Program (NPAP) 

requirement for annual verification of 

gaseous standards to the ORDrecommended certification periods 

identified in Table 2–3 of the EPA 

Traceability Protocol for Assay and 

Certification of Gaseous Calibration 

Standards (appendix B, section 6.0(4)) 

(88 FR 5668, January 27, 2023). The EPA 

received one comment supporting this 

change. As a result of the comment 

received and other general supportive 

comments regarding quality assurance, 

the EPA is finalizing the updated NPAP 

gaseous certification requirement in 

section 3.1.3.3 as proposed. 

The EPA proposed to adjust the 

minimum value required by appendix 

B, section 3.2.4, to be considered valid 

sample pairs for the PM2.5 Performance 

Evaluation Program (PEP) from 3 mg/m3

to 2 mg/m3 (88 FR 5668, January 27, 

2023). The EPA received comments in 

support and against the change. In the 

only opposing comment, the commenter 

expressed concern that the method 

detection limit (MDL) for PM2.5 is 2 mg/ 

m3. The commenter also indicated that 

the MDL ‘‘typically has minimal value 

per the definition of the MDL.’’ 40 CFR 

part 50, appendix L states, ‘‘The lower 

detection limit of the mass 

concentration measurement range is 

estimated to be approximately 2 mg/m3, 

based on noted mass changes in field 

blanks in conjunction with the 24 m3

nominal total air sample volume 

specified for the 24-hour sample’’. The 

EPA notes that field blanks currently 

average less than 10 mg nationally, and 

when divided by the 24 m3 nominal 

total air sample volume specified for a 

24-hour sample, the result is 0.4 mg/m3. 

The appendix L MDL referenced by the 

commenter was part of the 1997 PM 

NAAQS rulemaking more than 20 years 

ago (62 FR 38652, July 18, 1997); current 

data shows that the MDL is substantially 

lower than EPA’s original estimate. 

After review of the comments, and in 

consideration of the recently calculated 


ddrumheller on DSK120RN23PROD with RULES3
Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16349 

187Noah, G. (2023). Task 16 on PEP/NPAP Task 

Order: Bias and Precision DQOs for the PM2.5

Ambient Air Monitoring Network. Memorandum to 

the Rulemaking Docket for the Review of the 

National Ambient Air Quality Standards for 

Particulate Matter (EPA–HQ–OAR–2015–0072). 

Available at: https://www.regulations.gov/docket/ 

EPA-HQ-OAR-2015-0072. 

detection limit for the PM2.5 FRM that 

is substantially lower than our original 

estimate, the EPA is revising the 

minimum value for valid sample pairs 

for the PM2.5 Performance Evaluation 

Program (PEP) from 3 mg/m3 to 2 mg/m3

in appendix B, section 3.2.4 as 

proposed. 

c. Calculations for Data Quality 

Assessments 

The EPA proposed to change 

Equations 6 and 7 of appendix B, 

section 4.2.1 used for calculating the 

Collocated Quality Control Sampler 

Precision Estimate for PM10, PM2.5 and 

Pb (88 FR 5707, January 27, 2023). 

These new statistics are designed to 

address the high imprecision values that 

result from using these calculations to 

compare low concentrations that are 

now more routinely observed in the 

networks. The EPA received several 

comments in support of this change in 

general, but a couple commenters 

indicated that there could be an error in 

the new calculation that resulted in high 

imprecision from the calculation of the 

equation. The EPA reviewed the 

technical memorandum and discovered 

that a multiplier of 100 was 

unintentionally left in the proposed 

relative difference equation, Equation 6. 

Also, equation 6 was corrected from a 

normalized percent difference to a 

normalized relative percent difference 

that is appropriate for comparing 

collocated pairs at low concentrations. 

The technical memorandum titled 

‘‘Task 16 on PEP/NPAP Task Order: 

Bias and Precision DQOs for the PM2.5

Ambient Air Monitoring Network’’ was 

amended to correct the error and is 

included in the docket.187

As a result of the positive comments 

received and the correction to the 

equation made in response to those 

comments, the EPA is finalizing the 

update to Equation 6 and retaining 

Equation 7 as proposed for the 

calculation of the Collocated Quality 

Control Sampler Precision Estimate for 

PM10, PM2.5 and Pb in section 4.2.1. 

The EPA proposed to update the 

appendix B, section 4.2.5, Equation 8, 

calculation for the Performance 

Evaluation Program Bias Estimate for 

PM2.5 (88 FR 5668–59, January 27, 

2023). Because average ambient PM 

concentrations across the nation have 

steadily declined since the 

promulgation of the PM2.5 standard, the 

EPA proposed to replace the current 

percent difference equation with a 

relative difference equation. The EPA 

received several comments in support of 

this change in general, but some 

commenters identified a potential error 

in the new calculation that resulted in 

an artificially high estimate, which they 

do not support. The EPA reviewed the 

technical memorandum and discovered 

that a multiplier of 100 was left in the 

new relative difference equation used in 

the bias equation. The technical 

memorandum, ‘‘Task 16 on PEP/NPAP 

Task Order: Bias and Precision DQOs 

for the PM2.5 Ambient Air Monitoring 

Network’’ has been amended to correct 

the error and is included in the docket. 

The proposed Equation 8 (88 FR 5669, 

January 27, 2023) was: 


ddrumheller on DSK120RN23PROD with RULES3ER06MR24.033</GPH>

Equation 6 in the proposal (88 FR 5668, January 27, 2023) was: 

And the corrected Equation 6 is: 

meas-audit 

Si = -----;::::==-- X 100 

audit 

Equation 7 is below and is unchanged. 

k-1 

CV90NAAQS = 100 * kxl;f=1 tr-O:f=1 ti)2 X 

2k(k-1) NAAQS Concentration•xi.1,1c_1 
16350 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

As a result of the supportive 

comments received and the correction 

to the equation in response to some 

comments, the EPA is updating and 

finalizing Equation 8 as described for 

the calculation for the Performance 

Evaluation Program Bias Estimate for 

PM2.5, in section 4.2.5. 

d. References 

The EPA proposed to update the 

references and hyperlinks in appendix 

B, section 6 (88 FR 5669, January 27, 

2023) to provide accuracy in identifying 

and locating essential supporting 

documentation and delete references to 

historical documents that do not 

represent current practices. The EPA 

received only favorable comments, and 

as a result, the EPA is finalizing the 

updated the references and hyperlinks 

in appendix B, section 6, as proposed. 

The EPA also proposed to add a 

footnote to Table B–1 of part 58, 

appendix B—Minimum Data 

Assessment Requirements for NAAQS 

Related Criteria Pollutant PSD Monitors 

(88 FR 5669, January 27, 2023). The 

proposed footnote clarifies the 

allowable time (i.e., every two weeks, 

once a month, once a quarter, once 

every six months, or distributed over all 

four quarters depending on the check) 

between checks and encourages 

monitoring organizations to perform 

data assessments at regular intervals. 

The EPA received two comments 

regarding this proposal. One commenter 

indicated that this change is 

inconsistent with the QA Handbook. 

The EPA agrees with the commenter; 

because the QA Handbook is guidance, 

the EPA will revise it after this action 

is finalized to be consistent with the 

updated CFR provision. Another 

commenter does not support the 

addition of the footnote due to concerns 

about limiting flexibility. In response, 

the EPA reiterates that the proposed 

revision is intended to clarify intent and 

does not make any changes to the 

required frequencies or acceptance 

criteria for data assessment. A ‘‘weight 

of evidence’’ narrative is still found in 

40 CFR part 58, appendix B, section 

1.2.3. As a result of the comments 

received and the rationale discussed 

above, the EPA is adding the new 

footnote to Table B–1 of part 58, 

appendix B—Minimum Data 

Assessment Requirements for NAAQS 

Related Criteria Pollutant PSD Monitors 

as proposed. 

3. Amendments to PM Ambient Air 

Quality Methodology 

a. Revoking Approved Regional 

Methods (ARMs) 

The EPA proposed to remove 

provisions for approval and use of 

Approved Regional Methods (ARMs) 

throughout parts 50 and 58 of the CFR 

(88 FR 5669, January 27, 2023). ARMs 

are continuous PM2.5 methods that have 

been approved specifically within a 

State or local air agency monitoring 

network for purposes of comparison to 

the NAAQS and to meet other 

monitoring objectives. Currently, there 

are no approved ARMs. There are, 

however, more than a dozen approved 

Federal Equivalent Methods (FEMs) for 

PM2.5. These approved FEMs are eligible 

for comparison to the NAAQS and to 

meet other monitoring objectives. 

The EPA received comments from 

multiple State air programs in support 

of the proposal to remove provisions for 

approval and use of ARMs. One 

commenter cites that there are multiple 

FEMs available for monitoring agencies 

to work with and that the agency was 

never able to get a candidate ARM to 

meet the requirements for approval. 

With the availability of multiple FEMs 

that now work in the monitoring 

agency’s network, the commenting 

agency does not anticipate the need to 

ever pursue an ARM in the future and, 

therefore, suggests that the ARM 

provision is no longer needed. Another 

commenter strongly supported the 

proposed changes to remove the ARM 

provisions. The EPA also received 

comments from a few agencies that 

supported retaining the ARM provisions 

instead. One commenter cited the need 

to consider the rapid advancement of 

various new technologies and that, in 

some cases, approved continuous FEMs 

may have shortcomings, meaning that 

losing the ability to propose an ARM in 

the future may limit useful alternative 

options to monitoring agencies. Another 

commenter suggested that the removal 

of the ARM would take away the ability 

and right to use locally derived 

correction factors. 

After considering the comments for 

and against removing the provisions for 

ARMs, the EPA believes it is most 

appropriate to remove the ARM 

provisions. As described in the 

proposal, when the EPA first proposed 

the process for approving and using 

ARMs, there were no continuous FEMs 

approved. There are now over a dozen 

approved PM2.5 continuous FEMs and 

no approved ARMs. Therefore, the EPA 

is finalizing the removal of ARMs 

throughout 40 CFR parts 50 and 58 as 

proposed. 

b. Calibration of PM Federal Equivalent 

Methods (FEMs) 

The EPA proposed to modify its 

specifications for PM FEMs in appendix 

C to Part 58 (88 FR 5670–73, January 27, 

2023). Specifically, the EPA proposed 

that valid State, local, and Tribal (SLT) 

air monitoring data from Federal 

Reference Methods (FRMs) generated in 

routine networks and submitted to the 

EPA may be used to improve the PM 

concentration measurement 

performance of approved FEMs. This 

approach, initiated by instrument 

manufacturers, would be implemented 

as a national solution in factory 

calibrations of approved FEMs through 

a firmware update. This could apply to 

any PM FEM methods (i.e., PM10, PM2.5, 

and PM10–2.5). 

The EPA proposed this modification 

because there are some approved PM 

FEMs that are not currently meeting bias 

measurement quality objectives (MQOs) 

when evaluating data nationally as 

described in the 2022 PA (U.S. EPA, 

2022b, section 2.2.3.1), meaning that an 

update to factory calibrations may be 

appropriate; however, there is no clearly 

defined process to update the 

calibration of FEMs. While there are 

several types of data available to use as 

the reference for such updates (e.g., 

routinely operated FRMs, audit program 

FRMs, and chemical speciation sampler 


ddrumheller on DSK120RN23PROD with RULES3ER06MR24.034</GPH>

:I;!t S· meas - audit 

100 * ,=1 ' where St = ..Jaiiiilf x 100 

n.jNAAQS concentration audit 

and the corrected Equation 8 is: 

Ln s meas - audit 100 x i=l t where s, = ----- n,JNAAQS concentration ,/audit 
Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16351 

data), we proposed to use routinely 

operated SLT FRMs as the basis of 

comparison upon which to calibrate 

FEMs. The goal of updating factory 

calibrations would be to increase the 

number of routinely operating FEMs 

meeting bias MQOs across the networks 

in which they are operated. While there 

are other approaches that could improve 

data comparability between PM FEMs 

and collocated FRMs, the EPA believes 

that the proposed modification to 

calibrate PM FEMs represents the most 

reliable approach to update FEM factory 

calibrations, since the existing FRM 

network data that meet MQOs would be 

used to set updated factory calibrations. 

While the Agency proposed to add 

this language to more expressly define 

a process to update factory calibrations 

of approved PM FEMs, the EPA believes 

that the existing rules for updating 

approved FRMs and FEMs found at 40 

CFR 53.14 may also continue to be 

utilized for this purpose, as appropriate. 

40 CFR 53.14 allows instrument 

manufactures to submit to the EPA a 

‘‘Modification of a reference or 

equivalent method.’’ Submitting a 

modification request may be appropriate 

to ensure an approved FEM continues to 

meet 40 CFR 53.9, ‘‘Conditions of 

designation.’’ Specifically, 40 CFR 

53.9(c) requires that, ‘‘Any analyzer, 

PM10 sampler, PM2.5 sampler, or 

PM10–2.5 sampler offered for sale as part 

of an FRM or FEM shall function within 

the limits of the performance 

specifications referred to in § 53.20(a), 

§ 53.30(a), § 53.35, § 53.50, or § 53.60, as 

applicable, for at least 1 year after 

delivery and acceptance when 

maintained and operated in accordance 

with the manual referred to in 

§ 53.4(b)(3).’’ Thus, instrument 

manufacturers are encouraged to seek 

improvements to their approved FEM 

methods as needed to continue to meet 

data quality needs as operated across 

the network. 

There are several technical 

components to EPA’s proposed 

modification, including: the reference 

data to be used in the calibrations; 

implementing as a national solution in 

factory calibrations of approved FEMs 

through firmware updates; application 

to any PM FEM methods (i.e., PM10, 

PM2.5, and PM10–2.5); the appropriate 

range of data to be used to develop and 

test new factory calibrations, from just 

the most representative concentrations 

up to all available concentrations; the 

representative set of geographic 

locations that can be used; whether 

outliers may be included or not 

included; that new factory calibrations 

should be developed using data from at 

least 2 years and tested on data from a 

separate year or years; that updates to 

factory calibrations can occur as often as 

needed; that calibrations should be 

evaluated by monitoring agencies as 

part of routine data assessments, e.g., 

during certification of data and 5-year 

assessments; the EPA’s recognition that 

only data from existing operating sites is 

available; and finally, that an updated 

factory calibration does not have to 

work with the original field study data 

submitted that led to the original FEM 

designation. 

With the proposed modification, the 

EPA solicited input on these technical 

issues as well as the overall approach 

and any alternatives that could lead to 

more sites meeting the bias MQO with 

automated FEMs, especially for those 

sites that are near the level of the 

primary annual PM2.5 NAAQS, as 

proposed to be revised in section II 

above. In response, the EPA received 

comments from about two dozen 

entities, most of which were SLT air 

programs or Multi-Jurisdictional 

Organizations (MJOs) comprised of 

these entities. 

Overall, there was broad and strong 

support from a majority of commenters 

for the proposed requirement to use 

FRM data generated in routine networks 

and submitted to the EPA to update 

factory calibrations included as part of 

approved FEMs. There were a smaller 

number of critical comments on the 

proposed process as well as some 

commentors that supported the 

proposed requirement but also provided 

additional suggestions for the EPA’s 

consideration. Below, we address each 

of the areas on which the EPA requested 

comment regarding the calibration of 

PM FEMs, as well as a few additional 

areas where multiple commenters 

offered input on other areas related to 

our proposal. 

A majority of the commenters on the 

proposed PM FEM calibration process 

support the process to use valid State, 

local, and Tribal FRM data generated in 

routine networks and submitted to the 

EPA to improve the PM concentration 

measurement performance of approved 

FEMs. Some commenters suggested that 

this action is needed to ensure that data 

reported from FRMs and FEMs are 

comparable and correction methods 

applied to data from FEM monitors are 

defensible across the national PM 

monitoring network. Others stated that 

they agree with the EPA that this is a 

critical step in the right direction to 

account for the discrepancies between 

PM2.5 FRM data and PM2.5 FEM data. 

Some commented that applying 

corrections includes a recognition that, 

while different measurement principles 

may produce differences in the resulting 

data, having an approach that 

minimizes bias is extremely important. 

Finally, some stated their belief that a 

correction factor is necessary to preserve 

data integrity with the FRM. 

The EPA also received comments 

suggesting ways that the PM FEM 

correction could be performed, 

including through detailed analysis of 

data; by having PM FEM instrument 

manufacturers evaluate nationally 

available valid FRM data to update 

factory calibrations; and, by having the 

instrument manufacturers implement 

calibration adjustments at the factory. 

The EPA also received supportive 

comments on the PM FEMs calibration 

relating to comparability to the NAAQS. 

For example, a commenter stated that it 

is important to ensure bias MQOs are 

met for FEMs run at sites potentially 

affected by revised standards as well as 

the need to accurately designate areas as 

attaining or not attaining the NAAQS. 

There were comments supporting the 

correction of PM FEM data as helping 

the EPA and SLT monitoring programs 

continue to evolve toward more 

automated methods. For example, one 

commenter appreciates the EPA’s 

support for the ongoing move from 

filter-based PM2.5 FRMs to use of 

continuous FEMs, stating that they 

concur with the EPA’s assessment that 

there is monitoring bias between FRMs 

and FEMs, and commending EPA for 

recognizing ongoing data quality issues 

for FEMs and for taking action to 

improve these issues in collaboration 

with instrument manufacturers and SLT 

agencies. 

A small number of commenters were 

critical of the proposed FEM calibration 

approach. One commenter noted that 

EPA should further examine the 

handling of FEM PM2.5 data when used 

for comparison to the NAAQS. In 

response, we note that monitoring 

agencies and the EPA will continue to 

examine the comparability and use of 

FEM data used in comparison to the 

NAAQS. Another commenter suggested 

that the calibration process for a 

designated PM monitor should not be 

altered following Class III designation 

approval. The EPA disagrees as we 

believe it is appropriate for FEMs to be 

calibrated with routinely operated 

FRMs, because doing so is an efficient 

way to work towards FEM data meeting 

the bias MQO across the networks in 

which the FEMs are currently being 

operated. Also, having continuous PM 

FEMs meeting bias MQOs allows the 

use of the data in a variety of other ways 

that manually operated FRMs samplers 

cannot support. Another commenter 

stated that, if a particular FEM 

designated make or model of 


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instruments fails to meet MQOs, then 

that make or model should be removed 

from the designations altogether. The 

EPA agrees and clarifies that the 

modification would not prevent removal 

of FEM designation from a make or 

model of instrument under the existing 

40 CFR 53.11—Cancellation of reference 

or equivalent method designation. This 

may be appropriate if there are no other 

solutions to improve the method such 

that it achieves bias MQOs. 

A few commenters provided specific 

recommendations for how the 

regulatory language could be improved. 

These included comments that the new 

regulatory language proposed for 40 

CFR part 58, appendix C, section 2.2 

must ensure consistency and 

transparency when requesting changes 

to the factory calibration; that the EPA 

should incorporate binding regulatory 

language in 40 CFR part 58, appendix C, 

section 2.2 (i.e., it currently lacks 

‘‘shall’’ or ‘‘must’’) to ensure the 

language is not open to inconsistency 

and does not provide unique deference 

to instrument manufacturers without a 

mechanism for transparent 

communication of the changes being 

made and the supporting technical 

analysis. A commenter also requested 

that the EPA define the core 

requirements needed to ensure all 

requests for updating factory 

calibrations are required to follow the 

same process, using data of the same 

known quality, and evaluating the 

effectiveness of the resulting correction 

factors consistently. 

In response to these comments, while 

the EPA agrees that the proposed 

regulatory language for 40 CFR part 58, 

appendix C, section 2.2 must ensure 

consistency and transparency when 

entities request changes to factory 

calibrations, the EPA disagrees that the 

regulations cannot also provide some 

flexibility. For example, we believe that 

a degree of flexibility is appropriate 

regarding whether outliers in the data to 

be used for factory calibration should or 

should not be included, the range of 

data to be included, and in utilizing 

collocated FRM and FEM data for 

updated calibrations from a 

representative set of geographic areas in 

which it is produced. The EPA believes 

that the proposal defined the core 

requirements needed to ensure all 

requests for updating FEM factory 

calibrations will follow the same 

process, using data of the same known 

quality and evaluating the effectiveness 

of the resulting correction factors 

consistently. 

In its proposal, the EPA identified 

that while there are several types of data 

available to use as the reference for FEM 

calibration updates, including data from 

routinely operated FRMs, audit program 

FRMs, and PM2.5 chemical speciation 

samplers, the EPA proposed to use 

routinely operated State, local, and 

Tribal FRMs as the basis of comparison 

upon which to calibrate FEMs (88 FR 

5670–71, January 27, 2023). 

Importantly, routine SLT agency FRM 

data form the largest portion of the 

monitored air quality data used in 

epidemiologic studies that are being 

used to inform proposed decisions 

regarding the adequacy of the public 

health protection afforded by the 

primary PM2.5 NAAQS, as discussed in 

section II above. 

Overall, there was broad and strong 

support for utilizing collocated FRM 

data from routine SLT networks to 

provide calibrations of the continuous 

FEMs. For example, several commenters 

agree that valid SLT air monitoring data 

generated in routine networks and 

submitted to the EPA will improve the 

PM concentration measurement 

performance of approved FEMs. 

Another commenter provided support 

for PM FEM instrument manufacturers 

to evaluate nationally available valid 

FRM data as well as other data sets such 

as the performance evaluation audit 

program to update factory calibrations. 

The EPA believes that the routinely 

operated PM FRMs represent the best 

and largest source of data to calibrate 

continuous PM FEMs, and that 

performance evaluation audit program 

data should be kept independent of the 

calibration process. This will mean that 

assessments of the routine monitoring 

operations, including both the FRM and 

any future updated PM FEMs, will 

appropriately remain independent in 

evaluating whether updated methods 

are meeting bias MQOs. The EPA is, 

therefore, finalizing its approach to use 

routinely operated SLT FRMs as the 

basis of comparison upon which to 

calibrate continuous PM FEMs as 

proposed. 

Regarding the EPA’s proposed 

requirement to utilize factory 

calibrations (88 FR 5670–71, January 27, 

2023), several commenters agreed that 

factory calibrations provide the best 

option to improve PM FEMs. For 

example, one commenter stated that the 

correction factors are necessary to 

preserve data integrity with the FRM, 

and they support the proposal that the 

approach be initiated by instrument 

manufacturers and implemented as a 

national solution through firmware 

updates. 

Regarding the proposed requirement 

that calibrations be initiated by 

instrument manufacturers (88 FR 5671, 

January 27, 2023), most commenters 

were supportive of the proposed 

approach that recalibration of FEM PM 

instruments be initiated by instrument 

manufacturers. For example, one 

commenter stated they support allowing 

instrument companies submit 

improvements to their existing FEMs, as 

vendors should be encouraged to 

improve their methods. Another 

commenter noted that having a 

methodology initiated by the 

manufacturer will have nationwide 

consistency. A few of commenters 

recommended that SLT air agencies 

should have the additional ability to 

petition the EPA Administrator to 

initiate factory calibrations of FEMs to 

better meet MQOs when data collected 

by their agencies indicate disparities, 

because the monitoring agencies are 

responsible for the quality of the data 

from the specific makes and models of 

instrumentation used in their networks. 

While the EPA believes that, in most 

cases, the instrument companies should 

be the ones to initiate the process for 

calibration of FEMs to routinely 

operated FRMs, we agree with the 

commenters who suggested that other 

options should be available, including 

allowing monitoring agencies or MJOs 

to work independently or together to 

pursue improvements to designated 

FEMs. However, the EPA believes that 

any such improvements initiated by 

monitoring agencies or MJOs should 

still be facilitated through the 

responsible instrument company. Also, 

any such effort to improve data quality 

should be employed across all the 

networks in which the methods are 

operated and not limited to the 

networks operated by the agency(s) 

pursuing such improvements. 

Regarding how frequently factory 

calibrations should be updated, our 

proposal identified that it would be 

most appropriate to not define a specific 

time period for updates; rather, updates 

should be based on whether or not 

quality data is being produced across a 

given network (88 FR 5672, January 27, 

2023). Regarding this issue, one 

commenter recommended that 

instrument manufacturers be required to 

evaluate and, if necessary, adjust PM 

FEMs factory calibrations on an ongoing 

basis at regular intervals. The EPA notes 

that while it does not have the authority 

to require instrument companies to 

evaluate the quality of data from 

operating FEMs under 40 CFR part 58, 

the EPA does routinely participate in 

conferences and workshops and makes 

assessments of data quality specific to 

instrument makes and models publicly 

available. The EPA also regularly 

summarizes relevant FRM and FEM data 


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quality in documents such as the 2022 

PA (U.S. EPA, 2022b). Therefore, 

consistent with the proposal, we are not 

finalizing any specifics regarding how 

frequently factory calibrations should be 

updated but commit to continue to 

routinely provide information to SLT 

agencies regarding FEM data quality. 

The EPA proposed that the calibration 

of FEMs could apply to any of the PM 

FEM method indicators (i.e., PM10, 

PM2.5, and PM10–2.5) (88 FR 5670, 

January 27, 2023). The EPA received 

only supportive comments. All 

comments that included a discussion of 

three PM metrics support their 

inclusion for calibration of PM FEMs. 

Therefore, the EPA is finalizing the 

inclusion of all three PM indicators (i.e., 

PM10, PM2.5, and PM10–2.5) as proposed. 

The EPA proposed that either all data 

available or a range of data up to 125% 

of the 24-hour NAAQS for the PM 

indicator of interest may be used to 

establish new factory calibrations, (88 

FR 5671–73, January 27, 2023). The EPA 

received many comments supportive of 

the proposal and one comment offering 

a different approach on the range of data 

to use. One commenter recommends 

that the EPA should consider using all 

‘‘validated’’ data because how these 

instruments behave under normal 

operating ranges may be just as 

important as how they behave when 

monitoring conditions are low or 

elevated, and that the full range of data 

should be used when determining the 

appropriate level of the standard, just as 

the full range of data is used in 

determining if an area is attaining the 

standard. In response to this comment, 

the EPA believes that making 

allowances for some flexibilities will 

increase the likelihood of instrument 

companies pursuing such 

improvements. Also, even though there 

is flexibility, the EPA will still be able 

to evaluate the appropriateness of a 

range of concentration data included as 

part of each application submitted. 

Also, the EPA notes that in certain 

circumstances, States do petition the 

EPA to set aside data under the 

Exceptional Events Rule (§ 50.14, 

‘‘Treatment of air quality monitoring 

data influenced by exceptional events’’). 

Where approved, exceptional event data 

are set aside from use in regulatory 

decisions. Thus, there is a process to set 

aside certain high concentration data for 

certain purposes. Therefore, the EPA is 

finalizing the provision that factory 

calibrations may be based on a range of 

valid data as proposed. 

The EPA solicited comment on the 

representative set of geographic 

locations to use in the calibration of 

FEMs compared to collocated FEMs (88 

FR 5671, January 27, 2023). Most 

commenters were supportive of the 

approach of using representative sites in 

SLT networks from across the country. 

For example, several commenters 

provided their support for PM FEM 

instrument manufacturers to evaluate 

nationally available valid FRM data to 

update factory calibrations. Commenters 

disagreeing with a national geographic 

approach preferred to allow local 

solutions to correct data. For example, 

one commenter suggested having a local 

or regional option because PM 

instruments are impacted by, and 

respond differently to, a variety of local 

factors, including relative humidity, 

temperature, concentration levels, and 

particle composition. The EPA agrees 

that there are challenges in the response 

of PM FEMs to a variety of local factors; 

however, this can be true of many 

methods and are not specific to PM 

FEMs and, therefore, does not provide a 

reason to reject this approach in this 

instance. Another commenter stated that 

the proposed national correction factor 

is a ‘‘flawed concept,’’ suggesting that it 

is ‘‘widely understood throughout the 

monitoring community that monitors 

perform best with a local correction 

factor.’’ This commentor offered no 

record or citation supporting this point. 

The EPA counters that while monitoring 

agencies may statistically correct data 

from a PM continuous monitor for AQI 

purposes (40 CFR part 58, appendix G), 

there are both examples of well 

performing statistically corrected PM 

continuous monitors being used for AQI 

purposes; however, without proper 

attention and updates, there are also 

examples of poorly performing ones. 

Finally, another commenter believes 

that a national correction factor cannot 

possibly incorporate data to represent 

all the scenarios across the nation that 

have an impact on monitor performance 

and data quality. Although the EPA 

agrees that there are a variety of local 

scenarios that could affect monitor 

performance, the overall benefits of 

having nationally consistent 

measurement of PM concentrations and 

national calibration of data outweigh the 

potential advantages of locally specific 

calibrations. 

Several commenters also disagreed 

with using local and regional 

calibrations of data, including some 

monitoring agencies that asserted being 

unable to reinvest in the operation of 

FRMs that would be required to locally 

calibrate their own PM FEMs. Further, 

every approved PM FEM method 

designated today is effectively 

calibrated through demonstration of 

field testing in the areas in which it was 

required to be tested (40 CFR 

53.35(b)(1)). Moreover, the EPA 

proposed to require instrument 

manufacturers to demonstrate that they 

can improve the number of sites 

meeting bias MQOs by initiating a 

recalibration of an FEM. Thus, the use 

of a national set of sites where the 

methods are operated is essentially a 

fine-tuning of the PM FEMs 

performance across all sites where it is 

used. 

After considering all the comments 

received, the EPA believes it is 

appropriate to finalize as proposed with 

a representative set geographic locations 

at SLT sites to calibrate PM FEMs. 

Identification of such sites would be 

made by the applicant of the planned 

updated calibration, subject to EPA 

approval, and submitted to the EPA in 

accordance with the requirements and 

application instructions in 40 CFR part 

58, appendix C, sections 2.2 and 2.7. 

The EPA encourages early 

communication between an applicant 

seeking a method update and the EPA 

to facilitate the most appropriate sites 

are included in any updated application 

of the methods calibration. 

The EPA proposed that instrument 

companies may, but are not required to, 

check for and exclude any potential 

outliers that may exist in the validated 

State, local, and Tribal agency network 

data available from AQS that would be 

used to establish new factory 

calibrations. The EPA received two 

comments regarding potential outlier 

approaches. One commenter disagreed 

with the proposed approach and instead 

recommended the use of all ‘‘validated’’ 

data, because how these instruments 

behave under normal operating ranges 

may be just as important as how they 

behave when monitoring conditions are 

low or elevated. The EPA acknowledges 

this point; however, the proposal on 

outliers allows flexibility in using 

standard outlier tests if needed to 

include or exclude such data as part of 

the calibration process. Ultimately, the 

true test of success for an updated 

method calibration will be that a higher 

number of sites are meeting bias MQOs 

in the areas in which the method is 

used, which will include all routine 

valid data including any potential 

outliers. Another commenter asserted 

concerns with the ability of instrument 

manufacturers to analyze data within 

individual monitoring agencies. The 

EPA disagrees with the commenter 

because decisions whether to include or 

exclude outliers should be flexible and 

made on a case-by-case basis. Moreover, 

the expected substantially larger dataset 

from routinely operated collocated 

FRMs and FEMs compared to what was 


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used in the original FEM designation 

testing (§ 53.35 Test procedure for Class 

II and Class III methods for PM2.5 and 

PM10–2.5) will minimize the effect of any 

potential outliers. 

In contrast to these two comments, 

the EPA received many comments 

supportive of the proposed outlier 

approach overall. Therefore, the EPA is 

finalizing this part of the proposal that 

instrument companies may, but are not 

required to, check for and exclude any 

potential outliers that may exist in the 

validated State, local, and Tribal agency 

network data available from AQS that 

would be used to establish new factory 

calibrations. 

Several commenters offered input on 

statistical criteria and initial testing 

requirements for approval of candidate 

PM FEMs and the role of instrument 

manufacturers in this process. The EPA 

did not propose any changes related to 

these issues; however, these comments 

have been considered below. 

One commenter suggested that data 

quality objectives, bias, and precision 

estimators for different monitoring 

methods should be based on averages at 

both national and regional levels for 

purposes of comparison. Another 

commenter asked to strengthen the 

criteria for Class 3 Equivalency 

standards for candidate PM 

instrumentation. On testing 

requirements, one commenter 

recommended that the EPA consider 

updating the 40 CFR part 53 process for 

approving FEMs so that the testing 

process more closely reflects the 

regulatory deployment and data 

handling that generates NAAQScomparable data. Another commenter 

asked that the results from ‘‘summer’’ 

and ‘‘winter’’ field evaluations not be 

averaged together because it allows 

agencies to minimize the error of biased 

instruments by averaging poor results 

with data often biased in the other 

direction. The same commenter also 

recommended that candidate 

instruments data sets should not be 

averaged together as is done currently 

where data from triplicate instruments 

are averaged for each day. Another 

commenter asked that the EPA require 

FEM field comparability tests in the 

northwest (e.g., in EPA Region 10) in 

areas where particulate derived from 

biomass predominates to ensure that 

certified instruments will perform 

reliably in regions influenced by these 

sources. Related to the different 

measurement principles and the 

instrument companies’ role in PM 

FEMs, one commenter noted that FEMs 

may never align perfectly with the 

FRMs due to the use of different 

measurement principles. Another 

commenter asked that manufacturers of 

FEM instruments be held accountable 

for ensuring that they continue to meet 

FEM criteria, whether through 

calibration updates and/or follow-up 

evaluations. Another commenter 

suggested that instrument 

manufacturers should be required to 

further evaluate the FEM monitoring 

data at defined intervals including, but 

not limited to, the 2-year and 5-year 

approval anniversaries. 

The EPA did not propose to make 

modifications to the statistical criteria or 

testing requirements; however, we did 

solicit comment on any alternatives that 

would lead to more sites meeting the 

bias MQO with automated FEMs, 

especially for those sites that are near 

the level of the primary annual PM2.5

NAAQS as proposed (88 FR 5672–73, 

January 27, 2023). While the comments 

requesting that the statistical criteria be 

strengthened may have merit, doing so 

would not address the large inventory of 

already deployed PM FEMs used 

throughout the country. Also, without 

performing a detailed Data Quality 

Objective (DQO) design process, it is 

unclear how changing one or more 

statistical criteria would help improve 

the number of sites meeting the bias 

MQO now or in the future. Similarly, 

while the comments asking for changes 

to the locations of testing may also have 

merit, the EPA believes this could be a 

deterrent for instrument manufactures 

to seek additional improvements since 

more testing would be required, at least 

for candidate methods. Regarding the 

comment on the different measurement 

principles, the EPA concurs that 

different measurement principles may 

never align perfectly. Also, the EPA 

notes that the Agency has longstanding 

goals for acceptable measurement 

uncertainty of automated and manual 

PM2.5 methods in 40 CFR part 58, 

appendix A, section 2.3.1.1. Therefore, 

while having different measurement 

principles align is useful, meeting the 

goal for acceptable measurement 

uncertainty is the objective. 

Regarding the comments related to the 

instrument companies’ role in PM 

FEMs, the EPA notes that FEMs are 

already required to meet 40 CFR 53.9, 

‘‘Conditions of designation.’’ 

Specifically, 40 CFR 53.9(c) requires 

that, ‘‘Any analyzer, PM10 sampler, 

PM2.5 sampler, or PM10–2.5 sampler 

offered for sale as part of an FRM or 

FEM shall function within the limits of 

the performance specifications referred 

to in § 53.20(a), § 53.30(a), § 53.35, 

§ 53.50, or § 53.60, as applicable, for at 

least 1 year after delivery and 

acceptance when maintained and 

operated in accordance with the manual 

referred to in § 53.4(b)(3).’’ The EPA 

does not have the authority to require 

instrument manufacturers to further 

evaluate the FEM monitoring data at 

defined intervals, including but not 

limited to the 2-year and 5-year 

approval anniversaries, as one 

commenter suggested. 

In addition to these few 

recommendations, the EPA received 

many comments supportive of the 

proposal that valid State, local, and 

Tribal air monitoring data from FRMs 

generated in routine networks and 

submitted to the EPA may be used to 

improve the PM concentration 

measurement performance of approved 

FEMs; therefore, consistent with the 

proposal we are not finalizing any 

updates to the statistical criteria, testing 

requirements, or requirements on 

instrument manufactures as proposed. 

The EPA proposed that any new 

factory calibration should be developed 

using data from at least 2 years and 

tested on a separate year(s) of data (88 

FR 5672, January 27, 2023). Comments 

on this part of the proposal were 

generally supportive. One commenter 

requested that at least a 3-year dataset, 

rather than the proposed 2 years, be 

used for a representative design value 

comparison of the FEM and FRM 

datasets to be evaluated. Another 

commenter pointed out that as large a 

data set as possible should be used, but 

EPA should not limit it to only data 

collected by instruments that have 

operated for more than 2 years. 

In response to these comments, the 

EPA notes the broad support for the 

proposal as written. Also, the EPA notes 

that the 2-year period for using data to 

develop a factory calibration is a 

minimum, and that more years may be 

used as appropriate. Therefore, the EPA 

is finalizing its approach that any new 

factory calibration should be developed 

using data from at least 2 years and 

tested on a separate year(s) of data as 

proposed. 

The EPA proposed several aspects of 

the FEM calibration on which we did 

not receive specific comments, 

including a provision that FEM methods 

should be evaluated by monitoring 

agencies as part of routine data 

assessments, such as during certification 

of data and 5-year assessments; the fact 

that the EPA recognizes only data from 

existing operating sites are available for 

use in factory calibrations; and 

recognition that an updated factory 

calibration does not have to work with 

the original field study data submitted 

that led to the designation as an FEM. 

With the broad general support from 

commenters summarized above, the 

EPA is finalizing each of these 


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individual aspects of the FEM 

calibration as proposed. 

In the proposal, the EPA identified 

that we should expect a lag between the 

date when an already designated 

method is approved with a new factory 

calibration as an updated method by the 

EPA and when it can be implemented 

in the field. The EPA solicited comment 

on how to approach the data produced 

during this lag. Commenters provided 

input not only on how to address data 

during the lag, but also regarding how 

to address data already collected prior 

to a method update that has the 

potential to be used in regulatory 

decision making, particularly where 

such collected data do not meet the bias 

MQO. In response to this solicitation of 

comment, there was a consistent 

recommendation that calibrations of 

data associated with method updates 

should be applied to all relevant PM 

data prior to the EPA using it for 

designations under a final NAAQS. 

While the EPA appreciates these 

comments and recognizes their support 

for retroactive data correction, at this 

time and following this final rule, 

monitoring agencies should continue to 

report PM FEM data as measured. This 

component of this final rule is focused 

only on revising 40 CFR part 53, 

appendix C to implement an updated 

calibration for approved PM FEMs. The 

issue of how prior and future 

monitoring data will be used in the 

implementation of this NAAQS, such as 

for designations, and for air quality 

regulatory programs is outside the scope 

of this rulemaking and will therefore be 

addressed by the EPA in a subsequent 

relevant action or actions. 

The EPA received comments on 

whether updates to PM FEM methods 

should be required to be implemented 

or there would flexibility in when and 

if a monitoring agency implemented 

them. The commenters asked that EPA 

be flexible in allowing the use of 

updated method correction factors 

intended to improve the data 

comparability between the FRMs and 

FEMs. 

In most cases, the EPA expects that 

updating the FEMs will result in 

improved data quality and more sites 

meeting bias MQOs; however, the EPA 

is not finalizing an update requirement 

in this action. Monitoring agencies can 

assess their data and make decisions on 

an update based on whether they are 

meeting the bias MQOs. Such decisions 

on whether or not to update a method 

may efficiently be included in those 

agencies’ annual monitoring network 

plans under 40 CFR 58.10, ‘‘Annual 

monitoring network plan and periodic 

assessment,’’ which are already subject 

to EPA Regional office approval. In 

some circumstances, it is possible the 

original PM FEM may be revised in a 

manner where only the updated method 

has an active approved designation. In 

these cases, monitoring agencies would 

need to address updating their PM FEM 

in a timely manner. 

The EPA solicited input on any 

alternative approaches that could lead 

to more sites meeting the bias MQO 

with automated PM FEMs, especially for 

those sites that are near the level of the 

primary annual PM2.5 NAAQS as 

proposed to be revised in section II 

above. A few commentors provided 

input on potential options for 

alternative approaches and several 

others offered input on how a local or 

regional calibration of an FEM could 

work. Among alternative approaches, 

one commenter suggested that 

manufacturers of FEMs could provide 

settings that would allow for 

adjustments to make FEM data more 

‘‘FRM-like.’’ Another commenter 

suggested working with the 

manufacturers of FEM equipment to 

diagnose the cause of the bias and then 

to address it appropriately. 

