Integrated Review Plan for the Review of the 

Ozone National Ambient Air Quality Standards 

External Review Draft 
 
EPA-452/P-18-001 

October 2018 

Integrated Review Plan for the Review of the Ozone National Ambient 

Air Quality Standards 

External Review Draft 

U.S. Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Health and Environmental Impacts Division 

and 

National Center for Environmental Assessment 

Research Triangle Park, NC 
October 2018 i Draft – Do Not Quote or Cite

DISCLAIMER 

This document has been prepared by staff in the U.S. Environmental Protection Agency’s 

Office of Air Quality Planning and Standards and National Center for Environmental 

Assessment. Any findings and conclusions are those of the authors and do not necessarily reflect 

the views of the Agency. This document is being circulated to facilitate discussion with the 

Clean Air Scientific Advisory Committee and for public comment to inform the EPA’s 

consideration of the current review of the ozone national ambient air quality standards. This 

information is distributed for 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. Mention of trade names or commercial products does not 

constitute endorsement or recommendation for use. 

Questions or comments related to this document should be addressed to Dr. Deirdre 

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

C504-06, Research Triangle Park, North Carolina 27711 (email: murphy.deirdre@epa.gov). 
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TABLE OF CONTENTS 

LIST OF APPENDICES ........................................................................................................... iv 

LIST OF FIGURES .................................................................................................................. iv 

LIST OF TABLES .................................................................................................................... iv 

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

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

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

1.3 PLANNED PROCESS AND PROJECTED TIMELINE FOR THIS REVIEW ............. 1-7 

2 BACKGROUND ................................................................................................................. 2-1 

2.1 PRIOR REVIEWS OF AIR QUALITY CRITERIA AND STANDARDS FOR O3 ...... 2-1 

2.2 AMBIENT AIR MONITORING AND DATA HANDLING FOR THE CURRENT 

 STANDARDS .................................................................................................................. 2-6 

 2.2.1 Monitoring Requirements and the Current Monitoring Network ............................. 2-6 

 2.2.2 Data Analysis for Comparison to the Standards ....................................................... 2-9 

2.3 OVERVIEW OF OZONE AIR QUALITY ..................................................................... 2-9 

3 KEY POLICY-RELEVANT ISSUES FOR THE CURRENT REVIEW ..................... 3-1 

3.1 THE PRIMARY STANDARD ........................................................................................ 3-2

 3.1.1 Key Issues Related to the Primary Standard ............................................................ 3-2

 3.1.2 Background on the Current Primary Standard (Considerations and Conclusions 

 in the Last Review) ................................................................................................. 3-6

 3.1.2.1 Considering the Need for Revision .................................................................. 3-9

 3.1.2.2 Considering Revisions to the Standard .......................................................... 3-14

3.2   THE SECONDARY STANDARD ................................................................................ 3-21

 3.2.1 Key Issues Related to the Secondary Standard ....................................................... 3-21

 3.2.2 Background on the Current Secondary Standard (Considerations and 

 Conclusions in the Last Review) ........................................................................... 3-26

 3.2.2.1 Considering the Need for Revision ................................................................ 3-28

 3.2.2.2 Considering Revisions to the Standard .......................................................... 3-31

4 SCIENCE ASSESSMENT ................................................................................................. 4-1 

4.1 PURPOSE OF THE ISA .................................................................................................. 4-1

4.2 ORGANIZATION OF THE ISA ..................................................................................... 4-1

4.3 ASSESSMENT APPROACH .......................................................................................... 4-2

 4.3.1 Introduction ............................................................................................................... 4-2

 4.3.2 Scope of the ISA ....................................................................................................... 4-4

 4.3.3 Literature Search and Identification of Relevant Studies ......................................... 4-6

 4.3.3.1 Systematic Literature Search .......................................................................... 4-6

 4.3.3.2 Initial Screening (Level 1) of Studies from Literature Search ........................ 4-7

 4.3.3.3 Criteria of In-Scope Studies ............................................................................ 4-8

 4.3.4 Discipline-Specific Scoping, Searching and Screening ............................................ 4-9
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 4.3.4.1 Atmospheric Science ....................................................................................... 4-9

 4.3.4.2 Exposure Assessment....................................................................................... 4-9

 4.3.4.3 Health – Experimental Studies ....................................................................... 4-10

 4.3.4.4 Health – Observational (Epidemiologic) Studies .......................................... 4-11

 4.3.4.5 Welfare Effects – Ecological Studies ............................................................ 4-14

 4.3.4.6 Welfare – Effects on Climate ......................................................................... 4-16

 4.3.5 Identification of Policy-Relevant Studies ............................................................... 4-17

 4.3.6 Evaluation of Individual Study Quality .................................................................. 4-17

 4.3.7 Integration of Evidence and Determination of Causality ....................................... 4-18

 4.3.8 Peer Input Workshop .............................................................................................. 4-21

 4.3.9 Quality Management ............................................................................................... 4-22

4.4 SPECIFIC SCIENCE ISSUES TO BE ADDRESSED IN THE ISA ............................ 4-22

 4.4.1 Causality Determinations from 2013 ISA............................................................... 4-24

 4.4.2 Ambient Concentrations of O3 ................................................................................ 4-25

 4.4.3 Human Exposure ..................................................................................................... 4-26

 4.4.4 Health Effects .......................................................................................................... 4-27

 4.4.5 At-Risk Lifestages and Populations and Public Health Impact .............................. 4-29

 4.4.6 Welfare Effects ....................................................................................................... 4-29

 4.4.6.1 Ecological Effects .......................................................................................... 4-30

 4.4.6.2 Effects on Climate .......................................................................................... 4-32

4.5 SCIENTIFIC AND PUBLIC REVIEW ......................................................................... 4-32

5      QUANTITATIVE RISK AND EXPOSURE ASSESSMENTS ...................................... 5-1

5.1 ASSESSMENTS INFORMING REVIEW OF THE PRIMARY STANDARD .............. 5-3

 5.1.1 Overview of Assessments in Last Review ................................................................ 5-5

 5.1.1.1 Exposure-based Risk Analyses ....................................................................... 5-8

 5.1.1.2 Air Quality Epidemiologic Study Based Risk Analyses............................... 5-14

 5.1.2 Consideration of Assessments for this Review ........................................................ 5-16

5.2 ASSESSMENTS INFORMING REVIEW OF THE SECONDARY STANDARD .... 5-18

 5.2.1 Overview of Assessments in Last Review .............................................................. 5-19

 5.2.1.1 Growth-related Assessments .......................................................................... 5-20

 5.2.1.2 Foliar Injury Assessments .............................................................................. 5-24

 5.2.1.3 Additional Air Quality/Exposure and E-R Analyses ..................................... 5-25

 5.2.2 Consideration of Assessments for this Review ....................................................... 5-27

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

7 PROPOSED AND FINAL DECISIONS ............................................................................. 7-1 

8 REFERENCES ...................................................................................................................... 8-1 
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LIST OF APPENDICES 

5-A. Limitations and uncertainties of exposure and risk analyses developed in the last review of 

the primary standard and consideration of related newly available information and tools. 

5-B. Limitations and uncertainties of air quality, exposure and risk analyses developed in the 

last review of secondary standard and consideration of related newly available 

information and tools. 

