U. S. Environmental Protection Agency 

National Center for Environmental Assessment 

Office of Research and Development 

and 

Office of Air Quality Planning and Standards 

Office of Air and Radiation 

Research Triangle Park, North Carolina 27711 

April 2016 

EP United States 

Environmental 

Draft Integrated Review Plan for the 

National Ambient Air Quality 

Standards for Particulate Matter

EP United States EPA Environmental 

United States 

Environmental Protection Agency
April 2016 ii Draft – Do Not Quote or Cite

DISCLAIMER 

This draft integrated review plan has been prepared by staff in the U.S. Environmental Protection 

Agency’s (EPA) Office of Air Quality Planning and Standards (OAQPS) and National Center for 

Environmental Assessment (NCEA). Any findings and conclusions are those of the authors and 

do not necessarily reflect the views of the EPA. This draft document is being circulated to 

facilitate discussion with the Clean Air Scientific Advisory Committee and for public comment 

to inform the EPA’s review of the PM National Ambient Air Quality Standards. This 

information is distributed for the purposes of pre-dissemination peer review under applicable 

information quality guidelines. It does not represent, and should not be construed to represent, 

any Agency determination or policy. Questions or comments related to this draft document 

should be addressed to Dr. Scott Jenkins, U.S. Environmental Protection Agency, Office of Air 

Quality Planning and Standards, C539-06, Research Triangle Park, North Carolina 27711 (email: 

jenkins.scott@epa.gov) and Dr. Jason Sacks, U.S. Environmental Protection Agency, National 

Center for Environmental Assessment, MDB243-01, Research Triangle Park, North Carolina 

27711 (email: sacks.jason@epa.gov). 
April 2016 iii Draft – Do Not Quote or Cite

Table of Contents 

1 INTRODUCTION.............................................................................................. 1-1

1.1 LEGISLATIVE REQUIREMENTS .................................................................... 1-2 

1.2 OVERVIEW OF THE NAAQS REVIEW PROCESS ........................................ 1-4 

1.3 HISTORY OF REVIEWS OF THE PM NAAQS ............................................... 1-8 

1.4 GENERAL SCOPE OF THE CURRENT REVIEW ........................................ 1-15 

1.5 ANTICIPATED SCHEDULE FOR CURRENT REVIEW .............................. 1-17 

References ................................................................................................................... 1-19 

2 KEY POLICY-RELEVANT ISSUES IN THE CURRENT REVIEW ......... 2-1

2.1 PRIMARY STANDARDS .................................................................................. 2-2 

2.1.1 PM2.5 Standards ........................................................................................... 2-2

2.1.2 PM10 Standard ........................................................................................... 2-10

2.1.3 General Approach in the Current Review of the Primary Standards ........ 2-13

2.2 SECONDARY STANDARDS .......................................................................... 2-17 

2.2.1 Non-Visibility Welfare Effects.................................................................... 2-18

2.2.2 Visibility Effects ......................................................................................... 2-20

2.2.3 General Approach in the Current Review of the Secondary Standards .... 2-25

2.3 PM AMBIENT MONITORING ........................................................................ 2-29 

2.3.1 PM Monitoring Networks .......................................................................... 2-30

2.3.2 Consideration Of PM Monitoring Issues In The Current REVIEW .......... 2-37

References ................................................................................................................... 2-38 

3 SCIENCE ASSESSMENT ................................................................................ 3-1

3.1 SCOPE OF THE PM ISA .................................................................................... 3-1 

3.1.1 General Description ..................................................................................... 3-1

3.1.2 Defining Policy-Relevant Literature ............................................................ 3-4

3.2 ORGANIZATION OF THE PM ISA .................................................................. 3-6 

3.3 ASSESSMENT APPROACH .............................................................................. 3-7 

3.3.1 Introduction.................................................................................................. 3-7

3.3.2 Literature Search and Selection of Relevant Studies ................................. 3-10

3.3.3 Evaluation of Individual Study Quality...................................................... 3-11

3.4 SPECIFIC ISSUES TO BE ADDRESSED IN THE PM ISA ........................... 3-12 

3.4.1 Causal Determinations from 2009 PM ISA ............................................... 3-13

3.4.2 Uncertainties/Limitations Identified in 2009 PM ISA ............................... 3-14

3.4.3 Integration of Evidence and Causal Determinations ................................. 3-16

3.4.4 Atmospheric Chemistry and Ambient Concentrations of PM .................... 3-18

3.4.5 Human Exposure ........................................................................................ 3-19

3.4.6 Dosimetry ................................................................................................... 3-20

3.4.7 Modes of Action ......................................................................................... 3-21

3.4.8 Health Effects ............................................................................................. 3-22 
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3.4.9 At-Risk Lifestages and Populations and Public Health Impact ................. 3-25

3.4.10 Welfare Effects ........................................................................................... 3-26

3.5 QUALITY MANAGEMENT ............................................................................ 3-29 

3.6 SCIENTIFIC AND PUBLIC REVIEW ............................................................. 3-29 

 References.......................................................................................3-30 

4 HUMAN HEALTH RISK AND EXPOSURE ASSESSMENT ..................... 4-1

4.1 INTRODUCTION ............................................................................................... 4-1 

4.2 OVERVIEW OF HEALTH RISK AND EXPOSURE ASSESSMENT FROM 

THE LAST REVIEW .......................................................................................... 4-2 

4.2.1 Key Observations in the 2010 HREA ........................................................... 4-5

4.2.2 Key Uncertainties in the 2010 HREA .......................................................... 4-8

4.3 CONSIDERATION OF POTENTIAL QUANTITATIVE ASSESSMENTS FOR 

THIS REVIEW .................................................................................................. 4-10 

4.3.1 Epidemiology-Based Risk Assessment ....................................................... 4-12

4.3.2 Exposure Assessment ................................................................................. 4-22

4.4 SCIENTIFIC AND PUBLIC REVIEW ............................................................. 4-26 

References ................................................................................................................... 4-27 

5 WELFARE-RELATED RISK AND EXPOSURE ASSESSMENTS ............ 5-1

5.1 INTRODUCTION ............................................................................................... 5-1 

5.2 SUMMARY OF WELFARE RISK AND EXPOSURE ASSESSMENT FROM 

THE LAST REVIEW .......................................................................................... 5-2 

5.2.1 Quantitative Assessments of Urban Visibility Impairment in the Last Review 

........................................................................................................5-3

5.2.2 Consideration of Quantitative Assessments for Additional PM-related 

Welfare Effects in the Last Review ............................................................... 5-7

5.3 CONSIDERATION OF POTENTIAL QUANTITATIVE ASSESSMENTS FOR 

THIS REVIEW .................................................................................................... 5-8 

5.3.1 Visibility Effects ........................................................................................... 5-9

5.3.2 Ecological Effects ...................................................................................... 5-10

5.3.3 Climate Effects ........................................................................................... 5-13

5.3.4 Materials Effects ........................................................................................ 5-15

5.3.5 Characterization of Sources of Uncertainty and Consideration of Information 

Newly Available in this Review .................................................................. 5-16

5.4 SCIENTIFIC AND PUBLIC REVIEW ............................................................. 5-19 

References ................................................................................................................... 5-20 

6 POLICY ASSESSMENT AND RULEMAKING ........................................... 6-1

6.1 POLICY ASSESSMENT..................................................................................... 6-1 

6.2 RULEMAKING ................................................................................................... 6-3 

References ..................................................................................................................... 6-4 
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List of Tables 

Table 1-1. Overview of Documents Developed in NAAQS Reviews..............................1-5 

Table 1-2. Summary of National Ambient Air Quality Standards Promulgated for 

Particulate Matter 1971-2012 ................................................................................. 1-9

Table 1-3. Anticipated Schedule for the Review of the PM NAAQS .................................. 1-18

Table 4-1. Areas of Uncertainty Associated with Epidemiology-Based Risk Estimates and 

Potential Strategies for Addressing these Sources of Uncertainty .................... 4-19

Table 4-2. Uncertainties and Limitations to Input Data used by APEX to Estimate 

Exposures ................................................................................................................ 4-25

Table 5-1 Areas of Uncertainty in the Welfare Assessments and Potential Strategies to 

Address Them......................................................................................................... 5-17

List of Figures 

Figure 1-1. Overview of the EPA’s Process for Reviewing NAAQS............................1-4

Figure 2-1. Overview of General Approach for Review of Primary PM Standards ........ .2-14

Figure 2-2. Overview of General Approach for Review of Secondary PM Standards...... 2-26

Figure 3-1. General process for development of Integrated Science Assessments. ............. 3-9
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List of acronyms/abbreviations

AER Indoor air exchange rates 

APEX Air Pollutants Exposure model 

BenMAP-CE Environmental Benefits Mapping and Analysis Program – Community 

Edition 

CAA Clean Air Act 

CASAC Clean Air Scientific Advisory Committee 

CHAD Consolidated Human Activity Database 

CMAQ Community Multi-scale Air Quality modeling system 

CPL Candidate Protection Levels 

C-R Concentration-response 

CSN Chemical Speciation Network 

DEARS Detroit Exposure Aerosol Research Study 

EC Elemental carbon 

U.S. EPA United States Environmental Protection Agency 

FEM Federal Equivalent Method 

FRM Federal Reference Method 

HREA Health Risk and Exposure Assessment 

IHD Ischemic heart disease 

IMPROVE Interagency Monitoring of Protected Visual Environments 

IPCC Intergovernmental Panel on Climate Change 

IRP Integrated Review Plan 

ISA Integrated Science Assessment 

LML Lowest measured level 

METS Metabolic equivalents of work 

NCEA National Center for Environmental Assessment 

NAAQS National Ambient Air Quality Standards 

NO2 Nitrogen dioxide 

NOX Oxides of nitrogen 

OAR Office of Air and Radiation 

OAQPS Office of Air Quality Planning and Standards 

OC Organic carbon 

ORD Office of Research and Development 

PA Policy Assessment 

PM Particulate matter 

PM2.5 In general terms, particulate matter with an aerodynamic diameter less 

than or equal to a nominal 2.5 μm; a measurement of fine particles 

In regulatory terms, particles with an upper 50% cut-point of 2.5 μm 

aerodynamic diameter (the 50% cut point diameter is the diameter at 

which the sampler collects 50% of the particles and rejects 50% of the 

particles) and a penetration curve as measured by a reference method 

based on Appendix L of 40 CFR Part 50 and designated in accordance 

with 40 CFR Part 53, by an equivalent method designated in accordance 

 
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with 40 CFR Part 53, or by an approved regional method designated in 

accordance with Appendix C of 40 CFR Part 58 

PM10 In general terms, particulate matter with an aerodynamic diameter less 

than or equal to a nominal 10 μm; a measurement of thoracic particles 

(i.e., that subset of inhalable particles thought small enough to penetrate 

beyond the larynx into the thoracic region of the respiratory tract) 

In regulatory terms, particles with an upper 50% cut-point of 10± 0.5 μm 

aerodynamic diameter (the 50% cut point diameter is the diameter at 

which the sampler collects 50% of the particles and rejects 50% of the 

particles) and a penetration curve as measured by a reference method 

based on Appendix J of 40 CFR Part 50 and designated in accordance with 

40 CFR Part 53 or by an equivalent method designated in accordance with 

40 CFR Part 53 

PM10-2.5 In general terms, particulate matter with an aerodynamic diameter less 

than or equal to a nominal 10 μm and greater than a nominal 2.5 μm; a 

measurement of thoracic coarse particulate matter or the coarse fraction of 

PM10 

In regulatory terms, particles with an upper 50% cut-point of 10 μm 

aerodynamic diameter and a lower 50% cut-point of 2.5 μm aerodynamic 

diameter (the 50% cut point diameter is the diameter at which the sampler 

collects 50% of the particles and rejects 50% of the particles) as measured 

by a reference method based on Appendix O of 40 CFR Part 50 and 

designated in accordance with 40 CFR Part 53 or by an equivalent method 

designated in accordance with 40 CFR Part 53 

PRB Policy relevant background 

SES Socioeconomic status 

SHEDS-PM Stochastic Human Exposure and Dose Simulation for Particulate Matter 

SO2 Sulfur dioxide 

SOX Sulfur oxides 

SOPM Secondary Organic Particulate Matter 

TRIM Total Risk Integrated Methodology 

TSP Total Suspended Particles 

UFP Ultrafine Particles 

UFVA Urban-Focused Visibility Assessment 

VAQ Visual air quality 

VOC Volatile organic compound 

WHO World Health Organization 

WREA Welfare Risk and Exposure Assessment 
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1 1 INTRODUCTION 

2 The U.S. Environmental Protection Agency (EPA) is conducting a review of the existing 

3 air quality criteria for particulate matter (PM) and of the primary (health-based) and secondary 

4 (welfare-based) national ambient air quality standards (NAAQS) for PM. This review will 

5 provide an integrative assessment of relevant scientific information on PM and will focus on the 

basic elements of the PM NAAQS: the indicator, averaging time, form,1 6 and level. These 

7 elements, which together serve to define each NAAQS, are considered collectively in evaluating 

8 the protection to public health and public welfare afforded by the standards. The purpose of this 

9 draft Integrated Review Plan (IRP) is to communicate the plan for reviewing the air quality 

10 criteria and the primary and secondary NAAQS for PM. 

11 The Clean Air Scientific Advisory Committee (CASAC), an independent scientific 

12 advisory committee established under the Clean Air Act, will review this draft IRP at a public 

meeting and will provide advice to the EPA based on that review.2 13 The final IRP will reflect the 

14 EPA staff’s consideration of CASAC advice and of comments received from members of the 

15 public as part of the review process. As this review progresses, the plan described in the IRP may 

16 be modified to reflect information received during the review process, to address additional 

17 advice received from CASAC, and/or to address comments received from the public. 

18 This draft IRP is organized into six chapters. Chapter 1 presents the legislative 

19 requirements for the review of the NAAQS (section 1.1), background information on the 

20 NAAQS review process (section 1.2), an overview of the decisions made in past reviews of the 

21 PM NAAQS (section 1.3), the general scope of the current review (section 1.4), and the 

22 anticipated schedule for the current review (section 1.5). Chapter 2 summarizes the supporting 

23 rationales for the Administrator’s decisions in the last review of the PM NAAQS, including the 

24 important uncertainties and limitations in the scientific evidence and quantitative assessments in 

25 the last review, and the key policy-relevant issues that will frame the current review. Chapters 3 

26 through 6 discuss the key assessment documents for this review (i.e., Integrated Science 

27 Assessment (ISA), Risk and Exposure Assessments (REAs), Policy Assessment (PA)), the 

 

1

 The “form” of a standard defines the air quality statistic that is to be compared to the level of the standard in 

determining whether an area attains the standard. 

2

 See 

http://yosemite.epa.gov/sab/sabpeople.nsf/WebCommitteesSubcommittees/CASAC%20Particulate%20Matter%20R

eview%20Panel%20(2015-2018) for a list of the CASAC PM Panel members. 
April 2016 1-2 Draft - Do Not Quote or Cite

1 planned approaches for preparing these documents, and plans for their scientific and public 

2 review. Chapter 6 also includes an overview of the rulemaking process. 

3 1.1 LEGISLATIVE REQUIREMENTS 

4 Two sections of the Clean Air Act (CAA) govern the establishment and revision of the 

5 NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to identify and list “air 

6 pollutants” that, in his or her judgment, “cause or contribute to air pollution which may 

7 reasonably be anticipated to endanger public health and welfare” and whose “presence . . . in the 

8 ambient air results from numerous or diverse mobile or stationary sources” and to issue air 

9 quality criteria for air pollutants that are listed. 42 U.S.C. § 7408(a) & (b). Air quality criteria are 

10 intended to “accurately reflect the latest scientific knowledge useful in indicating the kind and 

11 extent of identifiable effects on public health or welfare which may be expected from the 

12 presence of [a] pollutant in ambient air . . . .” 42 U.S.C. § 7408(b). 

13 Section 109 (42 U.S.C. 7409) directs the Administrator to propose and promulgate 

14 “primary” and “secondary” NAAQS for pollutants for which air quality criteria are issued under 

15 section 108. 42 U.S.C. § 7409 (a). Section 109(b)(1) defines a primary standard as one “the 

16 attainment and maintenance of which in the judgment of the Administrator, based on such 

17 criteria and allowing an adequate margin of safety, are requisite to protect the public health.” 42 

U.S.C. § 7409(b)(1).3 18 A secondary standard, as defined in section 109(b)(2), “shall specify a 

19 level of air quality the attainment and maintenance of which in the judgment of the 

20 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.”4 21 

22 42 U.S.C. § 7409(b)(2). 

23 The requirement that primary standards provide an adequate margin of safety was 

24 intended to address uncertainties associated with inconclusive scientific and technical 

25 information available at the time of standard setting. It was also intended to provide a reasonable 

 

3

 The legislative history of section 109 indicates that a primary standard is to be set at “the maximum permissible 

ambient air level . . . which will protect the health of any [sensitive] group of the population,” and that for this 

purpose “reference should be made to a representative sample of persons comprising the sensitive group rather than 

to a single person in such a group” [S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970)]. 

4

 Welfare effects as defined in CAA section 302(h) [42 U.S.C. 7602(h)] include, but are not limited to, “effects on 

soils, water, crops, vegetation, man-made materials, animals, wildlife, weather, visibility and climate, damage to and 

deterioration of property, and hazards to transportation, as well as effects on economic values and on personal 

comfort and well-being.” 
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degree of protection against hazards that research has not yet identified.5 1 Both kinds of 

2 uncertainties are components of the risk associated with pollution at levels below those at which 

3 human health effects can be said to occur with reasonable scientific certainty. Thus, in selecting 

4 primary standards that provide an adequate margin of safety, the Administrator is seeking not 

5 only to prevent pollution levels that have been demonstrated to be harmful but also to prevent 

6 lower pollutant levels that may pose an unacceptable risk of harm, even if the risk is not 

7 precisely identified as to nature or degree. The CAA does not require the Administrator to 

8 establish a primary NAAQS at a zero-risk level or at background concentration levels, but rather 

9 at a level that reduces risk sufficiently so as to protect public health with an adequate margin of 

safety.6 10 

11 In addressing the requirement for an adequate margin of safety, the EPA considers such 

12 factors as the nature and severity of the health effects involved, the size of the sensitive group(s), 

13 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.7 14 

15 In setting standards that are “requisite” to protect public health and welfare, as provided in 

16 section 109(b), the EPA’s task is to establish standards that are neither more nor less stringent 

17 than necessary. In so doing, the EPA may not consider the costs of implementing the standards. 

18 See generally Whitman v. American Trucking Associations, 531 U.S. 457, 465-472, 475-76 

19 (2001). Likewise, “[a]ttainability and technological feasibility are not relevant considerations in 

20 the promulgation of national ambient air quality standards.” American Petroleum Institute v. 

21 Costle, 665 F. 2d at 1185. 

22 Section 109(d)(1) requires that “not later than December 31, 1980, and at 5-year 

23 intervals thereafter, the Administrator shall complete a thorough review of the criteria 

24 published under section 108 and the national ambient air quality standards . . . and shall make 

25 such revisions in such criteria and standards and promulgate such new standards as may be 

26 appropriate . . . .” 42 U.S.C. § 7409(d)(1). Sections 109(d)(2)(A) and 109(d)(2)(B) require that 

27 an independent scientific review committee “shall complete a review of the criteria . . . and the 

28 national primary and secondary ambient air quality standards . . . and shall recommend to the 

 

5

 See Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir 1980); American Petroleum Institute v. 

Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981); American Farm Bureau Federation v. EPA, 559 F. 3d 512, 533 (D.C. 

Cir. 2009); and Association of Battery Recyclers v. EPA, 604 F. 3d 613, 617-18 (D.C. Cir. 2010). 6

 See Lead Industries v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 723 F. 3d 246, 255, 262-63 (D.C. Cir. 

2013). 

7

 See Lead Industries Association v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 723 F. 3d at 265. 
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1 Administrator any new . . . standards and revisions of existing criteria and standards as may be 

2 appropriate . . . .” 42 U.S.C. § 7409(d)(2). Since the early 1980s, this independent review 

3 function has been performed by CASAC. 

4 1.2 OVERVIEW OF THE NAAQS REVIEW PROCESS 

5 The current process for reviewing the NAAQS includes five major phases: (1) the planning 

6 phase, (2) the science assessment phase, (3) the risk/exposure assessment phase, (4) the policy 

7 assessment phase, and (5) the rulemaking phase. Figure 1-1 provides an overview of the NAAQS 

8 review process and Table 1-1 provides an overview of each of the documents that are developed 

as part of this process.8 9 The phases of the NAAQS review process, and the documents 

10 developed, are described in more detail below. 

11 

12 Figure 1-1. Overview of the EPA’s Process for Reviewing NAAQS 

 

8

 The EPA maintains a website on which key documents developed for NAAQS reviews are made available 

(http://www3.epa.gov/ttn/naaqs/). 
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1 Table 1-1. Overview of Documents Developed in NAAQS Reviews 

Document Purpose CASAC Review 

Integrated Review Plan (IRP) 

Presents the anticipated schedule and process for the review, and the key policy-relevant science issues. Not intended to provide detailed scientific or technical information, plans for quantitative assessments, or conclusions on existing or alternative standard(s). 

One draft; consideraadvice and public coIRP is reflected in th

Integrated Science Assessment (ISA) 

Comprehensive review, synthesis and evaluation of the most policy-relevant science, including key science judgments. Does not 

present quantitative assessments or conclusions on existing or 

alternative standard(s). 

Consideration of CApublic comments on (multiple drafts, as wreflected in the fina

Risk and Exposure Assessment (REA) Planning Document(s) 

Drawing upon the information in the ISA, presents the planned approaches and scopes of the quantitative risk and exposure 

analyses that are warranted. Does not present conclusions on 

existing or alternative standard(s). 

Consideration of CApublic comments on document is reflecte

Health and Welfare REAs (HREA, WREA) 

Drawing upon the information in the ISA, presents quantitative assessments of risks and exposures, as warranted, under various air quality scenarios, including just meeting the existing and potential alternative standard(s). Does not present conclusions on existing or 

alternative standard(s). 

Consideration of CApublic comments on REA(s) (multiple drreflected in the fina

Policy Assessment (PA) 

Drawing upon the information in the ISA and REA(s), provides a transparent staff analysis of the scientific basis for policy options for consideration; facilitates CASAC advice to the Agency and recommendations to the Administrator on the adequacy of the existing standards and, as warranted, on the revisions that may be appropriate to consider. 

Consideration of CApublic comments on (multiple drafts, as wreflected in the fina

Proposed Rule Communicates the Administrator’s proposed decision(s); informed 

by the ISA, REA(s), PA and the advice of CASAC 

 Public comment period, including public hearings as warranted. 

Final Rule Communicates the Administrator’s final decision(s); informed by 

the ISA, REA(s), PA, advice of CASAC and public comments on the proposed rule. 

2 
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1 The planning phase of the NAAQS review process begins with a public workshop 

2 intended to provide the EPA an opportunity to receive input and advice on the key science and 

3 policy issues around which the review will be structured. Workshop participants are asked to 

4 highlight significant new and emerging research related to these key science and policy issues, 

5 and to make recommendations to Agency staff regarding the design and scope of the 

6 review. Drawing from workshop discussions, a draft IRP is prepared jointly by the EPA’s 

7 National Center for Environmental Assessment (NCEA), within the Office of Research and 

8 Development (ORD), and the EPA’s Office of Air Quality Planning and Standards (OAQPS), 

9 within the Office of Air and Radiation (OAR). The IRP presents the plan for the review, 

10 including the anticipated schedule, the key policy-relevant science issues that will guide the 

11 review, and the approach to developing the assessment documents that will inform the review. A 

12 draft of the IRP is made available for CASAC review and for public comment, and the final IRP 

13 reflects the EPA staff’s consideration of CASAC advice and public input. 

14 Following the IRP, the EPA’s NCEA develops the Integrated Science Assessment (ISA). 

15 The ISA provides a concise review, synthesis, and evaluation of the most policy-relevant science 

16 and includes key science judgments. The ISA provides the scientific foundation for the NAAQS 

17 review, and it is intended to provide information useful in forming judgments about air quality 

18 indicator(s), averaging time(s), form(s), and level(s) for the NAAQS. The schedule typically 

19 includes production of a first and, if needed, second draft ISA, both of which undergo CASAC 

review at public meetings.9 20 The final ISA reflects staff’s consideration of CASAC advice and of 

21 the public input provided on drafts of the ISA. Chapter 3 below provides a more detailed 

22 description of the planned scope, organization, and assessment approach for the ISA in the 

23 current review of the PM NAAQS. 

24 Building on the assessment of the evidence in the ISA, staff in the EPA’s OAQPS 

25 considers the extent to which there is support for the development of Health and/or Welfare Risk 

26 and Exposure Assessments (HREA, WREA). As an initial step in these considerations, staff 

27 prepares Risk and Exposure Assessment planning document(s) (REA Planning Document(s)) 

28 that consider the extent to which the available scientific evidence and tools/methodologies 

29 warrant the conduct of quantitative assessments. As discussed in Chapters 4 and 5 below, the 

30 REA Planning Document(s) will focus on the degree to which important uncertainties from 

31 previous reviews could be addressed by updated quantitative analyses, and on the degree to 

 

9

 The availability of draft documents (ISA, REA, PA), and a request for public input on those documents, is also 

announced in the Federal Register. 
April 2016 1-7 Draft - Do Not Quote or Cite

1 which updated or additional analyses could improve the understanding of pollutant exposures 

2 and/or risks. To the extent warranted, the REA Planning Document(s) will also outline a general 

3 plan, including scope and methods, for conducting quantitative assessments. REA Planning 

4 Documents are typically prepared in conjunction with the first or second draft ISA and are 

5 reviewed by CASAC at a public meeting. 

6 When developed, REAs provide concise presentations of exposure and/or risk 

7 assessments, including presentations of methods, key results, and uncertainties. One or more 

8 drafts of the REA(s) undergo CASAC review at public meetings. Staff considers the CASAC 

9 advice and public input received in preparing final REAs. Chapters 4 and 5 below provide more 

10 detailed descriptions of the approaches in this review of the PM NAAQS to considering the 

11 potential support for an HREA and WREA, respectively. 

12 Staff in the EPA’s OAQPS also prepares a Policy Assessment (PA), presenting staff’s 

13 considerations and conclusions regarding the adequacy of the current standards and, if warranted, 

14 the range of revised standards that could be supported by the available scientific evidence and 

15 exposure/risk information. The PA integrates and interprets the information from the ISA and 

16 REA(s) to frame policy options for consideration by the Administrator. Such an evaluation of 

17 policy options is intended to help bridge the gap between the Agency’s scientific assessments, 

18 presented in the ISA and REA(s) (when available), and the judgments required of the EPA 

19 Administrator in determining whether it is appropriate to retain or revise the NAAQS. The PA is 

20 also intended to facilitate CASAC’s advice to the Agency and recommendations to the 

21 Administrator on the adequacy of the existing standards and on revisions that may be appropriate 

22 to consider. Staff’s considerations and conclusions in the PA are based on the available scientific 

23 evidence and quantitative exposure and risk information, including the uncertainties and 

24 limitations in that evidence and information. The PA focuses on the evidence and information 

25 that is most pertinent to evaluating the basic elements of NAAQS: indicator, averaging time, 

26 form, and level. The schedule typically includes production of a first and, if needed, second draft 

27 PA, both of which undergo CASAC review at public meetings. Staff considers the CASAC 

28 advice and public input received in preparing the final PA. 

29 Following issuance of the final PA, the EPA develops and publishes a notice of proposed 

30 rulemaking that communicates the Administrator’s proposed decisions on the NAAQS. These 

31 proposed decisions are based on the Administrator’s consideration of the scientific evidence 

32 assessed in the ISA; quantitative exposure and risk information presented and assessed in the 

33 REAs, as available; staff’s considerations and conclusions based on the evidence and 

34 information, as presented in the PA; CASAC advice received during the development of the ISA, 

35 REA(s), and PA; and public input received on drafts of those assessment documents. Prior to 
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1 publication in the Federal Register, a draft proposal notice undergoes interagency review 

involving other federal agencies.10 2 Materials upon which proposed decisions are based, including 

3 the documents described above, are made available to the public in the regulatory docket for 

review.11 4 

5 A public comment period, during which one or more public hearings are generally held, 

6 follows publication of the notice of proposed rulemaking. Taking into account comments 

received on the proposed rule,12 7 the Agency develops a final rule, a draft version of which 

8 undergoes interagency review prior to publication in the Federal Register. Chapter 6 of this IRP 

9 discusses the development of the PA and the rulemaking steps for this review of the PM 

10 NAAQS. 

11 1.3 HISTORY OF REVIEWS OF THE PM NAAQS 

12 Particulate matter is the generic term for a broad class of chemically and physically 

13 diverse substances that exist as discrete liquid and/or solid particles over a wide range of sizes. 

14 Particles originate from a variety of anthropogenic stationary and mobile sources, as well as from 

15 natural sources. Particles may be emitted directly, or formed in the atmosphere by 

16 transformations of gaseous emissions such as sulfur oxides (SOX), oxides of nitrogen (NOX), and 

17 volatile organic compounds (VOC). Examples of secondary particle formation include: (1) the 

18 conversion of sulfur dioxide (SO2) to sulfuric acid (H2SO4) vapor that nucleates new particles or 

19 condenses on existing particles and further reacts with gaseous ammonia (NH3) to form various 

20 inorganic salts (e.g., ammonium sulfate, (NH4)2SO4, or ammonium bisulfate, NH4HSO4); (2) the 

21 conversion of nitrogen dioxide (NO2) to nitric acid (HNO3) vapor that condenses onto existing 

22 particles and reacts further with ammonia to form ammonium nitrate (NH4NO3); and (3) 

23 reactions involving gaseous VOC yielding organic compounds with low vapor pressures that 

24 nucleate or condense on existing particles to form secondary organic particulate matter (SOPM) 

 

10 Where implementation of the proposed decision would have an annual effect on the economy of $100 million of 

more, (e.g., by necessitating the implementation of emissions controls) the EPA also develops and releases a draft 

regulatory impact analysis (RIA) concurrent with the notice of proposed rulemaking. This activity is conducted 

under Executive Order 12866. The RIA is conducted independently of the rulemaking process and, by law, is not 

considered in decisions regarding the NAAQS. 

11 All documents in the docket are listed in the www.regulations.gov index. Publicly available docket materials are 

available either electronically at www.regulations.gov or in hard copy at the Air and Radiation Docket and 

Information Center. The docket ID number for this review of the PM NAAQS is EPA-HQ-OAR-2015-0072. 

12 When issuing the final rulemaking, the Agency responds to all significant comments on the proposed rule. 
April 2016 1-9 Draft - Do Not Quote or Cite

(U.S. EPA, 2004, Chapter 3).13 1 The chemical and physical properties of PM vary greatly with 

2 time, region, meteorology, and source category, complicating the assessment of health and 

3 welfare effects. 

4 This section summarizes the PM NAAQS that have been promulgated in past reviews 

5 (Table 1-2). Each of these reviews is discussed briefly below. 

6 Table 1-2. Summary of National Ambient Air Quality Standards Promulgated for 

Particulate Matter 1971-201214 7 

Review 

Completed Indicator Avg. Time Level Form 

1971 

Total 

Suspended 

Particles 

(TSP) 

24-hour 

260 μg/m3

(primary) 

150 μg/m3

(secondary)

Not to be exceeded more than once 

per year 

Annual 

75 μg/m3

(primary) 

60 μg/m3

(secondary)

Annual geometric mean 

1987 PM10

24-hour 150 μg/m3

Not to be exceeded more than once 

per year on average over a 3-year 

period 

Annual 50 μg/m3 Annual arithmetic mean, averaged 

over 3 years 

1997 

PM2.5

24-hour 65 μg/m3 98th percentile, averaged over 3 years 

Annual 15.0 μg/m3 Annual arithmetic mean, averaged 

over 3 years15

PM10

24-hour 150 μg/m3 99th percentile, averaged over 3 

years16 

Annual 50 μg/m3 Annual arithmetic mean, averaged 

over 3 years 

2006 PM2.5 24-hour 35 μg/m3 98th percentile, averaged over 3 years 

 

13 As discussed below (section 1.4), the ecological effects associated with deposited particulate NOX and SOX and 

their transformation products are addressed in the review of the secondary NOX and SOX NAAQS. 14 When not specified, primary and secondary standards are identical. 

15 The level of the 1997 annual PM2.5 standard was to be compared to 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”). Spatial averaging is 

discussed further in section 2.1.1, below. 

16 When the 1997 standards were vacated (see below), the form of the 1987 standards remained in place (i.e., not to 

be exceeded more than once per year on average over a 3-year period).
April 2016 1-10 Draft - Do Not Quote or Cite

Annual 15.0 μg/m3 Annual arithmetic mean, averaged 

over 3 years17

PM10 24-hour18 150 μg/m3 Not to be exceed more than once per 

year on average over a 3-year period 

2012 PM2.5

24-hour 35 μg/m3 98th percentile, averaged over 3 years 

Annual 

12.0 μg/m3

(primary) 

15.0 μg/m3

(secondary)

Annual mean, averaged over 3 

years19

PM10 24-hour 150 μg/m3 Not to be exceeded more than once 

per year on average over 3 years 

1 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).20 2 The federal reference 

3 method (FRM) specified for determining attainment of the original standards was the high4 volume sampler, which collects PM up to a nominal size of 25 to 45 micrometers (μm) (referred 

to as total suspended particulates or TSP). The primary standards were set at 260 μg/m3 5 , 24-hour 

average, not to be exceeded more than once per year, and 75 μg/m3 6 , annual geometric mean. The 

secondary standards were set at 150 μg/m3 7 , 24-hour average, not to be exceeded more than once 

per year, and 60 μg/m3 8 , annual geometric mean. 

9 In October 1979 (44 FR 56730, October 2, 1979), the EPA announced the first periodic 

10 review of the air quality criteria and NAAQS for PM. Revised primary and secondary standards 

11 were promulgated in 1987 (52 FR 24634, July 1, 1987). In the 1987 decision, the EPA changed 

12 the indicator for particles from TSP to PM10, in order to focus on the subset of inhalable particles 

13 small enough to penetrate to the thoracic region of the respiratory tract (including the 

tracheobronchial and alveolar regions), referred to as thoracic particles.21 14 The level of the 24-

hour standards (primary and secondary) was set at 150 μg/m3 15 , and the form was one expected 

 

17 The EPA tightened the constraints on the spatial averaging criteria by further limiting the conditions under which 

some areas may average measurements from multiple community-oriented monitors to determine compliance. 

Spatial averaging is discussed further in section 2.1.1, below. 

18 The EPA revoked the annual PM10 NAAQS in 2006. 

19 In the 2012 decision, the EPA eliminated the option for spatial averaging. 

20 Prior to the review initiated in 2007 (see below), the AQCD provided the scientific basis for the NAAQS. 

21 PM10 refers to particles with a nominal mean aerodynamic diameter less than or equal to 10 μm. More 

specifically, 10 μm is the aerodynamic diameter for which the efficiency of particle collection is 50 percent. Larger 

particles are not excluded altogether, but are collected with substantially decreasing efficiency while smaller 

particles are collected with increasing efficiency. 
April 2016 1-11 Draft - Do Not Quote or Cite

1 exceedance per year, on average over three years. The level of the annual standards (primary and 

secondary) was set at 50 μg/m3 2 , and the form was annual arithmetic mean, averaged over three 

3 years. 

4 In April 1994, the EPA announced its plans for the second periodic review of the air 

5 quality criteria and NAAQS for PM, and in 1997 the EPA promulgated revisions to the NAAQS 

6 (62 FR 38652, July 18, 1997). In the 1997 decision, the EPA determined that the fine and coarse 

7 fractions of PM10 should be considered separately. This determination was based on evidence 

8 that serious health effects were associated with short- and long-term exposures to fine particles in 

9 areas that met the existing PM10 standards. The EPA added new standards, using PM2.5 as the 

10 indicator for fine particles (with PM2.5 referring to particles with a nominal mean aerodynamic 

11 diameter less than or equal to 2.5 μm). These new standards were as follows: (1) an annual 

standard with a level of 15.0 μg/m3 12 , based on the 3-year average of annual arithmetic mean 

PM2.5 concentrations from single or multiple community-oriented monitors;22 13 and (2) a 24-hour 

standard with a level of 65 μg/m3, based on the 3-year average of the 98th 14 percentile of 24-hour 

15 PM2.5 concentrations at each monitor within an area. Also, the EPA established a new reference 

16 method for the measurement of PM2.5 in the ambient air and adopted rules for determining 

17 attainment of the new standards. To continue to address the coarse fraction of PM10 (referred to 

18 as thoracic coarse particles or PM10-2.5; generally including particles with a nominal mean 

19 aerodynamic diameter greater than 2.5 μm and less than or equal to 10 μm), the EPA retained the 

annual PM10 standard and revised the form of the 24-hour PM10 standard to be based on the 99th 20 

21 percentile of 24-hour PM10 concentrations at each monitor in an area. The EPA revised the 

22 secondary standards by making them identical in all respects to the primary standards. 

23 Following promulgation of the 1997 PM NAAQS, petitions for review were filed by a 

24 large number of parties, addressing a broad range of issues. In May 1999, the U.S. Court of 

25 Appeals for the District of Columbia Circuit (D.C. Circuit) upheld the EPA’s decision to 

26 establish fine particle standards, holding that "the growing empirical evidence demonstrating a 

27 relationship between fine particle pollution and adverse health effects amply justifies 

28 establishment of new fine particle standards." American Trucking Associations v. EPA, 175 F. 3d 

 

22 The level of the 1997 annual PM2.5 standard was to be compared to 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” as discussed in section 

2.1.1, below). In the last review (completed in 2012) the EPA replaced the term “community-oriented” monitor with 

the term “area-wide” monitor. Area-wide monitors are those sited at the neighborhood scale or larger, as well as 

those monitors sited at micro-or middle scales that are representative of many such locations in the same CBSA (78 

FR 3236, January 15, 2013). 
April 2016 1-12 Draft - Do Not Quote or Cite

1 1027, 1055-56 (D.C. Cir. 1999). The D.C. Circuit also found "ample support" for the EPA's 

2 decision to regulate coarse particle pollution, but vacated the 1997 PM10 standards, concluding 

3 that the EPA had not provided a reasonable explanation justifying use of PM10 as an indicator for 

4 coarse particles. 175 F. 3d at 1054-55. Pursuant to the D.C. Circuit’s decision, the EPA removed 

5 the vacated 1997 PM10 standards, and the pre-existing 1987 PM10 standards remained in place 

6 (65 FR 80776, December 22, 2000). The D.C. Circuit also upheld the EPA’s determination not to 

7 establish more stringent secondary standards for fine particles to address effects on visibility. 175 

8 F. 3d at 1027. 

9 The D.C. Circuit also addressed more general issues related to the NAAQS, including 

10 issues related to the consideration of costs in setting NAAQS and the EPA’s approach to 

11 establishing the levels of NAAQS. Regarding the cost issue, the court reaffirmed prior rulings 

12 holding that in setting NAAQS the EPA is “not permitted to consider the cost of implementing 

13 those standards.” Id. at 1040-41. Regarding the levels of NAAQS, the court held that the EPA’s 

14 approach to establishing the level of the standards in 1997 (i.e., both for PM and for the ozone 

15 NAAQS promulgated on the same day) effected “an unconstitutional delegation of legislative 

16 authority.” Id. at 1034-40. Although the court stated that “the factors EPA uses in determining 

17 the degree of public health concern associated with different levels of ozone and PM are 

18 reasonable,” it remanded the rule to the EPA, stating that when the EPA considers these factors 

19 for potential non-threshold pollutants “what EPA lacks is any determinate criterion for drawing 

20 lines” to determine where the standards should be set. 

21 The D.C. Circuit’s holding on the cost and constitutional issues were appealed to the 

22 United States Supreme Court. In February 2001, the Supreme Court issued a unanimous 

23 decision upholding the EPA’s position on both the cost and constitutional issues. Whitman v. 

24 American Trucking Associations, 531 U.S. 457, 464, 475-76. On the constitutional issue, the 

25 Court held that the statutory requirement that NAAQS be “requisite” to protect public health 

26 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.23 27 

 

23 The Supreme Court remanded the case to the Court of Appeals for resolution of any remaining issues that had not 

been addressed in that court’s earlier rulings. Id. at 475-76. In a March 2002 decision, the Court of Appeals 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). 
April 2016 1-13 Draft - Do Not Quote or Cite

1 In October 1997, the EPA published its plans for the third periodic review of the air 

2 quality criteria and NAAQS for PM (62 FR 55201, October 23, 1997). After CASAC and public 

3 review of several drafts, the EPA’s NCEA finalized the AQCD in October 2004 (U.S. EPA, 

4 2004). The EPA’s OAQPS finalized a Risk Assessment and Staff Paper in December of 2005 

(Abt, 2005; U.S. EPA, 2005).24 5 On December 20, 2005, the EPA announced its proposed 

6 decision to revise the NAAQS for PM, and solicited comment on a broad range of options (71 

7 FR 2620, January 17, 2006). 

8 On September 21, 2006, the EPA announced its final decisions to revise the primary and 

9 secondary NAAQS for PM to provide increased protection of public health and welfare, 

10 respectively (71 FR 61144, October 17, 2006). With regard to the primary and secondary 

11 standards for fine particles, the EPA revised the level of the 24-hour PM2.5 standards to 35 

μg/m3

, retained the level of the annual PM2.5 standards at 15.0 μg/m3 12 , and revised the form of the 

annual PM2.5 standards by narrowing the constraints on the optional use of spatial averaging.25 13 

14 With regard to the primary and secondary standards for PM10, the EPA retained the 24-hour 

standards, with levels at 150 μg/m3 15 , and revoked the annual standards. The Administrator judged 

16 that the available evidence generally did not suggest a link between long-term exposure to 

17 existing ambient levels of coarse particles and health or welfare effects. In addition, a new 

18 reference method was added for the measurement of PM10-2.5 in the ambient air, in order to 

19 provide a basis for approving federal equivalent methods (FEMs) and to promote the gathering 

20 of scientific data to support future reviews of the PM NAAQS. 

21 Several parties filed petitions for review following promulgation of the revised PM 

22 NAAQS in 2006. These petitions addressed the following issues: (1) selecting the level of the 

23 primary annual PM2.5 standard; (2) retaining PM10 as the indicator of a standard for thoracic 

24 coarse particles, retaining the level and form of the 24-hour PM10 standard, and revoking the 

25 PM10 annual standard; and (3) setting the secondary PM2.5 standards identical to the primary 

26 standards. On February 24, 2009, the U.S. Court of Appeals for the District of Columbia Circuit 

27 issued its opinion in the case American Farm Bureau Federation v. EPA, 559 F. 3d 512 (D.C. 

28 Cir. 2009). The court remanded the primary annual PM2.5 NAAQS to EPA because EPA failed to 

29 adequately explain why the standards provided the requisite protection from both short- and 

 

24 Prior to the review initiated in 2007, the Staff Paper, rather than the PA, 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. 

25 Spatial averaging is discussed in more detail in section 2.1.1, below. 
April 2016 1-14 Draft - Do Not Quote or Cite

1 long-term exposures to fine particles, including protection for at-risk populations. American 

2 Farm Bureau Federation v. EPA, 559 F. 3d 512, 520-27 (D.C. Cir. 2009). With regard to the 

3 standards for PM10, the court upheld EPA’s decisions to retain the 24-hour PM10 standard to 

4 provide protection from thoracic coarse particle exposures and to revoke the annual PM10

5 standard. American Farm Bureau Federation, 559 F. 2d at 533-38. With regard to the secondary 

6 PM2.5 standards, the court remanded the standards to EPA because the Agency failed to 

7 adequately explain why setting the secondary PM standards identical to the primary standards 

8 provided the required protection for public welfare, including protection from visibility 

9 impairment. American Farm Bureau Federation, 559 F. 2d at 528-32. The EPA responded to the 

10 court’s remands as part of the next review of the PM NAAQS, which was initiated in 2007 

11 (discussed below). 

12 In June 2007, the EPA initiated the fourth periodic review of the air quality criteria and 

13 the PM NAAQS by issuing a call for information in the Federal Register (72 FR 35462, June 28, 

2007). Based on the NAAQS review process, as revised in 2008 and again in 2009,26 14 the EPA 

15 held science/policy issue workshops on the primary and secondary PM NAAQS (72 FR 34003, 

16 June 20, 2007; 72 FR 34005, June 20, 2007), and prepared and released the planning and 

17 assessment documents that comprise the review process (i.e., IRP (U.S. EPA, 2008), ISA (U.S. 