The EPA received several comments 

on how to implement a local or regional 

calibration of FEMs. One commenter 

suggested that EPA could allow for SLT 

agencies to adjust FEM data to be more 

‘‘FRM-like’’ prior to submitting data to 

AQS. Another commenter suggested 

using a rolling 3-month linear regression 

based on a comparison of FEM data to 

PM2.5 levels measured by a 1-in-6-day 

FRM. Another commenter 

recommended that the EPA allow the 

application of a correction factor that is 

from an area with a similar climate and 

other conditions. Another commenter 

suggested that, for metropolitan 

statistical areas (MSAs) where the recalibrated FEMs still do not meet 

equivalency criteria, monitoring 

agencies should be able to use the 

rolling linear regression technique to 

further calibrate the FEMs within an 

MSA. Another commenter suggested 

that developing a simple linear 

regression could establish the 

relationship between FEM data and 

FRM data and be used to adjust the FEM 

data at each site where they are 

collocated. Another commenter 

suggested that averaging the results 

within a MSA and applying it on an 

MSA basis with the previous 2 years of 

data could provide an adjustment 

method for sites without a collocated 

FRM. Another commenter identified 

that a regional correction factor 

potentially could improve instrument 

accuracy to biomass sources, which are 

a large component of PM in many 

communities. 

Among the alternative approaches 

suggested, having settings that would 

allow for adjustments to make FEM data 

more ‘‘FRM-like’’ has merit, but 

assuming this was within a PM FEM 

itself, it would need to be separately 

incorporated into each make and model 

of FEM. If EPA were to pursue this 

alternative approach, the suggestion 

could be incorporated into a future 

regulatory action as a potential 

condition of designation because, 

without having the opportunity to 

thoughtfully consider how every step of 

such an approach would need to work, 

including what such requirements 

would look like and how potential 

settings adjustments would be made, it 

is not appropriate for the EPA to require 

the availability of such settings now, nor 

would it address the inventory of 

currently available PM FEMs already 

operating. 

Regarding the suggestion that the EPA 

and SLTs should work with the 

manufacturers of FEM equipment to 

diagnose the cause of any biases and 

then to address them appropriately, the 

EPA supports this recommendation, but 

does not believe a regulatory change is 

required to allow the monitoring 

community (EPA and SLTs) to work 

with instrument manufacturers in this 

way. 

Regarding the several comments on 

how to implement a local or regional 

calibration of FEMs, the EPA 

acknowledges the desire for this 

flexibility but believes that any such 

provisions for local or regional 

calibration of FEMs would need to be 

thoroughly thought out and proposed 

for consideration across the monitoring 

community. While several commenters 

support such an approach, the EPA also 

received adverse comments on the 

potential for local and regional 

calibration of PM FEMs instead of 

national. Most of the criticism of local 

and regional calibration of PM FEMs 

centered on both the lack of existing 

operating PM FRMs in commenters’ 

networks and monitoring agencies’ 

inability to staff the higher number of 

operating FRMs that would have to be 

collocated with PM FEMs to calibrate. 

Thus, the commenters that oppose local 

and regional calibrations of data prefer 

to utilize the national calibration of 

FEM data as proposed. Acknowledging 

all of these viewpoints, the EPA believes 

that it would not be appropriate to 

institute such an approach at this time. 

As discussed throughout this section, 

this final rule, the EPA is embarking on 

a new national approach to calibration 

of FEMs where valid State, local, and 


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188SES is a composite measure that includes 

metrics such as income, occupation, and education, 

and can play a role in populations’ access to 

healthy environments and healthcare. 

189Metropolitan and Micropolitan Statistical 

Areas are collectively referred to as ‘‘Core-Based 

Statistical Areas.’’ Metropolitan statistical areas 

have at least one urbanized area of 50,000 or more 

population, plus adjacent territory that has a high 

degree of social and economic integration with the 

core as measured by commuting ties. Micropolitan 

statistical areas are a set of statistical areas that have 

at least one urban cluster of at least 10,000 but less 

than 50,000 population, plus adjacent territory that 

has a high degree of social and economic 

integration with the core as measured by 

commuting ties. 

Tribal air monitoring data from FRMs 

generated in routine networks and 

submitted to the EPA may be used to 

improve the PM concentration 

measurement performance of approved 

FEMs. The EPA and the community of 

SLT monitoring agencies can further 

consider other solutions to improving 

PM FEM methods, including local and 

regional scale calibration of FEMs, in a 

future review of the PM NAAQS. 

In summary, the EPA is finalizing its 

proposal to allow valid State, local, and 

Tribal air monitoring data from PM 

FRMs and FEMs generated in routine 

networks and submitted to the EPA to 

update factory calibrations included as 

part of approved FEMs (40 CFR part 58, 

appendix C, sections 2.2 and 2.7). This 

approach, which will typically be 

initiated by instrument manufacturers 

but can also be spurred by monitoring 

agencies, MJOs of monitoring agencies, 

and the EPA itself, is to be implemented 

as a national solution in factory 

calibrations of approved FEMs through 

a firmware update, subject to EPA 

approval. FEM calibrations can apply to 

any PM FEM methods (i.e., PM10, PM2.5, 

and PM10–2.5). As part of this process, 

the EPA is finalizing that a range of data 

based on the most representative 

concentrations up to all available 

concentrations may be used in 

developing and testing a new factory 

calibration; that a representative set of 

geographic locations can be used; that 

outliers may be included or not 

included; that a new factory calibration 

should be developed using data from at 

least 2 years and tested on a separate 

year(s) of data; that updates to factory 

calibrations can occur as often as 

needed and should be evaluated by 

monitoring agencies as part of routine 

data assessments such as during 

certification of data and 5-year 

assessments; that the EPA recognizes 

only data from existing operating sites is 

available; and that an updated factory 

calibration does not have to work with 

the original field study data submitted 

that led to the designation as an FEM. 

The EPA is finalizing this approach as 

proposed with the intention of having 

more sites meet the bias MQOs with 

automated PM FEMs. 

4. Revisions to the PM2.5 Monitoring 

Network Design Criteria To Address AtRisk Communities 

To enhance protection of air quality 

in communities subject to 

disproportionate air pollution risk, 

particularly in light of the proposed 

range for a revised primary annual PM2.5

standard, the EPA proposed to modify 

the PM2.5 monitoring network design 

criteria to include an environmental 

justice (EJ) factor that accounts for 

proximity of at-risk populations (i.e., 

those identified in the 2019 ISA and ISA 

Supplement as being at increased risk of 

adverse health effects from PM2.5

exposures to sources of concern), 

consistent with the statutory 

requirement that the NAAQS protect the 

health of at-risk populations (88 FR 

5673, January 27, 2023). Specifically, 

the EPA proposed to modify the existing 

requirement at 40 CFR part 58, 

appendix D, section 4.7.1(b)(3)): ‘‘For 

areas with additional required SLAMS, 

a monitoring station is to be sited in an 

area of poor air quality,’’ to additionally 

address at-risk communities with a 

focus on anticipated exposures from 

local sources of emissions. The 

scientific evidence evaluated in the 

2019 ISA and ISA Supplement indicates 

that sub-populations at potentially 

greater risk from PM2.5 exposures 

include children, lower socioeconomic 

status (SES) 188 populations, minority 

populations (particularly Black 

populations), and people with certain 

preexisting diseases (particularly 

cardiovascular disease and asthma). The 

EPA proposed that communities with 

relatively higher proportions of subpopulations at greater risk from PM2.5

exposure within the jurisdiction of a 

State or local monitoring agency should 

be considered ‘‘at-risk communities’’ for 

these purposes. 

The PM2.5 network design criteria 

have led to a robust national network of 

PM2.5 monitoring stations. These 

monitoring stations are largely in CoreBased Statistical Areas (CBSAs) 189

across the country that include many 

PM2.5 monitoring sites in at-risk 

communities. Many of the 

epidemiologic studies evaluated in the 

2019 ISA and ISA Supplement, 

including those that provide evidence of 

disparities in PM2.5 exposure and health 

risk in minority populations and lowSES populations, often use data from 

these existing PM2.5 monitoring sites. 

However, we anticipate that with the 

more protective annual NAAQS 

finalized in section II above, 

characterizing localized air quality 

issues around local emission sources 

may become even more important. The 

EPA believes that adding a network 

design requirement to locate monitors in 

at-risk communities will improve our 

characterization of exposures for at-risk 

communities where localized air quality 

issues may contribute to air pollution 

exposures. Requiring that PM2.5

monitoring stations be sited in at-risk 

communities will allow other methods 

to be operated alongside PM2.5

measurements to support multiple 

monitoring objectives per 40 CFR part 

58, appendix D, section 1.1. The EPA 

believes that it is appropriate to 

formalize the monitoring network’s 

characterization of PM2.5 concentrations 

in communities at increased risk to 

provide such areas with the level of 

protection intended with the PM2.5

NAAQS. The addition of this 

requirement will also lead to enhanced 

local data that will allow air quality 

regulators help communities reduce 

exposures and inform future 

implementation and reviews of the 

NAAQS. 

The EPA received comments 

concerning the proposed requirement to 

modify the PM2.5 monitoring network 

design criteria to include an EJ factor 

that accounts for the proximity of 

populations at increased risk of adverse 

health effects from PM2.5 exposures to 

sources of concern. Commenters 

included State, local, and Tribal air 

agencies and multijurisdictional 

organizations (MJOs) comprised of those 

agencies; industry and industry groups; 

other Federal, State, and local 

government entities; public health, 

medical, and environmental 

nongovernmental organizations (NGOs); 

and private citizens. The EPA proposed 

to require that sites located in at-risk 

communities (particularly those whose 

air quality is potentially affected by 

local sources of concern) should 

nonetheless meet the requirements to be 

considered representative of ‘‘areawide’’ 

air quality as this is consistent with all 

other minimally required sites. There 

were several other technical 

components of the proposed 

requirement for which we asked for 

comment, including: how to identify atrisk communities; the PM sources of 

concern important to consider; the 

datasets that can be used to identify 

communities with high exposures; the 

most useful measurement methods to 

collocate with PM2.5 in at-risk 

communities; and the timeline to 

implement any new or moved sites. 

Overall, most commenters were very 

supportive of the EPA’s proposed 

modification to the PM2.5 monitoring 


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network design criteria to include an EJ 

factor that accounts for proximity of 

populations at increased risk of adverse 

health effects from PM2.5 exposures to 

sources of concern. A few commenters 

offered detailed supporting comments. 

For example, one commenter 

recommended targeting investment in 

regulatory monitors in EJ communities, 

opining that there is presently a lack of 

equitable distribution of these monitors 

in low-income and minority 

communities. Another commenter 

supports the inclusion of an EJ factor in 

PM2.5 monitoring network design 

criteria as a means to assess whether 

disparities in exposure are reduced in 

the future. The EPA appreciates the 

support for the proposed requirement 

and acknowledges the desirability of a 

goal to assess if disparities in exposure 

are reduced in the future as a result of 

these monitoring efforts. 

Some commenters were generally 

supportive of the proposed requirement 

but suggested that the EPA should recast 

the approach in a more specific way or 

offered additional examples of sources 

of concern. For example, one 

commenter stated that PM2.5 emissions 

from residential and commercial wood 

burning result in localized hotspots that 

are often not revealed by community air 

monitoring. Another commenter asked 

that the EPA adopt a strategy to monitor 

EJ communities near both larger wellknown point sources of PM2.5 and along 

traffic corridors as well as smaller 

sources that, when taken together, may 

create a large amount of emissions and 

health harms in the area. Another 

commenter stated that the national 

network of monitors operated by the 

EPA captures data used for generalized 

modeling, but overall monitoring is not 

as granular as one would expect, 

especially in urban areas. For instance, 

the commenter suggested that EPA 

could monitor suspected ‘‘hot spots’’ 

(e.g., residential development adjacent 

to highways and active construction 

sites) to better manage and mitigate 

PM2.5 pollution at their sites of origin, 

and that more extensive and granular 

monitoring data would also facilitate 

essential research and inform future 

evaluations and adjustments of the 

NAAQS. The EPA acknowledges these 

comments identifying other sources of 

concern, and we address these and other 

potential sources of concern below. 

Among adverse comments, a few 

commenters stated that ‘‘at-risk 

communities’’ is not well defined. The 

EPA disagrees and directs those 

commenters to the numerous places 

where this definition is covered, 

including in Section II.B.2 of the 

proposal where we explained the term 

related to a variety of at-risk populations 

(88 FR 5591–92, January 27, 2023) as 

well as section 12.5 of the 2019 ISA 

(U.S. EPA, 2019a) and section 3.3.3 of 

the ISA Supplement (U.S. EPA, 2022a). 

Other commenters oppose the addition 

of the proposed monitoring because 

they feel it would reduce flexibility for 

agencies in deciding where they should 

site monitors, advocating that 

monitoring agencies should be afforded 

maximum flexibility to identify where 

to site monitors for at-risk areas. 

Because the EPA recognizes the 

challenges cited by these commenters 

related to establishing new ambient air 

monitoring stations, the EPA is 

finalizing the modified requirement on 

PM2.5 monitoring network design 

criteria intended to address at-risk 

communities that allows flexibility 

regarding which EJ communities should 

be monitored. Finally, one commenter 

asked that the EPA clarify a specific 

metric to judge how to site monitors in 

at-risk communities. Instead, the EPA 

believes it is appropriate for agencies to 

recommend what they believe to be the 

most important things to consider for 

their sites to meet the PM2.5 network 

design requirements and, thus, applying 

a new metric could take away from local 

priorities for at-risk communities. 

A few commenters asked that the EPA 

require more monitoring than proposed. 

One commenter stated that it would be 

more beneficial to overburdened 

communities if air monitoring were 

required in all at-risk communities. A 

few commenters asked that EPA require 

additional monitoring for attainment of 

PM2.5 NAAQS in EJ communities. In 

response to these comments, the EPA 

supports the SLT agencies’ initiatives to 

conduct additional monitoring beyond 

the minimum monitoring requirements 

and network design criteria. In addition, 

the EPA supports agencies’ use of 

alternative datasets such as sensors and 

sensors networks, satellites, and other 

non-regulatory monitoring where 

appropriate for non-regulatory data 

uses. The EPA notes that many 

monitoring agencies already operate 

more monitoring sites than are 

minimally required, and we expect this 

to continue as agencies consider siting 

monitors in at-risk communities. 

However, the EPA also received 

substantial concerns from monitoring 

agencies about their resource 

constraints, including staffing to 

support any potential new monitoring. 

The EPA also notes that the existing and 

robust network of almost 1,000 PM2.5

sites nationally is designed to continue 

to protect all populations at the level of 

the NAAQS discussed in section II of 

this final action by always having at 

least one site in the area of expected 

maximum concentration for each CBSA 

where monitoring is required. As a 

result of the revisions to the annual 

PM2.5 NAAQS being finalized in this 

action, a small number of new 

monitoring sites will also be required 

under EPA’s current minimum 

monitoring requirements. With the 

monitoring network design changes 

finalized in this rule, many of these 

existing and new sites will form an 

important sub-component of the PM2.5

network by better characterizing air 

quality in at-risk communities, 

particularly with respect to sources of 

concern. 

The EPA concludes that the 

requirements in this final rule for siting 

of monitoring in at-risk communities 

will meaningfully improve the PM2.5

monitoring network and its 

characterization of air quality in at-risk 

communities, without placing 

substantial new resource burdens on 

States and their monitoring agencies 

that would be associated with 

requirements for additional monitoring 

sites. Therefore, the EPA is finalizing 

this part of the proposed action without 

requiring additional monitoring sites 

beyond what would be associated with 

the revised annual PM2.5 NAAQS 

described in section II as they pertain to 

the minimum requirements associated 

with Table D–5 of Appendix D to Part 

58—PM2.5 Minimum Monitoring 

Requirements. 

A few commenters asked that the EPA 

enhance monitoring in smaller cities 

and rural areas. One commenter asked 

for the EPA to extend the proposed 

monitoring network to Micropolitan 

Statistical Areas with populations of 

10,000–50,000 and to rural areas. 

Another commenter pointed out that 

current air quality monitoring networks 

focus on urban and densely populated 

areas; therefore, rural areas are often not 

captured in this existing monitoring 

infrastructure, despite well-documented 

examples of high PM concentration in 

rural communities. The commenter 

believes this results in inadequate 

assessment of air pollution exposures 

for a substantial segment of the U.S. 

population. The EPA disagrees that 

there needs to be additional 

requirements for small CBSA’s and rural 

areas. Regarding these comments, the 

EPA points out that we have a longstanding requirement for each State to 

monitor at background and transport 

sites (40 CFR part 58, appendix D, 

section 4.7.3—Requirement for PM2.5

Background and Transport Sites). Also, 

if an agency deems it appropriate to do 

so, monitoring coverage of rural areas 

can be accomplished with other tools 


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190MSA means a CBSA associated with at least 

one urbanized area of 50,000 population or greater. 

The central-county, plus adjacent counties with a 

high degree of integration, comprise the area. 

such as sensors and sensors networks, 

satellites, and other non-regulatory 

monitoring. Although there may be 

short-term high exposures in rural areas, 

there is no evidence that long-term 

averages are higher in rural areas 

compared to urban areas with 

significantly higher density of 

populations and emissions. For smaller 

cities or rural areas that may have 

concentrations near the level of the 

PM2.5 NAAQS finalized in section II 

above, monitoring agencies are 

encouraged to monitor and address 

emissions as appropriate. 

Some commenters disagree that the 

proposed revision to the PM2.5

monitoring network design criteria to 

address at-risk communities is needed. 

One commenter stated that including an 

EJ factor is not necessary because the 

current network is designed to protect 

all citizens. Another commenter stated 

that EJ factors could be cumbersome to 

implement. Another commenter 

asserted the proposal to add SLAMS in 

at-risk communities with higher PM2.5

concentrations might create more 

granular data and provide for a greater 

margin of safety for those communities 

and monitors in such a way that data 

from those areas could misrepresent the 

larger area represented by the network. 

In response to the comment on the 

current network protecting all citizens, 

the EPA agrees that by measuring in the 

community with the highest 

concentration of PM2.5 we protect other 

citizens; however, as stated in the 

proposal, the EPA believes that adding 

a requirement for sites with an EJ factor 

near sources of concern will enhance 

the overall network to the benefit of all 

citizens. Also, we anticipate that with 

the more protective annual NAAQS 

finalized in section II above, 

characterizing localized air quality 

issues will become even more important 

around local emission sources. As for EJ 

factors being cumbersome to implement, 

the EPA disagrees because there are 

many such locations already operating 

successfully in the current network. 

Regarding the comment that sites in atrisk communities may misrepresent the 

larger area represented by a particular 

network, the EPA notes that pursuant to 

40 CFR part 58, minimally required sites 

in a given network are to represent areawide air quality; therefore, sites in atrisk communities, by definition, would 

be representative of the communities 

within the network in which they are 

sited for the level of protection intended 

under the annual PM2.5 NAAQS. 

In the proposal, the EPA identified 

that, in light of the evidence of 

increased risk to at-risk communities, it 

would be appropriate to better 

characterize exposures for communities 

in proximity to local sources of concern 

(88 FR 5673–76, January 27, 2023). 

Thus, the EPA proposed that enhanced 

networks should include representation 

of at-risk communities living near 

emission sources of concern (e.g., major 

ports, rail yards, airports, industrial 

areas, or major transportation corridors). 

The EPA requested comment on the 

types of sources of concern most 

important to consider. In addition to 

supporting the types of sources the EPA 

identified in the proposal, commenters 

also identified several additional 

localized sources such as railroads, 

stationary sources, transportation 

facilities, and communities with high 

numbers of wood stoves. 

A few commenters suggested the 

inclusion of sources that are often 

considered line and/or area sources, 

e.g., traffic corridors and emissions from 

federally regulated facilities, military 

installations, and national forests. 

Commenters also identified other 

sources usually associated with longrange transport such as smoke from 

wildfire and prescribed fires and longdistance transport of PM, for example 

from Saharan dust and other 

international transport. As explained in 

the proposal, the site with the highest 

expected PM2.5 is already required to 

have a monitor by our long-standing 

requirement that monitors be placed 

‘‘. . . in the area of expected maximum 

concentration’’ (§ 58.1 and appendix D, 

section 4.7.1(b)(1)). The EPA expects 

that both sites with the expected 

maximum concentration and sites 

specifically placed in at-risk 

communities would be impacted by any 

long-range transport in the area. 

Therefore, the EPA believes any 

emphasis on the sources of concern 

should prioritize localized sources, 

including point, area, and line sources 

of concern impacting the at-risk 

community of interest. Therefore, based 

upon the comments, the EPA is 

finalizing a broader example list of 

sources of concern to include localized 

sources such as point sources and 

transportation facilities, since these are 

the most commonly expected additional 

sources of concern. In response to the 

other sources of concern suggested by 

commenters, the EPA notes that while it 

has provided examples, the siting of 

monitors in EJ communities would not 

be limited to these examples. Thus, the 

revised set of examples would include 

‘‘a major industrial area, point source(s), 

port, rail yard, airport, or other 

transportation facility or corridor.’’ In 

finalizing this modified list of examples, 

the EPA is not looking to prioritize one 

type of source category over another; 

rather, we intend to further illustrate the 

types of localized sources of pollution 

that might impact at-risk communities 

such that the siting of monitors nearby 

may be appropriate. 

One commenter noted that the 

proposal may have unintentionally 

taken out the requirement related to 

specific design criteria for PM2.5 in 40 

CFR part 58, appendix D, 4.7.1(b)(3) 

that, for an area with a requirement for 

an additional SLAMS monitor, it should 

‘‘be sited in an area of poor air quality.’’ 

Thus, the language as proposed neither 

requires that such monitors be sited in 

areas of poor air quality, nor does it 

require that the monitor be sited in an 

area that is anticipated to experience 

poor air quality from unspecified (and 

thus potentially relatively insignificant) 

sources in the area. The EPA agrees that 

this was not our intention; the EPA 

wants to protect populations in at-risk 

communities by ensuring they are 

protected by the NAAQS when there are 

sources of concern that may be 

impacting them (i.e., not insignificant 

sources). Thus, the EPA is reinstating 

this requirement in the network design 

language and combining it with the 

examples of the types of localized 

sources of concern: ‘‘For areas with 

additional required SLAMS, a 

monitoring station is to be sited in an atrisk community with poor air quality, 

particularly where there are anticipated 

effects from sources in the area (e.g., a 

major industrial area, point source(s), 

port, rail yard, airport, or other 

transportation facility or corridor).’’ 

To ensure minimally required 

monitoring sites appropriately represent 

exposures in at-risk communities, the 

EPA proposed that sites represent ‘‘areawide’’ air quality near local sources of 

concern (88 FR 5674, January 27, 2023). 

Sites representing ‘‘area-wide’’ air 

quality are those monitors sited at 

neighborhood, urban, and regional 

scales, as well as those monitors sited at 

either micro- or middle-scale that are 

identified as being representative of 

many such locations in the same 

Metropolitan Statistical Area (MSA).190

Most existing—as well as new or moved 

sites—are expected to be neighborhoodscale, which means that the monitoring 

stations would typically represent 

conditions throughout some reasonably 

homogeneous urban sub-region with 

dimensions of a few kilometers per part 

58, appendix D, section 4.7.1(c)(3). 

Additionally, as described in § 58.30, 


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Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16359 

191 Gantt, B. (2022). Analyses of Minimally 

Required PM2.5 Sites Under Alternative NAAQS. 

Memorandum to the Rulemaking Docket for the 

Review of the National Ambient Air Quality 

Standards for Particulate Matter (EPA–HQ–OAR– 

2015–0072). Available at: https://

www.regulations.gov/docket/EPA-HQ-OAR-2015- 

0072. 

192See: https://www.epa.gov/ejscreen. 193See: https://oehha.ca.gov/calenviroscreen. 194See: https://inmap.run/#home. 195Mention of commercial names does not 

constitute EPA endorsement. 196Mention of commercial names does not 

constitute EPA endorsement. 

sites representing ‘‘area-wide’’ air 

quality have a long-standing 

applicability to both the annual and 24- 

hour PM2.5 NAAQS. Our proposed 

requirement for siting monitors in 

communities representing ‘‘area-wide’’ 

air quality is consistent with other 

network design objectives pursuant to 

which we seek to have monitors located 

where people live, work, and play. 

The EPA received a few comments on 

its proposed requirement that minimally 

required sites represent ‘‘area-wide’’ air 

quality. One commenter stated that the 

inclusion of a provision for EJ would 

narrow the location of monitors to 

certain communities that may not best 

represent ‘‘areawide’’ air quality. 

Another commenter asked the EPA to 

consider removing requirements that 

sites be area-wide, since 24-hour and 

annual averaging times would miss 

short, elevated pollution events. A 

couple commenters had concerns with 

the difference in the scale of 

representation between EJ monitors 

using small scale and other NAAQS 

monitors using area-wide scale, in that 

area-wide scale would not protect those 

most at risk. However, another 

commenter agreed with the EPA that 

sites representing at-risk communities 

should represent area-wide air quality. 

In addition to these comments, the EPA 

received many comments with support 

for its proposed modifications to the 

network design criteria as whole. 

Regarding whether narrowing the 

location to certain communities may not 

best represent ‘‘area-wide’’ air quality, 

the EPA notes that sites are either 

identified as being area-wide or not; the 

EPA did not suggest it was seeking a 

best ‘‘area-wide’’ location. In response 

to the comment that area-wide site may 

miss short, elevated pollution events, 

the EPA is aware that there can be local, 

short-term spikes in PM2.5

concentrations. However, the network 

design criteria associated with 

minimally required sites is applicable to 

both the annual and 24-hour PM2.5

NAAQS, and the EPA believes it is 

appropriate to continue to ensure all 

minimally required sites have the most 

utility and remain applicable to both 

forms of the PM2.5 NAAQS. The 

identification of unique micro- and 

middle-scale sites was directed at 

discretionary efforts of any monitoring 

agency, with the recognition that such 

sites, (i.e., relatively unique micro-scale, 

or localized hot spot, or unique middlescale impact sites), are not applicable to 

the annual NAAQS as described in 

§ 58.30—Special consideration for data 

comparison to the NAAQS. 

After considering all the comments on 

this topic, the EPA is finalizing this part 

of the modification to the network 

design criteria to maintain, consistent 

with our long-standing network design 

criteria, that all minimally required sites 

are to represent area-wide air quality. 

In addition to using data from the 

robust network of almost 1,000 PM2.5

sites for NAAQS and AQI purposes, 

having a stable network of long-term 

sites is especially valuable to examine 

trends and to inform long-term health 

and epidemiology studies that support 

reviews of the PM NAAQS. Therefore, 

while we proposed to add a PM2.5

network design criterion to address atrisk communities, many sites are likely 

already in valuable locations meeting 

one of the existing network design 

criteria (i.e., being in an area-wide area 

of expected maximum concentration or 

collocated with near-road sites) and 

supporting multiple monitoring 

objectives. Also, in many communities, 

there may already be sites meeting the 

network design criterion we proposed 

for at-risk communities. Thus, 

acknowledging the value of having longterm data from a consistent set of 

network sites, the EPA believes that 

moving sites should be minimized, 

especially in MSAs with a small number 

of sites. However, because a small 

number of new sites are expected to be 

required due to the existing minimum 

monitoring requirements (40 CFR part 

58, appendix D, Table D–5) 191 and the 

revised primary annual PM2.5 NAAQS 

detailed in section II, and because sites 

occasionally have to be moved—due to, 

for example, loss of access to a site or 

a site no longer meeting siting criteria— 

the EPA believes it is appropriate to 

prioritize establishing sites in at-risk 

communities near sources of concern, 

whenever new sites are established, 

whether because it is a new site or a 

replacement for a prior site that must be 

moved. The EPA accordingly proposed 

that annual monitoring network plans 

(40 CFR 58.10(a)(1)) and 5-year 

assessments (40 CFR 58.10(d)) that 

include any of the few new sites that 

will be required include a commitment 

to examine the ability of existing and 

proposed sites to support air quality 

characterization for areas with at-risk 

populations in the community and the 

objective discussed herein. 

In the proposal, the EPA identified 

that assessing and prioritizing at-risk 

communities for monitoring can be 

accomplished through several 

approaches (88 FR 5675). The most 

critical aspect of prioritizing which 

communities to monitor is their 

representation of the at-risk populations 

described earlier in this section. The 

other major consideration is whether the 

community is near a source or sources 

of concern. While many CBSAs have 

one or more sources of concern 

described above, some CBSAs will not 

have a quantity of emissions from 

sources of concern that result in an 

elevated level of measured PM2.5

concentrations in surrounding 

communities. The siting criteria to be 

‘‘in the area of expected maximum 

concentration,’’ § 58.1 & appendix D, 

section 4.7.1(b)(1) ensures there is a 

monitoring site in the community with 

the highest exposure in each CBSA with 

a monitoring requirement. Some CBSAs 

may also have a requirement to 

collocate a PM2.5 monitor at a near-road 

NO2 station. Therefore, the EPA believes 

that for cases where an additional PM2.5

site is required, we should include a 

criterion that the site be in an at-risk 

community when there are no sources 

of concern identified in that CBSA, or 

such sources do exist but are not 

expected to lead to elevated levels of 

measured PM2.5 concentrations. 

In its proposal, the EPA highlighted 

that tools such as the EPA’s 

EJSCREEN 192 are available to identify 

the at-risk communities intended for 

monitoring as part of the proposed 

revision to the PM2.5 network design 

criteria (88 FR 5675–76, January 27, 

2023). The EPA solicited comment on 

other tools and/or datasets that can be 

utilized to identify at-risk communities. 

In addition to support for using 

EJSCREEN, commenters identified 

several other options to identify at-risk 

communities intended for monitoring as 

part of the proposed revision to the 

PM2.5 network design criteria. Among 

similar tools, one commentor suggesting 

using CalEnviroScreen.193 Commenters 

also identified different options for 

models including InMAP,194 satellitederived models that can be employed to 

help identify EJ communities, and 

hybrid models. A few commenters also 

suggested using sensors and sensor 

networks such as the BlueSky 195 and 

PurpleAir 196 sensors. 

The EPA supports the use of other 

State and local tools designed to help 

identify the at-risk communities that 


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197See: https://fire.airnow.gov/. 

198 Gantt, B. (2022). Analyses of Minimally 

Required PM2.5 Sites Under Alternative NAAQS. 

Memorandum to the Rulemaking Docket for the 

Review of the National Ambient Air Quality 

Standards for Particulate Matter (EPA–HQ–OAR– 

2015–0072). Available at: https://

www.regulations.gov/docket/EPA-HQ-OAR-2015- 

0072. 

should be monitored to meet the revised 

network design criteria. The EPA 

additionally agrees with commenters 

that the use of models as well as sensors 

and sensor networks may be appropriate 

and helpful in identifying the most 

appropriate at-risk communities in 

which to locate monitors. 

For at-risk communities, monitoring 

agencies need data that can best inform 

where there may be elevated levels of 

exposures from sources of concern. 

While we use FRMs and FEMs to 

determine compliance with the NAAQS, 

data from these methods will only be 

available at existing sites. However, 

there are several additional datasets 

available that may be useful in 

evaluating the potential for elevated 

levels of exposure to communities near 

sources of concern. In the proposal, EPA 

identified potential non-regulatory 

monitoring datasets such as CSN, 

IMPROVE, and AQI non-regulatory 

PM2.5 continuous monitors; modeling 

data that utilizes emission inventory 

and meteorological data; emerging 

sensor networks such as those that 

comprise EPA and the USFS’s Fire and 

Smoke Map; 197 and satellites that 

measure radiance and, with 

computational algorithms, can be used 

to estimate PM2.5 from aerosol optical 

depth (AOD) (88 FR 5675–76, January 

27, 2023). The EPA solicited comment 

on datasets most useful to identify 

communities with high exposures for 

PM2.5 NAAQS (i.e., annual or 24-hour). 

In addition to providing information 

about datasets that can inform the 

NAAQS comparison, commenters 

additionally identified several types of 

datasets that may be useful to identify 

where there may be elevated levels of 

exposures from sources of concern. 

These datasets include satellite 

measurements, sensors, and sensor 

network data, which may all be useful 

to find hot spots in communities. 

Commenters also identified EJScreen 

and CalEnviroScreen, which are 

screening and mapping tools that utilize 

several datasets. Another commenter 

stated that to better understand 

exposure differences in disadvantaged 

communities, shorter measurement 

intervals should be measured and 

reported. 

In considering the datasets identified 

in the proposal as well as the ones 

commenters provided, the EPA believes 

all the datasets have value to help 

inform where there may be elevated 

levels of exposures from sources of 

concern. However, each of them may 

also have limitations and, therefore, 

users should be careful not to rely solely 

on one dataset versus another for all 

purposes. Fortunately, many of the 

available datasets are becoming easier to 

work with and more accessible, which 

will allow interested parties and 

monitoring agencies the opportunity to 

efficiently review the datasets and 

determine best applicability. For all of 

these reasons, the EPA is not finalizing 

a requirement to use a specific dataset 

or tool to identify at risk communities; 

however, whatever datasets a 

monitoring agency elects to use, its plan 

to use such data for purposes of meeting 

the network design requirements will be 

subject to EPA approval as part of the 

40 CFR 58.10 annual monitoring 

network plan. Regarding the comment 

recommending shorter measurement 

intervals in measuring and reporting 

data to better understand exposure 

differences in disadvantaged 

communities, the EPA agrees and 

generally supports use of continuous 

methods. While we generally support 

use of continuous methods, approved 

filter-based technologies and methods 

also provide valuable air quality 

information. Therefore, the EPA is not 

requiring the use of automated 

continuous methods beyond what is 

already required in 40 CFR part 58, 

appendix D, section 4.7.2—Requirement 

for Continuous PM2.5 Monitoring. 

The monitoring methods appropriate 

for use at required PM2.5 sites in at-risk 

communities are FRMs and automated 

continuous FEMs (88 FR 5675–76, 

January 27, 2023). These are the 

methods eligible to compare to the PM2.5

NAAQS, which is the primary objective 

for collecting this data. There are several 

other monitoring objectives that would 

benefit from the use of automated 

continuous FEMs. For example, having 

hourly data available from automated 

continuous FEMs would allow sites to 

provide data in near-real time to support 

forecasting and near real-time reporting 

of the AQI. Automated continuous 

methods are also useful to support 

evaluation of other methods such as 

low-cost sensors. When used in 

combination with on-site wind speed 

and wind direction measurements, 

automated FEMs can provide useful 

pollution roses, which help in 

identifying the origin of emissions that 

affect a community. Additionally, when 

collocated with continuous carbon 

methods such as an aethalometer, 

automated FEMs can help identify 

potential local carbon sources 

contributing to increased exposure in 

the community. While either FRMs or 

automated FEMs may be used at a site 

for comparison to the PM2.5 NAAQS, the 

EPA supports use of automated 

continuous FEMs at sites in at-risk 

communities. 

The EPA requested comment on the 

measurement methods most useful to 

collocate with PM2.5 in at-risk 

communities (88 FR 5675–76, January 

27, 2023), and a few commenters 

provided input. One commenter 

recommended that the EPA should 

employ supplemental technologies and 

systems to increase coverage of the 

regulatory monitoring network and 

obtain more complete data to further 

protect public health and address 

environmental injustice in air pollution 

exposure. Another commenter 

recommended that the EPA invest in 

community-led monitoring and mobile 

air quality monitoring with a goal of 

recording block-level variabilities in 

data. And another commenter cited the 

value of community-deployed PM2.5

monitoring. 

The EPA appreciates the comments 

provided on the measurement methods 

most useful to collocate with PM2.5

monitoring sites in at-risk communities. 

Because the use of methods beyond the 

required PM2.5 FRMs or FEMs or other 

criteria pollutant measurements meeting 

a NAAQS monitoring requirement is 

voluntary, the establishment of PM2.5

NAAQS comparable sites in at-risk 

communities will allow for 

collaboration at multiple levels. The 

EPA strongly encourages such 

collaboration with impacted 

communities, and the measurement 

methods discussed here should be 

considered for use as appropriate. 

In the proposal, the EPA identified 

that, to meet the revised network design 

criteria, there will be only a few new 

sites required,198 plus any potentially 

moved sites in cases where an existing 

site lease is lost or otherwise requires 

relocation (88 FR 5675–76, January 27, 

2023). To handle these new or relocated 

sites, the EPA proposed to build upon 

our existing regulatory process for 

selecting and approving these sites 

under 40 CFR 58.10 (88 FR 5676, 

January 27, 2023). In the proposal, we 

stated it would be appropriate to 

provide at least 12 months from the 

effective date of the final rule to allow 

monitoring agencies to initiate planning 

to implement these measures by seeking 

input from communities and other 

interested parties and considering 

whether to revise their PM2.5 networks 


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or explain how their existing networks 

meet the objectives of the proposed 

modification to the network design 

criteria. Thus, the EPA proposed that 

monitoring agencies should address 

their approach to the question of 

whether any new or moved sites are 

needed and identify the potential 

communities in which the agencies are 

considering adding monitoring, if 

applicable, as well as identifying how 

they intend to meet the revised criteria 

for PM2.5 network design to address atrisk communities in the agencies’ 

annual monitoring network plans due to 

each applicable EPA Regional office no 

later than July 1, 2024 (see 40 CFR 

58.10). Specifics on the resulting new or 

moved sites for PM2.5 network design to 

address at-risk communities were 

proposed to be detailed in the annual 

monitoring network plans due to each 

applicable EPA Regional office no later 

than July 1, 2025 (40 CFR 58.10). The 

EPA proposed that any new or moved 

sites would be required to be 

implemented and fully operational no 

later than 24 months from the date of 

approval of a plan or January 1, 2027, 

whichever comes first, but the EPA 

solicited comment on whether less time 

is needed (e.g., 12 months from plan 

approval and/or January 1, 2026). 

The EPA received a few comments on 

its proposed timeline for monitoring 

agencies to identify, propose, and 

ultimately bring any new or moved sites 

online. One commenter asked that the 

timeline give states more time to start or 

move sites. A few commenters asked 

that the EPA only require meeting a 

timeline for identifying whether any 

new or moved sites are needed after the 

EPA has provided the monitoring 

agencies with guidance on the priority 

of the potential at-risk communities. 

One of those commenters further 

requests that the EPA allow at least 24 

months from the date of approval of a 

§ 58.10 monitoring plan identifying any 

relocation of monitoring sites or 

establishment of new monitoring sites to 

implement any changes to the network, 

citing the need for more time to work 

with local officials, procure monitoring 

equipment, and contract for services, all 

of which can cause significant delays in 

establishing a monitoring site. Another 

commenter asked that the EPA remain 

attentive to the challenges that States, 

and air agencies face regarding 

recruiting and retaining the specialized 

staff needed to support their existing 

regulatory monitoring networks and the 

capital resources needed to implement 

and sustain new monitoring stations in 

areas that are clearly meeting the 

existing PM NAAQS or any revised PM 

NAAQS. Another commenter stated that 

the July 1, 2024, timeline for a network 

evaluation this complex is insufficient, 

noting that they submit their draft 

annual monitoring network plan for 

public review and comment in midApril for 30 days. Because the final plan 

is due July 1 and must include all 

comments and responses and describe 

any changes based on those comments, 

the timeline does not take these 

requirements into consideration by 

allowing for the more extensive 

assessment of changes that may be 

needed to meet the proposed new 

monitoring requirements. The 

commenter stated that it would be 

appropriate to provide at least 12 

months from the effective date of this 

final rule for monitoring agencies to 

initiate planning to implement these 

measures, seek input, consider revisions 

to their PM2.5 networks, and explain 

how their existing networks meets the 

objectives of the final rule. The 

commenter notes that that SLT agencies 

should be provided a minimum of 18 

months after the final recommendation 

is published to add this information to 

their § 58.10 annual monitoring network 

plans. Another commenter encourages 

the EPA to retain the proposed deadline 

for any newly required monitoring 

stations in at-risk communities to be 

operational (i.e., 24 months after the 

July 2025 network plan approval or 

January 1, 2027, whichever is earlier). 

While the need for this data is urgent, 

the commenter stated that the process 

for procuring instrumentation, securing 

leases, and building permits, and other 

logistics in constructing new monitoring 

sites can take a significant amount of 

time, some of which are outside of 

agencies’ control. 

As stated earlier, the EPA received 

strong support for our proposal to 

modify the PM2.5 monitoring network 

design criteria to include an EJ factor 

that accounts for proximity of 

populations at increased risk of adverse 

health effects from PM2.5 exposures to 

sources of concern from a wide range of 

commenters. A few commenters support 

the timeline proposed, a few others 

support starting any new or moved sites 

sooner than proposed, while other 

commenters asked for more time or 

offered conditions regarding how to 

establish an appropriate timeline. 

The EPA disagrees with the 

commenter that suggested the EPA 

should only require agencies to meet a 

timeline to identify whether any new or 

moved sites are needed after the EPA 

has provided the monitoring agencies 

with guidance on the priority of the 

potential at-risk communities, because 

the regulatory text provides all the 

guidance required for agencies to begin 

this process. As we explained above, the 

EPA does not anticipate that many new 

or moved sites will be required based on 

the final rule because we think most 

sites are already in suitable locations 

and long-term sites are highly valued. 