LIST OF FIGURES 

Figure 2-1.Map of U.S. ambient air O3 monitoring sites reporting data to the EPA during the 

2015-2017 period. ...................................................................................................... 2-8

Figure 2-2.Trends in anthropogenic emissions of NOX and VOC (2002-2014). ...................... 2-12

Figure 2-3.2015-2017 O3 design values across the U.S.. .......................................................... 2-14

Figure 2-4.Trends in annual 4th highest daily maximum 8-hour average O3 at all sites across 

the U.S. with complete data (2000-2017). ............................................................... 2-15

Figure 3-1.Overview of general approach for review of the primary O3 standard. ..................... 3-5

Figure 3-2.Overview of general approach for review of the secondary O3 standard. ............... 3-25

Figure 4-1.General process for development of Integrated Science Assessments. ..................... 4-3

Figure 5-1.Summary of health risk assessment approaches that have been employed in 

NAAQS reviews. ....................................................................................................... 5-4

Figure 5-2.Conceptual model for 2014 O3 health risk assessment. Solid lines indicate 

processes included in the 2014 assessment. ............................................................... 5-5

Figure 5-3.Analytical approach for exposure-based risk analyses. Dashed lines and gray box 

indicates the sole lung function risk approach used prior to 2014 HREA. ................ 5-9

Figure 5-4.Analytical approach for epidemiologic-based analyses. .......................................... 5-16

LIST OF TABLES 

Table 1-1. Projected timeline for completion of the review. ....................................................... 1-9 

Table 2-1. Percentage of total emissions in top five source groups for NOX. ........................... 2-11

Table 2-2. Percentage of total emissions in top five source groups for VOC. .......................... 2-12

Table 4-1. PECOS statements for experimental studies. ........................................................... 4-11

Table 4-2. PECOS statements for epidemiologic studies. ......................................................... 4-13

Table 4-3. PECOS statements for ecological studies. ............................................................... 4-15

Table 4-4. PECOS statements for effects of tropospheric O3 on climate. ................................. 4-16

Table 4-5. Weight of evidence for causality determinations. .................................................... 4-19
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1 1 INTRODUCTION 

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

3 quality criteria and the national ambient air quality standards (NAAQS) for photochemical 

4 oxidants including ozone (O3). This draft Integrated Review Plan (IRP) contains the draft plans 

5 for this review. The review will provide an integrative assessment of relevant scientific 

6 information and will focus on key aspects of the O3 NAAQS, including the basic elements of the 

standards: the indicator,1 averaging time, form,2 7 and level. These elements, which together 

8 serve to define each ambient air quality standard, are considered collectively in evaluating the 

9 protection to public health and public welfare afforded by the standards. 

10 This document is organized into eight chapters. Chapter 1 presents introductory 

11 information on the legislative requirements for reviews of the NAAQS, an overview of the 

12 review process, and a summary of the status and projected schedule for the current review. 

13 Chapter 2 provides background information on prior reviews of the criteria and standards for 

14 photochemical oxidants, including O3, key aspects of the ambient air monitoring requirements, 

15 and an overview of current O3 air quality. Chapter 3 presents the general approach and a set of 

16 policy-relevant questions intended to focus this review on the critical scientific and policy issues. 

17 Chapters 4 through 7 discuss the planned scope and organization of key assessment documents, 

18 the planned approaches for preparing the documents, and plans for scientific and public review 

19 of the documents. The complete citations for references cited throughout the document are 

20 provided in chapter 8. 

21 1.1 LEGISLATIVE REQUIREMENTS 

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

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

24 pollutants and then to issue air quality criteria for those pollutants. The Administrator is to list 

25 those pollutants “emissions of which, in his judgment, cause or contribute to air pollution which 

26 may reasonably be anticipated to endanger public health or welfare”; “the presence of which in 

27 the ambient air results from numerous or diverse mobile or stationary sources”; and for which he 

28 “plans to issue air quality criteria....” (42 U.S.C. § 7408(a)(1)). Air quality criteria are intended 

 

1

 The “indicator” of a standard defines the chemical species or mixture that is to be measured in determining 

whether an area attains the standard. The indicator of the current NAAQS for photochemical oxidants is O3.

2

 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. For example, the form of the annual PM2.5 NAAQS is the threeyear average of the weighted annual mean PM2.5 concentrations, while the form of the current three-month Pb 

NAAQS is a three-month average concentration not to be exceeded during a three-year period. 
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1 to “accurately reflect the latest scientific knowledge useful in indicating the kind and extent of all 

2 identifiable effects on public health or welfare which may be expected from the presence of [a] 

3 pollutant in the ambient air....” 42 U.S.C. § 7408(a)(2). 

4 Section 109 [42 U.S.C. 7409] directs the Administrator to propose and promulgate 

5 “primary” and “secondary” NAAQS for pollutants for which air quality criteria are issued [42 

6 U.S.C. § 7409(a)]. Section 109(b)(1) defines primary standards as ones “the attainment and 

7 maintenance of which in the judgment of the Administrator, based on such criteria and allowing 

an adequate margin of safety, are requisite to protect the public health.”3 8 Under section 

9 109(b)(2), a secondary standard must “specify a level of air quality the attainment and 

10 maintenance of which, in the judgment of the Administrator, based on such criteria, is requisite 

11 to protect the public welfare from any known or anticipated adverse effects associated with the 

presence of [the] pollutant in the ambient air.”4 12 

13 In setting primary and secondary standards that are “requisite” to protect public health 

14 and welfare, respectively, as provided in section 109(b), the EPA’s task is to establish standards 

15 that are neither more nor less stringent than necessary. In so doing, the EPA may not consider the 

16 costs of implementing the standards. See generally, Whitman v. American Trucking Associations, 

17 531 U.S. 457, 465-472, 475-76 (2001). Likewise, “[a]ttainability and technological feasibility are 

18 not relevant considerations in the promulgation of national ambient air quality standards.” 

19 American Petroleum Institute v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981). At the same time, 

20 courts have clarified the EPA may consider “relative proximity to peak background ... 

21 concentrations” as a factor in deciding how to revise the NAAQS in the context of considering 

22 standard levels within the range of reasonable values supported by the air quality criteria and 

23 judgments of the Administrator. American Trucking Associations, Inc. v. EPA, 283 F.3d 355, 379 

24 (D.C. Cir. 2002). 

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

26 intended to address uncertainties associated with inconclusive scientific and technical 

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

28 degree of protection against hazards that research has not yet identified. See Lead Industries 

29 Association v. EPA, 647 F.2d 1130, 1154 (D.C. Cir 1980), cert. denied, 449 U.S. 1042 (1980); 

 

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

 Under CAA section 302(h) (42 U.S.C. § 7602(h)), effects on welfare include, but are not limited to, “effects on 

soils, water, crops, vegetation, manmade materials, animals, wildlife, weather, visibility, and climate, damage to 

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

comfort and well-being.” 
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1 American Petroleum Institute v. Costle, 665 F.2d at 1186 (D.C. Cir. 1981), cert. denied, 455 U.S. 

2 1034 (1982); Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 (D.C. Cir. 

3 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013). Both kinds of uncertainties are 

4 components of the risk associated with pollution at levels below those at which human health 

5 effects can be said to occur with reasonable scientific certainty. Thus, in selecting primary 

6 standards that include an adequate margin of safety, the Administrator is seeking not only to 

7 prevent pollution levels that have been demonstrated to be harmful but also to prevent lower 

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

9 identified as to nature or degree. The CAA does not require the Administrator to establish a 

10 primary NAAQS at a zero-risk level or at background concentration levels, see Lead Industries 

11 v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F.3d at 1351, but rather at a level that 

12 reduces risk sufficiently so as to protect public health with an adequate margin of safety. 

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

14 factors as the nature and severity of the health effects involved, the size of the sensitive 

15 population(s), and the kind and degree of uncertainties. The selection of any particular approach 

16 to providing an adequate margin of safety is a policy choice left specifically to the 

17 Administrator’s judgment. See Lead Industries Association v. EPA, 647 F.2d at 1161-62; 

18 Mississippi v. EPA, 744 F.3d at 1353. 

19 Section 109(d)(1) of the Act requires periodic review and, if appropriate, revision of 

20 existing air quality criteria to reflect advances in scientific knowledge on the effects of the 

21 pollutant on public health and welfare. Under the same provision, EPA is also to periodically 

review and, if appropriate, revise the NAAQS, based on the revised air quality criteria.5 22 

23 Section 109(d)(2) addresses the appointment and advisory functions of an independent 

24 scientific review committee. Section 109(d)(2)(A) requires the Administrator to appoint this 

25 committee, which is to be composed of “seven members including at least one member of the 

26 National Academy of Sciences, one physician, and one person representing State air pollution 

27 control agencies.” Section 109(d)(2)(B) provides that the independent scientific review 

28 committee “shall complete a review of the criteria...and the national primary and secondary 

29 ambient air quality standards...and shall recommend to the Administrator any new...standards 

30 and revisions of existing criteria and standards as may be appropriate....” Since the early 1980s, 

31 this independent review function has been performed by the Clean Air Scientific Advisory 

32 Committee (CASAC) of the EPA’s Science Advisory Board. A number of other advisory 

33 functions are also identified for the committee by section 109(d)(2)(C), which reads: 

 

5

 This section of the Act requires the Administrator to complete these reviews and make any revisions that may be 

appropriate “at five-year intervals.” 
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1 Such committee shall also (i) advise the Administrator of areas in which 

2 additional knowledge is required to appraise the adequacy and basis of existing, 

3 new, or revised national ambient air quality standards, (ii) describe the research 

4 efforts necessary to provide the required information, (iii) advise the 

5 Administrator on the relative contribution to air pollution concentrations of 

6 natural as well as anthropogenic activity, and (iv) advise the Administrator of any 

7 adverse public health, welfare, social, economic, or energy effects which may 

8 result from various strategies for attainment and maintenance of such national 

9 ambient air quality standards. 