18 EPA, 2009a), REA planning documents for health and welfare (U.S. EPA, 2009b, c), a 

quantitative health risk assessment (U.S. EPA, 2010a)27 19 and an urban-focused visibility 

assessment (U.S. EPA, 2010b),28 20 and PA (U.S. EPA, 2011)). In June 2012, the EPA announced 

21 its proposed decision to revise the NAAQS for PM (77 FR 38890, June 29, 2012). 

 

26 The history of the NAAQS review process, including revisions to the process, is discussed at 

http://www3.epa.gov/ttn/naaqs/review2.html. 27 The quantitative assessment of health risks conducted in the last review was presented in the Quantitative Health 

Risk Assessment for Particulate Matter (U.S. EPA, 2010a). In the current review, quantitative assessments for 

health-related exposures and risks, if warranted, would be presented in the Health Risk and Exposure Assessment 

(HREA). For consistency with the documents developed under the current NAAQS process, the Quantitative Health 

Risk Assessment for Particulate Matter (U.S. EPA, 2010a) from the last review will be referenced in this document 

as the 2010 HREA. 

28 The quantitative assessment of welfare effects conducted in the last review was presented, in part, in the UrbanFocused Visibility Assessment (U.S. EPA, 2010b). In the current review, quantitative assessments for welfare 

effects, if warranted, would be presented in the Welfare Risk and Exposure Assessment (WREA). The UrbanFocused Visibility Assessment (U.S. EPA, 2010b) from the last review will be referenced in this document as the 

2010 UFVA. 
April 2016 1-15 Draft - Do Not Quote or Cite

1 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).29 2 With 

regard to primary standards for PM2.5, the EPA revised the level of the annual PM2.5 standard30 3 to 

12.0 μg/m3

 and retained the 24-hour PM2.5 standard, with its level of 35 μg/m3 4 . For the primary 

5 PM10 standard, the EPA retained the 24-hour standard to continue to provide protection against 

6 effects associated with short-term exposure to thoracic coarse particles (i.e., PM10-2.5). With 

7 regard to the secondary PM standards, the EPA generally retained the 24-hour and annual PM2.5

standards31 8 and the 24-hour PM10 standard to address visibility and non-visibility welfare effects. 

9 On judicial review, the revised standards were upheld in all respects. NAM v EPA, 750 F.3d 921 

10 (D.C. Cir. 2014). 

11 1.4 GENERAL SCOPE OF THE CURRENT REVIEW 

12 With regard to scope, this review is focused on the air quality criteria for PM and on the 

13 primary and secondary NAAQS for PM2.5 and PM10. As discussed above, the current primary 

14 and secondary PM2.5 standards are meant to protect against the health and welfare effects, 

15 respectively, that have been associated with exposures to fine particles. The primary and 

16 secondary PM10 standards are meant to protect against the effects associated with exposures to 

17 thoracic coarse particles (i.e., PM10-2.5). Therefore, an important aspect of the current review will 

18 be the EPA’s assessment of the health and welfare effects that have been associated with size 

19 fractionated PM mass, with a particular focus on the PM2.5 and PM10-2.5 size fractions. In 

20 addition, as in the last review, the EPA will also assess the available scientific evidence for 

21 health or welfare effects associated with additional size fractions (e.g., ultrafine particles) and 

22 with particular PM components or groups of components, sources, or environments (e.g., urban 

23 and non-urban environments). 

24 Based on the available scientific information, the EPA will consider the extent to which 

25 the current PM2.5 and PM10 standards are requisite to protect public health and welfare, within 

26 the meaning of section 109(b) of the CAA (section 1.1, above). To the extent the available 

27 information calls into question the adequacy of the protection afforded by one or more of the 

28 existing PM standards, the EPA will consider potential alternatives that could be supported by 

 

29 The bases for these decisions are discussed further in sections 2.1 and 2.2, below. 

30 The EPA also eliminated the option for spatial averaging (section 2.1.1, below). 

31 Consistent with the primary standard, the EPA eliminated the option for spatial averaging with the annual 

standard (section 2.1.1, below). 
April 2016 1-16 Draft - Do Not Quote or Cite

1 the available scientific evidence and, as available, exposure-/risk-based information, in terms of 

2 the basic elements of the NAAQS (indicator, averaging time, form, level). 

3 With regard to the secondary standards in particular, as in the last review (77 FR 38991, 

4 June 29, 2012), this review of the PM NAAQS will consider the PM-related welfare effects that 

5 are not being addressed in the ongoing review of the secondary NOX and SOX NAAQS. 

6 Specifically, the review of the secondary NOX and SOX NAAQS is addressing the ecological 

7 effects of ecosystem loading of oxides of nitrogen and oxides of sulfur, which includes 

particulate nitrogen and sulfur compounds.32 8 In this review of the PM NAAQS, welfare effects to 

9 be considered include PM-related visibility impairment, climate effects, ecological effects (other 

10 than those attributable to deposition of NOX, SOX and their transformation products) and 

11 materials damage and soiling (i.e., materials effects). While N and/or S may contribute to these 

12 welfare effects, other components of PM play a role as well. Therefore, it is appropriate to 

13 consider these welfare effects in this review of the PM NAAQS to the extent that they are not 

14 being covered in the ongoing review of the secondary NOX and SOX NAAQS. In the case of 

15 materials effects, the impacts of gaseous and particulate N and S wet deposition cannot be clearly 

16 distinguished, so both will be considered in this review of the PM NAAQS, along with other 

17 materials effects of PM-related deposition. 

18 In contrast, as noted above, the ongoing review of the secondary NAAQS for ecological 

19 effects of NOX and SOX is considering the ecological effects that result specifically from the 

deposition of NOX and SOX and their transformation products.33 20 This approach is consistent with 

the previous review of the secondary NO2 and SO2 standards,34 21 in which the EPA conducted a 

22 joint review focused on the protection provided against the effects of deposition of NOX and SOX

23 to sensitive aquatic and terrestrial ecosystems. The approach in the last review recognized that 

24 NOX and SOX, and their associated transformation products, are linked from the perspectives of 

25 both atmospheric chemistry and ecological effects, providing “a strong basis for considering 

26 these pollutants together” (77 FR 20222, April 3, 2012). 

 

32 See http://www3.epa.gov/ttn/naaqs/standards/no2so2sec/index.html for more information on the current review 

of the secondary NOX and SOX NAAQS. 

33 More broadly, the review of the secondary NAAQS for ecological effects of NOX and SOX includes consideration 

of the ecological effects of ecosystem loading of particulate nitrogen and sulfur compounds. 

34 When discussing specific standards, we refer to those standards by their indicator species (i.e., in this case NO2

and SO2). As described in the draft NOX/SOX IRP (U.S. EPA, 2015, p. 1-1) the terms NO2 and SO2 are not 

interchangeable with the terms NOX and SOX. NOX and SOX include many more chemical species than just NO2 and 

SO2. 
April 2016 1-17 Draft - Do Not Quote or Cite

1 1.5 ANTICIPATED SCHEDULE FOR CURRENT REVIEW 

2 In December 2014, the EPA announced the initiation of the current periodic review of the 

3 air quality criteria for PM and of the PM2.5 and PM10 NAAQS and issued a call for information 

4 in the Federal Register (79 FR 71764, December 3, 2014). On February 9 to 11, 2015, the EPA’s 

5 NCEA and OAQPS held a public workshop to inform the planning for the current review of the 

6 PM NAAQS (announced in 79 FR 71764, December 3, 2014). This workshop was meant to 

7 provide the EPA an opportunity to receive input and advice on the key science and policy issues 

8 around which the PM NAAQS review will be structured. Workshop participants were asked to 

9 highlight significant new and emerging PM research related to these key science and policy 

10 issues, and to make recommendations to the Agency regarding the design and scope of this 

11 review. The input received at this workshop was considered by the EPA staff in developing this 

12 draft IRP. The EPA’s anticipated schedule for the remainder of this review is summarized in 

13 Table 1-3, below. 
April 2016 1-18 Draft - Do Not Quote or Cite

1 Table 1-3. Anticipated Schedule for the Review of the PM NAAQS 

Stage of 

Review

Major Milestone Actual or Target 

Date

Planning

Literature Search Ongoing

Federal Register Call for Information December 3, 2014

Workshop on Science/Policy Issues February 9-11, 2015

Release Draft IRP for CASAC/public review April 2016

CASAC Review Meeting for Draft IRP May 23, 2016

Release Final IRP September 2016

Science 

Assessment 

Release First Draft ISA for CASAC/public review Spring 2017

CASAC Review Meeting for First Draft ISA Summer 2017

Release Second Draft ISA for CASC/public review Winter 2018

CASAC Review Meeting for Second Draft ISA Spring 2018

Release Final ISA Spring 2019

Risk/Exposure 

Assessments

Release REA Planning Document(s) for CASAC/public 

review

Spring/Summer 2017

CASAC Review Meeting for REA Planning Document(s) Summer 2017

Release First Draft REA(s) for CASAC/Public Review Winter/Spring 2018

CASAC Review Meeting for First Draft REA(s) Spring 2018

Release Second Draft REA(s) for CASAC/Public Review Fall 2018

CASAC Review Meeting for Second Draft REA(s) Fall/Winter 2018

Release Final REA(s) Winter 2019

Policy 

Assessment/ 

Rulemaking

Release First Draft PA for CASAC/public review Fall 2018

CASAC Review Meeting on First Draft PA Fall/Winter 2018

Release Second Draft PA Fall 2019

CASAC Review/Public Comment on Second Draft PA Fall/Winter 2019

Release Final PA Winter 2020

Proposed Rulemaking 2020

Final Rulemaking 2021

2 
April 2016 1-19 Draft - Do Not Quote or Cite

1 REFERENCES 

2 DHEW (1969). Air Quality Criteria for Particulate Matter. U. S. Department of Health, 

3 Education, and Welfare. Public Health Service, Environmental Health Service, National 

4 Air Pollution Control Administration, Washington, D.C. January 1969. 

5 U.S. EPA (2004). Air Quality Criteria for Particulate Matter. National Center for Environmental 

6 Assessment, Office of Research and Development, U.S. Environmental Protection 

7 Agency, Research Triangle Park, NC 27711; report no. EPA/600/P-99/002aF and 

8 EPA/600/P-99/002bF. October 2004. Available: 

9 http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_cd.html. 

10 U.S. EPA (2005). Review of the National Ambient Air Quality Standards for Particulate Matter: 

11 Policy Assessment of Scientific and Technical Information, OAQPS Staff Paper. 

12 Research Triangle Park, NC 27711: Office of Air Quality Planning and Standards. Report 

13 No. EPA-452/R-05-005a. December 2005. Available: 

14 http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_sp.html. 

15 U.S. EPA (2008). Integrated Review Plan for the National Ambient Air Quality Standards for 

16 Particulate Matter. National Center for Environmental Assessment and Office of Air 

17 Quality Planning and Standards, U.S. Environmental Protection Agency, Research 

18 Triangle Park, NC. Report No. EPA 452/R-08-004. March 2008. Available: 

19 http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pd.html. 

20 U.S. EPA (2009a). Integrated Science Assessment for Particulate Matter: Final Report. National 

21 Center for Environmental Assessment-RTP Division, Office of Research and 

22 Development, Research Triangle Park, NC. EPA/600/R-08/139F. December 2009. 

23 Available: http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_isa.html. 

24 U.S. EPA (2009b). Particulate Matter National Ambient Air Quality Standards: Scope and 

25 Methods Plan for Health Risk and Exposure Assessment. Office of Air Quality Planning 

26 and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC. 

27 EPA-452/P-09-002. February 2009. Available: 

28 http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pd.html. 

29 U.S. EPA (2009c). Particulate Matter National Ambient Air Quality Standards: Scope and 

30 Methods Plan for Urban Visibility Impact Assessment. Office of Air Quality Planning 

31 and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC. 

32 EPA-452/P-09-001. February 2009. Available: 

33 http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pd.html. 

34 U.S. EPA (2010a). Quantitative Health Risk Assessment for Particulate Matter – Final Report. 

35 Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, 

36 Research Triangle Park, NC. EPA-452/R-10-005. June 2010. Available: 

37 http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html. 
April 2016 1-20 Draft - Do Not Quote or Cite

1 U.S. EPA (2010b). Particulate Matter Urban-Focused Visibility Assessment – Final Report. 

2 Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, 

3 Research Triangle Park, NC. EPA-452/R-10-004. July 2010. Available: 

4 http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html. 

5 U.S. EPA (2011). Policy Assessment for the Review of the Particulate Matter National Ambient 

6 Air Quality Standards. Office of Air Quality Planning and Standards, U.S. Environmental 

7 Protection Agency, Research Triangle Park, NC. EPA 452/R-11-003. April 2011. 

8 Available: http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. 

9 U.S. EPA (2015). Draft Integrated Review Plan for the Secondary National Ambient Air Quality 

10 Standards for Oxides of Nitrogen and Oxides of Sulfur. Office of Air Quality Planning 

11 and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC. 

12 October 2015. 

13 https://www3.epa.gov/ttn/naaqs/standards/no2so2sec/data/IRP_NOXSOX_CompleteDraf

14 t_102915_final.pdf

15 
April 2016 2-1 Draft - Do Not Quote or Cite

1 2 KEY POLICY-RELEVANT ISSUES IN THE CURRENT 

2 REVIEW 

3 In order to inform the Administrator’s decisions on the adequacy of the existing primary 

and secondary PM NAAQS, in the current review we35 4 will address the following overarching 

5 question: 

6  Does the currently available scientific evidence and exposure-/risk-based information 

7 support or call into question the adequacy of the protection afforded by the current 

8 primary and/or secondary PM standards?

9 If warranted, we will also address a second overarching question: 

10  What alternative standards are supported by the currently available scientific evidence 

11 and exposure-/risk-based information, and are appropriate for consideration? 

12 To inform our evaluation of these overarching questions, we will identify a number of more 

13 specific policy-relevant questions for consideration (see below). These policy-relevant questions 

14 will focus on key issues reflecting aspects of the health and welfare effects evidence, air quality 

15 information, and information from quantitative exposure and risk assessments that can inform 

16 the Administrator’s decisions in the current review. Questions will build upon the conclusions 

17 from the last review on the evidence and information, including conclusions on the uncertainties 

18 and limitations in that evidence and information. 

19 Sections 2.1 and 2.2 below summarize the decisions made in the last review of the PM 

20 NAAQS, the interpretations of the underlying scientific evidence and information supporting 

21 those decisions, the important uncertainties and limitations in the evidence and information in the 

last review, and potential key policy-relevant questions for the current review.36 22 Section 2.1 

23 focuses on the primary PM standards, and section 2.2 focuses on the secondary PM standards. 

24 Section 2.3 provides an overview of the PM ambient monitoring networks and the key 

25 monitoring-related issues to be considered as part of the current standards review. 

 

35 In this document, the terms “we” or “our” refer to staff in the EPA’s OAQPS and/or NCEA. 

36 Based on CASAC advice and public input on this draft IRP, we will modify these potential questions as 

appropriate for the final IRP. The key policy-relevant questions identified in the final PM IRP will provide an 

overall framework for the development of the PM ISA, REAs (if warranted), and the PA. 
April 2016 2-2 Draft - Do Not Quote or Cite

1 2.1 PRIMARY STANDARDS 

2 The last review of the primary PM NAAQS was completed in 2012 (78 FR 3086, January 

3 15, 2013). As noted above (section 1.3), in the last review the EPA lowered the level of the 

primary annual PM2.5 standard from 15.0 to 12.0 g/m3

,

37 4 retained the existing 24-hour PM2.5

standard with its level of 35 g/m3 5 , and retained the existing 24-hour PM10 standard with its level 

of 150 g/m3 6 . Sections 2.1.1 and 2.1.2 below discuss the primary PM2.5 and PM10 standards, 

7 respectively. These sections summarize the rationales for the decisions made in the last review, 

8 including the Agency’s consideration of important uncertainties and limitations in the scientific 

9 evidence and in the air quality and risk information. Section 2.1.3 provides an overview of the 

10 general approach and the potential key policy-relevant questions in the current review of the 

11 primary PM2.5 and PM10 standards. 

12 2.1.1 PM2.5 Standards 

13 The 2012 decision to strengthen the suite of primary PM2.5 standards was based on the 

14 Administrator’s consideration of the extensive body of scientific evidence assessed in the 2009 

15 PM ISA (U.S. EPA, 2009); the quantitative risk analyses presented in the 2010 HREA (U.S. 

EPA, 2010a);38 16 the advice and recommendations of CASAC (e.g., Samet, 2009; 2010a, b); and 

17 public comments on the proposed rule (78 FR 3086, January 15, 2013; U.S. EPA, 2012). In 

18 particular, the Administrator noted the “strong and generally robust body of evidence of serious 

19 health effects associated with both long- and short-term exposures to PM2.5” (78 FR 3120, 

20 January 15, 2013). This included epidemiologic studies reporting health effect associations based 

on long-term average PM2.5 concentrations ranging from about 15.0 g/m3 21 or above (i.e., at or 

22 above the level of the then-existing annual standard) to concentrations “significantly below the 

23 level of the annual standard” (78 FR 3120, January 15, 2013). The Administrator further 

24 observed that such studies were part of an overall pattern across a broad range of studies 

25 reporting positive associations, which were frequently statistically significant. Based on her 

26 “confidence in the association between exposure to PM2.5 and serious public health effects, 

27 combined with evidence of such an association in areas that would meet the current standards” 

 

37 The Agency also eliminated spatial averaging provisions as part of the form of the annual standard. 

38 In the last review, the EPA generated a quantitative health risk assessment for PM, and did not conduct an 

exposure assessment (U.S. EPA, 2010a). To be consistent with our general process for reviewing the NAAQS 

(section 1.2, above), and with our discussion of potential quantitative analyses in the current review (Chapter 4, 

below), we refer to the 2010 health risk assessment as the 2010 HREA. 
April 2016 2-3 Draft - Do Not Quote or Cite

1 (78 FR 3120, January 15, 2013), the Administrator concluded that revision of the suite of 

2 primary PM2.5 standards was necessary in order to provide increased public health protection. 

3 Specifically, she concluded that the then-existing suite of primary PM2.5 standards was not 

4 sufficient, and thus not requisite, to protect public health with an adequate margin of safety. This 

5 decision was consistent with advice received from CASAC (Samet, 2010b). 

6 The Administrator next considered what specific revisions to the existing primary PM2.5

7 standards were appropriate, given the available evidence and quantitative risk information. She 

8 considered both the annual and 24-hour PM2.5 standards, focusing on the basic elements of those 

9 standards (i.e., indicator, averaging time, form, and level). These considerations, and the 

10 Administrator’s conclusions, are summarized below. 

11 Indicator 

12 In initially setting standards for fine particles in 1997, the EPA concluded it was 

13 appropriate to control fine particles as a group, based on PM2.5 mass, rather than singling out any 

14 particular component or class of fine particles (62 FR 38667, July 18, 1997). In the review 

15 completed in 2006, based on similar considerations, the EPA concluded that the available 

16 information supported retaining the PM2.5 indicator and remained too limited to support a distinct 

17 standard for any specific PM2.5 component or group of components associated with particular 

18 source categories of fine particles (71 FR 61162 to 61164, October 17, 2006). 

19 In the last review, the EPA again considered issues related to the appropriate indicator for 

20 fine particles, with a focus on evaluating support for the existing PM2.5 mass-based indicator and 

21 for potential alternative indicators based on the ultrafine particle fraction or on fine particle 

composition (78 FR 3121, January 15, 2013).39 22 With regard to PM2.5 mass, as in the 1997 and 

23 2006 reviews, the health studies available during the last review continued to link adverse health 

24 outcomes (e.g., premature mortality, hospital admissions, emergency department visits) with 

25 long- and short-term exposures to fine particles indexed largely by PM2.5 mass (78 FR 3121, 

26 January 15, 2013). With regard to the ultrafine fraction of ambient PM, the PA noted the limited 

27 body of health evidence assessed in the ISA (summarized in U.S. EPA, 2009, section 2.3.5 and 

28 Table 2–6) and the limited monitoring information available to characterize ambient 

29 concentrations of ultrafine particles (U.S. EPA, 2011, section 1.3.2 and Appendix B, section 

30 B.1.3). With regard to PM composition, the ISA concluded that “the evidence is not yet 

 

39 In the last review, the ISA defined ultrafine particles as generally including particles with a mobility diameter less 

than or equal to 0.1 μm. Mobility diameter is defined as the diameter of a particle having the same diffusivity or 

electrical mobility in air as the particle of interest, and is often used to characterize particles of 0.5 μm or smaller 

(U.S. EPA, 2009a, pp. 3-2 to 3-3). 
April 2016 2-4 Draft - Do Not Quote or Cite

1 sufficient to allow differentiation of those constituents or sources that are more closely related to 

2 specific health outcomes” (U.S. EPA, 2009, pp. 2-26 and 6-212; 78 FR 3123, January 15, 2013). 

3 The PA further noted that “many different constituents of the fine particle mixture as well as 

4 groups of components associated with specific source categories of fine particles are linked to 

5 adverse health effects” (U.S. EPA, 2011, p. 2–55; 78 FR 3123, January 15, 2013). Consistent 

6 with the conclusions of the PA, CASAC advised that it was appropriate to consider retaining 

7 PM2.5 as the indicator for fine particles. CASAC specifically stated that “[t]here [is] insufficient 

8 peer-reviewed literature to support any other indicator at this time” (Samet, 2010a, p. 12). The 

9 Administrator concurred with the conclusions of the Policy Assessment and with CASAC 

10 recommendations, and concluded that it was “appropriate to retain PM2.5 as the indicator for fine 

11 particles” (78 FR 3123, January 15, 2013). 

12 Averaging time 

13 In 1997, the EPA set an annual PM2.5 standard to provide protection from health effects 

14 associated with long- and short-term exposures to PM2.5, and a 24-hour standard to supplement 

15 the protection afforded by the annual standard (62 FR 38667 to 38668, July, 18, 1997). In the 

16 2006 review, the EPA retained both annual and 24-hour averaging times (71 FR 61164, October 

17 17, 2006). 

18 In the last review, the EPA again considered issues related to the appropriate averaging 

19 times for PM2.5 standards, with a focus on evaluating support for the existing annual and 24-hour 

20 averaging times and for potential alternative averaging times based on sub-daily or seasonal 

21 metrics. Based on the evidence assessed in the ISA, the PA noted that the overwhelming 

22 majority of studies that had been conducted since the 2006 review continued to utilize annual (or 

23 multi-year) or 24-hour PM averaging periods (U.S. EPA, 2011, section 2.3.2). With regard to 

24 potential support for an averaging time shorter than 24-hours, the PA noted that studies of 

25 cardiovascular effects associated with sub-daily PM concentrations had evaluated a variety of 

26 PM metrics (e.g., PM2.5, PM10, PM10-2.5, ultrafine particles), averaging periods (e.g., 1, 2, and 4 

27 hours), and health outcomes (U.S. EPA, 2011, section 2.3.2). The PA concluded that this 

28 information, when viewed as a whole, was too uncertain to serve as a basis for establishing a 

29 primary PM2.5 standard with an averaging time shorter than 24-hours (U.S. EPA, 2011, p. 2-

57).40 30 With regard to potential support for a seasonal averaging time, few studies were available 

 

40 For respiratory effects specifically, the Administrator further noted the ISA conclusion that the strongest 

associations were observed with 24-hour average or longer exposures, not with exposures less than 24-hours (U.S. 

EPA, 2009, section 6.3). 
April 2016 2-5 Draft - Do Not Quote or Cite

1 to deduce a general pattern in PM2.5-related risk across seasons, and these studies did not provide 

2 information on health effects associated with season-long exposures to PM2.5 (U. S. EPA, 2011, 

3 p. 2-58; 78 FR 3124, January 15, 2013). 

4 The PA reached the overall conclusions that the available information provided strong 

5 support for considering retaining the current annual and 24-hour averaging times and did not 

6 provide support for considering alternative averaging times (U.S. EPA, 2011, p. 2-58). CASAC 

7 agreed that these conclusions were reasonable (Samet, 2010a, p. 13). The Administrator 

8 concurred with the PA conclusions and with CASAC’s advice. Specifically, she judged that it 

9 was “appropriate to retain the current annual and 24-hour averaging times for the primary PM2.5

10 standards to protect against health effects associated with long- and short-term exposure periods” 

11 (78 FR 3124, January 15, 2013). 

12 Form 

13 In 1997, the EPA established the form of the annual PM2.5 standard as an annual 

arithmetic mean, averaged over 3 years, from single or multiple community-oriented monitors.41 14 

15 That is, the level of the annual standard was to be compared to measurements made at each 

16 community-oriented monitoring site or, if specific criteria were met, measurements from 

17 multiple community-oriented monitoring sites could be averaged together (i.e., spatial 

averaging)42 18 (62 FR 38671 to 38672, July 18, 1997). In the 1997 review, the EPA also 

established the form of the 24-hour PM2.5 standard as the 98th 19 percentile of 24-hour 

20 concentrations at each monitor within an area (i.e., no spatial averaging), averaged over three 

21 years (62 FR at 38671 to 38674, July 18, 1997). In the 2006 review, the EPA retained these 

22 standard forms but tightened the criteria for using spatial averaging with the annual standard (78 

FR 3124, January 15, 2013).43 23 

 

41 As noted above (section 1.3), in the last review the EPA replaced the term “community-oriented” monitor with the 

term “area-wide” monitor. Area-wide monitors are those sited at the neighborhood scale or larger, as well as those 

monitors sited at micro-or middle scales that are representative of many such locations in the same CBSA (78 FR 

3236, January 15, 2013). CBSAs are required to have at least one area-wide monitor sited in the area of expected 

maximum PM2.5 concentration. 

42 The original criteria for spatial averaging included: (1) the annual mean concentration at each site shall be within 

20% of the spatially averaged annual mean, and (2) the daily values for each monitoring site pair shall yield a 

correlation coefficient of at least 0.6 for each calendar quarter (62 FR 38671 to 38672, July 18, 1997). 

43 Specifically, the Administrator revised spatial averaging criteria such that “(1) [t]he annual mean concentration at 

each site shall be within 10 percent of the spatially averaged annual mean, and (2) the daily values for each 

monitoring site pair shall yield a correlation coefficient of at least 0.9 for each calendar quarter (71 FR 61167, 

October 17, 2006). 
April 2016 2-6 Draft - Do Not Quote or Cite

1 In the last review, the EPA’s consideration of the form of the annual PM2.5 standard again 

2 included a focus on the issue of spatial averaging. An analysis of air quality and population 

3 demographic information indicated that the highest PM2.5 concentrations in a given area tended 

4 to be measured at monitors in locations where the surrounding populations were more likely to 

5 live below the poverty line and to include larger percentages of racial and ethnic minorities (U.S. 

6 EPA, 2011, p. 2-60). Based on this analysis, the PA concluded that spatial averaging could result 

7 in disproportionate impacts in at-risk populations, including minority populations and 

8 populations with lower socioeconomic status (SES). Therefore, the PA concluded that it was 

9 appropriate to consider revising the form of the annual PM2.5 standard such that it did not allow 

10 for the use of spatial averaging across monitors (U.S. EPA, 2011, p. 2-60). The CASAC agreed 

11 with staff conclusions that it was “reasonable” for the EPA to eliminate the spatial averaging 

12 provisions (Samet, 2010b, p. 2), stating the following: “Given mounting evidence showing that 

13 persons with lower SES levels are a susceptible group for PM-related health risks, CASAC 

14 recommends that the provisions that allow for spatial averaging across monitors be eliminated” 

15 (Samet, 2010a, p. 13). 

16 The Administrator concluded that public health would not be protected with an adequate 

17 margin of safety in all locations, as required by law, if disproportionately higher PM2.5

18 concentrations in low income and minority communities were averaged together with lower 

19 concentrations measured at other sites in a large urban area. Therefore, she concluded that the 

20 form of the annual PM2.5 standard should be revised to eliminate spatial averaging provisions. 

21 Thus, the level of the annual PM2.5 standard established in the last review is to be compared with 

22 measurements from each appropriate monitor in an area, with no allowance for spatial averaging. 

23 In the last review, the EPA also considered the form of the 24-hour PM2.5 standard. The 

Agency recognized that the existing 98th 24 percentile form for the 24-hour standard was originally 

25 selected to provide a balance between limiting the occurrence of peak 24-hour PM2.5

concentrations and identifying a stable target for risk management programs.44 26 Updated air 

27 quality analyses in the last review provided additional support for the increased stability of the 

98th percentile PM2.5 concentration, compared to the 99th 28 percentile (U.S. EPA, 2011, Figure 2-2, 

29 p. 2-62). Consistent with the PA conclusions based on this analysis, the Administrator concluded 

 

44 See ATA III, 283 F.3d at 374–376 which concludes that it is legitimate for the EPA to consider overall stability of 

the standard and its resulting promotion of overall effectiveness of NAAQS control programs in setting a standard 

that is requisite to protect the public health. 
April 2016 2-7 Draft - Do Not Quote or Cite

that it was appropriate to retain the 98th 1 percentile form for the 24-hour PM2.5 standard (78 FR 

2 3127, January 15, 2013). 

3 Level 

4 The EPA’s approach to considering alternative levels of the PM2.5 standards in the last 

5 review was based on evaluating the public health protection afforded by the annual and 24-hour 

6 standards, taken together, against mortality and morbidity effects associated with long-term or 

7 short-term PM2.5 exposures. This approach recognized that there is no bright line clearly 

8 directing the choice of level. Rather, the choice of what is appropriate is a public health policy 

9 judgment entrusted to the Administrator. In the last review, this judgment included consideration 

10 of the strengths and limitations of the evidence and the appropriate inferences to be drawn from 

11 the evidence and the exposure and risk assessments. 

12 In evaluating alternative standards, the Agency considered the extent to which specific 

13 alternative PM2.5 standard levels were likely to reduce the magnitudes of both long-term 

14 exposure-related mortality risk and short-term exposure-related mortality and morbidity risk. 

15 Results of the 2010 Health Risk and Exposure Assessment (HREA) indicated that, compared to 

16 revising the 24-hour standard, lowering the level of the annual standard would result in more 

17 consistent risk reductions across urban study areas, thereby potentially providing a more 

18 consistent degree of public health protection (U.S. EPA, 2010a, pp. 5-15 to 5-17; 78 FR 3128, 

19 January 15, 2013). Based on risk results, together with the available evidence, the Administrator 

20 concluded that it was appropriate to lower the level of the annual standard in order to increase 

21 protection against both long- and short-term PM2.5 exposures. She further concluded that it was 

22 appropriate to set the 24-hour standard in order to provide supplemental protection, particularly 

23 for areas with high peak-to-mean ratios of 24-hour PM2.5 concentrations (e.g., areas with 

24 important local or seasonal sources) and for PM2.5-related effects that may be associated with 

25 shorter-than daily exposure periods. The Administrator judged that this approach was the “most 

26 effective and efficient way to reduce total PM2.5-related population risk and to protect public 

27 health with an adequate margin of safety” (78 FR 3158, January 15, 2013). 

28 In selecting the level of the annual PM2.5 standard, the Administrator recognized the 

29 substantial increase in the number and diversity of studies available in the last review, including 

30 extended analyses of seminal studies of long-term PM2.5 exposures (i.e., American Cancer 

31 Society (ACS) and Harvard Six Cities studies), important new long-term exposure studies, and 

32 new U.S. multi-city studies that greatly expanded and reinforced our understanding of mortality 

33 and morbidity effects associated with short-term PM2.5 exposures. She placed the greatest 

34 emphasis on health endpoints for which the evidence was strongest, based on the assessment of 

35 the evidence in the ISA and on the ISA’s causality determinations (U.S. EPA, 2009, section 
April 2016 2-8 Draft - Do Not Quote or Cite

2.3.1).45 1 In particular, she noted that the evidence was sufficient to conclude a causal relationship 

2 exists between PM2.5 exposures and mortality and cardiovascular effects (i.e., for both long- and 

3 short-term exposures) and that the evidence was sufficient to conclude a causal relationship is 

4 “likely” to exist between PM2.5 exposures and respiratory effects (i.e., for both long- and short5 term exposures). The Administrator also noted additional, but more limited, evidence for a 

6 broader range of health endpoints including evidence “suggestive of a causal relationship” 

7 between long-term exposures and developmental and reproductive effects as well as 

8 carcinogenic effects (78 FR 3158, January 15, 2013). 

9 Based on information discussed and presented in the ISA, the Administrator recognized 

10 that health effects may occur over the full range of ambient PM2.5 concentrations observed in 

11 epidemiologic studies, since no discernible population-level threshold could be identified based 

12 on the evidence available in the last review (78 FR 3158, January 15, 2013; U.S. EPA, 2009, 

13 section 2.4.3). To inform her decisions on an appropriate level for the annual standard in the 

14 absence of a discernible population-level threshold, the Administrator considered the degree to 

15 which epidemiologic studies indicate confidence in the magnitude and significance of health 

16 effect associations over distributions of ambient PM2.5 concentrations. In doing so, she 

17 recognized that epidemiologic studies provide greater confidence in the magnitude and 

18 significance of observed associations for the part of the air quality distribution corresponding to 

19 the bulk of the health events evaluated, generally at and around the long-term mean PM2.5

20 concentrations. Accordingly, the Administrator weighed most heavily the long-term mean 

21 concentrations reported in key multi-city epidemiologic studies. She also took into account 

22 additional population-level information from a subset of studies, beyond the long-term mean 

23 concentrations, to identify a broader range of PM2.5 concentrations to consider in judging the 

need for public health protection.46 24 In doing so, the Administrator recognized that studies 

25 indicate diminished confidence in the magnitude and significance of observed associations in the 

26 lower part of the air quality distribution, corresponding to where a relatively small proportion of 

27 the health events are observed. 

In revising the level of the annual standard to 12.0 μg/m3 28 , the Administrator noted that 

29 such a level was below the long-term mean PM2.5 concentrations reported in key epidemiologic 

 

45 The ISA framework for reaching causality determinations is discussed in Chapter 3, below. 

46 This information characterized the distribution of health events in the studies, and the corresponding long-term 

mean PM2.5 concentrations (78 FR 3130 to3134, January 15, 2013). The additional population-level data helped 

inform the Administrator’s judgment of how far below the long-term mean concentrations to set the level of the 

annual standard (78 FR 3160). 
April 2016 2-9 Draft - Do Not Quote or Cite

1 studies that provided evidence of an array of serious health effects, including premature mortality 

2 and increased hospitalizations for cardiovascular and respiratory effects (78 FR 3161, January 

15, 2013). The Administrator further noted that 12.0 μg/m3 3 generally corresponded to the lower 

portions (i.e., about the 25th 4 percentile) of distributions of health events in the limited number of 

5 epidemiologic studies for which population-level information was available. The Administrator 

6 viewed this population information as helpful in guiding her determination as to where her 

7 confidence in the magnitude and significance of the PM2.5 associations were reduced to such a 

8 degree that a standard set at a lower level was not warranted. The Administrator also recognized 

that a level of 12.0 μg/m3 9 reflected placing some weight on studies of reproductive and 

10 developmental effects, for which the evidence was less strong (78 FR 3161-3162, January 15, 

2013).47 11 

In conjunction with a revised annual standard with a level of 12.0 μg/m3 12 , the 

Administrator concluded that the evidence supported retaining the 35 μg/m3 13 level of the 24-hour 

14 PM2.5 standard to provide supplemental protection (78 FR 3163, January 15, 2013). Specifically, 

15 she judged that by lowering the level of the annual standard, the distribution of 24-hour PM2.5

16 concentrations would be lowered as well, affording additional protection against effects 

associated with short-term PM2.5 exposures.48 17 

18 The Administrator recognized that uncertainties remained in the scientific information. 

19 She specifically noted uncertainties related to understanding the relative toxicity of the different 

20 components in the fine particle mixture, the role of PM2.5 in the complex ambient mixture, 

21 exposure measurement errors in epidemiologic studies, and the nature and magnitude of 

22 estimated risks related to relatively low ambient PM2.5 concentrations. Furthermore, the 

23 Administrator noted that epidemiologic studies had reported heterogeneity in responses both 

24 within and between cities and in geographic regions across the U.S. She recognized that this 

25 heterogeneity may be attributed, in part, to differences in fine particle composition in different 

26 regions and cities. With regard to evidence for reproductive and developmental effects, the 

27 Administrator recognized that there were a number of limitations associated with this body of 

 

47 With respect to cancer, mutagenic, and genotoxic effects, the Administrator observed that the PM2.5

concentrations reported in studies evaluating these effects generally included ambient concentrations that are equal 

to or greater than ambient concentrations observed in studies that reported mortality and cardiovascular and 

respiratory effects (U.S. EPA, 2009, section 7.5). Therefore, the Administrator concluded that, in selecting a 

standard level that provides protection from mortality and cardiovascular and respiratory effects, it is reasonable to 

anticipate that protection will also be provided for carcinogenic effects (78 FR 3161-3162, January 15, 2013). 

48 This judgment is supported by risk results presented in the 2010 HREA. For example, see section 4.2.2, and 

Figures 4-4 and 4-6 (U.S. EPA, 2010a). 
April 2016 2-10 Draft - Do Not Quote or Cite

1 evidence, including the following: the limited number of studies evaluating such effects; 

2 uncertainties related to identifying the relevant exposure time periods of concern; and limited 

3 toxicological evidence providing little information on the mode of action(s) or biological 

4 plausibility for an association between long-term PM2.5 exposures and adverse birth outcomes. 

5 On balance, the Administrator found that the available evidence, interpreted in light of 

6 the remaining uncertainties (noted above), did not justify an annual standard level set below 12.0 

μg/m3 7 as being “requisite” (i.e., neither more nor less stringent than necessary) to protect public 

8 health with an adequate margin of safety. Thus, the Administrator concluded that the available 

evidence and information supported an annual standard with a level of 12.0 μg/m3 9 , combined 

with a 24-hour standard with a level of 35 μg/m3 10 . She noted that this combination of standard 

11 levels was consistent with CASAC’s advice to consider an annual standard level within the range 

of 13 to 11 g/m3 and a 24-hour standard level from 35 to 30 g/m3 12 (Samet, 2010b). Taken 

13 together, the Administrator concluded that the revised annual PM2.5 standard, with its level of 

12.0 μg/m3 14 and a form that does not allow for spatial averaging, combined with the existing 24-

15 hour standard, would be requisite to protect the public health with an adequate margin of safety 

16 from effects associated with long- and short-term PM2.5 exposures. 

17 2.1.2 PM10 Standard 

18 In the last review the EPA retained the existing 24-hour primary PM10 standard, with its 

level of 150 g/m3 19 and its one-expected-exceedance form, in order to continue to provide public 

20 health protection against exposures to PM10-2.5. In support of this decision, the Administrator 

21 considered the extent to which a standard with a PM10 indicator can provide protection against 

22 exposures to PM10-2.5 and the degree of public health protection provided by the existing PM10

23 standard. Her consideration of each of these issues is summarized below. 

24 In reaching the conclusion that a standard with a PM10 indicator can provide appropriate 

25 protection against exposures to PM10-2.5, the Administrator noted that PM10 mass includes both 

26 coarse PM (PM10-2.5) and fine PM (PM2.5). As a result, the concentration of PM10-2.5 allowed by a 

27 PM10 standard set at a single level declines as the concentration of PM2.5 increases. Because 

28 PM2.5 concentrations tend to be higher in urban areas than rural areas (U.S. EPA, 2005, p. 2–54, 

29 and Figures 2–23 and 2–24), the Administrator observed that a PM10 standard will generally 

30 allow lower PM10-2.5 concentrations in urban areas than in rural areas. She judged it appropriate 

31 to maintain such a standard given that the large majority of the evidence for PM10-2.5 toxicity, 

32 particularly at relatively low particle concentrations, came from study locations where thoracic 

33 coarse particles were of urban origin, and given the possibility that PM10-2.5 contaminants in 

34 urban areas could increase particle toxicity. Thus, in the last review the Administrator concluded 
April 2016 2-11 Draft - Do Not Quote or Cite

1 that it remained appropriate to maintain a standard that allows lower ambient concentrations of 

2 PM10-2.5 in urban areas, where the evidence was strongest that thoracic coarse particles are linked 

3 to mortality and morbidity, and higher concentrations in non-urban areas, where the public health 

4 concerns were less certain. 

5 In specifically evaluating the degree of public health protection provided by the primary 

PM10 standard, with its level of 150 g/m3 6 and its one-expected-exceedance form, the 

7 Administrator recognized that the available health evidence and air quality information was 

8 much more limited for PM10-2.5 than for PM2.5. In particular, the strongest evidence for health 

9 effects attributable to PM10-2.5 exposures was for cardiovascular effects, respiratory effects, 

10 and/or premature mortality following short-term exposures. For each of these categories of 

11 effects, the ISA determined that the evidence was “suggestive of a causal relationship” (U.S. 

12 EPA, 2009, section 2.3.3). These determinations contrast with those for PM2.5, as described in 

13 section 2.1.1 above, which were judged in the ISA to be either “causal” or “likely to be causal” 

14 for mortality, cardiovascular effects, and respiratory effects (U.S. EPA, 2009, Tables 2-1 and 2-

15 2). 

16 The Administrator judged that the important uncertainties and limitations associated with 

17 the PM10-2.5 evidence and information raised questions as to whether additional public health 

18 improvements would be achieved by revising the existing PM10 standard. She specifically noted 

19 the following: 

20 (1) While PM10-2.5 effect estimates reported for mortality and morbidity were generally 

21 positive, most were not statistically significant, even in single-pollutant models. This 

22 included effect estimates reported in some study locations with PM10 concentrations 

23 above those allowed by the current 24-hour PM10 standard. 

24 (2) The number of epidemiologic studies that have employed co-pollutant models to address 

25 the potential for confounding, particularly by PM2.5, was limited. Therefore, the extent to 

26 which PM10-2.5 itself, rather than one or more co-pollutants, contributes to reported health 

27 effects remained uncertain. 

28 (3) Only a limited number of experimental studies provided support for the associations 

29 reported in epidemiologic studies, resulting in further uncertainty regarding the 

30 plausibility of the associations between PM10-2.5 and mortality and morbidity reported in 

31 epidemiologic studies. 

32 (4) Limitations in PM10-2.5 monitoring data and the different approaches used to estimate 

33 PM10-2.5 concentrations across epidemiologic studies resulted in uncertainty in the 

34 ambient PM10-2.5 concentrations at which the reported effects occur, increasing 
April 2016 2-12 Draft - Do Not Quote or Cite

1 uncertainty in estimates of the extent to which changes in ambient PM10-2.5 concentrations 

would likely impact public health.49 2 

3 (5) The composition of PM10-2.5, and the effects associated with the various components, 

4 were uncertain. Without more information on the chemical speciation of PM10-2.5, the 

5 apparent variability in associations across locations was difficult to characterize. 

6 With regard to these uncertainties and limitations, the Administrator noted in particular the 

7 considerable degree of uncertainty in the extent to which health effects reported in epidemiologic 

8 studies are due to PM10-2.5 itself, as opposed to one or more co-occurring pollutants. This 

9 uncertainty reflected the relatively small number of PM10-2.5 studies that had evaluated co10 pollutant models, particularly co-pollutant models that included PM2.5, and the very limited body 

11 of controlled human exposure evidence supporting the plausibility of PM10-2.5-attributable 

12 adverse effects at ambient concentrations. The Administrator noted that these important 

13 limitations in the overall body of health evidence introduce uncertainty into the interpretation of 

14 individual epidemiologic studies, particularly those studies reporting associations with PM10-2.5

15 that are not statistically significant. 

16 When she viewed the evidence as a whole, the Administrator concluded that the degree 

17 of public health protection provided against short-term exposures to PM10-2.5 should be 

18 maintained but did not need to be increased beyond that provided by the current PM10 standard. 

19 This conclusion emphasized (1) the important uncertainties and limitations associated with the 

20 overall body of health evidence and air quality information for PM10-2.5, as reflected in the ISA 

21 causal determinations; (2) information indicating that PM10-2.5 effect estimates for the most 

22 serious health effect, mortality, were not statistically significant in U.S. locations that met the 

23 current PM10 standard and where coarse particle concentrations were either directly measured or 

estimated based on co-located samplers;50 24 and (3) that PM10-2.5 effect estimates for morbidity 

25 endpoints were both positive and negative in locations that met the current standard, with most 

26 not statistically significant. Thus, the Administrator concluded that the existing 24-hour PM10

 

49 Such limitations also contributed to the decision not to conduct a quantitative risk assessment for PM10-2.5. The 

lack of a quantitative PM10-2.5 risk assessment further contributed to uncertainty regarding the extent to which any 

revisions to the current PM10 standard would be expected to improve the protection of public health, beyond the 

protection provided by the current standard. 