Also, monitoring agencies have 

discretion to provide to the EPA their 

recommendations regarding how they 

intend to meet the modifications to the 

PM2.5 monitoring network design 

criteria to include an EJ factor that 

accounts for proximity of populations at 

increased risk of adverse health effects 

from PM2.5 exposures to sources of 

concern. Overall, the EPA believes that 

having sites in the areas of expected 

maximum concentrations will best 

ensure that all communities are 

protected. Since there may be multiple 

choices for sites in EJ areas near sources 

of concern, the EPA acknowledges that 

there may be many locations that can 

meet the revised PM2.5 network design 

criteria. While, as we explained earlier, 

we want such sites to also be in areas 

of poor air quality, the sites in the area 

of maximum concentration will ensure 

that all communities are protected, there 

can be more flexibility afforded in the 

selection amongst at-risk communities 

to meet the revised requirements, since 

any alternative at-risk communities 

would already be protected. 

The EPA considered both the 

concerns and support for the timeline 

proposed and clarifies that the 

component of the proposed requirement 

regarding the need to identify potential 

new sites or an intention to move sites 

to be included in the annual monitoring 

network plan due to EPA on July 1, 

2024, would be satisfied with a 

statement of intent to pursue a new site 

per the revised network design criteria 

and in consideration of the minimum 

monitoring requirements. While 

monitoring agencies may provide as 

much detail as they deem appropriate 

regarding the revised PM2.5 network 

design criteria in their annual 

monitoring network plans due on July 1, 

2024, there is no expectation that any 

details on site-specific information 

would be included at that stage. We 

encourage agencies to provide their 

initial thinking on the communities they 

are most interested in monitoring 

pursuant to the revised network design 

criteria. Therefore, the EPA is finalizing 

the timeline as proposed, including the 

provision that monitoring agencies 

report their intention to add or move 

sites, where required, in their annual 

monitoring network plans due to each 

applicable EPA Regional office no later 

than July 1, 2024 (40 CFR 58.10). The 


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monitoring agencies will then provide 

specifics on any new or moved sites for 

PM2.5 network design to address at-risk 

communities in the annual monitoring 

network plans due to each applicable 

EPA Regional office no later than July 1, 

2025 (40 CFR 58.10). And any new or 

moved sites shall be implemented and 

fully operational no later than 24 

months from the date of approval of a 

§ 58.10 plan, or January 1, 2027, 

whichever comes first. 

In summary, the EPA is finalizing 

modifications to the PM2.5 network 

design criteria to include an EJ factor to 

address at-risk communities with a 

focus on exposures from sources of 

concern in areas of poor air quality. 

While this modification to the PM2.5

network design requires sites to be 

located in at-risk communities, 

particularly those whose air quality is 

potentially affected by local sources of 

concern, such sites must still meet the 

requirement for being considered ‘‘areawide’’ air quality. In finalizing this 

modification to the PM2.5 network 

design requirement, the EPA is making 

two changes in the final rule response 

to the comments received. First, the 

EPA is broadening our examples of 

‘‘sources of concern’’ to include 

localized sources such as point sources 

and major transportation facilities or 

corridors. Second, the EPA is reinstating 

‘‘poor air quality’’ in our requirement 

for the modified network design criteria, 

meaning the revised PM2.5 network 

design requirement now states: ‘‘For 

areas with additional required SLAMS, 

a monitoring station is to be sited in an 

at-risk community with poor air quality, 

particularly where there are anticipated 

effects from sources in the area (e.g., a 

major industrial area, point source(s), 

port, rail yard, airport, or other 

transportation facility or corridor).’’ All 

other aspects of the PM2.5 network 

design requirements are being finalized 

as proposed. 

5. Revisions to Probe and Monitoring 

Path Siting Criteria 

The EPA proposed changes to 

monitoring requirements in the 

Appendix E—Probe and Monitoring 

Path Siting Criteria for Ambient Air 

Quality Monitoring (88 FR 5676–78, 

January 27, 2023). Since 2006, the EPA 

finalized multiple rule revisions to 

establish siting requirements for 

PM10–2.5 and O3 monitoring sites (71 FR 

2748, January 17, 2006), Near-Road NO2

monitoring sites (75 FR 6535, February 

9, 2010), Near-Road CO monitoring sites 

(76 FR 54342, August 31, 2011), and 

Near-Road PM2.5 monitoring sites (78 FR 

3285, January 15, 2013). Through these 

previous revisions to the regulatory text, 

some requirements were inadvertently 

omitted, and, over time, the clarity of 

this appendix was reduced through 

those omissions that, in a few instances, 

led to unintended and conflicting 

regulatory requirements. The EPA 

proposed to reinstate portions of 

previous Probe and Monitoring Path 

Siting Criteria Requirements from 

previous rulemakings, where 

appropriate, to restore the original 

intent. 

The EPA only received a few 

comments on the proposed rulemaking 

pertaining to the proposed changes 

regarding probe and monitoring path 

siting criteria for ambient air quality 

monitoring, most of which were 

supportive of the proposed revisions. 

One commenter noted that the image for 

Figure E–1 in Appendix E to part 58 was 

distorted and of extremely poor quality, 

rendering the text in places almost 

unreadable (88 FR 5712, January 27, 

2023). The EPA makes several 

references to Figure E–1, which 

provides detailed information needed 

for assessing a range of acceptable probe 

distances from roadways based on a 

monitor’s spatial scale. The commenter 

also stated that a higher quality image 

is needed for the figure so that agencies 

can fully interpret the figure to the 

extent that EPA requires. The EPA 

agrees with the commenter that a higher 

quality image for Figure E–1 is 

important and needed. Based on this 

comment, the EPA is finalizing the 

revision to Figure E–1 to clearly 

communicate the requirements of 

appendix E. 

The EPA is revising appendix E in its 

entirety as proposed (88 FR 5709–5717, 

January 27, 2023) for clarity and as 

described in detail below. 

a. Separate Section for Open Path 

Monitoring Requirements 

The EPA proposed to relocate all open 

path monitor siting criteria 

requirements to a separate section in 

appendix E from those requirements for 

siting samplers and monitors that utilize 

probe inlets (88 FR 5676, January 27, 

2023). Separate sections for these 

distinct monitoring method types allows 

the EPA to more clearly articulate 

minimum technical siting requirements 

for each. 

The EPA received one supportive 

comment to adopt this change and 

received no adverse comments. Another 

commenter stated the regulatory text of 

the proposal improves the clarity of the 

appendix but encouraged the EPA to 

break the summary tables down further 

into more manageable components 

(perhaps by pollutant). The commenter 

stated that summary tables for the 

proposed appendix continue to be a 

‘‘jumbled mess of regulatory 

requirements.’’ The EPA agrees that the 

summary tables E–3 and E–6 in the 

proposal could be improved further. 

Also, the EPA found that footnote 3 of 

Table E–6 in the proposed rule was 

incomplete and corrected this editorial 

error. 

Therefore, the EPA is making editorial 

changes to both summary tables E–3 and 

E–6 and finalizing the remainder of the 

language as proposed with the open 

path monitor siting criteria 

requirements placed into a separate 

section of the appendix. 

b. Distance Precision for Spacing Offsets 

The EPA proposed to require that 

when rounding is performed to assess 

compliance with these siting 

requirements, the distance 

measurements will be rounded such as 

to retain at least two significant figures 

(88 FR 5676, January 27, 2023). The EPA 

proposed to communicate this rounding 

requirement in the regulatory text using 

footnotes in the tables of this appendix. 

The EPA received two supportive 

comments and no adverse comments 

regarding this proposed change. While 

supportive of the proposal, one of the 

two supporting comments suggested it 

would be clearer if EPA explicitly 

defined a decimal in the distance values 

and round to the nearest tenths place for 

these assessments. The EPA disagrees 

with this recommendation because in 

some cases it would be more restrictive 

and burdensome than the proposed 

requirement that was intended to 

provide both clarity and flexibility. 

Therefore, the EPA is finalizing the 

language as proposed. 

c. Summary Table of Probe Siting 

Criteria 

The EPA proposed to provide 

additional specificity and flexibility to 

the summary table for probe siting 

criteria by changing the ‘‘>’’ (greater 

than) symbols to ‘‘≥’’ (greater than or 

equal to) symbols in the summary table 

E–4 (88 FR 5676, January 27, 2023). 

Because one commenter pointed out to 

the EPA that in the prior version of the 

rule there was no table E–4, as a clerical 

matter, we have renumbered this 

summary table to table E–3 in the final 

rule. This proposed minor revision to 

the summary table more clearly 

expresses the EPA’s intent that the 

distance offsets provided in the 

summary tables in appendix E are 

acceptable for NAAQS compliance 

monitoring. 

The EPA received one comment 

supporting the proposal. The EPA 

received no adverse comments. Because 


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one commenter pointed out to the EPA 

that in the prior version of the rule there 

was no table E–4, as a clerical matter, 

we have renumbered this summary table 

to table E–3 in the final rule. Therefore, 

the EPA is updating the table numbering 

and otherwise finalizing the tables as 

proposed. 

d. Spacing From Minor Sources 

The EPA proposed to clarify and 

provide flexibility regarding siting 

monitors near minor sources by 

changing a requirement to a goal (88 FR 

5676–77, January 27, 2023). To 

accomplish this, the EPA proposed to 

replace the ‘‘must’’ in the regulation 

with a ‘‘should.’’ While the EPA 

proposed to change this requirement to 

a goal, the EPA reiterated in the 

proposal that it recommends that sites 

with minor sources be avoided 

whenever practicable and probe inlets 

should be spaced as far from minor 

sources as possible when alternative 

monitoring stations are not suitable. 

The EPA received one comment 

supporting the proposed revision and 

received no adverse comments. 

Therefore, the EPA is finalizing the 

language as proposed. 

e. Spacing From Obstructions and Trees 

The EPA proposed to clarify and 

redefine that the minimum arc required 

to be free of obstructions for a probe 

inlet or monitoring path is 270-degrees 

and that probe inlets must be no closer 

than 10-meters to the driplines of any 

trees (88 FR 5677, January 27, 2023). 

These changes were proposed because 

of inconsistencies introduced into the 

rule with the 2006 rulemaking. Both are 

discussed in more detail in the 

following sections. 

The majority of comments received 

were supportive of these proposed siting 

amendments and clarifications. Two 

commenters were not supportive of this 

proposal. One adverse comment focused 

on the potential that site modifications 

would be required if the minimum arc 

required to be free of obstructions for a 

probe inlet is 270-degrees. The second 

adverse comment pertained to the 

proposal to clarify distance 

requirements from tree driplines. The 

commenter stated they would expect 

significant challenges in meeting the 

proposed 20-meter tree dripline 

distance. This comment is not a 

substantive negative comment because 

the 20-meter distance provided in the 

proposal is a goal and not a 

requirement. As such, monitoring 

organizations should not expect 

additional challenges in meeting the 

probe siting requirements. One 

supportive commenter on the 270- 

degree minimum arc proposal also 

requested that the EPA acknowledge 

that some cases exist where monitoring 

is desired or necessary to protect the 

public health, but siting criteria cannot 

be met. 

Based on the only two negative 

comments received from monitoring 

agencies or organizations, one of which 

was not substantive, the EPA believes 

most sites already meet these proposed 

requirements related to the arc and 

distance from dripline. However, the 

EPA also acknowledges that there may 

be limited cases where this proposed 

revision may require site modifications, 

and some sites may not be able to be 

achieve the proposed siting 

requirements, even with modifications 

to the site. For cases where long-term 

trend sites or monitors that determine 

the design value for their area cannot 

reasonably meet these regulatory siting 

requirements, the EPA encourages 

monitoring organizations to work with 

their respective EPA Regional offices to 

determine if a waiver from this siting 

criteria would be appropriate under 

appendix E, section 10. 

These siting requirements are 

discussed in more detail below in 

sections VII.B.5.f and VII.B.5.h. 

f. Reinstating Minimum 270-Degree Arc 

and Clarified 180-Degree Arc 

The EPA proposed to correct 

identified inconsistencies in the 270- 

degree requirement for unrestricted 

airflow to the probe inlet by reinstating 

the requirement stated in appendix E, 

paragraph 4(b), and to clarify that the 

continuous 180-degree minimum arc of 

unrestricted airflow provision is 

reserved for monitors sited on the side 

of a building or wall to comply with 

network design criteria requirements 

specified in appendix D of part 58 (88 

FR 5677, January 27, 2023). 

The EPA received two comments 

regarding this proposal, with one being 

supportive and one being negative. The 

adverse comment focused on the 

potential that site modifications would 

be required if this revision was made. 

The commenter supporting the proposal 

also requested that the EPA 

acknowledge that some cases exist 

where monitoring is desired or 

necessary to protect the public health, 

but siting criteria cannot be met. The 

EPA agrees with both commenters and 

acknowledges that there does exist 

limited cases where this proposal would 

require site modifications and some 

sites may not be able to be achieve the 

proposed siting requirement even with 

modifications to the site. For these 

cases, and especially when long-term 

trend sites or monitors that determine 

the design value for their area cannot 

reasonably meet these regulatory siting 

requirements, the EPA encourages 

monitoring organizations to work with 

their respective EPA Regional Offices to 

determine if a waiver from this siting 

criteria is appropriate through the 

provisions found in Section 10 of this 

appendix. 

Based on the EPA only receiving a 

single negative comment regard the 270- 

degree and 180-degree provisions the 

EPA thinks most sites already meet 

these proposed requirements. 

Additionally, as stated above, the EPA 

is also retaining waiver provisions from 

these siting requirements for the 

remaining cases that can be exercised 

when appropriate. Therefore, the EPA is 

finalizing the language as proposed. 

g. Obstacles That Act as Obstructions 

The EPA proposed to clarify the 

definitions of ‘‘obstructions’’ and 

‘‘obstacles’’ in the regulatory text (88 FR 

5677, January 27, 2023). Stating that, 

‘‘[o]bstructions to the air flow of the 

probe inlet are those obstacles that are 

horizontally closer than twice the 

vertical distance the obstacle protrudes 

above the probe inlet and can be 

reasonably thought to scavenge reactive 

gases or to restrict the airflow for any 

pollutant,’’ the EPA proposed to 

reiterate that the EPA does not generally 

consider objects or obstacles such as flag 

poles or site towers used for NOy

convertors and meteorological sensors, 

etc., to be deemed obstructions. 

The EPA received one comment 

supporting the proposal and received no 

adverse comments. Therefore, the EPA 

is finalizing the definitions as proposed. 

h. 10-Meter Tree Dripline Requirement 

The EPA proposed to reconcile the 

conflicting requirements in 5(a) and the 

prior table E–4 footnote 3 by clarifying 

that the probe inlet must always be no 

closer than 10 meters to the tree dripline 

(88 FR 5677, January 27, 2023). The EPA 

also proposed to reinstate the goal ‘‘that 

monitor probe inlets should be at least 

20-meters from the driplines of trees,’’ a 

goal that was inadvertently omitted 

during previous rule revisions. In 

addition, the EPA proposed to clarify 

that if a tree or group of trees is 

considered an ‘‘obstruction,’’ section 

4(a) will apply. 

As described above, the majority of 

comments received were supportive of 

the EPA proposed amendments and 

clarification, with two commenters 

focused on the possibility that 

monitoring agencies may not be able to 

meet the revised siting requirements. 

Specific to the proposed dripline 

requirement, the EPA reiterates that the 


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199Available at https://fire.airnow.gov/. 

20-meter tree dripline offset is not a 

requirement, but rather a goal. 

Monitoring programs should as much as 

practicable attempt to meet this 20- 

meter tree dripline offset goal but are 

only required to be at least 10 meters 

removed from tree driplines. If these 

requirements cannot be met, the EPA 

encourages monitoring organizations to 

contact their respective EPA Regional 

offices to determine if a waiver from this 

siting criteria would be appropriate 

under appendix E, section 10. 

Another commenter recommended 

that the proposal should also include an 

elevation specification. For instance, if 

a monitor is on the roof of a shelter, a 

tree below that roof should not be 

considered an obstruction no matter the 

distance to the dripline. The EPA 

considers this scenario to occur in 

practice only rarely. The EPA agrees 

that when the overall tree height is less 

than the height of the probe inlet, the 

tree is not obstructing the airflow to the 

probe inlet. However, a tree in such 

proximity to the probe inlet in many 

cases is not likely to remain at a height 

lower than the probe inlet. The EPA 

considers a scenario such as this to be 

best addressed in the waiver provisions 

of this appendix due both to the rarity 

of this occurring as well as the need for 

the EPA to periodically reassess 

whether tree growth has adversely 

impacted the site conditions. 

For these reasons, the EPA is 

finalizing the language as proposed. 

i. Spacing Requirement for Microscale 

Monitoring 

The EPA proposed to require that 

microscale sites for any pollutant shall 

have no trees or shrubs blocking the 

line-of-sight fetch between the monitor’s 

probe inlet and the source under 

investigation (88 FR 5677, January 27, 

2023). This proposed revision would 

bring consistency between near-road 

monitoring stations and other 

microscale monitoring. 

The EPA received one comment on 

this proposed requirement expressing 

concerns regarding its practicality and 

legality. The commenter stated agencies 

may at times want to site a monitor 

close to a source, but the closest 

location will have trees in the line of 

sight on private property. Additionally, 

in some cases, the trees may have been 

planted for the purpose of reducing offproperty emissions from a source such 

as a Concentrated Animal Feeding 

Operation (CAFO). The commenter 

further stated that the proposal 

mandates that State agencies order the 

removal of trees from private property to 

collect valid data. 

The EPA disagrees that the proposed 

requirement is impractical or unlawful. 

The proposed requirement would not 

require, mandate, or otherwise empower 

monitoring agencies to force the 

removal of trees on private property. 

The EPA agrees with the commenter 

that trees may at times be planted as 

part of control strategies to reduce 

offsite emissions and thus protect the 

public, but the EPA disagrees with the 

commenter that the trees must be 

removed to perform ambient air 

monitoring in these locations. Rather, if 

trees or shrubs block the line-of-sight 

fetch between the monitor’s probe inlet 

and the source under investigation, it is 

the EPA’s position that, for most cases, 

a microscale designation does not 

accurately reflect the monitoring scale 

for this location, and instead the EPA 

would recommend that the monitoring 

scale be designated to a more 

representative monitoring scale such as 

middle scale or neighborhood scale. 

Moreover, for cases where long-term 

trend sites or monitors that determine 

the design value for an area cannot 

reasonably meet this regulatory siting 

requirement, the EPA encourages 

monitoring organizations to work with 

their respective EPA Regional offices to 

determine if a waiver from this siting 

criteria may be appropriate under 

appendix E, section 10. 

For these reasons, the EPA is 

finalizing the language as proposed. 

j. Waiver Provisions 

The EPA proposed to maintain the 

appendix E, section 10 waiver 

provisions in the current regulation for 

siting criteria, but to modify section 10.3 

to require that waivers from the probesiting criteria must be reevaluated and 

renewed minimally every 5 years (88 FR 

5677–78, January 27, 2023). 

The EPA received one comment 

supporting the proposal and no adverse 

comments. Therefore, the EPA is 

finalizing the language as proposed. 

k. Acceptable Probe Materials 

The EPA proposed to expand the list 

of acceptable probe materials for 

sampling reactive gases in appendix E, 

section 9, from just borosilicate glass 

and fluorinated ethylene propylene 

(FEP) Teflon®, or their equivalents. The 

EPA proposed to add polyvinylidene 

fluoride (PVDF), also known as Kynar®, 

polytetrafluoroethylene (PTFE), and 

perfluoroalkoxy (PFA) to the list of 

approved materials for efficiently 

transporting gaseous criteria pollutants, 

and the use of NafionTM upstream of 

ozone analyzers (88 FR 5678, January 

27, 2023). Mention of trade names or 

commercial products does not 

constitute endorsement. 

The EPA received two comments 

supporting the proposal and received no 

adverse comments. Therefore, the EPA 

is finalizing the language as proposed. 

D. Incorporating Data From Next 

Generation Technologies 

In the proposal, the EPA requested 

comment on how to incorporate data 

from next generation technologies into 

Agency efforts (88 FR 5678–80, January 

27, 2023). The near real-time integration 

of data from PM2.5 continuous monitors, 

sensors, and satellites has allowed the 

EPA to use data in certain informational 

applications such as EPA and USFS’s 

Fire and Smoke Map.199 This mapping 

product uses Application Program 

Interfaces (APIs) where data sets are 

automatically shared on prespecified 

computer servers. Given the success of 

the Fire and Smoke Map, the EPA 

indicated interest in exploring the use of 

next-generation technologies to develop 

additional approaches, products, and 

applications to help address important 

non-regulatory air quality data needs. 

Therefore, the EPA solicited comment 

on the most important data uses and 

data sets to consider in such future 

initiatives. Such approaches and/or 

products could utilize historical or near 

real-time data. The EPA sought this 

input and prioritization on use of next 

generation technologies to help improve 

the utility of data to better support air 

quality management to improve public 

health and the environment. 

The EPA received comments from 

about two dozen entities on its request 

for comments on how to incorporate 

data from next generation technologies. 

The entities that provided comment 

included federal agencies; 

representatives of industry and industry 

groups; public health, medical, and 

environmental organizations; State, 

local and related multi-state 

organizations involved in air program 

management; Tribes and Tribal 

organizations involved in air program 

management; and other State and local 

governments. 

While there were some differences 

across commenters, a majority of the 

commenters support use of next 

generation data for non-regulatory 

purposes, but not for regulatory decision 

making due to their inherent 

uncertainties and limitations. The EPA 

also received comments from some 

environmental organizations support 

using alternative data for regulatory 

decision making. 


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Many commenters pointed out that 

they are already successfully using 

sensor data and networks in 

supplemental and informational 

applications and support further 

expansion of these capabilities. Across 

many commenters, there was support 

for using next generation data as ‘‘fit for 

purpose,’’ filling in gaps, finding hot 

spots, identifying and addressing EJ 

concerns, and evaluating and informing 

network siting. The EPA acknowledges 

the successful examples of sensor data 

and networks for non-regulatory 

purposes. A few commenters support 

expanding the use of sensor data to 

provide real-time AQI; the EPA is 

interested in this use of next generation 

data as well. A few commenters pointed 

the need for the EPA to work closely 

with them and their communities to 

understand and use next generation 

data, while others expressed a desire for 

help developing best practices around 

collecting and using next generation 

data, developing products with data 

analysis/visualization, and developing 

appropriate QA/QC for sensor data. The 

EPA acknowledges each of these 

requests and expects to continue to 

work closely with SLTs and other 

stakeholders to understand and develop 

information on the collection and use of 

next generation data. 

A few commenters offered more 

detailed comments. Some recommended 

that the EPA repropose implementation 

provisions related to next generation 

technologies with greater clarity to 

provide for meaningful comment. For 

example, the use of low-cost sensor and 

satellite data could be used in drawing 

nonattainment area boundaries or 

identifying sources for emissions 

control, but doing so would be such a 

significant change from prior EPA 

policy that it warrants a more specific 

proposal, beyond the scope of this 

request for comment. In response to this 

comment, the EPA notes it did not 

propose or change the use of nonregulatory measurement data as part of 

this proposal, but instead opened an 

opportunity to comment about the use 

of next generation technologies. 

Another commenter stated that while 

low-cost sensor data can be invaluable 

for some purposes, the potentially 

overwhelming amount of data produced 

by sensors may present additional 

challenges to communities without the 

resources or expertise to analyze it. Cost 

is another concern associated with some 

next generation technologies of which 

some communities may not be aware, as 

the initial cost of the sensor alone is not 

indicative of the total cost of operation, 

which can include costs of internet 

access and servers. The EPA appreciates 

the need to consider all the costs of 

implementing and maintaining sensor 

data. 

Another commenter stated that having 

a dense sensor network collocated with 

FRMs and FEMs could help ensure 

timely maintenance of the regulatory 

measurements in the event there 

appears to be a divergence of data. The 

EPA appreciates the comment that 

emphasizes how sensors could be used 

to complement the FRM and FEM data 

with regard to ensuring timely 

maintenance. 

Another commentor strongly opposes 

incorporating sensor data into any EPA 

systems unless robust quality assurance 

(QA) practices are widely established 

and managed by qualified personnel. 

The EPA agrees that QA is necessary, 

and notes that the ‘‘fit for purpose’’ 

aspect of using sensor data will inform 

the appropriate QA associated with the 

intended use of such data. 

In summary, the EPA invited 

comment on how we should consider 

incorporating data from next generation 

technologies into our air monitoring 

efforts. In seeking comment on this 

topic, the EPA did not propose to add, 

edit, or delete any regulatory language 

associated with the PM NAAQS. The 

EPA received comments from a variety 

of entities that largely support using 

next generation data for a variety of 

purposes that supplement, but cannot 

replace, the measurement data from 

monitoring methods required (i.e., 

FRMs and FEMs) for regulatory decision 

making. Across many commenters, there 

was support for using next generation 

technologies and data as ‘‘fit for 

purpose,’’ filling in gaps, finding hot 

spots, identifying, and addressing EJ 

concerns, and evaluating and informing 

network siting. Quality assurance of the 

data will be an important component in 

the use of next generation technology 

data. The EPA will consider these 

comments as it continues its work with 

the co-regulated community comprised 

of SLT agencies and other stakeholders 

to understand and use next generation 

data and joint efforts to manage the 

nation’s ambient air. 

VIII. Clean Air Act Implementation 

Requirements for the Revised Primary 

Annual PM2.5 NAAQS 

The EPA’s revision to the primary 

annual PM2.5 NAAQS discussed in 

section II above triggers a number of 

implementation related activities that 

were described in the NPRM. The two 

most immediate implementation 

impacts following a final new or revised 

NAAQS are related to stationary source 

permitting and the initial area 

designations process. Permitting 

implications are discussed below in 

section VIII.E. With regard to initial area 

designations, the EPA is separately 

issuing a memorandum regarding the 

Initial Area Designations for the Revised 

Primary Annual Fine Particle National 

Ambient Air Quality Standard 

Memorandum (the ‘‘Annual PM2.5

NAAQS Designations Memorandum’’) 

that will provide information about the 

statutory schedule for the designations 

process. For other implementation 

related implications, please refer back to 

the NPRM section VIII. 

The NPRM also referred to the PM2.5

State Implementation Plan (SIP) 

Requirements Rule (81 FR 58010, 

August 24, 2016), which specifies 

planning requirements for areas 

designated as nonattainment for 

purposes of the PM2.5 NAAQS and 

includes a number of key 

recommendations for areas to consider 

implications of environmental justice 

through the attainment planning 

process, consistent with the 

identification of at-risk groups in the 

2019 ISA and ISA Supplement and the 

statutory requirement to protect the 

health of at-risk groups. As stated in the 

NPRM, State and local air agencies are 

encouraged to consider how they might 

develop implementation plans that 

encourage early emission reductions. 

A. Designation of Areas 

As discussed in section II, with 

respect to the PM2.5 NAAQS, the EPA is 

finalizing: (1) Revisions to the level of 

the primary annual PM2.5 NAAQS and 

retaining the current primary 24-hour 

PM2.5 NAAQS (section II.B.4); and (2) no 

change to the current secondary annual 

and 24-hour PM2.5 NAAQS at this time 

(section V.B.4). Upon promulgation of a 

new or revised NAAQS, States and the 

EPA must initiate the process for initial 

designations. 

The timeline for initial area 

designations begins with promulgation 

of the revised primary annual PM2.5

NAAQS, as stated in the CAA section 

107(d)(1)(B)(i). Through this process, 

which provides for input from States 

and others at various stages, the EPA 

identifies areas of the country that either 

meet or do not meet the revised primary 

annual PM2.5 NAAQS, along with the 

nearby areas contributing to NAAQS 

violations. The following includes 

additional information regarding the 

designations process described in the 

CAA. 

Section 107(d)(1) of the CAA states 

that, ‘‘By such date as the Administrator 

may reasonably require, but not later 

than 1 year after promulgation of a new 

or revised national ambient air quality 

standard for any pollutant under section 


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200While the CAA says ‘‘designating’’ with 

respect to the Governor’s letter, in the full context 

of the CAA section it is clear that the Governor 

actually makes a recommendation to which the EPA 

must respond via a specified process if the EPA 

does not accept it. 

201 In certain circumstances in which the 

Administrator has insufficient information to 

promulgate area designations within two years from 

the promulgation of the NAAQS, CAA section 

107(d)(1)(B)(i) provides that the EPA may extend 

the designations schedule by up to one year. 

202API v. Costle, 609 F.2d 20 (D.C. Cir. 1979). 

203 ‘‘Guidance to Regions for Working with Tribes 

during the National Ambient Air Quality Standards 

(NAAQS) Designations Process,’’ December 20, 

2011, Memorandum from Stephen D. Page to 

Regional Air Directors, Regions 1–X available at 

https://www.epa.gov/sites/default/files/2017-02/ 

documents/12-20-11_guidance_to_regions_for_

working_with_tribes_naaqs_designations.pdf. 

204 In certain circumstances in which the 

Administrator has insufficient information to 

promulgate area designations within two years from 

the promulgation of a new or revised NAAQS, CAA 

section 107(d)(1)(B)(i) provides the EPA may extend 

the designations schedule by up to one year. 

205See: https://www.epa.gov/particle-pollutiondesignations. 

109, the Governor of each State shall 

. . . submit to the Administrator a list 

of all areas (or portions thereof) in the 

State’’ and make recommendations for 

whether the EPA should designate those 

areas as nonattainment, attainment, or 

unclassifiable.200 The CAA provides the 

EPA with discretion to require States to 

submit their designations 

recommendations within a reasonable 

amount of time not exceeding one 

additional year.201 Section 107(d)(1)(A) 

of the CAA also states that ‘‘the 

Administrator may not require the 

Governor to submit the required list 

sooner than 120 days after promulgating 

a new or revised national ambient air 

quality standard.’’ Section 

107(d)(1)(B)(i) further provides, ‘‘Upon 

promulgation or revision of a NAAQS, 

the Administrator shall promulgate the 

designations of all areas (or portions 

thereof) . . . as expeditiously as 

practicable, but in no case later than 2 

years from the date of promulgation. 

Such period may be extended for up to 

one year in the event the Administrator 

has insufficient information to 

promulgate the designations.’’ With 

respect to the NAAQS setting process, 

courts have interpreted the term 

‘‘promulgation’’ to be signature and 

widespread dissemination of a final 

rule.202

If the EPA agrees with the 

designations recommendation of the 

State, then it may proceed to promulgate 

the designations for such areas. If, 

however, the EPA disagrees with the 

State’s recommendation, then the EPA 

may elect to make modifications to the 

recommended designations. By no later 

than 120 days prior to promulgating the 

final designations, the EPA is required 

to notify States of any intended 

modifications to the State designation 

recommendations for any areas or 

portions thereof, including the 

boundaries of areas, as the EPA may 

deem necessary. States then have an 

opportunity to comment on the EPA’s 

intended modification and tentative 

designation decision. If a State elects 

not to provide designation 

recommendations for any area, then the 

EPA must itself promulgate the 

designation that it deems appropriate. 

While section 107(d) of the CAA 

specifically addresses the designations 

process for States, the EPA intends to 

follow the same process for Tribes to the 

extent practicable, pursuant to section 

301(d) of the CAA regarding Tribal 

authority, and the Tribal Authority Rule 

(63 FR 7254, February 12, 1998). To 

provide clarity and consistency in doing 

so, the EPA issued a guidance 

memorandum to our Regional Offices on 

working with Tribes during the 

designations process.203

Consistent with the process used in 

previous area designations efforts, the 

EPA will evaluate each area on a caseby-case basis considering the specific 

facts and circumstances unique to the 

area to support area boundary decisions 

for the revised standard. The EPA 

intends to issue a designations 

memorandum which will provide 

information regarding the designations 

process. In broad overview, the EPA has 

historically used area-specific analyses 

to support nonattainment area boundary 

recommendations and final boundary 

determinations by evaluating factors 

such as air quality data, emissions and 

emissions-related data (e.g., population 

density and degree of urbanization, 

traffic and commuting patterns), 

meteorology, geography/topography, 

and jurisdictional boundaries. We 

expect to follow a similar process when 

establishing area designations for this 

revised PM2.5 NAAQS. CAA section 

107(d) explicitly requires that the EPA 

designate as nonattainment not only the 

area that is violating the pertinent 

standard, but also those nearby areas 

that contribute to the violation in the 

violating area. In the PM2.5 NAAQS 

Designations Memorandum, the EPA 

intends to include information 

regarding consideration of federal land 

boundaries that may be fully or partially 

included within the bounds of a county 

otherwise identified as nonattainment. 

As with past revisions of the PM2.5

NAAQS, the EPA intends to make the 

designations decisions for the revised 

primary annual PM2.5 NAAQS based on 

the most recent three years of qualityassured, certified air quality data in the 

EPA’s Air Quality System (AQS). 

Accordingly, the EPA recommends that 

States base their initial area designation 

recommendations on the most current 

available three years of complete and 

certified air quality data at the time of 

the recommendations. The EPA will 

then base the final designations on the 

most recent three consecutive years of 

complete, certified air quality 

monitoring data available at the time of 

final designations.204

Monitoring data are currently 

available from numerous existing PM2.5

Federal Equivalent Methods (FEM) and 

Federal Reference Methods (FRM) sites 

to determine violations of the revised 

primary annual PM2.5 NAAQS. As 

described in section VII.C.3.b, the EPA 

took comment on how to deal with 

cases where an FEM is approved by the 

EPA with an update and when it can be 

implemented in the field. The EPA took 

comment on how to approach the data 

produced during this lag and received 

input from over a dozen commenters. 

The commenters asked that the EPA be 

flexible in allowing the use of updated 

method correction factors intended to 

improve the data comparability between 

the FRMs and FEMs. The EPA will 

address any data correction issues 

between the FRMs and FEMs through a 

future Notice of Data Availability 

(NOA). 

Consistent with past practice and as 

noted in the NPRM, the EPA intends to 

provide additional information 

concerning the designations process, 

including information about the 

schedule and recommendations for 

determining area boundaries in the 

forthcoming Annual PM2.5 NAAQS 

Designations Memorandum. Other 

topics addressed in this memorandum 

include the schedule for preparing and 

submitting exceptional events initial 

notification and exceptional events 

demonstrations relevant to the 

designations process, and information 

related to wildfire and prescribed fire on 

wildlands as it pertains to initial area 

designations, as well as addressing 

back-correction of PM FEM data when 

a method has an approved factory 

calibration as part of a method update. 

The Annual PM2.5 NAAQS Designations 

Memorandum is intended to assist 

States and Tribes in formulating their 

area recommendations.205

As discussed in the proposal, the 

‘‘Treatment of Data Influenced by 

Exceptional Events; Final Rule,’’ (81 FR 

68216, October 3, 2016) and codified at 

40 CFR 50.1, 40 CFR 50.14, and 40 CFR 

51.930, contains instructions and 

requirements for air agencies that may 


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206See the EPA’s Exceptional Events homepage at 

https://www.epa.gov/air-quality-analysis/treatmentair-quality-data-influenced-exceptional-eventshomepage-exceptional. 

207See EPA’s ‘‘Final Guidance on the Preparation 

of Exceptional Events Demonstrations for Wildfire 

Events that May Influence Ozone Concentrations 

and EPA’s Exceptional Events Guidance: Prescribed 

Fire on Wildland that May Influence Ozone and 

Particulate Matter Concentrations,’’ found on EPA’s 

Exceptional Events homepage at https://

www.epa.gov/air-quality-analysis/treatment-airquality-data-influenced-exceptional-eventshomepage-exceptional. 

208See ‘‘Guidance on Infrastructure State 

Implementation Plan (SIP) Elements under Clean 

Air Act Sections 110(a)(1) and 110(a)(2)’’ September 

2013, Memorandum from Stephen D. Page to 

Regional Air Directors, Regions 1–10. 209CAA section 110(a)(2)(D)(i)(I). 

flag air quality data for certain days in 

the Air Quality System due to potential 

impacts from exceptional events (i.e., 

such as prescribed fires on wildland, 

wildfires, or high wind dust storms). 

Accordingly, for purposes of initial area 

designations for a new or revised 

NAAQS, an air agency may submit to 

the EPA an exceptional events 

demonstration with supporting 

information and analyses for each 

monitoring site and day the air agency 

claims the EPA should exclude from 

design value calculations for 

designations purposes. 

The EPA has provided tools to assist 

air agencies in preparing adequate 

exceptional events demonstrations.206

Further, the EPA will continue to work 

with air agencies as they identify 

exceptional events that may influence 

decisions related to the initial area 

designations process, and to prepare 

and submit exceptional events 

demonstrations if appropriate. 

Importantly, air quality monitoring data 

may be influenced by emissions from 

prescribed fires on wildland and 

wildfires. The EPA’s Exceptional Events 

Rule provides for both of these types of 

events to be considered as exceptional 

events, provided the affected air 

agencies submit exceptional events 

demonstrations that meet the procedural 

and technical requirements of the EPA’s 

Exceptional Events Rule. To that end, 

the EPA has issued guidance addressing 

development of exceptional events 

demonstrations for both wildfire and 

prescribed fires on wildland.207 In light 

of the growing frequency and severity of 

wildfire events, and expected increases 

in the application of prescribed fire as 

a means to achieve long-term reductions 

in high severity wildfire risk and 

associated smoke impacts, the EPA 

seeks to ensure that the Agency’s 

exceptional events process provides an 

efficient and clear pathway for 

excluding data that may be affected by 

such events in a manner that is 

consistent with the Clean Air Act and 

the public health objectives of the 

NAAQS. Accordingly, the EPA is 

continuing to explore opportunities to 

develop additional tools that could 

assist air agencies in preparing 

exceptional events demonstrations for 

wildfires and prescribed fires on 

wildland. In addition, EPA intends to 

continue engaging with the U.S. 

Department of Agriculture, U.S. 

Department of the Interior, air agencies, 

and other stakeholders on these issues. 

For more information regarding the 

exceptional events demonstration 

submission deadlines for the area 

designations process, please see Table 2 

to 40 CFR 50.14(c)(2)(vi)—‘‘Schedule for 

Initial Notification and Demonstration 

Submission for Data Influenced by 

Exceptional Events for Use in Initial 

Area Designations.’’ 

B. Section 110(a)(1) and (2) 

Infrastructure SIP Requirements 

As discussed in the NPRM, the CAA 

directs States to address basic SIP 

requirements to implement, maintain, 

and enforce the NAAQS. Under CAA 

sections 110(a)(1) and (2), states are 

required to have State implementation 

plans that provide the necessary air 

quality management infrastructure that 

provides for the implementation, 

maintenance, and enforcement of the 

NAAQS. After the EPA promulgates a 

new or revised NAAQS, States are 

required to make a new SIP submission 

to establish that they meet the necessary 

structural requirements for such new or 

revised NAAQS or make changes to do 

so. The EPA refers to this type of SIP 

submission as an ‘‘infrastructure SIP 

submission.’’ Under CAA section 

110(a)(1), all States are required to make 

these infrastructure SIP submissions 

within three years after the effective 

date of a new or revised primary 

standard. While the CAA authorizes the 

EPA to set a shorter time for States to 

make these SIP submissions, the EPA is 

requiring submission of infrastructure 

SIPs within three years of the effective 

date of this revised primary annual 

PM2.5 NAAQS. 

The EPA has provided general 

guidance to States concerning its 

interpretation of these requirements of 

CAA section 110(a)(1) and (2) in the 

context of infrastructure SIP 

submissions for a new or revised 

NAAQS.208 The EPA encourages States 

to use this guidance when developing 

their infrastructure SIPs for this revised 

primary annual PM2.5 NAAQS. 

As a reminder, the EPA notes that 

States are not required to address 

nonattainment plan requirements for 

purposes of the revised primary annual 

PM2.5 NAAQS on the same schedule as 

infrastructure SIP requirements. The 

EPA interprets the CAA such that two 

elements identified in section 110(a)(2) 

are not subject to the 3-year submission 

deadline of section 110(a)(1) and thus 

States are not required to address them 

in the context of an infrastructure SIP 

submission. The elements pertain to 

part D, in title I of the CAA, which 

addresses additional SIP requirements 

for nonattainment areas. Therefore, for 

the reasons explained below, the 

following section 110(a)(2) elements are 

considered by the EPA to be outside the 

scope of infrastructure SIP actions: (1) 

The portion of section 110(a)(2)(C), 

programs for enforcement of control 

measures and for construction or 

modification of stationary sources that 

applies to permit programs applicable in 

designated nonattainment areas (known 

as ‘‘nonattainment new source review’’) 

under part D; and (2) section 

110(a)(2)(I), which requires a SIP 

submission pursuant to part D, in its 

entirety. 