10 As previously noted, the Supreme Court has held that section 109(b) “unambiguously bars cost 

11 considerations from the NAAQS-setting process” (Whitman v. Am. Trucking Associations, 531 

12 U.S. 457, 471 [2001]). Accordingly, some of these issues regarding which Congress has directed 

13 CASAC to advise the Administrator are ones that are relevant to the standard setting process. 

14 Issues that are not relevant to standard setting may be relevant to implementation of the NAAQS 

once they are established.6 15 

16 1.2 OVERVIEW OF THE NAAQS REVIEW PROCESS 

17 The process for reviewing the NAAQS has three general phases: (1) planning, (2) 

18 assessment, and (3) decision making. Each of these phases is described in this section. The 

19 Agency maintains a web site on which key documents developed in each phase of each NAAQS 

20 review are made available (https://www.epa.gov/naaqs). This website also makes available 

21 information regarding the process for NAAQS reviews, including the May 2018 memorandum 

22 from the Administrator to Assistant Administrators (Pruitt, 2018) that describes five areas for 

23 emphasis (principles) in the reviews and that builds on prior memoranda concerning the process 

24 for NAAQS reviews (Peacock, 2006; Jackson, 2009). 

25 The planning phase of each NAAQS review begins with a call for information and the 

26 identification of issues and questions to frame the review. Drawing on this information and 

27 issues raised in the last review, a draft IRP is prepared jointly by the EPA’s National Center for 

28 Environmental Assessment (NCEA), within the Office of Research and Development (ORD), 

 

6

 Some aspects of CASAC advice may not be relevant to EPA’s process of setting primary and secondary standards 

that are requisite to protect public health and welfare. Indeed, were EPA to consider costs of implementation 

when reviewing and revising the standards “it would be grounds for vacating the NAAQS.” Whitman, 531 U.S. at 

471 n.4. At the same time, the Clean Air Act directs CASAC to provide advice on “any adverse public health, 

welfare, social, economic, or energy effects which may result from various strategies for attainment and 

maintenance” of the NAAQS to the Administrator under section 109(d)(2)(C)(iv). In Whitman, the Court 

clarified that most of that advice would be relevant to implementation but not standard setting, as it “enable[s] the 

Administrator to assist the States in carrying out their statutory role as primary implementers of the NAAQS.” Id. 

at 470 (emphasis in original). However, the Court also noted that CASAC’s “advice concerning certain aspects of 

‘adverse public health ... effects’ from various attainment strategies is unquestionably pertinent” to the NAAQS 

rulemaking record and relevant to the standard setting process. Id. at 470 n.2. 
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1 and the EPA’s Office of Air Quality Planning and Standards (OAQPS), within the Office of Air 

2 and Radiation (OAR). This draft document presents the current plan, projected timeline, and 

3 process for conducting the review, and also identifies key policy-relevant issues or questions 

4 intended to guide the review. The draft IRP is made available for consultation with the CASAC 

5 and for public comment. The final IRP is prepared in consideration of CASAC and public 

6 comments. 

7 The assessment phase of the review involves assessments of scientific information, 

8 exposure or risk, and policy, which are described in key documents for the review. The 

9 Integrated Science Assessment (ISA), prepared by the NCEA, provides a focused review, 

10 synthesis, and evaluation of the most policy-relevant scientific information, including key 

11 scientific judgments that are important to the design and scope of any exposure and risk 

assessments, as well as other aspects of the NAAQS review. The ISA7 12 provides a comprehensive 

13 assessment of the current scientific literature pertaining to known and anticipated effects on 

14 public health and welfare associated with the presence of the pollutant in the ambient air, 

15 emphasizing information that has become available since the last air quality criteria review in 

16 order to reflect the current state of knowledge. As such, the ISA forms the scientific foundation 

17 for each NAAQS review and is intended to provide information useful in forming policy18 relevant judgments about air quality indicator(s), form(s), averaging time(s) and level(s) for the 

19 NAAQS. Prior to its completion in final form, the ISA, in draft form, is reviewed by the CASAC 

20 and made available for public comment. Chapter 4 below provides a more detailed description of 

21 the planned scope, organization and assessment approach for the ISA and its supporting 

22 materials in this review of the air quality criteria and O3 NAAQS. 

23 Based on the information and conclusions presented in the ISA, the EPA considers the 

24 support provided for the development of quantitative assessments of the risks and/or exposures 

25 for health and/or welfare effects. In so doing, the EPA considers the extent to which newly 

26 available scientific evidence and tools/methodologies may warrant the conduct of new 

quantitative risk and exposure assessments for the review.8 27 Key to the EPA’s decision on 

28 exposure or risk analyses that may be appropriate to develop in the review is consideration of the 

29 newly available data, methods and tools in light of areas of uncertainty in the assessments 

 

7

 The ISA functions in the current NAAQS review process as the Air Quality Criteria Document (AQCD) did in 

reviews completed prior to 2009. 

8

 In some reviews this consideration, and, as warranted, a general plan, including scope and methods, for conducting 

the assessments, have been described in a planning document (e.g., REA Planning Document) that has been 

provided to the CASAC for consultation and made available for public comment. The EPA is not planning to 

prepare such a separate document in this review of the O3 NAAQS; the EPA’s general considerations for 

identifying the quantitative air quality exposure and risk analyses to be performed in this review are discussed in 

this draft IRP (Chapter 5). 
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1 conducted for the last review and of the potential for new or updated assessments to provide 

2 notably different exposure and/or risk estimates with lower associated uncertainty. Any 

3 exposure/risk analyses performed for the review, and/or exposure/risk information developed in 

4 the prior review that remains relevant in the current review, are considered in the policy 

5 assessment (PA) for the review. The details regarding methods, key results, observations, and 

related uncertainties are documented in a separate document accompanying the PA9 6 or in an 

7 appendix to the PA. Chapter 5 includes preliminary consideration of quantitative human health8 and welfare-related assessments for this review. 

9 The PA, prepared by the OAQPS, is a document that provides a transparent analysis 

10 regarding the adequacy of the current standards and, as appropriate, potential alternatives for 

11 Agency consideration prior to the issuance of proposed and final decisions. The PA integrates 

12 and interprets the information from the ISA and from any risk and exposure analyses to frame 

13 policy options for consideration by the Administrator. Such an evaluation of policy implications 

14 is intended to help “bridge the gap” between the Agency’s scientific assessments, presented in 

15 the ISA and quantitative analyses, and the judgments required of the EPA Administrator in 

16 determining whether it is appropriate to retain or revise the NAAQS. In so doing, the PA is also 

17 intended to facilitate CASAC advice to the Agency and recommendations to the Administrator 

18 on the adequacy of the existing standards or revisions that may be appropriate to consider, as 

19 provided for in the CAA. In evaluating the adequacy of the current standards and, as appropriate, 

20 a range of alternative standards, the PA considers the available scientific evidence and, as 

21 available, quantitative risk-based analyses, together with related limitations and uncertainties. 