50 The Administrator noted that the study by Zanobetti and Schwartz (2009) was the only study to estimate ambient 

PM10-2.5 concentrations as the difference between county-wide PM10 mass and county-wide PM2.5 mass (78 FR 3178, 

January 15, 2013). As discussed in the PA, it is not clear how such computed PM10-2.5 measurements compare with 

the PM10-2.5 concentrations obtained in other studies either by direct measurement or by calculating the difference 

using co-located samplers (U.S. EPA, 2009, section 6.5.2.3). 
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standard, with its one-expected exceedance form and a level of 150 g/m3 1 , is requisite to protect 

2 public health with an adequate margin of safety against effects that have been associated with 

3 PM10-2.5. In light of this conclusion, the EPA retained the existing PM10 standard. 

4 2.1.3 General Approach in the Current Review of the Primary Standards 

5 The approach for this review will build on the substantial body of work done during the 

6 course of the last review, taking into account the more recent scientific information and air 

7 quality data now available to inform our understanding of the key policy-relevant issues. The 

8 approach summarized below is most fundamentally based on using the EPA’s assessment of the 

9 current scientific evidence, quantitative assessments of exposures and/or risks, and other 

10 associated analyses (e.g., air quality analyses) to inform the Administrator’s judgments regarding 

11 primary standards for PM that are requisite to protect public health with an adequate margin of 

12 safety. This approach will involve translating scientific and technical information into the basis 

13 for addressing a series of key policy-relevant questions using both evidence- and exposure-/riskbased considerations.51 14 

15 Figure 2-1 summarizes the general approach in the current review to reaching conclusions 

16 on the current primary standards and on potential alternative standards, if appropriate. The ISA, 

17 HREA (if developed), and PA developed in this review will provide the basis for addressing the 

18 key policy-relevant questions and will inform the Administrator’s decisions as to whether to 

19 retain or revise the primary PM NAAQS. The four basic elements of the NAAQS (i.e., indicator, 

20 averaging time, form, and level) will be considered collectively in evaluating the health 

21 protection afforded by the current standards, and by any alternatives considered. 

22 

 

51 Evidence-based considerations include those related to the health effects evidence assessed and characterized in 

the ISA. Exposure-/risk-based considerations draw from the results of the quantitative assessments. 
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1 

2 Figure 2-1. Overview of General Approach for Review of Primary PM Standards 
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1 The final decisions on the adequacy of the current standards and, if appropriate, potential 

2 alternative standards, are largely public health policy judgments to be made by the 

3 Administrator. The Administrator’s final decisions will draw upon the scientific evidence for 

4 PM-related health effects, quantitative analyses of population exposures and/or health risks, and 

5 judgments about how to consider the uncertainties and limitations that are inherent in the 

6 scientific evidence and quantitative analyses. To inform the Administrator’s public health policy 

7 judgments and decisions, we will consider the support for, and the potential implications of, 

8 placing more or less emphasis on various aspects of the scientific evidence, the exposure-/risk9 based information, and the associated uncertainties and limitations. 

10 The current review of the primary PM2.5 and PM10 standards will build upon the 

11 conclusions from the last review, taking into account the updated evidence and information that 

12 has become available since that review. Our consideration of the evidence and information will 

13 inform the answer to the following overarching question: 

14  Does the currently available scientific evidence and exposure-/risk-based information 

15 support or call into question the adequacy of the public health protection afforded by 

16 the current primary PM2.5 or PM10 standards? 

17 In order to answer this overarching question, we will consider a series of more specific policy18 relevant questions related to the health effects and health risks of short- and long-term PM 

19 exposures. These questions will focus on PM exposures indexed by PM2.5 and PM10-2.5 mass, as 

20 well as PM exposures indexed by other metrics such as other size fractions (e.g., ultrafine 

21 fraction), PM characteristics other than size (e.g., chemical composition), PM from particular 

22 sources, or PM present in particular types of environments (e.g., urban versus non-urban, various 

geographic areas). Potential policy-relevant questions52 23 include the following: 

24  To what extent has new evidence strengthened or otherwise altered the scientific support for 

25 the occurrence of adverse health effects or premature mortality as a result of exposures to 

26 particles in the ambient air? To what extent have important uncertainties in the evidence from 

27 the last review been addressed, and have new uncertainties emerged? 

28  To what extent has new evidence improved our understanding of human lifestages and 

29 populations that are at increased risk of experiencing health effects associated with exposures 

30 to ambient PM? 

 

52 As noted above, based on CASAC advice and public input on this draft IRP, we will modify these potential 

questions as appropriate for the final IRP. The key policy-relevant questions identified in the final PM IRP will 

provide an overall framework for the development of the PM ISA, REAs (if warranted), and the PA. 
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1  What does the evidence indicate with regard to confidence in the occurrence of adverse 

2 health effects attributable to exposures to ambient PM concentrations that would likely have 

3 met the current primary standards? 

4  What do available epidemiologic studies indicate with regard to the shapes of 

5 PM concentration-response functions and confidence in PM health effect 

6 associations at ambient particle concentrations that would meet the existing 

7 standards? Do studies indicate departures from linearity at such low 

8 concentrations? 

9  When taken together, what do available epidemiologic and experimental 

10 studies indicate with regard to confidence in PM-attributable adverse effects at 

11 ambient PM concentrations that would meet existing standards? 

12  To what extent does the evidence indicate that health effects are associated with exposures to 

13 PM2.5 or PM10-2.5 themselves (or other indicator of PM), as opposed to one or more co14 occurring pollutants, particularly at relatively low ambient PM concentrations that would 

15 meet the existing standards? 

16  To what extent are PM-attributable health effects larger and/or more serious with co17 exposures to other pollutants or other stressors? 

18  Is new information available to better inform our understanding of factors other than 

19 pollutant exposures that might influence the associations between ambient PM concentrations 

20 and health effects (e.g., weather-related factors, behavioral factors, heating/air conditioning 

21 use, driving patterns, time-activity patterns)? 

22  Do studies examining the potential for effect modification, either by co-occurring pollutants 

23 or by factors other than pollutant exposures, improve our understanding of the geographic 

24 heterogeneity in epidemiologic associations with PM? 

25  Is new information available to improve our understanding of PM exposures, and how those 

26 exposures relate to the ambient concentrations often used as exposure surrogates in 

27 epidemiologic studies? Does our understanding of geographic variability in exposure 

28 measurement error inform our understanding of geographic heterogeneity in epidemiologic 

29 associations? 

30  Is new information available to improve our understanding of the PM exposure periods (e.g., 

31 exposure durations and/or windows of exposure) that are most relevant for PM-associated 

32 mortality and/or morbidity? 

33  To what extent do quantitative estimates indicate that PM-attributable health risks of public 

34 health importance could occur at ambient PM concentrations meeting the current primary 

35 standards? To what extent do uncertainties and limitations in the underlying health evidence 

36 or in the assessment approaches affect our interpretation of these quantitative estimates? 

37 If the available evidence and exposure/risk information call into question the adequacy of 

38 the public health protection afforded by the current primary PM2.5 or PM10 standards, we will 

39 also consider the following overarching question: 
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1  What alternative standards are supported by the currently available scientific evidence 

2 and exposure-/risk-based information, and are appropriate for consideration? 

3 The answer to this second overarching question will also be informed by our consideration of a 

4 series of more specific questions focused on the basic elements of the NAAQS (indicator, 

5 averaging time, form, level). In addressing these more specific questions, we will draw from our 

6 consideration of the scientific evidence and from the results of any additional air quality, 

exposure, and/or risk analyses that focus on potential alternative standards.53 7 We will consider 

8 the elements of the NAAQS collectively in evaluating the health protection afforded by potential 

9 alternative standards. Specific policy-relevant questions on potential alternative standards will 

10 include the following: 

11  Do the available health effects evidence and air quality information provide support for 

12 consideration of indicators for fine and thoracic coarse particles in addition to, or in place of, 

13 PM2.5 and PM10, respectively? Does the evidence support an alternative approach for 

14 defining particle pollution, including in terms of other size fractions, specific components, 

15 source-related mixtures, or specific environments? 

16  Do the available health effects evidence, air quality information, and exposure/risk 

17 information provide support for considering averaging times in addition to, or in place of, the 

18 current 24-hour and annual averaging times? 

19  To what extent do the available health effects evidence, air quality information, and 

20 exposure/risk information provide support for consideration of alternative standard forms? 

21  What range of alternative standard levels could be supported based on the scientific evidence, 

22 air quality analyses, and exposure/risk assessments? 

23  What are the important uncertainties and limitations in the available evidence and in the 

24 available quantitative analyses, and how might these uncertainties and limitations be taken 

25 into consideration in identifying alternative standard indicators, averaging times, forms 

26 and/or levels? 

27 2.2 SECONDARY STANDARDS 

28 In the last review, the EPA generally retained the existing suite of secondary PM 

standards (78 FR 3228, January 15, 2013).54 29 As described below, the EPA retained the 

 

53 Such additional quantitative analyses can inform conclusions on the magnitude of the public health improvement 

that would be expected upon just meeting various potential alternative standards, and on the exposures and/or risks 

expected to remain. 

54 The previous review of the secondary PM NAAQS focused on the welfare effects associated with particles present 

in the ambient air, including the effects of ambient particulate species of NOX and SOX. The review did not focus on 

the deposition related effects of particulate species of NOX and SOX. The EPA reviewed the secondary NO2 and SO2
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secondary 24-hour PM2.5 standard, with its level of 35 μg/m3 1 , and the 24-hour PM10 standard, 

with its level of 150 μg/m3 2 . The EPA also retained the secondary annual PM2.5 standard, with its 

level of 15.0 μg/m3 3 , except for a change to the form of the annual standard. Consistent with the 

4 change to the form of the primary annual PM2.5 standard (section 2.1.1 above), the EPA removed 

5 the option for spatial averaging from the form of the secondary annual PM2.5 standard (78 FR 

6 3228, January 15, 2013). Key aspects of the Administrator’s decisions on the secondary PM 

7 standards for non-visibility welfare effects and visibility effects are described below in sections 

8 2.2.1 and 2.2.2, respectively. Key policy-relevant issues for the current review are discussed in 

9 section 2.2.3. 

10 2.2.1 Non-Visibility Welfare Effects 

11 In the last review of the PM NAAQS, the Administrator concluded that it was important 

12 to maintain an appropriate degree of control of both fine and coarse particles to address non13 visibility welfare effects. Lacking information that would support revised standards, she further 

14 concluded that it was appropriate to retain the existing suite of secondary standards to protect 

15 against such effects. Non-visibility welfare effects considered include climate effects, ecological 

effects (e.g., effects on plants, soil and nutrient cycling, wildlife and water),55 16 and materials 

17 effects. The Administrator’s consideration of each of these types of effects is discussed below. 

18 With regard to the role of PM in climate, the Administrator considered whether it was 

19 appropriate to establish any distinct secondary PM standards to address welfare effects 

20 associated with climate impacts. In considering the scientific evidence, she noted the ISA 

21 conclusion “that a causal relationship exists between PM and effects on climate” and that 

aerosols56 22 alter climate processes directly through radiative forcing and by indirect effects on 

23 cloud brightness, changes in precipitation, and possible changes in cloud lifetimes (U.S. EPA, 

24 2009, section 9.3.10). Additionally, the ISA noted that the major aerosol components 

25 contributing to climate processes (i.e., black carbon (BC), organic carbon (OC), sulfates, nitrates 

26 and mineral dusts) vary in their reflectivity, forcing efficiencies, and direction of climate forcing, 

 

NAAQS separately and evaluated the ecosystem-related welfare effects that result from the deposition of particulate 

species of NOX, SOX, and their transformation products. 

55 As described above, the on-going NOX/SOX review includes consideration of the ecological effects of ecosystem 

loading of particulate nitrogen and sulfur compounds. Thus, these endpoints are not discussed in this review. 

56 The term aerosol is used in this document when discussing suspended ambient particles in the context of climate 

impacts. 
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1 though there is an overall net climate cooling associated with aerosols in the global atmosphere 

2 (U.S. EPA, 2009, section 9.3.10). 

3 Noting the strong evidence indicating that aerosols affect climate, the Administrator 

4 further considered whether there was sufficient information to revise the secondary PM 

5 standards. She noted that a number of uncertainties in the scientific information were identified 

6 in the ISA and PA. For example, the ISA and PA noted the spatial and temporal heterogeneity of 

7 PM components that contribute to climate forcing, uncertainties in the measurement of aerosol 

8 components, inadequate consideration of aerosol impacts in climate modeling, insufficient data 

9 on local and regional microclimate variations and heterogeneity of cloud formations. In light of 

10 these uncertainties and the lack of sufficient data, the PA concluded that it was not feasible in the 

11 last review “to conduct a quantitative analysis for the purpose of informing revisions [to the 

12 secondary PM NAAQS] based on climate” (U.S. EPA, 2011, pp. 5-11 to 5-12) and that there was 

13 insufficient information available to base a national ambient standard on climate impacts 

14 associated with ambient concentrations of PM or its constituents (U.S. EPA, 2011, section 5.2.3). 

15 The Administrator agreed with this conclusion (78 FR 3225-3226, January 15, 2013). 

16 The Administrator also considered ecological effects in the last review, including direct 

17 effects on metabolic processes of plants; contribution to total metal loading resulting in alteration 

18 of soil biogeochemistry and microbiology, plant and animal growth and reproduction; and 

19 contribution to total organics loading resulting in bioaccumulation and biomagnification across 

20 trophic levels. The ISA determined that the evidence was sufficient to conclude that “a causal 

21 relationship is likely to exist between deposition of PM and a variety of effects on individual 

22 organisms and ecosystems” (U.S. EPA, 2009, p. 2-30; sections 2.5.3 and 9.4.7). However, the 

23 ISA also noted that it is generally difficult to characterize the nature and magnitude of effects 

24 and to quantify relationships between ambient concentrations of PM and ecosystem responses. 

25 Such difficulty is due to significant data gaps and uncertainties, as well as to the considerable 

26 variability that exists in the components of PM and their various ecological effects (U.S. EPA, 

27 2009, p. 9-193). Given uncertainties and limitations in the available evidence, the PA concluded 

28 that the information available at the time of the last review was insufficient for the purposes of 

29 assessing the adequacy of the protection for ecosystems afforded by the existing suite of 

30 secondary PM standards or for establishing a distinct national PM standard based on ecosystem 

31 effects of particulates (U.S. EPA, 2011, p. 5-24). The Administrator agreed with this conclusion 

32 (78 FR 3225-3226, January 15, 2013). 

33 With regard to materials effects, the Administrator also considered effects associated with 

34 the deposition of PM (i.e., dry and wet deposition), including both physical damage (materials 

35 effects) and impaired aesthetic qualities (soiling effects). The deposition of PM can physically 
April 2016 2-20 Draft - Do Not Quote or Cite

1 affect materials, adding to the effects of natural weathering processes, by promoting or 

2 accelerating the corrosion of metals; by degrading paints; and by deteriorating building materials 

3 such as stone, concrete, and marble (U.S. EPA, 2009, section 9.5). Additionally, the deposition 

4 of ambient PM can reduce the aesthetic appeal of buildings and objects through soiling. The ISA 

5 concluded that evidence was sufficient to support a causal relationship between PM and effects 

6 on materials (U.S. EPA, 2009, sections 2.5.4 and 9.5.4). However, the PA noted that quantitative 

7 relationships are lacking between particle size, concentrations, and frequency of repainting and 

8 repair of surfaces and that considerable uncertainty exists in the contributions of co-occurring 

9 pollutants to materials damage and soiling processes (U.S. EPA, 2011, p. 5-29). The PA 

10 concluded that none of the evidence available in the last review called into question the adequacy 

11 of the existing secondary PM standards to protect against material effects and that such effects 

12 could play no quantitative role in determining whether revisions to the secondary PM NAAQS 

13 were appropriate (U.S. EPA, 2011, p. 5-29). The Administrator agreed with this conclusion (78 

14 FR 3225-3226, January 15, 2013). 

15 In considering non-visibility welfare effects in the last review, as discussed above, the 

16 Administrator concluded that, while it is important to maintain an appropriate degree of control 

17 of fine and coarse particles to address non-visibility welfare effects, “[i]n the absence of 

18 information that would support any different standards...it is appropriate to retain the existing 

19 suite of secondary standards” (FR 78 3225 to 3226, January 15, 2013). Her decision was 

20 consistent with CASAC advice related to non-visibility effects. Specifically, CASAC agreed 

21 with the PA conclusions that, while these effects are important, “there is not currently a strong 

22 technical basis to support revisions of the current standards to protect against these other welfare 

23 effects” (Samet, 2010a, p. 5). Thus, the Administrator concluded that it was appropriate to retain 

24 all aspects of the existing 24-hour PM2.5 and PM10 secondary standards. With regard to the 

25 secondary annual PM2.5 standard, the Administrator concluded that it was appropriate to retain a 

level of 15.0 μg/m3 26 while revising only the form of the standard to remove the option for spatial 

27 averaging, consistent with this change to the primary annual PM2.5 standard (78 FR 3225-3226, 

28 January 15, 2013). 

29 2.2.2 Visibility Effects 

30 The Administrator also considered the level of protection that would be requisite to 

31 protect public welfare with regard to visual air quality and whether to adopt a distinct secondary 

32 standard to achieve this level of protection. In reaching her final decision that the existing 24-

33 hour PM2.5 standard provides sufficient protection against PM-related visibility impairment (78 

34 FR 3228, January 15, 2013), the Administrator considered the evidence assessed in the ISA (U.S. 
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1 EPA, 2009) and the analyses included in the Urban Focused Visibility Assessment (UFVA) 

2 (U.S. EPA, 2010b) and the PA (U.S. EPA, 2011). She also considered the degree of protection 

3 for visibility that would be provided by the existing secondary standard, focusing specifically on 

the secondary 24-hour PM2.5 standard of 35 μg/m3 4 . These considerations, and the 

5 Administrator’s conclusions regarding visibility, are discussed in more detail below. 

6 In the last review, the ISA concluded that, “collectively, the evidence is sufficient to 

7 conclude that a causal relationship exists between PM and visibility impairment” (U.S. EPA, 

8 2009, p. 2-28). Visibility impairment is caused by light scattering and absorption by suspended 

particles and gases, including water content of aerosols.57 9 The available evidence in the last 

10 review indicated that various components of PM have been shown to contribute to visibility 

11 impairment. For example, at sufficiently high relative humidity values, sulfate and nitrate are the 

12 particulate species that contribute most efficiently to visibility impairment. Elemental carbon 

13 (EC) and organic carbon (OC) also are important contributors, especially in the northwestern 

14 U.S. Crustal material can be significant contributors to visibility impairment, particularly for 

15 remote areas in the arid southwestern U.S. (U.S. EPA, 2009, section 2.5.1). 

16 Visibility impairment can have implications for people’s enjoyment of daily activities and 

17 for their overall sense of well-being (U.S. EPA, 2009, section 9.2). In consideration of the 

18 potential public welfare implications of various degrees of visibility impairment, the 

19 Administrator considered the available visibility preference studies which were reviewed by 

EPA in the 2010 UFVA (U.S. EPA, 2010b, chapter 2).58 20 These preference studies provided 

21 information about the potential public welfare implications of visibility impairment from survey 

22 studies in which participants were asked questions about their preferences or the values they 

23 placed on various visibility conditions, as displayed to them in scenic photographs or in images 

24 with a range of known light extinction levels. 

 

57 All particles scatter light and, although a larger particle scatters more light than a similarly shaped smaller particle 

of the same composition, the light scattered per unit of mass is greatest for particles with diameters from ~0.3-1.0 

μm (U.S. EPA, 2009, section 2.5.1). Particles with hygroscopic components (e.g., particulate sulfate and nitrate) 

contribute more light extinction at higher relative humidity than at lower relative humidity because they change size 

in the atmosphere in response to ambient relative humidity conditions. 

58 Preference studies were available in four urban areas in the last review. Three western preference studies were 

available, including one in Denver, Colorado (Ely et al., 1991), one in the lower Fraser River valley near Vancouver, 

British Columbia, Canada (Pryor, 1996), and one in Phoenix, Arizona (BBC Research & Consulting, 2003). A pilot 

focus group study was also conducted for Washington, DC (Abt Associates Inc., 2001), and a replicate study with 

26 participants was also conducted for Washington, DC (Smith and Howell, 2009). 
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1 In noting the relationship between ambient PM and PM-related light extinction, the 

2 Administrator focused on identifying an adequate level of protection against visibility-related 

3 welfare effects. She first concluded that a standard based on a PM2.5 visibility index would 

4 provide a measure of protection against PM-related light extinction that directly takes into 

5 account the factors (i.e., species composition and relative humidity) that influence the 

6 relationship between PM2.5 in ambient air and PM-related visibility impairment. A PM2.5

7 visibility index standard would afford a relatively high degree of uniformity of visual air quality 

8 protection in areas across the country by directly incorporating the effects of differences of PM2.5

9 composition and relative humidity. In defining a target level of protection based on a PM2.5

10 visibility index, as discussed below, the Administrator considered specific elements of the index, 

11 including the appropriate indicator, averaging time, level, and form. 

12 With regard to the indicator of a visibility index, the Administrator concluded that a 

13 calculated PM2.5 light extinction indicator that utilized an adjusted version of the original 

IMPROVE algorithm,59 14 in conjunction with monthly average relative humidity data based on 

15 long-term climatological means, would be the most appropriate indicator for a PM2.5 visibility 

16 index standard (78 FR 3226, January 15, 2013). In reaching her final decision, the Administrator 

17 further noted the CASAC conclusion that it was reasonable to rely on a calculated PM2.5 light 

18 extinction indicator based on PM2.5 chemical composition and relative humidity. The 

19 Administrator also considered the PM2.5 mass indicator and directly measured PM2.5 light 

20 extinction. She concluded that a PM2.5 mass-based indicator would not be appropriate because 

21 the available mass monitoring methods did not include measurement of the full water content of 

22 ambient PM2.5, nor did they provide information on the composition of PM2.5, both of which 

23 contribute to visibility impacts (77 FR 38980, June 29, 2012). In addition, at the time of the 

24 proposal, the Administrator provisionally concluded that directly measured PM2.5 light extinction 

25 was not an appropriate option because a suitable specification of available equipment or 

26 performance-based verification procedures for direct measurement of light extinction could not 

27 be developed in the time frame of the review (77 FR 38980-38981, June 29, 2012). 

28 With regard to averaging time of the index, the Administrator concluded that a 24-hour 

29 averaging time would be appropriate for a visibility index (78 FR 3226, January 15, 2013). She 

30 concluded that hourly or sub-daily (4- to 6-hour) averaging times, within daylight hours and 

 

59 The IMPROVE algorithm uses PM mass concentration measurements and relative humidity estimates to calculate 

light extinction. For more information about the derivation of and input data required for the original and revised 

IMPROVE algorithms, see 78 FR 3186-3177, January 15, 2013. 
April 2016 2-23 Draft - Do Not Quote or Cite

1 excluding hours with high relative humidity, are more directly related to the short-term nature of 

2 the perception of PM-related visibility impairment and the relevant exposure periods for 

3 segments of the viewing public than a 24-hour averaging time. However, she also noted the data 

4 quality uncertainties associated with the instruments used to provide the hourly PM2.5 mass 

5 measurements required for an averaging time shorter than 24 hours. The Administrator also 

6 considered comparative analyses of 24-hour and 4-hour averaging times in conjunction with a 

7 calculated PM2.5 indicator in the PA (U.S. EPA, 2011, pp. 4-55 - 4-56, Appendix G, section G.4). 

8 These analyses showed good correlation between 24-hour and 4-hour average PM2.5 light 

9 extinction, as evidenced by reasonably high city-specific and pooled R-squared values, generally 

10 in the range of over 0.6 to over 0.8. The Administrator considered and agreed with the PA 

11 conclusion that at a 24-hour averaging time would be a reasonable and appropriate surrogate for 

12 a sub-daily averaging time. 

13 With regard to form of the index, the Administrator concluded that a multi-year 

14 percentile-based form offered greater stability to the air quality management process by reducing 

15 the possibility that statistically unusual indicator values would lead to transient violations in the 

16 standard. She noted that a three-year average form provided stability from the occasional effects 

17 of inter-annual meteorological variability that can result in unusually high pollution levels for a 

particular year (U.S. EPA, 2011, p. 4-58). In the UFVA, 90th, 95th, and 98th 18 percentile forms were 

19 assessed for alternative PM light extinction standards (U.S. EPA, 2010b, chapter 4). In 

20 considering these alternative percentiles, the PA noted that the Regional Haze Program targets 

21 the 20 percent most impaired days for improvements in visual air quality in Federal Class I areas. 

A focus on improving the 20 percent most impaired days suggests that the 90th 22 percentile, which 

23 represents the median of the distribution of the 20 percent worst days, would be an appropriate 

24 form to consider. Strategies that are implemented so that 90 percent of days would have visual 

25 air quality that is at or below the level of the standard would reasonably be expected to lead to 

26 improvements in visual air quality for the 20 percent most impaired days. Given that the 

27 preference studies did not provide information with regard to the frequency of time that visibility 

28 levels should be below these values, the PA found no basis to conclude that it would be 

29 appropriate to consider limiting the occurrence of days with peak PM-related light extinction in 

30 urban areas to a greater degree (U.S. EPA, 2011, p. 4-59). Based on the above considerations, the 

Administrator concluded that a 90th 31 percentile form was the most appropriate form (78 FR 3226, 

32 January 15, 2013). 

33 With regard to level of the index, the Administrator considered the visibility preference 

34 studies conducted in four urban areas (U.S. EPA, 2011, p. 4-61) and further quantitative analyses 

35 of visibility conditions for 15 urban study areas (U.S. EPA, 2011, Appendix G, Tables G-7 and 
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G-8). Based on these studies the PA identified a range of levels from 20 to 30 deciviews (dv)60 1 as 

2 being a reasonable range of “candidate protection levels” (CPLs). In considering this range of 

3 CPLs, the Administrator noted the uncertainties and limitations in public preference studies, 

4 including the small number of stated preference studies available; the relatively small number of 

5 study participants and the extent to which the study participants may not be representative of the 

6 broader study area population in some of the studies; and the variations in the specific materials 

7 and methods used in each study. She concluded that the substantial degrees of variability and 

8 uncertainty in the public preference studies should be reflected in a target protection level at the 

9 upper end of the range of CPLs than if the information were more consistent and certain. 

10 Therefore, the Administrator concluded that it was appropriate to set a target level of protection 

11 in terms of a 24-hour PM2.5 visibility index at 30 dv (78 FR 3226-3227, January 15, 2013). 

12 Based on her considerations and conclusions summarized above, the Administrator 

13 concluded that the protection provided by a secondary standard defined in terms of a PM2.5

visibility index with a 24-hour averaging time, a 90th 14 percentile form averaged over 3 years, and 

15 a level of 30 dv, would be requisite to protect public welfare with regard to visual air quality (78 

16 FR 3227, January 15, 2013). Having reached this conclusion, she next determined whether to 

17 adopt such a visibility index as a distinct secondary standard, particularly in the context of the 

18 full suite of existing secondary standards. 

19 In determining whether a distinct secondary standard was needed, the Administrator 

20 considered the degree of protection from visibility impairment afforded by the existing 

21 secondary standards. She considered both whether the existing 24-hour PM2.5 standard of 35 

μg/m3 22 is sufficient (i.e., not under-protective) and whether it is not more stringent than necessary 

23 (i.e., not over-protective). In doing so, she noted that the air quality analyses showed that all 

areas meeting the existing 24-hour PM2.5 standard, with its level of 35 μg/m3 24 , had visual air 

25 quality at least as good as 30 dv (based on the visibility index defined above) (Kelly et al., 

26 2012a, 2012b). Thus, the secondary 24-hour PM2.5 standard would likely be controlling relative 

27 to a 24-hour visibility index set at a level of 30 dv. Additionally, areas would be unlikely to 

28 exceed the target level of protection for visibility of 30 dv without also exceeding the existing 

secondary 24-hour standard.61 29 Thus, the Administrator judged that the 24-hour PM2.5 standard 

 

60 Deciview 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. 61 The Administrator also recognized that air quality analyses indicated that “the 24-hour PM2.5 standard of 35 g/m3

also would likely achieve more than the target level of protection of visual air quality (30 dv) in some areas” (78 FR 

3227, January 15, 2013). 
April 2016 2-25 Draft - Do Not Quote or Cite

1 “provides sufficient protection in all areas against the effects of visibility impairment—i.e., that 

2 the existing 24-hour PM2.5 standard would provide at least the target level of protection for 

3 visual air quality of 30 dv which the Administrator judges appropriate” (78 FR 3227, January 15, 

4 2013). She further judged that “[s]ince sufficient protection from visibility impairment would be 

5 provided for all areas of the country without adoption of a distinct secondary standard, and 

6 adoption of a distinct secondary standard will not change the degree of over-protection provided 

7 for some areas of the country...adoption of such a distinct secondary standard is not needed to 

8 provide requisite protection for both visibility and nonvisibility related welfare effects” (78 FR 

9 3228, January 15, 2013). 

10 2.2.3 General Approach in the Current Review of the Secondary Standards 

11 The approach for this review builds on the substantial body of work completed during the 

12 course of the last review, taking into account the more recent scientific information and air 

13 quality data now available to inform our understanding of the key-policy relevant issues. The 

14 approach described below is most fundamentally based on using the EPA’s assessment of the 

15 current scientific evidence and associated quantitative analyses to inform the Administrator’s 

16 judgments regarding secondary standards for PM that are requisite to protect the public welfare 

17 from any known or anticipated adverse effects. 

18 Figure 2-2 summarizes the general approach, including consideration of the policy19 relevant questions which will frame the current review. The ISA, WREA (if warranted), and PA 

20 developed in this review will provide the basis for addressing the key policy-relevant questions 

21 and will inform the Administrator’s judgments as to the adequacy of the current secondary PM2.5

22 and PM10 standards, as well as his/her decisions as to whether to retain or revise these standards. 
April 2016 2-26 Draft - Do Not Quote or Cite

1 
April 2016 2-27 Draft - Do Not Quote or Cite

1 Figure 2-2. Overview of General Approach for Review of Secondary PM Standards 

2 The current review of the secondary PM2.5 and PM10 standards will build upon the 

3 conclusions from the last review, taking into account the updated evidence and information that 

4 has become available since that review. Our consideration of the evidence and information will 

5 inform the answer to the following overarching question: 

6  Does the currently available scientific evidence and quantitative information support or 

7 call into question the adequacy of the welfare protection afford by the current 

8 secondary PM2.5 or PM10 standards? 

9 In order to answer this overarching question, we will consider a series of more specific policy10 relevant questions related to the available scientific evidence and information from quantitative 

assessments. Potential policy-relevant questions62 11 include the following: 

12  To what extent has new scientific evidence improved our understanding of the nature and 

13 magnitude of visibility, climate, ecosystem responses, and materials effects to ambient PM, 

14 including the variability associated with such responses? To what extent have important 

15 uncertainties in the evidence from the last review been addressed, and have new uncertainties 

16 emerged? 

17  To what extent is new information available that changes or enhances our understanding of 

18 the physics of light extinction and/or its quantification in urban and non-urban areas (e.g., 

19 through light extinction or other monitoring methods or through algorithms such as 

20 IMPROVE)? 

21  To what extent are new studies available on the nature of the relationship between PM22 attributable visibility impairment and public perceptions of such impairment? What 

23 information is available to inform judgments about the potential adversity to public welfare 

24 of PM-attributable visibility impairment? 

25  Is evidence available from recent studies in additional urban and non-urban 

26 areas, beyond the studies in the previous review? 

27  To what extent is evidence available that distinguishes visibility preferences 

28 from health preferences? 

29  To what extent is evidence available that evaluates the sensitivity of visibility 

30 preferences to survey methods? 

31  To what extent is evidence available that examines the potential relationship 

32 between intensity versus frequency of visibility impairment in stated public 

33 preferences? 

 

62 As noted above (section 2.1), based on CASAC advice and public input on this draft IRP, we will modify these 

potential questions as appropriate for the final IRP. The key policy-relevant questions identified in the final PM IRP 

will provide an overall framework for the development of the PM ISA, REAs (if warranted), and the PA. 
April 2016 2-28 Draft - Do Not Quote or Cite

1  To what extent is new information available that changes or enhances our understanding of 

2 the climate impacts of PM-related aerosols, particularly regarding the quantification of 

3 anthropogenic aerosol effects on radiative forcing? 

4  To what extent is new information available to link PM (excluding nitrates and sulfates) to 

5 ecological effects? 

6  To what extent is new information available to link PM to materials effects, including 

7 degradation of surfaces, and deterioration of materials such as metal, stone, concrete and 

8 marble? Are there studies linking perceptions of reduced aesthetic appeal of buildings and 

9 other objects to PM or wet deposition of N and S species? 

10  Does the available evidence and/or quantitative analyses suggest that PM-induced visibility 

11 impairment or other PM-related welfare effects could occur with ambient concentrations of 

12 PM that meet the current standards? If so, could these effects be of sufficient magnitude 

13 and/or frequency such that they might reasonably be judged to be adverse to public welfare? 

14 To what degree would updated or additional analyses improve our understanding of the 

15 welfare effects that could be allowed by the current standards? 

16  To what extent have important uncertainties identified in the last review been reduced and/or 

17 have new uncertainties emerged? 

18 If the available evidence and information from quantitative analyses call into question the 

19 adequacy of the welfare protection afforded by the current secondary PM2.5 or PM10 standards, 

20 we will also consider the following overarching question: 

21  What alternative standards are supported by the currently available scientific evidence 

22 and quantitative information and are appropriate for consideration? 

23 To answer this second overarching question, we will also consider a series of more 

24 specific questions focused on the basic elements of the NAAQS (indicator, averaging time, form, 

25 level). We will consider these elements collectively in evaluating the public welfare protection 

26 afforded by potential alternative standards. With regard to consideration of alternative standards, 

27 the specific policy-relevant questions will include the following: 

28  Do the available welfare effects evidence and air quality information provide support for 

29 consideration of indicators in addition to, or in place of, the current mass-based indicators? 

30 Do the evidence and information from the quantitative analyses, if warranted, support 

31 alternative indicators based on light extinction, chemical composition, or other factors? 

32  Do the available welfare effects evidence, air quality information, and information from 

33 quantitative assessments, provide support for considering averaging times in addition to, or 

34 in place of, the current 24-hour and annual averaging times? Do the evidence and 

35 information support a sub-daily or other alternative averaging time? 

36  To what extent do air quality analyses or information from quantitative assessments provide 

37 support for considering alternative standard forms? To what extent do assessments support an 

38 alternative form based on daylight hours or other metric? 
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1  What range of levels should be considered, based on the scientific evidence, air quality 

2 analyses, and quantitative assessments? At what concentrations of ambient PM do adverse 

3 visibility impairment and/or other environmental effects of concern for public welfare occur? 

4  What are the important uncertainties and limitations in the available evidence, analyses, and 

5 assessments and how might those uncertainties and limitations be taken into consideration in 

6 identifying alternative standard indicators, averaging times, forms, and/or levels? 

7 2.3 PM AMBIENT MONITORING 

8 Achieving the degree of public health and welfare protection intended for the NAAQS 

9 depends, in large part, on appropriate ambient monitoring networks. In the case of PM, existing 

10 monitoring networks provide data for a variety of objectives as part of an iterative process in 

11 managing air quality. These objectives include: (1) determining compliance with the NAAQS; 

12 (2) characterizing air quality status, including providing the public with timely reports and 

13 forecasts of the Air Quality Index (AQI); (3) supporting air quality analyses used to conduct 

14 assessments of exposure, health risks, and welfare effects; (4) developing and evaluating 

15 emissions control strategies; and (5) measuring trends and overall progress for the air pollution 

16 control program. 

17 Federal rules that regulate ambient monitoring programs are found in 40 CFR parts 50, 

18 53 and 58. The EPA amended these regulations in the 2006 and 2012 reviews of the PM 

19 NAAQS, in part to support changes necessary for implementation of the revised PM standards. 

20 The EPA expects to follow a similar process for monitoring rule changes during this review, if 

appropriate. Potential monitoring rule changes include the Federal Reference Methods (FRMs)63 21 

22 that exist as appendices to part 50, the procedures for approval of Federal Reference and Federal 

23 Equivalent Methods (FEMs) contained in part 53, and the rules applicable to ambient monitoring 

24 network planning and operations that are the basis for part 58 and Appendices A through E. 

25 Section 2.3.1 below provides an overview of the PM monitoring networks and their 

26 history. Section 2.3.2 summarizes the key potential monitoring-related issues in the current 

27 review. 

 

63 FRMs provide the methodological basis for comparison to the NAAQS and also serve as the “gold-standard” for 

the comparison of other methods being reviewed for potential approval as equivalent methods. The EPA keeps a 

complete list of designated reference and equivalent methods available on its Ambient Monitoring Technology 

Information Center (AMTIC) website (http://www3.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalentmethods-list.pdf). 
April 2016 2-30 Draft - Do Not Quote or Cite

1 2.3.1 PM Monitoring Networks 

2 The EPA and its partners at state, local, and tribal monitoring agencies manage and 

3 operate the nations’ ambient air monitoring networks. The EPA provides minimum monitoring 

4 requirements for criteria pollutants and related monitoring (e.g., the Chemical Speciation 

5 Network (CSN)) including identification of an FRM for criteria pollutants and guidance 

6 documents to support implementation and operation of the networks. Monitoring agencies carry 

7 out and perform ambient air monitoring in accordance with the EPA’s requirements and 

8 guidance as well as often meeting their own state monitoring needs that may go beyond the 

9 minimum federal requirements. This partnership results in a nationally consistent ambient air 

10 monitoring program that supports the objectives listed above. Data from the ambient air 

11 monitoring networks are available from two national databases. The AirNow database provides 

12 data used in public reporting and forecasting of the AQI, while the Air Quality System (AQS) 

13 database is the EPA’s long-term repository of ambient air monitoring data. 

14 The EPA and states currently operate robust national networks for both PM10 and PM2.5, 

15 as these are the two measurement programs directly supporting NAAQS. PM10 measurements 

16 are based on gravimetric mass, while PM2.5 measurements include gravimetric mass and 

17 chemical speciation. A smaller network of stations is operating and reporting data for PM10-2.5

18 gravimetric mass and a small number of monitors are operated to support special projects, 

19 including pilot studies, for continuous speciation and particle count data. Monitoring networks 

20 and additional monitoring efforts for each of the various PM size fractions and for PM 

composition are discussed below.64 21 All sampler and monitor counts provided below are based on 

22 data submitted to the EPA for calendar year 2014, unless otherwise noted. 

23 Total Suspended Particulates (TSP) Sampling 

24 The EPA first established NAAQS for PM in 1971, based on the original air quality 

25 criteria document (DHEW, 1969). The reference method specified for determining attainment of 

26 the original standards was the high-volume sampler, which collects PM up to a nominal size of 

27 25 to 45 micrometers (μm) (referred to as total suspended particles or TSP). TSP was replaced by 

28 PM10 as the indicator for particles in a 1987 final notice (52 FR 24854, July 1, 1987). However, 

29 TSP sampling remains in operation to provide the aerosol needed for TSP lead sampling as well 

30 as for cases where a state may continue to have state standards for TSP. The size of the TSP 

31 network peaked in the mid-1970s when over 4,300 TSP samplers were in operation. Today, there 

 

64 More information on ambient monitoring networks can be found at http://www3.epa.gov/ttn/amtic/. 
April 2016 2-31 Draft - Do Not Quote or Cite

1 are about 200 TSP samplers still in operation as part of the lead monitoring program; of these, 

2 about 50 also report TSP mass. 

3 PM10 Monitoring 

4 As a result of the 1987 standard for PM10, the EPA and its state and local partners 

5 implemented the first size-selective PM monitoring network in 1990 with the establishment of a 

6 PM10 network consisting of mainly high-volume samplers. The PM10 monitoring network peaked 

7 in size in 1995 with 1,665 stations reporting data. 

8 Approximately 750 PM10 stations with samplers and monitors are currently in operation 

9 to support comparison of the PM10 data to the NAAQS, trends, and reporting and forecasting of 

10 the AQI. Though the current PM10 network is relatively stable, monitoring agencies may 

11 continue divesting of some of the PM10 monitoring stations where concentration levels are low 

12 relative to the NAAQS. 

13 While the PM10 network is national in scope, there are areas of the west such as 

14 California and Arizona with substantially higher PM10 station density than the rest of the 

15 country. In the PM10 mass network, about 320 of the stations operate continuous mass monitors 

16 approved as FEMs and 480 operate FRMs. Thus, about 50 of the PM10 stations have collocation 

17 with both continuous FEMs and FRMs. Almost half of the PM10 stations with FRMs operate on a 

18 sample frequency of one in every sixth day, with about 140 operating every third day and just 

19 over 100 operating every day. 

20 PM2.5 Monitoring 

21 After setting the first PM2.5 NAAQS in 1997, the EPA and states implemented a PM2.5 

22 network consisting of ambient air monitoring sites with mass and/or chemical speciation 

23 measurements. Network operation began in 1999 with nearly 1000 monitoring stations operating 

24 FRMs to provide fine particle mass. The PM2.5 monitoring program remains one of the major 

25 ambient air monitoring programs operated across the country. 

26 There are three main components of the current PM2.5 monitoring program including 

27 FRMs, PM2.5 continuous mass monitors, and Chemical Speciation Network (CSN) samplers. The 

28 FRMs are primarily used for comparison to the NAAQS, but also serve other important purposes 

29 such as developing trends and evaluating the performance of PM2.5 continuous mass monitors. 

30 PM2.5 continuous mass monitors are primarily used to support forecasting and reporting of the 

31 AQI, but are also used for comparison to the NAAQS, where approved as FEMs. The CSN and 

32 related Interagency Monitoring of Protected Visual Environments (IMPROVE) network are used 

33 to provide chemical composition of the aerosol which serve a variety of objectives. This section 

34 provides an overview of each of these components of the PM2.5 monitoring program. 
April 2016 2-32 Draft - Do Not Quote or Cite

1 As noted above, the PM2.5 monitoring network began operation in 1999 with nearly 1,000 

2 monitoring stations operating FRMs. The PM2.5 FRM network peaked in operation in 2001 with 

3 over 1,150 monitoring stations. In the current PM2.5 network, there are approximately 773 FRM 

4 filter-based samplers that provide 24-hour PM2.5 mass concentration data. Of these operating 

5 FRMs, 111 are providing daily PM2.5 data, 535 every third day, and 125 every sixth day. Close to 

6 800 continuous PM2.5 mass monitors provide hourly data on a near real-time basis. About 363 of 

7 the PM2.5 continuous monitors are FEMs and therefore used for comparison with the NAAQS 

8 and AQI, and another 424 are methods not approved as FEMs and therefore used just for the 

9 AQI. 

10 Due to the complex nature of fine particles, in 2000 the EPA and states implemented the 

11 CSN to better understand the components of fine particle mass at selected locations across the 

12 country. The CSN was first piloted at 13 sites. After the pilot phase, the program continued with 

13 deployment of the Speciation Trends Network (STN) in the fall of 2000. The CSN ultimately 

14 grew to 54 trends sites and peaked in operation in 2005 with 252 stations; the 54 trends stations 

15 and nearly 200 supplemental stations. The original CSN program had multiple sampler 

16 configurations including the Thermo Andersen RAAS, Met One SASS/SuperSASS, and URG 

17 MASS. During the 2000s, the EPA and states worked to align the network to one common 

18 sampler for elements and ions, which was the Met One SASS/SuperSASS. In 2005, CASAC 

19 provided recommendations to the EPA for making changes to the CSN. These changes were 

20 intended to improve data comparability with the rural IMPROVE carbon concentration data. To 

21 accomplish this, the EPA replaced the existing carbon channel sampling and analysis methods 

22 with a new modified IMPROVE version III module C sampler, the URG 3000N. Implementation 

23 of the new carbon sampler and analysis was broken into three phases starting in May of 2007 

24 through October of 2009. 

25 In the current PM2.5 CSN, long-term measurements are made at about 90 largely urban 

locations comprised of either the STN or the National Core (NCore) network.65 26 NCore is a 

27 multipollutant network measuring particles, gases, and basic meteorology that has been in formal 

28 operation since January 1, 2011. Particle measurements made at NCore include PM2.5 filter-based 

29 mass, which is largely the FRM, except in some rural locations which utilize the IMPROVE 

30 program PM2.5 mass filter-based measurement; PM2.5 speciation using either the CSN program or 

31 IMPROVE program; and PM10-2.5 mass utilizing an FRM, FEM or IMPROVE for some of the 

 

65 In most cases where a city has an STN station, it is located at the same site as the NCore station. In a few cases, a 

city may have an STN station located at a different location than the NCore station. 
April 2016 2-33 Draft - Do Not Quote or Cite

1 rural locations. The NCore network includes a total of 78 stations of which 63 are in urban or 

2 suburban stations designed to provide representative population exposure and another 15 rural 

3 stations designed to provide background and transport information. The NCore network is 

4 deployed in all 50 States, DC, and Puerto Rico with at least one station in each state and two or 

5 more stations in larger population states. Since 2008, the EPA’s Office of Research and 

6 Development has approved ten models of PM2.5 continuous monitors as FEMs; about 75% of the 

7 reporting PM2.5 continuous FEMs in the country are the Met One BAM 1020. 