Accordingly, the EPA does not expect 

States to address the requirement for a 

new or revised NAAQS in the 

infrastructure SIP submissions to 

include regulations or emissions limits 

developed specifically for attaining the 

relevant standard in areas designated 

nonattainment for the revised primary 

annual PM2.5 NAAQS. States are 

required to submit infrastructure SIP 

submissions for the revised primary 

annual PM2.5 NAAQS before they will 

be required to submit nonattainment 

plan SIP submissions to demonstrate 

attainment with the same NAAQS. 

States are required to submit 

nonattainment plan SIP submissions to 

provide for attainment and maintenance 

of a revised primary annual PM2.5

NAAQS within 18 months from the 

effective date of nonattainment area 

designations as required under CAA 

section 189(a)(2)(B). The EPA reviews 

and acts upon these later SIP 

submissions through a separate process. 

For this reason, the EPA does not expect 

States to address new nonattainment 

area emissions controls per section 

110(a)(2)(I) in their infrastructure SIP 

submissions. 

One of the required infrastructure SIP 

elements is that each State SIP must 

contain adequate provisions to prohibit, 

consistent with the provisions of title I 

of the CAA, emissions from within the 

State that will significantly contribute to 

nonattainment in, or interfere with 

maintenance by, any other State of the 

primary or secondary NAAQS.209 This 


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210CAA section 110(a)(2)(D)(i)(II) also addresses 

certain interstate effects that states must address 

and thus is also sometimes referred to as relating 

to ‘‘interstate transport.’’ 

211See North Carolina v. EPA, 531 F.3d 896, 909– 

11 (D.C. Cir. 2008). 

212See id. 911–13. See also Wisconsin v. EPA, 

938 F.3d 303, 313–20 (D.C. Cir. 2019); Maryland v. 

EPA, 958 F.3d 1185, 1203–04 (D.C. Cir. 2020). 

213A ‘‘certification’’ approach would not be 

appropriate for the interstate pollution control 

requirements of CAA section 110(a)(2)(D)(i). 

214https://cdx.epa.gov/. 

215Provides guidance on developing 

demonstrations under section 189(e) intended to 

show that a certain PM2.5 precursor in a particular 

nonattainment area does not significantly 

contribute to PM2.5 concentrations that exceed the 

standard. 

element is often referred to as the ‘‘good 

neighbor’’ or ‘‘interstate transport’’ 

provision.210 The provision has two 

prongs: significant contribution to 

nonattainment (prong 1), and 

interference with maintenance (prong 

2). The EPA and States must give 

independent significance to prong 1 and 

prong 2 when evaluating downwind air 

quality problems under CAA section 

110(a)(2)(D)(i)(I).211 Further, case law 

has established that the EPA and States 

must implement requirements to meet 

interstate transport obligations in 

alignment with the applicable statutory 

attainment schedule of the downwind 

areas impacted by upwind-state 

emissions.212 Thus, the EPA anticipates 

that States will need to address 

interstate transport obligations 

associated with this revised PM 

NAAQS, in alignment with the 

provisions of subpart 4 of part D of the 

CAA, as discussed in more detail in 

section VIII.C below. Specifically, States 

must implement any measures required 

to address interstate transport 

obligations as expeditiously as 

practicable and no later than the next 

statutory attainment date, i.e., for this 

NAAQS revision as expeditiously as 

practicable, but no later than the end of 

the sixth calendar year following 

nonattainment area designations. See 

CAA section 188(c). States may find it 

efficient to make SIP submissions to 

address the interstate transport 

provisions separately from other 

infrastructure SIP elements. 

Each State has the authority and 

responsibility to review its air quality 

management program’s existing SIP 

provisions in light of a new or revised 

NAAQS to determine if any revisions 

are necessary to implement the new or 

revised NAAQS. Most States have 

revised and updated their SIPs in recent 

years to address requirements associated 

with other revised NAAQS. For certain 

infrastructure elements, some States 

may believe they already have adequate 

State regulations adopted and approved 

into the SIP to address a particular 

requirement with respect to the revised 

primary annual PM2.5 NAAQS. 

If a State determines that existing SIPapproved provisions are adequate in 

light of this revised primary annual 

PM2.5 NAAQS with respect to a given 

infrastructure SIP element (or subelement), then the State may make an 

infrastructure SIP submission 

‘‘certifying’’ that the existing State’s 

existing EPA approved SIP already 

contains provisions that address one or 

more specific section 110(a)(2) 

infrastructure elements.213 In the case of 

such a submission, the State does not 

have to include a copy of the relevant 

provision (e.g., rule or statute) itself. 

Rather, this certification submission 

should provide citations to the SIPapproved State statutes, regulations, or 

non-regulatory measures, as 

appropriate, in or referenced by the 

already EPA-approved SIP that meet 

particular infrastructure SIP element 

requirements. The State’s infrastructure 

SIP submission should also include an 

explanation as to how the State has 

determined that those existing 

provisions meet the relevant 

requirements. 

Like any other SIP submission, that 

State can make such an infrastructure 

SIP submission certifying that it has 

already met some or all of the applicable 

requirements only after it has provided 

reasonable notice and opportunity for 

public hearing. This ‘‘reasonable notice 

and opportunity for public hearing’’ 

requirement for infrastructure SIP 

submissions is to meet the requirements 

of CAA sections 110(a) and 110(l). 

Under the EPA’s regulations at 40 CFR 

part 51, if a public hearing is held, an 

infrastructure SIP submission must 

include a certification by the State that 

the public hearing was held in 

accordance with the EPA’s procedural 

requirements for public hearings. See 40 

CFR part 51, appendix V, section 2.1(g), 

and see 40 CFR 51.102. 

In consultation with the EPA’s 

Regional office, a State should follow all 

applicable EPA regulations governing 

infrastructure SIP submissions in 40 

CFR part 51—e.g., subpart I (Review of 

New Sources and Modifications), 

subpart J (Ambient Air Quality 

Surveillance), subpart K (Source 

Surveillance), subpart L (Legal 

Authority), subpart M 

(Intergovernmental Consultation), 

subpart O (Miscellaneous Plan Content 

Requirements), subpart P (Protection of 

Visibility), and subpart Q (Reports). For 

the EPA’s general criteria for 

infrastructure SIP submissions, refer to 

40 CFR part 51, appendix V, Criteria for 

Determining the Completeness of Plan 

Submissions. For additional information 

on infrastructure SIP submission 

requirements, refer to the EPA’s 2013 

guidance entitled ‘‘Guidance on 

Infrastructure State Implementation 

Plan (SIP) Elements under Clean Air Act 

Sections 110(a)(1) and 110(a)(2).’’ The 

EPA recommends that States 

electronically submit their 

infrastructure SIPs to the EPA through 

the State Plan Electronic Collaboration 

System (SPeCS),214 an online system 

available through the EPA’s Central Data 

Exchange. 

C. Implementing Revised Primary 

Annual PM2.5 NAAQS in Nonattainment 

Areas 

As discussed in the NPRM, the EPA 

issued a SIP Requirements Rule for 

implementing the PM2.5 NAAQS (81 FR 

58010, August 24, 2016) (PM2.5 SIP 

Requirements Rule). It provides 

guidance and establishes additional 

regulatory requirements for States 

regarding development of attainment 

plans for nonattainment areas for the 

1997, 2006, and 2012 revisions of the 

PM2.5 NAAQS. The guidance and 

regulations in the SIP Requirements 

Rule also apply to any States for which 

the EPA promulgates nonattainment 

area designations for the new revised 

primary annual PM2.5 NAAQS. 

The PM2.5 SIP Requirements Rule 

provides comprehensive information 

regarding nonattainment plan 

requirements including, among other 

things: nonattainment area emissions 

inventories; policies regarding PM2.5

precursor pollutants (i.e., SO2, NOX, 

VOC, and ammonia); control strategies 

(such as reasonably available control 

measures and reasonably available 

control technology for direct PM2.5 and 

relevant precursors); air quality 

modeling; attainment demonstrations; 

reasonable further progress 

requirements; quantitative milestones; 

and contingency measures. Information 

provided in the PM2.5 SIP Requirements 

Rule is supplemented by other EPA 

documents, including guidance on 

emissions inventory development (80 

FR 8787, February 19, 2015; U.S. EPA, 

2017), optional PM2.5 precursor 

demonstrations (U.S. EPA, 2019b),215

and guidance on air quality modeling 

for meeting air quality goals for the 

ozone and PM2.5 NAAQS and regional 

haze program (U.S. EPA, 2018b). 

As stated in the NPRM, the PM2.5 SIP 

Requirements Rule provides 

recommendations to States regarding 

consideration of environmental justice 

in the context of PM2.5 attainment 


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216For more information on the EPA’s 

recommendations and examples, see 81 FR 58010, 

58137, August 24, 2016. 

217CAA Sections 110(a) and 172 contain general 

nonattainment planning provisions, regarding the 

public review, adoption, submittal, and content of 

implementation plans. CAA Section 189 specifies 

additional plan provisions for particulate matter 

nonattainment areas. General Preamble for the 

Implementation of Title I of the Clean Air Act 

Amendments of 1990 provides a detailed 

discussion of the EPA’s interpretation of the Title 

I requirements (57 FR 13498, April 16, 1992; 59 FR 

41998, August 16, 1994). 

218 40 CFR 51.166(i)(2) and 52.21(i)(2). 

219On July 29, 2022, the EPA issued ‘‘Final 

Guidance for Ozone and Fine Particulate Matter 

Permit Modeling,’’ available at https://

www.epa.gov/system/files/documents/2022-07/ 

Guidance_for_O3_PM25_Permit_Modeling.pdf. This 

guidance provides the EPA’s recommendations for 

how a stationary source seeking a PSD permit may 

demonstrate that it will not cause or contribute to 

a violation of the National Ambient Air Quality 

Standards for Ozone and PM2.5 and PSD increments 

for PM2.5, as required under section 165(a)(3) of the 

Clean Air Act and 40 CFR 51.166(k) and 52.21(k). 

The EPA has also previously issued two technical 

guidance documents for use in conducting these 

demonstrations: ‘‘Guidance on the Development of 

Modeled Emission Rates for Precursors (MERPs) as 

a Tier 1 Demonstration Tool for Ozone and PM2.5

under the PSD Permitting Program,’’ available at 

https://www.epa.gov/sites/default/files/2020-09/ 

documents/epa-454_r-19-003.pdf, and ‘‘Guidance 

on the Use of Models for Assessing the Impacts of 

Emissions from Single Sources on the Secondarily 

Formed Pollutants: Ozone and PM2.5,’’ available at 

https://www.epa.gov/sites/default/files/2020-09/ 

documents/epa-454_r-16-005.pdf. 

planning. Some of the considerations for 

States include: (1) Identifying areas with 

overburdened communities where more 

ambient monitoring may be warranted; 

(2) targeting emissions reductions that 

may be needed to attain the PM2.5

NAAQS; and (3) increasing 

opportunities for meaningful 

involvement for overburdened 

populations (see 88 FR 5558, 5684, 

January 27, 2023; 80 FR 58010, 58136, 

August 25, 2016). In light of the 

identification of at-risk populations for 

this reconsideration, the EPA 

encourages States to consider these and 

other factors as part of their attainment 

plan SIP development process. 

The PM2.5 SIP Requirements Rule 

outlines some examples of how States 

can elect to implement these 

recommendations.216 For instance, 

States can use modeling and screening 

tools to better understand where sources 

of PM2.5 or PM2.5 precursor emissions 

are located and identify areas that may 

be candidates for additional ambient 

monitoring. Furthermore, once these 

target areas are identified, States can 

prioritize direct PM2.5 or PM2.5

precursor control measures and 

enforcement strategies in these areas to 

reduce ambient PM2.5 and achieve the 

NAAQS. As articulated in the NPRM 

and the PM2.5 SIP Requirements Rule, 

the EPA recognizes that States have 

flexibility under the CAA to concentrate 

State resources on controlling sources of 

PM2.5 emissions in light of 

environmental justice considerations 

(see 88 FR 5558, 5684, January 27, 2023; 

81 FR 58010, 58137, August 24, 2016). 

Moreover, States can establish 

opportunities to bolster meaningful 

involvement in a number of ways, such 

as communicating in appropriate 

languages, ensuring access to draft SIPs 

and other information, and developing 

enhanced notice-and-comment 

opportunities, as appropriate (see 88 FR 

5558, 5684, January 27, 2023; 80 FR 

58010, 58136, August 25, 2016). 

As previously mentioned, the PM2.5

SIP Requirements Rule provides 

guidance and regulatory requirements 

for remaining nonattainment areas for 

the 1997, 2006, and 2012 revisions of 

the PM2.5 NAAQS, as well as for 

nonattainment areas designated 

pursuant to any future revisions of the 

PM2.5 NAAQS, including the revised 

annual PM2.5 NAAQS being finalized in 

this action. The EPA is not making any 

changes to the current PM2.5 SIP 

Requirements Rule. 

D. Implementing the Primary and 

Secondary PM10 NAAQS 

As summarized in sections III.B.4 and 

V.B.4 above, the EPA is retaining the 

current primary and secondary 24-hour 

PM10 NAAQS to protect against the 

health effects associated with short-term 

exposures to thoracic coarse particles 

and against the welfare effects 

considered in this reconsideration (i.e., 

visibility, climate, and materials effects). 

The EPA is retaining the existing 

implementation strategy for meeting the 

CAA requirements for the PM10

NAAQS. States and emissions sources 

should continue to follow the existing 

regulations and guidance for 

implementing the current standards.217

E. Prevention of Significant 

Deterioration and Nonattainment New 

Source Review Programs for the Revised 

Primary Annual PM2.5 NAAQS 

The CAA, at parts C and D of title I, 

contains preconstruction review and 

permitting programs applicable to new 

major stationary sources and major 

modifications of existing major sources. 

The preconstruction review of each new 

major stationary source and major 

modification applies on a pollutantspecific basis, and the requirements that 

apply for each pollutant depend on 

whether the area in which the source is 

situated is designated as attainment (or 

unclassifiable) or nonattainment for that 

pollutant. In areas designated 

attainment or unclassifiable for a 

pollutant, the Prevention of Significant 

Deterioration (PSD) requirements under 

part C apply to construction at major 

sources. In areas designated 

nonattainment for a pollutant, the 

Nonattainment New Source Review 

(NNSR) requirements under part D 

apply to construction at major sources. 

Collectively, those two sets of permit 

requirements are commonly referred to 

as the ‘‘major New Source Review’’ or 

‘‘major NSR’’ programs. 

Until the EPA designates an area with 

respect to the revised primary annual 

PM2.5 NAAQS, the NSR provisions 

applicable under an area’s current 

designation for the 1997, 2006, and 2012 

PM2.5 NAAQS would continue to apply. 

See 40 CFR 51.166(i)(2) and 52.21(i)(2). 

That is, for areas designated as 

attainment/unclassifiable for the 1997, 

2006, and 2012 PM2.5 NAAQS, PSD will 

apply to new major stationary sources 

and major modifications that trigger 

major source permitting requirements 

for PM2.5. For areas designated 

nonattainment for the 1997, 2006, or 

2012 PM2.5 NAAQS, NNSR 

requirements will apply for new major 

stationary sources and major 

modifications that trigger major source 

permitting requirements for PM2.5. 

When the initial area designations for 

this revised primary annual PM2.5

NAAQS become effective, those 

designations will further determine 

whether PSD or NNSR applies to PM2.5

in a particular area, depending on the 

designation status. New major sources 

and major modifications will be subject 

to the PSD program requirements for 

PM2.5 if they are located in an area that 

does not have a current nonattainment 

designation under CAA section 107 for 

PM2.5.218

Under the PSD program, the permit 

applicant must demonstrate that the 

new or modified source emissions 

increase does not cause or contribute to 

a NAAQS violation. In 2017, the EPA 

revised the Guideline on Air Quality 

Models (published as appendix W to 40 

CFR part 51) to address primary and 

secondary PM2.5 impacts in making this 

demonstration. The EPA has since 

provided associated technical guidance, 

models and tools, such as the recent 

‘‘Final Guidance for Ozone and Fine 

Particulate Matter Permit Modeling’’ 

(July 29, 2022).219 Additionally, in light 

of this NAAQS revision, the EPA is 

updating its guidance that provides 

recommended significant impact levels 

(SILs) for PM2.5 and expects that an 

updated SIL for the revised primary 

annual PM2.5 NAAQS will be available 


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220Sulfur dioxide is a precursor to PM2.5 in all 

attainment and unclassifiable areas. NOX is 

presumed to be a precursor to PM2.5 in all 

attainment and unclassifiable areas, unless a state 

or the EPA demonstrates that emissions of NOX

from sources in a specific area are not a significant 

contributor to that area’s ambient PM2.5

concentrations. VOC is presumed not to be a 

precursor to PM2.5 in any attainment or 

unclassifiable area, unless a state or the EPA 

demonstrates that emissions of VOC from sources 

in a specific area are a significant contributor to that 

area’s ambient PM2.5 concentrations. 

221By establishing the maximum allowable level 

of ambient pollutant concentration increase in a 

particular area, an increment defines ‘‘significant 

deterioration’’ of air quality in that area. Increments 

are defined by the CAA as maximum allowable 

increases in ambient air concentrations above a 

baseline concentration and are specified in the PSD 

regulations by pollutant and area classification 

(Class I, II and III). 40 CFR 51.166(c), 40 CFR 

52.21(c); 75 FR 64864 (October 20, 2010). 

222Congress established certain Class I areas in 

section 162(a) of the CAA, including international 

parks, national wilderness areas, and national parks 

that meet certain criteria. Such Class I areas, known 

as mandatory Federal Class I areas, are afforded 

special protection under the CAA. In addition, 

States and Tribal governments may establish Class 

I areas within their own political jurisdictions to 

provide similar special air quality protection. 

223See 40 CFR part 51, appendix W; 82 FR 5182 

(January 17, 2017); See also U.S. EPA, 2021d. The 

EPA provided an initial version of the 2021 

guidance for public comment on February 10, 2020. 

Upon consideration of the comments received, and 

consistent with Executive Order 13990, the EPA 

revised the initial draft guidance and posted the 

revised version for additional public comment. 

224This exemption was referred to as 

‘‘grandfathering’’ in the 2015 Ozone NAAQS and 

the D.C. Circuit’s Murray Energy Corp. decision on 

that exemption. See 80 FR 65292, 65431 (October 

26, 2015); Murray Energy Corp. v. EPA, 936 F.3d 

597, 627 (D.C. Cir. 2019). The EPA refers to this 

‘‘grandfathering’’ provision in this action as an 

exemption provision. 

225While the specifics of this case involved the 

2015 ozone NAAQS, the case was based upon an 

interpretation of CAA section 165(a) and therefore 

applies equally to any PSD permitting exemption 

provision for a new or revised NAAQS. 

on or before the effective date of the 

final NAAQS. 

The statutory requirements for a PSD 

permit program set forth under part C of 

title I of the CAA (sections 160 through 

169) are addressed by the EPA’s PSD 

regulations found at 40 CFR 51.166 

(minimum requirements for an 

approvable PSD SIP) and 40 CFR 52.21 

(PSD permitting program for permits 

issued under the EPA’s Federal 

permitting authority). These regulations 

already apply to PM2.5 in areas that are 

designated attainment or unclassifiable 

for PM2.5 whenever a proposed new 

major source or major modification 

triggers PSD requirements for PM2.5. 

For PSD, a ‘‘major stationary source’’ 

is one with the potential to emit 250 

tons per year (tpy) or more of any 

regulated NSR pollutant, unless the new 

or modified source is classified under a 

list of 28 source categories contained in 

the statutory definition of ‘‘major 

emitting facility’’ in section 169(1) of 

the CAA. For those 28 source categories, 

a ‘‘major stationary source’’ is one with 

the potential to emit 100 tpy or more of 

any regulated NSR pollutant. A ‘‘major 

modification’’ is a physical change or a 

change in the method of operation of an 

existing major stationary source that 

results, first, in a significant emissions 

increase of a regulated NSR pollutant 

and, second, in a significant net 

emissions increase of that pollutant. See 

40 CFR 51.166(b)(2)(i), 40 CFR 

52.21(b)(2)(i). The EPA PSD regulations 

define the term ‘‘regulated NSR 

pollutant’’ to include any pollutant for 

which a NAAQS has been promulgated 

and any pollutant identified by the EPA 

as a constituent or precursor to such 

pollutant. See 40 CFR 51.166(b)(49), 40 

CFR 52.21(b)(50). These regulations 

identify SO2 and NOX as precursors to 

PM2.5 in attainment and unclassifiable 

areas. See 40 CFR 51.166(b)(49)(i)(b), 40 

CFR 52.21(b)(50)(i)(b).220 Thus, for 

PM2.5, the PSD program currently 

requires the review and control of 

emissions of direct PM2.5 emissions and 

SO2 and NOX (as precursors to PM2.5), 

absent a demonstration otherwise for 

NOX. Among other things, for each 

regulated NSR pollutant emitted or 

increased in a significant amount, the 

PSD program requires a new major 

stationary source or a major 

modification to apply the ‘‘best 

available control technology’’ (BACT) to 

limit emissions and to conduct an air 

quality impact analysis to demonstrate 

that the proposed major stationary 

source or major modification will not 

cause or contribute to a violation of any 

NAAQS or PSD increment.221 See CAA 

section 165(a)(3) and (4), 40 CFR 

51.166(j) and (k), 40 CFR 52.21(j) and 

(k). The PSD requirements may also 

include, in appropriate cases, an 

analysis of potential adverse impacts on 

Class I areas. See CAA sections 162(a) 

and 165(d), 40 CFR 51.166(p); 40 CFR 

52.21(p)).222 The EPA developed the 

Guideline on Air Quality Models and 

other documents to, among other things, 

provide methods and guidance for 

demonstrating that increased emissions 

from construction will not cause or 

contribute to exceedances of the PM2.5

NAAQS and PSD increments for 

PM2.5.223

Upon the effective date of the revised 

primary annual PM2.5 NAAQS, the 

demonstration required under CAA 

Section 165(a)(3), and the associated 

regulations, must include the revised 

primary annual PM2.5 NAAQS. In past 

NAAQS revision rules, including the 

2012 PM2.5 NAAQS (78 FR 3086, 

January 15, 2013) and 2015 Ozone 

NAAQS (80 FR 65292, October 26, 

2015), the EPA included limited 

provision that exempted certain sources 

with pending PSD permit applications 

(those that had reached a particular 

stage in the permitting process at the 

time the revised NAAQS was 

promulgated or became effective) from 

the requirement to demonstrate that the 

proposed emissions increases would not 

cause or contribute to a violation of the 

revised NAAQS.224 In August 2019, the 

U.S. Court of Appeals for the D.C. 

Circuit vacated the exemption provision 

in the PSD rules for the 2015 Ozone 

NAAQS, finding that the provision 

contradicted ‘‘Congress’s ‘express policy 

choice’ not to allow construction which 

will ‘cause or contribute to’ 

nonattainment of ‘any’ effective 

NAAQS, regardless of when they are 

adopted or when a permit was 

completed.’’ Murray Energy Corp. v. 

EPA, 936 F.3d 597, 627 (D.C. Cir. 

2019).225 Based on that court decision, 

the EPA is not establishing any PSD 

permitting exemption provision in this 

action. Some commenters requested that 

the EPA provide the same kind of relief 

for pending PSD permit applications by 

extending the effective date of this new 

revised NAAQS beyond the 60 days that 

the EPA has traditionally used for such 

rules. Such comments are addressed in 

the Response to Comments portion of 

this action. The EPA is making this 

revised primary annual PM2.5 NAAQS 

effective in 60 days. 

The EPA anticipates that the existing 

PM2.5 air quality in some areas will not 

be in attainment with the revised 

primary annual PM2.5 NAAQS, and the 

EPA will designate these areas as 

nonattainment at a later date, consistent 

with the designation process described 

in the preceding sections. However, 

until such nonattainment designation 

occurs, proposed new major sources and 

major modifications located in any area 

currently designated attainment or 

unclassifiable for all preexisting PM2.5

NAAQS will continue to be subject to 

the PSD program requirements for 

PM2.5. Any proposed major stationary 

source or major modification triggering 

PSD requirements for PM2.5 that does 

not receive its PSD permit by the 

effective date of a new nonattainment 

designation for the area where the 

source would locate would then be 

required to satisfy applicable NNSR 

preconstruction permit requirements for 

PM2.5. 

In areas where air pollution exceeds 

the level of the revised primary annual 

PM2.5 NAAQS, a PSD permit applicant 

must demonstrate that the source or 

modification will not cause or 


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226 40 CFR 51.166(k) states that SIPs must require 

that the owner or operator of the proposed source 

or modification demonstrate that allowable 

emission increases from the proposed source or 

modification, in conjunction with all other 

applicable emissions increases or reductions 

(including secondary emissions), would not cause 

or contribute to air pollution in violation of: (i) Any 

national ambient air quality standard in any air 

quality control region; or (ii) any applicable 

maximum allowable increase over the baseline 

concentration in any area. 

227See, e.g., Memorandum from Stephen D. Page, 

Director, Office of Air Quality Planning and 

Standards to Regional Air Division Directors, 

Guidance Concerning Implementation of the 1-hour 

SO2 NAAQS for the Prevention of Significant 

Deterioration Program. August 23, 2010. Office of 

Air Quality Planning and Standards U.S. EPA, 

Research Triangle Park. Available at: https://

www.epa.gov/sites/default/files/2015-07/ 

documents/appwso2.pdf; 44 FR 3274, 3278, January 

16, 1979; See also In re Interpower of New York, 

Inc., 5 E.A.D. 130, 141 (EAB 1994) (describing an 

EPA Region 2 PSD permit that relied in part on 

offsets to demonstrate the source would not cause 

or contribute to a violation of the NAAQS). 52 FR 

24634, 24684, July 1, 1987; 78 FR 3085, 3261–62, 

January 15, 2013. The EPA has recognized the 

ability of sources to obtain offsets in the context of 

PSD though the PSD provisions of the Act do not 

expressly reference offsets as the NNSR provisions 

of the Act do. See 80 FR 65292, 65441, October 26, 

2015. 

228All of these pollutants are identified as 

precursors to PM2.5 in NNSR regulations. See 40 

CFR 51.165(a)(1)(xxxvii)(C)(2). No significant 

emission rate is established by the EPA for 

ammonia, and states are required to define 

‘‘significant’’ for ammonia for their respective areas 

unless the state pursues the optional precursor 

demonstration to exclude ammonia from planning 

requirements. See 40 CFR 51.165(a)(1)(x)(F); 40 CFR 

51.165(a)(13). 

contribute to a violation of the NAAQS. 

Section 165(a)(3)(B) of the CAA states 

that a proposed source may not 

construct unless it demonstrates that it 

will not cause or contribute to a 

violation of any NAAQS. This statutory 

requirement is implemented through a 

provision contained in the PSD 

regulations at 40 CFR 51.166(k) and 

52.21(k).226 If a source cannot make this 

demonstration, or if its initial air quality 

impact analysis shows that the source’s 

impact would cause or contribute to a 

violation, the reviewing authority may 

not issue a PSD permit to that source. 

However, a PSD permit applicant may 

be able to make this demonstration if it 

compensates for the adverse impact that 

would otherwise cause or contribute to 

a violation of the NAAQS. In contrast to 

the NSR requirements for nonattainment 

areas, the PSD regulations do not 

explicitly specify remedial actions that 

a prospective source must take to 

address such a situation, but the EPA 

has historically recognized that sources 

applying for PSD permits may utilize 

offsetting reductions in emissions as 

part of the required PSD demonstration 

under CAA section 165(a)(3)(B).227

Part D of title I of the CAA includes 

preconstruction review and permitting 

requirements applicable to new major 

stationary sources and major 

modifications located in areas 

designated nonattainment for a 

pollutant for which the EPA has 

established a NAAQS (i.e., a criteria 

pollutant). The relevant part D 

requirements are typically referred to as 

the nonattainment NSR (NNSR) 

program. The EPA’s regulations for the 

NNSR program are contained in 40 CFR 

51.165 and 52.24 and part 51, appendix 

S. Specifically, the EPA has developed 

minimum program requirements for a 

NNSR program that is approvable in a 

SIP, and those requirements, which 

include requirements for PM2.5, are 

contained in 40 CFR 51.165. In addition, 

40 CFR part 51, appendix S, contains 

requirements constituting an interim 

NNSR program. This interim program 

enables NNSR permitting in 

nonattainment areas by States that lack 

a SIP-approved NNSR permitting 

program during the time between the 

date of the relevant designation and the 

date that the EPA approves into the SIP 

a NNSR program. See 40 CFR part 51, 

appendix S, section I; 40 CFR 52.24(k). 

For NNSR, ‘‘major stationary source’’ 

is generally defined as a source with the 

potential to emit at least 100 tpy of the 

regulated NSR pollutant for which the 

area is designated nonattainment. In 

some cases, however, the CAA and the 

NNSR regulations define ‘‘major 

stationary source’’ for NNSR in terms of 

a lower rate dependent on the pollutant 

and degree of nonattainment in the area. 

For purposes of the PM2.5NAAQS, in 

addition to the general threshold level 

of 100 tpy in Moderate PM2.5

nonattainment areas, a lower major 

source threshold of 70 tpy applies in 

Serious PM2.5 nonattainment areas 

pursuant to subpart 4 of part D, title I 

of the CAA. See 40 CFR 

51.165(a)(1)(iv)(A)(1)(vii) and (viii); 40 

CFR part 51, appendix S, II.A.4(i)(a)(7) 

and (8). 

Under the NNSR program, direct 

PM2.5 emissions and emissions of each 

PM2.5 precursor are considered 

separately in accordance with the 

applicable major source threshold. For 

example, the threshold for Serious PM2.5

nonattainment areas is 70 tpy of direct 

PM2.5, as well as for the PM2.5

precursors SO2, NOX, VOC, and 

ammonia.228 See 40 CFR 

51.165(a)(1)(iv)(A)(1)(vii) and (viii); 40 

CFR part 51, appendix S, II.A.4.(i)(a)(7) 

and (8). A source qualifies as major for 

nonattainment NSR in a PM2.5

nonattainment area if it emits or has the 

potential to emit direct PM2.5 or any 

PM2.5 precursor in an amount equal to 

or greater than the applicable threshold. 

For modifications, NNSR applies to 

proposed physical changes or changes 

in the method of operation of an 

existing stationary source where (1) the 

source is major for the nonattainment 

pollutant (or a precursor for that 

pollutant) and (2) the physical change or 

change in the method of operation of a 

major stationary source results, first, in 

a significant emissions increase of a 

regulated NSR pollutant and, second, in 

a significant net emissions increase of 

that same nonattainment pollutant (or 

same precursor for that pollutant). See 

40 CFR 51.165(a)(1)(v)(A); 40 CFR part 

51, appendix S, II.A.5.(i). For example, 

to qualify as a major modification for 

SO2 (as a PM2.5 precursor) in a Moderate 

PM2.5 nonattainment area, the existing 

source would have to have the potential 

to emit 100 tpy or more of SO2, and the 

project would have to result in an 

increase in SO2 emissions of 40 tpy or 

more. See 40 CFR 51.165(a)(1)(x)(A). 

New major stationary sources and 

major modifications for PM2.5 subject to 

NNSR must comply with the ‘‘lowest 

achievable emission rate’’ (LAER), as 

defined in the CAA and NNSR rules. 

Such sources must also perform other 

analyses and obtain emission offsets, as 

required under section 173 of the CAA 

and applicable regulations. 

Following the promulgation of this 

revised primary annual PM2.5 NAAQS, 

some new areas may be designated 

nonattainment for PM2.5. Where a State 

does not have an existing NNSR 

program or where the current NNSR 

program does not apply to PM2.5, that 

State will be required to submit the 

necessary SIP revisions to ensure that 

new major stationary sources and major 

modifications for PM2.5 or a PM2.5

precursor undergo preconstruction 

review pursuant to the NNSR program. 

States with designated nonattainment 

areas for the revised primary annual 

PM2.5 NAAQS are required to make SIP 

submissions to meet nonattainment plan 

requirements within 18 months from the 

effective date of designations, as 

required under CAA section 

189(a)(2)(B). States that have existing 

NNSR program requirements that 

cannot be interpreted to apply at the 

time of designation to the revised 

primary annual PM2.5 NAAQS may, in 

the interim, issue permits in accordance 

with the applicable nonattainment 

permitting requirements contained in 40 

CFR part 51, appendix S, which would 

apply to the revised primary annual 

PM2.5 NAAQS upon its effective date. 

See 73 FR 28321, 28340, May 16, 2008. 

Finally, the EPA has released several 

documents that discuss air permitting 


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229Memorandum from Joseph Goffman, Principal 

Deputy Assistant Administrator, Office of Air and 

Radiation, to Air and Radiation Division Directors, 

‘‘Principles for Addressing Environmental Justice in 

Air Permitting’’ (December 22, 2022), available at 

https://www.epa.gov/caa-permitting/ej-airpermitting-principles-addressing-environmentaljustice-concerns-air. 

230 Id., Attachment, ‘‘EJ in Air Permitting: 

Principles for Addressing Environmental Justice 

Concerns in Air Permitting’’ (December 2022), 

available at https://www.epa.gov/caa-permitting/ejair-permitting-principles-addressingenvironmental-justice-concerns-air. 

231 40 CFR part 93, subpart A. 232 40 CFR part 93 subpart B. 

233Further, the EPA’s current Unified Agenda 

and Regulatory Plan includes its intention to issue 

a proposed rule to amend the General Conformity 

Regulations. The EPA intends to address in that 

regulatory action topics regarding prescribed fire, 

including consideration of smoke management 

approaches such as those discussed in the 

Exceptional Events Rule, among other topics. See, 

e.g., https://www.reginfo.gov/public/do/eAgenda

ViewRule?pubId=202310&RIN=2060-AV28. 

and environmental justice, including, 

for example, a memorandum 229 and 

attached permitting principles.230 The 

EPA recommends that PSD and NNSR 

permitting authorities review this 

memorandum and the principles and 

consider applying them in their air 

permitting actions as appropriate to 

help identify, analyze, and address 

environmental justice concerns in those 

air permitting actions to help ensure 

that the NAAQS achieve their intended 

health benefits for at-risk populations. 

F. Transportation Conformity Program 

Transportation conformity is required 

under CAA section 176(c) to ensure that 

transportation plans, transportation 

improvement programs (TIPs) and 

federally supported highway and transit 

projects will not cause or contribute to 

any new air quality violation, increase 

the frequency or severity of any existing 

violation, or delay timely attainment or 

any required interim emissions 

reductions or other milestones. 

Transportation conformity applies to 

areas that are designated as 

nonattainment or nonattainment areas 

that have been redesignated to 

attainment with an approved CAA 

section 175A maintenance plan (i.e., 

maintenance areas) for transportationrelated criteria pollutants: carbon 

monoxide, ozone, NO2, PM2.5, and PM10. 

Transportation conformity for the 

revised primary annual PM2.5 NAAQS 

does not apply until one year after the 

effective date of nonattainment 

designations for that NAAQS. See CAA 

section 176(c)(6) and 40 CFR 93.102(d)). 

The EPA’s Transportation Conformity 

Rule 231 establishes the criteria and 

procedures for determining whether 

transportation activities conform to the 

SIP. No changes are being made to the 

transportation conformity rule in this 

final rulemaking. The EPA notes that 

the transportation conformity rule 

already addresses the PM2.5 and PM10

NAAQS. However, in the future, the 

EPA intends to review the need to issue 

or revise guidance describing how the 

current conformity rule applies in 

nonattainment and maintenance areas 

for the revised primary annual PM2.5

NAAQS, as needed. 

G. General Conformity Program 

The conformity requirement under 

CAA section 176(c) ensures that federal 

activities implemented by federal 

agencies will not interfere with a State’s 

ability to attain and maintain the 

NAAQS. Under CAA 176(c)(1), the 

requirement prohibits Federal agencies 

from approving, permitting, licensing, 

or funding activities that do not conform 

to the purpose of the applicable SIP for 

the control and prevention of air 

pollution. See CAA 176(c)(1)(A). Under 

CAA 176(c)(1)(B), conformity to an 

implementation plan means that federal 

activities will not cause or contribute to 

any new violations of the NAAQS, 

increase the frequency or severity of any 

existing NAAQS violation, or delay 

timely attainment or any required 

interim emissions reductions or other 

milestones contained in the applicable 

SIP. 

The general conformity program 232

implements CAA section 176(c)(4)(A), 

and the criteria and procedures for 

determining conformity of federal 

activities to the applicable SIP are 

established under 40 CFR part 93 

subpart B, sections 93.150 through 

93.165. General Conformity applies to 

federal activities that (1) would cause 

emissions of relevant criteria or 

precursor pollutants to originate within 

nonattainment areas or areas that have 

been redesignated to attainment with an 

approved CAA section 175A 

maintenance plan (i.e., maintenance 

areas), as set forth under 40 CFR 93.153, 

and (2) are not Federal Highway 

Administration (FHWA) or Federal 

Transit Administration (FTA) 

transportation projects as defined in 40 

CFR 93.101 under the transportation 

conformity requirements. See 40 CFR 

93.153. General conformity for the 

revised primary annual PM2.5 NAAQS 

does not apply until one year after the 

effective date of the nonattainment 

designation for that NAAQS. See 40 

CFR 93.153(k). 

With regard to issues regarding 

prescribed fires, which were addressed 

earlier in this action, here is some 

additional information regarding 

prescribed fires and General Conformity 

regulations. Under the General 

Conformity regulations at 40 CFR 

93.153(c)(4), a conformity evaluation is 

not required to support a decision by a 

federal agency to conduct or carry out 

prescribed burning when the burn is 

consistent with the terms of a land 

management plan or other plan that 

includes the prescribed burn at issue, 

where the overall plan that includes the 

burn was previously evaluated under 40 

CFR part 93 subpart B by the 

responsible federal agency, and the 

agency found the plan conforms under 

CAA paragraphs 176(c)(1)(A) and (1)(B). 

This assumes the burn at issue will be 

conducted by meeting any conditions 

specified as necessary for meeting 

conformity in the agency’s decision to 

approve the plan. Alternatively, a 

presumption of conformity applies also 

under 40 CFR 93.153(i)(2) for prescribed 

fires conducted in accordance with a 

Smoke Management Program that meets 

the requirements of the EPA’s 1998 

Interim Air Quality Policy on Wildland 

and Prescribed Fires or an equivalent 

replacement EPA policy. The preamble 

to the Exceptional Events Rule explains 

that the EPA adapted language 

associated with the six basic 

components of a certifiable Smoke 

Management Program for exceptional 

events purposes from the 1998 Interim 

Air Quality Policy on Wildland and 

Prescribed Fires (see, e.g., 81 FR 68216, 

68252 (including footnote 75), 68256, 

October 2, 2016). The Exceptional 

Events Rule at 40 CFR 50.14(a)(3)(ii)(A) 

also indicates that certain requirements 

within the Exceptional Events Rule can 

be satisfied if a prescribed fire is 

conducted under a certified Smoke 

Management Program or using 

appropriate basic smoke management 

practices such as those identified in 

Table 1 to 40 CFR 50.14 (see e.g., 81 FR 

68216, 68250–68257, 68277–68278, 

October 3, 2016). 

No changes are being made to the 

general conformity regulations in this 

final rulemaking and the EPA notes that 

the courts recognize the regulations 

constitute control for the established 

PM2.5 and PM10 NAAQS. However, in 

the future, the EPA intends to review 

the need to issue or revise guidance 

describing how the current General 

Conformity regulations apply within 

nonattainment and maintenance areas 

for the revised primary annual PM2.5

NAAQS, as needed.233

IX. Statutory and Executive Order 

Reviews 

Additional information about these 

statutes and Executive orders can be 


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found at https://www.epa.gov/lawsregulations/laws-and-executive-orders. 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 14094: Modernizing Regulatory 

Review 

This action is ‘‘significant regulatory 

action’’ as defined under section 3(f)(1) 

of Executive Order 12866, as amended 

by Executive Order 14094. Accordingly, 

the EPA submitted this action to the 

Office of Management and Budget 

(OMB) for review. Documentation of 

any changes made in response to the 

Executive Order 12866 review is 

available in the docket. The EPA 

prepared an illustrative analysis of the 

potential costs and benefits associated 

with this action. This analysis, 

‘‘Regulatory Impact Analysis for the 

Reconsideration of the National 

Ambient Air Quality Standards for 

Particulate Matter,’’ is available in the 

Regulatory Impact Analysis (RIA) 

docket (EPA–HQ–OAR–2019–0587) and 

briefly summarized below. However, the 

CAA and judicial decisions make clear 

that the economic and technical 

feasibility of attaining ambient 

standards are not to be considered in 

setting or revising NAAQS, although 

such factors may be considered in the 

development of State plans to 

implement the standards. Accordingly, 

although an RIA has been prepared, the 

results of the RIA have not been 

considered in issuing this final rule. 