22 The PA focuses on the information that is most pertinent to evaluating the basic elements of 

23 NAAQS: indicator, averaging time, form, and level. The PA, in draft form, is released for 

24 CASAC review and public comment prior to completion of the final PA. 

25 The May 2018 NAAQS process memorandum identified a set of general charge questions 

26 to be posed to the CASAC in the NAAQS review process, while recognizing that these would be 

27 supplemented with more detailed requests as necessary (Pruitt, 2018). The general questions 

28 cited in the May 2018 memo are as follows: 

29  Are there areas in which additional knowledge is required to appraise the 

30 adequacy and basis of existing, new, or revised NAAQS? Please describe the 

31 research efforts necessary to provide the required information.

32  What scientific evidence has been developed since the last review to indicate 

33 if the current primary and/or secondary NAAQS need to be revised or if an 

34 alternative level or form of these standards is needed to protect public health 

 

9

 In reviews conducted since 2008, the separate, stand-alone document presenting these analyses has been termed 

the Risk and Exposure Assessment (REA). 
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1 and/or public welfare? Please recommend to the Administrator any new 

2 NAAQS or revisions of existing criteria and standards as may be appropriate. 

3 In providing advice, please consider a range of options for standard setting, in 

4 terms of indicators, averaging times, form, and ranges of levels for any 

5 alternative standards, along with a description of the alternative underlying 

6 interpretations of the scientific evidence and risk/exposure information that 

7 might support such alternative standards and that could be considered by the 

8 Administrator in making NAAQS decisions. 

9  Do key studies, analyses, and assessments which may inform the 

10 Administrator's decision to revise the NAAQS properly address or 

11 characterize uncertainty and causality? Are there appropriate criteria to ensure 

12 transparency in the evaluation, assessment and characterization of key 

13 scientific evidence for this review? 

14  What is the relative contribution to air pollution concentrations of natural as 

15 well as anthropogenic activity? In providing advice on any recommended 

16 NAAQS levels, please discuss relative proximity to peak background levels. 

17  Please advise the Administrator of any adverse public health, welfare, social, 

18 economic, or energy effects which may result from various strategies for 

19 attainment and maintenance of such NAAQS. 

20 The memo recognized that the last two charge questions may elicit information which is not 

21 relevant to the standard-setting process under the interpretation of section 109(b) articulated by 

22 the Supreme Court in Whitman , noting that the EPA should consider an appropriate mechanism, 

23 including opportunities after the CASAC has provided its final advice on the standards, to 

24 facilitate robust feedback on these topics (Pruitt, 2018). In order to facilitate meaningful advice 

25 on these questions, the EPA issued a call for information in June 2018 that requested interested 

26 parties to submit information on any adverse public health, welfare, social, economic, or energy 

27 effects which may result from various strategies for attainment and maintenance of existing, 

28 new, or revised NAAQS for consideration by the CASAC (83 FR 29784, June 26, 2018). 

29 Separately, the EPA issued a separate call for scientific and policy-relevant information for the 

30 current O3 NAAQS review, as noted in section 1.3 below (83 FR 29785, June 26, 2018). 

31 Following issuance of the final PA and consideration of conclusions presented therein, 

32 the Agency develops and publishes a notice in the Federal Register that communicates the 

33 Administrator’s proposed decisions regarding the review. A draft of this notice may undergo 

34 interagency review involving other federal agencies prior to publication (e.g., in cases when the 

proposed decision in a NAAQS review involves revision of a standard).10 35 Materials upon which 

 

10 Where the proposed or final action involves NAAQS revisions for which implementation would have a large 

economic effect (e.g., an annual effect on the economy of $100 million or more), such as by necessitating the 

implementation of emissions controls, EPA develops and releases a regulatory impact analysis (RIA) concurrent 

with the notice of proposed or final action. This activity is conducted under Executive Order 12866. The RIA is 
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1 this proposed decision is based, including the documents described above, are made available to 

2 the public in the docket for the review. A public comment period, during which public hearings 

3 are generally held, follows publication of the notice of the proposed action. Taking into account 

comments received on the proposed decision,11 4 the Agency develops a notice of its final action, 

5 which communicates the Administrator’s final decisions on the review. As with the notice of 

6 proposed action, a draft of this notice may undergo interagency review prior to publication in the 

7 Federal Register to complete the process. Chapter 6 discusses the development of the PA and 

8 Chapter 7 the anticipated steps for issuing a proposed and then final decision for the review. 

9 1.3 PLANNED PROCESS AND PROJECTED TIMELINE FOR THIS 

10 REVIEW 

11 In May 2018, the Administrator directed his Assistant Administrators to initiate this 

12 review of the O3 NAAQS (Pruitt, 2018). In conveying this direction, the Administrator further 

13 directed the EPA staff to expedite the review, implementing an accelerated schedule to ensure 

14 completion of the review in 2020 (Pruitt, 2018). Accordingly, the EPA took immediate steps to 

15 proceed with the review. In June 2018, the EPA’s NCEA announced the initiation of the current 

16 periodic review of the air quality criteria for photochemical oxidants and the O3 NAAQS and 

17 issued a call for information in the Federal Register (83 FR 29785, June 26, 2018). Two types of 

18 information were called for: information regarding significant new O3 research to be considered 

19 for the ISA for the review, and policy-relevant issues for consideration in this NAAQS review. 

20 Based in part on the information received in response to the call for information, EPA has 

21 developed this draft IRP. The draft IRP is being made available for consultation with the 

22 CASAC and for public comment. 

23 Under the plan outlined here, the current review of the O3 NAAQS is progressing on an 

24 accelerated schedule and the EPA is incorporating a number of efficiencies in various aspects of 

25 the review process to ensure completion within the statutorily required period (Pruitt, 2018). For 

26 example, the kick-off workshop has been replaced with the addition of a call for policy-relevant 

27 information coincident with the call for scientific information that traditional initiates a NAAQS 

28 review (83 FR 29785, June 26, 2018). The EPA is not planning to develop a Risk and Exposure 

29 Assessment (REA) Planning Document in this review; key considerations with regard to 

30 development of quantitative analyses are discussed in Chapter 5 of this document, which will be 

 

conducted completely independent of and, by statute, is not considered in decisions regarding the review of the 

NAAQS. 

11 When issuing the final action, the Agency responds to all significant comments on the proposed decision. Where a 

separate Response to Comments document is created for this purpose, it is added to the public docket for the 

review, along with any additional materials upon which the final decision is based. 
October 2018 1-9 Draft – Do Not Quote or Cite

1 the subject of a consultation with the CASAC. Further, the EPA has also considered combining 

2 the reviews by the CASAC and the public for some of the main documents in a review (Pruitt, 

3 2018). As a result, the EPA is planning to incorporate the REA-related analyses into the PA, 

4 combining what had been two documents into a single document for review by the CASAC and 

5 the public. Further, we are striving to ensure that initial draft documents are sufficiently robust 

6 and complete to support a single, full review by the CASAC and the public. The successfulness 

7 of these and other efficiencies implemented in this review will be considered by the EPA in 

8 planning for other future NAAQS reviews (Pruitt, 2018). 

9 Also coincident with preparation of this draft IRP, the EPA has begun review of the 

10 literature for consideration in the ISA, as described in Chapter 4 below. The current timeline 

11 projects release of a draft ISA for CASAC review and public comment in Spring 2019. In 

12 addition to informing any revisions to the ISA, that review step and the associated comments and 

13 advice from the CASAC and the public will also inform development of the draft PA. Comments 

14 and recommendations from the CASAC, and public comment, on the draft PA later in the Fall 

15 will then inform completion of the final PA, including its presentation of options appropriate for 

16 the Administrator to consider in this review of the O3 NAAQS. The current timeline projects a 

17 proposed decision in Spring 2020 and completion of the review with a final decision in late 

18 2020. 