8 Both the STN and NCore networks are intended to remain in operation indefinitely. The 

9 CSN measurements at NCore and STN stations operate every third day. Another approximately 

10 70 CSN stations are known as supplemental sites are intended to be potentially less permanent 

11 locations used to support State Implementation Plan (SIP) development and other monitoring 

objectives.66 12 Supplemental CSN stations typically operate every sixth day. In 2014, about 40 

13 supplemental CSN stations that are largely located in the eastern half of the country stopped 

14 operations to ensure a sustainable CSN network moving forward. Specific components of fine 

particles are also measured through the IMPROVE monitoring program67 15 which supports 

16 regional haze characterization and tracks changes in visibility in Class I areas as well as many 

17 other rural and some urban areas. The base IMPROVE network includes 110 monitoring 

18 locations and. another approximately 20 locations are operated as IMPROVE protocol sites 

19 where a monitoring agency has requested participation in the program. These IMPROVE 

20 protocol sites are operated the same way as the IMPROVE program, but they may serve a 

21 number of monitoring objectives (i.e., the same objectives as the CSN) and are not explicitly tied 

22 to the Regional Haze Program. Samplers at IMPROVE stations operate every third day. In 

23 January of 2016, 8 IMPROVE protocol stations stopped operating to ensure a sustainable 

24 IMPROVE program moving forward. Details on the process and outcomes of the CSN 

25 supplemental and IMPROVE protocol assessments used to identify sites that would no longer be 

 

66 See http://www.epa.gov/ttn/amtic/speciepg.html for more information on the PM2.5 speciation monitoring 

program. 

67Recognizing the importance of visual air quality, Congress included legislation in the 1977 Clean Air Act to 

prevent future and remedy existing visibility impairment in Class I areas. To aid the implementation of this 

legislation, the IMPROVE program was initiated in 1985 and substantially expanded in 2000-2003. This program 

implemented an extensive long term monitoring program to establish the current visibility conditions, track changes 

in visibility and determine causal mechanism for the visibility impairment in the National Parks and Wilderness 

Areas. For more information see http://www.epa.gov/ttn/amtic/visdata.html. 
April 2016 2-34 Draft - Do Not Quote or Cite

funded are available on an interactive website68 1 . Together, the CSN and IMPROVE data provide 

2 chemical species information for fine particles that are critical for use in health and 

epidemiologic studies to help inform reviews of the PM NAAQS.69 3 

4 Key changes made to the EPA’s monitoring requirements as a result of the 2012 PM 

5 NAAQS review included the addition of PM2.5 monitoring at near road locations in CBSAs over 

6 1 million in population; the clarification of terms used in siting of PM2.5 monitors and their 

7 applicability to the NAAQS; and providing flexibility on data uses to monitoring agencies where 

8 their PM2.5 continuous monitors are not providing data that meets the performance criteria used 

9 to approve the continuous method as an FEM. The addition of PM2.5 monitoring at near road 

10 locations is being phased in 2015 and 2017. By January 1, 2015, 22 CBSAs with a population of 

11 2.5 million or more were to have a PM2.5 FRM or FEM operating at a near-road monitoring 

12 station. By January 1, 2017, 30 CBSAs with a population between 1 million and 2.5 million are 

13 to have a PM2.5 FRM or FEM operating are a near-road monitoring station. The terms clarified in 

14 the 2012 rule ensure consistency with other NAAQS and long standing definitions used by the 

15 EPA (78 FR 3234, January 15, 2013). The flexibility provided to monitoring agencies such that 

16 data from certain PM2.5 continuous FEM monitors are not applicable to the NAAQS, where 

17 appropriate, ensures that the incentives of utilizing PM2.5 continuous monitors (e.g., efficiencies 

18 in operation and availability of hourly data in near-real time) are realized without having 

19 potentially poor performing data be misused in a NAAQS decision (78 FR 3241, January 15, 

2013).70 20 

21 PM10-2.5 Monitoring 

22 In the 2006 PM NAAQS review, the EPA promulgated a new FRM for the measurement 

23 of PM10-2.5 mass in ambient air. Although the standard for thoracic coarse particles uses a PM10 

 

68 See the Chemical Speciation Network Assessment Interactive Website at: 

https://www.sdas.battelle.org/CSNAssessment/html/Default.html 

69 These data could also be used to better understand visibility through calculation of light extinction using the 

IMPROVE algorithm. 

70 Although not explicitly required under any existing monitoring regulations, the EPA and state and local agencies 

have also been working together to pilot additional PM methods at near-road monitoring stations that may be of 

interest to data users. These methods include such techniques as particle counters, particle size distribution, and 

black carbon by aethalometer. These methods and their rationale for use at near-road monitoring stations are 

described in a Technical Assistance Document (TAD) on NO2 near road monitoring (Near-road NO2 Monitoring 

Technical Assistance Document, EPA-454/B-12-002, June 2012). 
April 2016 2-35 Draft - Do Not Quote or Cite

1 indicator, a new FRM for PM10-2.5 mass was developed to provide a basis for approving FEMs 

2 and to promote the gathering of scientific data to support future reviews of the PM NAAQS. The 

3 PM10-2.5 FRM (or approved FEMs, where available) was implemented at required NCore stations 

4 by January 1, 2011. In addition to NCore, there are other collocated PM10 and PM2.5 low-volume 

5 FRMs operating across the country that are essentially providing the PM10-2.5 FRM measurement 

6 by the difference method. 

7 PM10-2.5 measurements are currently performed across the country at NCore stations, 

8 IMPROVE monitoring stations, and at a few additional locations where state or local agencies 

9 choose to operate the method. For urban NCore stations and other State and Local Air 

10 Monitoring Stations (SLAMS) the method employed is either a PM10-2.5 FRM, which is 

11 performed using a low-volume PM10 FRM collocated with a low volume PM2.5 FRM of the same 

12 make and model, or FEMs for PM10-2.5, including filter-based dichotomous methods and 

13 continuous methods of which several makes and models are approved. Filter-based PM10-2.5

14 measurements at NCore (i.e., the FRM or dichotomous filter-based FEM) operate every third 

15 day, while continuous methods have data available every hour of every day. PM10-2.5 filter-based 

16 methods at other SLAMS typically operate every third or sixth day. For IMPROVE, which is 

17 largely a rural network, PM10-2.5 measurements are made with two sample channels; one each for 

18 PM10 and the other PM2.5. All IMPROVE program samplers operate every third day. All together 

19 there approximately 250 stations where PM10-2.5 data are being reported to the EPA. 

20 There is no operating chemical speciation network for characterizing the specific 

21 components of thoracic coarse particles. Washington University at St. Louis recently reported on 

22 a coarse particle speciation pilot study with several objectives aimed at addressing this issue, 

23 such as evaluating a coarse particle species analyte list and evaluating sampling and analytical 

methods.71 24 The coarse particle speciation pilot study should provide useful information for any 

25 organization wishing to pursue coarse particle speciation. 

26 Additional PM Metrics 

27 There are additional PM measurement metrics made at a much smaller number of 

28 stations. These measurements may be associated with special projects or are complementary 

29 measurements to other networks where the monitoring agency has prioritized having the

 

71 Pilot Study on Coarse PM Monitoring, EPA-454/R-15-001, February 2015. 

http://www3.epa.gov/ttn/amtic/files/pm10pilot/PMc_ReportEPA454R-15-001.pdf
April 2016 2-36 Draft - Do Not Quote or Cite

1 measurements. None of these measurements are required by regulation. They include PM 

2 measurements such as particle counts, continuous carbon, and continuous sulfate. 

3 Particle count measurements are being implemented at several near-road monitoring 

4 stations as part of the recent deployment of the near-road monitoring program, but may be used 

5 at other locations. Particle counts are one of several measurements identified as being a 

6 secondary priority for multipollutant monitoring at near-road monitoring stations. Details on 

7 priorities for multipollutant monitoring at near-road monitoring stations can be found in Section 

16 of the Near-road NO2 Monitoring Technical Assistance Document (TAD)72 8 .

9 Aethalometer data has been measured and submitted to AQS for many years. Data uses 

10 include characterizing black carbon and wood smoke. Ambient air monitoring stations that may 

11 have aethalometers include some of the near-road monitoring stations and National Air Toxics 

12 Trends Stations (NATTS). About 40 monitoring sites across the county are reporting data from 

13 aethalometers. While aethalometer data is available at high time resolutions, it is typically 

14 reported to the AQS data base in one hour periods. 

15 Continuous elemental and organic carbon data are monitored at select locations 

16 participating in a pilot of the Sunset EC/OC analyzer as well as a few additional sites that were 

17 already operating before the EPA initiated the pilot study. The Sunset EC/OC analyzer provides 

18 high time resolution carbon data, typically every hour, but in some remote locations the 

19 instrument is programmed to run every 2 hours to ensure collection of enough aerosol. The data 

20 from the Sunset is being compared to filter-based carbon methods from the carbon channel of the 

21 CSN program. The six sites participating in the study are Washington DC, Chicago IL, St. Louis 

22 MO, Houston TX, Las Vegas NV, and Los Angeles CA. Each of the six sites participating in the 

23 pilot study are operating for at least three years. 

24 Continuous sulfate continues to be measured at four locations. One station each in North 

25 Carolina, Tennessee, Indiana, and Maine are reporting data. The continuous sulfate analyzer 

26 provides hourly data and data can be readily compared to 24-hour sulfate data collected from the 

27 ion channel in the CSN program. 

28 In addition, over the last few years, the EPA has investigated the use of a number of PM 

29 sensor technologies as one of several areas of research intended to address the next generation of 

30 air measurements. The investigation into air sensors is envisioned to work towards near real-time 

31 or continuous measurement options that are smaller, cheaper, and more portable than traditional 

 

72 Near-road NO2 Monitoring Technical Assistance Document, Publication No. EPA-454/B-12-002, June 2012. 

Available on EPA’s Web site at: http://www3.epa.gov/ttn/amtic/files/nearroad/NearRoadTAD.pdf 
April 2016 2-37 Draft - Do Not Quote or Cite

1 FRM or FEM methods. These sensor devices have the potential to be used in a number of 

2 applications such as identifying hot spots, informing network design, providing personal 

3 exposure monitoring, supporting risk assessments, and providing background concentration data 

4 for permitting. The EPA has hosted workshops and published a number of documents and peer5 reviewed articles on this work (See the EPA’s web site at: http://www2.epa.gov/air6 research/next-generation-air-measuring-research). 

7 2.3.2 Consideration Of PM Monitoring Issues In The Current Review 

8 This review of the PM NAAQS will include the consideration of policy-relevant issues 

9 associated with measuring and characterizing PM in ambient air. These issues include the design 

10 of the PM network, the performance of existing FRMs and FEMs, the performance of continuous 

11 monitors, PM chemical speciation, PM monitoring near major roadways, the availability and 

12 performance of low-cost PM sensors, and data reporting requirements. The EPA will draw upon 

13 the information presented in the ISA to inform the evaluation of appropriate ambient monitoring 

14 methods and network design for PM. If there is a need for formal scientific input on important 

15 changes to ambient air monitoring, the EPA may request convening the Ambient Air Monitoring 

16 Subcommittee of the Clean Air Scientific Advisory Committee. This subcommittee has worked 

17 closely with the EPA during past reviews, where appropriate, to provide the scientific review of 

monitoring options under consideration as part of the PM NAAQS review process.73 18 Input and 

19 development of options to improve ambient air monitoring are also based on input from 

20 monitoring agencies and other interested stakeholders. 

 

73 The EPA will draw upon the information presented in the ISA to inform the evaluation of appropriate ambient 

monitoring methods and network design for PM. Input and development of options to improve ambient air 

monitoring are also based on input from monitoring agencies and other interested stakeholders. 
April 2016 2-38 Draft - Do Not Quote or Cite

1 REFERENCES 

2 Abt Associates, Inc. (2001). Assessing Public Opinions on Visibility Impairment due to Air 

3 Pollution: Summary Report. Prepared for EPA Office of Air Quality Planning and 

4 Standards; funded under EPA Contract No. 68-D-98-001. Abt Associates Inc., Bethesda, 

5 MD. January 2001. Available: http://www.epa.gov/ttncaaa1/t1/reports/vis_rpt_final.pdf 

6 Abt Associates, Inc. (2005). Particulate Matter Health Risk Assessment For Selected Urban 

7 Areas. Prepared for EPA Office of Air Quality Planning and Standards; funded under 

8 EPA Contract No. 68-D-03-002. Abt Associates Inc., Bethesda, MD. December 2005. 

9 Available: http://www3.epa.gov/ttn/naaqs/standards/pm/data/PMrisk20051220.pdf 

10 BBC Research & Consulting (2003). Phoenix Area Visibility Survey. Draft Report. Available: 

11 http://www.azdeq.gov/environ/air/download/vis_021903f.pdf. 

12 Ely, D.W., J.T. Leary, T.R. Stewart, and D.M. Ross (1991). The Establishment of the Denver 

13 Visibility Standard. Presented at the 84th Annual Meeting & Exhibition of the Air and 

14 Waste Management Association. June 16-21. 

15 Pryor, S.C. (1996). Assessing public perception of visibility for standard setting exercises. 

16 Atmospheric Environment 30(15):2705-2716. 

17 Samet J (2009). Letter from Dr. Jonathan M. Samet, Chair, Clean Air Scientific Advisory 

18 Committee to the Honorable Lisa P. Jackson, Administrator, U.S. EPA. CASAC Review 

19 of EPA’s Integrated Science Assessment for Particulate Matter –Second External Review 

20 Draft (July 2009). November 24, 2009. Docket ID number. EPA-HQ-ORD-2007-0517-

21 0121. Available: 

22 http://yosemite.epa.gov/sab/sabproduct.nsf/264cb1227d55e02c85257402007446a4/151B

23 1F83B023145585257678006836B9/$File/EPA-CASAC-10-001-unsigned.pdf. 

24 Samet J (2010a). Letter from Dr. Jonathan M. Samet, Chair, Clean Air Scientific Advisory 

25 Committee to the Honorable Lisa P. Jackson, Administrator, U.S. EPA. CASAC Review 

26 of Policy Assessment for the Review of the PM NAAQS – First External Review Draft 

27 (March 2010). May 17, 2010. Docket ID number EPA-HQ-OAR-2007-0492-0113. 

28 Available: 

29 http://yosemite.epa.gov/sab/sabproduct.nsf/264cb1227d55e02c85257402007446a4/E504

30 EE3276D87A9E8525772700647AFB/$File/EPA-CASAC-10-011-unsigned.pdf. 

31 Samet J (2010b). Letter from Dr. Jonathan M. Samet, Chair, Clean Air Scientific Advisory 

32 Committee to the Honorable Lisa P. Jackson, Administrator, U.S. EPA. CASAC Review 

33 of Policy Assessment for the Review of the PM NAAQS – Second External Review Draft 

34 (June 2010). September 10, 2010. Docket ID number EPA-HQ-OAR-2007-0492-0256. 

35 Available: 

36 http://yosemite.epa.gov/sab/sabproduct.nsf/264cb1227d55e02c85257402007446a4/CCF9

37 F4C0500C500F8525779D0073C593/$File/EPA-CASAC-10-015-unsigned.pdf. 

38 U.S. EPA (2005). Review of the National Ambient Air Quality Standards for Particulate Matter: 

39 Policy Assessment of Scientific and Technical Information, OAQPS Staff Paper. 

40 Research Triangle Park, NC 27711: Office of Air Quality Planning and Standards. Report 
April 2016 2-39 Draft - Do Not Quote or Cite

1 No. EPA-452/R-05-005a. December 2005. Available: 

2 http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_sp.html. 

3 U.S. EPA (2009). Integrated Science Assessment for Particulate Matter: Final Report. National 

4 Center for Environmental Assessment-RTP Division, Office of Research and 

5 Development, Research Triangle Park, NC. EPA/600/R-08/139F. December 2009. 

6 Available: http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_isa.html. 

7 U.S. EPA (2010a). Quantitative Health Risk Assessment for Particulate Matter – Final Report. 

8 Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, 

9 Research Triangle Park, NC. EPA-452/R-10-005. June 2010. Available: 

10 http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html. 

11 U.S. EPA (2010b). Particulate Matter Urban-Focused Visibility Assessment – Final Report. 

12 Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, 

13 Research Triangle Park, NC. EPA-452/R-10-004. July 2010. Available: 

14 http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html. 

15 U.S. EPA (2011). Policy Assessment for the Review of the Particulate Matter National Ambient 

16 Air Quality Standards. Office of Air Quality Planning and Standards, U.S. Environmental 

17 Protection Agency, Research Triangle Park, NC. EPA 452/R-11-003. April 2011. 

18 Available: http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_pa.html. 

19 U.S. EPA (2012). Responses to Significant Comments on the 2012 Proposed Rule on the 

20 National Ambient Air Quality Standards for Particulate Matter (June 29, 2012; 77 FR 

21 38890). Available: http://www3.epa.gov/ttn/naaqs/standards/pm/data/20121214rtc.pdf

22 Zanobetti A; Schwartz J (2009). The effect of fine and coarse particulate air pollution on 

23 mortality: A national analysis. Environ Health Perspect, 117: 898-903. 
April 2016 3-1 Draft – Do Quote or Cite

1 3 SCIENCE ASSESSMENT 

2 Integrated Science Assessments (ISAs) serve as the scientific foundation of the NAAQS 

3 review process and are developed by the EPA’s National Center for Environmental Assessment 

4 in Research Triangle Park (NCEA-RTP). As outlined in sections 1.2 and 1.4 above, the 

5 particulate matter (PM) ISA will inform the review of both the primary (health-based) and 

6 secondary (welfare-based) PM standards. 

7 3.1 SCOPE OF THE PM ISA 

8 3.1.1 General Description 

9 The ISA provides a comprehensive synthesis and evaluation of the most policy-relevant 

10 science, including key science judgments that are important to inform the development of the 

11 risk and exposure assessments, as well as other aspects of the NAAQS review (U.S. EPA, 2016). 

12 The shift to the ISA away from the Air Quality Criteria Documents, which originally provided 

13 the scientific basis of the NAAQS review process, was initiated in 2006. This change was rooted 

14 in the “broad recognition that the Criteria Document is typically ‘encyclopedic’ in nature, which 

15 is seen by many as contributing to an unnecessarily lengthy process for preparing document 

16 drafts and for reviews by CASAC [(Clean Air Scientific Advisory Committee)] and the public, 

17 and obscuring a focus on the most policy-relevant scientific information” (U.S. EPA, 2006). 

18 CASAC provided positive feedback on this transition to the ISA, encouraging “the development 

19 of a more timely and more concise integrated assessment of the policy-relevant science that 

20 would replace the voluminous air quality criteria document”(Peacock, 2008). The purpose of the 

21 PM ISA is to provide a critical evaluation and synthesis of the current scientific literature on 

22 health and welfare evidence necessary to support the PM NAAQS review process. 

23 The PM ISA is not intended to provide a detailed review of all studies of the 

24 aforementioned topics but rather, will draw from the available evidence to synthesize the current 

25 state of knowledge of the most policy-relevant issues for the review of the primary and 

26 secondary PM NAAQS. The PM ISA will update the scientific assessment upon which the last 

27 PM NAAQS review was based. Thus, the PM ISA will build on the conclusions of the last 

28 review of the air quality criteria for PM as presented in the 2009 PM ISA and focus on peerreviewed literature published since that document,74 29 as well as on any new interpretations of 

 

74 The 2009 PM ISA included studies through May 2009. 
April 2016 3-2 Draft – Do Quote or Cite

1 previously available literature. Key findings, conclusions, and uncertainties from the 2009 PM 

2 ISA will be briefly summarized at the beginning of the PM ISA and at the beginning of 

3 individual sections. Important older studies may be discussed in detail to reinforce key concepts 

4 and conclusions and/or if they are open to reinterpretation in light of newer data. Older studies 

5 also may be the primary focus in some subject areas or scientific disciplines (e.g., epidemiology, 

6 controlled human exposure, animal toxicology, atmospheric science, exposure science, visibility 

7 impairment, climate, materials effects, and ecological effects) where research efforts have 

8 subsided, and these older studies remain the definitive works available in the literature. 

9 In order to provide a more focused evaluation of the scientific evidence for health- and 

10 welfare-related effects, the PM ISA will discuss the most important topics that address policy11 relevant questions. Therefore, the PM ISA will more fully evaluate those health and welfare 

12 effects for which the evidence in the 2009 PM ISA was less certain (i.e., effects where the causal 

13 determination was “likely to be causal”, “suggestive”, or “inadequate” as detailed below in 

14 section 3.4.3) and where there is now a larger body of evidence (e.g., diabetes, nervous system 

15 effects, etc.). For those health and welfare effects where the 2009 PM ISA concluded that the 

16 evidence was sufficient to infer a causal relationship (i.e., health: short- and long-term PM2.5

17 exposures and cardiovascular effects; short- and long-term PM2.5 exposures and mortality; and 

18 welfare: PM exposures and effects on materials, visibility, and climate), the PM ISA will focus 

19 the evaluation on characterizing the extent to which new studies address key uncertainties and 

20 limitations identified in the previous review or provide insight on new issues. For the 

21 epidemiologic studies the focus of the evaluation will include, but not be limited to, addressing 

22 the impact of exposure assessment techniques on associations observed; evaluating potential 

23 copollutant confounding; assessing the impact of PM components and sources on associations 

24 observed; and examining heterogeneity in PM associations. Those epidemiologic studies that do 

25 not directly address uncertainties and limitations as identified in the 2009 PM ISA, but still meet 

26 our inclusion criteria will be detailed in tables presenting information for consideration in 

27 evaluating the adequacy of the current PM NAAQS (e.g., averaging time of exposure, 

28 distribution of PM concentrations, etc.). For experimental studies, specifically toxicological 

29 studies, the evaluation will focus on those studies that also address key uncertainties and 

30 limitations in the evidence identified in the previous review. For example, does the new evidence 

31 further our understanding of the biological mechanisms by which PM elicits a health effect or 

32 provide coherence for the effects observed in epidemiologic studies? Similar to the 

33 epidemiologic studies, a more detailed description of key pieces of information from the 

34 toxicological studies will be detailed in tables. 
April 2016 3-3 Draft – Do Quote or Cite

1 Consistent with the goal of making the PM ISA a focused assessment of the current state 

2 of the science with respect to PM, this ISA incorporates CASAC advice received during the 

review of External Review Drafts of the 2009 PM ISA.75 3 For example, CASAC provided 

4 specific advice about the evaluation of PM10. Although the indicator for coarse particles is 

PM10,

76 5 studies that examine the health or welfare effects of exposures to only PM10 are limited 

6 in their ability to inform the health and welfare effects of PM10-2.5 or PM2.5. As a result, CASAC 

7 suggested revisions to the PM ISA to “remove the impression that PM10 is a separate pollutant 

8 from PM2.5 and PM10-2.5”, specifically the removal of causal determinations for short- and long9 term exposures to PM10 (Samet, 2009) Additionally, CASAC suggested detailing “when 

10 possible, the particle size distribution of the PM10 mixture” (Samet, 2009). As such, the final 

11 2009 PM ISA had minimal discussion of PM10 health effect studies, and these studies were used 

12 as supporting evidence of PM2.5-related health effects primarily due to the majority of PM10

13 studies being conducted in large urban areas where, in many locations, ambient monitoring 

14 typically indicates that PM2.5 comprises a large percent of PM10 mass. Therefore, consistent with 

15 the previous CASAC panel’s recommendation, the current PM ISA will focus on the evaluation 

16 of health studies of PM2.5 and PM10-2.5 as the most pertinent to addressing the key policy-relevant 

17 questions of the PM NAAQS review. This is also consistent with the 2012 PM Final Rule, which 

18 stated that “... the extent to which PM10 effect estimates reflect associations with PM10-2.5 versus 

19 PM2.5 can be highly uncertain [and, as a result]... it is preferable to consider PM10-2.5 studies 

20 when such studies are available” (78 FR 3086). Therefore, the evaluation of PM10 studies will be 

21 limited to those that specifically address remaining uncertainties or limitations in the PM2.5 or 

22 PM10-2.5 health or welfare effects evidence or evaluate a new health or welfare effect not 

23 previously examined. 

24 The general scope presented above will allow the PM ISA to primarily focus on scientific 

25 evaluations that are pertinent to the key policy-relevant questions described in Chapter 2 

26 (sections 2.1 and 2.2) above. Within the discussion of the health and welfare effects information, 

27 other scientific information will also be presented and evaluated in order to provide a better 

28 understanding of the following issues: (1) the sources of PM in the ambient air; (2) fate and 

29 transport of PM in the environment; (3) measurement of PM and recent ambient concentrations

 

75 External Review Drafts of the PM ISA are available at: 

http://www3.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_isa.html 76 As discussed above (sections 1.3 and 2.1.2), the purpose of the PM10 standard is to protect against exposures to 

PM10-2.5. 
April 2016 3-4 Draft – Do Quote or Cite

1 of PM; (4) the validity of inferences that can be drawn about PM health and welfare effects 

2 based on exposure assessment methodology; (5) the potential influence of other factors (e.g., 

3 other pollutants in the ambient mixture, ambient temperature) shown to be correlated with PM 

4 and health or welfare effects; (6) the shape of the concentration-response (C-R) relationship at 

5 PM concentrations at the low end of the distribution; and (7) populations and lifestages at 

6 greatest risk of PM-related health effects. The process for evaluating and synthesizing scientific 

7 literature and addressing key policy questions is detailed in the Preamble to the Integrated 

8 Science Assessments(U.S. EPA, 2015). Collectively, the approach outlined for the health and 

9 welfare effects will allow for the EPA to provide a focused assessment of the scientific evidence 

10 that more directly informs policy-relevant considerations detailed in the Policy Assessment (PA) 

11 and Risk and Exposure Assessment(s) (REA) as detailed in Chapter 2. 

12 3.1.2 Defining Policy-Relevant Literature 

13 PM is unique among the criteria pollutants in that it is composed of multiple components 

14 and size fractions. PM is often examined using mass-based metrics for overall mass and the mass 

15 of individual components within the following size fractions: PM10 (thoracic PM; particulate 

16 matter with a nominal mean aerodynamic diameter less than or equal to 10 μm), PM2.5 (fine PM; 

17 particulate matter with a nominal mean aerodynamic diameter less than or equal to 2.5 μm), and 

18 PM10-2.5 (thoracic coarse or coarse PM; particulate matter with a nominal mean aerodynamic 

19 diameter greater than 2.5 μm and less than or equal to 10 μm). Ultrafine particles (UFP, 

20 generally considered as particulates with a diameter less than or equal to 0.1 μm [typically based 

21 on physical size, thermal diffusivity or electrical mobility] (U.S. EPA, 2009)) are examined 

22 using multiple indices such as mass, number concentration, and surface area. As discussed in 

23 Chapter 1, the current indicators of the PM NAAQS are mass-based for both fine particles, 

24 represented by PM2.5, and for thoracic coarse particles, represented by PM10. 

25 Given the array of PM size fractions and components examined in studies of atmospheric 

26 chemistry, exposure, health effects, and welfare effects, it is important to clearly define the types 

27 of studies that will be evaluated within the PM ISA to ensure its conclusions are pertinent to the 

28 key questions of the PM NAAQS review, specifically: 

29  Does exposure to PM cause a direct health or welfare effect (i.e., an independent 

30 effect)?

31  Are the current indicators (i.e., PM2.5 for fine particles and PM10 for thoracic 

32 coarse particles), averaging times (e.g., 24-hour average, annual average), and 

33 levels appropriate?

34 When evaluating the broad body ofliterature across scientific disciplines, the EPA 

35 considers whether the studies fall within the scope of the PM ISA (i.e., provide information 
April 2016 3-5 Draft – Do Quote or Cite

1 which can address the above questions). As a result, the focus of the PM ISA with respect to the 

2 health evidence is on studies conducted at concentrations of PM that are relevant to the range of 

human exposures across ambient microenvironments (up to 2 mg/m3 3 PM, which is one to two 

orders of magnitude above ambient concentrations) and (1) include a composite measure of PM77 4 

5 or (2) apply some approach to assess the independent effect of PM when the exposure of interest 

6 is a source-based mixture (e.g., diesel exhaust, gasoline exhaust, wood smoke). However, the 

7 scope of experimental studies included in the PM ISA may be broader when examining modes of 

8 action for PM, and may include in vitro studies, studies examining relative toxicity, and studies 

conducted at concentrations > 2 mg/m3 9 . In the case of (1), if a study focuses on a single 

10 component, group of components, or source, the study must also examine a composite measure 

11 of PM (e.g., mass of PM2.5, PM10-2.5, ultrafine particles [UFP]) to be included in this review. 

12 This requirement ensures that the study is relevant to the scope of the PM ISA and evaluated in 

13 the proper context; specifically, this approach will facilitate a comparison of effects or 

14 associations observed to the current mass-based PM indicator. Case (2) primarily applies to 

15 experimental studies that attempt to disentangle the independent effects of PM from a complex 

16 air pollution mixture of particles, gases, and semi-volatile components. This may be 

17 accomplished by using filtration (e.g., a particle trap) or other approaches to distinguish between 

18 effects due to the mixture and effects due to the particles alone. 

19 With respect to identifying policy-relevant studies for welfare effects (ecological effects, 

20 visibility, materials effects, and climate) PM composition and/or size fraction are important to 

21 consider. For ecological effects, studies that focus on deposited components of PM (including 

22 metals and organics, but not nitrogen (N) and sulfur (S)-containing compounds or their 

transformation products)78 23 at ambient or near-ambient concentrations of PM will be emphasized, 

24 leading to the evaluation of the direct and indirect effects of PM on vegetation, soils, and biota. 

25 Generally, components of PM drive ecosystem responses, although size fraction may play a role 

26 in the direct effects of PM such as deposition to vegetation. As a result, the emphasis will be on 

27 studies of ecological effects detailing non-nutrient (N and S) particle chemistry/composition 

28 (e.g., cations, trace metals, semi-volatile organics), associated size fraction, and magnitude and 

29 rates of wet and dry deposition across the landscape. 

 

77 Composite measures of PM may include mass, volume, surface area, or number concentration. 78 The PM ISA will not evaluate nutrient N and S ecosystem effects, which will be addressed in the concurrent 

review of the secondary NOx and SOx NAAQS for ecological effects, see: 

http://www3.epa.gov/ttn/naaqs/standards/no2so2sec/index.html. 
April 2016 3-6 Draft – Do Quote or Cite

1 For materials effects, visibility, and climate effects, this ISA will build on information 

2 available during the last review describing the role of PM on materials damage and soiling, 

3 visibility impairment, and in radiative forcing resulting in global and regional climate change. 

4 For effects on materials, studies will be included that examine the role of PM, or related N and S 

5 species as discussed in Section 1.4, on materials damage and soiling. Specifically, studies will be 

6 considered that examine gaseous and particle contributions from nitrogen and sulfur along with 

7 other PM components due to the difficulty in determining whether an effect on materials is due 

8 to the particle or gaseous component of NOx and SOx and because the NOx/SOx secondary ISA 

9 focuses only on ecological effects, not other welfare effects. For visibility, studies will be 

10 included which advance our understanding of visual impairment of airborne PM, including 

11 studies of atmospheric chemistry, visibility preference, or other measures of adversity to public 

12 welfare, in urban and rural settings. For climate, the ISA will focus on climate as the welfare 

13 effect as listed in the 1977 Amendment to the Clean Air Act and will not focus on downstream 

14 ecosystem effects, human health effects, or future air quality projections resulting from changes 

15 in climate (Clean Air Act Amendments of 1977). The primary literature base for the evaluation 

16 of the effects of airborne and deposited PM on climate will come from recent national and 

17 international climate assessments such as the National Climate Assessment (Melillo et al., 2014) 

18 and International Panel on Climate Change (IPCC, 2014). Focus will be on studies that inform 

19 the independent role of PM in climate forcing as well as effects on U.S. national and regional 

20 climate. 

21 3.2 ORGANIZATION OF THE PM ISA 

22 The broad organization of the PM ISA will be consistent with that used in the recent 

23 assessments for other criteria pollutants (e.g., ISA for Ozone and Related Photochemical 

24 Oxidants, U.S. EPA, 2013c). The detailed description of the procedures for the assessment of 

25 scientific information within the ISA for each of the criteria air pollutants can be found in the 

26 Preamble to the Integrated Science Assessments (U.S. EPA, 2015). The PM ISA will begin with 

27 a Preface discussing major legal and historical aspects of prior PM NAAQS reviews. An 

28 executive summary will succinctly summarize the conclusions of the ISA. An integrative 

29 synthesis chapter will provide a more detailed summary of the key information for each topic 

30 area, including the causal determinations for relationships between exposure to PM and health 

31 and welfare effects, information describing the extent to which health and welfare effects can be 

32 attributable specifically to PM, evaluation of at-risk lifestages and populations, and other 

33 uncertainties related to the interpretation of scientific information. The integrative synthesis 

34 chapter also will discuss policy-relevant issues such as the exposure averaging times and the lag 
April 2016 3-7 Draft – Do Quote or Cite

1 structure of associations for health effects; the C-R relationships including the overall shape and 

2 whether or not the evidence supports identification of a discernible threshold below which 

3 effects are not likely to occur; and the public health and welfare impact of effects associated with 

4 exposure to PM. Subsequent chapters will be organized by subject area with the health evidence 

5 presented in separate chapters by PM size fraction (i.e., PM2.5, PM10-2.5, and UFPs) and all of the 

6 welfare (i.e., ecological, visibility, materials, and climate) effects evidence discussed in a 

7 separate chapter(s). Each of the chapters will contain an evaluation of results from recent policy8 relevant studies integrated with previous findings (see section 3.4 below for specific issues to be 

9 addressed). Sections for each broad health effect category (e.g., respiratory effects) will conclude 

10 with a causal determination about the relationship with relevant exposures to PM for health 

outcomes that are specific to the size fraction being evaluated (e.g., PM2.5, PM10-2.5, UFPs).79 11 

12 Sections on welfare will conclude with causal determinations for ecological effects, visibility, 

13 materials effects, and climate. The PM ISA will conclude with a chapter that examines studies 

14 for evidence of differential exposure and risk for PM-related health effects to draw conclusions 

15 about potential at-risk lifestages and populations. 

16 The PM ISA may be supplemented with other materials (e.g., tables, figures) if additional 

17 documentation is required to support information contained within the PM ISA. These 

18 supplementary materials may include more detailed and comprehensive coverage of relevant 

19 publications and may accompany the PM ISA or be available in electronic form as output from 

20 the Health and Environmental Research Online (HERO) database developed by the EPA 

(http://hero.epa.gov).80 21 Supplementary information that is available in the HERO database will 

22 be presented as electronic links in the PM ISA. 

23 3.3 ASSESSMENT APPROACH 

24 3.3.1 Introduction 

25 In each NAAQS review, development of the ISA begins with a “Call for Information” 

26 published in the Federal Register. This notice, which for PM was published December 3, 2014 

27 (79 FR 71764), announces the EPA’s initiation of activities in the preparation of an ISA for the 

28 specific NAAQS review and invites the public to assist in this process through the submission of

 

79 Within the evaluation of the evidence for specific size fractions, the ISA will also evaluate the strength of 

evidence for PM components and sources. 

80 For more information on the HERO database, see http://hero.epa.gov/. 
April 2016 3-8 Draft – Do Quote or Cite

1 research studies in the identified subject areas. This and subsequent key components of the 

2 process currently followed for the development of an ISA are presented in Figure 3-1 and are 

3 described in greater detail in the Preamble to the Integrated Science Assessments (U.S. EPA, 

4 2015). Section 1.2 (above) briefly describes how the PM ISA fits into the larger PM NAAQS 

5 review. Important aspects of the development of the PM ISA are described in the sections below, 

6 including the approach for searching the literature, identifying relevant publications, and 

7 informing specific policy-relevant questions that are intended to guide the assessment. These 

8 responsibilities are undertaken by expert authors of the PM ISA chapters, which include the EPA 

9 staff in NCEA-RTP with extensive knowledge in their respective fields and extramural scientists 

10 solicited by the EPA for their expertise in specific fields. The process for scientific and public 

11 review of drafts of the PM ISA is described in section 3.5 below. 

12 
April 2016 3-9 Draft – Do Quote or Cite

1 

2 Source: Modified from Figure II of the Preamble to the Integrated Science Assessments 

3 (U.S. EPA, 2015). 

4 Figure 3-1. General process for development of Integrated Science Assessments. 

Integrated Science Assessment Development Process

Literature Search and

Study Selection

(See Figure III)

Develop Initial Sections Review and summarize conclusions from 

previous assessments and new study results

and findings by discipline and category of 

outcome/effect (e.g., toxicological studies of lung 

function or biogeochemical studies of forests)

Development of Scientific Conclusions and Causal Determinations Characterize weight of evidence and develop judgments regarding causality for health or welfare effect categories. 

Develop conclusions regarding concentration- or dose-response relationships, potentially at-risk populations, 

lifestages, or ecosystems.

Draft Integrated Science Assessment Evaluation and integration of newly published studies

Clean Air Scientific Advisory Committee

Independent review of draft documents for 

scientific quality and sound implementation of 

causal framework during public meetings.

Final Integrated Science Assessment

Evaluation, Synthesis, and Integration of Evidence Integrate evidence from scientific disciplines. Evaluate evidence for related groups of endpoints or outcomes to 

draw conclusions for specific health or welfare effect categories, integrating health or welfare effects evidence with 

information on mode of action and exposure assessment.

Public Comments

Comments on draft ISA solicited by the U.S. EPA

Evaluation of Individual Study Quality After study selection, the quality of individual studies is evaluated by U.S. EPA or outside experts in the fields of 

atmospheric science, exposure assessment, dosimetry, animal toxicology, controlled human exposure, 

epidemiology, biogeochemistry, terrestrial and aquatic ecology, and other welfare effects, considering the design, 

methods, conduct, and documentation of each study. Strengths and limitations of individual studies that may affect 

the interpretation of the study are considered. 

Peer Input Consultation Review of initial draft materials by scientists 

from both outside and within the U.S. EPA in 

public meeting or public teleconference.
April 2016 3-10 Draft – Do Quote or Cite

1 3.3.2 Literature Search and Selection of Relevant Studies 

2 The EPA uses a structured approach to identify relevant studies for consideration and 

3 inclusion in the ISAs. To be deemed relevant, studies must fall within the scope of the PM ISA, 

4 as detailed in Section 3.1. A Federal Register notice is published that announces the initiation of 

5 a review and requests information, including relevant literature, from the public (79 FR 71764). 

6 In addition, the EPA identifies publications by conducting a recursive multi-tiered literature 

7 search process that includes extensive manual and computer-aided citation mining of computer 

8 databases (e.g., PubMed, Web of Science) on specific topics in a variety of disciplines. The 

9 search strategies are designed a priori and iteratively modified to optimize identification of 

10 pertinent published papers. 

11 For this PM ISA, a broad search string will be developed with keywords including, but 

12 not limited to, particulate, PM, PM10, PM2.5, PM10-2.5, and ultrafine particles with the specific 

13 syntax tailored for each database (e.g., PubMed, Web of Science). The broad search string will 

14 then be used in combination with additional search strings to identify references relevant to 

15 health and welfare topic areas. In addition to the use of the broad PM search strings, papers will 

16 be identified for inclusion in several additional ways: specialized searches on specific topics; 

17 relational searches that identify recent publications that have cited references from previous 

18 assessments; identification of relevant literature by expert scientists; recommendations from the 

19 public and CASAC during the call for information and external review process; and review of 

20 citations in previous assessments. These search methods will be used to identify recent research 

21 published or accepted for publication since the 2009 PM ISA, i.e., starting in January 2009, 

22 through approximately two months before the release of the second external review draft of the 

23 PM ISA (see Table 1-3 for target dates). Studies published after the PM ISA cut-off date may 

24 also be considered in subsequent phases of the NAAQS review, after assessing whether they 

25 provide new information that impacts key scientific issues. 

26 Once studies are identified through the multipronged search strategy, PM ISA authors 

27 (EPA staff and extramural scientists) will review the studies for relevance. Relevant to the 

28 review of the primary PM NAAQS are epidemiologic, toxicological, and controlled human 

29 exposure studies or reports that examine health effects in relation to exposure to PM as well as 

30 studies or reports that examine sources, emissions, atmospheric chemistry, human exposure, 

31 dosimetry, and modes of action. For the review of the secondary PM NAAQS relevant studies 

32 are those that examine ecological effects, visibility, effects on materials and climate. Specific 

33 information detailing the scope of the PM ISA, and subsequently those studies that will be 

34 evaluated within it are detailed above in section 3.1.2. 
April 2016 3-11 Draft – Do Quote or Cite

1 To be included in the PM ISA, relevant studies and reports must have undergone scientific 

2 peer review and have been published or accepted for publication. Some publications retrieved 

3 from the literature search will be excluded as not being relevant (e.g., do not meet 

4 aforementioned criteria) based on screening of the title or citation (e.g., not about air pollution, 

5 conference abstract, review articles, commentaries) and will not documented further. For other 

6 publications, decisions about relevance require reading beyond the title. These publications will 

7 be labeled as “considered” for inclusion in the PM ISA and are listed in the HERO database 

8 (http://hero.epa.gov). 

9 From the group of “considered” references, studies and reports will be selected for 

10 inclusion in the PM ISA based on review of the abstract and full text. The selection process will 

11 be based on the extent to which the study is potentially policy-relevant and informative. 

12 Potentially policy-relevant and informative studies will include those that provide a basis for or 

13 describe the relationship between PM and effects, in particular, those studies that reduce 

14 uncertainty or address limitations of critical issues. Also pertinent are studies that offer 

15 innovation in method or design or present novel information on effects or issues previously not 

16 identified. Uncertainty can be addressed, for example, by analyses of potential confounding or 

17 effect modification by copollutants or other factors, analyses of C-R or dose-response 

18 relationships, or analyses related to time between exposure and response. In keeping with the 

19 purpose to accurately reflect the latest scientific knowledge, a majority of the discussion in the 

20 PM ISA will describe studies published since the 2009 PM ISA. However, evidence from 

21 previous studies will be included and integrated with results from recent studies. In some cases, 

22 evidence from previous studies may be the key policy-relevant information in a particular subject 

23 area or scientific discipline. Analyses conducted by the EPA using publicly available data, for 

24 example, air quality and emissions data, will also be considered for inclusion in the PM ISA. 

25 Informative studies will not be limited to specific study designs, model systems, or outcomes. 

26 While study quality is important, it is not the sole criteria for study inclusion. The combination of 

27 approaches described above are intended to produce a comprehensive collection of pertinent 

28 studies needed to address the key scientific issues that form the basis of the PM ISA. References 

29 will be cited in the PM ISA by a hyperlink to the HERO database and also are compiled into 

30 reference lists. 

31 3.3.3 Evaluation of Individual Study Quality 

32 After selecting studies for inclusion, individual study quality is evaluated by considering 

33 the design, methods, conduct, and documentation of each study, but not by considering whether 

34 the study results are positive, negative, or null. In the PM ISA, conclusions about the strength of 
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1 inference from study results will be made by weighing the authors’ conclusions and 

2 independently evaluating study quality as detailed in the Preamble to the Integrated Science 

3 Assessments (U.S. EPA, 2015). This uniform approach aims to consider the strengths, 

4 limitations, and possible roles of chance, confounding, and other biases that may affect the 

5 interpretation of the results from individual studies for both health and welfare. In assessing the 

6 scientific quality of studies, the following broad parameters will be considered: 

7  How clearly were the study design, study groups, methods, data, and results presented to 

8 allow for study evaluation? 

9  To what extent are the air quality data, exposure, or dose metrics of adequate quality to 

10 serve as credible exposure indicators? 

11  Were the study populations, species, subjects, or animal models adequately selected, and 

12 are they sufficiently well-defined to allow for meaningful comparisons between study or 

13 exposure groups? 

14  Are the statistical analyses appropriate, properly performed, and properly interpreted? Do 

15 the analytical methods provide adequate sensitivity and precision to support study 

16 conclusions? 