The RIA estimates the costs and 

monetized human health benefits in 

2032, after implementing existing and 

expected regulations and assessing 

emissions reductions to meet the 

current primary annual and 24-hour 

particulate matter NAAQS (12/35 mg/ 

m3), associated with applying national 

control strategies for the revised annual 

and 24-hour standard levels of 9/35 mg/ 

m3, as well as the following less and 

more stringent alternative standard 

levels: (1) A less stringent alternative 

annual standard level of 10 mg/m3 in 

combination with the current 24-hour 

standard (i.e., 10/35 mg/m3), (2) a more 

stringent alternative annual standard 

level of 8 mg/m3 in combination with the 

current 24-hour standard (i.e., 8/35 mg/ 

m3), and (3) a more stringent alternative 

24-hour standard level of 30 mg/m3 in 

combination with an annual standard 

level of 10 mg/m3 (i.e., 10/30 mg/m3). 

Table 3 provides a summary of the 

estimated monetized benefits, costs, and 

net benefits associated with applying 

national control strategies toward 

reaching the revised and alternative 

standard levels. 

TABLE 3—ESTIMATED MONETIZED BENEFITS, COSTS, AND NET BENEFITS OF THE ILLUSTRATIVE CONTROL STRATEGIES

APPLIED TOWARD THE PRIMARY REVISED AND ALTERNATIVE ANNUAL AND DAILY STANDARD LEVELS OF 10/35 μg/m3, 

10/30 μg/m3, 9/35 μg/m3, AND 8/35 μg/m3 IN 2032 FOR THE U.S. 

[Millions of 2017$] 

10/35 10/30 9/35 8/35 

Benefits a ........................... $8,500 and $17,000 .......... $10,000 and $21,000 ........ $22,000 and $46,000 ........ $48,000 and $99,000. 

Costs b ............................... $200 .................................. $340 .................................. $590 .................................. $1,500. 

Net Benefits ................ $8,300 and $17,000 .......... $9,900 and $21,000 .......... $22,000 and $46,000 ........ $46,000 and $97,000. 

Notes: Rows may not appear to add correctly due to rounding. We provide a snapshot of costs and benefits in 2032, using the best available 

information to approximate social costs and social benefits recognizing uncertainties and limitations in those estimates. The estimated costs and 

monetized human health benefits associated with applying national control strategies do not fully account for all the emissions reductions needed 

to reach the final and more stringent alternative standard levels for some standard levels analyzed. aWe assume that there is a cessation lag between the change in PM exposures and the total realization of changes in mortality effects. Specifically, we assume that some of the incidences of premature mortality related to PM2.5 exposures occur in a distributed fashion over the 20 

years following exposure, which affects the valuation of mortality benefits at different discount rates. Similarly, we assume there is a cessation 

lag between the change in PM exposures and both the development and diagnosis of lung cancer. The benefits are associated with two point 

estimates from two different epidemiologic studies, and we present the benefits calculated at a real discount rate of 3 percent. The monetized 

benefits exclude additional health and welfare benefits that could not be quantified. bThe costs are annualized using a 7 percent interest rate. 

B. Paperwork Reduction Act (PRA) 

This action does not impose any new 

information collection burden under the 

PRA. OMB has previously approved the 

information collection activities 

contained in the existing regulations 

and has assigned OMB control number 

2060–0084. The data collected through 

this information collection consist of 

ambient air concentration 

measurements for the seven air 

pollutants with national ambient air 

quality standards (i.e., ozone, sulfur 

dioxide, nitrogen dioxide, lead, carbon 

monoxide, PM2.5 and PM10), ozone 

precursors, air toxics, meteorological 

variables at a select number of sites, and 

other supporting measurements. 

Accompanying the pollutant 

concentration data are quality 

assurance/quality control data and air 

monitoring network design information. 

The EPA and others (e.g., State and local 

air quality management agencies, tribal 

entities, environmental organizations, 

academic institutions, industrial groups) 

use the ambient air quality data for 

many purposes including informing the 

public and other interested parties of an 

area’s air quality, judging an area’s air 

quality in comparison with the 

established health or welfare standards, 

evaluating an air quality management 

agency’s progress in achieving or 

maintaining air pollutant levels below 

the national and local standards, 

developing and revising State 

Implementation Plans (SIPs), evaluating 

air pollutant control strategies, 

developing or revising national control 

policies, providing data for air quality 

model development and validation, 

supporting enforcement actions, 

documenting episodes and initiating 

episode controls, assessing air quality 

trends, and conducting air pollution 

research. 

C. 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 final rule 

establishes 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), rev’d in part on other grounds, 

Whitman v. American Trucking 

Associations, 531 U.S. 457 (2001). 


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D. Unfunded Mandates Reform Act 

(UMRA) 

This action does not contain an 

unfunded mandate of $100 million or 

more as described in the Unfunded 

Mandates Reform Act (UMRA), 2 U.S.C. 

1531–1538, and does not significantly or 

uniquely affect small governments. 

Furthermore, as indicated previously, in 

setting a NAAQS the EPA cannot 

consider the economic or technological 

feasibility of attaining ambient air 

quality standards, although such factors 

may be considered to a degree in the 

development of State plans to 

implement the standards. See also 

American Trucking Associations v. 

EPA, 175 F. 3d at 1043 (noting that 

because the EPA is precluded from 

considering costs of implementation in 

establishing NAAQS, preparation of the 

RIA pursuant to the Unfunded 

Mandates Reform Act would not furnish 

any information that the court could 

consider in reviewing the NAAQS). 

E. Executive Order 13132: Federalism 

This action 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. However, the EPA 

recognizes that States will have a 

substantial interest in this action and 

any future revisions to associated 

requirements. 

F. 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 as Tribes are not obligated 

to adopt or implement any NAAQS. In 

addition, Tribes are not obligated to 

conduct ambient monitoring for PM or 

to adopt the ambient monitoring 

requirements of 40 CFR part 58. Thus, 

Executive Order 13175 does not apply 

to this action. However, consistent with 

the EPA Policy on Consultation and 

Coordination with Indian Tribes, the 

EPA offered consultation to all 574 

Federally Recognized Tribes during the 

development of this action. Although no 

Tribes requested consultation, the EPA 

provided informational meetings 

including an informational meeting 

with the Pueblo de San Ildefonso and 

provided information on the monthly 

National Tribal Air Association calls. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

Executive Order 13045 directs federal 

agencies to include an evaluation of the 

health and safety effects of the planned 

regulation on children in federal health 

and safety standards and explain why 

the regulation is preferable to 

potentially effective and reasonably 

feasible alternatives. This action is 

subject to Executive Order 13045 

because it is a significant regulatory 

action under section 3(f)(1) of Executive 

Order 12866, and the EPA believes that 

the environmental health or safety risk 

addressed by this action may have a 

disproportionate effect on children. 

Accordingly, we have evaluated the 

environmental health or safety effects of 

PM exposures on children. The 

protection offered by these standards 

may be especially important for 

children because childhood represents a 

lifestage associated with increased 

susceptibility to PM-related health 

effects. Because children have been 

identified as a susceptible population, 

we have carefully evaluated the 

environmental health effects of 

exposure to PM pollution among 

children. Children make up a 

substantial fraction of the U.S. 

population, and often have unique 

factors that contribute to their increased 

risk of experiencing a health effect due 

to exposures to ambient air pollutants 

because of their continuous growth and 

development. As described in the 2019 

Integrated Science Assessment, children 

may be particularly at risk for health 

effects related to ambient air PM2.5

exposures compared with adults 

because they have (1) a developing 

respiratory system, (2) increased 

ventilation rates relative to body mass 

compared with adults, and (3) an 

increased proportion of oral breathing, 

particularly in boys, relative to adults. 

More detailed information on the 

evaluation of the scientific evidence and 

policy considerations pertaining to 

children, including an explanation for 

why the Administrator judges the 

revised standards to be requisite to 

protect public health, including the 

health of children, with an adequate 

margin of safety, are contained in 

section II.A.2. ‘‘Overview of the Health 

Effects Evidence’’, section II.A.2.b 

‘‘Public Health Implications and At-Risk 

Populations’’ and II.B ‘‘Conclusions on 

the Primary PM2.5 Standards’’ of this 

preamble. Copies of all documents have 

been placed in the public docket for this 

action. The Administrator judges that 

revising the primary annual PM2.5

standard to a level of 9.0 mg/m3 and 

retaining the primary 24-hour PM2.5

standard provides requisite public 

health protection with an adequate 

margin of safety, including for children. 

Furthermore, the Policy on Children’s 

Health also applies to this action. 

Information on how the Policy was 

applied is described in section II.A.2 

‘‘Overview of the Health Effects 

Evidence’’, section II.A.2.b ‘‘Public 

Health Implications and At-Risk 

Populations’’ and II.B ‘‘Conclusions on 

the Primary PM2.5 Standards’’ of this 

preamble. 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution or Use 

This action is not a ‘‘significant 

energy action’’ because it is not likely to 

have a significant adverse effect on the 

supply, distribution, or use of energy. 

The purpose of this action is to revise 

level of the primary annual PM2.5

NAAQS. The action does not prescribe 

specific pollution control strategies by 

which these ambient standards and 

monitoring revisions will be met. Such 

strategies will be developed by States on 

a case-by-case basis, and the EPA cannot 

predict whether the control options 

selected by States will include 

regulations on energy suppliers, 

distributors, or users. Thus, the EPA 

concludes that this action does not 

constitute a significant energy action as 

defined in Executive Order 13211. 

I. National Technology Transfer and 

Advancement Act (NTTAA) 

This rulemaking involved 

environmental monitoring or 

measurement. The EPA has decided it 

will continue to use the existing 

indicators for fine (PM2.5) and coarse 

(PM10) particles. The indicator for fine 

particles is measured using the 

Reference Method for the Determination 

of Fine Particulate Matter as PM2.5 in the 

Atmosphere (appendix L to 40 CFR part 

50), which is known as the PM2.5 FRM, 

and the indicator for coarse particles is 

measured using the Reference Method 

for the Determination of Particulate 

Matter as PM10 in the Atmosphere 

(appendix J to 40 CFR part 50), which 

is known as the PM10 FRM. 

To the extent feasible, the EPA 

employs a Performance-Based 

Measurement System (PBMS), which 

does not require the use of specific, 

prescribed analytic methods. The PBMS 

is defined as a set of processes wherein 

the data quality needs, mandates or 

limitations of a program or project are 

specified and serve as criteria for 

selecting appropriate methods to meet 

those needs in a cost-effective manner. 


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It is intended to be more flexible and 

cost effective for the regulated 

community; it is also intended to 

encourage innovation in analytical 

technology and improved data quality. 

Though the FRM defines the particular 

specifications for ambient monitors, 

there is some variability with regard to 

how monitors measure PM, depending 

on the type and size of PM and 

environmental conditions. Therefore, it 

is not practically possible to fully define 

the FRM in performance terms to 

account for this variability. 

Nevertheless, our approach in the past 

has resulted in multiple brands of 

monitors being approved as FRM for 

PM, and we expect this to continue. 

Also, the FRMs described in 40 CFR 

part 50 and the equivalency criteria 

described in 40 CFR part 53, constitute 

a performance-based measurement 

system for PM, since methods that meet 

the field testing and performance 

criteria can be approved as FEMs. Since 

finalized in 2006 (71 FR 61236, October 

17, 2006) the new field and performance 

criteria for approval of PM2.5 continuous 

FEMs has resulted in the approval of 13 

approved FEMs. In summary, for 

measurement of PM2.5 and PM10, the 

EPA relies on both FRMs and FEMs, 

with FEMs relying on a PBMS approach 

for their approval. The EPA is not 

precluding the use of any other method, 

whether it constitutes a voluntary 

consensus standard or not, as long as it 

meets the specified performance 

criteria. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations and Executive 

Order 14096: Revitalizing Our Nation’s 

Commitment to Environmental Justice 

for All 

The EPA believes that the human 

health or environmental conditions 

associated with the primary PM2.5

NAAQS that exist prior to this action 

result in or have the potential to result 

in disproportionate and adverse human 

health or environmental effects on 

communities with environmental justice 

concerns. There is strong evidence for 

racial and ethnic disparities in PM2.5

exposures and PM2.5-related health risk, 

as assessed in the 2019 Integrated 

Science Assessment and with even more 

evidence available since the literature 

cutoff date for the 2019 Integrated 

Science Assessment and evaluated in 

the Supplement to the 2019 Integrated 

Science Assessment. There is strong 

evidence demonstrating that Black and 

Hispanic populations, in particular, 

have higher PM2.5 exposures than nonHispanic White populations. Black 

populations or individuals that live in 

predominantly Black neighborhoods 

experience higher PM2.5 exposures, in 

comparison to non-Hispanic White 

populations. There is also consistent 

evidence across multiple studies that 

demonstrate increased risk of PM2.5- 

related health effects, with the strongest 

evidence for health risk disparities for 

mortality. There is also evidence of 

health risk disparities for both Hispanic 

and non-Hispanic Black populations 

compared to non-Hispanic White 

populations for cause-specific mortality 

and incident hypertension. 

Socioeconomic status (SES) is a 

composite measure that includes 

metrics such as income, occupation, or 

education, and can play a role in access 

to healthy environments as well as 

access to healthcare. SES may be a 

factor that contributes to differential risk 

from PM2.5-related health effects. 

Studies assessed in the 2019 Integrated 

Science Assessment and Supplement to 

the 2019 Integrated Science Assessment 

provide evidence that lower SES 

communities are exposed to higher 

concentrations of PM2.5 compared to 

higher SES communities. Studies using 

composite measures of neighborhood 

SES consistently demonstrated a 

disparity in both PM2.5 exposure and the 

risk of PM2.5-related health outcomes. 

There is some evidence that supports 

associations larger in magnitude 

between mortality and long-term PM2.5

exposures for those with low income or 

living in lower income areas compared 

to those with higher income or living in 

higher income neighborhoods. 

Additionally, evidence supports 

conclusions that lower SES is associated 

with cause-specific mortality and 

certain health endpoints (i.e., HI and 

CHF), but less so for all-cause or total 

(non-accidental) mortality. 

The EPA believes that this action is 

likely to reduce existing 

disproportionate and adverse effects on 

communities with environmental justice 

concerns. 

The EPA additionally identified and 

addressed environmental justice 

concerns by providing opportunities for 

public input on the proposed decisions. 

The EPA held a multi-day virtual public 

hearing for the public to provide oral 

testimony and there was a 60-day public 

comment period for the proposed 

action. As described in section II.A.3 

above, the EPA conducted a risk 

assessment to support this action that 

included an at-risk analysis that 

evaluates exposure and PM2.5 mortality 

risk for older adults (e.g., 65 years and 

older), stratified for White, Black, Asian, 

Native American, Non-Hispanic, and 

Hispanic individuals. This at-risk 

analysis found that compared to a 

primary annual PM2.5 standard with a 

level of 12.0 mg/m3, meeting a revised 

annual standard with a level of 9.0 mg/ 

m3 is estimated to reduce PM2.5- 

associated health risks in the 30 study 

areas controlled by the annual standard 

by about 22–28% and is expected to 

reduce disparities in exposure and risk 

among these populations. 

The information supporting this 

Executive Order review is is contained 

in sections II.A.2, II.B.3.a, II.B.3.c, II.B.2, 

and II.B.4. of this preamble and also in 

the 2019 Integrated Science Assessment, 

Supplement to the 2019 Integrated 

Science Assessment, and 2022 Policy 

Assessment. The EPA has carefully 

evaluated the potential impacts on 

minority populations and low SES 

populations as discussed in sections 

II.A.2, II.A.3, II.B.2, and II.B.4 of this 

preamble. The 2019 Integrated Science 

Assessment, Supplement to the 

Integrated Science Assessment, and 

2022 Policy Assessment contain the 

evaluation of the scientific evidence, 

quantitative risk analyses and policy 

considerations that pertain to these 

populations. These documents are 

available in the public docket for this 

action. 

K. 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. This action meets the criteria set 

forth in 5 U.S.C. 804(2). 

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List of Subjects 

40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

40 CFR Part 53 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Reporting and 

recordkeeping requirements. 

40 CFR Part 58 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 

relations, Reporting and recordkeeping 

requirements. 

Michael S. Regan, 

Administrator. 

For the reasons set forth in the 

preamble, chapter I of title 40 of the 

Code of Federal Regulations is amended 

as follows: 

PART 50—NATIONAL PRIMARY AND 

SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

■ 1. The authority citation for part 50 

continues to read as follows: 

Authority: 42 U.S.C. 7401, et seq. 

■ 2. Add § 50.20 to read as follows: 

§ 50.20 National primary ambient air 

quality standards for PM2.5. 

(a) The national primary ambient air 

quality standards for PM2.5 are 9.0 

micrograms per cubic meter (mg/m3) 

annual arithmetic mean concentration 

and 35 mg/m3 24-hour average 

concentration measured in the ambient 

air as PM2.5 (particles with an 

aerodynamic diameter less than or equal 

to a nominal 2.5 micrometers) by either: 

(1) A reference method based on 

appendix L to this part and designated 

in accordance with part 53 of this 

chapter; or 

(2) An equivalent method designated 

in accordance with part 53 of this 

chapter. 

(b) The primary annual PM2.5

standard is met when the annual 

arithmetic mean concentration, as 

determined in accordance with 

appendix N to this part, is less than or 

equal to 9.0 mg/m3. 

(c) The primary 24-hour PM2.5

standard is met when the 98th 

percentile 24-hour concentration, as 

determined in accordance with 

appendix N to this part, is less than or 

equal to 35 mg/m3. 

■ 3. Amend appendix K to part 50 by: 

■ a. In section 1.0 revising paragraph 

(b); 

■ b. In section 2.3 adding paragraph (d); 

and 

■ c. In section 3.0 adding paragraphs (a) 

and (b). 

The revision and additions read as 

follows: 

Appendix K to Part 50—Interpretation 

of the National Ambient Air Quality 

Standards for Particulate Matter 

1.0 General 

* * * * * 

(b) The terms used in this appendix are 

defined as follows: 

Average refers to the arithmetic mean of 

the estimated number of exceedances per 

year, as per section 3.1 of this appendix. 

Collocated monitors refer to two or more 

air measurement instruments for the same 

parameter (e.g., PM10 mass) operated at the 

same site location, and whose placement is 

consistent with part 53 of this chapter. For 

purposes of considering a combined site 

record in this appendix, when two or more 

monitors are operated at the same site, one 


ddrumheller on DSK120RN23PROD with RULES3
Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16381 

monitor is designated as the ‘‘primary’’ 

monitor with any additional monitors 

designated as ‘‘collocated.’’ It is implicit in 

these appendix procedures that the primary 

monitor and collocated monitor(s) are all 

reference or equivalent methods; however, it 

is not a requirement that the primary and 

collocated monitors utilize the same specific 

sampling and analysis method. 

Combined site data record is the data set 

used for performing computations in this 

appendix and represents data for the primary 

monitors augmented with data from 

collocated monitors according to the 

procedure specified in section 3.0(a) of this 

appendix. 

Daily value for PM10 refers to the 24-hour 

average concentration of PM10 calculated or 

measured from midnight to midnight (local 

time). 

Exceedance means a daily value that is 

above the level of the 24-hour standard after 

rounding to the nearest 10 mg/m3 (i.e., values 

ending in 5 or greater are to be rounded up). 

Expected annual value is the number 

approached when the annual values from an 

increasing number of years are averaged, in 

the absence of long-term trends in emissions 

or meteorological conditions. 

Primary monitors are suitable monitors 

designated by a State or local agency in their 

annual network plan as the default data 

source for creating a combined site data 

record. If there is only one suitable monitor 

at a particular site location, then it is 

presumed to be a primary monitor. 

Year refers to a calendar year. 

* * * * * 

2.3 Data Requirements 

* * * * * 

(d) 24-hour average concentrations will be 

computed from submitted hourly PM10

concentration data for each corresponding 

day of the year and the result will be stored 

in the first, or start, hour (i.e., midnight, hour 

‘0’) of the 24-hour period. A 24-hour average 

concentration shall be considered valid if at 

least 75 percent of the hourly averages (i.e., 

18 hourly values) for the 24-hour period are 

available. In the event that fewer than all 24 

hourly average concentrations are available 

(i.e., fewer than 24 but at least 18), the 24- 

hour average concentration shall be 

computed on the basis of the hours available 

using the number of available hours within 

the 24-hour period as the divisor (e.g., the 

divisor is 19 if 19 hourly values are 

available). 24-hour periods with 7 or more 

missing hours shall also be considered for 

computations in this appendix if, after 

substituting zero for all missing hourly 

concentrations, the resulting 24-hour average 

daily value exceeds the level of the 24-hour 

standard specified in § 50.6 after rounding to 

the nearest 10 mg/m3. 

* * * * * 

3.0 Computational Equations for the 24- 

Hour Standards 

(a) All computations shown in this 

appendix shall be implemented on a sitelevel basis. Site level concentration data shall 

be processed as follows: 

(1) The default dataset for PM10 mass 

concentrations for a site shall consist of the 

measured concentrations recorded from the 

designated primary monitor(s). All daily 

values produced by the primary monitor are 

considered part of the site record. 

(2) If a daily value is not produced by the 

primary monitor for a particular day, but a 

value is available from a single collocated 

monitor, then that collocated monitor value 

shall be considered part of the combined site 

data record. If daily value data is available 

from two or more collocated monitors, the 

average of those collocated values shall be 

used as the daily value. The data record 

resulting from this procedure is referred to as 

the ‘‘combined site data record.’’ 

(b) In certain circumstances, including but 

not limited to site closures or relocations, 

data from two nearby sites may be combined 

into a single site data record for the purpose 

of calculating a valid design value. The 

appropriate Regional Administrator may 

approve such combinations if the Regional 

Administrator determines that the measured 

concentrations do not differ substantially 

between the two sites, taking into 

consideration factors such as distance 

between sites, spatial and temporal patterns 

in air quality, local emissions and 

meteorology, jurisdictional boundaries, and 

terrain features. 

* * * * * 

■ 4. Amend appendix L to part 50 by 

revising section 7.3.4 and adding 

section 7.3.4.5 to read as follows: 

Appendix L to Part 50—Reference 

Method for the Determination of Fine 

Particulate Matter as PM2.5 in the 

Atmosphere 

* * * * * 

7.3.4 Particle size separator. The sampler 

shall be configured with one of the three 

alternative particle size separators described 

in this section. One separator is an impactortype separator (WINS impactor) described in 

sections 7.3.4.1, 7.3.4.2, and 7.3.4.3 of this 

appendix. One alternative separator is a 

cyclone-type separator (VSCCTM) described 

in section 7.3.4.4 of this appendix. The other 

alternative separator is also a cyclone-type 

separator (TE–PM2.5C) described in section 

7.3.4.5 of this appendix. 

* * * * * 

7.3.4.5 A second cyclone-type separator is 

identified as a Tisch TE–PM2.5C Cyclone 

particle size separator specified as part of 

EPA-designated reference method RFPS– 

1014–219 and as manufactured by Tisch 

Environmental Incorporated, 145 S. Miami 

Avenue, Village of Cleves, Ohio 45002. 

* * * * * 

■ 5. Amend appendix N to part 50 by: 

■ a. In section 1.0 revising paragraph (a); 

■ b. In section 3.0 adding paragraph 

(d)(3); 

■ c. In section 4.1 revising paragraph (a); 

and 

■ d. In section 4.2 revising paragraph 

(a). 

The addition and revisions read as 

follows. 

Appendix N to Part 50—Interpretation 

of the National Ambient Air Quality 

Standards for PM2.5

1.0 General 

(a) This appendix explains the data 

handling conventions and computations 

necessary for determining when the national 

ambient air quality standards (NAAQS) for 

PM2.5 are met, specifically the primary and 

secondary annual and 24-hour PM2.5 NAAQS 

specified in §§ 50.7, 50.13, 50.18, and 50.20. 

PM2.5 is defined, in general terms, as particles 

with an aerodynamic diameter less than or 

equal to a nominal 2.5 micrometers. PM2.5

mass concentrations are measured in the 

ambient air by a Federal Reference Method 

(FRM) based on appendix L to this part, as 

applicable, and designated in accordance 

with part 53 of this chapter or by a Federal 

Equivalent Method (FEM) designated in 

accordance with part 53 of this chapter. Only 

those FRM and FEM measurements that are 

derived in accordance with part 58 of this 

chapter (i.e., that are deemed ‘‘suitable’’) 

shall be used in comparisons with the PM2.5

NAAQS. The data handling and computation 

procedures to be used to construct annual 

and 24-hour NAAQS metrics from reported 

PM2.5 mass concentrations, and the 

associated instructions for comparing these 

calculated metrics to the levels of the PM2.5

NAAQS, are specified in sections 2.0, 3.0, 

and 4.0 of this appendix. 

* * * * * 

3.0 Requirements for Data Use and Data 

Reporting for Comparisons With the NAAQS 

for PM2.5

* * * * * 

(d) * * * 

(3) In certain circumstances, including but 

not limited to site closures or relocations, 

data from two nearby sites may be combined 

into a single site data record for the purpose 

of calculating a valid design value. The 

appropriate Regional Administrator may 

approve such site combinations if the 

Regional Administrator determines that the 

measured concentrations do not differ 

substantially between the two sites, taking 

into consideration factors such as distance 

between sites, spatial and temporal patterns 

in air quality, local emissions and 

meteorology, jurisdictional boundaries, and 

terrain features. 

* * * * * 

4.1 Annual PM2.5 NAAQS 

(a) Levels of the primary and secondary 

annual PM2.5 NAAQS are specified in 

§§ 50.7, 50.13, 50.18, and 50.20 as applicable. 

* * * * * 

4.2 Twenty-Four-Hour PM2.5 NAAQS 

(a) Levels of the primary and secondary 24- 

hour PM2.5 NAAQS are specified in §§ 50.7, 

50.13, 50.18, and 50.20 as applicable. 

* * * * * 


ddrumheller on DSK120RN23PROD with RULES3
16382 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

PART 53—AMBIENT AIR MONITORING 

REFERENCE AND EQUIVALENT 

METHODS 

■ 6. The authority citation for part 53 

continues to read as follows: 

Authority: Sec. 301(a) of the Clean Air Act 

(42 U.S.C. 1857g(a)), as amended by sec. 

15(c)(2) of Pub. L. 91–604, 84 Stat. 1713, 

unless otherwise noted. 

Subpart A—General Provisions 

■ 7. Amend § 53.4 by: 

■ a. Revising paragraph (a); 

■ b. Adding paragraph (b)(7); and 

■ c. Revising paragraph (d). 

The revisions and addition read as 

follows: 

§ 53.4 Applications for reference or 

equivalent method determinations. 

(a) Applications for FRM or FEM 

determinations and modification 

requests of existing designated 

instruments shall be submitted to: U.S. 

Environmental Protection Agency, 

Director, Center for Environmental 

Measurement and Modeling, Reference 

and Equivalent Methods Designation 

Program (MD–D205–03), 109 T.W. 

Alexander Drive, P.O. Box 12055, 

Research Triangle Park, North Carolina 

27711 (commercial delivery address: 

4930 Old Page Road, Durham, North 

Carolina 27703). 

* * * * * 

(b) * * * 

(7) All written materials for new FRM 

and FEM applications and modification 

requests must be submitted in English 

in MS Word format. For any calibration 

certificates originally written in a nonEnglish language, the original nonEnglish version of the certificate must 

be submitted to EPA along with a 

version of the certificate translated to 

English. All laboratory and field data 

associated with new FRM and FEM 

applications and modification requests 

must be submitted in MS Excel format. 

All worksheets in MS Excel must be 

unprotected to enable full inspection as 

part of the application review process. 

* * * * * 

(d) For candidate reference or 

equivalent methods or for designated 

instruments that are the subject of a 

modification request, the applicant, if 

requested by EPA, shall provide to EPA 

a representative sampler or analyzer for 

test purposes. The sampler or analyzer 

shall be shipped free on board (FOB) 

destination to Director, Center for 

Environmental Measurements and 

Modeling, Reference and Equivalent 

Methods Designation Program (MD 

D205–03), U.S. Environmental 

Protection Agency, 4930 Old Page Road, 

Durham, North Carolina 27703, 

scheduled to arrive concurrently with or 

within 30 days of the arrival of the other 

application materials. This sampler or 

analyzer may be subjected to various 

tests that EPA determines to be 

necessary or appropriate under § 53.5(f), 

and such tests may include special tests 

not described in this part. If the 

instrument submitted under this 

paragraph (d) malfunctions, becomes 

inoperative, or fails to perform as 

represented in the application before the 

necessary EPA testing is completed, the 

applicant shall be afforded the 

opportunity to repair or replace the 

device at no cost to the EPA. Upon 

completion of EPA testing, the sampler 

or analyzer submitted under this 

paragraph (d) shall be repacked by EPA 

for return shipment to the applicant, 

using the same packing materials used 

for shipping the instrument to EPA 

unless alternative packing is provided 

by the applicant. Arrangements for, and 

the cost of, return shipment shall be the 

responsibility of the applicant. The EPA 

does not warrant or assume any liability 

for the condition of the sampler or 

analyzer upon return to the applicant. 

■ 8. Amend § 53.8 by revising paragraph 

(a) to read as follows: 

§ 53.8 Designation of reference and 

equivalent methods. 

(a) A candidate method determined 

by the Administrator to satisfy the 

applicable requirements of this part 

shall be designated as an FRM or FEM 

(as applicable) by and upon publication 

of the designation in the Federal 

Register. Applicants shall not publicly 

announce, market, or sell the candidate 

sampler and analyzer as an approved 

FRM or FEM (as applicable) until the 

designation is published in the Federal 

Register. 

* * * * * 

■ 9. Amend § 53.14 by revising 

paragraphs (c)(4), (5), and (6) to read as 

follows: 

§ 53.14 Modification of a reference or 

equivalent method. 

* * * * * 

(c) * * * 

(4) Send notice to the applicant that 

additional information must be 

submitted before a determination can be 

made and specify the additional 

information that is needed (in such 

cases, the 90-day period shall 

commence upon receipt of the 

additional information). 

(5) Send notice to the applicant that 

additional tests are necessary and 

specify which tests are necessary and 

how they shall be interpreted (in such 

cases, the 90-day period shall 

commence upon receipt of the 

additional test data). 

(6) Send notice to the applicant that 

additional tests will be conducted by 

the Administrator and specify the 

reasons for and the nature of the 

additional tests (in such cases, the 90- 

day period shall commence 1 calendar 

day after the additional tests are 

completed). 

* * * * * 

■ 10. Revise table A–1 to subpart A of 

part 53 to read as follows: 

TABLE A–1 TO SUBPART A OF PART 53—SUMMARY OF APPLICABLE REQUIREMENTS FOR REFERENCE AND EQUIVALENT

METHODS FOR AIR MONITORING OF CRITERIA POLLUTANTS

Pollutant Reference or equivalent Manual or automated 

Applicable 

appendix of 

part 50 of this 

chapter 

Applicable subparts of this part 

A B C D E F 

SO2 ............ Reference ........................... Manual ............................... A–2 

Automated .......................... A–1 ✓ ✓ 

Equivalent .......................... Manual ............................... A–1 ✓ ................ ✓ 

Automated .......................... A–1 ✓ ✓ ✓ 

CO .............. Reference ........................... Automated .......................... C ✓ ✓ 

Equivalent .......................... Manual ............................... C ✓ ................ ✓ 

Automated .......................... C ✓ ✓ ✓ 

O3 ............... Reference ........................... Automated .......................... D ✓ ✓ 

Equivalent .......................... Manual ............................... D ✓ ................ ✓ 

Automated .......................... D ✓ ✓ ✓ 

NO2 ............ Reference ........................... Automated .......................... F ✓ ✓ 

Equivalent .......................... Manual ............................... F ✓ ................ ✓ 

Automated .......................... F ✓ ✓ ✓ 


ddrumheller on DSK120RN23PROD with RULES3
Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16383 

TABLE A–1 TO SUBPART A OF PART 53—SUMMARY OF APPLICABLE REQUIREMENTS FOR REFERENCE AND EQUIVALENT

METHODS FOR AIR MONITORING OF CRITERIA POLLUTANTS—Continued 

Pollutant Reference or equivalent Manual or automated 

Applicable 

appendix of 

part 50 of this 

chapter 

Applicable subparts of this part 

A B C D E F 

Pb ............... Reference ........................... Manual ............................... G 

Equivalent .......................... Manual ............................... G ✓ ................ ✓ 

Automated .......................... G ✓ ................ ✓ 

PM10-Pb ..... Reference ........................... Manual ............................... Q 

Equivalent .......................... Manual ............................... Q ✓ ................ ✓ 

Automated .......................... Q ✓ ................ ✓ 

PM10 ........... Reference ........................... Manual ............................... J ✓ ................ ................ ✓ 

Equivalent .......................... Manual ............................... J ✓ ................ ✓ ✓ 

Automated .......................... J ✓ ................ ✓ ✓ 

PM2.5 .......... Reference ........................... Manual ............................... L ✓ ................ ................ ................ ✓ 

Equivalent Class I .............. Manual ............................... L ✓ ................ ✓ ................ ✓ 

Equivalent Class II ............. Manual ............................... L 1 ✓ ................ 2✓ ................ ✓ 1 2✓ 

Equivalent Class III ............ Automated .......................... L 1 ✓ ................ ✓ ................ ✓ 1✓ 

PM10–2.5 ..... Reference ........................... Manual ............................... L,2 O ✓ ................ ................ ................ ✓ 

Equivalent Class I .............. Manual ............................... L,2 O ✓ ................ ✓ ................ ✓ 

Equivalent Class II ............. Manual ............................... L,2 O ✓ ................ 2✓ ................ ✓ 1,2✓ 

Equivalent Class III ............ Automated .......................... 1 L, 1 2O ✓ ................ ✓ ................ ✓ 1✓ 

1Some requirements may apply, based on the nature of each particular candidate method, as determined by the Administrator. 

2Alternative Class III requirements may be substituted. 

Subpart B—Procedures for Testing 

Performance Characteristics of 

Automated Methods for SO2, CO, O3, 

and NO2

■ 11. Amend table B–1 to subpart B of 

part 53 by revising footnote 4 to read as 

follows: 

Table B–1 to Subpart B of Part 53— 

Performance Limit Specifications for 

Automated Methods 

* * * * * 4For nitric oxide interference for the SO2

ultraviolet fluorescence (UVF) method, 

interference equivalent is ±0.003 ppm for the 

lower range. 

* * * * * 

■ 12. Revise table B–3 to subpart B of 

part 53 to read as follows: 


ddrumheller on DSK120RN23PROD with RULES3
16384 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

TABLE B–3 TO

SUBPART B OF

PART 53—INTERFERENT

TEST

CONCENTRATION 1

[Parts per million] 

Pollutant Analyzer type 2

Hydrochloric 

acid 

Ammonia Hydrogen sulfide Sulfur dioxide Nitrogen dioxide Nitric oxide Carbon dioxide Ethylene Ozone M-xylene Water vapor Carbon monoxide Methane Ethane Naphthalene 

SO2 .............. Ultraviolet fluorescence ..................................................................... .............. .............. 5 0.1 4 0.14 0.5 0.5 ................ ................ 0.5 0.2 20,000 ................ .............. .............. 6 0.05 SO2 .............. Flame photometric ............................................................................. .............. .............. 0.01 4 0.14 ................ .............. 750 ................ .............. ................ 3 20,000 50 .............. .............. ......................SO2 .............. Gas chromatography ......................................................................... .............. .............. 0.1 4 0.14 ................ .............. 750 ................ .............. ................ 3 20,000 50 .............. .............. ......................SO2 .............. Spectrophotometric-wet chemical (pararosanaline) .......................... 0.2 0.1 0.1 4 0.14 0.5 .............. 750 ................ 0.5 ................ .............. ................ .............. .............. ......................SO2 .............. Electrochemical ................................................................................. 0.2 0.1 0.1 4 0.14 0.5 0.5 ................ 0.2 0.5 ................ 3 20,000 ................ .............. .............. ......................SO2 .............. Conductivity ....................................................................................... 0.2 0.1 ................ 4 0.14 0.5 .............. 750 ................ .............. ................ .............. ................ .............. .............. ......................SO2 .............. Spectrophotometric-gas phase, including DOAS .............................. .............. .............. ................ 4 0.14 0.5 0.5 ................ ................ 0.5 0.2 .............. ................ .............. .............. ......................O3 ................ Ethylene Chemiluminescence ........................................................... .............. .............. 3 0.1 .............. ................ .............. 750 ................ 4 0.08 ................ 3 20,000 ................ .............. .............. ......................O3 ................ NO-chemiluminescence .................................................................... .............. .............. 3 0.1 .............. 0.5 .............. 750 ................ 4 0.08 ................ 3 20,000 ................ .............. .............. ......................O3 ................ Electrochemical ................................................................................. .............. 3 0.1 ................ 0.5 0.5 .............. ................ ................ 4 0.08 ................ 3 20,000 ................ .............. .............. ......................O3 ................ Spectrophotometric-wet chemical (potassium iodide) ...................... .............. 3 0.1 ................ 0.5 0.5 3 0.5 ................ ................ 4 0.08 ................ .............. ................ .............. .............. ......................O3 ................ Spectrophotometric-gas phase, including ultraviolet absorption and 

DOAS.

.............. .............. ................ 0.5 0.5 3 0.5 ................ ................ 4 0.08 0.02 20,000 ................ .............. .............. ......................

CO ............... Non-dispersive Infrared ..................................................................... .............. .............. ................ .............. ................ .............. 750 ................ .............. ................ 20,000 4 10 .............. .............. ......................CO ............... Gas chromatography with flame ionization detector ......................... .............. .............. ................ .............. ................ .............. ................ ................ .............. ................ 20,000 4 10 .............. 0.5 ......................CO ............... Electrochemical ................................................................................. .............. .............. ................ .............. ................ 0.5 ................ 0.2 .............. ................ 20,000 4 10 .............. .............. ......................CO ............... Catalytic combustion-thermal detection ............................................ .............. 0.1 ................ .............. ................ .............. 750 0.2 .............. ................ 20,000 4 10 5.0 0.5 ......................CO ............... IR fluorescence ................................................................................. .............. .............. ................ .............. ................ .............. 750 ................ .............. ................ 20,000 4 10 .............. 0.5 ......................CO ............... Mercury replacement-UV photometric .............................................. .............. .............. ................ .............. ................ .............. ................ 0.2 .............. ................ .............. 4 10 .............. 0.5 ......................NO2 .............. Chemiluminescent ............................................................................. .............. 3 0.1 ................ 0.5 4 0.1 0.5 ................ ................ .............. ................ 20,000 ................ .............. .............. ......................NO2 .............. Spectrophotometric-wet chemical (azo-dye reaction) ....................... .............. .............. ................ 0.5 4 0.1 0.5 750 ................ 0.5 ................ .............. ................ .............. .............. ......................NO2 .............. Electrochemical ................................................................................. 0.2 3 0.1 ................ 0.5 4 0.1 0.5 750 ................ 0.5 ................ 20,000 50 .............. .............. ......................NO2 .............. Spectrophotometric-gas phase ......................................................... .............. 3 0.1 ................ 0.5 4 0.1 0.5 ................ ................ 0.5 ................ 20,000 50 .............. .............. ......................

1 Concentrations of interferent listed must be prepared and controlled to ±10 percent of the stated value. 2Analyzer types not listed will be considered by the Administrator as special cases. 3 Do not mix interferent with the pollutant. 4 Concentration of pollutant used for test. These pollutant concentrations must be prepared to ±10 percent of the stated value. 5 If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test. 6 If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full-scale response, reattach the scrubber, and evaluate response for interference. 


ddrumheller on DSK120RN23PROD with RULES3
Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16385 

■ 13. Amend appendix A to subpart B 

of part 53 by revising figures B–3 and 

B–5 to read as follows: 

Appendix A to Subpart B of Part 53— 

Optional Forms for Reporting Test 

Results 

* * * * * 

Figure B–3 to Appendix A to Subpart B of 

Part 53—Form for Test Data and 

Calculations for Lower Detectable Limit 

(LDL) and Interference Equivalent (IE) (see 

§ 53.23(c) and (d)) 

LDL Interference Test Data 

Applicant llllllllllllllll

Analyzer llllllllllllllll

Date llllllllllllllllll

Pollutant llllllllllllllll

* * * * * Figure B–5 to Appendix A to Subpart B of 

Part 53—Form for Calculating Zero Drift, 

Span Drift and Precision (see § 53.23(e)) 

Calculation of Zero Drift, Span Drift, and 

Precision 

Applicant llllllllllllllll

Analyzer llllllllllllllll

Date llllllllllllllllll

Pollutant llllllllllllllll


ddrumheller on DSK120RN23PROD with RULES3ER06MR24.035</GPH> ER06MR24.036</GPH>

TEST READING or 

PARAMETER CALCUIATION 1 

Bz 

LOWER Bi. DmCTABLE UMIT LDL= Bi.-Bz 

Rs 

1 Ra. 

IE=Ra.-R, 

Rz 

2 Ra 

IE=Ra-R, 

R. 