19 Table 1-1. Projected timeline for completion of the review. 

Key Milestones in the Review 

May 2018 Administrator’s memo directing initiation of the review 

June 2018 Announcement and Call for Information in Federal Register

August 2018 End comment period for Call for Information 

October 2018 Draft IRP for CASAC and public comment 

November 2018 CASAC consultation on draft IRP 

Early 2019 Final IRP 

Spring 2019 Draft ISA for CASAC review and public comment 

Fall 2019 Draft PA for CASAC review and public comment 

Early Spring 2020 

Final ISA

Final PA

Spring 2020 Proposed decision

Late 2020 Final decision

20 

21 
October 2018 2-1 Draft – Do Not Quote or Cite

1 2 BACKGROUND 

2 Air quality criteria were developed for photochemical oxidants in 1970 (U.S. DHEW, 

3 1970; 35 FR 4768, March 19, 1970), and primary and secondary NAAQS were first established 

4 in 1971 (36 FR 8186, April 30, 1971). Based on the scientific information in the 1970 air quality 

5 criteria document (AQCD), the EPA set both primary and secondary standards at 0.08 parts per 

6 million (ppm), as a 1-hour average of total photochemical oxidants, not to be exceeded more 

7 than one hour per year. As summarized in section 2.1, the EPA has reviewed the air quality 

8 criteria and standards a number of times since then, with the most recent review being completed 

9 in 2015. An overview of the requirements for ambient air monitoring and data handling 

10 established for the 2015 standard are summarized in section 2.2 and current ozone air quality is 

11 summarized in section 2.3. 

12 2.1 PRIOR REVIEWS OF AIR QUALITY CRITERIA AND STANDARDS 

13 FOR PHOTOCHEMICAL OXIDANTS INCLUDING O3

14 The EPA initiated the first periodic review of the NAAQS for photochemical oxidants in 

15 1977. Based on the 1978 AQCD (U.S. EPA, 1978), the EPA published proposed revisions to the 

16 original NAAQS in 1978 (43 FR 26962, June 22, 1978) and final revisions in 1979 (44 FR 8202, 

17 February 8, 1979). At that time, the EPA changed the indicator from photochemical oxidants to 

18 O3, revised the level of the primary and secondary standards from 0.08 to 0.12 ppm and revised 

19 the form of both standards from a deterministic (i.e., not to be exceeded more than one hour per 

20 year) to a statistical form. With these changes, attainment of the standards was defined to occur 

21 when the average number of days per calendar year (across a 3-year period) with maximum 

22 hourly average O3 concentration greater than 0.12 ppm equaled one or less (44 FR 8202, 

23 February 8, 1979; 43 FR 26962, June 22, 1978). 

24 Following the EPA’s decision in the 1979 review, the city of Houston challenged the 

25 Administrator’s decision arguing that the standard was arbitrary and capricious because natural 

26 O3 concentrations and other physical phenomena in the Houston area made the standard 

27 unattainable in that area. The U.S. Court of Appeals for the District of Columbia Circuit (D.C. 

28 Circuit) rejected this argument, holding (as noted in section 1.1 above) that attainability and 

29 technological feasibility are not relevant considerations in the promulgation of the NAAQS 

30 (American Petroleum Institute v. Costle, 665 F.2d at 1185). The court also noted that the EPA 

31 need not tailor the NAAQS to fit each region or locale, pointing out that Congress was aware of 

32 the difficulty in meeting standards in some locations and had addressed this difficulty through 

33 various compliance related provisions in the CAA (id. at 1184-86). 
October 2018 2-2 Draft – Do Not Quote or Cite

1 The next periodic reviews of the criteria and standards for O3 and other photochemical 

2 oxidants began in 1982 and 1983, respectively (47 FR 11561, March 17, 1982; 48 FR 38009, 

3 August 22, 1983). The EPA subsequently published the 1986 AQCD (U.S. EPA, 1986) and the 

4 1989 Staff Paper (U.S. EPA, 1989). Following publication of the 1986 AQCD, a number of 

5 scientific abstracts and articles were published that appeared to be of sufficient importance 

6 concerning potential health and welfare effects of O3 to warrant preparation of a supplement to 

7 the 1986 AQCD (U.S. EPA, 1992). In August of 1992, the EPA proposed to retain the existing 

8 primary and secondary standards based on the health and welfare effects information contained 

9 in the 1986 AQCD and its 1992 Supplement (57 FR 35542, August 10, 1992). In March 1993, 

10 the EPA announced its decision to conclude this review by affirming its proposed decision to 

11 retain the standards, without revision (58 FR 13008, March 9, 1993). 

12 In the 1992 notice of its proposed decision in that review, the EPA announced its 

13 intention to proceed as rapidly as possible with the next review of the air quality criteria and 

14 standards for O3 and other photochemical oxidants in light of emerging evidence of health effects 

15 related to 6- to 8-hour O3 exposures (57 FR 35542, August 10, 1992). The EPA subsequently

16 published the AQCD and Staff Paper for that next review (U.S. EPA, 1996a, b). In December 

17 1996, the EPA proposed revisions to both the primary and secondary standards (61 FR 65716, 

18 December 13, 1996). With regard to the primary standard, the EPA proposed to replace the then19 existing 1-hour primary standard with an 8-hour standard set at a level of 0.08 ppm (equivalent 

20 to 0.084 ppm based on the proposed data handling convention) as a 3-year average of the annual 

21 third-highest daily maximum 8-hour concentration. The EPA proposed to revise the secondary 

22 standard either by setting it identical to the proposed new primary standard or by setting it as a 

23 new seasonal standard using a cumulative form. The EPA completed this review in 1997 by 

24 setting the primary standard at a level of 0.08 ppm, based on the annual fourth-highest daily 

25 maximum 8-hour average concentration, averaged over three years, and setting the secondary 

26 standard identical to the revised primary standard (62 FR 38856, July 18, 1997). 

27 On May 14, 1999, in response to challenges by industry and others to the EPA’s 1997 

28 decision, the D.C. Circuit remanded the O3 NAAQS to the EPA, finding that section 109 of the 

29 CAA, as interpreted by the EPA, effected an unconstitutional delegation of legislative authority 

30 (American Trucking Assoc. v. EPA, 175 F.3d 1027, 1034-1040 [D.C. Cir. 1999]). In addition, the 

31 court directed that, in responding to the remand, the EPA should consider the potential beneficial 

32 health effects of O3 pollution in shielding the public from the effects of solar ultraviolet (UV) 

33 radiation, as well as adverse health effects (id. at 1051-53). In 1999, the EPA petitioned for 

34 rehearing en banc on several issues related to that decision. The court granted the request for 

35 rehearing in part and denied it in part, but declined to review its ruling with regard to the 

36 potential beneficial effects of O3 pollution (American Trucking Assoc. v. EPA,195 F.3d 4, 10 
October 2018 2-3 Draft – Do Not Quote or Cite

1 [D.C Cir., 1999]). On January 27, 2000, the EPA petitioned the U.S. Supreme Court for 

2 certiorari on the constitutional issue (and two other issues), but did not request review of the 

3 ruling regarding the potential beneficial health effects of O3. On February 27, 2001, the U.S. 

4 Supreme Court unanimously reversed the judgment of the D.C. Circuit on the constitutional 

5 issue. Whitman v. American Trucking Assoc., 531 U. S. 457, 472-74 (2001) (holding that section 

6 109 of the CAA does not delegate legislative power to the EPA in contravention of the 

7 Constitution). The Court remanded the case to the D.C. Circuit to consider challenges to the O3

8 NAAQS that had not been addressed by that court’s earlier decisions. On March 26, 2002, the 

9 D.C. Circuit issued its final decision on remand, finding the 1997 O3 NAAQS to be “neither 

10 arbitrary nor capricious,” and so denying the remaining petitions for review. See American 

11 Trucking Associations, Inc. v. EPA, 283 F.3d 355, 379 (D.C Cir. 2002, hereafter referred to as 

12 “ATA III”). 