17  Are likely covariates (i.e., potential confounding factors, modifying factors) adequately 

18 controlled for or taken into account in the study design or statistical analyses? 

19  Are the endpoint measurements meaningful, valid, and reliable? 

20 Additional considerations in evaluating individual study quality specific to particular 

21 scientific disciplines (e.g. epidemiology, toxicology, ecology) are discussed in detail in the 

22 Preamble to the Integrated Science Assessments (U.S. EPA, 2015). 

23 3.4 SPECIFIC ISSUES TO BE ADDRESSED IN THE PM ISA 

24 The PM ISA will contain information relevant to considering whether it is appropriate to 

25 retain or revise the current primary and secondary PM NAAQS. Decisions on the specific 

26 content of the PM ISA will be guided by policy-relevant questions that frame the entire review of 

27 the primary and secondary PM NAAQS as outlined in Chapter 2 above. These policy-relevant 

28 questions are related to two overarching issues. The first overarching issue is whether new 

29 evidence reinforces or calls into question the evidence presented and evaluated in the last PM 

30 NAAQS review: (1) with respect to factors such as the plausibility of health effects caused by 

31 exposure to PM, specifically PM2.5 and PM10-2.5, and concentrations of PM associated with health 

32 effects; and (2) with respect to welfare-related effects of PM. The second overarching issue is 

33 whether uncertainties from the last review have been reduced and/or whether new uncertainties 

34 have emerged, such as whether other measures of PM (e.g., different size fractions or 
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1 components) are better at capturing the health and welfare effects attributed to PM exposures 

2 than those used as the indicators for the current NAAQS. The PM ISA also will address a set of 

3 more specific policy-relevant questions related to the available scientific evidence that stem from 

4 these issues. These questions were derived from the last PM NAAQS review, as well as from 

5 discussions of the scientific evidence that occurred at the February 2015 science policy 

6 workshop which initiated this PM NAAQS review (79 FR 71764, December 3, 2014). The PM 

7 ISA may include supplementary material if additional documentation is required to support 

8 information contained in the PM ISA. As in recently completed ISAs, such supplementary 

9 material will be available in the online HERO database and referenced in the PM ISA as 

10 electronic links to HERO. 

11 3.4.1 Causal Determinations from 2009 PM ISA 

12 In the 2009 PM ISA, the EPA concluded that the findings of epidemiologic, controlled 

13 human exposure, and animal toxicological studies collectively provided evidence of a “causal 

14 relationship” for short- and long-term PM2.5 exposures and cardiovascular effects as well as 

15 mortality (U.S. EPA, 2009, as summarized in Chapter 2). In evaluating a broader range of health 

16 effects for PM2.5, the 2009 PM ISA concluded there was evidence of a “likely to be causal 

17 relationship” for short- and long-term PM2.5 exposures and respiratory effects (U.S. EPA, 2009, 

18 as summarized in Chapter 2). Additionally, there was evidence “suggestive of a causal 

19 relationship” for long-term PM2.5 exposures and other health effects, including developmental 

20 and reproductive effects (e.g., low birth weight, infant mortality) and carcinogenic, mutagenic, 

21 and genotoxic effects (e.g., lung cancer mortality) (U.S. EPA, 2009, as summarized in Chapter 

22 2). The 2009 PM ISA also formed causal determinations for exposures to PM10-2.5 and ultrafine 

23 particles. With respect to PM10-2.5, the 2009 PM ISA concluded that the evidence was 

24 “suggestive of a causal relationship” for short-term exposures and cardiovascular effects, 

25 respiratory effects, and mortality, with the evidence “inadequate to infer a causal relationship” 

26 for long-term PM10-2.5 exposures and all health effects (U.S. EPA, 2009, as summarized in 

27 Chapter 2). For ultrafine particles, the 2009 PM ISA concluded the evidence was “suggestive of 

28 a causal relationship” for short-term exposure and cardiovascular and respiratory effects, but 

29 “inadequate to infer a causal relationship” for short-term exposures and central nervous system 

30 effects and mortality, and for long-term exposure and all health effects (U.S. EPA, 2009, as 

31 summarized in Chapter 2). For welfare effects, the evidence indicated a “causal relationship” for 

32 PM exposures and effects on visibility, climate, and materials; and a “likely to be causal 

33 relationship” for PM exposures and ecological effects (U.S. EPA, 2009, as summarized in 

34 Chapter 2). 
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1 The causal determinations in the 2009 PM ISA, based on the causal framework and 

2 integration of available evidence from previous and recent studies, were presented with a 

3 summary of the available evidence at the end of the sections for each broad health and welfare 

4 effect category and in the integrative synthesis chapter at the beginning of the PM ISA. In the 

5 current review, specific policy-relevant questions related to the causal determinations that will be 

6 addressed include: 

7  Does the evidence base from recent studies contain new information to support or re8 evaluate the causal determinations made for relationships between PM exposure and 

9 various health and welfare effects in the 2009 PM ISA? 

10  Does new evidence confirm or extend biological plausibility of PM-related health 

11 effects? 

12  What is the strength of inference from epidemiologic studies based on the extent to which 

13 they have: 

14 o Examined exposure metrics that capture the spatial and/or temporal pattern of PM in 

15 the study area? 

16 o Assessed potential confounding by other pollutants and factors? 

17  What information is available to support a rationale for forming causal determinations for 

18 PM size fractions other than PM2.5, specific PM components, or PM from specific 

19 sources? 

20  What information is available regarding the health impacts of a decrease in ambient PM 

21 concentrations to inform causal determinations? 

22 3.4.2 Uncertainties/Limitations Identified in 2009 PM ISA 

23 The causal determinations described above for the relationships between PM2.5, PM10-2.5, 

24 and UFPs exposure and health and welfare effects were informed by uncertainties and limitations 

25 in the evidence. For example, the 2009 PM ISA noted a number of uncertainties and limitations 

26 in the health evidence, such as the heterogeneity often observed in multi-city epidemiologic 

27 studies and whether this can be attributed to exposure differences or regional differences in PM 

28 components or sources; and uncertainty related to the use of ambient PM concentrations from 

29 central-site monitors and their ability to represent personal ambient PM exposure (U.S. EPA, 

30 2009, Chapter 2). In each of the health and welfare effects sections, and the integrative synthesis 

31 chapter, the PM ISA will evaluate uncertainties and limitations in the scientific data. These 

32 uncertainties also will inform causal determinations. The PM ISA will evaluate potential 

33 confounding by other ambient pollutants. To assess the independent effects of PM, the PM ISA 

34 will examine whether epidemiologic associations with PM are observed in copollutant models. 

35 Copollutant models are the predominant method used in air pollution epidemiology to estimate 
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1 the effect of one pollutant for a given concentration of a copollutant. The PM ISA also will 

2 evaluate whether PM has interactions with copollutants or joint effects in associations with 

3 health outcomes. The assessment of potential confounding, interactions, or joint effects will draw 

4 upon results from health effects studies, available information on copollutant interactions in the 

5 atmosphere that influence the spatial distributions of PM and copollutants, as well as information 

6 from experimental studies that examine the health effects of PM exposures alone and PM in 

7 combination with other pollutants. In the absence of these aforementioned methods, the PM ISA 

8 will examine whether single-pollutant epidemiologic associations with health effects in a given 

9 study differ between PM and copollutants, and if insights regarding potential copollutant 

10 confounding can be gained by examining the magnitude of correlation between pollutants. 

11 Drawing from discussion about the strengths and limitations of various exposure 

12 assessment methods, the PM ISA will evaluate the strength of inference in epidemiologic studies 

13 by considering information such as the exposure duration being examined, the extent of spatial 

14 and/or temporal variability in PM in the study area, the distribution of monitoring sites in the 

15 study area, the performance of exposure models used, and time-activity patterns of the study 

16 population. Additionally, monitoring data will be used to characterize the spatial distributions in 

17 ambient PM concentrations and human exposures, and in turn, the potential exposure 

18 measurement error in particular study areas based on the particular method of exposure 

19 assessment used. The adequacy of exposure assessment in epidemiologic studies will be 

20 considered in weighing the quality of evidence, and in turn, forming causal determinations. 

21 Epidemiologic evidence is unlikely to completely address the uncertainties mentioned 

22 above. Any individual study is unlikely to evaluate all potentially correlated copollutants, and the 

23 limitations of epidemiologic methods in separating effects of highly correlated pollutants or 

24 separating the effects of more than two pollutants in the same model are well recognized. With 

25 respect to exposure measurement error, few studies with personal ambient exposure 

26 measurements are likely to be available. Thus, coherence with other lines of evidence may 

27 strengthen inferences when there are uncertainties in epidemiologic evidence due to copollutant 

28 confounding or exposure measurement error. Controlled human exposure and toxicological 

29 studies that demonstrate similar effects at relevant PM exposures may provide coherence with 

30 epidemiologic evidence. Further, experimental results that identify key events in the mode of 

31 action may provide biological plausibility. 

32 In the 2009 PM ISA, a number of uncertainties were identified with respect to quantitative 

33 relationships between PM and effects on welfare. Although ecological effects due to PM 

34 deposition were described in the previous ISA, there was a lack of information linking specific 

35 ambient concentrations of PM to impairment of ecological receptors. For materials effects, the 
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1 rate of deposition of PM onto surfaces was identified as an uncertainty as was the role of co2 pollutants in soiling effects. The 2009 PM ISA identified uncertainties in assessment of climate 

3 effects including quantification and modeling of aerosols and relating observed effects to 

4 measured concentrations of ambient PM. For visibility in the 2009 PM ISA, there were 

5 uncertainties associated with the visibility preference studies and insufficient urban monitoring 

6 data. The PM ISA will evaluate the status of these uncertainties and limitations in each of the 

7 welfare effects sections and this information will be used in the development of causal 

8 determinations. 

9 3.4.3 Integration of Evidence and Causal Determinations 

10 As described in the Preamble to the Integrated Science Assessments {U.S. EPA, 2015, 

11 3037426}, the EPA uses a structured framework to provide a consistent and transparent basis for 

12 classifying the weight of available evidence for health and welfare effects according to a five13 level hierarchy: (1) causal relationship; (2) likely to be a causal relationship; (3) suggestive, but 

14 not sufficient, to infer a causal relationship; (4) inadequate to infer a causal relationship; and (5) 

15 not likely to be a causal relationship. In the framework, key considerations in drawing 

16 conclusions about causality include consistency of findings for an endpoint across studies, 

17 coherence of the evidence across disciplines and across related endpoints, and biological 

18 plausibility. As judged by these parameters, evidence that rules out chance, confounding, and 

19 other biases with reasonable confidence is sufficient to infer a causal relationship. Increasing 

20 uncertainty due to limited available information, inconsistency, and/or limited coherence and 

21 biological plausibility leads to conclusions lower in the hierarchy. Causality determinations are 

22 based on the confidence in the integrated body of evidence, considering study design and quality 

23 and strengths and weaknesses in the overall collection of previous and recent studies across 

24 disciplines. In discussing each determination of causality, the EPA characterizes the evidence 

25 upon which the judgment is based, including the weight of evidence for individual endpoints 

26 within the health or welfare effect category or group of related endpoints. 

27 For evaluation of human health effects, determinations of causality are made for major 

28 health effect categories (e.g., respiratory effects) or more specific groups of related outcomes and 

29 for the range of exposure concentrations of PM defined to be relevant to ambient concentrations 

(e.g., up to 2 mg/m3 30 ). Findings based on higher exposure concentrations may be considered if 

31 they add to the understanding of biological plausibility, provided that they do not reflect different 

32 biological mechanisms operating at higher concentrations. The main lines of evidence for use in 

33 causal determinations for human health are controlled human exposure, epidemiologic, and 

34 toxicological studies. Evidence is integrated from previous and recent studies. Other information 
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1 including mechanistic evidence, toxicokinetics, and exposure assessment may be drawn upon if 

2 relevant to the evaluation of health effects and if of sufficient importance to affect the overall 

3 evaluation. The relative importance of different sources of evidence to the conclusions varies by 

4 pollutant or assessment, as does the availability of different sources of evidence when making a 

5 determination regarding causality. In judgments of causality, scientists will also evaluate 

6 uncertainty in the scientific evidence, considering issues such as generalizing results from a 

7 small number of controlled human exposure subjects to the larger population; extrapolations of 

8 observed pollutant-induced pathophysiological alterations from laboratory animals to humans; 

9 confounding by co-exposure to other ambient pollutants, meteorological factors, or other factors; 

10 the potential for effects to be due to exposure to air pollution mixtures; and the influence of 

11 exposure measurement error on epidemiologic study findings. Judgments of causality also are 

12 informed by the extent to which uncertainty in one line of evidence (e.g., potential copollutant 

13 confounding in epidemiologic results) is addressed by another line of evidence (e.g., coherence 

14 of effects observed in epidemiologic studies with experimental findings, mode of action 

15 information). Thus, evidence integration is not a unidirectional process but occurs iteratively 

16 within and across scientific disciplines and related outcomes. 

17 A similar process is used for the integration of evidence and determination of causality for 

18 welfare-related effects. For ecological effects, several types of experimental approaches (e.g. 

19 field studies, mesocosm, controlled laboratory exposures, gradient) may be relevant for assessing 

20 PM effects to ecosystems. Field studies measure biological changes in uncontrolled situations 

21 and describe an association between a disturbance and an ecological effect. They are most useful 

22 for linking stressors with effects when stressor and effect levels are measured concurrently. The 

23 presence of confounding factors can make it difficult to attribute observed effects to specific 

24 stressors. Mesocosm studies are performed in the natural environment while controlling for 

25 some, but not all, of the environmental conditions and can be considered a hybrid between a field 

26 and a laboratory study. Controlled exposure studies provide the strongest evidence for causality, 

27 however the scope of inference to conditions in natural systems may be limited. Studies where 

28 effects are observed at relevant ambient concentrations are weighted more heavily, however, 

29 studies conducted at higher concentrations can inform the understanding of mode of action in 

30 aquatic and terrestrial organisms. The EPA evaluates evidence relevant to understanding the 

31 quantitative relationships between pollutant exposures and ecological effects. Integration of 

32 evidence for visibility and climate draws upon modeling and monitoring data as well as 

33 experimental approaches designed to characterize the role of PM in atmospheric processes. In the 

34 case of visibility, new preference studies designed to quantitatively assess the impact of PM on 

35 visibility will also be incorporated if available. This includes evaluating the C-R or deposition- 
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1 response relationships and, to the extent possible, drawing conclusions on the levels at which 

2 effects are observed. For materials effects, evidence of corrosive and soiling effects on stone, 

3 metals, paint and other surfaces at relevant concentrations of PM will be considered. Generally, a 

4 causal determination is made based on many lines of evidence that reinforce each other and are 

5 based on integrating evidence from both previous and recent studies. 

6 3.4.4 Atmospheric Chemistry and Ambient Concentrations of PM 

7 The PM ISA will present and evaluate data related to ambient concentrations of PM and 

8 its components; sources leading to the presence of PM in the atmosphere; the deposition of PM; 

9 and chemical reactions that determine the formation, transformation, and lifetime of PM in the 

10 atmosphere. Specific policy-relevant questions related to atmospheric chemistry and ambient 

11 concentrations that will be addressed in the PM ISA include: 

12  What are the strengths and limitations of existing and new measurement methods and 

13 approaches (including low cost sensors and remote sensing) for both advancing 

14 science and providing routine measurements of particulate matter? What are the 

15 important biases and uncertainties associated with existing and new monitoring 

16 methods? How do new methods and approaches compare with or complement routine 

17 measurement tools (e.g., FRMs)? 

18  What changes have occurred in primary PM and PM precursor emissions rates over 

19 time? Can the impacts of biomass burning (including occurrence of more fires due to 

20 climate change and potentially identification of more fires with more sensitive 

21 sensors on satellites), new transportation and industrial technologies, improved 

22 industrial and motor vehicle emission controls, and strategies for accounting for semi23 volatile organic emissions be observed in current atmospheric PM concentrations? 

24 What are the uncertainties associated with source apportionment approaches, whereby 

25 individual PM sources can be distinguished as a function of the chemical composition 

26 of the emitted PM? 

 What changes have occurred in the quantities of sulfate (SO4

2-), nitrate (NO3- 27 ), and 

28 secondary organic PM (SOPM) formed in the atmosphere over time? What new 

29 information is available regarding mechanisms of their production, including aqueous 

30 phase SOPM formation, the role of sulfur and nitrogen in SOPM formation, and the 

31 role of semi-volatile species? Have the relative contributions of primary and 

32 secondary PM components changed as a result of decreases in precursor emissions? 

33  What are the current ambient concentrations of different size fractions of PM (i.e., 

34 PM2.5, PM10-2.5, UFP), and how have they changed over time? What are the 

35 uncertainties in the spatial and temporal distributions of PM? How do those patterns 

36 vary with particle size fraction? 

37  What uncertainties exist concerning the sources and atmospheric chemistry of PM 

38 components (both primary and secondary particles)? How does PM composition 

39 change over various spatial and temporal scales? What new information is available 
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1 regarding the composition of the PM size fractions that could not be well 

2 characterized during the last review (e.g., coarse PM, ultrafine PM)? 

3  How does intercontinental or trans-boundary transport of pollutants contribute to 

4 background PM concentrations? What information is available to quantify the 

5 emissions from natural PM sources that contribute to background PM concentrations? 

6 Is there enough information to estimate background concentrations of PM species? 

7 The information provided in the Atmospheric Chemistry and Ambient Concentrations 

8 chapter will provide context for understanding ambient concentration data used as surrogates for 

9 human exposure and for interpretation of the health and welfare effects evidence in subsequent 

10 chapters. 

11 3.4.5 Human Exposure 

12 The PM ISA will evaluate methods for estimating exposure to ambient PM, the 

13 factors that influence exposure to PM, as well as the ability to make inferences about personal 

14 exposure to ambient PM when extrapolating from ambient concentration data, particularly in the 

15 context of interpreting results from epidemiologic studies, and from controlled human exposure, 

16 animal toxicology, and in vitro chamber studies of near-atmospheric conditions. The issues 

17 surrounding the ability to make inferences about personal exposure differ by the exposure period 

18 of interest. Short-term exposure studies (i.e., exposures ranging from hours to days to weeks) 

19 primarily rely on temporal variation in exposure while long-term exposure studies (i.e., 

20 exposures ranging from months to years) rely on spatial variability of exposure. The PM ISA 

21 will consider the available information on differential exposures to UFPs, PM2.5, and PM10-2.5

22 and on particle characteristics such as chemical composition, size, surface area, number, and 

23 source. Specific policy-relevant questions related to human exposure that will be addressed in the 

24 PM ISA include: 

25  Are new models or other techniques available to estimate human exposure to ambient 

26 PM? What are the strengths and limitations of existing and new techniques? Do any 

27 of these techniques characterize PM by size fraction or composition? 

28  What techniques are available to replicate human exposure to ambient PM in 

29 controlled human exposure, animal toxicology, and in vitro experiments? What 

30 factors lead to uncertainty in the exposure conditions established within these 

31 experiments? What factors affect inference about human exposure from such 

32 experiments? 

33  What are the limitations in our understanding of exposures to PM components, size 

34 fractions, sources/environments, and relationships between exposure to specific PM 

35 size fractions and/or components and corresponding exposure to gaseous co36 pollutants? What uncertainties exist regarding the spatial and temporal patterns of PM 
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1 concentrations, size fractions, and components as they relate to patterns of human 

2 exposure? 

3  What are the uncertainties when extrapolating from stationary PM monitoring 

4 instruments to personal exposure to PM of ambient origin, especially in populations 

5 at increased risk of PM-related health effects? How do these uncertainties vary for 

6 particles of different size or composition? Issues include measurement error in 

7 outdoor ambient monitors, the use of centralized monitors for estimating community 

8 concentrations, and the use of centralized monitors as a surrogate for personal 

9 exposure to PM of ambient origin. 

10  What are the uncertainties associated with using measurements of ambient 

11 concentrations of PM to provide an estimate of ambient exposures for health studies, 

12 an indicator of personal exposure to PM, and/or an indicator of exposure to other 

13 pollutants or pollutant mixtures? 

14  Higher sampler density is more likely for saturation samplers or low-costs sensors 

15 than for other measurement tools with greater sensitivity and precision. How does 

16 sampler density affect data quality objectives needed for individual samplers, 

17 particularly for saturation samplers or low-cost sensors? 

18  What limitations exist for interpretation of sub-daily, daily, and longer-term PM 

19 concentration data used to estimate exposures? How do factors such as air exchange 

20 rates, indoor sources, ambient sources, and methods for measuring personal 

21 exposures to ambient PM influence interpretation of PM concentration data over 

22 different sampling periods? Does this information differ by PM characteristics (e.g., 

23 size, chemical composition, sources/environments)? 

24  What new developments have occurred with respect to chemical transport modeling 

25 of short-term and long-term PM concentrations for use in exposure assessment? How 

26 might modeling and satellite data supplement monitoring data for understanding 

27 human exposures? What are the limitations of using modeling or satellite data in lieu 

28 of monitoring data? What advancements have been made with respect to techniques 

29 for fusing modeling, monitoring, and/or satellite data for assessing long-term 

30 exposures to ambient PM? What are the uncertainties in data from chemical transport 

31 models and satellites at the extremes of the concentration distribution, such as in low 

32 concentration areas? 

33  What new developments have been made in use of source apportionment techniques 

34 for assessing human exposure to ambient PM? 

35  What new developments have been made in assessing and/or correcting the influence 

36 of exposure measurement error on health effect estimates for epidemiologic studies of 

37 short-term and long-term exposure? How do these methods reduce the uncertainty 

38 and/or bias in the health effect estimates for PM exposure? 

39 3.4.6 Dosimetry 

40 The PM ISA will evaluate literature focusing on dosimetry that may underlie the health 

41 outcomes associated with exposure to PM. In Chapter 4 of the 2009 PM ISA, in relation to 
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1 particle deposition in the body, it was concluded that the first line of defense for protecting the 

2 lower respiratory tract from inhaled particles is the nose and mouth; that the lower respiratory 

3 tract of children receive a higher surface dose of ambient PM compared to adults; and that people 

4 with COPD generally have greater total deposition and more heterogeneous deposition patterns 

5 compared to healthy individuals. In relation to particle clearance, it was concluded that particles 

6 depositing on the olfactory mucosa may translocate to the brain; and that a small, but statistically 

7 significant fraction (<1% of deposited material) of poorly soluble particles deposited in the 

8 alveolar region may translocate into circulation. A number of factors such as age and respiratory 

9 disease were recognized as affecting both rates of deposition and clearance. Interspecies 

10 differences in deposition and clearance were also discussed. In the current review, specific 

11 policy-relevant questions related to dosimetry that will be addressed in the PM ISA include: 

12  Are there new data that better quantify extrathoracic and thoracic deposition of 

13 particles in children, adults, and among species? 

14  Are new data available evaluating the effects of respiratory disease on deposition and 

15 clearance? 

16  What new data or models are available that help facilitate interspecies comparisons? 

17  Is there new information related to the translocation of particles into circulation or the 

18 olfactory bulb? 

19 3.4.7 Modes of Action 

20 The PM ISA will evaluate literature focusing on modes of action that may underlie the 

21 health outcomes associated with exposure to PM. These topic areas will be developed using both 

22 human and animal data. In the current review, specific policy-relevant questions related to modes 

23 of action that will be addressed include: 

24  How do physical-chemical particle characteristics influence biological responses to 

25 inhaled PM? 

26  What new evidence is available to characterize biologic responses of various PM size 

27 fractions or PM from different sources? 

28  What are the time-courses of biological responses to inhaled PM? What are the 

29 implications of these time-courses for health effects associated with short-and long30 term exposure to PM? 

31  What are the interspecies differences in biological responses to inhaled PM? What are 

32 the implications of interspecies differences for extrapolation of results to humans? 

33  Is there a common underlying biological response (e.g., inflammation) that supports 

34 an effect of PM exposure on an array of health outcomes (e.g., cardiovascular, 

35 respiratory, or nervous system)? 
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1  What new evidence is available to elucidate mechanisms by which pulmonary 

2 deposition of PM leads to extrapulmonary effects? 

3 3.4.8 Health Effects 

4 In the 2009 PM ISA, the health effects evidence for PM largely focused on PM2.5; 

5 fewer studies were available for PM10-2.5, UFPs, and sources/components of PM. The evidence 

6 indicated that a “causal relationship exists” for short- and long-term exposures to PM2.5 and 

7 cardiovascular effects and mortality, and a “likely to be causal relationship exists” for short- and 

8 long-term PM2.5 exposures and respiratory effects. More limited evidence with a larger degree of 

9 uncertainty formed the basis for the determinations for other health effects and other PM size 

10 fractions (i.e., PM10-2.5 and UFPs). Additionally, with respect to sources and PM components, the 

11 EPA concluded “that many PM [components] can be linked with differing health effects and the 

12 evidence is not yet sufficient to allow differentiation of those [components] or sources that are 

13 more closely related to specific health outcomes (U.S. EPA, 2009, p. 2-26)” The EPA will build 

14 on the 2009 PM ISA by evaluating the newly available literature related to PM exposures and 

15 health effects, including, but not limited to respiratory, cardiovascular, neurologic, reproductive 

16 and developmental effects, mortality, and cancer. Depending on data availability, other health 

17 effects may be evaluated. 

18 The PM ISA will evaluate health effects that occur following both short- and long-term 

19 exposures as examined in epidemiologic, controlled human exposure, and animal toxicological 

20 studies. Efforts will be directed towards identifying the concentrations at which effects are 

21 observed, particularly in potential at-risk lifestages and populations, and assessing the role of PM 

22 within the broader mixture of ambient air pollutants. The data will be reviewed for relevance to 

23 the current elements of the PM NAAQS (i.e., indicator, averaging time, form, and level) in order 

24 to support decisions regarding the adequacy of the current primary standard. Additionally, the 

25 evidence will be evaluated to determine whether specific chemical components and/or 

26 source(s)/environment(s) (e.g., near-road) more fully explain PM health effects. The discussion 

27 of health effects will be integrated with relevant information on dosimetry and modes of action, 

28 as well as with information from the exposure chapter. 

29 In light of recent International Agency for Research on Cancer (IARC) conclusions on 

30 the carcinogenicity of outdoor air pollution, and specifically of PM, it is important for the EPA to 

31 clearly outline its approach for evaluating the relationship between long-term PM exposures and 

32 cancer in the PM ISA (Loomis et al., 2013). While IARC focuses on whether a chemical is a 

33 carcinogen at any concentration and through any route of exposure, the PM ISA focuses on 

34 whether PM can directly cause cancer through inhalation exposures at ambient and near-ambient 
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1 concentrations. When evaluating the epidemiologic evidence for cancer, the EPA will focus on 

2 those studies with composite measures of PM and with exposures consistent with the overall 

3 scope of the PM ISA detailed in Section 3.1, i.e., concentrations of PM that represent the range 

of human exposures across ambient microenvironments (up to 2 mg/m3 4 PM, which is one to two 

5 orders of magnitude above ambient concentrations). For the toxicological evidence, the EPA will 

6 describe in vitro studies and other mechanistic studies related to cancer in the Mode of Action 

7 chapter because these types of studies are pertinent to the biological pathways underlying cancer. 

8 Consistent with previous ISAs, in vivo studies that examine cancer-related endpoints will be 

9 discussed in combination with the epidemiologic evidence in the health chapters. The evaluation 

10 of toxicological studies related to mutagenicity, genotoxicity, epigenetic changes, and 

11 carcinogenicity will focus on inhalation exposures primarily at ambient relevant concentrations 

(i.e., up to 2 mg/m3 12 ) with the potential inclusion of exposures above this concentration. Lastly, 

13 the evaluation of cancer will not focus on studies that use PM filter extracts because they may 

14 not mimic what is bioavailable in vivo, or on studies of individual PM components due to the 

15 inability to compare effects to the current mass-based PM indicator. 

16 In the current review, specific policy-relevant questions that will be addressed in 

17 consideration of health effects associated with short- and long-term exposure to PM, include the 

18 following: 

19 Short-Term Exposure: 

20  What new evidence is available to inform policy-relevant considerations of the PM 

21 NAAQS (e.g., considerations related to indicator, averaging time, form, level) for 

22 those health categories where the 2009 PM ISA concluded that a “causal relationship” 

23 exists (i.e., short-term PM2.5 exposures and mortality and cardiovascular effects)? Do 

24 new controlled human exposure and toxicological studies continue to provide support 

25 for biologically plausible relationships between short-term PM exposures and 

26 cardiovascular health effects? Do new studies report PM-attributable effects at lower 

27 PM concentrations than indicated by studies available in the last review? 

28  How do results of recent studies expand understanding of the relationship between 

29 short-term exposure to PM and respiratory effects, such as exacerbation of asthma or 

30 chronic obstructive pulmonary disease and respiratory infection? Does recent 

31 evidence improve coherence across disciplines for lung function changes, pulmonary 

32 inflammation, host defense mechanisms, and outcomes such as symptoms, hospital 

33 admissions, or emergency department visits? 

34  To what extent is short-term exposure to PM related to or associated with the 

35 progression of diabetes, other metabolic diseases, and/or to other endocrine system 

36 effects? To what extent are new health outcomes related to or associated with PM 

37 exposures? 
April 2016 3-24 Draft – Do Quote or Cite

1  Across the evaluated health effects, what new evidence is available on effects 

2 occurring from exposures at averaging times different (e.g., 1-hour) than the current 

3 24-hour average PM NAAQS? 

4 Long-Term Exposure: 

5  What new evidence is available to inform policy-relevant considerations of the PM 

6 NAAQS (e.g., considerations related to indicator, averaging time, form, level) for 

7 those health categories where the 2009 PM ISA concluded that a “causal relationship” 

8 exists (i.e., long-term PM2.5 exposures and mortality and cardiovascular effects)? Do 

9 new controlled human exposure and toxicological studies continue to provide support 

10 for biologically plausible relationships between long-term PM exposures and 

11 cardiovascular health effects? Do new studies report PM-attributable effects at lower 

12 PM concentrations than indicated by studies available in the last review? 

13  To what extent do recent studies improve understanding of the relationships between 

14 long-term PM exposure and the development of asthma or to the impairment of lung 

15 development? Do recent studies improve coherence across disciplines for respiratory 

16 disease incidence, pulmonary inflammation and oxidative stress, and development of 

17 allergic responses? 

18  To what extent do recent studies improve understanding of the relationship between 

19 long-term PM exposure and reproductive and developmental health outcomes, such 

20 as adverse birth outcomes, fertility and pregnancy outcomes (e.g., infertility, sperm 

21 quality, preeclampsia, gestational hypertension), or developmental outcomes (e.g., 

22 neurocognitive ability)? 

23  To what extent does new literature support a biologically plausible relationship 

24 between long-term PM exposures and a number of nervous system effects (e.g., 

25 cognitive decline and autism)? 

26  How do results of recent studies expand our understanding of the relationship 

27 between long-term PM exposure and cancer, mutagenic, genotoxic, and epigenetic 

28 effects? To what extent does the evidence indicate that long-term exposure to PM 

29 can increase the incidence of cancer? 

30  To what extent is long-term exposure to PM related to or associated with the 

31 development of diabetes and other metabolic diseases, as well as to health effects in 

32 the endocrine system or other organ systems? To what extent are new health 

33 outcomes related to or associated with PM exposures? 

34 Additional Policy-Relevant Considerations: 

35  Across the evaluated health effects, to what extent does new evidence inform the 

36 understanding of differential effects from exposures to various PM size fractions (i.e., 

37 PM2.5, PM10-2.5, UFPs)? 

38  Do studies of mortality, hospital admissions, or emergency department visits provide 

39 new information to improve our understanding of the potential heterogeneity in 

40 effects observed across the U.S. in multi-city studies? 
April 2016 3-25 Draft – Do Quote or Cite

1  How do recent studies support the attribution of health effects to one or more PM 

2 component(s) or source(s), in addition to PM mass, for health effects for which there 

3 is sufficient evidence of a strong relationship (e.g., cardiovascular effects, mortality) 

4 with PM exposure? 

5  How do the results of recent studies inform the shape of the concentration-response 

6 relationship for PM and various health outcomes (e.g., mortality, hospital admissions, 

7 etc.), especially for concentrations near or below the levels of the current PM 

8 NAAQS? 

9  What new evidence adds to the understanding of which lifestages and populations are 

10 at risk of PM-related health effects? 

11  What new evidence supports evaluation of inter-individual variability in response to 

12 PM exposures? 

13  Across health effects, what new information is available to delineate the effects of 

14 chronic exposure to lower concentrations versus acute, repeated exposures to higher 

15 concentrations of PM? 

16  What is the nature of health effects in persons exposed to multipollutant mixtures that 

17 contain PM in comparison to exposure to PM alone? 

18 3.4.9 At-Risk Lifestages and Populations and Public Health Impact 

19 The PM ISA will evaluate an array of factors that may contribute to increased risk of 

20 PM-related health effects for various lifestages or populations. These factors reflect the multiple 

21 avenues through which a lifestage or population may be at increased risk of an air pollutant22 related health effect, specifically: intrinsic factors (e.g., biological factors such as age or genetic 

23 variants), extrinsic factors (nonbiological factors such as diet, low SES), and/or factors affecting 

24 dose or exposure (e.g., sex, age, outdoor activity or work, low SES, physical activity). It is also 

25 important to recognize the interconnectedness among these factors that may also confer 

26 increased risk, an example being pre-existing diseases or conditions and socioeconomic status. 

27 The 2009 PM ISA evaluated studies that provided evidence that children, older adults, people 

28 with pre-existing cardiopulmonary diseases, and people with lower SES are at increased risk of 

29 PM-related health effects (U.S. EPA, 2009, Chapter 8). Since completion of the 2009 PM ISA, 

30 the EPA has developed a more detailed framework to provide a consistent and transparent basis 

31 for communicating the overall confidence in the evidence that a particular factor may increase 

32 the risk of an air pollutant-related health effect for a lifestage or population according to one of 

33 four levels: adequate evidence, suggestive evidence, inadequate evidence, and evidence of no 

34 effect (U.S. EPA, 2013c, Table 8-1). Key considerations in characterizing the evidence include 

35 consistency of findings for a factor within a discipline and, where available, coherence of the 

36 evidence across disciplines as well as biological plausibility. Several lines of evidence inform 
April 2016 3-26 Draft – Do Quote or Cite

1 conclusions about at-risk lifestages and populations, but primarily include observational or 

2 experimental studies that compare exposure to PM or relationships with health effects among 

3 groups that vary by some characteristic, such as pre-existing disease or age (i.e., exposure or 

4 effect modification). Also relevant are comparisons of results among observational or 

5 experimental studies that examine different lifestage or population characteristics or time 

6 windows of exposure and experimental studies that examine health effects of PM in a group with 

7 a particular characteristic (e.g., genetic background, pre-existing disease). Where possible, the 

8 discussion will also include evaluation of the adversity of the health effects potentially associated 

9 with exposure to PM. The assessment of public health impact also may include, as appropriate, 

10 an estimation of the sizes of potential at-risk lifestages and populations. Further, to the extent 

11 that evidence is available, the at-risk chapter of the PM ISA will discuss what evidence is 

12 available regarding interrelationships among risk factors in a particular lifestage or population as 

13 described in the preceding section that may add to the understanding of the public health impact 

14 of exposure to PM. Specific questions include: 

15  Is there new information that identifies a combination of factors (i.e., co-occurring) 

16 that can lead to one lifestage or population being at greater risk compared to another? 

17  Have recent studies characterized whether certain lifestages or populations experience 

18 differential exposures to PM mass, PM components or PM sources, which may 

19 contribute to them being at increased risk? 

20  What new evidence is available regarding additional lifestages or populations (e.g., 

21 pre-existing diseases such as diabetes, pregnant women) potentially at increased risk 

22 of a PM-related health effect? 

23  Have recent studies been able to delineate whether those lifestages or populations at 

24 increased risk of a PM-related health effect differ depending on exposures to different 

25 PM size fractions, PM components, or PM sources? 

26 3.4.10 Welfare Effects 

27 In the 2009 PM ISA, the welfare effects evidence for PM focused on the effects of 

28 airborne PM, including NOx and SOx, on visibility and climate, and on the effects of deposition 

29 of PM constituents other than NOx and SOx, primarily metals and carbonaceous compounds on 

30 materials and ecology. The evidence indicated that a “causal relationship exists” for PM 

31 exposures and effects on visibility, climate, and materials; and a “likely to be causal relationship 

32 exists” for PM exposures and ecological effects. The EPA will build on the 2009 PM ISA by 

33 evaluating the newly available literature related to PM exposures and welfare effects, specifically 

34 ecological effects, visibility, climate, and materials. 

35 Ecological Effects 
April 2016 3-27 Draft – Do Quote or Cite

1 The PM ISA will summarize studies of impacts of PM deposition with the secondary 

2 NOx/SOx ISA covering the depositional effects of N and S on ecological receptors. As indicated 

3 in the previous ISA, chemical components of PM determine ecological response to a greater 

4 degree than PM size class. However, size fraction may play a role in direct effects such as 

5 deposition on vegetative surfaces near PM sources. In regards to effects of PM on ecosystem 

6 components (e.g. plants, soils, wildlife, nutrient cycling), both direct and indirect effects of PM 

7 deposition will be considered. PM deposited directly on vegetative surfaces may impair 

8 photosynthesis and gas exchange between the plant and the atmosphere. Indirect effects of trace 

9 metals and organics associated with PM deposition on ecosystem receptors including aquatic and 

10 terrestrial organisms, as well as effects on ecosystem structure and function, will be evaluated. If 

11 studies are available that link PM deposition to ecosystem services these findings will be 

12 discussed. Specific questions include: 

13  Is there new information on PM that can be correlated to a specific physiological 

14 response in aquatic and terrestrial biota? 

15  Do new studies show effects on ecosystem structure, function and biodiversity or PM 

16 impacts to ecosystem services? 

17 Visibility Effects 

18 The PM ISA will summarize long-known information needed for placing current 

19 information in context with respect to visibility. Previous evaluations have indicated that 

20 anthropogenic sulfate and nitrate particles are responsible for most of the regional haze in the 

21 eastern U.S. In the West, sulfates may be generally of less importance since less sulfur dioxide is 

22 emitted there, but important sub-regional and seasonal differences still exist. Nitrates are a 

23 leading factor in haze generation in specific areas such as southern California in winter, for 

24 example, while in many of the less-populous areas of the West, airborne soil particles can be the 

25 largest contributor to visibility extinction during the spring. 

26 In addition, anthropogenic nitrates and organic species, either emitted directly or formed 

27 secondarily from other emissions, can be significant factors in haze formation. Other sources of 

28 visibility extinction (e.g., dust, smoke, sea salt) have anthropogenic, biogenic, and geogenic 

29 sources that vary in strength and significance sub-regionally as well. Smoke from wildfires and 

30 prescribed burnings contribute to visibility extinction both directly and indirectly, though much 

31 less is known of these indirect effects through SOPM generation. The PM ISA will evaluate 

32 newly available evidence summarizing the recent important policy-relevant findings and will 

33 include sections for aerosol optical characteristics, spatial and temporal trends, and causes of 

34 haze. Specific questions include: 
April 2016 3-28 Draft – Do Quote or Cite

1  What relationships exist between ambient PM and visibility impairment? How do 

2 these relationships vary between urban and rural settings? 

3  What metrics and algorithms are available to estimate visibility impairment as a 

4 function of PM mass, optical characteristics, and composition? 

5  What role do spatial patterns (e.g., elevated particulate nitrate in the midwestern U.S. 

6 and enhancement of sulfate concentrations in the eastern U.S.), seasonal patterns, 

7 multi-year trends, emissions changes, and meteorology play with respect to PM 

8 effects on visibility? 

9  What new preference studies or other metrics are available to describe the extent to 

10 which PM-related visibility impairment may be considered adverse to public welfare? 

11 Climate Effects 

12 The PM ISA will present information on radiative forcing and climate change impacts of 

13 PM and PM components. Background information on the physics of climate and radiative 

14 forcing and observed trends in anthropogenic and natural PM will be presented as well as climate 

15 impacts of changes in anthropogenic PM since preindustrial times. Focus will be on information 

16 necessary for interpretation of effects described in the chapter and on newly available 

17 information since the last ISA. PM size/effect dependencies (e.g., cloud formation and 

18 precipitation) and PM composition/effect dependencies (e.g., black carbon and sulfate aerosol) 

19 will be addressed as they pertain to climate effects. 

20  What are the important and relevant climate metrics used to quantify PM-related 

21 climate effects and what are their strengths and limitations in assessing climate effects 

22 at different temporal and spatial scales? 

23  What is the current understanding of the magnitude and direction (warming vs. 

24 cooling) of PM climate impacts and what important uncertainties remain? 

25  To what extent do we understand the independent effect of PM on climate in the 

26 broader context of other climate forcers? 

27  What is the current state of knowledge of PM size and composition in relation to 

28 climate forcing and what is the relationship between PM metrics (e.g., size, 

29 composition) and important and relevant climate metrics used to quantify PM-related 

30 climate effects? 

31  What recent advancements have been made in understanding PM effects on regional 

32 climate in the U.S.? 

33 Materials Effects: 

34 The PM ISA will summarize soiling and deterioration of materials attributable to PM and 

35 related N and S components because of the difficulty associated with isolating the effects of 

36 gaseous and particulate N and S wet deposition and the NOx/SOx secondary ISA only focuses 
April 2016 3-29 Draft – Do Quote or Cite

1 on ecological effects, not other welfare effects (See Section 1.4). Previous PM assessments 

2 indicated that stone, paint, metal and other materials can become discolored and/or undergo 

3 corrosion processes from particle pollution. Culturally important items such as buildings, statues 

4 or works of art can be physically damaged and/or lose aesthetic appeal. The focus of the PM ISA 

5 will be on whether there is new information on soiling and corrosion, dose-response relationships 

6 between PM or related N and S species and damage to surfaces. Any new areas of research 

7 focus on PM-related materials effects will also be emphasized in the PM ISA. 

8  Is there new information on soiling of stone, metal and paint surfaces due to PM or 

9 related N and S species? Corrosive effects? 

10  Do new studies show effects on additional types of materials? 

11 3.5 QUALITY MANAGEMENT 

12 Within the EPA, Quality Management Plans (QMP) are developed to ensure that all 

13 Agency materials meet a high standard for quality. NCEA-RTP participates in the Agency-wide 

14 Quality Management System, which requires the development of a QMP. Implementation of the 

15 NCEA QMP ensures that all data generated or used by NCEA scientists are “of the type and 

16 quality needed and expected for their intended use” and that all information disseminated by 

17 NCEA adheres to a high standard for quality including objectivity, utility, and integrity. Quality 

18 assurance (QA) measures detailed in the QMP will be employed for the development of the PM 

19 ISA. NCEA QA staff will be responsible for the review and approval of quality-related 

20 documentation. NCEA scientists will be responsible for the evaluation of all inputs to the PM 

21 ISA, including primary (new) and secondary (existing) data, to ensure their quality is appropriate 

22 for their intended purpose. NCEA adheres to Data Quality Objectives, which identify the most 

23 appropriate inputs to the science assessment and provide QA instruction for researchers citing 

24 secondary information. The approaches utilized to search the literature and criteria applied to 

25 select and evaluate studies were detailed in the two preceding subsections. Generally, NCEA 

26 scientists rely on scientific information found in peer-reviewed journal articles, books, and 

27 government reports. The PM ISA also can include information that is integrated or summarized 

28 from multiple sources to create new figures, tables, or summation, which is subject to rigorous 

29 quality assurance measures to ensure their accuracy. 

30 3.6 SCIENTIFIC AND PUBLIC REVIEW 

31 Drafts of the PM ISA will be made available for review by the CASAC and the public as 

32 indicated in Figure 1-1 and Tables 1-1 and 1-3 above. Availability of draft documents will be 

33 announced in the Federal Register. The CASAC will review the draft PM ISA documents and 
April 2016 3-30 Draft – Do Quote or Cite

1 discuss its comments in public meetings that will be announced in the Federal Register. The 

2 EPA will take into account comments, advice, and recommendations received from the CASAC 

3 and from the public in revising the draft PM ISA documents. The EPA has established a public 

docket for the development of the PM ISA.81 4 After appropriate revision based on comments 

5 received from the CASAC and the public, the final document will be made available on the EPA 

6 website. A notice announcing the availability of the final PM ISA will be published in the 

7 Federal Register. 

8 

 

81 The ISA docket can be accessed at www.regulations.gov using Docket ID number EPA-HQ-ORD-2014-0859. 
April 2016 3-31 Draft – Do Quote or Cite

1 REFERENCES 

2 Clean Air Act Amendments of 1977. U.S. Government Printing Office; 1977. 

3 IPCC (2014). Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II, 

4 and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. 