INTER- 3* Rs 

FERENCE IE=Re-R. 

EQUIV- R. 

ALENT 4* R,. 

IE=Ra-R. 

Rs 

5* Re 

IE=R.,-R. 

.. TOTAL* z'.11e,1 

*If required. 

TEST 

PARAMmR 

ZERO 

12 

HOUR 

DRIFT 24 

HOUR 

SPAN 24 

DRIFT HOUR 

20% 

URL 

PREC- (P20) 

1=1 

CALCUIATION 

12ZD = c....-c.... 

Z =(Lt+ LJ/2 

24ZD•Z.-Z..., 

24ZD=Z'.-Z',.., 

SD,. = S,. -S..-i 

S..-1 

X100% 

SD,.= S,.-S',._i 

S',._1 

X100% 

P20 =%STANDARD 

DEVIATION of (P, ... P,) 

TEST NUMBER 

2 3 4 5 6 7 8 9 10 11 12 13 14 15 

TESTDAY(n) 

1 2 3 4 5 6 7 8 9 10 11 U 13 14 15 

1------------+---+----+---t---+---+---t---,t---+--+----+---t---+---t,---+---t ISION 80% 

URL 

(Pao) 

P20 =%STANDARD 

DEVIATION of (P.,."P12) 
16386 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

* * * * * 

Subpart C—Procedures for 

Determining Comparability Between 

Candidate Methods and Reference 

Methods 

■ 14. Amend § 53.35 by revising 

paragraph (b)(1)(ii)(D) to read as follows: 

§ 53.35 Test procedure for Class II and 

Class III methods for PM2.5 and PM10¥2.5. 

* * * * * 

(b) * * * 

(1) * * * 

(ii) * * * 

(D) Site D shall be in a large city east 

of the Mississippi River, having 

characteristically high humidity levels. 

* * * * * 

■ 15. Revise table C–4 to subpart C of 

part 53 to read as follows: 

TABLE C–4 TO SUBPART C OF PART 53—TEST SPECIFICATIONS FOR PM10, PM2.5, AND PM10–2.5 CANDIDATE

EQUIVALENT METHODS

Specification PM10

PM2.5 PM10–2.5

Class I Class II Class III Class II Class III 

Acceptable concentration range (Rj), μg/ 

m3.

5–300 ....................... 3–200 ....................... 3–200 ....................... 3–200 ....................... 3–200 ....................... 3–200. 

Minimum number of 

test sites.

2 ............................... 1 ............................... 2 ............................... 4 ............................... 2 ............................... 4. 

Minimum number of 

candidate method 

samplers or analyzers per site.

3 ............................... 3 ............................... 31 ............................. 31 ............................. 31 ............................. 3.1

Number of reference 

method samplers 

per site.

3 ............................... 3 ............................... 31 ............................. 31 ............................. 31 ............................. 3.1

Minimum number of 

acceptable sample 

sets per site for 

PM10 methods: 

Rj < 20 μg/m3 ...... 3 ............................... .................................. .................................. .................................. ..................................

Rj > 20 μg/m3 ...... 3 ............................... .................................. .................................. .................................. ..................................

Total ............. 10 ............................. .................................. .................................. .................................. ..................................

Minimum number of 

acceptable sample 

sets per site for 

PM2.5 and PM10–2.5

candidate equivalent 

methods: 

Rj < 15 μg/m3 for 

24-hr or Rj < 8 

μg/m3 for 48-hr 

samples..

.................................. 3 ............................... 3 ............................... 3 ............................... 3 ............................... 3. 

Rj > 15 μg/m3 for 

24-hr or Rj > 8 

μg/m3 for 48-hr 

samples.

.................................. 3 ............................... 3 ............................... 3 ............................... 3 ............................... 3. 

Each season ........ .................................. 10 ............................. 23 ............................. 23 ............................. 23 ............................. 23. 

Total, each 

site.

.................................. 10 ............................. 23 ............................. 23 (46 for two-season sites).23 ............................. 23 (46 for two-season sites). 

Precision of replicate 

reference method 

measurements, PRj

or RPRj, respectively; RP for Class 

II or III PM2.5 or 

PM10–2.5, maximum.

5 μg/m3 or 7%. ........ 2 μg/m3 or 5%. ........ 10%2 ........................ 10%2 ........................ 10%2 ........................ 10%.2

Precision of PM2.5 or 

PM10–2.5 candidate 

method, CP, each 

site.

.................................. .................................. 10%2 ........................ 15%2 ........................ 15%2 ........................ 15%.2

Slope of regression relationship.1 ±0.10 ..................... 1 ±0.05 ..................... 1 ±0.10 ..................... 1 ±0.10 ..................... 1 ±0.10 ..................... 1 ±0.12. 

Intercept of regression 

relationship, μg/m3.

0 ±5 .......................... 0 ±1 .......................... Between: 13.55— 

(15.05 × slope), 

but not less than— 

1.5; and 16.56— 

(15.05 × slope), 

but not more than 

+1.5.

Between: 15.05— 

(17.32 × slope), 

but not less than— 

2.0; and 15.05— 

(13.20 × slope), 

but not more than 

+2.0.

Between: 62.05— 

(70.5 × slope), but 

not less than—3.5; 

and 78.95—(70.5 

× slope), but not 

more than +3.5.

Between: 70.50— 

(82.93 × slope), 

but not less than— 

7.0; and 70.50— 

(61.16 × slope), 

but not more than 

+7.0. 

Correlation of reference method and 

candidate method 

measurements.

≥ 0.97 ....................... ≥ 0.97 ....................... ≥ 0.93—for CCV ≤ 0.4; 

≥ 0.85 + 0.2 × CCV—for 0.4 ≤ CCV ≤ 0.5; 

≥ 0.95—for CCV ≥ 0.5 

1Some missing daily measurement values may be permitted; see test procedure. 

2 Calculated as the root mean square over all measurement sets. 


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Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16387 

Subpart D—Procedures for Testing 

Performance Characteristics of 

Methods for PM10

■ 16. Amend § 53.43 by revising the 

formula in paragraph (a)(2)(xvi) and the 

formula in paragraph (c)(2)(iv) to read as 

follows: 

§ 53.43 Test procedures. 

(a) * * * 

(2) * * * 

(xvi) * * * 

* * * * * 

(c) * * * 

(2) * * * 

(iv) * * * 

if C ̄ j is below 80 mg/m3, or 

if C ̄ j is above 80 mg/m3. 

Subpart E—Procedures for Testing 

Physical (Design) and Performance 

Characteristics of Reference Methods 

and Class I and Class II Equivalent 

Methods for PM2.5 or PM10–2.5 

■ 17. Amend § 53.51 by revising 

paragraph (d)(2) to read as follows: 

§ 53.51 Demonstration of compliance with 

design specifications and manufacturing 

and test requirements. 

* * * * * 

(d) * * * 

(2) VSCC and TE–PM2.5C separators. 

For samplers and monitors utilizing the 

BGI VSCC or Tisch TE–PM2.5C particle 

size separators specified in sections 

7.3.4.4 and 7.3.4.5 of appendix L to part 

50 of this chapter, respectively, the 

respective manufacturers shall identify 

the critical dimensions and 

manufacturing tolerances for the 

separator, devise appropriate test 

procedures to verify that the critical 

dimensions and tolerances are 

maintained during the manufacturing 

process, and carry out those procedures 

on each separator manufactured to 

verify conformance of the manufactured 

products. The manufacturer shall also 

maintain records of these tests and their 

test results and submit evidence that 

this procedure is incorporated into the 

manufacturing procedure, that the test is 

or will be routinely implemented, and 

that an appropriate procedure is in 

place for the disposition of units that 

fail this tolerance tests. 

* * * * * 

Subpart F—Procedures for Testing 

Performance Characteristics of Class II 

Equivalent Methods for PM2.5

■ 18. Amend § 53.61 by revising 

paragraph (g) introductory text, the first 

sentence of paragraph (g)(1), the first 

sentence of (g)(1)(i), (g)(2)(i) and adding 

paragraph (g)(2)(iii) to read as follows: 

§ 53.61 Test conditions. 

* * * * * 

(g) Vibrating Orifice Aerosol 

Generator (VOAG) and Flow-Focusing 

Monodisperse Aerosol Generator 

(FMAG) conventions. This section 

prescribes conventions regarding the 

use of the vibrating orifice aerosol 

generator (VOAG) and the flow-focusing 

monodisperse aerosol generator (FMAG) 

for the size-selective performance tests 

outlined in §§ 53.62, 53.63, 53.64, and 

53.65. 

(1) Particle aerodynamic diameter. 

The VOAG and FMAG produce nearmonodisperse droplets through the 

controlled breakup of a liquid jet. * * * 

(i) The physical diameter of a 

generated spherical particle can be 

calculated from the operational 

parameters of the VOAG and FMAG as: 

* * * * * 

(2) * * * 

(i) Solid particle tests performed in 

this subpart shall be conducted using 

particles composed of ammonium 

fluorescein. For use in the VOAG or 

FMAG, liquid solutions of known 

volumetric concentration can be 

prepared by diluting fluorescein powder 

(C2OH12O5, FW = 332.31, CAS 2321–07– 

5) with aqueous ammonia. Guidelines 

for preparation of fluorescein solutions 

of the desired volume concentration 

(Cvol) are presented in Vanderpool and 

Rubow (1988) (Reference 2 in appendix 

A to this subpart). For purposes of 

converting particle physical diameter to 

aerodynamic diameter, an ammonium 

fluorescein particle density of 1.35 g/ 

cm3 shall be used. 

* * * * * 

(iii) Calculation of the physical 

diameter of the particles produced by 

the VOAG and FMAG requires 


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16388 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

knowledge of the liquid solution’s 

volume concentration (Cvol). Because 

uranine is essentially insoluble in oleic 

acid, the total particle volume is the 

sum of the oleic acid volume and the 

uranine volume. The volume 

concentration of the liquid solution 

shall be calculated as: 

Where: 

Vu = uranine volume, ml; 

Voleic = oleic acid volume, ml; 

Vsol = total solution volume, ml; 

Mu = uranine mass, g; 

Pu = uranine density, g/cm3; 

Moleic = oleic acid mass, g; and 

Poleic = oleic acid density, g/cm3. 

* * * * * 

PART 58—AMBIENT AIR QUALITY 

SURVEILLANCE 

■ 19. The authority citation for part 58 

continues to read as follows: 

Authority: 42 U.S.C. 7403, 7405, 7410, 

7414, 7601, 7611, 7614, and 7619. 

Subpart A—General Provisions 

■ 20. Amend § 58.1 by: 

■ a. Removing the definition for 

‘‘Approved regional method (ARM)’’; 

and 

■ b. Revising the definition for 

‘‘Traceable.’’ 

The revision reads as follows: 

§ 58.1 Definitions. 

* * * * * 

Traceable means a measurement 

result from a local standard whereby the 

result can be related to the International 

System of Units (SI) through a 

documented unbroken chain of 

calibrations, each contributing to the 

measurement uncertainty. Traceable 

measurement results must be compared 

and certified, either directly or via not 

more than one intermediate standard, to 

a National Institute of Standards and 

Technology (NIST)-certified reference 

standard. Examples include but are not 

limited to NIST Standard Reference 

Material (SRM), NIST-traceable 

Reference Material (NTRM), or a NISTcertified Research Gas Mixture (RGM). 

Traceability to the SI through other 

National Metrology Institutes (NMIs) in 

addition to NIST is allowed if a 

Declaration of Equivalence (DoE) exists 

between NIST and that NMI. 

* * * * * 

Subpart B—Monitoring Network 

■ 21. Amend § 58.10 by: 

■ a. Revising paragraphs (a)(1) and 

(b)(10) and (13); 

■ b. Adding paragraph (b)(14); and 

■ c. Revising paragraph (d). 

The revisions and addition read as 

follows: 

§ 58.10 Annual monitoring network plan 

and periodic network assessment. 

(a)(1) Beginning July 1, 2007, the 

State, or where applicable local, agency 

shall submit to the Regional 

Administrator an annual monitoring 

network plan which shall provide for 

the documentation of the establishment 

and maintenance of an air quality 

surveillance system that consists of a 

network of SLAMS monitoring stations 

that can include FRM and FEM 

monitors that are part of SLAMS, NCore, 

CSN, PAMS, and SPM stations. The 

plan shall include a statement of 

whether the operation of each monitor 

meets the requirements of appendices 

A, B, C, D, and E to this part, where 

applicable. The Regional Administrator 

may require additional information in 

support of this statement. The annual 

monitoring network plan must be made 

available for public inspection and 

comment for at least 30 days prior to 

submission to the EPA and the 

submitted plan shall include and 

address, as appropriate, any received 

comments. 

* * * * * 

(b) * * * 

(10) Any monitors for which a waiver 

has been requested or granted by the 

EPA Regional Administrator as allowed 

for under appendix D or appendix E to 

this part. For those monitors where a 

waiver has been approved, the annual 

monitoring network plan shall include 

the date the waiver was approved. 

* * * * * 

(13) The identification of any PM2.5

FEMs used in the monitoring agency’s 

network where the data are not of 

sufficient quality such that data are not 

to be compared to the national ambient 

air quality standards (NAAQS). For 

required SLAMS where the agency 

identifies that the PM2.5 Class III FEM 

does not produce data of sufficient 

quality for comparison to the NAAQS, 

the monitoring agency must ensure that 

an operating FRM or filter-based FEM 

meeting the sample frequency 

requirements described in § 58.12 or 

other Class III PM2.5 FEM with data of 

sufficient quality is operating and 

reporting data to meet the network 

design criteria described in appendix D 

to this part. 

(14) The identification of any site(s) 

intended to address being sited in an atrisk community where there are 

anticipated effects from sources in the 

area as required in section 4.7.1(b)(3) of 

appendix D to this part. An initial 

approach to the question of whether any 

new or moved sites are needed and to 

identify the communities in which they 

intend to add monitoring for meeting 

the requirement in this paragraph 

(b)(14), if applicable, shall be submitted 

in accordance with the requirements of 

section 4.7.1(b)(3) of appendix D to this 

part, which includes submission to the 

EPA Regional Administrator no later 

than July 1, 2024. Specifics on the 

resulting proposed new or moved sites 

for PM2.5 network design to address atrisk communities, if applicable, would 

need to be detailed in annual 

monitoring network plans due to each 

applicable EPA Regional office no later 

than July 1, 2025. The plan shall 

provide for any required sites to be 

operational no later than 24 months 

from date of approval of a plan or 

January 1, 2027, whichever comes first. 

* * * * * 

(d) The State, or where applicable 

local, agency shall perform and submit 

to the EPA Regional Administrator an 

assessment of the air quality 

surveillance system every 5 years to 

determine, at a minimum, if the network 

meets the monitoring objectives defined 

in appendix D to this part, whether new 

sites are needed, whether existing sites 

are no longer needed and can be 

terminated, and whether new 

technologies are appropriate for 

incorporation into the ambient air 

monitoring network. The network 

assessment must consider the ability of 

existing and proposed sites to support 

air quality characterization for areas 

with relatively high populations of 


ddrumheller on DSK120RN23PROD with RULES3ER06MR24.040</GPH>

Equation 5 to Paragraph (g)(2)(iii) 

Vu+ Voleic 

Cvol = V. 

sol 

(Mu/ Pu)+ (Moleic/ PoleiJ 

Vsol 
Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16389 

susceptible individuals (e.g., children 

with asthma) and other at-risk 

populations, and, for any sites that are 

being proposed for discontinuance, the 

effect on data users other than the 

agency itself, such as nearby States and 

Tribes or health effects studies. The 

State, or where applicable local, agency 

must submit a copy of this 5-year 

assessment, along with a revised annual 

network plan, to the Regional 

Administrator. The assessments are due 

every 5 years beginning July 1, 2010. 

* * * * * 

■ 22. Amend § 58.11 by revising 

paragraphs (a)(2) and (e) to read as 

follows: 

§ 58.11 Network technical requirements. 

(a) * * * 

(2) Beginning January 1, 2009, State 

and local governments shall follow the 

quality assurance criteria contained in 

appendix A to this part that apply to 

SPM sites when operating any SPM site 

which uses an FRM or an FEM and 

meets the requirements of appendix E to 

this part, unless the Regional 

Administrator approves an alternative to 

the requirements of appendix A with 

respect to such SPM sites because 

meeting those requirements would be 

physically and/or financially 

impractical due to physical conditions 

at the monitoring site and the 

requirements are not essential to 

achieving the intended data objectives 

of the SPM site. Alternatives to the 

requirements of appendix A may be 

approved for an SPM site as part of the 

approval of the annual monitoring plan, 

or separately. 

* * * * * 

(e) State and local governments must 

assess data from Class III PM2.5 FEM 

monitors operated within their network 

using the performance criteria described 

in table C–4 to subpart C of part 53 of 

this chapter, for cases where the data are 

identified as not of sufficient 

comparability to a collocated FRM, and 

the monitoring agency requests that the 

FEM data should not be used in 

comparison to the NAAQS. These 

assessments are required in the 

monitoring agency’s annual monitoring 

network plan described in § 58.10(b) for 

cases where the FEM is identified as not 

of sufficient comparability to a 

collocated FRM. For these collocated 

PM2.5 monitors, the performance criteria 

apply with the following additional 

provisions: 

(1) The acceptable concentration 

range (Rj), mg/m3 may include values 

down to 0 mg/m3. 

(2) The minimum number of test sites 

shall be at least one; however, the 

number of test sites will generally 

include all locations within an agency’s 

network with collocated FRMs and 

FEMs. 

(3) The minimum number of methods 

shall include at least one FRM and at 

least one FEM. 

(4) Since multiple FRMs and FEMs 

may not be present at each site, the 

precision statistic requirement does not 

apply, even if precision data are 

available. 

(5) All seasons must be covered with 

no more than 36 consecutive months of 

data in total aggregated together. 

(6) The key statistical metric to 

include in an assessment is the bias 

(both additive and multiplicative) of the 

PM2.5 continuous FEM(s) compared to a 

collocated FRM(s). Correlation is 

required to be reported in the 

assessment, but failure to meet the 

correlation criteria, by itself, is not 

cause to exclude data from a continuous 

FEM monitor. 

■ 23. Amend § 58.12 by revising 

paragraph (d)(1): 

§ 58.12 Operating schedules. 

* * * * * 

(d) * * * 

(1)(i) Manual PM2.5 samplers at 

required SLAMS stations without a 

collocated continuously operating PM2.5

monitor must operate on at least a 1-in3 day schedule unless a waiver for an 

alternative schedule has been approved 

per paragraph (d)(1)(ii) of this section. 

(ii) For SLAMS PM2.5 sites with both 

manual and continuous PM2.5 monitors 

operating, the monitoring agency may 

request approval for a reduction to 1-in6 day PM2.5 sampling or for seasonal 

sampling from the EPA Regional 

Administrator. Other requests for a 

reduction to 1-in-6 day PM2.5 sampling 

or for seasonal sampling may be 

approved on a case-by-case basis. The 

EPA Regional Administrator may grant 

sampling frequency reductions after 

consideration of factors (including but 

not limited to the historical PM2.5 data 

quality assessments, the location of 

current PM2.5 design value sites, and 

their regulatory data needs) if the 

Regional Administrator determines that 

the reduction in sampling frequency 

will not compromise data needed for 

implementation of the NAAQS. 

Required SLAMS stations whose 

measurements determine the design 

value for their area and that are within 

plus or minus 10 percent of the annual 

NAAQS, and all required sites where 

one or more 24-hour values have 

exceeded the 24-hour NAAQS each year 

for a consecutive period of at least 3 

years are required to maintain at least a 

1-in-3 day sampling frequency until the 

design value no longer meets the criteria 

in this paragraph (d)(1)(ii) for 3 

consecutive years. A continuously 

operating FEM PM2.5 monitor satisfies 

the requirement in this paragraph 

(d)(1)(ii) unless it is identified in the 

monitoring agency’s annual monitoring 

network plan as not appropriate for 

comparison to the NAAQS and the EPA 

Regional Administrator has approved 

that the data from that monitor may be 

excluded from comparison to the 

NAAQS. 

(iii) Required SLAMS stations whose 

measurements determine the 24-hour 

design value for their area and whose 

data are within plus or minus 5 percent 

of the level of the 24-hour PM2.5 NAAQS 

must have an FRM or FEM operate on 

a daily schedule if that area’s design 

value for the annual NAAQS is less than 

the level of the annual PM2.5 standard. 

A continuously operating FEM or PM2.5

monitor satisfies the requirement in this 

paragraph (d)(1)(iii) unless it is 

identified in the monitoring agency’s 

annual monitoring network plan as not 

appropriate for comparison to the 

NAAQS and the EPA Regional 

Administrator has approved that the 

data from that monitor may be excluded 

from comparison to the NAAQS. The 

daily schedule must be maintained until 

the referenced design values no longer 

meets the criteria in this paragraph 

(d)(1)(iii) for 3 consecutive years. 

(iv) Changes in sampling frequency 

attributable to changes in design values 

shall be implemented no later than 

January 1 of the calendar year following 

the certification of such data as 

described in § 58.15. 

* * * * * 

■ 24. Revise § 58.15 to read as follows: 

§ 58.15 Annual air monitoring data 

certification. 

(a) The State, or where appropriate 

local, agency shall submit to the EPA 

Regional Administrator an annual air 

monitoring data certification letter to 

certify data collected by FRM and FEM 

monitors at SLAMS and SPM sites that 

meet criteria in appendix A to this part 

from January 1 to December 31 of the 

previous year. The head official in each 

monitoring agency, or his or her 

designee, shall certify that the previous 

year of ambient concentration and 

quality assurance data are completely 

submitted to AQS and that the ambient 

concentration data are accurate to the 

best of her or his knowledge, taking into 

consideration the quality assurance 

findings. The annual data certification 

letter is due by May 1 of each year. 

(b) Along with each certification 

letter, the State shall submit to the 

Regional Administrator an annual 


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16390 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

summary report of all the ambient air 

quality data collected by FRM and FEM 

monitors at SLAMS and SPM sites. The 

annual report(s) shall be submitted for 

data collected from January 1 to 

December 31 of the previous year. The 

annual summary serves as the record of 

the specific data that is the object of the 

certification letter. 

(c) Along with each certification 

letter, the State shall submit to the 

Regional Administrator a summary of 

the precision and accuracy data for all 

ambient air quality data collected by 

FRM and FEM monitors at SLAMS and 

SPM sites. The summary of precision 

and accuracy shall be submitted for data 

collected from January 1 to December 31 

of the previous year. 

Subpart C—Special Purpose Monitors 

■ 25. Amend § 58.20 by revising 

paragraphs (b) through (e) to read as 

follows: 

§ 58.20 Special purpose monitors (SPM). 

* * * * * 

(b) Any SPM data collected by an air 

monitoring agency using a Federal 

reference method (FRM) or Federal 

equivalent method (FEM) must meet the 

requirements of §§ 58.11 and 58.12 and 

appendix A to this part or an approved 

alternative to appendix A. Compliance 

with appendix E to this part is optional 

but encouraged except when the 

monitoring agency’s data objectives are 

inconsistent with the requirements in 

appendix E. Data collected at an SPM 

using a FRM or FEM meeting the 

requirements of appendix A must be 

submitted to AQS according to the 

requirements of § 58.16. Data collected 

by other SPMs may be submitted. The 

monitoring agency must also submit to 

AQS an indication of whether each SPM 

reporting data to AQS monitor meets the 

requirements of appendices A and E. 

(c) All data from an SPM using an 

FRM or FEM which has operated for 

more than 24 months are eligible for 

comparison to the relevant NAAQS, 

subject to the conditions of §§ 58.11(e) 

and 58.30, unless the air monitoring 

agency demonstrates that the data came 

from a particular period during which 

the requirements of appendix A, 

appendix C, or appendix E to this part 

were not met, subject to review and EPA 

Regional Office approval as part of the 

annual monitoring network plan 

described in § 58.10. 

(d) If an SPM using an FRM or FEM 

is discontinued within 24 months of 

start-up, the Administrator will not base 

a NAAQS violation determination for 

the PM2.5 or ozone NAAQS solely on 

data from the SPM. 

(e) If an SPM using an FRM or FEM 

is discontinued within 24 months of 

start-up, the Administrator will not 

designate an area as nonattainment for 

the CO, SO2, NO2, or 24-hour PM10

NAAQS solely on the basis of data from 

the SPM. Such data are eligible for use 

in determinations of whether a 

nonattainment area has attained one of 

these NAAQS. 

* * * * * 

■ 26. Amend appendix A to part 58 by: 

■ a. Revising section 2.6.1 and adding 

sections 2.6.1.1 and 2.6.1.2; 

■ b. Removing section 3.1.2.2 and 

redesignating sections 3.1.2.3, 3.1.2.4, 

3.1.2.5, and 3.1.2.6 as sections 3.1.2.2, 

3.1.2.3, 3.1.2.4, and 3.1.2.5, respectively; 

■ c. Revising sections 3.1.3.3, 3.2.4, 

4.2.1, and 4.2.5; and 

■ d. In section 6 revising References (1), 

(4), (6), (7), (9), (10), and (11) and table 

A–1. 

The revisions and additions read as 

follows: 

Appendix A to Part 58—Quality 

Assurance Requirements for Monitors 

used in Evaluations of National 

Ambient Air Quality Standards 

* * * * * 

2.6.1 Gaseous pollutant concentration 

standards (permeation devices or cylinders of 

compressed gas) used to obtain test 

concentrations for CO, SO2, NO, and NO2

must be EPA Protocol Gases certified in 

accordance with one of the procedures given 

in Reference 4 of this appendix. 

2.6.1.1 The concentrations of EPA 

Protocol Gas standards used for ambient air 

monitoring must be certified with a 95- 

percent confidence interval to have an 

analytical uncertainty of no more than ±2.0 

percent (inclusive) of the certified 

concentration (tag value) of the gas mixture. 

The uncertainty must be calculated in 

accordance with the statistical procedures 

defined in Reference 4 of this appendix. 

2.6.1.2 Specialty gas producers 

advertising certification with the procedures 

provided in Reference 4 of this appendix and 

distributing gases as ‘‘EPA Protocol Gas’’ for 

ambient air monitoring purposes must adhere 

to the regulatory requirements specified in 40 

CFR 75.21(g) or not use ‘‘EPA’’ in any form 

of advertising. Monitoring organizations must 

provide information to the EPA on the 

specialty gas producers they use on an 

annual basis. PQAOs, when requested by the 

EPA, must participate in the EPA Ambient 

Air Protocol Gas Verification Program at least 

once every 5 years by sending a new unused 

standard to a designated verification 

laboratory. 

* * * * * 

3.1.3.3 Using audit gases that are verified 

against the NIST standard reference methods 

or special review procedures and validated 

per the certification periods specified in 

Reference 4 of this appendix (EPA 

Traceability Protocol for Assay and 

Certification of Gaseous Calibration 

Standards) for CO, SO2, and NO2 and using 

O3 analyzers that are verified quarterly 

against a standard reference photometer. 

* * * * * 

3.2.4 PM2.5 Performance Evaluation 

Program (PEP) Procedures. The PEP is an 

independent assessment used to estimate 

total measurement system bias. These 

evaluations will be performed under the 

national performance evaluation program 

(NPEP) as described in section 2.4 of this 

appendix or a comparable program. A 

prescribed number of Performance evaluation 

sampling events will be performed annually 

within each PQAO. For PQAOs with less 

than or equal to five monitoring sites, five 

valid performance evaluation audits must be 

collected and reported each year. For PQAOs 

with greater than five monitoring sites, eight 

valid performance evaluation audits must be 

collected and reported each year. A valid 

performance evaluation audit means that 

both the primary monitor and PEP audit 

concentrations are valid and equal to or 

greater than 2 mg/m3. Siting of the PEP 

monitor must be consistent with section 

3.2.3.4(c) of this appendix. However, any 

horizontal distance greater than 4 meters and 

any vertical distance greater than one meter 

must be reported to the EPA regional PEP 

coordinator. Additionally for every monitor 

designated as a primary monitor, a primary 

quality assurance organization must: 

* * * * * 

4.2.1 Collocated Quality Control Sampler 

Precision Estimate for PM10, PM2.5, and Pb. 

Precision is estimated via duplicate 

measurements from collocated samplers. It is 

recommended that the precision be 

aggregated at the PQAO level quarterly, 

annually, and at the 3-year level. The data 

pair would only be considered valid if both 

concentrations are greater than or equal to 

the minimum values specified in section 4(c) 

of this appendix. For each collocated data 

pair, calculate ti, using equation 6 to this 

appendix: 


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Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16391 

Where Xi is the concentration from the 

primary sampler and Yi is the concentration 

value from the audit sampler. The coefficient 

of variation upper bound is calculated using 

equation 7 to this appendix: 

Where k is the number of valid data pairs 

being aggregated, and X2

0.1,k-1 is the 10th 

percentile of a chi-squared distribution with 

k-1 degrees of freedom. The factor of 2 in the 

denominator adjusts for the fact that each ti

is calculated from two values with error. 

* * * * * 

4.2.5 Performance Evaluation Programs 

Bias Estimate for PM2.5. The bias estimate is 

calculated using the PEP audits described in 

section 3.2.4. of this appendix. The bias 

estimator is based on, si, the absolute 

difference in concentrations divided by the 

square root of the PEP concentration. 

* * * * * 

6. References 

(1) American National Standard Institute— 

Quality Management Systems For 

Environmental Information And 

Technology Programs—Requirements 

With Guidance For Use. ASQ/ANSI E4– 

2014. February 2014. Available from 

ANSI Webstore https://webstore.ansi.

org/. 

* * * * * 

(4) EPA Traceability Protocol for Assay and 

Certification of Gaseous Calibration 

Standards. EPA–600/R–12/531. May, 

2012. Available from U.S. Environmental 

Protection Agency, National Risk 

Management Research Laboratory, 

Research Triangle Park NC 27711. 

https://www.epa.gov/nscep. 

* * * * * 

(6) List of Designated Reference and 

Equivalent Methods. Available from U.S. 

Environmental Protection Agency, 

Center for Environmental Measurements 

and Modeling, Air Methods and 

Characterization Division, MD–D205–03, 

Research Triangle Park, NC 27711. 

https://www.epa.gov/amtic/airmonitoring-methods-criteria-pollutants. 

(7) Transfer Standards for the Calibration of 

Ambient Air Monitoring Analyzers for 

Ozone. EPA–454/B–13–004 U.S. 

Environmental Protection Agency, 

Research Triangle Park, NC 27711, 

October, 2013. https://www.epa.gov/ 

sites/default/files/2020-09/documents/ 

ozonetransferstandardguidance.pdf. 

* * * * * 

(9) Quality Assurance Handbook for Air 

Pollution Measurement Systems, Volume 

1—A Field Guide to Environmental 

Quality Assurance. EPA–600/R–94/038a. 

April 1994. Available from U.S. 

Environmental Protection Agency, ORD 

Publications Office, Center for 

Environmental Research Information 

(CERI), 26 W. Martin Luther King Drive, 

Cincinnati, OH 45268. https://

www.epa.gov/amtic/ambient-airmonitoring-qualityassurance#documents. 

(10) Quality Assurance Handbook for Air 

Pollution Measurement Systems, Volume 

II: Ambient Air Quality Monitoring 

Program Quality System Development. 

EPA–454/B–13–003. https://

www.epa.gov/amtic/ambient-airmonitoring-qualityassurance#documents. 

(11) National Performance Evaluation 

Program Standard Operating Procedures. 

https://www.epa.gov/amtic/ambient-airmonitoring-quality-assurance#npep. 

TABLE A–1 TO SECTION 6 OF APPENDIX A—MINIMUM DATA ASSESSMENT REQUIREMENTS FOR NAAQS RELATED

CRITERIA POLLUTANT MONITORS

Method Assessment method Coverage Minimum 

frequency 

Parameters 

reported 

AQS assessment 

type 

Gaseous Methods 

(CO, NO2, SO2, O3): 


ddrumheller on DSK120RN23PROD with RULES3ER06MR24.041</GPH> ER06MR24.042</GPH> ER06MR24.043</GPH>

Equation 6 to Section 4.2.1 of Appendix A 

Equation 7 to Section 4.2.1 of Appendix A 

ICV90NAAQS = 100 * k x 1:r=l tf - c1:r=1 ti) 2 k - 1 

2k(k - 1) x NAAQS Concentration* Xli,k-l 

Equation 8 to Section 4.2.5 of Appendix A 

In s meas - audit 

100 x t=l t where St = ----- n,JNAAQS concentration .../audit 
16392 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

TABLE A–1 TO SECTION 6 OF APPENDIX A—MINIMUM DATA ASSESSMENT REQUIREMENTS FOR NAAQS RELATED

CRITERIA POLLUTANT MONITORS—Continued 

Method Assessment method Coverage Minimum 

frequency 

Parameters 

reported 

AQS assessment 

type 

One-Point QC for 

SO2, NO2, O3, 

CO.

Response check at 

concentration 

0.005–0.08 ppm 

SO2, NO2, O3, and.

0.5 and 5 ppm CO ....

Each analyzer ........... Once per 2 weeks 5 .. Audit concentration 1

and measured concentration.2.

One-Point QC. 

Annual performance 

evaluation for SO2, 

NO2, O3, CO.

See section 3.1.2 of 

this appendix.

Each analyzer ........... Once per year ........... Audit concentration 1

and measured concentration 2 for 

each level.

Annual PE. 

NPAP for SO2, NO2, 

O3, CO.

Independent Audit ..... 20% of sites each 

year.

Once per year ........... Audit concentration1

and measured concentration 2 for 

each level.

NPAP. 

Particulate Methods: 

Continuous 4

method—collocated quality 

control sampling 

PM2.5.

Collocated samplers 15% ........................... 1-in-12 days .............. Primary sampler concentration and duplicate sampler 

concentration.3.

No Transaction reported as raw data. 

Manual method— 

collocated quality control sampling PM10, 

PM2.5, Pb-TSP, 

Pb-PM10.

Collocated samplers 15% ........................... 1-in-12 days .............. Primary sampler concentration and duplicate sampler 

concentration.3.

No Transaction reported as raw data. 

Flow rate 

verification 

PM10 (low Vol) 

PM2.5, Pb-PM10.

Check of sampler 

flow rate.

Each sampler ............ Once every month 5 .. Audit flow rate and 

measured flow rate 

indicated by the 

sampler.

Flow Rate 

Verification. 

Flow rate 

verification 

PM10 (HighVol), Pb-TSP.

Check of sampler 

flow rate.

Each sampler ............ Once every quarter 5 Audit flow rate and 

measured flow rate 

indicated by the 

sampler.

Flow Rate 

Verification. 

Semi-annual flow 

rate audit PM10, 

TSP, PM10–2.5, 

PM2.5, Pb-TSP, 

Pb-PM10.

Check of sampler 

flow rate using 

independent standard.

Each sampler ............ Once every 6 

months 5.

Audit flow rate and 

measured flow rate 

indicated by the 

sampler.

Semi Annual Flow 

Rate Audit. 

Pb analysis audits 

Pb-TSP, PbPM10.

Check of analytical 

system with Pb 

audit strips/filters.

Analytical ................... Once each quarter 5 .. Measured value and 

audit value (ug Pb/ 

filter) using AQS 

unit code 077.

Pb Analysis Audits. 

Performance Evaluation Program 

PM2.5.

Collocated samplers (1) 5 valid audits for 

primary QA orgs, 

with ≤5 sites.

(2) 8 valid audits for 

primary QA orgs, 

with >5 sites.

(3) All samplers in 6 

years.

Distributed over all 4 

quarters 5.

Primary sampler concentration and performance evaluation sampler concentration.PEP. 

Performance Evaluation Program 

Pb-TSP, PbPM10.

Collocated samplers (1) 1 valid audit and 4 

collocated samples 

for primary QA 

orgs, with ≤5 sites.

(2) 2 valid audits and 

6 collocated samples for primary QA 

orgs with >5 sites.

Distributed over all 4 

quarters 5.

Primary sampler concentration and performance evaluation sampler concentration. Primary 

sampler concentration and duplicate 

sampler concentration.

PEP. 

1 Effective concentration for open path analyzers. 2 Corrected concentration, if applicable for open path analyzers. 3 Both primary and collocated sampler values are reported as raw data. 4 PM2.5 is the only particulate criteria pollutant requiring collocation of continuous and manual primary monitors. 5 EPA’s recommended maximum number of days that should exist between checks to ensure that the checks are routinely conducted over 

time and to limit data impacts resulting from a failed check. 

* * * * * 

■ 27. Amend appendix B to part 58 by: 

■ a. Revising section 2.6.1 and adding 

sections 2.6.1.1 and 2.6.1.2; 

■ b. Removing and reserving section 

3.1.2.2; 


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■ c. Revising sections 3.1.3.3 and 3.2.4; 

■ d. Adding sections 3.2.4.1 through 

3.2.4.3; 

■ e. Revising sections 4.2.1, and 4.2.5; 

and 

■ f. In section 6 revising References (1), 

(4), (6), (7), (9), (10), and (11) and table 

B–1. 

The revisions and additions read as 

follows: 

Appendix B to Part 58—Quality 

Assurance Requirements for Prevention 

of Significant Deterioration (PSD) Air 

Monitoring 

* * * * * 

2.6.1 Gaseous pollutant 

concentration standards (permeation 

devices or cylinders of compressed gas) 

used to obtain test concentrations for 

CO, SO2, NO, and NO2 must be EPA 

Protocol Gases certified in accordance 

with one of the procedures given in 

Reference 4 of this appendix. 

2.6.1.1 The concentrations of EPA 

Protocol Gas standards used for ambient 

air monitoring must be certified with a 

95-percent confidence interval to have 

an analytical uncertainty of no more 

than ±2.0 percent (inclusive) of the 

certified concentration (tag value) of the 

gas mixture. The uncertainty must be 

calculated in accordance with the 

statistical procedures defined in 

Reference 4 of this appendix. 

2.6.1.2 Specialty gas producers 

advertising certification with the 

procedures provided in Reference 4 of 

this appendix and distributing gases as 

‘‘EPA Protocol Gas’’ for ambient air 

monitoring purposes must adhere to the 

regulatory requirements specified in 40 

CFR 75.21(g) or not use ‘‘EPA’’ in any 

form of advertising. The PSD PQAOs 

must provide information to the PSD 

reviewing authority on the specialty gas 

producers they use (or will use) for the 

duration of the PSD monitoring project. 

This information can be provided in the 

QAPP or monitoring plan but must be 

updated if there is a change in the 

specialty gas producers used. 

* * * * * 

3.1.3.3 Using audit gases that are 

verified against the NIST standard 

reference methods or special review 

procedures and validated per the 

certification periods specified in 

Reference 4 of this appendix (EPA 

Traceability Protocol for Assay and 

Certification of Gaseous Calibration 

Standards) for CO, SO2, and NO2 and 

using O3 analyzers that are verified 

quarterly against a standard reference 

photometer. 

* * * * * 

3.2.4 PM2.5 Performance Evaluation 

Program (PEP) Procedures. The PEP is 

an independent assessment used to 

estimate total measurement system bias. 

These evaluations will be performed 

under the NPEP as described in section 

2.4 of this appendix or a comparable 

program. Performance evaluations will 

be performed annually within each 

PQAO. For PQAOs with less than or 

equal to five monitoring sites, five valid 

performance evaluation audits must be 

collected and reported each year. For 

PQAOs with greater than five 

monitoring sites, eight valid 

performance evaluation audits must be 

collected and reported each year. A 

valid performance evaluation audit 

means that both the primary monitor 

and PEP audit concentrations are valid 

and equal to or greater than 2 mg/m3. 

Siting of the PEP monitor must be 

consistent with section 3.2.3.4(c) of this 

appendix. However, any horizontal 

distance greater than 4 meters and any 

vertical distance greater than one meter 

must be reported to the EPA regional 

PEP coordinator. Additionally for every 

monitor designated as a primary 

monitor, a primary quality assurance 

organization must: 

3.2.4.1 Have each method 

designation evaluated each year; and, 

3.2.4.2 Have all FRM and FEM 

samplers subject to a PEP audit at least 

once every 6 years, which equates to 

approximately 15 percent of the 

monitoring sites audited each year. 

3.2.4.3 Additional information 

concerning the PEP is contained in 

Reference 10 of this appendix. The 

calculations for evaluating bias between 

the primary monitor and the 

performance evaluation monitor for 

PM2.5 are described in section 4.2.5 of 

this appendix. 