13 Specifically, in ATA III, the D.C. Circuit upheld the EPA’s decision on the 1997 O3

14 standard as the product of reasoned decision making. With regard to the primary standard, the 

15 court made clear that the most important support for the EPA’s decision to revise the standard 

16 was the health evidence of insufficient protection afforded by the then-existing standard (“the 

17 record is replete with references to studies demonstrating the inadequacies of the old one-hour 

18 standard”), as well as extensive information supporting the change to an 8-hour averaging time 

19 (id. at 378). The court further upheld the EPA’s decision not to select a more stringent level for 

20 the primary standard noting “the absence of any human clinical studies at ozone concentrations 

21 below 0.08 [ppm]” which supported the EPA’s conclusion that “the most serious health effects 

22 of ozone are ‘less certain’ at low concentrations, providing an eminently rational reason to set the 

23 primary standard at a somewhat higher level, at least until additional studies become available” 

24 (id. at 378, internal citations omitted). The court also pointed to the significant weight that the 

25 EPA properly placed on the advice it received from the CASAC (id. at 379). In addition, the 

26 court noted that “although relative proximity to peak background O3 concentrations did not, in 

27 itself, necessitate a level of 0.08 [ppm], the EPA could consider that factor when choosing 

28 among the three alternative levels” (id. at 379). 

29 Coincident with the continued litigation of the other issues, the EPA responded to the 

30 court’s 1999 remand to consider the potential beneficial health effects of O3 pollution in 

31 shielding the public from effects of UV radiation (66 FR 57268, Nov. 14, 2001; 68 FR 614, 

32 January 6, 2003). The EPA provisionally determined that the information linking changes in 

33 patterns of ground-level O3 concentrations to changes in relevant patterns of exposures to UV 

34 radiation of concern to public health was too uncertain, at that time, to warrant any relaxation in 

35 1997 O3 NAAQS. The EPA also expressed the view that any plausible changes in UV-B 

36 radiation exposures from changes in patterns of ground-level O3 concentrations would likely be 
October 2018 2-4 Draft – Do Not Quote or Cite

1 very small from a public health perspective. In view of these findings, the EPA proposed to leave 

2 the 1997 primary standard unchanged (66 FR 57268, Nov. 14, 2001). After considering public 

3 comment on the proposed decision, the EPA published its final response to this remand in 2003, 

4 re-affirming the 8-hour primary standard set in 1997 (68 FR 614, January 6, 2003). 

5 The EPA initiated the fourth periodic review of the air quality criteria and standards for 

6 O3 and other photochemical oxidants with a call for information in September 2000 (65 FR 

7 57810, September 26, 2000). The schedule for completion of that review was ultimately 

8 governed by a consent decree resolving a lawsuit filed in March 2003 by plaintiffs representing 

9 national environmental and public health organizations, who maintained that the EPA was in 

10 breach of a nondiscretionary duty to complete review of the O3 NAAQS within a statutorily 

11 mandated deadline. In 2007, the EPA proposed to revise the level of the primary standard within 

12 a range of 0.075 to 0.070 ppm (72 FR 37818, July 11, 2007). The EPA proposed to revise the 

13 secondary standard either by setting it identical to the proposed new primary standard or by 

14 setting it as a new seasonal standard using a cumulative form. Documents supporting these 

15 proposed decisions included the 2006 AQCD (U.S. EPA, 2006a) and 2007 Staff Paper (U.S 

16 EPA, 2007) and related technical support documents. The EPA completed the review in March 

17 2008 by revising the levels of both the primary and secondary standards from 0.08 ppm to 0.075 

18 ppm while retaining the other elements of the prior standards (73 FR 16436, March 27, 2008). 

19 In May 2008, state, public health, environmental, and industry petitioners filed suit 

20 challenging the EPA’s final decision on the 2008 O3 standards. On September 16, 2009, the EPA 

announced its intention to reconsider the 2008 O3 standards,12 21 and initiated a rulemaking to do 

22 so. At the EPA’s request, the court held the consolidated cases in abeyance pending the EPA’s 

23 reconsideration of the 2008 decision. 

24 In January 2010, the EPA issued a notice of proposed rulemaking to reconsider the 2008 

25 final decision (75 FR 2938, January 19, 2010). In that notice, the EPA proposed that further 

26 revisions of the primary and secondary standards were necessary to provide a requisite level of 

27 protection to public health and welfare. The EPA proposed to revise the level of the primary 

28 standard from 0.075 ppm to a level within the range of 0.060 to 0.070 ppm, and to revise the 

29 secondary standard to one with a cumulative, seasonal form. At the EPA’s request, the CASAC 

30 reviewed the proposed rule at a public teleconference on January 25, 2010 and provided 

31 additional advice in early 2011 (Samet, 2010, 2011). After considering comments from the 

32 CASAC and the public, the EPA prepared a draft final rule, which was submitted for interagency 

33 review pursuant to Executive Order 12866. On September 2, 2011, consistent with the direction 

 

12 The press release of this announcement is available at: 

https://archive.epa.gov/epapages/newsroom_archive/newsreleases/85f90b7711acb0c88525763300617d0d.html. 
October 2018 2-5 Draft – Do Not Quote or Cite

1 of the President, the Administrator of the Office of Information and Regulatory Affairs, Office of 

2 Management and Budget (OMB), returned the draft final rule to the EPA for further 

3 consideration. In view of this return and the fact that the Agency’s next periodic review of the O3 

4 NAAQS required under CAA section 109 had already begun (as announced on September 29, 

5 2008), the EPA decided to consolidate the reconsideration with its statutorily required periodic 

review.13 6 

7 In light of the EPA’s decision to consolidate the reconsideration with the current review, 

8 the D.C. Circuit proceeded with the litigation on the 2008 final decision. On July 23, 2013, the 

9 court upheld the EPA’s 2008 primary O3 standard, but remanded the 2008 secondary standard to 

10 the EPA (Mississippi v. EPA, 744 F. 3d 1334 [D.C. Cir. 2013]). With respect to the primary

11 standard, the court first rejected arguments that the EPA should not have lowered the level of the 

12 existing primary standard, holding that the EPA reasonably determined that the existing primary 

13 standard was not requisite to protect public health with an adequate margin of safety, and 

14 consequently required revision. The court went on to reject arguments that the EPA should have 

15 adopted a more stringent primary standard. With respect to the secondary standard, the court held 

16 that the EPA’s explanation for the setting of the secondary standard identical to the revised 8-

17 hour primary standard was inadequate under the CAA because the EPA had not adequately 

18 explained how that standard provided the required public welfare protection. 

19 At the time of the court’s decision, the EPA had already completed significant portions of 

20 its next statutorily required periodic review of the O3 NAAQS. This review had been formally 

21 initiated in 2008 with a call for information in the Federal Register (73 FR 56581, Sept. 29, 

22 2008). In late 2014, based on the Integrated Science Assessment (ISA), Risk and Exposure 

Assessments (REAs) for health and welfare, and PA14 23 developed for this review, the EPA 

24 proposed to revise the 2008 primary and secondary standards by reducing the level of both 

25 standards to within the range of 0.07 to 0.065 ppm (79 FR 75234, December 17, 2014). Public 

26 comments were received on the proposal during the subsequent public comment period, which 

included three public hearings.15 27 

28 The EPA’s final decision in this review was published in October 2015, establishing the 

29 now-current standards (80 FR 65292, October 26, 2015).In this decision, based on consideration 

30 of the health effects evidence on respiratory effects of O3 in at-risk populations, the EPA revised 

 

13 This rulemaking, completed in 2015, concluded the reconsideration process. 

14 The final versions of these documents, released in August 2014, were developed with consideration of the 

comments and recommendations from the CASAC, as well as comments from the public on the draft documents 

(U.S. EPA 2014a; U.S. EPA, 2014b; U.S. EPA, 2014c; Frey, 2014a; Frey, 2014b; Frey, 2014c). 