5 [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland. 

6 Loomis D, Groose Y, Lauby-Secretan B, El Ghissassi F, et al. (2013). The carcinogenicity of 

7 outdoor air pollution. Lancet Oncol 14(13):1262-3. 

8 Melillo JM, Richmond TC, Yohe GW, Eds. (2014). Climate Change Impacts in the United 

9 States: The Third National Climate Assessment. U.S. Global Change Research Program. 

10 Available: 

11 http://s3.amazonaws.com/nca2014/low/NCA3_Climate_Change_Impacts_in_the_United

12 %20States_LowRes.pdf?download=1. 

13 Peacock M (2008). Letter from Marcus Peacock, Deputy Administrator, U.S. EPA to Rogene 

14 Henderson, Chair, Clean Air Scientific Advisory Committee, September 8, 2008. 

15 Available: http://www3.epa.gov/ttn/naaqs/pdfs/PeacocklettertoCASAC090808.pdf. 

16 Samet J (2009). Letter from Dr. Jonathan M. Samet, Chair, Clean Air Scientific Advisory 

17 Committee to the Honorable Lisa P. Jackson, Administrator, U.S. EPA. CASAC Review 

18 of EPA’s Integrated Science Assessment for Particulate Matter – First External Review 

19 Draft, December 2008. (May 21, 2009). Available: 

20 http://ofmpub.epa.gov/eims/eimscomm.getfile?p_download_id=508493. 

21 U.S. EPA (2006). Review of the Process for Setting National Ambient Air Quality Standards. 

22 NAAQS Process Review Workgroup, U.S. Environmental Protection Agency, March 

23 2006. Available: 

24 http://www3.epa.gov/ttn/naaqs/pdfs/naaqs_process_report_march2006.pdf. 

25 U.S. EPA (2009). Final Report: Integrated Science Assessment for Particulate Matter. U.S. 

26 Environmental Protection Agency, Washington, DC, EPA/600/R-08/139F. Available: 

27 https://cfpub.epa.gov/ncea/risk/recordisplay.cfm?deid=216546&CFID=50518184&CFTO

28 KEN=64007426. 

29 U.S. EPA (2015). Preamble to the Integrated Science Assessments. U.S. Environmental 

30 Protection Agency, Washington, DC, EPA/600/R-15/067. Available: 

31 https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=310244. 

32 U.S. EPA (2016). Process of Reviewing the National Ambient Air Quality Standards. March 4, 

33 2016. Available: http://www3.epa.gov/ttn/naaqs/review.html. 
April 2016 4-1 Draft – Do Quote or Cite

1 4 HUMAN HEALTH RISK AND EXPOSURE 

2 ASSESSMENT 

3 4.1 INTRODUCTION 

4 Within the context of NAAQS reviews, a quantitative health risk and exposure assessment 

5 (HREA) is designed to estimate human exposure and health risks associated with the existing 

6 primary standards and with potential alternative primary standard(s), if any, that might be 

7 appropriate to consider. This assessment can inform conclusions on the adequacy of the public 

8 health protection provided by just meeting these standards. The purpose of this chapter is to 

9 highlight key findings from the quantitative risk assessments conducted in the last review of the 

10 PM NAAQS and to identify key issues to be addressed in planning for any additional 

11 quantitative assessments that might be warranted for the current review. The scope of any HREA 

12 would be informed by the scientific evidence in the upcoming PM ISA; existing and historical 

13 air quality patterns and trends; the availability of improved data, methods, tools, and models that 

14 may better characterize important uncertainties or provide additional insights beyond those 

15 provided by prior HREAs; and available resources. 

16 In the upcoming HREA Planning Document (discussed in sections 1.2 and 1.5, above), the 

17 EPA will evaluate newly available information within the context of the 2010 HREA from the 

18 last review of the PM NAAQS to determine 1) the extent to which important uncertainties may 

19 be better characterized by information newly available for the current review and 2) the extent to 

20 which this new information may affect the health risks estimated in the 2010 HREA in important 

21 ways or suggest new quantitative analyses that can increase our understanding of the health risks 

22 associated with ambient PM exposures. The HREA Planning Document will also describe the 

23 scope and methods for any new or updated quantitative assessments warranted for this review. 

24 CASAC advice and public comments on this draft IRP will be considered in developing the 

25 HREA Planning Document, which will also be subject to CASAC review and will be made 

26 available for public comment. If warranted, one or more drafts of an HREA would then be 

27 prepared and released for CASAC review and public comment prior to completion of a final 

28 HREA. 

29 Section 4.2 describes the key analyses, findings and uncertainties from the 2010 HREA. 

30 Section 4.3 describes the two major components of potential new quantitative health risk 

31 assessments (i.e., epidemiology-based risk assessment and exposure assessment) that the EPA 

32 will further evaluate in the HREA Planning Document. Section 4.4 describes the process for 

33 obtaining scientific and public review of the HREA Planning Document and the HREA itself. 
April 2016 4-2 Draft – Do Quote or Cite

1 4.2 OVERVIEW OF HEALTH RISK AND EXPOSURE ASSESSMENT 

2 FROM THE LAST REVIEW 

3 In the last review of the PM NAAQS, the EPA conducted a quantitative, epidemiology4 based risk assessment for selected health endpoints to provide information and insights that 

5 could help inform decisions on the primary standards, namely, the degree of protection provided 

6 by the then-existing suite of primary standards, as summarized below (in sections 4.2.1 and 

7 4.2.2). The 2010 HREA (U.S. EPA, 2010a) characterized risk associated with ambient fine 

8 particle (PM2.5) concentrations because at that time it was decided that any risks estimated using 

9 the limited data available for thoracic coarse (i.e., PM10-2.5) and ultrafine particles would have 

10 uncertainties too large to provide reasonable and informative results for the review. The 2010 

11 HREA quantified endpoints from health effect categories classified as having a “causal” or 

12 “likely to be causal” relationship with exposure to PM2.5 in the 2009 PM ISA (U.S. EPA, 2009a). 

13 Endpoints included total, cardiopulmonary, and lung cancer mortality associated with long-term 

14 (≥ 1 year) exposure to PM2.5, and mortality (total non-accidental, cardiovascular, and respiratory) 

15 and morbidity (hospital admissions for cardiovascular and respiratory causes and respiratory 

16 symptoms not requiring hospitalizations) associated with short-term (24-hour) PM2.5 exposures. 

17 The 2010 HREA quantified health risks associated with ambient PM2.5 concentrations (as a 

surrogate for population exposure) in fifteen urban study areas (U.S. EPA, 2010a, section 3.1).82 18 

19 In addition, the 2010 HREA included a representativeness analysis that contrasted these urban 

20 study areas against national-level patterns and trends for key PM related attributes (e.g., PM2.5 

21 composition, demographics, and weather) and the total number of deaths that could be attributed 

22 to PM2.5 (U.S. EPA 2010a, sections 4.4.1 and 4.4.2). The 2010 HREA focused on estimating risk 

23 remaining with PM2.5 concentrations adjusted to just meet the then-existing primary PM2.5

standards (15.0 μg/m3

 annual and 35 μg/m3 24-hour, which are abbreviated here as 15/3583 24 ), as 

25 well as risk reductions from just meeting alternative suites of standard levels (relative to 15/35). 

26 The EPA did not conduct a quantitative population-based, microenvironmental exposure 

27 assessment (such as described below in section 4.3.2) due to concerns related to the utility of 

28 such an assessment from both staff and CASAC (Samet, 2009a). 

 

82 The 2010 HREA only estimated remaining risk and risk reductions for the urban areas where PM2.5 improvements 

were needed to just meet the then-existing standards and alternative suites of standards. As described below, two of 

the 15 urban study areas did not exceed existing standards. 

83 In discussing the then-existing standards and alternative suites of PM2.5 standards, the notation used throughout 

this document is “annual standard/24-hour standard” in units of μg/m3

. 
April 2016 4-3 Draft – Do Quote or Cite

1 The health endpoints evaluated in the 2010 HREA were selected based on: 1) the overall 

2 weight of evidence and subsequently causal determinations in the 2009 PM ISA for general 

3 health effect categories that is based on the collective body of evidence from epidemiologic, 

4 controlled human exposure, and toxicological studies; 2) the extent to which particular health 

5 endpoints within these broader health effect categories are considered important from a public 

6 health perspective; 3) the availability of epidemiologic studies providing effect estimates for 

7 specific health endpoints; 4) the availability of air quality monitoring data in urban areas that 

8 were evaluated in the epidemiologic studies; 5) the availability of baseline incidence and 

prevalence data84 9 to support population risk (incidence) modeling; and 6) the anticipated 

usefulness of developing quantitative risk estimates85 10 for the health endpoint(s) to inform 

11 decision-making in the context of the PM NAAQS review. The 2010 HREA also considered 

12 information in the 2009 PM ISA on at-risk populations, which identified the life stages of 

13 children and older adults, people with pre-existing cardiovascular and respiratory diseases, and 

14 people with lower socioeconomic status as populations at increased risk for PM-related health 

15 effects. 

16 Criteria used in selecting the urban study areas included 1) the availability of air quality 

17 monitoring data; 2) inclusion in an epidemiologic study providing effect estimates; 3) the 

18 availability of study area-specific baseline incidence and prevalence data; 4) the potential for risk 

19 reductions by adjusting PM2.5 concentrations to just meet alternative annual and 24-hour standard 

20 levels being considered, using the lowest of the alternative suite of standards as the cutoff

 

84 The baseline incidence rate is the number of new cases of a health effect at a given time (often per 10,000 or 

100,000 of the general population in a year) in a given location due to all causes, including air pollution. These data 

are an integral part of the C-R functions used to determine the number of new cases that can be attributed to changes 

in PM2.5 concentrations. For some health endpoints (e.g., asthma exacerbation), we also use prevalence rates to 

define the applicable population with a given health condition (e.g., asthmatics). Prevalence refers to the rate of all 

cases in the population (both new and pre-existing). 

85 The term “risk estimate” as used here refers to a quantitative, model-derived estimate of the likelihood for adverse 

health effects within a defined population following exposure to a specific chemical agent or agents. In the context 

of NAAQS reviews, risk estimates typically take the form of incidence (count) estimates for specific morbidity or 

mortality endpoints for a defined population. 
April 2016 4-4 Draft – Do Quote or Cite

(12/25); and 5) regional representation.86 After selecting the 15 urban study areas,87 1 the 2010 

2 HREA identified the “spatial template” to use in defining the geographical area associated with 

3 each urban study area, including which counties and PM2.5 monitors were associated with a 

4 particular urban study area. A national-scale assessment evaluated the representativeness of the 

5 risk estimates for the urban study areas compared to national-level results. First, the national6 scale assessment considered key PM2.5 risk-related attributes to determine whether the selected 

7 urban study areas are nationally representative or more focused on a particular portion of the 

8 population distribution for a given attribute (e.g., demographics) (U.S. EPA, 2010a, section 

9 4.4.1). Second, the national-scale assessment analyzed estimates of mortality associated with 

recent long-term PM2.5 concentrations,88 10 to assess the extent to which the 15 urban study areas 

11 reflected locations within the U.S. likely to experience the highest PM2.5-related risk (U.S. EPA, 

12 2010a, section 4.4.2). 

13 The 2010 HREA quantified risks using concentration-response (C-R) functions derived 

14 from effect estimates reported in epidemiologic studies identified in the 2009 PM ISA. These 

studies generally used ambient air quality data from fixed-site, population-oriented monitors;89 15 

16 thus, the appropriate application of these estimates in C-R functions for a PM risk assessment 

17 similarly required the use of ambient air quality data at fixed-site, population-oriented monitors. 

18 The 2010 HREA adjusted ambient air quality concentrations using several different methods, 

including a proportional “rollback” approach,90 19 to simulate the distribution of PM2.5 ambient 

 

86 The goal of the 2010 HREA was to select at least one urban study area from each of the seven geographic regions 

identified in the 1996 PM Criteria Document (U.S. EPA, 1996, section 6.4) (i.e., PM regions). The selected urban 

study areas represented six of these seven regions, with the Upper Midwest not represented. 

87 The 15 urban study areas were Tacoma, Fresno, Los Angeles, Phoenix, Salt Lake City, Dallas, Houston, St. Louis, 

Birmingham, Atlanta, Detroit, Pittsburgh, Baltimore, Philadelphia, and New York. Collectively, these 15 urban 

study areas comprised 31 counties. 

88 These impacts were estimated down to policy relevant background (PRB) for short-term exposure and the lowest 

measured level (LML) observed in the epidemiologic study for long-term exposure. PRB concentrations have 

historically been defined by the EPA as those concentrations that would occur in the U.S. in the absence of 

anthropogenic emissions in continental North America, defined as the U.S., Canada, and Mexico. The 2010 HREA 

used regional PRB estimates generated using a combination of chemical transport modeling tools, as discussed in 

the 2009 PM ISA (section 3.2.2). LML refers to the lowest measured PM2.5 concentration within an epidemiologic 

study. 

89 Most studies applied the composite monitor approach, where PM2.5 levels were averaged across monitors within 

an urban study area in order to produce a single, generalizable PM2.5 distribution for that urban area. 90 “Rollback” is a mathematical approach to lower recent concentrations at ambient monitors in order to simulate 

just meeting the “design value” for various standards. “Design values” are the metrics (i.e., statistics) that are 

compared to the NAAQS levels to determine compliance. The proportional rollback method, which had been used in 

previous risk assessments, reflected a uniform percentage of reduction in ambient PM2.5 concentrations across all 

monitors in an urban study area (U.S EPA, 2010a, section 3.2.3). 
April 2016 4-5 Draft – Do Quote or Cite

1 concentrations that would “just meet” the then-existing and alternative suites of standards. In 

2 addition, two alternative approaches (locally focused and a hybrid of local and proportional 

3 rollback approaches) were applied to improve our understanding of the uncertainty associated 

with the air quality adjustment (U.S. EPA, 2010a, section 3.2.3).91 4 The general PM health risk 

5 model combined PM2.5 concentrations in specific urban areas, C-R functions derived from 

6 epidemiologic studies, baseline incidence data for specific health endpoints, and urban area 

7 population estimates to derive estimates of the annual incidence of specified health effects 

8 attributable to ambient PM concentrations under different air quality scenarios. The human 

9 health risks were estimated using the risk assessment component of the EPA’s Total Risk 

Integrated Methodology model (TRIM.Risk).92 10 The analyses conducted in the 2010 HREA 

11 focused on estimating changes in risks associated with air quality adjusted to just meet the then12 existing primary standards, as well as any additional risk reductions estimated to occur upon just 

13 meeting alternative suites of standards. In the short-term exposure analysis, the 2010 HREA 

14 modeled risk down to PRB, while the long-term exposure analysis modeled down to the LML 

15 observed in the epidemiologic study. 

16 4.2.1 Key Observations in the 2010 HREA 

17 Section 5.2 of the 2010 HREA discussed the key observations from the quantitative 

18 analyses (U.S. EPA, 2010a, pp. 5-9 to 5-10). In summary, the 2010 HREA emphasized 

19 cardiovascular-related health effects due to the greater degree of confidence in these endpoints 

20 stemming from information provided in the 2009 PM ISA relative to other health effect 

21 outcomes, including respiratory effects (U.S. EPA 2010a, p. 5-9). Thus, for long-term exposure-

 

91 The locally focused rollback approach reflected a local pattern of reduction in ambient PM2.5 concentrations 

focused exclusively on those monitors within an urban study area that exceeded the 24-hour standard under 

consideration. As such, this approach was only applied to the subset of urban study areas where no adjustment was 

needed to meet the annual standard. The hybrid approach reflected a combination of a more localized pattern of 

rollback focused on source-oriented monitors with relatively elevated ambient PM2.5 levels, followed by a more 

generalized regional pattern of rollback across all monitors in the study area to just meet the standards. For one 

urban study area (Pittsburgh), the 2010 HREA also applied a refined rollback approach that used a dual-zone 

approach to take into account monitor locations and the related topography in that area (U.S. EPA, 2010a, section 

3.2.3) 

92 Additional information on the risk characterization module in TRIM can be found at: 

http://www2.epa.gov/fera/total-risk-integrated-methodology-trim-trimrisk 
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1 related risk, the focus of the key observations was on ischemic heart disease (IHD)-related 

2 mortality, and for short-term exposures the focus was on cardiovascular-related mortality and 

3 morbidity. The 2010 HREA assessed risks associated with PM2.5 concentrations adjusted to just 

4 meet the then-existing standards and for PM2.5 levels meeting an alternative suite of annual and 

5 24-hour standards. The 2010 HREA also estimated the risks that remained after attaining 

alternative annual standard levels of between 10 to 14 μg/m3 6 . This choice accounts both for 

7 epidemiologic evidence in the ISA and advice from CASAC. The 2010 HREA recognized that 

8 the risks reported at the lower end of the standard levels were subject to greater uncertainty. The 

9 2010 HREA evaluated five suites of annual and 24-hour standard levels to provide a range of 

10 combinations across the urban study areas. These air quality analyses found that the annual 

11 standard was controlling in some urban study areas, and the 24-hour standard was controlling in 

others.93 12 The analysis reported separate risk estimates for years 2005, 2006 and 2007. However, 

13 the 2010 HREA focused on the 2007-based estimates, as these estimates for 2007 fell in the 

14 middle of the risk estimates, in terms of the magnitude of risk (U.S. EPA, 2010a, p. 4-5). 

15 Consequently, 2007-based risk estimates are reflected in the key observations below. 

16 

17 Magnitude of risk associated with air quality adjusted to just meet the then-existing PM2.5 

18 standards (15/35) 

19 In considering PM2.5-related risks associated with air quality just meeting the then20 existing PM2.5 standards in the 15 urban study areas, the 2010 HREA focused on the 13 urban 

study areas that would not meet 15/35 based on air quality from 2005 to 2007.94 21 These 13 urban 

22 study areas had annual and/or 24-hour design values that were above the levels of the then23 existing standards. Based on the risk estimates for these areas, the 2010 HREA made the 

following key observations regarding the magnitude of risk remaining95 24 upon just meeting 15/35 

25 (using proportional rollback): 

 

93 The controlling standard in a particular area is the one that, if met, ensures that the other standard would also be 

met in that area. Because the NAAQS for PM2.5 includes standards using both 24-hour and annual averages, the 

controlling standard in an area was determined by the annual or 24-hour average requiring the largest adjustment or 

“rollback” to meet the existing or potential alternative standard(s), if any. In general, only one of the two standards 

(daily or annual) would be the controlling standard in a particular area and 3-year period. 

94The two urban study areas that did not exceed either the then-existing annual and/or 24-hour design values were 

Dallas and Phoenix. Thus, the 2010 HREA did not estimate risk reductions in this analysis for these urban study 

areas. PM2.5 concentrations in these two areas were not “rolled up” to the then-existing standards. 

95 Risk remaining refers to the estimation of total risk from the then-existing standard down to the lowest bound for 

the analysis (either PRB or LML). 
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1  Long-term exposure-related mortality risk remaining: The analysis estimated that across the 

2 urban study areas, the IHD-related mortality attributable to long-term PM2.5 exposure ranged 

3 from less than 100 to approximately 2,000 cases per year, with this variability reflecting, to a 

4 great extent, differences in the population size of each urban study area. These estimates 

5 represented from 4 to 17 percent of all IHD-related mortality in a given year across the urban 

6 study areas, which is a measure of risk that takes into account differences in population size 

7 and baseline mortality rates. 

8  Short-term exposure-related mortality and morbidity risk remaining: The analysis estimated 

9 that across the urban study areas, the cardiovascular-related mortality attributable to short10 term PM2.5 exposure ranged from <10 to 500 cases per year. These estimates represented 

11 from approximately 1 to 2 percent of total cardiovascular-related mortality in a given year 

12 across the urban study areas. In terms of morbidity risk, cardiovascular-related hospital 

13 admissions ranged from approximately 10 to 800 cases per year across the study areas, 

14 representing less than one percent of total cardiovascular-related hospital admissions in each 

15 study area. 

16 Magnitude of risk reductions associated with air quality adjusted to just meet alternative 

17 PM2.5 standards 

18 In characterizing PM2.5-related risks associated with air quality adjusted to just meet the 

19 alternative annual standards (14/35, 13/35 and 12/35), the 2010 HREA estimated both the 

magnitude of risk reductions (relative to risk remaining upon just meeting 15/35)96 20 and the 

magnitude of risk associated with air quality adjusted to just meet the alternative standards.97 21 

22 More uniform risk reductions were estimated to result from just meeting the alternative annual 

standard levels than just meeting alternative 24-hour standard levels.98 23 Thus, in discussing these 

24 risks, the 2010 HREA focused on the set of urban study areas that would have risk reductions by 

25 just meeting each alternative annual standard. 

26  Reductions in long-term exposure-related mortality risk: Upon just meeting the alternative 

27 annual standard levels considered in conjunction with the existing 24-hour standard (denoted 

28 as 14/35, 13/35 and 12/35), the analysis estimated reductions in long-term exposure-related 

mortality for 1299 29 of the 15 urban study areas, with the degree of risk reduction increasing

 

96 Risk reductions refers to the difference in risk between the then-existing standard (in this case 15/35) and the 

alternative standard being analyzed. 

97 The 2010 HREA also estimated risks at 10 μg/m3

 for the annual standard; however, the HREA noted the larger 

degree of uncertainty in these estimates and therefore, these results are not presented here (U.S. EPA, 2010a, section 

5.2.2). 

98 The degree of estimated risk reduction in long-term exposure-related IHD mortality risk provided by the 

alternative 24-hour standards was highly variable across the study areas, in part due to different rollback approaches 

(U.S. EPA, 2010a, section 4.2.2 and section 5.2.3). 

99 For 12/35, three of the urban study areas (i.e., Tacoma, Fresno and Salt Lake City) did not experience any 

decreases in risk when the annual standard was rolled back, as the design values in these urban study areas met the 

annual standard but exceeded the 24-hour standard, and thus would only experience risk reductions under alternative 

24-hour standards. 
April 2016 4-8 Draft – Do Quote or Cite

1 incrementally across the alternative standard levels (both in terms of the number of study 

2 areas experiencing risk reduction and the magnitude of those reductions). For the alternative 

annual standard level of 12.0 μg/m3 3 (in conjunction with the existing 24-hour standard), the 

4 analysis estimated that these urban study areas have reductions in mortality risk (relative to 

5 risk remaining upon just meeting then-existing suite of standards) ranging from about 11 to 

6 35 percent. 

7  Reductions in short-term exposure-related mortality and morbidity risk: For the alternative 

annual standard level of 12.0 μg/m3 8 (in conjunction with the existing 24-hour standard), the 

9 analysis estimated that reductions in both short-term exposure-related cardiovascular 

mortality and morbidity risk ranged from 5 to 23 percent across the urban study areas. 100 10 

11 4.2.2 Key Uncertainties in the 2010 HREA 

12 The EPA recognized that both the inputs to the risk assessment and the risk model used 

13 were each subject to uncertainties that affected the magnitude and distribution of the results. The 

14 2010 HREA followed the World Health Organization (WHO, 2008) framework for 

15 characterizing uncertainty in the analyses and incorporated elements intended to increase the 

16 EPA’s overall confidence in the risks estimated in the fifteen urban study areas (U.S. EPA, 

17 2010a, section 3.5.1 and 4.3), including: 

18  The use of a rigorous, transparent, and fully documented process subject to peer and public 

19 review in developing all key elements of the 2010 HREA (U.S. EPA, 2010a, section 5.1.1); 

20  Integration of key sources of variability into the design and interpretation of results; 

21  Assessment of the degree to which the urban study areas included in the 2010 HREA are 

22 representative of areas in the U.S. experiencing higher PM2.5-related risk; and 

23  Identification and assessment of the impact (i.e., magnitude and direction) of important 

24 sources of uncertainty on risk estimates. 

25 The 2010 HREA included both qualitative (U.S. EPA, 2010a, section 3.5.3) and 

26 quantitative sensitivity analyses (U.S. EPA, 2010a, section 3.5.4 and 4.3) designed to identify 

27 sources of uncertainty most influencing the estimated risk. These analyses characterized the 

28 sources of uncertainty likely to substantially impact the estimated risk. Since quantitative 

29 information was not available to characterize overall levels of confidence in alternative model 

 

100 The patterns of risk reduction (as a percentage) were similar for both sets of endpoints because the rollback was 

the same for both and the C-R functions were assumed to be linear (U.S. EPA 2010a, section 4.2.2). 
April 2016 4-9 Draft – Do Quote or Cite

1 inputs, the uncertainty characterization in the 2010 HREA did not include a full probabilistic 

2 assessment of uncertainty and its impact on risk estimates. 

3 

4 Key observations from analyses of uncertainty 

5 The qualitative analysis of uncertainty identified the following sources of uncertainty as 

potentially having a medium to high impact101 6 on risk estimates (see Table 3-6, p. 3-72 of the 

7 2010 HREA for additional detail, U.S. EPA, 2010a): 

8  Intra-urban variability in PM2.5 exposure in epidemiologic studies: Use of composite 

9 monitors as exposure surrogates in epidemiologic studies may not fully capture spatial 

10 variability in PM concentrations within an urban area, and introduces exposure measurement 

11 error and uncertainty into the effect estimates obtained from the epidemiologic studies. 

12  Shape and statistical fit of the C-R functions: There is uncertainty in the shape of the C-R 

13 function, particularly at lower ranges of concentrations due to a lack of observations. In 

14 addition, we consider the statistical fit of the of the C-R function. The term "statistical fit" as 

15 used here indicates the precision of a statistical model for capturing the observations. It can 

16 be influenced by a variety of factors, including exposure measurement error, sample size, and 

17 control for confounders. 

18  Lag structure (short-term exposure studies): Different lags may have varying degrees of 

19 association with a particular health endpoint, and it may be difficult to clearly identify the 

20 specific lag that produces the majority of a PM-related effect. A lack of information 

21 regarding the specific lag(s) most associated with a particular health endpoint adds 

22 uncertainty into risk estimates for that endpoint. 

23  Transferability of C-R functions from epidemiologic study areas to HREA urban study areas 

24 (long-term exposure health endpoints): The use of effects estimates based on data collected 

25 in a particular location(s) as part of the underlying epidemiologic study in different locations 

26 (the focus of the risk assessment) introduces uncertainty into the analysis. 

27  Single-city versus multi-city epidemiologic studies: Often both single-city and multi-city 

28 epidemiologic studies are available (for a given health endpoint) for the derivation of C-R 

29 functions. Each of these study types has advantages and disadvantages, and both study types 

30 can introduce uncertainty into the analysis. 

31  Historical PM2.5 exposures: Long-term exposure studies of mortality suggest that different 

32 time periods of PM exposure can produce substantially different effects estimates, 

33 introducing uncertainty in identifying the exposure window that is most strongly associated 

34 with mortality. 

 

101 The 2010 HREA classified sources as “medium impact” if they have the potential to change the interpretation of 

the risk estimates in the context of the PM NAAQS review, and classified sources as “high impact” if they are likely 

to influence the interpretation of risk. 
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1  PM compositional differences: The composition of PM can differ across urban study areas. If 

2 these compositional differences contribute to different effect estimates (i.e., heterogeneity), 

3 then substantial uncertainty may be introduced into risk estimates if these compositional 

4 differences are not explicitly addressed. 

5 

6 The 2010 HREA included two types of quantitative sensitivity analyses: single-factor and 

7 multi-factor. The single-factor sensitivity analysis varied one source of uncertainty at a time and 

identified the following sources of uncertainty as having a moderate to large impact102 8 on risk 

9 estimates (U.S. EPA, 2010a, section 4.3.1): 

10  Long-term exposure-related mortality: (a) different C-R function model choices (e.g., fixed 

11 versus random effects, log-linear versus log-log, single- versus co-pollutant), (b) modeling 

12 risk down to PRB rather than LML, (c) use of C-R functions from different epidemiologic 

13 studies, and (d) nature of the spatial pattern of ambient PM2.5 reductions (i.e., rollback 

14 method). 

15  Short-term exposure-related mortality and morbidity: (a) use of seasonally differentiated 

16 versus annual-based C-R functions and (b) different models, lag structures and single-versus 

17 co-pollutant model forms. 

18 The multi-factor sensitivity analysis varied multiple sources of uncertainty simultaneously and 

19 covered both long-term exposure-related mortality and short-term exposure-related mortality and 

20 morbidity. This multi-factor analysis found that a number of sources of uncertainty, identified 

21 above, could work in concert to affect risk estimates, and that these combined effects would be 

22 more than additive in certain circumstances (U.S. EPA, 2010a, section 4.3.1.2). 

23 

24 4.3 CONSIDERATION OF POTENTIAL QUANTITATIVE 

25 ASSESSMENTS FOR THIS REVIEW 

26 The goal of an HREA is to provide information relevant to answering questions regarding 

27 the adequacy of the existing PM primary standards and, if appropriate, the potential 

28 improvements in public health from meeting potential alternative standards. A quantitative 

29 HREA for this review, if warranted, would build on the approaches used and lessons learned in 

30 the 2010 HREA and would focus on improving the characterization of PM exposure and

 

102 The 2010 HREA classified factors as “moderate to large impact” if they had a 20 percent to greater than a 100 

percent difference on risk results. 
April 2016 4-11 Draft – Do Quote or Cite

1 associated health risks, including related uncertainties, by incorporating a number of 

2 enhancements, in terms of newly available models, methods, tools, and data. 

3 Characterizing health risks for the current review of the primary PM NAAQS could 

4 include conducting air quality analyses to support quantitative assessments of risk and exposure 

5 in specific urban study areas as well as putting the results into a national public health 

6 perspective. We anticipate that, consistent with the large body of available scientific and air 

7 quality evidence available to support quantitative assessments, the most likely focus of 

8 quantitative analyses in an HREA would be PM2.5. These assessments would be designed to 

9 estimate human exposures and/or to characterize the potential health risks that are associated 

10 with recent ambient levels, with ambient PM2.5 concentrations adjusted to just meet the now11 existing standards (12/35) and, if appropriate, with ambient concentrations adjusted to just meet 

12 alternative standard(s) that may be considered. To the extent that information becomes available 

13 in the PM ISA for the current review that could support quantitative analyses of PM2.5

14 components, PM10-2.5, or ultrafine particles, additional quantitative analyses could be considered 

15 which focus on these other size fractions or components. Similar to considerations for PM2.5 risk 

16 analyses, we would evaluate whether sufficient information is available for other size fractions 

17 and/or components, including air quality information, endpoints with causal or likely to be causal 

18 determinations in the ISA, availability of concentration-response functions, and baseline 

19 incidence data. The considerations for all potential quantitative epidemiology-based risk analyses 

20 are described in greater detail in section 4.3.1 below. 

21 As described further in this section, an HREA for the current review would consider a 

22 variety of health endpoints for which, in staff’s judgment, there would be adequate information 

23 to develop quantitative risk estimates that can meaningfully inform the review of the primary PM 

24 NAAQS. We also intend to evaluate the distribution of risks and patterns of risk reduction (e.g., 

25 among urban case study areas and across ambient PM2.5 concentrations) and uncertainties in 

26 those risk estimates. In presenting the results of any new assessments, we plan to evaluate the 

27 influence of various inputs and assumptions on the exposure and risk estimates to more clearly 

28 differentiate potential alternative standard(s) that might be considered, including potential 

29 impacts on various at-risk populations and lifestages. In addition, we will consider characterizing 

30 risk on a national scale. This assessment could also evaluate the distribution of PM-related health 

31 risk across the U.S. (and the factors that influence the distribution of risk) to assess the degree to 

32 which risks estimated in each urban study area are representative of national-level risks. 

33 An important issue associated with conducting exposure and human health risk 

34 assessments is the treatment of variability and the characterization of uncertainty. Variability 

35 refers to the inherent differences in a population or variable of interest (e.g., residential air 
April 2016 4-12 Draft – Do Quote or Cite

1 exchange rates); it cannot be reduced through further research, but it may be better characterized 

2 with additional measurement. However, models can account for variability. Uncertainty refers to 

3 the lack of knowledge regarding both the actual values of model input variables (i.e., parameter 

4 uncertainty) and the physical systems or relationships (i.e., model uncertainty – e.g., the shapes 

5 of concentration-response relationships). Uncharacterized spatial and temporal variability in 

6 PM2.5 mass or species concentrations is often an important source of uncertainty in PM risk 

7 assessments. As part of such analyses, variability and uncertainty will be explicitly addressed, 

8 where feasible, in any new air quality, exposure, and health risk assessments. 

9 Consistent with the 2010 HREA, we are considering following the WHO framework 

10 (WHO, 2008) to characterize uncertainty and variability, which presents a multi-tiered approach 

11 for characterizing uncertainty (and to a lesser extent variability) in the context of a risk 

12 assessment. This framework provides a method for systematically linking the characterization of 

13 uncertainty to the level of complexity of the underlying risk assessment. Ultimately, the decision 

14 as to which tier of uncertainty characterization to include in this risk assessment will depend on 

15 the availability of information for characterizing the various sources of uncertainty. The four 

16 WHO tiers range from application of default uncertainty factors up to probabilistic assessments. 

17 Similar to the 2010 HREA, we anticipate that the uncertainty characterization in any HREA for 

18 the current review would likely have sufficient data to conduct uncertainty assessments in the 

19 middle tiers (e.g., qualitative assessments of direction/magnitude of the effect on results and 

20 sensitivity analyses). 

21 Below, we outline the two major components of the quantitative health risk 

22 characterization (i.e., epidemiology-based risk assessments and exposure assessment) that will be 

23 described in more detail in the HREA Planning Document. Preparation of this planning 

24 document will draw from the assessments of the scientific evidence in the first draft PM ISA to 

25 facilitate the integration of policy-relevant science into the planning document. In particular, the 

26 availability of air quality, exposure-response, concentration-response, and baseline incidence and 

27 prevalence data as well as consideration of available resources will influence the type of risk and 

28 exposure assessments that would be developed. 

29 4.3.1 Epidemiology-Based Risk Assessment 

30 The goals of an updated epidemiology-based risk assessment for PM would be to: (1) 

31 estimate the potential magnitude of mortality and morbidity risks in urban study areas for recent 

32 ambient concentrations as well as for PM concentrations adjusted to meet the existing standards 

33 and, if appropriate, potential alternative standard(s); (2) develop a better understanding of how 

34 various inputs and assumptions influence the risk estimates, including a characterization of the 
April 2016 4-13 Draft – Do Quote or Cite

1 confidence in the risk estimates ; and (3) gain insights into the distribution of risks and patterns 

2 of risk reduction across modeled populations (including at-risk populations and lifestages) as 

3 well as the spatial distribution of risk within and across modeled study areas. 

4 The approach for an epidemiology-based risk assessment for the current review would 

5 build upon the methods developed for and insights gained from the 2010 HREA (section 4.2 

6 above). Section 4.3.1.1 below describes the ambient PM concentrations for recent conditions and 

7 adjustments to just meet the existing and potential alternative standard(s). Section 4.3.1.2 

8 discusses factors to be considered in designing any new epidemiology-based risk assessment. 

9 Section 4.3.1.3 identifies the sources of uncertainty that had a substantial impact on risk 

10 estimates in the 2010 HREA and discusses potential strategies for reducing and characterizing 

11 those sources of uncertainty in the current review. Each of these design factors and strategies for 

12 reducing uncertainties, including data availability, would be further evaluated in the HREA 

13 Planning Document. 

14 4.3.1.1 Ambient PM Concentrations 

15 Recent Ambient Conditions 

16 For an HREA for the current review, we anticipate relying on data from the national air 

17 quality monitoring networks described above (in section 2.3) to characterize ambient PM 

18 concentrations for the time period and metric of interest (e.g., size fraction, form). As described 

19 above (in section 4.2), the 2010 HREA used data from these monitoring networks to characterize 

20 PM2.5 concentrations and associated health risks during 2005 to 2007 for 15 urban study areas. A 

21 similar degree of PM2.5 monitoring is now available for more recent years that could be 

22 considered in the current review. We would select specific urban study areas to assess in this 

23 HREA by considering the available monitoring data in conjunction with the new epidemiologic 

24 studies in the upcoming PM ISA, and this new information may suggest we consider the same 

25 study areas in the upcoming HREA that were assessed in the 2010 HREA. Criteria for 

26 identifying urban study areas will be identified in the HREA Planning Document. 

27 In the 2010 HREA, the air quality analyses focused on PM2.5. Due to the substantial 

28 uncertainties and data gaps that were identified, the 2010 HREA did not include quantitative 

29 analyses for additional PM size fractions (e.g., PM10-2.5 or ultrafine particles). For PM10-2.5, these 

30 uncertainties were related to (1) concerns that the monitoring data for 2005 to 2007 would not be 

31 similar to the data used in the epidemiologic studies that provided the effect estimates for the 

32 C-R functions; (2) uncertainty in the prediction of ambient concentrations under the then-existing 

33 and alternative standards; (3) concerns that the 15 urban study areas in the 2010 HREA may not 

34 be representative of areas experiencing the highest PM10-2.5 concentrations; and (4) concerns 
April 2016 4-14 Draft – Do Quote or Cite

1 about the much smaller number of relevant epidemiologic studies for PM10-2.5 compared to the 

2 larger number of studies for PM2.5. The 2010 HREA also concluded that the available 

3 information was too limited to support quantitative assessments for ultrafine particles. A lack of 

information on the appropriate metric103 4 for characterizing ultrafine particles in risk assessments, 

5 as well as the lack of a national monitoring network for ultrafine particles, are likely to continue 

6 to challenge our ability to conduct a quantitative assessment for ultrafine particles in the current 

7 review. In the current review, we will consider whether new information on these areas of 

8 uncertainty has become available since the last review of the PM NAAQS that would be 

9 sufficient to support quantitative air quality assessments of recent ambient conditions for PM10-2.5

10 or ultrafine particles. If so, we will consider in the HREA Planning Document the degree to 

11 which these ambient concentrations could be adequately characterized using available data from 

12 the national monitoring networks (or other datasets in the case of ultrafine particles) to support 

13 the HREA. 

14 The 2010 HREA did not evaluate PM2.5 composition differences quantitatively because 

C-R functions for specific PM2.5 components and sources had not been identified at that time.104 15 

16 In the HREA Planning Document, we will consider whether new information on C-R functions 

17 for components and/or sources has become available in the PM ISA for the current review and 

18 could necessitate the characterization of ambient PM2.5 components and/or sources. If warranted 

19 for the HREA for the current review, we would consider the degree to which measurements from 

20 the CSN and IMPROVE networks described in Chapter 2 could be used to adequately 

21 characterize PM2.5 components or sources. 

22 Approach for Adjusting PM Concentrations to Just Meet the Existing and Potential 

23 Alternative Standard(s) 

24 In order to estimate changes in health risks from just meeting the existing PM2.5 standards, 

25 and potential alternative standard(s) if appropriate, we would first need to adjust recent ambient 

26 concentrations to reflect the distribution of PM2.5 concentrations in an urban area that could 

27 correspond to these air quality scenarios. Since there are multiple combinations of emissions 

28 profiles that could result in just meeting the current standards or any potential alternative 

29 standards, there are many possible scenarios of various spatially and temporally distributed PM2.5

 

103 For example, total particle number, solid particle number, active particle surface area, Brunauer–Emmett–Teller 

surface area, or mass of particles in a certain size range. 104 However, the 2010 HREA indirectly considered PM composition differences by applying region- or city-specific 

effect estimates in modeling health risks for short-term exposure to ambient PM2.5. 
April 2016 4-15 Draft – Do Quote or Cite

1 concentrations that would correspond to the target air quality scenarios. Given this fact, the 2010 

2 HREA used multiple air quality adjustment approaches (i.e., proportional, local, and hybrid 

3 rollback) to estimate the effects on the distribution of PM2.5 concentrations at monitors in an 

4 urban study area of just meeting the then-existing and alternative standards. Use of multiple 

5 adjustment approaches enabled exploration of the sensitivity of risk results to different spatial 

6 patterns of PM2.5 reduction, with the goal of largely bounding potential risks. 

7 In the HREA Planning Document, we will consider whether it would be appropriate to 

8 use additional or different approaches to adjusting air quality concentrations including methods 

9 based on results from chemical transport modeling of national emission reduction scenarios. 

10 These additional or different adjustment approaches would aim to improve the representation of 

11 PM2.5 reduction patterns over an area. If estimates of PRB concentrations are needed for the air 

12 quality adjustments in the HREA, we would consider information provided in the PM ISA for 

13 the current review. In selecting methods for air quality adjustment, we would consider the ability 

14 of the approaches to represent the relationship between the annual and 24-hour NAAQS. 

15 There is relatively limited information on PM10-2.5, specific PM components and sources, 

16 or ultrafine particles compared with PM2.5 to develop approaches for adjusting their 

17 concentrations to meet the existing PM2.5 standards and potential alternative standard(s), if 

18 appropriate. We will consider in the HREA Planning Document the ability of various adjustment 

19 approaches, including those based on statistical and air quality modeling relationships, to provide 

20 reasonable estimates of adjusted concentrations for PM10-2.5, specific PM components and 

21 sources, and ultrafine particles, if warranted. 

22 4.3.1.2 Factors to Consider in Designing any New Epidemiology-based Risk Assessment 

23  PM size fractions, components, and sources: We anticipate focusing any new quantitative 

24 risk assessments primarily on PM2.5 but will consider, to the extent sufficient air quality and 

25 epidemiologic data become available in the PM ISA for the current review, characterizing 

26 risks associated with PM10-2.5 and ultrafine particles as well as risks associated with specific 

27 PM components and/or sources. 

28  Health endpoints and health input data: For any quantitative risk assessment, we would 

29 follow a multi-step process for each health endpoint considered. First, we would identify 

30 those health endpoints that the PM ISA for the current review determines have either a 

31 “causal” or “likely to be causal” relationship with PM exposure. Next, we would use 

32 information from the PM ISA and the epidemiologic studies to specify C-R functions for 

33 those health endpoints, including details on model form and associated effect estimates. Last, 

34 we would obtain the necessary baseline incidence and/or prevalence rate data for each 

35 endpoint. These rates originate from a variety of sources and often reflect varying spatial 

36 scales. 
April 2016 4-16 Draft – Do Quote or Cite

1  Exposure metrics: Similar to previous PM HREAs, we anticipate modeling health endpoints 

2 associated with both short-term (24-hour average) and long-term (annual average) exposure 

3 depending on the epidemiologic study data available in the PM ISA for this review. 

4 Furthermore, for short-term exposure-related endpoints, we could consider whether to 

5 differentiate risk estimates through application of seasonally differentiated effect estimates 

6 paired with seasonal mortality rates, if available. 

7  Spatial scale of the analysis: Based on the epidemiologic studies available in the PM ISA for 

8 the current review and the available air quality monitoring data, we would anticipate 

9 analyzing a set of study areas in detail and applying effect estimates specific to each study 

10 area. We would also anticipate adjusting recent PM2.5 concentrations to just meet the existing 

11 and, if appropriate, potential alternative standard(s), in each urban area, including 

12 consideration of various adjustment approaches (e.g., proportional rollback). In addition, we 

13 will consider whether a national-scale analysis of risk for recent ambient concentrations 

14 (such as completed in the 2010 HREA) is useful for evaluating the representativeness of the 

15 study areas. We will also consider whether to estimate risk reductions at the national-level 

16 from just meeting potential alternative standard level(s), if any. However, adjusting air 

17 quality can be more challenging at broader regional or national scales than at the urban study 

18 area scale. We anticipate that any national-level estimates, if generated, would supplement 

19 the higher confidence risks estimated for the study areas. 

20  At-risk populations and lifestages: Based on information from the PM ISA, we would 

21 consider whether data would be sufficient to generate more refined risk estimates for these 

22 populations groups, including whether effect estimates and baseline incidence rates for these 

23 at-risk populations and lifestages are available. 

24  Risk metrics: The 2010 HREA quantified risks using several risk metrics, including (a) 

25 PM2.5-related incidence (for both morbidity and mortality endpoints), (b) PM2.5-related 

26 incidence per 100,000 exposed individuals, representing a standardized risk metric that can 

27 be readily compared across urban study areas, (c) percent of baseline incidence attributable to 

28 PM2.5 exposure, and (d) percent reduction in PM-related risk from just meeting the then29 existing and alternative standard levels. We anticipate including similar risk metrics in any 

30 new risk assessment, and we will consider including some additional risk metrics based on 

31 information from the PM ISA to more fully characterize PM-related mortality risk. For 

32 example, we may consider metrics that characterize person-level risk, such as life years 

33 gained (from just meeting the existing and potential alternative standard(s), if any), life years 

34 lost (from exposure to recent PM concentrations), and/or estimation of population-level 

35 changes in life expectancy. 