* * * * * 

4.2.1 Collocated Quality Control 

Sampler Precision Estimate for PM10,

PM2.5, and Pb. Precision is estimated via 

duplicate measurements from collocated 

samplers. It is recommended that the 

precision be aggregated at the PQAO 

level quarterly, annually, and at the 3- 

year level. The data pair would only be 

considered valid if both concentrations 

are greater than or equal to the 

minimum values specified in section 

4(c) of this appendix. For each 

collocated data pair, calculate ti, using 

equation 6 to this appendix: 

Where Xi is the concentration from 

the primary sampler and Yi is the 

concentration value from the audit 

sampler. The coefficient of variation 

upper bound is calculated using 

equation 7 to this appendix: 

Where k is the number of valid data 

pairs being aggregated, and X2

0.1,k-1 is 

the 10th percentile of a chi-squared 

distribution with k-1 degrees of 

freedom. The factor of 2 in the 

denominator adjusts for the fact that 


ddrumheller on DSK120RN23PROD with RULES3ER06MR24.044</GPH> ER06MR24.045</GPH>

Equation 6 to Section 4.2.1 of Appendix B 

Equation 7 to Section 4.2.1 of Appendix B 

I k X If=t ff - (If=t ti) 2 k - 1 

CV90NAAQS = 100 * 2k(k -1) X 2 NAAQS Concentration* Xo.t,k-t 
16394 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

each ti is calculated from two values 

with error. 

* * * * * 

4.2.5 Performance Evaluation 

Programs Bias Estimate for PM2.5. The 

bias estimate is calculated using the PEP 

audits described in section 3.2.4. of this 

appendix. The bias estimator is based 

on, si, the absolute difference in 

concentrations divided by the square 

root of the PEP concentration. 

* * * * * 

6. References 

(1) American National Standard Institute— 

Quality Management Systems For 

Environmental Information And 

Technology Programs—Requirements 

With Guidance For Use. ASQ/ANSI E4– 

2014. February 2014. Available from 

ANSI Webstore https://webstore.ansi.

org/. 

* * * * * 

(4) EPA Traceability Protocol for Assay and 

Certification of Gaseous Calibration 

Standards. EPA–600/R–12/531. May, 

2012. Available from U.S. Environmental 

Protection Agency, National Risk 

Management Research Laboratory, 

Research Triangle Park NC 27711. 

https://www.epa.gov/nscep. 

* * * * * 

(6) List of Designated Reference and 

Equivalent Methods. Available from U.S. 

Environmental Protection Agency, 

Center for Environmental Measurements 

and Modeling, Air Methods and 

Characterization Division, MD–D205–03, 

Research Triangle Park, NC 27711. 

https://www.epa.gov/amtic/airmonitoring-methods-criteria-pollutants. 

(7) Transfer Standards for the Calibration of 

Ambient Air Monitoring Analyzers for 

Ozone. EPA–454/B–13–004 U.S. 

Environmental Protection Agency, 

Research Triangle Park, NC 27711, 

October, 2013. https://www.epa.gov/ 

sites/default/files/2020-09/documents/ 

ozonetransferstandardguidance.pdf. 

* * * * * 

(9) Quality Assurance Handbook for Air 

Pollution Measurement Systems, Volume 

1—A Field Guide to Environmental 

Quality Assurance. EPA–600/R–94/038a. 

April 1994. Available from U.S. 

Environmental Protection Agency, ORD 

Publications Office, Center for 

Environmental Research Information 

(CERI), 26 W. Martin Luther King Drive, 

Cincinnati, OH 45268. https://

www.epa.gov/amtic/ambient-airmonitoring-qualityassurance#documents. 

(10) Quality Assurance Handbook for Air 

Pollution Measurement Systems, Volume 

II: Ambient Air Quality Monitoring 

Program Quality System Development. 

EPA–454/B–13–003. https://

www.epa.gov/amtic/ambient-airmonitoring-qualityassurance#documents. 

(11) National Performance Evaluation 

Program Standard Operating Procedures. 

https://www.epa.gov/amtic/ambient-airmonitoring-quality-assurance#npep. 

TABLE B–1 TO SECTION 6 OF APPENDIX B- MINIMUM DATA ASSESSMENT REQUIREMENTS FOR NAAQS RELATED CRITERIA

POLLUTANT PSD MONITORS

Method Assessment 

method Coverage Minimum frequency 

Parameters 

reported 

AQS 

Assessment type 

Gaseous Methods 

(CO, NO2, SO2, O3): 

One-Point QC for 

SO2, NO2, O3, 

CO.

Response check at 

concentration 

0.005–0.08 ppm 

SO2, NO2, O3, & 

0.5 and 5 ppm CO.

Each analyzer ........... Once per 2 weeks5 ... Audit concentration1

and measured concentration2.

One-Point QC. 

Quarterly performance evaluation 

for SO2, NO2, 

O3, CO.

See section 3.1.2 of 

this appendix.

Each analyzer ........... Once per quarter5 ..... Audit concentration1

and measured concentration2 for each 

level.

Annual PE. 

NPAP for SO2, 

NO2, O3, CO3.

Independent Audit ..... Each primary monitor Once per year ........... Audit concentration1

and measured concentration2 for each 

level.

NPAP. 

Particulate Methods: 

Collocated sampling PM10, 

PM2.5, Pb.

Collocated samplers 1 per PSD Network 

per pollutant.

Every 6 days or every 

3 days if daily monitoring required.

Primary sampler concentration and duplicate sampler 

concentration4.

No Transaction reported as raw data. 

Flow rate 

verification 

PM10, PM2.5, 

Pb.

Check of sampler 

flow rate.

Each sampler ............ Once every month5 ... Audit flow rate and 

measured flow rate 

indicated by the 

sampler.

Flow Rate 

Verification. 

Semi-annual flow 

rate audit PM10, 

PM2.5, Pb.

Check of sampler 

flow rate using 

independent standard.

Each sampler ............ Once every 6 months 

or beginning, middle and end of 

monitoring5.

Audit flow rate and 

measured flow rate 

indicated by the 

sampler.

Semi Annual Flow 

Rate Audit. 


ddrumheller on DSK120RN23PROD with RULES3ER06MR24.046</GPH>

Equation 8 to Section 4.2.5 of Appendix B 

,1:n s meas - audit 

100 x 1=1 ' where s - ----- n,/NAAQS concentration ' - -../audit 
Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16395 

TABLE B–1 TO SECTION 6 OF APPENDIX B- MINIMUM DATA ASSESSMENT REQUIREMENTS FOR NAAQS RELATED CRITERIA

POLLUTANT PSD MONITORS—Continued 

Method Assessment 

method Coverage Minimum frequency 

Parameters 

reported 

AQS 

Assessment type 

Pb analysis audits 

Pb-TSP, PbPM10.

Check of analytical 

system with Pb 

audit strips/filters.

Analytical ................... Each quarter5 ............ Measured value and 

audit value (ug Pb/ 

filter) using AQS 

unit code 077 for 

parameters:.

14129—Pb (TSP) LC 

FRM/FEM.

85129—Pb (TSP) LC 

Non-FRM/FEM..

Pb Analysis Audits. 

Performance Evaluation Program 

PM2.53.

Collocated samplers (1) 5 valid audits for 

PQAOs with <= 5 

sites..

(2) 8 valid audits for 

PQAOs with > 5 

sites..

(3) All samplers in 6 

years.

Over all 4 quarters5 .. Primary sampler concentration and performance evaluation sampler concentration.PEP. 

Performance Evaluation Program 

Pb 3.

Collocated samplers (1) 1 valid audit and 4 

collocated samples 

for PQAOs, with 

<=5 sites..

(2) 2 valid audits and 

6 collocated samples for PQAOs 

with >5 sites..

Over all 4 quarters5 .. Primary sampler concentration and performance evaluation sampler concentration. Primary 

sampler concentration and duplicate 

sampler concentration.

PEP. 

1 Effective concentration for open path analyzers. 2 Corrected concentration, if applicable for open path analyzers. 3 NPAP, PM2.5, PEP, and Pb-PEP must be implemented if data is used for NAAQS decisions otherwise implementation is at PSD reviewing 

authority discretion. 4 Both primary and collocated sampler values are reported as raw data 5 A maximum number of days should be between these checks to ensure the checks are routinely conducted over time and to limit data impacts resulting from a failed check. 

■ 28. Amend appendix C to part 58 by: 

■ a. Adding sections 2.2 and 2.2.1 

through 2.2.19; 

■ b. Removing and reserving sections 

2.4, 2.4.1; 

■ c. Removing sections 2.4.1.1 through 

2.4.1.7; and 

■ d. Revising section 2.7.1. 

The additions and revision reads as 

follows: 

Appendix C to Part 58—Ambient Air 

Quality Monitoring Methodology 

* * * * * 

2.2 PM10, PM2.5, or PM10–2.5 continuous 

FEMs with existing valid designations may 

be calibrated using network data from 

collocated FRM and continuous FEM data 

under the following provisions: 

2.2.1 Data to demonstrate a calibration 

may include valid data from State, local, or 

Tribal air agencies or data collected by 

instrument manufacturers in accordance with 

40 CFR 53.35 or other data approved by the 

Administrator. 

2.2.2 A request to update a designated 

methods calibration may be initiated by the 

instrument manufacturer of record or the 

EPA Administrator. State, local, Tribal, and 

multijusistincional organizations of these 

entities may work with an instrument 

manufacture to update a designated method 

calibration. 

2.2.3 Requests for approval of an updated 

PM10, PM2.5, or PM10–2.5 continuous FEM 

calibration must meet the general submittal 

requirements of section 2.7 of this appendix. 

2.2.4 Data included in the request should 

represent a subset of representative locations 

where the method is operational. For cases 

with a small number of collocated FRMs and 

continuous FEMs sites, an updated candidate 

calibration may be limited to the sites where 

both methods are in use. 

2.2.5 Data included in a candidate 

method updated calibration may include a 

subset of sites where there is a large grouping 

of sites in one part of the country such that 

the updated calibration would be 

representative of the country as a whole. 

2.2.6 Improvements should be national in 

scope and ideally implemented through a 

firmware change. 

2.2.7 The goal of a change to a methods 

calibration is to increase the number of sites 

meeting measurements quality objectives of 

the method as identified in section 2.3.1.1 of 

appendix A to this part. 

2.2.8 For meeting measurement quality 

objectives (MQOs), the primary objective is to 

meet the bias goal as this statistic will likely 

have the most influence on improving the 

resultant data collected. 

2.2.9 Precision data are to be included, 

but so long as precision data are at least as 

good as existing network data or meet the 

MQO referenced in section 2.2.8 of this 

appendix, no further work is necessary with 

precision. 

2.2.10 Data available to use may include 

routine primary and collocated data. 

2.2.11 Audit data may be useful to 

confirm the performance of a candidate 

updated calibration but should not be used 

as the basis of the calibration to keep the 

independence of the audit data. 

2.2.12 Data utilized as the basis of the 

updated calibration may be obtained by 

accessing EPA’s AQS database or future 

analogous EPA database. 

2.2.13 Years of data to use in a candidate 

method calibration should include two 

recent years where we are past the 

certification period for the previous year’s 

data, which is May 1 of each year. 

2.2.14 Data from additional years is to be 

used to test an updated calibration such that 

the calibration is independent of the test 

years of interest. Data from these additional 

years need to minimally demonstrate that a 

larger number of sites are expected to meet 

bias MQO especially at sites near the level of 

the NAAQS for the PM indicator of interest. 

2.2.15 Outliers may be excluded using 

routine outlier tests. 

2.2.16 The range of data used in a 

calibration may include all data available or 

alternatively use data in the range from the 

lowest measured data available up to 125% 

of the 24-hour NAAQS for the PM indicator 

of interest. 


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2.2.17 Other improvements to a PM 

continuous method may be included as part 

of a recommended update so long as 

appropriate testing is conducted with input 

from EPA’s Office of Research and 

Development (ORD) Reference and 

Equivalent (R&E) Methods Designation 

program. 

2.2.18 EPA encourages early 

communication by instrument manufacturers 

considering an update to a PM method. 

Instrument companies should initiate such 

dialogue by contacting EPA’s ORD R&E 

Methods Designation program. The contact 

information for this can be found at 40 CFR 

53.4. 

2.2.19 Manufacturers interested in 

improving instrument’s performance through 

an updated factory calibration must submit a 

written modification request to EPA with 

supporting rationale. Because the testing 

requirements and acceptance criteria of any 

field and/or lab tests can depend upon the 

nature and extent of the intended 

modification, applicants should contact 

EPA’s R&E Methods Designation program for 

guidance prior to development of the 

modification request. 

* * * * * 

2.7.1 Requests for approval under 

sections 2.2, 2.4, 2.6.2, or 2.8 of this 

appendix must be submitted to: Director, 

Center for Environmental Measurement and 

Modeling, Reference and Equivalent Methods 

Designation Program (MD–D205–03), U.S. 

Environmental Protection Agency, P.O. Box 

12055, Research Triangle Park, North 

Carolina 27711. 

■ 29. Amend appendix D to part 58 by 

revising sections 1 and 1.1(b), the 

introductory text before the table in 

section 4.7.1(a), and sections 4.7.1(b)(3) 

and 4.7.2 to read as follows: 

Appendix D to Part 58—Network 

Design Criteria for Ambient Air Quality 

Monitoring 

* * * * * 

1. Monitoring Objectives and Spatial Scales 

The purpose of this appendix is to describe 

monitoring objectives and general criteria to 

be applied in establishing the required 

SLAMS ambient air quality monitoring 

stations and for choosing general locations 

for additional monitoring sites. This 

appendix also describes specific 

requirements for the number and location of 

FRM and FEM sites for specific pollutants, 

NCore multipollutant sites, PM10 mass sites, 

PM2.5 mass sites, chemically-speciated PM2.5

sites, and O3 precursor measurements sites 

(PAMS). These criteria will be used by EPA 

in evaluating the adequacy of the air 

pollutant monitoring networks. 

1.1 * * * 

(b) Support compliance with ambient air 

quality standards and emissions strategy 

development. Data from FRM and FEM 

monitors for NAAQS pollutants will be used 

for comparing an area’s air pollution levels 

against the NAAQS. Data from monitors of 

various types can be used in the development 

of attainment and maintenance plans. 

SLAMS, and especially NCore station data, 

will be used to evaluate the regional air 

quality models used in developing emission 

strategies, and to track trends in air pollution 

abatement control measures’ impact on 

improving air quality. In monitoring 

locations near major air pollution sources, 

source-oriented monitoring data can provide 

insight into how well industrial sources are 

controlling their pollutant emissions. 

* * * * * 

4.7.1 * * * 

(a) State and where applicable, local, 

agencies must operate the minimum number 

of required PM2.5 SLAMS sites listed in table 

D–5 to this appendix. The NCore sites are 

expected to complement the PM2.5 data 

collection that takes place at non-NCore 

SLAMS sites, and both types of sites can be 

used to meet the minimum PM2.5 network 

requirements. For many State and local 

networks, the total number of PM2.5 sites 

needed to support the basic monitoring 

objectives of providing air pollution data to 

the general public in a timely manner, 

support compliance with ambient air quality 

standards and emission strategy 

development, and support for air pollution 

research studies will include more sites than 

the minimum numbers required in table D– 

5 to this appendix. Deviations from these 

PM2.5 monitoring requirements must be 

approved by the EPA Regional 

Administrator. 

* * * * * 

(b) * * * 

(3) For areas with additional required 

SLAMS, a monitoring station is to be sited in 

an at-risk community with poor air quality, 

particularly where there are anticipated 

effects from sources in the area (e.g., a major 

industrial area, point source(s), port, rail 

yard, airport, or other transportation facility 

or corridor). 

* * * * * 

4.7.2 Requirement for Continuous PM2.5

Monitoring. The State, or where appropriate, 

local agencies must operate continuous PM2.5

analyzers equal to at least one-half (round 

up) the minimum required sites listed in 

table D–5 to this appendix. At least one 

required continuous analyzer in each MSA 

must be collocated with one of the required 

FRM/FEM monitors, unless at least one of the 

required FRM/FEM monitors is itself a 

continuous FEM monitor in which case no 

collocation requirement applies. State and 

local air monitoring agencies must use 

methodologies and quality assurance/quality 

control (QA/QC) procedures approved by the 

EPA Regional Administrator for these 

required continuous analyzers. 

* * * * * 

■ 30. Revise appendix E to part 58 to 

read as follows: 

Appendix E to Part 58—Probe and 

Monitoring Path Siting Criteria for 

Ambient Air Quality Monitoring 

1. Introduction 

2. Monitors and Samplers with Probe Inlets 

3. Open Path Analyzers 

4. Waiver Provisions 

5. References 

1. Introduction 

1.1 Applicability 

(a) This appendix contains specific 

location criteria applicable to ambient air 

quality monitoring probes, inlets, and optical 

paths of SLAMS, NCore, PAMS, and other 

monitor types whose data are intended to be 

used to determine compliance with the 

NAAQS. These specific location criteria are 

relevant after the general location has been 

selected based on the monitoring objectives 

and spatial scale of representation discussed 

in appendix D to this part. Monitor probe 

material and sample residence time 

requirements are also included in this 

appendix. Adherence to these siting criteria 

is necessary to ensure the uniform collection 

of compatible and comparable air quality 

data. 

(b) The probe and monitoring path siting 

criteria discussed in this appendix must be 

followed to the maximum extent possible. It 

is recognized that there may be situations 

where some deviation from the siting criteria 

may be necessary. In any such case, the 

reasons must be thoroughly documented in a 

written request for a waiver that describes 

whether the resulting monitoring data will be 

representative of the monitoring area and 

how and why the proposed or existing siting 

must deviate from the criteria. This 

documentation should help to avoid later 

questions about the validity of the resulting 

monitoring data. Conditions under which the 

EPA would consider an application for 

waiver from these siting criteria are 

discussed in section 4 of this appendix. 

(c) The pollutant-specific probe and 

monitoring path siting criteria generally 

apply to all spatial scales except where noted 

otherwise. Specific siting criteria that are 

phrased with ‘‘shall’’ or ‘‘must’’ are defined 

as requirements and exceptions must be 

granted through the waiver provisions. 

However, siting criteria that are phrased with 

‘‘should’’ are defined as goals to meet for 

consistency but are not requirements. 

2. Monitors and Samplers with Probe Inlets 

2.1 Horizontal and Vertical Placement 

(a) For O3 and SO2 monitoring, and for 

neighborhood or larger spatial scale Pb, PM10, 

PM10–2.5, PM2.5, NO2, and CO sites, the probe 

must be located greater than or equal to 2.0 

meters and less than or equal to 15 meters 

above ground level. 

(b) Middle scale CO and NO2 monitors 

must have sampler inlets greater than or 

equal to 2.0 meters and less than or equal to 

15 meters above ground level. 

(c) Middle scale PM10–2.5 sites are required 

to have sampler inlets greater than or equal 

to 2.0 meters and less than or equal to 7.0 

meters above ground level. 

(d) Microscale Pb, PM10, PM10–2.5, and 

PM2.5 sites are required to have sampler 

inlets greater than or equal to 2.0 meters and 

less than or equal to 7.0 meters above ground 

level. 

(e) Microscale near-road NO2 monitoring 

sites are required to have sampler inlets 

greater than or equal to 2.0 meters and less 

than or equal to 7.0 meters above ground 

level. 

(f) The probe inlets for microscale carbon 

monoxide monitors that are being used to 


ddrumheller on DSK120RN23PROD with RULES3
Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16397 

measure concentrations near roadways must 

be greater than or equal to 2.0 meters and less 

than or equal to 7.0 meters above ground 

level. Those probe inlets for microscale 

carbon monoxide monitors measuring 

concentrations near roadways in downtown 

areas or urban street canyons must be greater 

than or equal to 2.5 meters and less than or 

equal to 3.5 meters above ground level. The 

probe must be at least 1.0 meter vertically or 

horizontally away from any supporting 

structure, walls, parapets, penthouses, etc., 

and away from dusty or dirty areas. If the 

probe is located near the side of a building 

or wall, then it should be located on the 

windward side of the building relative to the 

prevailing wind direction during the season 

of highest concentration potential for the 

pollutant being measured. 

2.2 Spacing From Minor Sources 

(a) It is important to understand the 

monitoring objective for a particular site in 

order to interpret this requirement. Local 

minor sources of a primary pollutant, such as 

SO2, lead, or particles, can cause high 

concentrations of that particular pollutant at 

a monitoring site. If the objective for that 

monitoring site is to investigate these local 

primary pollutant emissions, then the site 

will likely be properly located nearby. This 

type of monitoring site would, in all 

likelihood, be a microscale-type of 

monitoring site. If a monitoring site is to be 

used to determine air quality over a much 

larger area, such as a neighborhood or city, 

a monitoring agency should avoid placing a 

monitor probe inlet near local, minor 

sources, because a plume from a local minor 

source should not be allowed to 

inappropriately impact the air quality data 

collected at a site. Particulate matter sites 

should not be located in an unpaved area 

unless there is vegetative ground cover yearround, so that the impact of windblown dusts 

will be kept to a minimum. 

(b) Similarly, local sources of nitric oxide 

(NO) and ozone-reactive hydrocarbons can 

have a scavenging effect causing 

unrepresentatively low concentrations of O3

in the vicinity of probes for O3. To minimize 

these potential interferences from nearby 

minor sources, the probe inlet should be 

placed at a distance from furnace or 

incineration flues or other minor sources of 

SO2 or NO. The separation distance should 

take into account the heights of the flues, 

type of waste or fuel burned, and the sulfur 

content of the fuel. 

2.3 Spacing From Obstructions 

(a) Obstacles may scavenge SO2, O3, or 

NO2, and can act to restrict airflow for any 

pollutant. To avoid this interference, the 

probe inlet must have unrestricted airflow 

pursuant to paragraph (b) of this section and 

should be located at a distance from 

obstacles. The horizontal distance from the 

obstacle to the probe inlet must be at least 

twice the height that the obstacle protrudes 

above the probe inlet. An obstacle that does 

not meet the minimum distance requirement 

is considered an obstruction that restricts 

airflow to the probe inlet. The EPA does not 

generally consider objects or obstacles such 

as flag poles or site towers used for NOy 

convertors and meteorological sensors, etc. to 

be deemed obstructions. 

(b) A probe inlet located near or along a 

vertical wall is undesirable because air 

moving along the wall may be subject to 

removal mechanisms. A probe inlet must 

have unrestricted airflow with no 

obstructions (as defined in paragraph (a) of 

this section) in a continuous arc of at least 

270 degrees. An unobstructed continuous arc 

of 180 degrees is allowable when the 

applicable network design criteria specified 

in appendix D of this part require monitoring 

in street canyons and the probe is located on 

the side of a building. This arc must include 

the predominant wind direction for the 

season of greatest pollutant concentration 

potential. For particle sampling, there must 

be a minimum of 2.0 meters of horizontal 

separation from walls, parapets, and 

structures for rooftop site placement. 

(c) A sampling station with a probe inlet 

located closer to an obstacle than required by 

the criteria in this section should be 

classified as middle scale or microscale, 

rather than neighborhood or urban scale, 

since the measurements from such a station 

would more closely represent these smaller 

scales. 

(d) For near-road monitoring stations, the 

monitor probe shall have an unobstructed air 

flow, where no obstacles exist at or above the 

height of the monitor probe, between the 

monitor probe and the outside nearest edge 

of the traffic lanes of the target road segment. 

2.4 Spacing From Trees 

(a) Trees can provide surfaces for SO2, O3, 

or NO2 adsorption or reactions and surfaces 

for particle deposition. Trees can also act as 

obstructions in locations where the trees are 

between the air pollutant sources or source 

areas and the monitoring site and where the 

trees are of a sufficient height and leaf 

canopy density to interfere with the normal 

airflow around the probe inlet. To reduce this 

possible interference/obstruction, the probe 

inlet should be 20 meters or more from the 

drip line of trees and must be at least 10 

meters from the drip line of trees. If a tree 

or group of trees is an obstacle, the probe 

inlet must meet the distance requirements of 

section 2.3 of this appendix. 

(b) The scavenging effect of trees is greater 

for O3 than for other criteria pollutants. 

Monitoring agencies must take steps to 

consider the impact of trees on ozone 

monitoring sites and take steps to avoid this 

problem. 

(c) Beginning January 1, 2024, microscale 

sites of any air pollutant shall have no trees 

or shrubs located at or above the line-of-sight 

fetch between the probe and the source under 

investigation, e.g., a roadway or a stationary 

source. 

2.5 Spacing From Roadways 

TABLE E–1 TO SECTION 2.5 OF APPENDIX E—MINIMUM SEPARATION

DISTANCE BETWEEN ROADWAYS AND

PROBES FOR MONITORING NEIGHBORHOOD AND URBAN SCALE

OZONE (O3) AND OXIDES OF NITROGEN (NO, NO2, NOX, NOy) 

Roadway 

average daily 

traffic, 

vehicles per day 

Minimum 

distance1 3

(meters) 

Minimum 

distance123

(meters) 

≤1,000 ............... 10 10 

10,000 ............... 10 20 

15,000 ............... 20 30 

20,000 ............... 30 40 

40,000 ............... 50 60 

70,000 ............... 100 100 

≥110,000 ........... 250 250 

1 Distance from the edge of the nearest traffic lane. The distance for intermediate traffic 

counts should be interpolated from the table 

values based on the actual traffic count./ 

TNOTE≤ 2Applicable for ozone monitors whose 

placement was not approved as of December 

18, 2006. 3All distances listed are expressed as having 2 significant figures. When rounding is performed to assess compliance with these siting 

requirements, the distance measurements will 

be rounded such as to retain at least two significant figures. 

2.5.1 Spacing for Ozone Probes 

In siting an O3 monitor, it is important to 

minimize destructive interferences from 

sources of NO, since NO readily reacts with 

O3. Table E–1 of this appendix provides the 

required minimum separation distances 

between a roadway and a probe inlet for 

various ranges of daily roadway traffic. A 

sampling site with a monitor probe located 

closer to a roadway than allowed by the 

Table E–1 requirements should be classified 

as middle scale or microscale, rather than 

neighborhood or urban scale, since the 

measurements from such a site would more 

closely represent these smaller scales. 

2.5.2 Spacing for Carbon Monoxide Probes 

(a) Near-road microscale CO monitoring 

sites, including those located in downtown 

areas, urban street canyons, and other nearroad locations such as those adjacent to 

highly trafficked roads, are intended to 

provide a measurement of the influence of 

the immediate source on the pollution 

exposure on the adjacent area. 

(b) Microscale CO monitor probe inlets in 

downtown areas or urban street canyon 

locations shall be located a minimum 

distance of 2.0 meters and a maximum 

distance of 10 meters from the edge of the 

nearest traffic lane. 

(c) Microscale CO monitor probe inlets in 

downtown areas or urban street canyon 

locations shall be located at least 10 meters 

from an intersection, preferably at a midblock 

location. Midblock locations are preferable to 

intersection locations because intersections 

represent a much smaller portion of 

downtown space than do the streets between 


ddrumheller on DSK120RN23PROD with RULES3
16398 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

them. Pedestrian exposure is probably also 

greater in street canyon/corridors than at 

intersections. 

(d) Neighborhood scale CO monitor probe 

inlets in downtown areas or urban street 

canyon locations shall be located according 

to the requirements in Table E–2 of this 

appendix. 

TABLE E–2 TO SECTION 2.5.2 OF APPENDIX E—MINIMUM SEPARATION

DISTANCE BETWEEN ROADWAYS AND

PROBES FOR MONITORING NEIGHBORHOOD SCALE CARBON MONOXIDE

Roadway average 

daily traffic, 

vehicles per day 

Minimum distance 1 2

(meters) 

≤10,000 ................. 10 

15,000 ................... 25 

20,000 ................... 45 

30,000 ................... 80 

40,000 ................... 115 

50,000 ................... 135 

≥60,000 ................. 150 

1 Distance from the edge of the nearest traffic lane. The distance for intermediate traffic 

counts should be interpolated from the table 

values based on the actual traffic count. 

2All distances listed are expressed as having 2 significant figures. When rounding is performed to assess compliance with these siting 

requirements, the distance measurements will 

be rounded such as to retain at least two significant figures. 

2.5.3 Spacing for Particulate Matter (PM2.5, 

PM2.5–10, PM10, Pb) Inlets 

(a) Since emissions associated with the 

operation of motor vehicles contribute to 

urban area particulate matter ambient levels, 

spacing from roadway criteria are necessary 

for ensuring national consistency in PM 

sampler siting. 

(b) The intent is to locate localized hot-spot 

sites in areas of highest concentrations, 

whether it be caused by mobile or multiple 

stationary sources. If the area is primarily 

affected by mobile sources and the maximum 

concentration area(s) is judged to be a traffic 

corridor or street canyon location, then the 

monitors should be located near roadways 

with the highest traffic volume and at 

separation distances most likely to produce 

the highest concentrations. For microscale 

traffic corridor sites, the location must be 

greater than or equal 5.0 meters and less than 

or equal to 15 meters from the major 

roadway. For the microscale street canyon 

site, the location must be greater than or 

equal 2.0 meters and less than or equal to 10 

meters from the roadway. For the middle 

scale site, a range of acceptable distances 

from the roadway is shown in Figure E–1 of 

this appendix. This figure also includes 

separation distances between a roadway and 

neighborhood or larger scale sites by default. 

Any PM probe inlet at a site, 2.0 to 15 meters 

high, and further back than the middle scale 

requirements will generally be neighborhood, 

urban or regional scale. For example, 

according to Figure E–1 of this appendix, if 

a PM sampler is primarily influenced by 

roadway emissions and that sampler is set 

back 10 meters from a 30,000 ADT (average 

daily traffic) road, the site should be 

classified as microscale, if the sampler’s inlet 

height is between 2.0 and 7.0 meters. If the 

sampler’s inlet height is between 7.0 and 15 

meters, the site should be classified as 

middle scale. If the sampler is 20 meters from 

the same road, it will be classified as middle 

scale; if 40 meters, neighborhood scale; and 

if 110 meters, an urban scale. 

2.5.4 Spacing for Nitrogen Dioxide (NO2) 

Probes 

(a) In siting near-road NO2 monitors as 

required in section 4.3.2 of appendix D of 

this part, the monitor probe shall be as near 

as practicable to the outside nearest edge of 

the traffic lanes of the target road segment but 

shall not be located at a distance greater than 

50 meters, in the horizontal, from the outside 

nearest edge of the traffic lanes of the target 

road segment. Where possible, the near-road 

NO2 monitor probe should be within 20 

meters of the target road segment. 

(b) In siting NO2 monitors for 

neighborhood and larger scale monitoring, it 

is important to minimize near-road 

influences. Table E–1 of this appendix 

provides the required minimum separation 

distances between a roadway and a probe 

inlet for various ranges of daily roadway 

traffic. A site with a monitor probe located 

closer to a roadway than allowed by the 

Table E–1 requirements should be classified 


ddrumheller on DSK120RN23PROD with RULES3ER06MR24.047</GPH>

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Figure E-1. Distance of PM Samplers to nearest traffic lane (meters) 

Notes: Microscale street canyon sites must reside between 2 and 10 meters from the roadway. 

Near-Road sites must be within 50 meters of the roadway. 

The slopes of the lines between monitoring scales are one to one. 
Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16399 

as microscale or middle scale rather than 

neighborhood or urban scale. 

2.6 Probe Material and Pollutant Sampler 

Residence Time 

(a) For the reactive gases (SO2, NO2, and 

O3), approved probe materials must be used 

for monitors. Studies25 34 have been 

conducted to determine the suitability of 

materials such as polypropylene, 

polyethylene, polyvinyl chloride, Tygon®, 

aluminum, brass, stainless steel, copper, 

borosilicate glass, polyvinylidene fluoride 

(PVDF), polytetrafluoroethylene (PTFE), 

perfluoroalkoxy (PFA), and fluorinated 

ethylene propylene (FEP) for use as intake 

sampling lines. Of the above materials, only 

borosilicate glass, PVDF, PTFE, PFA, and 

FEP have been found to be acceptable for use 

as intake sampling lines for all the reactive 

gaseous pollutants. Furthermore, the EPA 25

has specified borosilicate glass, FEP Teflon®, 

or their equivalents as the only acceptable 

probe materials for delivering test 

atmospheres in the determination of 

reference or equivalent methods. Therefore, 

borosilicate glass, PVDF, PTFE, PFA, FEP, or 

their equivalents must be the only material 

in the sampling train (from probe inlet to the 

back of the monitor) that can be in contact 

with the ambient air sample for reactive gas 

monitors. NafionTM, which is composed 

primarily of PTFE, can be considered 

equivalent to PTFE; it has been shown in 

tests to exhibit virtually no loss of ozone at 

20-second residence times.35

(b) For volatile organic compound (VOC) 

monitoring at PAMS, FEP Teflon® is 

unacceptable as the probe material because of 

VOC adsorption and desorption reactions on 

the FEP Teflon®. Borosilicate glass, stainless 

steel, or their equivalents are the acceptable 

probe materials for VOC and carbonyl 

sampling. Care must be taken to ensure that 

the sample residence time is kept to 20 

seconds or less. 

(c) No matter how nonreactive the 

sampling probe material is initially, after a 

period of use, reactive particulate matter is 

deposited on the probe walls. Therefore, the 

time it takes the gas to transfer from the 

probe inlet to the sampling device is critical. 

Ozone in the presence of nitrogen oxide (NO) 

will show significant losses, even in the most 

inert probe material, when the residence time 

exceeds 20 seconds.26 Other 

studies 27 28indicate that a 10-second or less 

residence time is easily achievable. 

Therefore, sampling probes for all reactive 

gas monitors for SO2, NO2, and O3 must have 

a sample residence time less than 20 seconds. 

2.7 Summary 

Table E–3 of this appendix presents a 

summary of the general requirements for 

probe siting criteria with respect to distances 

and heights. Table E–3 requires different 

elevation distances above the ground for the 

various pollutants. The discussion in this 

appendix for each of the pollutants describes 

reasons for elevating the monitor or probe 

inlet. The differences in the specified range 

of heights are based on the vertical 

concentration gradients. For source oriented 

and near-road monitors, the gradients in the 

vertical direction are very large for the 

microscale, so a small range of heights are 

used. The upper limit of 15 meters is 

specified for the consistency between 

pollutants and to allow the use of a single 

manifold for monitoring more than one 

pollutant. 

TABLE E–3 TO SECTION 2.7 OF APPENDIX E—SUMMARY OF PROBE SITING CRITERIA

Pollutant Scale 9

Height 

from 

ground to 

probe 8

(meters) 

Horizontal or 

vertical distance 

from supporting 

structures 1 8 to 

probe inlet (meters) 

Distance 

from drip 

line of 

trees to 

probe 8

(meters) 

Distance from roadways to probe 8

(meters) 

SO2

2345 ............................................. Middle, Neighborhood, Urban, and 

Regional.

2.0–15 ≥1.0 ≥10 N/A. 

CO346 ................................................ Micro [downtown or street canyon 

sites].

2.5–3.5 2.0–10 for downtown areas or street 

canyon microscale. 

Micro [Near-Road sites] ...................... 2.0–7.0 ≥1.0 ≥10 ≤50 for near-road microscale. 

Middle and Neighborhood .................. 2.0–15 See Table E–2 of this appendix for 

middle and neighborhood scales. 

O3

234 .................................................. Middle, Neighborhood, Urban, and 

Regional.

2.0–15 ≥1.0 ≥10 See Table E–1. 

Micro ................................................... 2.0–7.0 ≤50 for near-road micro-scale. 

NO2

234 ............................................... Middle, Neighborhood, Urban, and 

Regional.

2.0–15 ≥1.0 ≥10 See Table E–1. 

PAMS234 Ozone precursors .............. Neighborhood and Urban ................... 2.0–15 ≥1.0 ≥10 See Table E–1. 

PM, Pb 2347 ....................................... Micro ................................................... 2.0–7.0 

Middle, Neighborhood, Urban and 

Regional.

2.0–15 ≥2.0 (horizontal 

distance only) 

≥10 See Figure E–1. 

N/A—Not applicable. 

1When a probe is located on a rooftop, this separation distance is in reference to walls, parapets, or penthouses located on the roof. 

2Should be greater than 20 meters from the dripline of tree(s) and must be 10 meters from the dripline. 

3 Distance from sampler or probe inlet to obstacle, such as a building, must be at least twice the height the obstacle protrudes above the sampler or probe inlet. 

Sites not meeting this criterion may be classified as microscale or middle scale (see paragraphs 2.3(a) and 2.3(c)). 4 Must have unrestricted airflow in a continuous arc of at least 270 degrees around the probe or sampler; 180 degrees if the probe is on the side of a building or a 

wall for street canyon monitoring. 5The probe or sampler should be away from minor sources, such as furnace or incineration flues. The separation distance is dependent on the height of the minor 

source emission point(s), the type of fuel or waste burned, and the quality of the fuel (sulfur, ash, or lead content). This criterion is designed to avoid undue influences 

from minor sources. 6For microscale CO monitoring sites, the probe must be ≥10 meters from a street intersection and preferably at a midblock location. 7 Collocated monitor inlets must be within 4.0 meters of each other and at least 2.0 meters apart for flow rates greater than 200 liters/min or at least 1.0 meter apart 

for samplers having flow rates less than 200 liters/min to preclude airflow interference, unless a waiver has been granted by the Regional Administrator pursuant to 

paragraph 3.3.4.2(c) of appendix A of part 58. For PM2.5, collocated monitor inlet heights should be within 1.0 meter of each other vertically. 8All distances listed are expressed as having 2 significant figures. When rounding is performed to assess compliance with these siting requirements, the distance 

measurements will be rounded such as to retain at least two significant figures. 9See section 1.2 of appendix D for definitions of monitoring scales. 

3. Open Path Analyzers 

3.1 Horizontal and Vertical Placement 

(a) For all O3 and SO2 monitoring sites and 

for neighborhood or larger spatial scale NO2, 

and CO sites, at least 80 percent of the 

monitoring path must be located greater than 

or equal 2.0 meters and less than or equal to 

15 meters above ground level. 

(b) Middle scale CO and NO2 sites must 

have monitoring paths greater than or equal 

2.0 meters and less than or equal to 15 meters 

above ground level. 

(c) Microscale near-road monitoring sites 

are required to have monitoring paths greater 

than or equal 2.0 meters and less than or 

equal to 7.0 meters above ground level. 

(d) For microscale carbon monoxide 

monitors that are being used to measure 

concentrations near roadways, the 

monitoring path must be greater than or 

equal 2.0 meters and less than or equal to 7.0 

meters above ground level. If the microscale 

carbon monoxide monitors measuring 

concentrations near roadways are in 

downtown areas or urban street canyons, the 

monitoring path must be greater than or 

equal 2.5 meters and less than or equal to 3.5 

meters above ground level and at least 90 


ddrumheller on DSK120RN23PROD with RULES3
16400 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

percent of the monitoring path must be at 

least 1.0 meter vertically or horizontally 

away from any supporting structure, walls, 

parapets, penthouses, etc., and away from 

dusty or dirty areas. If a significant portion 

of the monitoring path is located near the 

side of a building or wall, then it should be 

located on the windward side of the building 

relative to the prevailing wind direction 

during the season of highest concentration 

potential for the pollutant being measured. 

3.2 Spacing From Minor Sources 

(a) It is important to understand the 

monitoring objective for a particular site in 

order to interpret this requirement. Local 

minor sources of a primary pollutant, such as 

SO2 can cause high concentrations of that 

particular pollutant at a monitoring site. If 

the objective for that monitoring site is to 

investigate these local primary pollutant 

emissions, then the site will likely be 

properly located nearby. This type of 

monitoring site would, in all likelihood, be 

a microscale type of monitoring site. If a 

monitoring site is to be used to determine air 

quality over a much larger area, such as a 

neighborhood or city, a monitoring agency 

should avoid placing a monitoring path near 

local, minor sources, because a plume from 

a local minor source should not be allowed 

to inappropriately impact the air quality data 

collected at a site. 

(b) Similarly, local sources of nitric oxide 

(NO) and ozone-reactive hydrocarbons can 

have a scavenging effect causing 

unrepresentatively low concentrations of O3

in the vicinity of monitoring paths for O3. To 

minimize these potential interferences from 

nearby minor sources, at least 90 percent of 

the monitoring path should be at a distance 

from furnace or incineration flues or other 

minor sources of SO2 or NO. The separation 

distance should take into account the heights 

of the flues, type of waste or fuel burned, and 

the sulfur content of the fuel. 

3.3 Spacing From Obstructions 

(a) Obstacles may scavenge SO2, O3, or 

NO2, and can act to restrict airflow for any 

pollutant. To avoid this interference, at least 

90 percent of the monitoring path must have 

unrestricted airflow and should be located at 

a distance from obstacles. The horizontal 

distance from the obstacle to the monitoring 

path must be at least twice the height that the 

obstacle protrudes above the monitoring 

path. An obstacle that does not meet the 

minimum distance requirement is considered 

an obstruction that restricts airflow to the 

monitoring path. The EPA does not generally 

consider objects or obstacles such as flag 

poles or site towers used for NOy convertors 

and meteorological sensors, etc. to be deemed 

obstructions. 