15 All significant comments on the proposed decision, along with the EPA’s responses, were documented in the 

notice of the final decision or in the separate Response to Comments document (U.S. EPA, 2015a). 
October 2018 2-6 Draft – Do Not Quote or Cite

1 the primary standard from a level of 0.075 ppm to a level of 0.070 ppm, while retaining all the 

2 other elements of the standard (80 FR 65292, October 26, 2015). The EPA’s decision on the 

3 level for the standard was based on the weight of the scientific evidence and quantitative 

4 exposure/risk information. The level of the secondary standard was also revised from 0.075 ppm 

5 to 0.070 ppm based on the scientific evidence of O3 effects on welfare, particularly the evidence 

of O3 impacts on vegetation, and quantitative analyses available in the review.16 6 The other 

7 elements of the standard were retained. This decision on the secondary standard also 

8 incorporated the EPA’s response to the D.C. Circuit’s remand of the 2008 secondary standard in 

9 Mississippi v. EPA, 744 F.3d 1344 (D.C. Cir. 2013). The 2015 revisions to the NAAQS were 

10 accompanied by revisions to the data handling procedures, and the ambient air monitoring 

requirements17 (80 FR 65292, October 26, 2015).18 11 

12 After publication of the final rule, a number of industry groups, environmental and health 

13 organizations, and certain states filed petitions for judicial review in the D.C. Circuit. The 

14 industry and state petitioners filed briefs arguing that the revised standards are too stringent, 

15 while the environmental and health petitioners’ brief argued that the revised standards are not 

16 stringent enough to protect public health and welfare as the Act requires. A number of industry 

17 groups, states, and environmental and health groups have also intervened in these challenges, 

18 with some supporting the revised standards and others opposing them. Oral arguments in this 

19 litigation are scheduled for December 18, 2018 (Murray Energy v. EPA, No. 15-1385, Order, 

20 Doc. No. 1752391 [D.C. Cir. Sept. 24, 2018]). 

21 2.2 AMBIENT AIR MONITORING AND DATA HANDLING FOR THE 

22 CURRENT STANDARDS 

23 2.2.1 Monitoring Requirements and the Current Monitoring Network 

24 State and local environmental agencies operate O3 monitors at state or local air 

25 monitoring stations (SLAMS) as part of the SLAMS network. The requirements for the SLAMS 

network depend on the population and most recent O3 design values19 26 in the area. The minimum 

27 number of O3 monitors required in a metropolitan statistical area (MSA) ranges from zero for 

 

16 The current NAAQS for O3 are specified at 40 CFR 50.19. 

17 The current federal regulatory measurement methods for O3 are specified in 40 CFR 50, Appendix D and 40 CFR 

part 53. Consideration of ambient air measurements with regard to judging attainment of the standards is 

specified in 40 CFR 50, Appendix U. The O3 monitoring network requirements are specified in 40 CFR 58. 

18 This decision additionally announced revisions to the exceptional events scheduling provisions, as well as changes 

to the air quality index and the regulations for the prevention of significant deterioration permitting program. 

19 A design value is a statistic that describes the air quality status of a given area relative to the level of the standard, 

taking the averaging time and form into account. Design values are typically used to classify nonattainment areas, 

assess progress towards meeting the NAAQS, and develop control strategies. 
October 2018 2-7 Draft – Do Not Quote or Cite

1 areas with a population less than 350,000 and no recent history of an O3 design value greater 

2 than 85 percent of the level of the standard, to four for areas with a population greater than 10 

million and an O3 design value greater than 85 percent of the standard level.20 3 Within an O3

4 monitoring network, at least one site for each MSA must be designed to record the maximum 

5 concentration for that particular metropolitan area. Since the highest O3 concentrations tend to be 

6 associated with a particular season for various locations, the EPA requires O3 monitoring during 

7 specific O3 monitoring seasons which vary by state from five months (May to September in 

Oregon and Washington) to all twelve months (in a number of states).21 8 

9 Most of the state, local, and tribal air monitoring stations that report data to the EPA use 

10 ultraviolet Federal Equivalent Methods. The Federal Reference Method (FRM) was revised in 

11 2015 to include a new chemiluminescence by nitric oxide (NO-CL) method. The previous 

12 ethylene (ET-CL) method is no longer used due to lack of availability and safety concerns with 

ethylene.22 13 The NO-CL method is beginning to be implemented in the SLAMS network. 

14 In 2017, there were over 1,300 federal, state, local, and tribal ambient air monitors 

15 reporting O3 concentrations to the EPA. Figure 2-1 shows the locations of the monitoring sites 

16 reporting data to the EPA at any time during the 2015-2017 period. The blue-green dots which 

17 make up about 80% of the O3 monitoring network are SLAMS monitors, which are operated by 

18 state and local governments to meet regulatory requirements and provide air quality information 

19 to public health agencies. Thus, the SLAMS monitoring sites are largely focused on urban and 

20 suburban areas. 

21 The magenta dots highlight two important subsets of monitoring sites within the SLAMS 

22 network: the “National Core” (NCore) multi-pollutant monitoring network and the 

23 “Photochemical Assessment Monitoring Stations” (PAMS) network. Each state is required to 

24 have at least one NCore station, and O3 monitors at NCore sites are required to operate year25 round. At each NCore site located in a CBSA with a population of 1 million or more (based on 

the most recent census), a PAMS network site is required.23 26 At each PAMS monitor ambient air 

 

20 The SLAMS minimum monitoring requirements to meet the O3 design criteria are specified in 40 CFR Part 58, 

Appendix D. The minimum O3 monitoring network requirements for urban areas are listed in Table D-2 of 

Appendix D to 40 CFR Part 58 (accessible at https://www.ecfr.gov). 

21 The required O3 monitoring seasons for each state are listed in Table D-3 of Appendix D to 40 CFR Part 58. 

22 The current FRM for O3 (established in 2015) is a chemiluminescence method. This is an automated method 

allowing for the measurement of O3 concentrations in ambient air using continuous (real-time) sampling and 

analysis. This method is based on continuous automated measurement of the intensity of the characteristic 

chemiluminescence released by the gas phase reaction of O3 in sampled air with either ethylene or nitric oxide 

gas. This method is fully described in Appendix D to 40 CFR Part 50. 

23 The requirements for PAMS, which were most recently updated in 2015, is fully described in Appendix D to 40 

CFR Part 58. 
October 2018 2-8 Draft – Do Not Quote or Cite

1 concentration measurements of O3 and O3 precursors are collected, as well as data representing 

2 local meteorological conditions. At a minimum, PAMS monitoring is required during the months 

3 of June, July and August. 

4 While the SLAMS network has a largely urban and population-based focus, there are 

5 monitoring sites in other networks that can be used to track compliance with the NAAQS in rural 

6 areas. For example, the dark yellow dots in Figure 2-1 represent the Clean Air Status and Trends 

7 Network (CASTNET) monitors which are located in rural areas. There were about 80 CASTNET 

8 sites operating in 2017, with most of the sites in the eastern U.S. being operated by the EPA, and 

9 most of the sites in the western U.S. being operated by the National Park Service (NPS). Finally, 

10 the black dots represent “Special Purpose Monitoring Stations” (SPMs), which are not required 

11 but often operate for short periods of time (less than 3 years) to collect data for human health and 

12 welfare studies, as well as other types of monitoring sites, including monitors operated by tribes 

and industrial sources. The SPMs are typically not used to assess compliance with the NAAQS.24 13 

14 

15 Figure 2-1. Map of U.S. ambient air O3 monitoring sites reporting data to the EPA during 

16 the 2015-2017 period. 

 

24 However, SPMs that use federal reference or equivalent methods, meet all applicable requirements in 40 CFR Part 

58, and operate continuously for at least 3 years may be used to assess compliance with the NAAQS. 
October 2018 2-9 Draft – Do Not Quote or Cite

1 2.2.2 Data Analysis for Comparison to the Standards 

2 To assess whether a monitoring site or geographic area meets or exceeds a NAAQS, the 

3 monitoring data are analyzed consistent with the established regulatory requirements for the 

4 handling of monitoring data for the purposes of deriving a design value. A design value 

5 expresses ambient air concentrations in terms of the averaging time and form for a given 

6 standard such that its comparison to the level of the standard indicates whether the location 

7 meets or exceeds the standard. Consistent with the form and averaging time of the O3 standards, 

8 O3 design values are calculated as the 3-year average of the annual fourth highest daily 

9 maximum 8-hour average O3 concentration. 