36  Differentiating risk estimates by PM concentration: Based on information from the PM ISA, 

37 we would consider whether it would be appropriate and informative to differentiate the 

38 magnitude of risks or overall confidence in risk estimates for different concentration ranges. 

39 Such an analysis could reflect, among other factors, reduced confidence in specifying C-R 

40 functions at lower PM concentrations where less data exist. For such an analysis, we would 

41 explore methods such as (a) binning risk estimates into categories of confidence based on 

42 data available for C-R functions including differences in the size of standard errors at various 
April 2016 4-17 Draft – Do Quote or Cite

1 PM concentrations, and (b) using other types of evidence (clinical and/or toxicological) to 

2 differentiate risk estimates semi-quantitatively within specified PM2.5 concentration ranges.

3  BenMAP-CE: Consistent with the HREA conducted for the last ozone NAAQS review (U.S. 

4 EPA, 2014), we anticipate using the environmental Benefits Mapping and Analysis Program 

5 – Community Edition (BenMAP-CE) (U.S. EPA, 2015), the EPA’s GIS-based computer 

program for the estimation of health impacts associated with changes in air pollution.105 6 

7 BenMAP-CE draws upon a database of population, baseline incidence and prevalence, and 

effect estimates to automate the calculation of health impacts. 106 8 In addition, BenMAP-CE 

can utilize standard errors from the effect estimates to generate 95th 9 percentile confidence 

10 intervals around point estimates. These confidence intervals reflect the statistical fit 

11 (precision) of the underlying epidemiologic models, assuming that the form of the model is 

12 correct. 

13 4.3.1.3 Characterization of Sources of Uncertainty and Consideration of Information Newly 

14 Available in this Review 

15 As described above, the 2010 HREA primarily used sensitivity analyses (including both 

16 single and multi-factor) to evaluate the impact of uncertainty in specific input factors on the risk 

17 estimates, while a probabilistic analysis (i.e., Monte Carlo) was used to cascade the impact of 

standard errors in the effect estimates from epidemiologic studies into 95th 18 percentile confidence 

19 intervals around the risk estimates. Although Monte Carlo analyses could be used to reflect the 

20 combined impact of multiple sources of uncertainty affecting the risk estimates, this type of 

21 analysis would require information not previously available, including the specification of 

22 defensible distributions of values for key inputs and any potential correlations between those 

23 inputs. For an HREA in the current review, we anticipate relying on single and multi-factor 

24 sensitivity analysis as the primary method for characterizing the combined impact of uncertainty 

25 from multiple key inputs on the risk estimates, although we will consider additional tools should 

26 they become available. Based on information from the PM ISA for the current review, we would 

27 anticipate conducting additional sensitivity analyses to better characterize uncertainties related to 

28 the specification of C-R functions (e.g., model specification, treatment of copollutants, and 

29 treatment of lag for short-term exposure endpoints). 

 

105 The BenMAP software and associated documentation are available for download at 

http://www2.epa.gov/benmap. 106 As noted in section 4.2, the 2010 HREA used the TRIM model to estimate human health risks (U.S. EPA, 

2010a). For subsequent NAAQS epidemiology-based risk assessments (U.S. EPA, 2014), we utilize BenMAP-CE, 

which provides a more efficient and flexible platform for modeling PM-related health risk. For example, BenMAPCE allows users to readily conduct sensitivity analyses exploring the impact of alternative modeling choices (e.g., 

effect estimates, thresholds, age ranges for target populations, units of spatial aggregation) on risk estimates. 
April 2016 4-18 Draft – Do Quote or Cite

1 In designing an HREA for the current review, our goal will be to reduce and/or better 

2 characterize the impact of the sources of uncertainty identified in the 2010 HREA to the extent 

3 possible given available information. In addition, we would address any newly identified sources 

4 of uncertainty. In Table 4-1 below, we identify several important sources of uncertainty and 

5 discuss our considerations for potentially addressing these sources of uncertainty. These 

6 strategies would be further described in the HREA Planning Document. 
April 2016 4-19 Draft – Do Quote or Cite

1 Table 4-1. Areas of Uncertainty Associated with Epidemiology-Based Risk Estimates and 

2 Potential Strategies for Addressing these Sources of Uncertainty 

Category of 

Uncertainty/ 

Limitation 

Description of Uncertainty/ 

Limitation in the Last Review 

Consideration of Information 

Newly Available in this 

Review 

Transferability of 

C-R functions from 

epidemiologic study 

areas to HREA 

urban study areas 

At times, the EPA applies effect 

estimates to urban study areas that 

were not included in the original 

epidemiologic study providing those 

effect estimates. This geographic 

extrapolation can introduce 

uncertainty into the risk estimates for 

those urban study areas. 

New epidemiologic studies in 

the PM ISA and other available 

data may provide additional 

perspective on which urban 

areas or regions are likely to 

have similar population-level 

responses to PM pollution and 

which are likely to have 

different responses. This 

information may guide 

decision-making regarding the 

transferability of effect 

estimates to broader 

geographic areas. 

Transferability of 

C-R functions from 

an earlier time 

period (in the 

epidemiologic 

study) to a more 

recent time period in 

the HREA 

Depending on the degree to which 

conditions associated with PM 

exposure and risk (e.g., population 

behavior and mobility, baseline 

health effects incidence rates, 

ambient urban PM profiles) changed 

over time, uncertainty may be 

associated with the temporal 

extrapolation of effect estimates. 

New epidemiologic studies in 

the PM ISA may provide 

additional perspective on the 

degree to which effect 

estimates vary (for the same 

geographic location) over time 

and hence the degree to which 

uncertainty associated with this 

type of temporal extrapolation 

of effect estimates may 

introduce uncertainty into the 

risk results. 

Use of composite 

monitors as 

exposure surrogates 

in the epidemiologic 

studies 

The 2010 HREA asserted that the 

exposure surrogate used in 

epidemiologic studies (i.e.., a single 

composite monitor representing an 

entire urban study area) could impact 

the magnitude of the effect estimates. 

New epidemiologic studies in 

the PM ISA may characterize 

the degree to which different 

spatial scales (used in defining 

exposure surrogates) introduce 

exposure measurement error, 

depending on the endpoint, 

location and pollutant being 

evaluated. 

Spatial 

heterogeneity in 

effect estimates in 

the epidemiologic 

studies 

In the 2010 HREA, effect estimates 

for short-term mortality and 

morbidity endpoints demonstrated 

considerable spatial heterogeneity. 

At that time, the EPA noted that this 

New epidemiologic studies in 

the PM ISA might provide 

information regarding which 

factors drive spatial 

heterogeneity in effect 
April 2016 4-20 Draft – Do Quote or Cite

Category of 

Uncertainty/ 

Limitation 

Description of Uncertainty/ 

Limitation in the Last Review 

Consideration of Information 

Newly Available in this 

Review 

heterogeneity could reflect a number 

of factors (e.g., differences in 

behavior across/within cities, PM 

compositional/source differences, 

different degrees of exposure 

measurement error).

estimates. This information 

could help us to differentiate 

between effect estimates (and 

hence the risk estimates) in 

terms of overall confidence. 

Shape and statistical 

fit of the C-R 

functions107 

There is uncertainty in the shape and 

statistical fit of the C-R functions, 

particularly at lower ranges of 

concentrations. Although the 2010 

HREA applied linear C-R functions, 

these functions could have nonlinearities, thresholds or 

concentrations with reduced 

confidence in the effect estimate. 

New epidemiologic studies in 

the PM ISA could provide 

additional information 

regarding risk across the range 

of PM concentrations or 

possible non-linearities in the 

C-R function, particularly in 

lower PM concentrations 

where there tends to be less 

data. Greater data density at 

lower concentrations could also 

result in increased precision 

(and hence statistical fit) within 

that range. 

Lag structure in 

short-term exposure 

epidemiologic 

studies 

Different lags have varying degrees 

of association with a particular health 

endpoint, and it may be difficult to 

clearly identify the specific lag that 

produces the majority of a PMrelated effect. A lack of information 

regarding the specific lag(s) most 

associated with a particular health 

endpoint adds uncertainty into risk 

estimates for that endpoint. For 

short-term mortality, the 2010 HREA 

used an average of 0 and 1-day lags, 

and other lags, where available, were 

considered in sensitivity analyses. 

New epidemiologic studies in 

the PM ISA could provide 

additional information 

regarding the lag structure most 

strongly associated with 

specific health endpoints. 

 

107 The term "statistical fit" as used here indicates the precision of a statistical model for capturing the observations. 

It can be influenced by a variety of factors, including exposure measurement error, sample size, and control for 

confounders. 
April 2016 4-21 Draft – Do Quote or Cite

Category of 

Uncertainty/ 

Limitation 

Description of Uncertainty/ 

Limitation in the Last Review 

Consideration of Information 

Newly Available in this 

Review 

Single pollutant 

models versus 

copollutant models 

Ozone and other ambient 

copollutants may confound effect 

estimates associated with PM. 

New epidemiologic studies in 

the PM ISA could provide 

additional information 

regarding potential 

confounding by copollutants. 

Single-city versus 

multi-city 

epidemiologic 

studies 

For long-term exposure mortality, the 

2010 HREA employed effect 

estimates from multi-city 

epidemiologic studies. For short-term 

exposure mortality, the 2010 HREA 

used a combination of single and 

multi-city epidemiologic studies, 

acknowledging advantages and 

disadvantages with both types of 

studies. 

New epidemiologic studies in 

the PM ISA could provide 

additional information 

regarding whether either type 

of study (or a combination of 

both) would be preferred in 

modeling specific endpoints. 

New single-city epidemiologic 

studies in the PM ISA could 

add confidence to the 

individual city risk results 

estimated using multi-city 

effect estimates. 

Historical PM 

exposure in longterm exposure 

studies 

Long-term epidemiologic studies 

suggest that different time periods of 

PM exposure can produce 

substantially different effects 

estimates. This can introduce 

uncertainty in identifying the 

exposure window is most strongly 

associated with an endpoint. 

New epidemiologic studies in 

the PM ISA could provide 

additional information 

regarding which exposure 

windows are most strongly 

associated with long-term 

exposure endpoints, 

recognizing potential 

measurement error linked to 

population mobility (within 

and between cities) over time. 

Differences in PM 

composition or 

sources 

The composition of PM and overall 

pollution mixture can differ across 

urban study areas. If these 

compositional differences contribute 

to different effect estimates (i.e., 

heterogeneity), then substantial 

uncertainty may be introduced into 

the risk estimates if these 

compositional differences are not 

explicitly addressed. 

New epidemiologic, 

toxicological and clinical 

studies in the PM ISA could 

provide additional information 

regarding differences in PM 

composition, including whether 

there is sufficient evidence to 

model risk by applying 

differential effect estimates. 
April 2016 4-22 Draft – Do Quote or Cite

1 4.3.2 Exposure Assessment 

2 As noted above, neither the 2010 HREA nor any prior review of the PM NAAQS 

3 included a quantitative population-based, microenvironmental exposure assessment, largely 

4 because of the uncertainties surrounding the purpose of such an assessment and the insights it 

5 could provide in better understanding PM-related health risks beyond those provided using the 

6 epidemiology-based approach. In the current review, we will again consider whether to perform 

7 an exposure assessment. In doing so, we will consider the extent to which the results of such an 

8 assessment could be informative when interpreted in light of important uncertainties. Key factors 

9 that we will consider when determining whether to perform an exposure assessment include our 

10 technical ability to generate estimates of PM exposure on temporal and spatial scales that would 

11 be informative for the review, quantitative and qualitative characterizations of uncertainties in 

12 exposure estimates, and the availability in this review of PM human exposure studies that 

13 provide insight into the potential public health importance of particular PM exposures. 

14 Unlike an epidemiology-based risk assessment that often uses composite ambient monitor 

15 concentrations to represent generalized exposure over an urban area (as described in section 

16 4.3.1), an exposure assessment could estimate the magnitude, frequency, duration, and pattern of 

17 exposures to PM concentrations, considering activity patterns and microenvironments. Such 

18 quantitative assessments could range from a data simplified analysis up to a modeled population19 based, microenvironmental exposure assessment. A detailed exposure assessment could provide 

20 insight into the populations with the highest PM exposures. In addition, if available human 

21 exposure studies provide a basis for identifying ambient-relevant PM exposure concentrations 

22 that result in effects of potential public health concern, an exposure assessment could also 

23 include a comparison of PM exposure estimates to the exposure concentrations shown to cause 

24 such effects (i.e., an exposure benchmark analysis). 

25 If a quantitative population-based, microenvironmental exposure assessment is warranted, 

26 exposure calculations would rely largely on measurements of ambient PM concentrations 

27 potentially supplemented with information from air quality models and databases of daily 

28 time/location/activity patterns and personal attributes, as well as information on physical factors 

29 that influence microenvironmental concentrations, exposures, and internal doses. Important 

30 exposure factors include information on atmospheric chemistry and components of PM, temporal 

31 and spatial distributions of air quality data, time spent in particular microenvironments and at 

32 varying exertion levels, and information on at-risk populations and lifestages. Such an 

33 assessment, if warranted, would likely focus primarily on ambient-related exposures to PM2.5 but 

34 could consider, to the extent sufficient information becomes available in the PM ISA, exposures 

35 associated with other PM size fractions and/or particular PM components. If warranted, such an 
April 2016 4-23 Draft – Do Quote or Cite

1 exposure assessment would build upon the information presented in the PM ISA, could employ a 

2 modeling approach recently used for other NAAQS reviews (Nitrogen Dioxide: U.S. EPA, 2008; 

3 Sulfur Dioxide: U.S. EPA, 2009b; Carbon Monoxide: U.S. EPA, 2010c; Ozone: U.S. EPA, 

4 2014), and would depend upon available data and resources. 

5 Section 4.3.2.1 below discusses factors to be considered in designing an exposure 

6 assessment. Section 4.3.2.2 presents challenges in conducting such an assessment and sources of 

7 uncertainty. These design factors and strategies for reducing uncertainties would be further 

8 evaluated in the HREA Planning Document. In the HREA Planning Document, we will consider 

9 whether these strategies, including information from the PM ISA for the current review and 

10 available air quality monitoring data, would provide sufficient confidence to move forward with 

11 an exposure assessment and the associated air quality analyses in the current review. 

12 4.3.2.1 Factors to Consider in Designing an Exposure Assessment 

13  Characterizing ambient PM concentrations. The air quality data used in exposure 

14 assessments generally correspond to the finest spatial and temporal scales in ambient 

15 concentrations that substantially influence important exposure metrics and 

microenvironments of interest.108 16 Developing highly resolved PM2.5 concentrations from 

17 existing monitoring and modeling data to support a population-based, microenvironmental 

18 exposure would present a number of challenges, including adequately representing temporal 

19 and spatial variability in concentrations at fine scales over urban areas with a variety of 

20 microenvironments. The challenges would be greater for PM2.5 components and/or different 

21 PM size fractions, because less monitoring is available, gradients may be sharper, and 

22 modeling may be less reliable for PM2.5 components and different size fractions compared 

23 with PM2.5. 

24  Exposure model: If a population-based, microenvironmental exposure is warranted for 

the current review, we would likely use the APEX model109 25 to estimate exposures. APEX is a 

26 probabilistic human exposure model that simulates the movement of individuals through time 

27 and space within a study area. Exposures are calculated in APEX by accounting for the 

28 complete time series of exposure concentrations and doses as they occur in the simulated 

29 individual (minute by minute), and then typically, they are aggregated to a metric of interest 

30 at the individual level and summarized for the population simulated in the study area. APEX 

31 can use any geographic frame of reference (e.g., census tract centroids, 1x1 km grid blocks)

 

108 For example, the temporal and spatial scales used in recent APEX simulations for ozone (U.S. EPA, 2014) were 

hourly concentrations at the census tract level, due to observed health effects resulting from 6-8 hour ozone 

exposures in controlled human exposure studies and the availability of comprehensive population demographics. 109 Additional information on APEX can be found at: http://www2.epa.gov/fera/human-exposure-modeling-airpollutants-exposure-model. 
April 2016 4-24 Draft – Do Quote or Cite

1 and time period (e.g., minute, hourly) to represent ambient concentrations for calculating 

2 exposures. 

3  Population exposures: An APEX-based exposure assessment could be designed to estimate 

4 population exposures to ambient PM in urban study areas that represent a variety of study 

5 groups of interest (e.g., at-risk populations). Such an assessment could evaluate the influence 

6 of factors that contribute to temporal/spatial patterns of PM concentrations (e.g., climatic 

7 conditions) and variability in activity patterns and microenvironmental concentrations. 

8 Further, because APEX estimates both short-term and long-term exposures for each 

9 simulated individual, we could determine the importance of short-term exposure variability 

10 with respect to long-term cumulative exposure, which could improve the understanding of 

11 the relationship between short-term and long-term exposure health effects via their linked 

12 exposure profiles. 

13  Ambient concentrations and human activity patterns: An APEX-based exposure assessment 

14 can use ambient concentrations with a range of time or spatial scales, and we could design an 

15 exposure assessment to focus on the scale of other APEX model input data (e.g., population 

16 demographics at the census tract level) and the exposure output of interest. The human 

17 activity data used by APEX are drawn from the EPA’s Consolidated Human Activity 

18 Database (CHAD; McCurdy et al., 2000; U.S. EPA, 2002). The CHAD includes data from 

19 several surveys sampling at city, state, and national levels, including diaries of their daily 

20 activities, locations visited, activities performed, time-of-day and their durations. We could 

21 consider whether it would be informative to conduct a child-specific analysis that calculates 

22 exposure for various lifestages utilizing activity patterns and exposure parameters specific to 

23 these lifestages. However, we note that we are currently unaware of exposure parameters for 

24 early lifestages (age < 5), and therefore such an analysis may not be feasible for this review. 

25  Microenvironmental exposure: An APEX-based exposure assessment estimates exposures 

26 that individuals experience while in indoor, outdoor, and inside-vehicle microenvironments 

27 using the ambient concentrations and other input variables specific to microenvironments. In 

28 addition to drawing on information from the PM ISA for the current review to parameterize 

29 these and other input variables, we could consider including specific microenvironments and 

30 variables in other exposure model analyses (e.g., SHEDS-PM; Burke et al., 2001). Because 

31 APEX has a flexible, probabilistic approach to estimating exposure and dose, we could 

32 consider developing appropriate distributions representing variability and uncertainty for 

33 various inputs (e.g., air exchange rates, decay rates, and physiological parameters). 

34  Spatial and temporal scope of exposure assessment: The overall scope of an exposure 

35 assessment, if warranted, could focus on a subset of urban study areas selected for the 

36 epidemiology-based assessment. We could evaluate exposures associated with the recent air 

37 quality conditions and, if feasible and informative, evaluate exposures associated with just 

38 meeting the existing and potential alternative standard(s), if any. We note that there are likely 

39 to be additional challenges and uncertainties associated with characterizing ambient PM 

40 concentrations at high spatial and temporal resolution for just meeting the existing and any 

41 potential alternative standard(s). 

42 
April 2016 4-25 Draft – Do Quote or Cite

1 4.3.2.2 Characterization of Sources of Uncertainty and Consideration of Information Newly 

2 Available in this Review 

3 Conducting an exposure assessment for PM2.5 using APEX presents a number of 

4 challenges in obtaining the inputs for the model. Incomplete or uncertain model inputs result in 

5 uncertainties in the estimated exposures. The primary limitations in input data for APEX are 

6 listed in Table 4-2. Identification of available model inputs and data limitations is one of the first 

7 steps in determining whether an APEX-based exposure assessment is warranted for this review. 

8 These considerations will be discussed, along with possible strategies for addressing the 

9 uncertainties and limitations, in the HREA Planning Document. 

10 Table 4-2. Uncertainties and Limitations to Input Data used by APEX to Estimate 

11 Exposures 

Category of 

Uncertainty/ 

Limitation 

Description of Uncertainty/ Limitation 

Spatial variability of 

ambient PM2.5

concentrations 

APEX requires spatially variable ambient concentrations in order to 

capture the spatial variation of exposure across the study area, and 

home/work differential exposures. The spatial resolution of other 

APEX inputs would be at a census tract level, so ideally tract-to-tract 

spatial variation of ambient PM2.5 would be available for input to 

APEX. 

PM aerodynamic 

properties 

The aerodynamic properties of particles affect indoor infiltration 

rates and deposited PM in different lung regions. The breakdown 

into size fractions and density of the ambient PM2.5 (which are input 

to APEX) are likely to be fairly uncertain. 

Near-road and insidevehicle exposures For some PM size fractions (e.g., PM10-2.5, ultrafine particles), the 

near-road and in-vehicle microenvironments may be important 

contributors to high ambient-related exposures. How these 

concentrations might be characterized could have varying degrees of 

uncertainty depending on the data available or the approach taken. 

Activity pattern 

representativeness 

The extent to which the human activity database (CHAD) provides a 

balanced representation of the population being modeled will vary 

across simulated study areas. APEX constructs activity sequences to 

account for the effects of population demographics and local climate, 

though this adjustment procedure does not fully capture all of the 

variability within an urban study area that exists in the people's 

activities that actually reside in the study area. Also, the extent to 

which a particular at-risk population or lifestage group is represented 

by the complete range of CHAD diaries may be uncertain. 

Approach for 

developing 

longitudinal diary 

profile 

One limitation of the CHAD activity pattern data used in our 

exposure model is that, on average, study participants provided less 

than two days of diary data. Thus, annual activity sequences of days 

for each simulated individual in APEX are a combination of diary 
April 2016 4-26 Draft – Do Quote or Cite

Category of 

Uncertainty/ 

Limitation 

Description of Uncertainty/ Limitation 

days from a single or multiple similarly related subjects (e.g., age, 

sex), generated using a statistical algorithm and appropriate linking 

variables (e.g., daily time spent outdoors). Performance evaluations 

of the APEX algorithms have been limited by the small number of 

CHAD individuals having multiday diaries. 

Indoor air exchange 

rates (AER) 

Because people generally spend a significant amount of time 

indoors, AER are important in estimating exposures. APEX uses 

statistical distributions of AER that account for regional variability 

and several temperature ranges and considers separately the presence 

of mechanical and non-mechanical building ventilation, to the extent 

possible. One limitation is that it does not account for spatial 

differences that may exist in AER within an urban study area (e.g., 

based on differences in age of residence or building). In addition, 

while the set of existing AER distributions were developed from 

measurement studies conducted in specific geographic areas in the 

U.S., they may not include all specific study areas that could be 

selected for a population-based exposure assessment. 

1 

2 4.4 SCIENTIFIC AND PUBLIC REVIEW 

3 The HREA Planning Document will be distributed to the CASAC for review and 

4 provided to the public for review and comment. The document will be the subject of a review 

5 with the CASAC at a public meeting or teleconference that will be announced in the Federal 

6 Register. The EPA does not produce a final HREA Planning Document, but instead considers 

7 CASAC recommendations and public comments in the design and when conducting the 

8 quantitative assessments either in a new HREA or in updating or expanding the last assessment 

9 as part of the PA. In either case, staff would prepare at least one draft of the assessment for 

10 CASAC review and public comment. CASAC would review the document and discuss it at a 

11 public meeting that would be announced in the Federal Register. Based on past practice, the 

12 EPA expects that CASAC would summarize key advice and recommendations for revision of the 

13 assessment in a letter to the EPA Administrator. In revising any draft HREA, the EPA would 

14 take into account any such recommendations and also consider comments received from the 

15 public, both at the meeting itself and directly in writing. A final assessment would then be made 

16 available on an EPA website, with its public availability announced in the Federal Register. 
April 2016 4-27 Draft – Do Quote or Cite

1 REFERENCES 

2 Abt Associates Inc (2008). Environmental Benefits Mapping and Analysis Program (Version 

3 3.0). Bethesda, MD. Prepared for Environmental Protection Agency, Office of Air 

4 Quality Planning and Standards, Air Benefits and Costs Group. Research Triangle Park, 

5 NC. 

6 Byun, D., and K.L. Schere (2006). Review of the Governing Equations, Computational 

7 Algorithms, and Other Components of the Models-3 Community Multiscale Air Quality 

8 (CMAQ) Modeling System. Applied Mechanics Reviews, 59, 51-77. 

9 Burke, J., Zufall, M., Özkaynak, H. (2001). A population exposure model for particulate matter: 

10 Case study results for PM2.5 in Philadelphia, PA. J Expos Anal Environ Epidemiol. 11: 

11 470-489. 

12 McCurdy, T.; G. Glen; L. Smith and Y. Lakkadi (2000). The National Exposure Research 

13 Laboratory's consolidated human activity database. Journal Expos Anal Environ 

14 Epidemiol. 10:566-578. 

15 Samet (2009a). Letter from Dr. Jonathan M. Samet, Chair Clean Air Scientific Advisory 

16 Committee, to Lisa P. Jackson, Administrator U.S. Environmental Protection Agency 

17 regarding “Consultation on EPA’s Particulate Matter National Ambient Air Quality 

18 Standards: Scope and Methods Plan for Health Risk and Exposure Assessment.” May, 

19 21, 2009. (letter in docket #EPA-HQ-OAR-2007-0492). 

20 U.S. EPA (2002). Consolidated Human Activities Database (CHAD) Users Guide: Extracting 

21 Human Activity Information from CHAD on the PC. Written for EPA National Exposure 

22 Research Laboratory by ManTech Environmental Technologies. Modified in June 2002. 

23 Available: https://www.epa.gov/sites/production/files/2015-

24 02/documents/chad_manual.pdf. 

25 U.S. EPA (2004). Air Quality Criteria for Particulate Matter. U.S. EPA, National Center for 

26 Environmental Assessment, Office of Research and Development. Research Triangle 

27 Park, NC, EPA/600/P-99/002aF and EPA/600/P-99/002bF. October 2004. Available: 

28 http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_cd.html. 

29 U.S. EPA (2009a). Integrated Science Assessment (ISA) for Particulate Matter (Final Report). 

30 U.S. EPA, National Center for Environmental Assessment, Office of Research and 

31 Development. Research Triangle Park, NC. December, 2009. EPA/600/R-08/139F. 

32 Available at: http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=216546. 

33 U.S. EPA (2011). Policy Assessment for the Review of the Particulate Matter National Ambient 

34 Air Quality Standards. U.S. EPA, Office of Air Quality Planning and Standards. 

35 Research Triangle Park, NC, EPA 452/R-11-003. April, 2011. Available at: 

36 http://www.epa.gov/ttn/naaqs/standards/pm/data/20110419pmpafinal.pdf. 

37 U.S. EPA (2008). Risk and Exposure Assessment to Support the Review of the NO2 Primary 

38 National Ambient Air Quality Standard. U.S. EPA, Office of Air Quality Planning and 

39 Standards. Research Triangle Park, NC. EPA 452/R-08-008a/b. November 2008. 

40 Available: http://www.epa.gov/ttn/naaqs/standards/nox/s_nox_cr_rea.html. 
April 2016 4-28 Draft – Do Quote or Cite

1 U.S. EPA (2009b). Risk and Exposure Assessment to Support the Review of the SO2 Primary 

2 National Ambient Air Quality Standard. U.S. EPA, Office of Air Quality Planning and 

3 Standards. Research Triangle Park, NC. July 2009. EPA-452/R-09-007. Available: 

4 http://www.epa.gov/ttn/naaqs/standards/so2/data/200908SO2REAFinalReport.pdf. 

5 U.S. EPA (2010a). Quantitative Risk Assessment for Particulate Matter – Final Report. U.S. 

6 EPA, Office of Air Quality Planning and Standards. Research Triangle Park, NC. EPA7 452/R- 10-005. June 2010. Available: 

8 http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html

9 U.S. EPA (2010b). Particulate Matter Urban-Focused Visibility Assessment – Final Report. 

10 Office of Air Quality Planning and Standards. Research Triangle Park, NC. EPA-452/R11 10-004. June 2010. Available: 

12 http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html. 

13 U.S. EPA (2010c). Quantitative Risk and Exposure Assessment for Carbon Monoxide – 

14 Amended. U.S. EPA, Office of Air Quality Planning and Standards. Research Triangle 

15 Park, NC EPA-452/R-10-009. July 2010. Available: 

16 http://www.epa.gov/ttn/naaqs/standards/co/data/CO-REA-Amended-July2010.pdf. 

17 U.S. EPA (2014). Health Risk and Exposure Assessment for Ozone, Final Report. U.S. EPA, 

18 Office of Air Quality Planning and Standards. Research Triangle Park, NC. EPA-452/R19 14-004a. August 2014. Available: 

20 http://www.epa.gov/ttn/naaqs/standards/ozone/data/20140829healthrea.pdf. 

21 U.S. EPA (2015). BenMAP-CE User’s Manual Appendices. U.S. EPA, Office of Air Quality 

22 Planning and Standards. Research Triangle Park, NC. Available: 

23 http://www2.epa.gov/benmap/manual-and-appendices-benmap-ce

24 World Health Organization (2008). Part 1: Guidance Document on Characterizing and 

25 Communicating Uncertainty in Exposure Assessment, Harmonization Project Document 

26 No. 6. Published under joint sponsorship of the World Health Organization, the 

27 International Labour Organization and the United Nations Environment Programme. 

28 WHO Press, World Health Organization, 20 Avenue Appia, 1211 Geneva 27, 

29 Switzerland (tel.: +41 22 791 2476). 
April 2016 5-1 Draft – Do Quote or Cite

1 5 WELFARE-RELATED RISK AND EXPOSURE 

2 ASSESSMENTS 

3 5.1 INTRODUCTION 

4 Within the context of NAAQS reviews, a quantitative welfare risk and exposure 

assessment (WREA) is designed to estimate exposure and risks to public welfare110 5 associated 

6 with the existing secondary standards and with potential alternative secondary standard(s), if any, 

7 that might be appropriate to consider. This assessment can inform conclusions on the adequacy 

8 of the public welfare protection provided by these standards. The purpose of this chapter is to 

9 highlight key findings from the quantitative risk assessments conducted in the last review of the 

10 PM NAAQS and to identify key issues to be addressed in planning for any additional 

11 quantitative assessments that might be warranted for the current review. The scope of any 

12 WREA would be informed by the scientific evidence in the upcoming PM ISA; existing and 

13 historical air quality patterns and trends; the availability of improved data, methods, tools, and 

14 models that may better characterize important uncertainties or provide additional insights beyond 

15 those provided by previous assessments; and available resources. 

16 In the last review of the PM NAAQS, the quantitative welfare assessments focused 

17 primarily on urban visibility and were presented in the Urban-Focused Visibility Assessment 

18 (2010 UFVA, U.S. EPA, 2010b) and the Policy Assessment (2011 PA, U.S. EPA, 2011). The 

19 EPA did not conduct quantitative assessments for other welfare-related effects from PM (e.g., 

ecological effects, climate effects, and materials effects).111 20 

21 In the upcoming WREA Planning Document (discussed in sections 1.2 and 1.5, above), 

22 the EPA will evaluate the newly available information within the context of the 2010 UFVA and 

23 2011 PA analyses from the last review of the PM NAAQS to determine 1) the extent to which 

24 important uncertainties may be better characterized by information newly available for the 

25 current review and 2) the extent to which new information may affect the results of the 

26 quantitative analyses from the last review in important ways or may suggest additional 

 

110 Welfare effects as defined in CAA section 302(h) [42 U.S.C. 7602(h)] include, but are not limited to, “effects on 

soils, water, crops, vegetation, man-made materials, animals, wildlife, weather, visibility and climate, damage to and 

deterioration of property, and hazards to transportation, as well as effects on economic values and on personal 

comfort and well-being.” 

111 The on-going NOX/SOX review includes consideration of the ecological effects of ecosystem loading of 

particulate nitrogen and sulfur compounds. Thus, these endpoints are not discussed in this review. 
April 2016 5-2 Draft – Do Quote or Cite

1 quantitative analyses that can improve our understanding of the welfare exposures and risks 

2 associated with PM, including consideration of welfare effects beyond visibility impairment. The 

3 WREA Planning Document will also include a preliminary determination of whether a distinct 

4 WREA document would be needed. CASAC advice and public comments on this draft IRP will 

5 be considered in developing the WREA Planning Document, which will also be subject to 

6 CASAC review and will be made available for public comment. If warranted, one or more drafts 

7 of the WREA would then be prepared and released for CASAC review and public comment prior 

8 to completion of a final WREA. 

9 Section 5.2 below describes the key analyses, findings and uncertainties from the last 

10 review of the PM NAAQS. Section 5.3 describes the effect categories to be considered for 

11 potential welfare assessments (i.e., visibility impairment, ecological effects, climate effects, and 

12 materials effects) that the EPA will further evaluate in the WREA Planning Document. Section 

13 5.4 describes the process for obtaining scientific and public review of the WREA Planning 

14 Document and the WREA itself, if warranted. 

15 5.2 SUMMARY OF WELFARE RISK AND EXPOSURE ASSESSMENT 

16 FROM THE LAST REVIEW 

17 In the last review of the PM NAAQS, as summarized below, the EPA conducted 

18 quantitative assessments of PM-related visibility impairment in urban areas, recognizing that a 

19 secondary PM NAAQS developed with an urban focus would complement the EPA’s Regional 

Haze Program, which focuses on Class I areas.112 20 The purpose of these assessments was to 

21 provide information and insights to help inform decisions on the secondary standards. As noted 

22 above, these quantitative analyses were provided in two documents for the last review of the PM 

23 NAAQS: the 2010 UFVA and the 2011 PA. The 2010 UFVA combined information from urban24 focused public preference studies, urban air quality data, and urban visibility protection 

25 programs. The 2011 PA incorporated some additional air quality analyses and used more recent 

26 air quality data. These assessments (described in section 5.2.1) included evaluations of different 

27 indicators, levels, averaging times, and forms for their appropriateness in the context of 

28 measuring urban visibility impairment and its impacts on public welfare. 

 

112 In 1977, Congress established as a national goal ‘‘the prevention of any future, and the remedying of any 

existing, impairment of visibility in mandatory Federal Class I areas which impairment results from manmade air 

pollution’’, CAA section 169(a)(1). Currently, 156 national parks and wilderness areas are designated as Class I 

areas. For more information regarding the EPA’s Regional Haze Program, please see 

http://www3.epa.gov/visibility/program.html. 
April 2016 5-3 Draft – Do Quote or Cite

1 As noted above (in section 5.1), in the last review of the PM NAAQS the EPA did not 

2 quantify welfare risks for effects other than visibility impairment. At the time of the last review, 

3 the Agency determined that any risks estimated using the limited data available for other PM4 related welfare effects would have uncertainties too large to provide reasonable and informative 

5 results for the review. We discuss below (in section 5.2.2) the uncertainties identified in the last 

6 review of the PM NAAQS regarding additional PM-related welfare effects. 

7 5.2.1 Quantitative Assessments of Urban Visibility Impairment in the Last Review 

8 5.2.1.1 Reanalyses of Public Preference Survey Studies 

9 The 2010 UFVA included a detailed reanalysis of the available urban visibility 

10 preference studies. These studies are surveys designed to elicit information about the public’s 

11 preferences regarding visibility impairment by asking participants to rate the acceptability or 

12 unacceptability of a haze-obscured scene from each urban area. Each of these studies 

13 investigated the same question, “What level of visibility degradation is acceptable?” The studies 

14 available for the last review of the PM NAAQS were composed of three completed western 

15 urban visibility preference survey studies plus a pair of smaller focus studies designed to explore 

16 and further develop urban visibility survey methods. The three western studies included one in 

17 Denver, Colorado (Ely et al., 1991), one in the lower Fraser River valley near Vancouver, British 

18 Columbia, Canada (Pryor, 1996), and one in Phoenix, Arizona (BBC Research & Consulting, 

19 2003). A pilot focus group with 9 participants was conducted in Washington, DC (Abt 

20 Associates, Inc., 2001), and a replicate study with 26 participants was also conducted for 

21 Washington, DC (Smith and Howell, 2009). When taken together, these studies included 

acceptability ratings regarding visual air quality (VAQ)113 22 from a total of 852 individuals. 

23 The approaches in the four studies were all derived from the method first developed for 

24 the Denver urban visibility study. Although the approaches used in the four preference studies 

25 were similar, the specific materials and methods used in each study introduced uncertainties to 

26 be considered when interpreting the results of the comparison of the studies. Key differences

 

113 VAQ is defined as the visibility effect caused solely by air quality conditions and excluding those associated with 

meteorological conditions like fog and precipitation. It is commonly measured as either light extinction (in terms of 

inverse megameters, Mm-1) or the haziness index (in terms of deciview, dv) (Pitchford and Malm, 1993). The 

deciview scale was developed for use in visibility perception studies because it has a more linear relationship to 

perceived changes in haze compared with light extinction. A dv is defined as ten times the natural logarithmic of one 

tenth of the light extinction in Mm-1 (Pitchford and Malm, 1993). 
April 2016 5-4 Draft – Do Quote or Cite

1 between the studies include the following: 1) scene characteristics; 2) image presentation 

methods (e.g., projected slides of actual photos, projected images generated using WinHaze114 2 or 

3 use of a computer monitoring screen); 3) number of participants in each study; 4) participant 

4 representativeness of the general populations of the relevant metropolitan area; and 5) specific 

5 wording used to frame the questions used in the group interview process. 

6 Given the similarities in the approaches used in the preference studies, the EPA used 

7 regression analysis to reanalyze the results of the preferences studies individually and in 

8 combination. Specifically, the 2010 UFVA used a logit model to develop response curves that 

9 depicted the percentage of participants in each study that rated the VAQ (measured in dv) as 

10 either “acceptable” or “unacceptable”, for each of the visual images presented for a given area. 

11 The 2010 UFVA adopted the “50% acceptability” criterion from the Denver preference study 

12 (Ely et al., 1991), which identifies the inflection point along the response curve above which the 

13 visibility impairment of the scenes was found unacceptable by at least 50% of the participants 

14 and below which the level of VAQ in the images was found acceptable by at least 50% of 

15 participants, as a useful index for comparing the results across studies. 

16 These results were used to identify similarities in public preferences regarding VAQ and 

to inform the range of candidate protection levels (CPLs)115 17 in the 2010 UFVA and the 2011 PA. 

18 Specifically, the 2010 UFVA used the 50 percent acceptance criteria from each of the four urban 

19 preference studies to identify the range of CPLs from 20 dv to 30 dv, with a midpoint of 25 

dv.116 20 These values bracketed the daytime urban light extinction conditions that were judged 

21 unacceptable by 50 percent of the participants in the preference studies for those urban scenes, 

22 and provided low, medium, and high cut-points for use in the remaining assessments (U.S. EPA, 

23 2010b, pp. 2-28 to 2-29). 

24 5.2.1.2 Analyses of Visibility Conditions 

25 Building on prior reviews of the PM NAAQS, in the last review the EPA conducted 

26 quantitative analyses to further characterize ambient PM conditions in terms of PM 

 

114 The WinHaze model (Molenar et al., 1994) uses image processing technology to apply user-specified changes in 

light extinction values to the same base photograph with set scene and lighting characteristics. 

115 The term CPL refers to the target levels of visibility within a range that the EPA staff deemed appropriate for 

consideration that could, in conjunction with other elements of the standard, including indicator, averaging time, and 

form, potentially provide an appropriate degree of visibility protection. 

116 For comparison, 20 dv, 25 dv, and 30 dv are equivalent to 64, 112, and 191 Mm-1, respectively. 
April 2016 5-5 Draft – Do Quote or Cite

1 concentrations and light extinction in 15 urban study areas. Analyses of visibility conditions 

2 were conducted as a part of the 2010 UFVA and the 2011 PA. Analyses in the 2010 UFVA 

3 evaluated calculated PM10 light extinction and were expanded in the 2011 PA to evaluate 

calculated PM2.5 light extinction.117 4 

5 The 2010 UFVA focused on calculated PM10 light extinction as the indicator for the 

6 quantitative assessments based on the data collected from the IMPROVE network (as described 

7 in section 2.3 above) and the requirements of the EPA’s Regional Haze Program (U.S. EPA, 

2009a, sections 9.2.3.2 and 9.2.3.4). These assessments used a combination of several datasets118 8 

as inputs into the IMPROVE algorithm119 9 to estimate the daily maximum 1-hour PM10 light 

10 extinction using data from 2005 to 2007. The goals of these air quality assessments were to: 1) 

11 improve understanding of the extent, patterns, and causes of PM-related impairment of urban 

12 visibility during daylight hours; 2) create the basis for projections of PM2.5 mass and PM10 light 

13 extinction under potential alternative standard scenarios; and 3) examine the correlation between 

14 PM10 light extinction and potential alternative indicator(s) based on PM2.5 mass concentration 

15 (U.S. EPA, 2010b, chapters 3, 4, and Appendix D; U.S. EPA, 2011, Appendix F). 

16 The 2011 PA contained additional air quality analyses in order to estimate PM2.5 light 

17 extinction. PM2.5 is the size fraction of PM responsible for the visibility impairment in most 

18 urban areas, and the methods for estimating PM2.5 light extinction were simpler than the methods 

19 used in the 2010 UFVA for estimating PM10 light extinction. Benefits of moving to a simpler 

20 approach included: 1) more transparency in the required calculations; 2) less intensive data 

21 processing; 3) an increase in the number of monitoring sites that could meet the data 

22 requirements of the approach without adding sampling equipment or additional laboratory

 

117 Analyses in the 2010 UFVA and 2011 PA used the same 15 urban study areas as the 2010 HREA: Tacoma, 

Fresno, Los Angeles, Phoenix, Salt Lake City, Dallas, Houston, St. Louis, Birmingham, Atlanta, Detroit, Pittsburgh, 

Baltimore, Philadelphia, and New York. Collectively, these 15 study areas comprised 31 counties. 118 Several datasets were combined for the 2010 UFVA analysis: 1) continuous hourly averaged PM2.5 mass 

concentrations and relative humidity; 2) 24-hour average filter measurements for PM2.5 composition; 3) hourly 

daylight PM2.5 components generated using the Community Multi-scale Air Quality modeling system (CMAQ); and, 

4) estimated/measured hourly PM10-2.5 concentrations (U.S. EPA, 2010b, section 3). The resulting estimates of 

hourly averaged PM2.5 component concentrations, PM10-2.5 mass concentration, and the measured relative humidity 

were used in the IMPROVE algorithm to estimate hourly PM10 light extinction to be calculated for daylight hours 

with relative humidity no greater than 90 percent (U.S. EPA, 2010b, section 3.2, 3.3). 119 The IMPROVE algorithm is based on data generated at monitoring sites in the IMPROVE network to estimate 

light extinction as a function of PM concentrations and relative humidity levels. For more detailed information on 

the IMPROVE algorithm, see section 9.2.2.2 of the 2009 PM ISA (U.S. EPA, 2009a). As noted above (in section 

2.2.2), there is both an original and revised version of the IMPROVE algorithm (Pitchford et al. 2007). The revised 

version was developed to address observed biases in rural areas under certain light extinction conditions. 
April 2016 5-6 Draft – Do Quote or Cite

1 analysis; and 4) an increase in the number of days per year for which the calculation of PM light 

2 extinction could be conducted (U.S. EPA, 2011, Appendix F). 

3 The quantitative analyses of visibility conditions can be considered as two distinct 

4 analyses, covering recent conditions and adjustments to just meet the existing standards. The first 

5 analysis presented in both the 2010 UFVA and the 2011 PA characterized the recent visibility 

6 conditions in the 15 urban study areas. However, the 2010 UFVA focused on PM10 light 

7 extinction using monitoring data from 2005 to 2007, whereas the 2011 PA focused on PM2.5 light 

8 extinction using monitoring data from 2007 to 2009. The estimates of the percentage of daily 

9 maximum hourly PM2.5 light extinction values exceeding the CPLs from the 2011 PA were 

10 somewhat lower than for the PM10 light extinction values from the 2010 UFVA. However, the 

11 patterns of the estimates across the 15 urban study areas were similar. Specifically, 13 of the 15 

12 urban study areas in the 2011 PA (with the exception of two Texas and non-California western 

13 urban study areas) were estimated to have exceeded the high CPL (30 dv) from about 10 to 50 

14 percent of the days based on the PM2.5 light extinction analysis, while all 15 urban study areas 

15 were estimated to exceed the low CPL (20 dv) from 10 to 90 percent of the days (U.S. EPA, 

16 2011, pp. 4-29 to 4-30). 