(b) A monitoring path located near or along 

a vertical wall is undesirable because air 

moving along the wall may be subject to 

removal mechanisms. At least 90 percent of 

the monitoring path for open path analyzers 

must have unrestricted airflow with no 

obstructions (as defined in paragraph (a) of 

this section) in a continuous arc of at least 

270 degrees. An unobstructed continuous arc 

of 180 degrees is allowable when the 

applicable network design criteria specified 

in appendix D of this part require monitoring 

in street canyons and the monitoring path is 

located on the side of a building. This arc 

must include the predominant wind 

direction for the season of greatest pollutant 

concentration potential. 

(c) Special consideration must be given to 

the use of open path analyzers given their 

inherent potential sensitivity to certain types 

of interferences and optical obstructions. A 

monitoring path must be clear of all trees, 

brush, buildings, plumes, dust, or other 

optical obstructions, including potential 

obstructions that may move due to wind, 

human activity, growth of vegetation, etc. 

Temporary optical obstructions, such as rain, 

particles, fog, or snow, should be considered 

when siting an open path analyzer. Any of 

these temporary obstructions that are of 

sufficient density to obscure the light beam 

will negatively affect the ability of the open 

path analyzer to continuously measure 

pollutant concentrations. Transient, but 

significant obscuration of especially longer 

measurement paths, could occur as a result 

of certain meteorological conditions (e.g., 

heavy fog, rain, snow) and/or aerosol levels 

that are of a sufficient density to prevent the 

open path analyzer’s light transmission. If 

certain compensating measures are not 

otherwise implemented at the onset of 

monitoring (e.g., shorter path lengths, higher 

light source intensity), data recovery during 

periods of greatest primary pollutant 

potential could be compromised. For 

instance, if heavy fog or high particulate 

levels are coincident with periods of 

projected NAAQS-threatening pollutant 

potential, the representativeness of the 

resulting data record in reflecting maximum 

pollution concentrations may be 

substantially impaired despite the fact that 

the site may otherwise exhibit an acceptable, 

even exceedingly high, overall valid data 

capture rate. 

(d) A sampling station with a monitoring 

path located closer to an obstacle than 

required by the criteria in this section should 

be classified as middle scale or microscale, 

rather than neighborhood or urban scale, 

since the measurements from such a station 

would more closely represent these smaller 

scales. 

(e) For near-road monitoring stations, the 

monitoring path shall have an unobstructed 

air flow, where no obstacles exist at or above 

the height of the monitoring path, between 

the monitoring path and the outside nearest 

edge of the traffic lanes of the target road 

segment. 

3.4 Spacing From Trees 

(a) Trees can provide surfaces for SO2, O3, 

or NO2 adsorption or reactions. Trees can 

also act as obstructions in locations where 

the trees are located between the air pollutant 

sources or source areas and the monitoring 

site, and where the trees are of a sufficient 

height and leaf canopy density to interfere 

with the normal airflow around the 

monitoring path. To reduce this possible 

interference/obstruction, at least 90 percent 

of the monitoring path should be 20 meters 

or more from the drip line of trees and must 

be at least 10 meters from the drip line of 

trees. If a tree or group of trees could be 

considered an obstacle, the monitoring path 

must meet the distance requirements of 

section 3.3 of this appendix. 

(b) The scavenging effect of trees is greater 

for O3 than for other criteria pollutants. 

Monitoring agencies must take steps to 

consider the impact of trees on ozone 

monitoring sites and take steps to avoid this 

problem. 

(c) Beginning January 1, 2024, microscale 

sites of any air pollutant shall have no trees 

or shrubs located at or above the line-of-sight 

fetch between the monitoring path and the 

source under investigation, e.g., a roadway or 

a stationary source. 

3.5 Spacing from Roadways 

TABLE E–4 OF SECTION 3.5 OF APPENDIX E—MINIMUM SEPARATION

DISTANCE BETWEEN ROADWAYS AND

MONITORING PATHS FOR MONITORING NEIGHBORHOOD AND URBAN

SCALE OZONE (O3) AND OXIDES OF

NITROGEN (NO, NO2, NOX, NOy) 

Roadway 

average daily traffic, 

vehicles per day 

Minimum 

distance 1 3

(meters) 

Minimum 

distance 123

(meters) 

≤1,000 ....................... 10 10 

10,000 ....................... 10 20 

15,000 ....................... 20 30 

20,000 ....................... 30 40 

40,000 ....................... 50 60 

70,000 ....................... 100 100 

≥110,000 ................... 250 250 

1 Distance from the edge of the nearest traffic lane. The distance for intermediate traffic 

counts should be interpolated from the table 

values based on the actual traffic count. 2Applicable for ozone open path monitors 

whose placement was not approved as of December 18, 2006. 3All distances listed are expressed as having 2 significant figures. When rounding is performed to assess compliance with these siting 

requirements, the distance measurements will 

be rounded such as to retain at least two significant figures. 

3.5.1 Spacing for Ozone Monitoring Paths 

In siting an O3 open path analyzer, it is 

important to minimize destructive 

interferences form sources of NO, since NO 

readily reacts with O3. Table E–4 of this 

appendix provides the required minimum 

separation distances between a roadway and 

at least 90 percent of a monitoring path for 

various ranges of daily roadway traffic. A 

monitoring site with a monitoring path 

located closer to a roadway than allowed by 

the Table E–4 requirements should be 

classified as microscale or middle scale, 

rather than neighborhood or urban scale, 

since the measurements from such a site 

would more closely represent these smaller 

scales. The monitoring path(s) must not cross 

over a roadway with an average daily traffic 

count of 10,000 vehicles per day or more. For 

locations where a monitoring path crosses a 

roadway with fewer than 10,000 vehicles per 

day, monitoring agencies must consider the 

entire segment of the monitoring path in the 

area of potential atmospheric interference 

from automobile emissions. Therefore, this 


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calculation must include the length of the 

monitoring path over the roadway plus any 

segments of the monitoring path that lie in 

the area between the roadway and minimum 

separation distance, as determined from 

Table E–4 of this appendix. The sum of these 

distances must not be greater than 10 percent 

of the total monitoring path length. 

3.5.2 Spacing for Carbon Monoxide 

Monitoring Paths 

(a) Near-road microscale CO monitoring 

sites, including those located in downtown 

areas, urban street canyons, and other nearroad locations such as those adjacent to 

highly trafficked roads, are intended to 

provide a measurement of the influence of 

the immediate source on the pollution 

exposure on the adjacent area. 

(b) Microscale CO monitoring paths in 

downtown areas or urban street canyon 

locations shall be located a minimum 

distance of 2.0 meters and a maximum 

distance of 10 meters from the edge of the 

nearest traffic lane. 

(c) Microscale CO monitoring paths in 

downtown areas or urban street canyon 

locations shall be located at least 10 meters 

from an intersection, preferably at a midblock 

location. Midblock locations are preferable to 

intersection locations because intersections 

represent a much smaller portion of 

downtown space than do the streets between 

them. Pedestrian exposure is probably also 

greater in street canyon/corridors than at 

intersections. 

(d) Neighborhood scale CO monitoring 

paths in downtown areas or urban street 

canyon locations shall be located according 

to the requirements in Table E–5 of this 

appendix. 

TABLE E–5 SECTION 3.5.2 OF APPENDIX E—MINIMUM SEPARATION DISTANCE BETWEEN ROADWAYS AND

MONITORING PATHS FOR MONITORING NEIGHBORHOOD SCALE

CARBON MONOXIDE

Roadway average 

daily traffic, 

vehicles per day 

Minimum 

distance 1 2

(meters) 

≤10,000 ................................. 10 

15,000 ................................... 25 

20,000 ................................... 45 

30,000 ................................... 80 

40,000 ................................... 115 

TABLE E–5 SECTION 3.5.2 OF APPENDIX E—MINIMUM SEPARATION DISTANCE BETWEEN ROADWAYS AND

MONITORING PATHS FOR MONITORING NEIGHBORHOOD SCALE

CARBON MONOXIDE—Continued 

Roadway average 

daily traffic, 

vehicles per day 

Minimum 

distance 1 2

(meters) 

50,000 ................................... 135 

≥60,000 ................................. 150 

1 Distance from the edge of the nearest traffic lane. The distance for intermediate traffic 

counts should be interpolated from the table 

values based on the actual traffic count. 2All distances listed are expressed as having 2 significant figures. When rounding is performed to assess compliance with these siting 

requirements, the distance measurements will 

be rounded such as to retain at least two significant figures. 

3.5.3 Spacing for Nitrogen Dioxide (NO2) 

Monitoring Paths 

(a) In siting near-road NO2 monitors as 

required in section 4.3.2 of appendix D of 

this part, the monitoring path shall be as near 

as practicable to the outside nearest edge of 

the traffic lanes of the target road segment but 

shall not be located at a distance greater than 

50 meters, in the horizontal, from the outside 

nearest edge of the traffic lanes of the target 

road segment. 

(b) In siting NO2 open path monitors for 

neighborhood and larger scale monitoring, it 

is important to minimize near-road 

influences. Table E–5 of this appendix 

provides the required minimum separation 

distances between a roadway and at least 90 

percent of a monitoring path for various 

ranges of daily roadway traffic. A site with 

a monitoring path located closer to a roadway 

than allowed by the Table E–4 requirements 

should be classified as microscale or middle 

scale rather than neighborhood or urban 

scale. The monitoring path(s) must not cross 

over a roadway with an average daily traffic 

count of 10,000 vehicles per day or more. For 

locations where a monitoring path crosses a 

roadway with fewer than 10,000 vehicles per 

day, monitoring agencies must consider the 

entire segment of the monitoring path in the 

area of potential atmospheric interference 

form automobile emissions. Therefore, this 

calculation must include the length of the 

monitoring path over the roadway plus any 

segments of the monitoring path that lie in 

the area between the roadway and minimum 

separation distance, as determined from 

Table E–5 of this appendix. The sum of these 

distances must not be greater than 10 percent 

of the total monitoring path length. 

3.6 Cumulative Interferences on a 

Monitoring Path 

The cumulative length or portion of a 

monitoring path that is affected by minor 

sources, trees, or roadways must not exceed 

10 percent of the total monitoring path 

length. 

3.7 Maximum Monitoring Path Length 

The monitoring path length must not 

exceed 1.0 kilometer for open path analyzers 

in neighborhood, urban, or regional scale. For 

middle scale monitoring sites, the monitoring 

path length must not exceed 300 meters. In 

areas subject to frequent periods of dust, fog, 

rain, or snow, consideration should be given 

to a shortened monitoring path length to 

minimize loss of monitoring data due to 

these temporary optical obstructions. For 

certain ambient air monitoring scenarios 

using open path analyzers, shorter path 

lengths may be needed in order to ensure that 

the monitoring site meets the objectives and 

spatial scales defined in appendix D to this 

part. The Regional Administrator may require 

shorter path lengths, as needed on an 

individual basis, to ensure that the SLAMS 

sites meet the appendix D requirements. 

Likewise, the Administrator may specify the 

maximum path length used at NCore 

monitoring sites. 

3.8 Summary 

Table E–6 of this appendix presents a 

summary of the general requirements for 

monitoring path siting criteria with respect to 

distances and heights. Table E–6 requires 

different elevation distances above the 

ground for the various pollutants. The 

discussion in this appendix for each of the 

pollutants describes reasons for elevating the 

monitoring path. The differences in the 

specified range of heights are based on the 

vertical concentration gradients. For source 

oriented and near-road monitors, the 

gradients in the vertical direction are very 

large for the microscale, so a small range of 

heights are used. The upper limit of 15 

meters is specified for the consistency 

between pollutants and to allow the use of 

a monitoring path for monitoring more than 

one pollutant. 

TABLE E–6 SECTION 3.8 OF APPENDIX E—SUMMARY OF MONITORING PATH SITING CRITERIA

Pollutant Maximum monitoring path length 9 10

Height from 

ground to 80% 

of monitoring 

path 1 8

(meters) 

Horizontal or 

vertical distance from 

supporting 

structures 2 to 

90% of monitoring path 1 8

(meters) 

Distance from 

trees to 90% 

of monitoring 

path 1 8

(meters) 

Distance from roadways to monitoring path 1 8

(meters) 

SO2 3456 .......................................... <= 300 m for Middle ........................

<= 1.0 km for Neighborhood, Urban, 

and Regional 

2.0–15 ≥1.0 ≥10 N/A. 

CO457 .............................................. <= 300 m for Micro [downtown or 

street canyon sites].

2.5–3.5 ≥1.0 ≥10 2.0–10 for downtown areas or street 

canyon microscale. 

<= 300 m for Micro [Near-Road 

sites].

2.0–7.0 ≤50 for near-road microscale. 


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TABLE E–6 SECTION 3.8 OF APPENDIX E—SUMMARY OF MONITORING PATH SITING CRITERIA—Continued 

Pollutant Maximum monitoring path length 9 10

Height from 

ground to 80% 

of monitoring 

path 1 8

(meters) 

Horizontal or 

vertical distance from 

supporting 

structures 2 to 

90% of monitoring path 1 8

(meters) 

Distance from 

trees to 90% 

of monitoring 

path 1 8

(meters) 

Distance from roadways to monitoring path 1 8

(meters) 

<= 300 m for Middle ........................ 2.0–15 See Table E–5 of this appendix for 

middle and neighborhood scales. 

<= 1.0 km for Neighborhood.

O3

345 .............................................. <= 300 m for Middle.

<= 1.0 km for Neighborhood, Urban, 

and Regional.

2.0–15 ≥1.0 ≥10 See Table E–4. 

NO2

345 ........................................... Between 50 m–300 m for Micro 

(Near-Road).

2.0–7.0 ≤50 for near-road micro-scale. 

<= 300 m for Middle ........................ ≥1.0 ≥10 

<= 1.0 km for Neighborhood, Urban, 

and Regional.

2.0–15 See Table E–4. 

PAMS345 Ozone precursors .......... <= 1.0 km for Neighborhood and 

Urban.

2.0–15 ≥1.0 ≥10 See Table E–4. 

N/A—Not applicable. 

1 Monitoring path for open path analyzers is applicable only to middle or neighborhood scale CO monitoring, middle, neighborhood, urban, and regional scale NO2

monitoring, and all applicable scales for monitoring SO2, O3, and O3 precursors. 2When the monitoring path is located on a rooftop, this separation distance is in reference to walls, parapets, or penthouses located on roof. 

3At least 90 percent of the monitoring path should be greater than 20 meters from the dripline of tree(s) and must be 10-meters from the dripline. 

4 Distance from 90 percent of monitoring path to obstacle, such as a building, must be at least twice the height the obstacle protrudes above the monitoring path. 

Sites not meeting this criterion may be classified as microscale or middle scale (see text). 5 Must have unrestricted airflow 270 degrees around at least 90 percent of the monitoring path; 180 degrees if the monitoring path is adjacent to the side of a building or a wall for street canyon monitoring. 6The monitoring path should be away from minor sources, such as furnace or incineration flues. The separation distance is dependent on the height of the minor 

source’s emission point (such as a flue), the type of fuel or waste burned, and the quality of the fuel (sulfur, ash, or lead content). This criterion is designed to avoid 

undue influences from minor sources. 7For microscale CO monitoring sites, the monitoring path must be ≥10. meters from a street intersection and preferably at a midblock location. 8All distances listed are expressed as having 2 significant figures. When rounding is performed to assess compliance with these siting requirements, the distance 

measurements will be rounded such as to retain at least two significant figures. 9See section 1.2 of appendix D for definitions of monitoring scales. 

10See section 3.7 of this appendix. 

4. Waiver Provisions 

Most sampling probes or monitors can be 

located so that they meet the requirements of 

this appendix. New sites, with rare 

exceptions, can be located within the limits 

of this appendix. However, some existing 

sites may not meet these requirements and 

may still produce useful data for some 

purposes. The EPA will consider a written 

request from the State, or where applicable 

local, agency to waive one or more siting 

criteria for some monitoring sites providing 

that the State or their designee can 

adequately demonstrate the need (purpose) 

for monitoring or establishing a monitoring 

site at that location. 

4.1 For a proposed new site, a waiver 

may be granted only if both the following 

criteria are met: 

4.1.1 The proposed new site can be 

demonstrated to be as representative of the 

monitoring area as it would be if the siting 

criteria were being met. 

4.1.2 The monitor or probe cannot 

reasonably be located so as to meet the siting 

criteria because of physical constraints (e.g., 

inability to locate the required type of site the 

necessary distance from roadways or 

obstructions). 

4.2 For an existing site, a waiver may be 

granted if either the criterion in section 4.1.1 

or the criterion in 4.1.2 of this appendix is 

met. 

4.3 Cost benefits, historical trends, and 

other factors may be used to add support to 

the criteria in sections 4.1.1 and 4.1.2 of this 

appendix; however, by themselves, they will 

not be acceptable reasons for the EPA to grant 

a waiver. Written requests for waivers must 

be submitted to the Regional Administrator. 

Granted waivers must be renewed minimally 

every 5 years and ideally as part of the 

network assessment as defined in § 58.10(d). 

The approval date of the waiver must be 

documented in the annual monitoring 

network plan to support the requirements of 

§ 58.10(a)(1) and 58.10(b)(10). 

5. References 

1. Bryan, R.J., R.J. Gordon, and H. Menck. 

Comparison of High Volume Air Filter 

Samples at Varying Distances from Los 

Angeles Freeway. University of Southern 

California, School of Medicine, Los Angeles, 

CA. (Presented at 66th Annual Meeting of Air 

Pollution Control Association. Chicago, IL. 

June 24–28, 1973. APCA 73–158.) 

2. Teer, E.H. Atmospheric Lead 

Concentration Above an Urban Street. Master 

of Science Thesis, Washington University, St. 

Louis, MO. January 1971. 

3. Bradway, R.M., F.A. Record, and W.E. 

Belanger. Monitoring and Modeling of 

Resuspended Roadway Dust Near Urban 

Arterials. GCA Technology Division, 

Bedford, MA. (Presented at 1978 Annual 

Meeting of Transportation Research Board, 

Washington, DC. January 1978.) 

4. Pace, T.G., W.P. Freas, and E.M. Afify. 

Quantification of Relationship Between 

Monitor Height and Measured Particulate 

Levels in Seven U.S. Urban Areas. U.S. 

Environmental Protection Agency, Research 

Triangle Park, NC. (Presented at 70th Annual 

Meeting of Air Pollution Control Association, 

Toronto, Canada. June 20–24, 1977. APCA 

77–13.4.) 

5. Harrison, P.R. Considerations for Siting 

Air Quality Monitors in Urban Areas. City of 

Chicago, Department of Environmental 

Control, Chicago, IL. (Presented at 66th 

Annual Meeting of Air Pollution Control 

Association, Chicago, IL. June 24–28, 1973. 

APCA 73–161.) 

6. Study of Suspended Particulate 

Measurements at Varying Heights Above 

Ground. Texas State Department of Health, 

Air Control Section, Austin, TX. 1970. p.7. 

7. Rodes, C.E. and G.F. Evans. Summary of 

LACS Integrated Pollutant Data. In: Los 

Angeles Catalyst Study Symposium. U.S. 

Environmental Protection Agency, Research 

Triangle Park, NC. EPA Publication No. EPA– 

600/4–77–034. June 1977. 

8. Lynn, D.A. et al. National Assessment of 

the Urban Particulate Problem: Volume 1, 

National Assessment. GCA Technology 

Division, Bedford, MA. U.S. Environmental 

Protection Agency, Research Triangle Park, 

NC. EPA Publication No. EPA–450/3–75– 

024. June 1976. 

9. Pace, T.G. Impact of Vehicle-Related 

Particulates on TSP Concentrations and 

Rationale for Siting Hi-Vols in the Vicinity of 

Roadways. OAQPS, U.S. Environmental 

Protection Agency, Research Triangle Park, 

NC. April 1978. 

10. Ludwig, F.L., J.H. Kealoha, and E. 

Shelar. Selecting Sites for Monitoring Total 

Suspended Particulates. Stanford Research 

Institute, Menlo Park, CA. Prepared for U.S. 

Environmental Protection Agency, Research 

Triangle Park, NC. EPA Publication No. EPA– 

450/3–77–018. June 1977, revised December 

1977. 

11. Ball, R.J. and G.E. Anderson. Optimum 

Site Exposure Criteria for SO2 Monitoring. 

The Center for the Environment and Man, 


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Inc., Hartford, CT. Prepared for U.S. 

Environmental Protection Agency, Research 

Triangle Park, NC. EPA Publication No. EPA– 

450/3–77–013. April 1977. 

12. Ludwig, F.L. and J.H.S. Kealoha. 

Selecting Sites for Carbon Monoxide 

Monitoring. Stanford Research Institute, 

Menlo Park, CA. Prepared for U.S. 

Environmental Protection Agency, Research 

Triangle Park, NC. EPA Publication No. EPA– 

450/3–75–077. September 1975. 

13. Ludwig, F.L. and E. Shelar. Site 

Selection for the Monitoring of 

Photochemical Air Pollutants. Stanford 

Research Institute, Menlo Park, CA. Prepared 

for U.S. Environmental Protection Agency, 

Research Triangle Park, NC. EPA Publication 

No. EPA–450/3–78–013. April 1978. 

14. Lead Analysis for Kansas City and 

Cincinnati, PEDCo Environmental, Inc., 

Cincinnati, OH. Prepared for U.S. 

Environmental Protection Agency, Research 

Triangle Park, NC. EPA Contract No. 66–02– 

2515, June 1977. 

15. Barltrap, D. and C.D. Strelow. Westway 

Nursery Testing Project. Report to the Greater 

London Council. August 1976. 

16. Daines, R. H., H. Moto, and D. M. 

Chilko. Atmospheric Lead: Its Relationship to 

Traffic Volume and Proximity to Highways. 

Environ. Sci. and Technol., 4:318, 1970. 

17. Johnson, D. E., et al. Epidemiologic 

Study of the Effects of Automobile Traffic on 

Blood Lead Levels, Southwest Research 

Institute, Houston, TX. Prepared for U.S. 

Environmental Protection Agency, Research 

Triangle Park, NC. EPA–600/1–78–055, 

August 1978. 

18. Air Quality Criteria for Lead. Office of 

Research and Development, U.S. 

Environmental Protection Agency, 

Washington, DC EPA–600/8–83–028 aF–dF, 

1986, and supplements EPA–600/8–89/049F, 

August 1990. (NTIS document numbers 

PB87–142378 and PB91–138420.) 

19. Lyman, D. R. The Atmospheric 

Diffusion of Carbon Monoxide and Lead from 

an Expressway, Ph.D. Dissertation, 

University of Cincinnati, Cincinnati, OH. 

1972. 

20. Wechter, S.G. Preparation of Stable 

Pollutant Gas Standards Using Treated 

Aluminum Cylinders. ASTM STP. 598:40– 

54, 1976. 

21. Wohlers, H.C., H. Newstein and D. 

Daunis. Carbon Monoxide and Sulfur 

Dioxide Adsorption On and Description 

From Glass, Plastic and Metal Tubings. J. Air 

Poll. Con. Assoc. 17:753, 1976. 

22. Elfers, L.A. Field Operating Guide for 

Automated Air Monitoring Equipment. U.S. 

NTIS. p. 202, 249, 1971. 

23. Hughes, E.E. Development of Standard 

Reference Material for Air Quality 

Measurement. ISA Transactions, 14:281–291, 

1975. 

24. Altshuller, A.D. and A.G. Wartburg. 

The Interaction of Ozone with Plastic and 

Metallic Materials in a Dynamic Flow 

System. Intern. Jour. Air and Water Poll., 

4:70–78, 1961. 

25. Code of Federal Regulations. 40 CFR 

53.22, July 1976. 

26. Butcher, S.S. and R.E. Ruff. Effect of 

Inlet Residence Time on Analysis of 

Atmospheric Nitrogen Oxides and Ozone, 

Anal. Chem., 43:1890, 1971. 

27. Slowik, A.A. and E.B. Sansone. 

Diffusion Losses of Sulfur Dioxide in 

Sampling Manifolds. J. Air. Poll. Con. Assoc., 

24:245, 1974. 

28. Yamada, V.M. and R.J. Charlson. Proper 

Sizing of the Sampling Inlet Line for a 

Continuous Air Monitoring Station. Environ. 

Sci. and Technol., 3:483, 1969. 

29. Koch, R.C. and H.E. Rector. Optimum 

Network Design and Site Exposure Criteria 

for Particulate Matter, GEOMET 

Technologies, Inc., Rockville, MD. Prepared 

for U.S. Environmental Protection Agency, 

Research Triangle Park, NC. EPA Contract 

No. 68–02–3584. EPA 450/4–87–009. May 

1987. 

30. Burton, R.M. and J.C. Suggs. 

Philadelphia Roadway Study. Environmental 

Monitoring Systems Laboratory, U.S. 

Environmental Protection Agency, Research 

Triangle Park, N.C. EPA–600/4–84–070 

September 1984. 

31. Technical Assistance Document for 

Sampling and Analysis of Ozone Precursors. 

Atmospheric Research and Exposure 

Assessment Laboratory, U.S. Environmental 

Protection Agency, Research Triangle Park, 

NC 27711. EPA 600/8–91–215. October 1991. 

32. Quality Assurance Handbook for Air 

Pollution Measurement Systems: Volume IV. 

Meteorological Measurements. Atmospheric 

Research and Exposure Assessment 

Laboratory, U.S. Environmental Protection 

Agency, Research Triangle Park, NC 27711. 

EPA 600/4–90–0003. August 1989. 

33. On-Site Meteorological Program 

Guidance for Regulatory Modeling 

Applications. Office of Air Quality Planning 

and Standards, U.S. Environmental 

Protection Agency, Research Triangle Park, 

NC 27711. EPA 450/4–87–013. June 1987F. 

34. Johnson, C., A. Whitehill, R. Long, and 

R. Vanderpool. Investigation of Gaseous 

Criteria Pollutant Transport Efficiency as a 

Function of Tubing Material. U.S. 

Environmental Protection Agency, Research 

Triangle Park, NC 27711. EPA/600/R–22/212. 

August 2022. 

35. Hannah Halliday, Cortina Johnson, Tad 

Kleindienst, Russell Long, Robert 

Vanderpool, and Andrew Whitehill. 

Recommendations for Nationwide Approval 

of NafionTM Dryers Upstream of UVAbsorption Ozone Analyzers. U.S. 

Environmental Protection Agency, Research 

Triangle Park, NC 27711. EPA/600/R–20/390. 

November 2020. 

■ 31. Revise appendix G to part 58 to 

read as follows: 

Appendix G to Part 58—Uniform Air 

Quality Index (AQI) and Daily 

Reporting 

1. General Information 

2. Reporting Requirements 

3. Data Handling 

1. General Information 

1.1 AQI Overview. The AQI is a tool that 

simplifies reporting air quality to the public 

in a nationally uniform and easy to 

understand manner. The AQI converts 

concentrations of pollutants, for which the 

EPA has established a national ambient air 

quality standard (NAAQS), into a uniform 

scale from 0–500. These pollutants are ozone 

(O3), particulate matter (PM2.5, PM10), carbon 

monoxide (CO), sulfur dioxide (SO2), and 

nitrogen dioxide (NO2). The scale of the 

index is divided into general categories that 

are associated with health messages. 

2. Reporting Requirements 

2.1 Applicability. The AQI must be 

reported daily for a metropolitan statistical 

area (MSA) with a population over 350,000. 

When it is useful and possible, it is 

recommended, but not required for an area to 

report a sub-daily AQI as well. 

2.2 Contents of AQI Report. 

2.2.1 Content of AQI Report 

Requirements. An AQI report must contain 

the following: 

a. The reporting area(s) (the MSA or 

subdivision of the MSA). 

b. The reporting period (the day for which 

the AQI is reported). 

c. The main pollutant (the pollutant with 

the highest index value). 

d. The AQI (the highest index value). 

e. The category descriptor and index value 

associated with the AQI and, if choosing to 

report in a color format, the associated color. 

Use only the following descriptors and colors 

for the six AQI categories: 

TABLE 1 TO SECTION 2 OF APPENDIX G—AQI CATEGORIES

For this AQI Use this descriptor And this color 1

0 to 50 ................ ‘‘Good’’ ..................................................................................... Green. 

51 to 100 ............ ‘‘Moderate’’ ............................................................................... Yellow. 

101 to 150 .......... ‘‘Unhealthy for Sensitive Groups’’ ............................................ Orange. 

151 to 200 .......... ‘‘Unhealthy’’ .............................................................................. Red. 

201 to 300 .......... ‘‘Very Unhealthy’’ ..................................................................... Purple. 

301 and above ... ‘‘Hazardous’’ ............................................................................. Maroon1. 

1Specific color definitions can be found in the most recent reporting guidance (Technical Assistance Document for the Reporting of Daily Air 

Quality), which can be found at https://www.airnow.gov/publications/air-quality-index/technical-assistance-document-for-reporting-the-daily-aqi/. 


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f. The pollutant specific sensitive groups 

for any reported index value greater than 100. 

The sensitive groups for each pollutant are 

identified as part of the periodic review of 

the air quality criteria and the NAAQS. For 

convenience, the EPA lists the relevant 

groups for each pollutant in the most recent 

reporting guidance (Technical Assistance 

Document for the Reporting of Daily Air 

Quality), which can be found at https://www.

airnow.gov/publications/air-quality-index/ 

technical-assistance-document-for-reportingthe-daily-aqi/. 

2.2.2 Contents of AQI Report When 

Applicable. When appropriate, the AQI 

report may also contain the following, but 

such information is not required: 

a. Appropriate health and cautionary 

statements. 

b. The name and index value for other 

pollutants, particularly those with an index 

value greater than 100. 

c. The index values for sub-areas of your 

MSA. 

d. Causes for unusually high AQI values. 

e. Pollutant concentrations. 

f. Generally, the AQI report applies to an 

area’s MSA only. However, if a significant air 

quality problem exists (AQI greater than 100) 

in areas significantly impacted by the MSA 

but not in it (for example, O3 concentrations 

are often highest downwind and outside an 

urban area), the report should identify these 

areas and report the AQI for these areas as 

well. 

2.3. Communication, Timing, and 

Frequency of AQI Report. The daily AQI 

must be reported 7 days per week and made 

available via website or other means of 

public access. The daily AQI report 

represents the air quality for the previous 

day. Exceptions to this requirement are in 

section 2.4 of this appendix. 

a. Reporting the AQI sub-daily is 

recommended, but not required, to provide 

more timely air quality information to the 

public for making health-protective 

decisions. 

b. Submitting hourly data in real-time to 

the EPA’s AirNow (or future analogous) 

system is recommended, but not required, 

and assists the EPA in providing timely air 

quality information to the public for making 

health-protective decisions. 

c. Submitting hourly data for appropriate 

monitors (referenced in section 3.2 of this 

appendix) satisfies the daily AQI reporting 

requirement because the AirNow system 

makes daily and sub-daily AQI reports 

widely available through its website and 

other communication tools. 

d. Forecasting the daily AQI provides 

timely air quality information to the public 

and is recommended but not required. Subdaily forecasts are also recommended, 

especially when air quality is expected to 

vary substantially throughout the day, like 

during wildfires. Long-term (multi-day) 

forecasts can also be made available when 

useful. 

2.4. Exceptions to Reporting 

Requirements. 

a. If the index value for a particular 

pollutant remains below 50 for a season or 

year, then it may be excluded from the 

calculation of the AQI in section 3 of this 

appendix. 

b. If all index values remain below 50 for 

a year, then the AQI may be reported at the 

discretion of the reporting agency. In 

subsequent years, if pollutant levels rise to 

where the AQI would be above 50, then the 

AQI must be reported as required in section 

2 of this appendix. 

c. As previously mentioned in section 2.3 

of this appendix, submitting hourly data in 

real-time from appropriate monitors 

(referenced in section 3.2 of this appendix) 

to the EPA’s AirNow (or future analogous) 

system satisfies the daily AQI reporting 

requirement. 

3. Data Handling. 

3.1 Relationship of AQI and pollutant 

concentrations. For each pollutant, the AQI 

transforms ambient concentrations to a scale 

from 0 to 500. As appropriate, the AQI is 

associated with the NAAQS for each 

pollutant. In most cases, the index value of 

100 is associated with the numerical level of 

the short-term standard (i.e., averaging time 

of 24-hours or less) for each pollutant. The 

index value of 50 is associated with the 

numerical level of the annual standard for a 

pollutant, if there is one, at one-half the level 

of the short-term standard for the pollutant 

or at the level at which it is appropriate to 

begin to provide guidance on cautionary 

language. Higher categories of the index are 

based on the potential for increasingly 

serious health effects to occur following 

exposure and increasing proportions of the 

population that are likely to be affected. The 

reported AQI corresponds to the pollutant 

with the highest calculated AQI. For the 

purposes of reporting the AQI, the subindexes for PM10 and PM2.5 are to be 

considered separately. The pollutant 

responsible for the highest index value (the 

reported AQI) is called the ‘‘main’’ pollutant 

for that day. 

3.2 Monitors Used for AQI Reporting. 

Concentration data from State/Local Air 

Monitoring Station (SLAMS) or parts of the 

SLAMS required by 40 CFR 58.10 must be 

used for each pollutant except PM. For PM, 

calculate and report the AQI on days for 

which air quality data has been measured 

(e.g., from continuous PM2.5 monitors 

required in appendix D to this part). PM 

measurements may be used from monitors 

that are not reference or equivalent methods 

(for example, continuous PM10 or PM2.5

monitors). Detailed guidance for relating nonapproved measurements to approved 

methods by statistical linear regression is 

referenced here: 

Reference for relating non-approved PM 

measurements to approved methods (Eberly, 

S., T. Fitz-Simons, T. Hanley, L. Weinstock., 

T. Tamanini, G. Denniston, B. Lambeth, E. 

Michel, S. Bortnick. Data Quality Objectives 

(DQOs) For Relating Federal Reference 

Method (FRM) and Continuous PM2.5

Measurements to Report an Air Quality Index 

(AQI). U.S. Environmental Protection 

Agency, Research Triangle Park, NC. EPA– 

454/B–02–002, November 2002). 

3.3 AQI Forecast. The AQI can be 

forecasted at least 24-hours in advance using 

the most accurate and reasonable procedures 

considering meteorology, topography, 

availability of data, and forecasting expertise. 

The guidance document, ‘‘Guidelines for 

Developing an Air Quality (Ozone and PM2.5) 

Forecasting Program,’’ can be found at 

https://www.airnow.gov/publications/ 

weathercasters/guidelines-developing-airquality-forecasting-program/. 

3.4 Calculation and Equations. 

a. The AQI is the highest value calculated 

for each pollutant as follows: 

i. Identify the highest concentration among 

all of the monitors within each reporting area 

and truncate as follows: 

(A) Ozone—truncate to 3 decimal places 

PM2.5—truncate to 1 decimal place 

PM10—truncate to integer 

CO—truncate to 1 decimal place 

SO2—truncate to integer 

NO2—truncate to integer 

(B) [Reserved] 

ii. Using table 2 to this appendix, find the 

two breakpoints that contain the 

concentration. 

iii. Using equation 1 to this appendix, 

calculate the index. 

iv. Round the index to the nearest integer. 

TABLE 2 TO SECTION 3.4 OF APPENDIX G—BREAKPOINTS FOR THE AQI 

These breakpoints Equal these AQI’s 

O3 (ppm) 8-hour O3 (ppm) 1- 

hour1

PM2.5 (μg/m3) 

24-hour 

PM10 (μg/m3) 

24-hour 

CO 

(ppm) 8-hour 

SO2

(ppb) 

1-hour 

NO2

(ppb) 

1-hour 

AQI Category 

0.000–0.054 ..... ........................ 0.0–9.0 0–54 0.0–4.4 0–35 0–53 0–50 Good. 

0.055–0.070 ..... ........................ 9.1–35.4 55–154 4.5–9.4 36–75 54–100 51–100 Moderate. 

0.071–0.085 ..... 0.125–0.164 35.5–55.4 155–254 9.5–12.4 76–185 101–360 101–150 Unhealthy for Sensitive 

Groups. 

0.086–0.105 ..... 0.165–0.204 55.5–125.4 255–354 12.5–15.4 3 186–304 361–649 151–200 Unhealthy. 

0.106–0.200 ..... 0.205–0.404 125.5—225.4 355–424 15.5–30.4 3 305–604 650–1249 201–300 Very Unhealthy. 


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Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 16405 

TABLE 2 TO SECTION 3.4 OF APPENDIX G—BREAKPOINTS FOR THE AQI—Continued 

These breakpoints Equal these AQI’s 

O3 (ppm) 8-hour O3 (ppm) 1- 

hour1

PM2.5 (μg/m3) 

24-hour 

PM10 (μg/m3) 

24-hour 

CO 

(ppm) 8-hour 

SO2

(ppb) 

1-hour 

NO2

(ppb) 

1-hour 

AQI Category 

0.201¥(2) ........ 0.405+ 225.5+ 425+ 30.5+ 3 605+ 1250+ 301+ 4 Hazardous. 

1Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour ozone 

values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be calculated, and the 

maximum of the two values reported. 2 8-hour O3 concentrations do not define higher AQI values (≤301). AQI values > 301 are calculated with 1-hour O3 concentrations. 3 1-hr SO2 concentrations do not define higher AQI values (≥200). AQI values of 200 or greater are calculated with 24-hour SO2 concentration. 4AQI values between breakpoints are calculated using equation 1 to this appendix. For AQI values in the hazardous category, AQI values greater than 500 should 

be calculated using equation 1 and the concentration specified for the AQI value of 500. The AQI value of 500 are as follows: O3 1-hour—0.604 ppm; PM2.5 24- 

hour—325.4 μg/m3; PM10 24-hour—604 μg/m3; CO ppm—50.4 ppm; SO2 1-hour—1004 ppb; and NO2 1-hour—2049 ppb. 

b. If the concentration is equal to a 

breakpoint, then the index is equal to the 

corresponding index value in table 2 to this 

appendix. However, equation 1 to this 

appendix can still be used. The results will 

be equal. If the concentration is between two 

breakpoints, then calculate the index of that 

pollutant with equation 1. It should also be 

noted that in some areas, the AQI based on 

1-hour O3 will be more precautionary than 

using 8-hour values (see footnote 1 to table 

2). In these cases, the 1-hour values as well 

as 8-hour values may be used to calculate 

index values and then use the maximum 

index value as the AQI for O3.

Where: 

Ip = the index value for pollutantp. 

Cp = the truncated concentration of 

pollutantp. 

BPHi = the breakpoint that is greater than or 

equal to Cp. 

BPLo = the breakpoint that is less than or 

equal to Cp. 

IHi = the AQI value corresponding to BPHi. 

Ilo = the AQI value corresponding to BPLo. 

c. If the concentration is larger than the 

highest breakpoint in table 2 to this appendix 

then the last two breakpoints in table 2 may 

be used when equation 1 to this appendix is 

applied. 

Example: 

d. Using table 2 and equation 1 to this 

appendix, calculate the index value for each 

of the pollutants measured and select the one 

that produces the highest index value for the 

AQI. For example, if a PM10 value of 210 mg/ 

m3 is observed, a 1-hour O3 value of 0.156 

ppm, and an 8-hour O3 value of 0.130 ppm, 

then do this: 

i. Find the breakpoints for PM10 at 210 mg/ 

m3 as 155 mg/m3 and 254 mg/m3, 

corresponding to index values 101 and 150; 

ii. Find the breakpoints for 1-hour O3 at 

0.156 ppm as 0.125 ppm and 0.164 ppm, 

corresponding to index values 101 and 150; 

iii. Find the breakpoints for 8-hour O3 at 

0.130 ppm as 0.116 ppm and 0.374 ppm, 

corresponding to index values 201 and 300; 

iv. Apply equation 21 to this appendix for 

210 mg/m3, PM10: 

v. Apply equation 3 to this appendix for 

0.156 ppm, 1-hour O3: 

vi. Apply equation 4 to this appendix for 

0.130 ppm, 8-hour O3: 


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Equation 1 to Appendix G to Part 58 

Equation 2 to Appendix G to Part 58 

150 - 101 

254 _ 155 (210 - 155) + 101 = 128 

Equation 3 to Appendix G to Part 58 

150 - 101 

0.164 _ 0.125 (0.156 - 0.125) + 101 = 140 

Equation 4 to Appendix G to Part 58 

300 - 201 

0.374 _ 0.116 (0.130 - 0.116) + 201 = 206 
16406 Federal Register / Vol. 89, No. 45 / Wednesday, March 6, 2024 / Rules and Regulations 

vii. Find the maximum, 206. This is the 

AQI. A minimal AQI report could read: 

‘‘Today, the AQI for my city is 206, which 

is Very Unhealthy, due to ozone.’’ It would then reference the associated sensitive 

groups. 

[FR Doc. 2024–02637 Filed 3–5–24; 8:45 am] 

BILLING CODE 6560–50–P 


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