17 The second analysis characterized visibility conditions adjusted to just meet the existing 

18 secondary standards. This adjustment applied a proportional rollback approach that uniformly 

19 reduced all PM2.5 components to just meet the target levels (with a lower bound on potential air 

20 quality adjustments at PRB). For PM2.5 light extinction in the 2011 PA, the analysis was 

21 designed to assess the likelihood that PM-related visibility impairment would exceed CPLs upon 

just meeting the suite of secondary PM2.5 NAAQS (15.0 μg/m3 annual and 35 μg/m3 22 24-hour 

(98th percentile form)) (U.S. EPA, 2011, p. 4-33).120 23 The PM2.5 light extinction was estimated in 

24 terms of both daily maximum 1-hour average values and multi-hour (i.e., 4-hour) average values 

for daylight hours (U.S. EPA, 2011, p. 4-33).121 25 When adjusted to just meet the existing 

26 standards, the daily maximum 1-hour average PM2.5 light extinction values in a majority of the 

27 urban study areas were estimated to show improvement across the CPLs (U.S. EPA, 2011, 

28 Figures 4-4 and 4-7, Tables 4-3 and 4-6). Similar patterns of visibility improvement were also

 

120 The 2010 UFVA included additional analyses characterizing visibility conditions on PM10 light extinction after 

adjustments to just meet an alternative suite of secondary PM2.5 standards (i.e., 12.0 μg/m3

 annual and 25 μg/m3 24-

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

121 As mentioned in section 2.2.2, the EPA focused on sub-daily averaging times as the relevant exposure periods for 

segments of the viewing public due to the short-term nature of the perception of PM-related visibility impairment. 
April 2016 5-7 Draft – Do Quote or Cite

1 shown with daily maximum 4-hour average PM2.5 light extinction values (U.S. EPA, 2011, 

2 Figure 4-8, Appendix G, Figure G-3). 

3 5.2.1.3 Key Uncertainties in the 2010 UFVA and 2011 PA 

4  Visibility Preferences: In the last review of the PM NAAQS, a number of uncertainties were 

5 identified with respect to the public preference studies. There was substantial variability 

6 between studies in the range of visibility conditions that were judged by study participants to 

7 be “unacceptable” (U.S. EPA, 2010, Chapter 2). The relative importance of the degree of 

8 visibility impairment (e.g., light extinction level) versus the frequency and duration of 

9 visibility impairment in preference studies was unclear. Additionally, differences in the 

10 strength of preference for visibility conditions across the study areas (e.g., why visibility 

11 impairment that is acceptable in one area is not acceptable in another area) were not well 

12 understood. Moreover, the value of visibility in the preference studies is not distinctly 

13 defined beyond acceptable or unacceptable, and consideration of other metrics of valuing 

14 visibility may be needed. A better understanding of the reasons for differences in preferences 

15 across studies, differences in preference in different urban study areas, and alternative 

16 measures of acceptable visibility impairments could influence the interpretation of the results 

17 in defining additional or alternative range of CPLs for visibility impairment. 

18  Calculation of Visibility Conditions: In the last review of the PM NAAQS, a number of 

19 uncertainties were identified regarding the estimation of recent visibility conditions and 

20 adjustments to just meet alternative scenarios. Calculated light extinction estimates have 

21 uncertainties compared to directly measured PM light extinction. A number of uncertainties 

22 were identified with the inputs for the IMPROVE algorithm, including the use of measured 

23 or estimated PM concentrations and components and the influence of relative humidity in the 

24 IMPROVE calculations. The methods used to temporally apportion 24-hour PM2.5

25 components to calculate hourly-averaged values relied on monthly-averaged PM2.5

26 component variations from chemical transport modeling. The mass balance method used in 

27 this modeling to estimate organic carbon concentrations and the loss of nitrate was 

28 considered to be a reasonable approach but is not likely to be precise. Moreover, the 

29 IMPROVE algorithm was designed for use under the Regional Haze Program, which applies 

30 in rural areas, and the algorithm has not been validated for urban areas. Uncertainties remain 

31 with respect to the timeframe (e.g., 1-hour, 4-hour, 24-hour) for evaluating visibility 

32 conditions, particularly given that the public preference studies did not provide insight on the 

33 frequency or duration of visibility impairment that would be acceptable. In addition, the 

34 rollback approach uniformly reduced all non-PRB PM2.5 components, but emission control 

35 programs in practice would not likely operate in this manner. 

36 5.2.2 Consideration of Quantitative Assessments for Additional PM-related Welfare 

37 Effects in the Last Review 

38 Based on the determination in the 2009 PM ISA (U.S. EPA, 2009a) that there was a 

39 “causal” relationship between PM and effects on visibility, climate, and materials, as well as a 

40 “likely to be causal” determination with ecological effects, the 2009 Scope and Methods Plan 

41 (U.S. EPA, 2009c) considered whether adequate evidence was available to conduct quantitative 
April 2016 5-8 Draft – Do Quote or Cite

1 assessments for each of these categories of effects. This Plan also noted that the chemical 

2 components of PM (to a greater extent than PM mass or size fraction) largely determine the 

3 nature, degree, and direction of the non-visibility welfare effects. After a careful evaluation of 

4 these categories of effects, the EPA determined in the 2009 PM Scope and Methods Plan that 

5 data were not available to conduct quantitative assessments for non-visibility welfare effects in 

6 the last review, including the following: 

7  Ecological effects from deposition of particulate organics and metals: Data to link PM 

8 toxicity to ecosystem function effects were available for only one tree species, and limited 

9 data on other species hindered additional analyses. Toxic effects of some PM components on 

10 plant leaves and soil were not well-characterized and it was difficult to isolate these 

11 endpoints from other environmental stressors. A lack of data on the site-specific composition 

12 of PM and soil-associated biota prevented quantitative analysis of population shifts from 

13 deposited PM. A lack of data on bioavailability of PM components and uncertainties in 

14 cumulative exposure effects prevented quantitative analysis of PM metal toxicity to biota. 

15 Data on PM components were only available for a few urban areas. Data were unavailable 

16 regarding seasonal composition of near roadway PM and trophic transfer to animals that 

17 forage on roadsides. 

18  Climate effects: Representation of aerosols in climate models was needed to more accurately 

19 predict changes in climate. Complex interactions of aerosols and linkages between clouds 

20 and the overall climate system limited the feasibility of conducting a quantitative analysis. 

21 Insufficient data on local and regional microclimate variations for many regions of the U.S. 

22 and heterogeneity of cloud formations limited the feasibility of conducting a quantitative 

23 analysis. 

24  Materials effects: New evidence was not sufficient to conduct a quantitative assessment of 

25 materials damage or soiling from PM deposition onto surfaces. 

26 

27 5.3 CONSIDERATION OF POTENTIAL QUANTITATIVE 

28 ASSESSMENTS FOR THIS REVIEW 

29 The goals of a WREA in the current review would be to provide information relevant to 

30 answering questions regarding the adequacy of the existing PM secondary standards and, if 

31 appropriate, the potential improvements in public welfare that could be achieved from meeting 

32 potential alternative standard(s). Any quantitative WREA for the current review would build on 

33 the approaches used and lessons learned in the last review and would focus on whether newly 

34 available models, methods, tools, and data would substantially reduce the previously identified 

35 uncertainties. 

36 As described further in this section, any WREA for the current review would consider the 

37 main categories of PM-related welfare effects (e.g., visibility effects, ecological effects, climate 
April 2016 5-9 Draft – Do Quote or Cite

1 effects, and materials) for which, in staff’s judgment, there would be adequate information to 

2 develop quantitative risk estimates that can meaningfully inform the review of the secondary PM 

3 NAAQS. These considerations will be described in more detail in the WREA Planning 

4 Document. Preparation of the WREA Planning Document will draw from the assessments of the 

5 scientific evidence in the first draft PM ISA to facilitate the integration of policy-relevant science 

6 into the planning document. In particular, consideration of the availability of new data regarding 

7 each of these effects as well as consideration of available resources to conduct such assessments 

8 will influence whether we develop additional quantitative assessments in a new WREA. We 

9 anticipate that any quantitative analyses, if warranted, could focus on PM2.5, but we would also 

10 consider the extent to which information becomes available in the PM ISA for the current review 

11 that could support quantitative analyses of PM components, PM10-2.5, or ultrafine particles. 

12 5.3.1 Visibility Effects 

13 As described above (in section 5.2), the EPA conducted extensive quantitative 

14 assessments of the impact of visibility impairment on public welfare based on public preferences 

15 for VAQ and visibility conditions in 15 urban study areas in the 2010 UFVA and 2011 PA. The 

16 goals of any new or additional quantitative assessment for visibility would be to utilize newly 

17 available information to address the key uncertainties identified in the last review regarding the 

18 two key aspects of those analyses: visibility preference studies and air quality analyses. We 

19 discuss both of these below. 

20 In the last review, the EPA heavily relied on surveys of public preferences regarding the 

21 acceptability of various levels of urban VAQ. Quantitative reanalysis of those survey results in 

22 the 2010 UFVA informed the development of the range of visibility CPLs (i.e., 20 to 30 dv). 

23 Based on information discussed during the 2015 PM NAAQS kick-off workshop (79 FR 71764), 

24 limited new evidence related to visibility preferences in urban areas has been identified in the 

25 available literature. The WREA Planning Document will evaluate any new visibility preference 

26 studies identified in the PM ISA for the current review to determine whether additional 

27 quantitative assessments would be warranted. To the extent that additional urban preference 

28 studies are identified in the PM ISA, we would consider whether an updated quantitative analysis 

29 that considered any new information provided by the new studies would substantially change the 

30 range of CPLs identified in the last review. If no additional visibility preference studies in urban 

31 areas become available in this review, we anticipate that the upcoming PA would continue to 

32 rely upon the quantitative analyses of the preference studies available in the last review. In 

33 addition, the PA would consider any additional information beyond preference studies that may 
April 2016 5-10 Draft – Do Quote or Cite

1 become available in the PM ISA relevant to assessing adversity to public welfare from urban 

2 visibility impairment. 

3 In addition, we would consider in the WREA Planning Document whether providing 

4 updated air quality assessments to include more recent PM monitoring data would substantially 

5 reduce the uncertainties identified in the last review of the PM NAAQS regarding the 

6 characterization of recent visibility conditions and adjustment approaches to just meet the 

7 existing standards. The determination whether to conduct additional quantitative assessments of 

8 visibility conditions in the current review could depend on (1) whether updated information is 

9 available in the PM ISA to warrant new air quality assessments, such as significantly updated 

10 understanding of the variables that affect light extinction including relative humidity, PM species 

11 composition, PM10-2.5 data, or the IMPROVE algorithm (Pitchford et al, 2007), and (2) whether 

12 we determine that an updated reanalysis of the preference studies would be warranted and would 

13 substantially change the range of the CPLs identified in the last review (20 to 30 dv). New 

14 quantitative air quality assessments, if warranted, could also use more recent air quality 

15 monitoring data, including data for PM concentrations and components to determine the extent to 

16 which the existing secondary standards are adequate given recent ambient concentrations. For 

17 example, such assessments could determine whether exceedances of the range of CPLs would be 

18 estimated to occur under more recent ambient conditions and under the existing secondary 

19 standards, if warranted. 

20 In the WREA Planning Document, we will consider any new data that have become 

21 available in the PM ISA for the current review on preference studies and estimating visibility 

22 conditions in order to determine whether conducting additional quantitative analyses on visibility 

23 effects would meaningfully inform the current review of the secondary PM NAAQS. The EPA 

24 may determine that certain quantitative assessments are more appropriate in a PA rather than in a 

25 WREA. 

26 5.3.2 Ecological Effects 

27 As noted above, the EPA did not include a quantitative ecological risk assessment for any 

28 prior reviews of the PM NAAQS. For the current review, we are considering whether 

29 information would now be sufficient to conduct a quantitative ecological risk assessment 

30 associated with deposition of PM, primarily particulate organics and metals, including whether 

31 information and data are now available to address the uncertainties identified in the last review 

32 of the PM NAAQS that prohibited a quantitative ecological risk assessment. As noted above (in 

33 section 5.2.2), these uncertainties included limited data to link deposition of PM organics and 

34 metals to ecosystem functions and specific ecological receptors (e.g., soils, vegetation, and 
April 2016 5-11 Draft – Do Quote or Cite

1 fauna) effects, and other site-specific data. In the WREA Planning Document, we would consider 

2 whether any new information in the PM ISA for the current review substantially reduces the 

3 uncertainties identified in the last review of the PM NAAQS. In particular, we would consider 

4 any new information that 1) quantitatively elucidates pathways of exposure, 2) quantitatively 

characterizes ecological effects122 5 from deposition of PM mass or PM components (e.g., specific 

6 organic and metal PM) on ecological receptors, and 3) provides information sufficient to 

7 quantify relationships between ambient PM, deposited PM, specific ecological effects, and, to 

8 the extent feasible, associated ecosystem services. 

9 The first step in potentially assessing ecological effects associated with deposition of 

10 organic and metal PM would require the identification and quantification of the pathways of 

11 exposure. Pathways of PM exposure for ecological receptors can include direct deposition to the 

12 receptor surface via wet, dry or occult (e.g., fog) deposition, or transfer from one environmental 

13 compartment (e.g., aquatic, terrestrial) or organism to another. The last review of the PM 

14 NAAQS noted that the chemical constituents that make up PM (rather than PM mass) largely 

15 determine the nature, degree, and direction of the ecological effects (U.S. EPA, 2009c, Appendix 

16 A). The components that make up a given mass concentration of PM can vary substantially both 

17 temporally and spatially, which would affect the nature and magnitude of resulting ecological 

18 effects. Uncertainty associated with heterogeneity in deposition of PM components, along with 

19 significant data gaps, confounded efforts to evaluate PM-related ecological effects on ecosystem 

20 function at the ecosystem, regional, watershed, or national scale in the last review of the PM 

21 NAAQS (U.S. EPA, 2009c, Appendix A). In the WREA Planning Document, we would evaluate 

22 whether sufficient data have become available to estimate the spatial heterogeneity in site23 specific PM deposition (across and within ecosystems), particularly of organic and metal 

24 components, and then to quantitatively link this deposition to ecological effects in various types 

25 of ecosystems. 

26 In the last review of the PM NAAQS, vegetation was shown to be sensitive to heavy 

27 metal deposition (U.S. EPA, 2009c, Appendix A). For example, PM dry deposition to leaf 

28 surfaces and the inner canopy is well documented (U.S. EPA, 2009a, section 3.3.4.3). Impacts on 

29 vegetation from PM metal deposition include growth suppression, toxicity to root colonizing 

30 microorganisms, impairment of root development and induction of phytochelatins (U.S. EPA, 

 

122 As described above, the on-going NOX/SOX review includes consideration of the ecological effects of ecosystem 

loading of particulate nitrogen and sulfur compounds. Thus, these endpoints are not discussed in this review. 
April 2016 5-12 Draft – Do Quote or Cite

2009a, section 9.4.5.3).123 1 However, the last review of the PM NAAQS also noted that toxic 

2 effects of some components of PM on plants are not well characterized, and it was difficult to 

3 isolate these endpoints from other environmental stressors (U.S. EPA, 2009c, Appendix A). 

4 Similarly, based on information discussed during the 2015 PM NAAQS kick-off workshop, 

5 limited new evidence has been identified related to PM effects on vegetation, but most of those 

6 plants were not native to the U.S. In addition, workshop participants also noted that new studies 

7 and models are available to quantify the removal of ambient PM by plants (e.g., air filtration by 

8 dry deposition on leaves), but this is a distinct issue from quantifying how PM deposition affects 

9 those plants (e.g., PM deposition on leaf surfaces that inhibits photosynthesis). In the WREA 

10 Planning document, we would evaluate whether sufficient data and tools are available to apply 

11 environmental modeling techniques to PM concentration and deposition data to quantify the 

12 responses in U.S. vegetation and the extent to which such information would provide additional 

13 insight regarding how PM toxicity affects ecosystem functioning considering the inherent 

14 uncertainties in such approaches. 

15 Information from the last review of the PM NAAQS indicated a variety of potential 

16 adverse effects from PM deposition on soils (U.S. EPA, 2009a, section 9.4.5.2). For example, 

17 PM effects on soil and soil-associated microfauna from heavy metals (such as zinc, copper, and 

18 cadmium) have been shown to be toxic to soil fungi and bacteria, which could have broader 

19 implications at the ecosystem level because many plant species are dependent upon fungal and 

20 bacterial associations to obtain nutrients from the rhizosphere (U.S. EPA, 2009a). Shifts in soil 

21 microflora populations can also affect nutrient and organic matter cycling and carbon utilization. 

22 In addition, vegetation can take up heavy metals from the soil, which could lead to effects on 

23 wildlife and humans who consume those plants. These effects were not quantified in the last 

24 review of the PM NAAQS because of a lack of sufficient data. In the WREA Planning 

25 Document, we could evaluate whether sufficient data now exist to quantify the site-specific 

26 composition of PM deposition and the resulting population shifts in soil-associated biota in 

27 response to ecological stressors such as PM deposition. 

28 The last review of the PM NAAQS indicated that fauna may also be an ecological 

29 receptor for deposition of PM organics and metals through ingestion, absorption, trophic transfer, 

30 transfer between aquatic to terrestrial compartments, and bioaccumulation and biomagnification 

31 of heavy metals across trophic levels (U.S. EPA, 2009a, section 9.4.5.6). However the last 

 

123 Phytochelatins are intracellular metal-binding peptides that act as specific indicators of metal stress (U.S. EPA, 

2009a). 
April 2016 5-13 Draft – Do Quote or Cite

1 review of the PM NAAQS noted that quantitative assessment of particulate metal toxicity to 

2 biota was limited due to the heterogeneous composition of PM, lack of data on the bioavailability 

3 of PM components, and uncertainties in cumulative exposure effects (U.S. EPA, 2009c, 

4 Appendix A). In the WREA Planning Document, we could consider whether new data and 

5 information from the PM ISA in the current review would provide sufficient information on the 

6 effects of PM on aquatic and terrestrial invertebrates, amphibians, birds and mammals to conduct 

7 quantitative analyses of these ecological effects and associated ecosystem services. 

8 In the WREA Planning Document, we could consider whether sufficient data have 

9 become available in the PM ISA for the current review to conduct quantitative analyses on the 

10 PM-related ecological risks to vegetation, soils, and fauna associated with PM deposition in light 

11 of inherent uncertainties. Although we do not anticipate that sufficient information would 

12 become available to reduce these uncertainties, if it did, we would give consideration to whether 

13 conducting such quantitative analyses of PM-related ecological risks and associated ecosystem 

14 services would be practicable and would meaningfully inform this review. If uncertainties and 

15 data gaps prevent the EPA from conducting a quantitative ecological risk assessment for this 

16 review, we anticipate that the upcoming PA would rely on information from the PM ISA 

17 regarding PM-related ecological effects. 

18 5.3.3 Climate Effects 

19 As noted above, the EPA did not include a quantitative climate risk assessment for any 

20 prior reviews of the PM NAAQS. The last review of the PM NAAQS noted that the addition of 

anthropogenic aerosols124 21 to the atmosphere perturbs the Earth’s energy balance and constitutes 

22 an aerosol climate forcing (U.S. EPA, 2009c, Appendix A). The influence of this forcing on 

23 various climate metrics across a wide range of scales is an active area of research, and studies 

24 have identified both direct and indirect aerosol forcing pathways that can lead to effects on 

25 climate (U.S. EPA, 2009a, section 9.3.1). Aerosol direct effects on climate can occur when 

26 changes in ambient aerosol properties (e.g., concentration, composition, and size distribution) 

27 alter atmospheric radiative budgets by modifying the scattering and absorption of radiation by 

28 particles. Aerosol indirect effects on climate can occur when changes in aerosol properties alter 

29 radiative budgets by modifying cloud amount, lifetime, and microphysical and radiative 

30 properties. Aerosol indirect effects include the effects on clouds resulting from the role of 

 

124 As noted above (in section 2.2.1), the term aerosol is used in this document when discussing suspended ambient 

particles in the context of climate impacts. 
April 2016 5-14 Draft – Do Quote or Cite

1 particles in the formation of cloud droplets and ice. An aerosol semi-direct effect can occur when 

2 clouds are affected by changes in atmospheric properties (e.g., relative humidity and vertical 

3 temperature structure) resulting from the absorption of radiation by particles. 

4 Recent climate assessments characterize the current scientific understanding of aerosol 

5 effects on climate (Bond et al., 2013; IPCC, 2014; National Climate Assessment, 2014), and we 

6 anticipate that the evaluation of climate evidence in the PM ISA for the current review would be 

7 informed largely by findings of these major assessments. Improvements in measurements and 

8 modeling of atmospheric aerosol properties are continually occurring and will contribute to the 

9 understanding of aerosol effects on climate in this review. For example, remote sensing 

measurements from satellite systems and surface-based networks (e.g., AERONET125 10 ) can 

11 facilitate more advanced analysis of variables relevant to aerosol-climate interactions. We 

12 anticipate that atmospheric models with improved algorithms will be available in this review to 

13 better represent the interactions of atmospheric particles with radiation and clouds and improve 

14 the understanding of the role of aerosols in global climate change. 

15 However, our current understanding is that considerable uncertainty in the effects of 

16 aerosols on climate still exists (Bond et al., 2013; IPCC, 2014). Large spatial and temporal 

17 heterogeneities in direct and indirect aerosol climate forcings occur due to the variety of aerosol 

18 sources, the intermittency of these sources, the short atmospheric lifetime of aerosols relative to 

19 the major greenhouse gases, and the chemical and microphysical processing that occurs in the 

20 atmosphere. These features lead to greater uncertainty in quantitative estimates of the effect of 

21 aerosols on climate relative to that of the major greenhouse gases. This uncertainty is especially 

22 large at the local and regional scales in the U.S. that would likely be most relevant to a 

23 quantitative assessment of the potential effects of a national PM standard on climate in this 

24 review. In the WREA Planning Document, we will consider the extent to which information in 

25 the PM ISA for the current review substantially reduces this uncertainty and whether information 

26 and tools would be sufficient to quantify the local and regional effects from aerosols in this 

27 review. Specifically, the WREA Planning Document will consider the extent to which 

28 conducting a quantitative climate assessment would provide meaningful information for this 

29 review beyond the information available in the PM ISA and the other major scientific 

30 assessments and considering the inherent uncertainties in such an assessment. For example, we 

31 would consider whether a quantitative assessment in the current review (1) could quantify how 

32 the PM NAAQS alone would affect climate and (2) how changes in PM (via changes in the 

 

125 See http://aeronet.gsfc.nasa.gov/ 
April 2016 5-15 Draft – Do Quote or Cite

1 standards) would result in changes to climate. If uncertainties suggest that it would not be of 

2 value to the current review for the EPA to conduct a quantitative climate risk assessment for this 

3 review, we anticipate that the upcoming PA would rely on information from the PM ISA 

4 regarding PM-related climate effects. 

5 5.3.4 Materials Effects 

6 As noted above, the EPA did not include a quantitative risk assessment on materials for 

7 any prior reviews of the PM NAAQS due to a lack of data. PM-related materials effects are 

8 generally classified into two categories: materials damage and soiling. 

Materials damage associated with deposited PM, particularly sulfates and nitrates,126 9 

10 include the corrosion of metals, degradation of painted surfaces, deterioration of building 

11 materials, and weakening of material components. Because of their electrolytic, hygroscopic, and 

12 acidic properties and their ability to sorb corrosive gases, particles contribute to materials 

13 damage by adding to the effects of natural weathering processes. Deposited pollutants often 

14 undergo chemical transformations and are commonly oxidized to acids, leading to materials 

15 damage particularly from reactions between materials and NOx and SO2. Wet and dry deposition 

16 contribute to PM accumulation and surface damage, although the presence of moisture can 

17 accelerate some materials damage (e.g., corrosion of metals). Generally, SO2 is more corrosive 

18 than NOx, but mixtures of SO2, NOx, and other PM components and species can corrode certain 

19 metals at a faster rate than individual pollutants alone. In the last review of the PM NAAQS, 

20 sufficient evidence was not available to conduct a quantitative assessment of PM-attributable 

21 materials damage (U.S. EPA, 2004, 2009a, b). While ambient particles play a role in the 

22 corrosion of metals and in the weathering of materials, no quantitative relationships between 

23 ambient particle concentrations and rates of damages had been established (U.S. EPA, 2009a). If 

24 this information becomes available in the PM ISA, we would consider whether this information 

25 would be sufficient to support a quantitative assessment for PM-attributable materials damage. 

26 Deposition of PM onto surfaces, such as metal, paint, and stone, can lead to soiling. 

27 Soiling is the result of PM accumulation on an object that alters the optical characteristics 

28 (appearance). The presence of PM may alter light transmission or change the reflectivity of a 

29 surface. These soiling effects can impact the aesthetic value of a structure or result in reversible

 

126 In the case of materials effects, it is difficult to isolate the effects of gaseous and particulate N and S wet 

deposition so both will be considered along with other PM-related deposition effects on materials in this review of 

the PM NAAQS. 
April 2016 5-16 Draft – Do Quote or Cite

1 or irreversible damage to the surface. The presence of air pollution can increase the frequency 

2 and duration of cleaning and can enhance biodeterioration processes on the surface of materials. 

3 For example, deposition of carbonaceous components of PM can lead to the formation of black 

crusts on surfaces, and the buildup of microbial biofilms127 4 can discolor surfaces by trapping PM 

5 more efficiently (U.S. EPA, 2009a, p. 9-195). Additionally, the organic or nutrient content of 

6 deposited PM may enhance microbial growth on surfaces. However, in the last review of the PM 

7 NAAQS, sufficient evidence was not available to conduct a quantitative assessment of PM8 attributable soiling effects (U.S. EPA, 2004, 2009a,b). While soiling associated with fine and 

9 coarse particles can result in increased cleaning frequency and repainting of surfaces, no 

10 quantitative relationships between particle characteristics and the frequency of cleaning or 

11 repainting had been established (U.S. EPA, 2011). Additionally, despite the limited new data in 

12 the last review of the PM NAAQS on the role of microbial colonizers in biodeterioration 

13 processes and contributions of black crust to soiling, these data were not sufficient for 

14 quantitative analyses (U.S. EPA, 2011). Should new information about cleaning frequency and 

15 repainting of surfaces and the role of microbial colonizers in soiling effects become available in 

16 this review, we would consider whether this information would support a quantitative 

17 assessment. 

18 In the WREA Planning Document, we will consider whether sufficient data have become 

19 available in the PM ISA for the current review to conduct quantitative analyses on either 

20 category of PM-related materials effects. If data gaps and uncertainties prevent the EPA from 

21 conducting a quantitative materials effects assessment for this review, we anticipate that the 

22 upcoming PA would rely on information from the PM ISA regarding PM-related materials 

23 effects. 

24 5.3.5 Characterization of Sources of Uncertainty and Consideration of Information Newly 

25 Available in this Review 

26 Table 5-1 summarizes the potentially important uncertainties related to the quantification 

27 of PM-related welfare effects where additional information, if available, could improve our 

28 understanding of PM-related welfare risks and/or reduce uncertainties identified in the last 

29 review of the PM NAAQS. To the extent that we would not be able to reduce these uncertainties 

30 in the current review, the utility of conducting additional quantitative welfare assessments would 

 

127 Microbial biofilms, primarily composed of fungi, can stain exposed rock surfaces with yellow, orange, brown, 

gray, or black colors. 
April 2016 5-17 Draft – Do Quote or Cite

1 be limited. We will further discuss these issues, including whether additional information has 

2 come available in the PM ISA for this review, in the WREA Planning Document. 

3 Table 5-1 Areas of Uncertainty in the Welfare Assessments and Potential Strategies to 

4 Address Them 

Major 

Uncertainty or 

Limitation 

Uncertainty/Limitation Remaining 

from Last Review 

Consideration of Information 

Newly Available in this Review 

Variability in 

“acceptable” 

VAQ levels 

across 

preference 

studies and 

urban areas 

There was substantial variability in 

the range of VAQ judged to be 

“acceptable” by the preference study 

participants across the urban areas 

studied. Limited studies and data 

were available to better inform our 

understanding of the factors leading 

to this variability. In addition, 

differences in the strength of 

preferences across urban areas were 

not well understood. 

If new preference studies are 

available in the PM ISA and include 

additional explanatory variables 

regarding this issue, we would 

consider whether incorporating these 

studies into another quantitative 

assessment of the preference studies 

would be warranted for this review. 

Degree of 

visibility 

impairment 

versus 

frequency and 

duration 

The relative importance of the 

degree of visibility impairment (e.g., 

light extinction level) compared to 

the frequency and duration of 

visibility impairment in the 

preference studies was unclear. 

If new preference studies are 

available in the PM ISA and include 

additional explanatory variables 

regarding this issue, we would 

consider whether incorporating these 

studies into another quantitative 

assessment of the preference studies 

would be warranted for this review. 

Recent 

visibility 

conditions 

Calculated estimates of PM light 

extinction have inherent 

uncertainties. For example, the 

IMPROVE algorithm uses measured 

or estimated PM concentrations and 

components, is influenced by 

relative humidity, and had not been 

validated for urban areas at the time 

of the last review. Limited hourly 

PM10-2.5 monitoring, continuous 

PM2.5 speciation monitoring, and 

direct measurement of PM10 light 

extinction contributed to 

uncertainties in estimating the light 

extinction. In addition, the time 

frame for assessing visibility 

conditions was uncertain because the 

If a new quantitative analysis of 

visibility conditions is warranted for 

this review (i.e., based on the 

availability of new preference 

studies), we would evaluate whether 

new or different methods are 

available to reduce this uncertainty 

in characterizing PM2.5 light 

extinction for recent conditions. 
April 2016 5-18 Draft – Do Quote or Cite

Major 

Uncertainty or 

Limitation 

Uncertainty/Limitation Remaining 

from Last Review 

Consideration of Information 

Newly Available in this Review 

preference studies did not provide 

insights on frequency or duration. 

Visibility 

conditions 

under existing 

and alternative 

standards 

The proportional rollback approach 

used to just meet the existing (and 

alternative) standards uniformly 

reduced all PM2.5 components, but 

emission control programs in 

practice would not likely operate in 

this manner. 

If a new quantitative analysis of 

visibility conditions under 

alternative scenarios is warranted for 

this review, we would evaluate 

whether new or different methods 

are available to reduce this 

uncertainty in estimating PM2.5 light 

extinction upon just meeting the 

existing standards and, if 

appropriate, potential alternative 

standards under evaluation. 

Heterogeneity 

and data gaps 

in quantifying 

ecological 

effects 

Ecological effects from PM 

deposition on soils, flora, and fauna 

vary greatly depending on the 

specific PM component, the 

ecosystem affected, and other sitespecific information. However, data 

were limited on PM components 

outside of urban areas (other than 

near specific point sources) and the 

associated ecosystem responses from 

deposition of metals and organics. 

If studies and data become available 

in the PM ISA to sufficiently reduce 

the uncertainties associated with 

quantifying the ecological effects, 

we would consider whether this new 

information would be sufficient to 

warrant a quantitative ecological 

assessment for this review. 

Aerosols in 

climate models 

and 

local/regional 

variability 

Improved representation of aerosols 

in climate models is essential to 

more accurate predictions of the role 

of PM in climate forcing. Most 

climate model simulations available 

did not consider local variations in 

climate forcing due to emissions 

sources and local meteorological 

patterns. 

If the PM ISA provides an improved 

understanding of the local and 

regional effects of aerosols on 

climate, we would evaluate whether 

this new information would be 

sufficient to warrant a quantitative 

climate assessment for this review. 

Materials 

damage and 

soiling effects 

No quantitative relationships were 

available between ambient particle 

concentrations and rates of corrosion 

of metals and in the weathering of 

materials. In addition, no 

quantitative relationships were 

available between particles and the 

frequency of cleaning or repainting 

of surfaces. Data on the role of 

If additional studies become 

available in the PM ISA to 

substantially reduce the uncertainty 

in quantifying materials damage and 

soiling effects, we would consider 

whether this new information would 

be sufficient to warrant a 

quantitative materials assessment for 

this review. 
April 2016 5-19 Draft – Do Quote or Cite

Major 

Uncertainty or 

Limitation 

Uncertainty/Limitation Remaining 

from Last Review 

Consideration of Information 

Newly Available in this Review 

microbial colonizers in 

biodeterioration processes and 

contributions of black crust to 

soiling were not sufficient for 

quantitative analyses. 

1 

2 5.4 SCIENTIFIC AND PUBLIC REVIEW 

3 The WREA Planning Document will be distributed to the CASAC for review and provided 

4 to the public for review and comment. The document will be the subject of a review with the 

5 CASAC at a public meeting or teleconference that will be announced in the Federal Register. 

6 The EPA does not produce a final WREA Planning Document but instead considers CASAC 

7 recommendations and public comments in the design and when conducting the quantitative 

8 assessments either in a new WREA or in updating or expanding the last assessment as part of the 

9 PA. In either case, staff would prepare at least one draft of the assessment for CASAC review 

10 and public comment. CASAC would review the document and discuss it at a public meeting that 

11 would be announced in the Federal Register. Based on past practice, the EPA expects that 

12 CASAC would summarize key advice and recommendations for revision of the assessment in a 

13 letter to the EPA Administrator. In revising any draft WREA, the EPA would take into account 

14 any such recommendations and also consider comments received from the public, both at the 

15 meeting itself and directly in writing. A final assessment would then be made available on an 

16 EPA website, with its public availability announced in the Federal Register. 
April 2016 5-20 Draft – Do Quote or Cite

1 REFERENCES 

2 Abt Associates, Inc. (2001). Assessing Public Opinions on Visibility Impairment due to Air 

3 Pollution: Summary Report. Prepared for EPA Office of Air Quality Planning and 

4 Standards; funded under EPA Contract No. 68-D-98-001. Abt Associates Inc., Bethesda, 

5 MD. January 2001. Available: http://www.epa.gov/ttncaaa1/t1/reports/vis_rpt_final.pdf 

6 BBC Research & Consulting (2003). Phoenix Area Visibility Survey. Draft Report. Available: 

7 http://www.azdeq.gov/environ/air/download/vis_021903f.pdf. 

8 Bond, T. C., et al. (2013), Bounding the role of black carbon in the climate system: A scientific 

9 assessment, J. Geophys. Res. Atmos., 118, 5380–5552. 

10 Ely, D.W., J.T. Leary, T.R. Stewart, and D.M. Ross (1991). The Establishment of the Denver 

11 Visibility Standard. Presented at the 84th Annual Meeting & Exhibition of the Air and 

12 Waste Management Association. June 16-21. 

13 Intergovernmental Panel on Climate Change (IPCC). (2013) Climate Change 2013: The Physical 

14 Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the 

15 Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. 

16 Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. 

17 Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 

18 1535 pp, doi:10.1017/CBO9781107415324. 

19 Molenar, J.V., W.C. Malm, and C.E. Johnson (1994). Visual air quality simulation techniques. 

20 Atmospheric Environment 28(5):1055-1063. 

21 Pitchford, M. and W. Malm (1993). Development and applications of a standard visual index. 

22 Atmospheric Environment 28(5):1049-1054. 

23 Pitchford M; Malm W; Schichtel B; Kumar N; Lowenthal D; Hand J (2007). Revised algorithm 

24 for estimating light extinction from IMPROVE particle speciation data. J Air Waste 

25 Manag Assoc 57: 1326–36. 

26 Pryor, S.C. (1996). Assessing public perception of visibility for standard setting exercises. 

27 Atmospheric Environment 30(15):2705-2716. 

28 Smith, A.E. and S. Howell. (2009). An Assessment of the Robustness of Visual Air Quality 

29 Preference Study Results. CRA International, Washington, DC. March 30. Prepared for 

30 the Utility Air Regulatory Group. Submitted as supplemental material to presentation by 

31 Anne Smith to the public meeting of the EPA Clean Air Science Advisory Council. April 

32 2. 

33 U.S. EPA (2004). Air Quality Criteria for Particulate Matter. U.S. EPA, National Center for 

34 Environmental Assessment, Office of Research and Development. Research Triangle 

35 Park, NC, EPA/600/P-99/002aF and EPA/600/P-99/002bF. October 2004. Available: 

36 http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_cd.html. 

37 U.S. EPA (2009a). Integrated Science Assessment (ISA) for Particulate Matter (Final Report). 

38 U.S. EPA, National Center for Environmental Assessment, Office of Research and 

39 Development. Research Triangle Park, NC. December, 2009. EPA/600/R-08/139F. 

40 Available at: http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=216546. 
April 2016 5-21 Draft – Do Quote or Cite

1 U.S. EPA (2011). Policy Assessment for the Review of the Particulate Matter National Ambient 

2 Air Quality Standards. U.S. EPA, Office of Air Quality Planning and Standards. 

3 Research Triangle Park, NC, EPA 452/R-11-003. April, 2011. Available at: 

4 http://www.epa.gov/ttn/naaqs/standards/pm/data/20110419pmpafinal.pdf. 

5 U.S. EPA (2009c). Particulate Matter National Ambient Air Quality Standards: Scope and 

6 Methods Plan for Urban Visibility Impact Assessment. U.S. EPA, Office of Air Quality 

7 Planning and Standards, Research Triangle Park, NC. EPA-452/P-09-001. February 

8 2009. Available: 

9 http://www3.epa.gov/ttn/naaqs/standards/pm/data/20090227PMNAAQSWelfare.pdf. 

10 U.S. EPA (2010b). Particulate Matter Urban-Focused Visibility Assessment – Final Report. 

11 Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, 

12 Research Triangle Park, NC. EPA-452/R-10-004. June 2010. Available: 

13 http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_2007_risk.html. 

14 U.S. Global Change Research Program (2014). Climate Change Impacts in the United States: 

15 The Third National Climate Assessment. May. Available: 

16 http://nca2014.globalchange.gov/. 

17 

18 
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1 6 POLICY ASSESSMENT AND RULEMAKING 

2 As outlined in section 1.2 above, the PA and rulemaking comprise the final phases of the 

3 review. These phases are described briefly in sections 6.1 (PA) and 6.2 (rulemaking) below. 

4 6.1 POLICY ASSESSMENT 

5 The PA will provide a transparent analysis and presentation of staff conclusions 

6 regarding the adequacy of the existing PM standards and the potential alternatives, if any, that 

7 are appropriate to consider in the current review. The PA will integrate and interpret the 

8 information from the ISA and, if available, REA(s) to frame policy options for consideration by 

9 the Administrator. The PA is also intended to facilitate CASAC’s advice to the Agency, and 

10 recommendations to the Administrator, on the adequacy of the existing standards and on 

11 revisions that may be appropriate to consider, as provided for in the CAA. Staff conclusions will 

12 be based on the assessment of the scientific evidence in the PM ISA; the results of exposure and 

13 risk assessments in the REA(s), as available; and any additional staff evaluations and analyses 

14 that are included in the PA. 

15 The discussion in the PA will be framed by consideration of a series of policy-relevant 

16 questions drawn from those presented in sections 2.1.3 and 2.2.3 above. These will include 

17 questions on the adequacy of the current PM standards and, as appropriate, on the elements of 

18 potential alternative standards (i.e., indicator, averaging time, form, level). The PA will identify 

19 evidence-based and exposure-/risk-based approaches for addressing these policy-relevant 

20 questions and for reaching public health and welfare policy judgments. 

21 The PA will identify the range of policy options that the staff concludes could be 

22 supported by the available scientific evidence and the information from available quantitative 

23 assessments. In so doing, the PA will describe the interpretations of this evidence and 

24 information that could support various policy options, as appropriate, and that could be 

25 considered by the Administrator in making decisions on the PM NAAQS. This will include the 

26 identification of key uncertainties and limitations in the underlying scientific evidence and in the 

information available from quantitative assessments.128 27 

28 In identifying ranges of primary and secondary standard options for consideration, the PA 

29 will recognize that the Administrator’s final decisions will reflect public health and public 

 

128 In addition to presenting staff conclusions on the NAAQS, the PA will also highlight areas for future health- and 

welfare-related research, model development, and data collection. 
April 2016 6-2 Draft – Do Not Quote or Cite

1 welfare policy judgments. It will further recognize that the Administrator’s final decisions will 

2 draw upon scientific information and analyses of health or welfare effects and risks, as well as 

3 judgments about how to deal with the range of uncertainties that are inherent in the scientific 

4 evidence and analyses. 

5 Our approach in the PA to informing the Administrator’s judgments on the primary PM 

6 standards will recognize that the available health effects evidence reflects a continuum consisting 

7 of PM exposure concentrations at which scientists generally agree that health effects are likely to 

8 occur, through lower exposure concentrations at which the likelihood and magnitude of the 

9 response become increasingly uncertain. This approach is consistent with the requirements of the 

10 NAAQS provisions of the CAA and with how the EPA and the courts have historically 

11 interpreted the CAA. These provisions require the Administrator to establish primary standards 

12 that are requisite to protect public health and that are neither more nor less stringent than 

13 necessary for this purpose. As discussed in section 1.1 above, the provisions do not require that 

14 primary standards be set at a zero-risk level, but rather at a level that avoids unacceptable risks to 

15 public health, including the health of at-risk populations. 

16 Similarly, our approach in the PA to informing the Administrator’s judgments on the 

17 secondary PM standards will recognize that a final decision must draw upon scientific evidence 

18 and analyses about effects on public welfare, as well as judgments about how to deal with the 

19 range of uncertainties that are inherent in the relevant information. As is the case for the primary 

20 standards discussed above, this approach is consistent with the requirements of the NAAQS 

21 provisions of the CAA and with how the EPA and the courts have historically interpreted the 

22 CAA. These provisions require the Administrator to establish secondary standards that are 

23 requisite to protect public welfare from any known or anticipated adverse effects associated with 

24 the presence of the pollutant in the ambient air. In so doing, the Administrator seeks to establish 

25 standards that are neither more nor less stringent than necessary for this purpose. The provisions 

26 do not require that secondary standards be set to eliminate all welfare effects, but rather at a level 

27 that protects public welfare from those effects that are judged to be adverse. 

28 Staff will prepare at least one draft of the PA for CASAC review and public comment. 

29 CASAC will review the draft PA at a public meeting that will be announced in the Federal 

30 Register. Based on past practice by CASAC, the EPA expects that CASAC will summarize key 

31 advice and recommendations for revision of the document in a letter to the EPA Administrator. 

32 In revising the draft PA, we will take into account such recommendations, comments received 

33 from CASAC and from the public at the meeting itself, and any written comments received. The 

34 final document will be made available on the EPA website, with its public availability 

35 announced in the Federal Register. 
April 2016 6-3 Draft – Do Not Quote or Cite

1 6.2 RULEMAKING 

2 Following the issuance of the final PA and the EPA management consideration of staff 

3 analyses and conclusions presented therein, and taking into consideration CASAC advice and 

4 recommendations, the Agency will develop a notice of proposed rulemaking. The notice of 

5 proposed rulemaking will convey the Administrator’s proposed conclusions regarding the 

6 adequacy of the current standard(s) and any revision that may be appropriate. A draft notice of 

7 proposed rulemaking will be submitted to the Office of Management and Budget (OMB) for 

8 interagency review, in which OMB and other federal agencies are provided the opportunity for 

9 review and comment. After the completion of interagency review, the EPA will publish the 

10 notice in the Federal Register seeking comment on the proposed agency action – namely 

11 whether or not to revise the current standard, and if so, how. Monitoring rule changes associated 

12 with review of the PM standards will be developed and proposed, as appropriate, in conjunction 

13 with the NAAQS rulemaking. 

14 At the time of publication of the notice of proposed rulemaking, all materials on which the 

proposal is based will be made available in the public docket of the rulemaking.129 15 Publication of 

16 the proposal notice is followed by a public comment period, generally lasting 60 to 90 days, 

17 during which the public is invited to submit comments on the proposal to the rulemaking docket. 

18 Taking into account comments received on the proposed rule, the Agency will then 

19 develop a notice of final rulemaking, which again undergoes OMB-coordinated interagency 

20 review prior to issuance by the EPA of the final rule. At the time of the final rulemaking, the 

Agency responds to all significant comments on the proposed rule.130 21 Publication of the final rule 

22 in the Federal Register completes the rulemaking process. 

23 

 

129 The rulemaking docket for the current PM NAAQS review is identified as the EPA-HQ-OAR-2015-0072. This 

docket has incorporated the ISA docket (EPA-HQ-ORD-2014-0859) by reference. Both dockets are publicly 

accessible at www.regulations.gov. 

130 For example, Agency responses to all significant comments on the 2009 notice of proposed rulemaking in the last 

review were provided in the preamble to the final rule and in a document titled Responses to Significant Comments 

on the 2012 Proposed Rule on the National Ambient Air Quality Standards for Particulate Matter (June 29, 2012; 

77 FR 28890) (U.S. EPA, 2012). 
April 2016 4 Draft – Do Not Quote or Cite

REFERENCES 

U.S. EPA (2012). Responses to Significant Comments on the 2012 Proposed Rule on the 

National Ambient Air Quality Standards for Particulate Matter. U.S. Environmental 

Protection Agency, Research Triangle Park, NC. Available at: 

http://www.epa.gov/ttn/naaqs/standards/pm/data/20121214rtc.pdf