10 Hourly average O3 concentrations at the monitoring sites used for assessing compliance 

11 with the NAAQS are required to be reported in ppm to the third decimal place, with additional 

12 digits truncated, consistent with the typical measurement precision associated with most O3

13 monitoring instruments. The hourly average concentrations are used to compute moving 8-hour 

14 average concentrations for each day, with the daily maximum 8-hour average identified as the 

highest of the 17 consecutive, valid25 15 8-hour averages that begin with the 8-hour period from 

7am to 3pm and end with the 8-hour period from 11pm to 7am the subsequent day.26 16 An O3

17 monitoring site meets the standard if its design value is less than or equal to the level of the 

18 standard. A geographic area meets the NAAQS if all ambient air monitoring sites in the area 

have valid27 19 design values meeting the standard, and if one or more monitors has a design value 

20 exceeding the standard, then the area exceeds the NAAQS. 

21 2.3 OVERVIEW OF OZONE AIR QUALITY 

22 Ozone is a gas composed of three oxygen atoms (O3). It is naturally present in the Earth’s 

23 atmosphere, both in the stratospheric layer occurring roughly 10 to 30 miles above the Earth’s 

24 surface as well as in the closer tropospheric layer. The stratosphere contains a large reservoir of 

25 O3 (i.e. the “ozone layer”) that results naturally from photochemical reactions between ultraviolet 

 

25 An 8-hour average is considered valid if at least six of the hourly concentrations are available or if substitution of 

zero for the missing hourly concentrations yields an 8-hour average above the level of the standard. The 8-hour 

averages are required to be reported to three decimal places with additional digits to right of third decimal place 

truncated (Appendix U to 40 CFR Part 50). 

26 A daily maximum concentration is considered valid if at least 13 of the 17 consecutive 8-hour averages are 

available or if the daily maximum based on fewer than 13 is greater than the level of the standard (Appendix U to 

40 CFR Part 50). 

27 An O3 design value less than or equal to the level of the standard is valid if daily maximum values are available 

for at least 90% of the days in the O3 monitoring season on average over the 3 years, with a minimum of 75% 

data completeness in any individual year (Appendix U to 40 CFR Part 50). A design value greater than the level 

of the standard is always valid. 
October 2018 2-10 Draft – Do Not Quote or Cite

light (UV) and molecular oxygen (O2).28 1 Under specific meteorological conditions, this reservoir 

2 can contribute to O3 concentrations at the Earth’s surface (Langford et al., 2017). Ozone is also 

3 produced near the earth’s surface due to chemical interactions involving solar radiation and 

4 pollution resulting from human activity. These chemical reactions involve specific O3 precursors, 

5 such as nitrogen oxides (NOX), volatile organic compounds (VOC), and carbon monoxide (CO), 

which can be emitted from both natural and anthropogenic sources.29 6 

7 Global air quality models have estimated that natural sources of O3 precursors, such as 

8 vegetation, lightning, and wildfires, can produce daily 8-hour peak O3 concentrations of 15-35 

9 parts per billion by volume (ppb) across the U.S. during the warm season (2014 PA, section 

10 2.4.1). Human activity from combustion of fossil fuels or biomass and the use of industrial and 

11 consumer chemicals can also lead to emissions of these O3 precursors, which can then yield O3

12 concentrations substantially above naturally occurring levels. The EPA conducted air quality 

13 modeling analyses in the last review to assess the role of natural sources (i.e., natural 

14 background) and the combined impacts of natural background plus anthropogenic sources 

outside of the U.S. (i.e., U.S. background) on O3 concentrations (2014 PA, section 2.4)30 15 . These 

16 2007-based annual modeling analyses (presented in the 2014 PA) estimated that seasonal mean 

17 natural background levels ranged from 15 to 35 ppb over the U.S. This modeling also estimated 

18 that seasonal mean daily maximum 8-hour average concentrations of U.S. background O3ranged 

19 from 25 to 50 ppb. While the majority of modeled events greater than 70 ppb were primarily 

20 driven by local and regional O3 precursor emissions, there were some events with substantial 

21 U.S. background contributions where O3 concentrations approached or exceeded 75 ppb (80 FR 

65300, October 26, 2015). 31 22 

23 As part of the current review, the EPA plans to utilize state-of-the-science air quality 

24 modeling. for a more recent time period, 2016, to provide updated estimates of the relative 

25 contributions of natural and anthropogenic sources of O3 in the U.S. Specifically, the EPA 

26 intends to use the Community Multiscale Air Quality (CMAQ) modeling system (Appel et al., 

27 2017) over a Northern Hemisphere domain to provide boundary conditions for a finer-scale 

 

28 This layer of O3 in the upper atmosphere helps to protect the earth’s populations and ecosystems from the 

damaging effects of UV radiation (Norval et al., 2011; Bais et al., 2017). 

29 Methane (CH4) emissions can also contribute to O3 formation, but its impacts are more frequently observed at the

global scale over longer time periods (e.g., decadal scale). 

30 The difference between natural and U.S. background is that U.S. background also includes, along with 

contributions from natural sources, the impacts from anthropogenic emissions outside the U.S. 

31 Noting the infrequency of such events, and of the EPA policies that allow for the exclusion of air quality 

monitoring data from design value calculations when they are substantially affected by certain background 

influences, the EPA explained in the 2015 decision that background concentrations of O3 were not expected to 

preclude attainment of a revised O3 standard with a level of 70 ppb (80 FR 65328, October 26, 2015). 
October 2018 2-11 Draft – Do Not Quote or Cite

1 national application of CMAQ to estimate current levels of background ozone using recently 

available emissions estimates and meteorological data.32 2 Using this model configuration, the 

3 EPA plans to conduct, evaluate, and summarize the results of a series of “zero-out” sensitivity 

runs33 designed to isolate natural background and U.S. background34 4 . 

5 Based on estimates compiled in version 2 of the 2014 National Emissions Inventory 

(NEI) (U.S. EPA, 2018a), biogenic and fire emissions comprise 78 percent35 6 of the total VOC 

7 emissions in the U.S., but only 9% of the NOX emissions. Tables 2-1 and 2-2 show the top 

8 anthropogenic source groups that emit NOX and VOC, respectively, within the U.S., based on the 

9 2014 NEI. Mobile sources, such as on-road vehicles and non-road equipment, are the largest 

10 contributors to both NOX and VOC. Anthropogenic emissions of NOX and VOC have been 

11 trending downward over the U.S., as shown in Figure 2-2. Emissions of NOX decreased by more 

12 than 40% and VOC emissions by more than 15% since 2002. 

13 Table 2-1. Percentage of total emissions in top five source groups for NOX.

 

NOx 

(% of total U.S. anthropogenic emissions) 

Mobile Sources 61.5 

External Combustion Boilers 15.8 

Industrial Processes 10.6 

Stationary Source Fuel Combustion 5.8 

Internal Combustion Engines 5.3 

Source: EPA National Emissions Inventory 2014 version 2 

14 

15 

 

32 The modeling analyses conducted in the review completed in 2015 used boundary conditions from the global 

GEOS-Chem model (Henderson et al., 2014) as inputs into regional models (e.g., CMAQ) to estimate background 

levels (2014 PA, section 2.4). 

33 Zero-out sensitivity modeling refers to a commonly used method for isolating the O3 impacts of specific emissions 

source categories or sources from specific regions. To accomplish this, O3 concentrations are estimated from 

model simulations in which emissions of interest are set to zero. As an example, natural background could be 

estimated from a simulation in which all anthropogenic emissions are zeroed out in the simulation. 

34 These analyses can be used to facilitate CASAC advice on CAA Section 109(d)(2)(c)(iii) (e.g., as discussed Pruitt 

[2018]). 

35 In locations near large concentrations of anthropogenic VOC sources (e.g., in certain urban areas or oil and gas 

development basins), the relative contribution of anthropogenic sources can be much higher than the national 

average. 
October 2018 2-12 Draft – Do Not Quote or Cite

1 Table 2-2. Percentage of total emissions in top five source groups for VOC. 

 

VOC 

(% of total U.S. anthropogenic emissions) 

Mobile Sources 33.4 

Industrial Processes 29.3 

Solvent Utilization 23.2 

Storage and Transport 4.5 

Stationary Source Fuel Combustion 3.1 

Source: EPA National Emissions Inventory 2014 version 2 

2 

3 

4 Figure 2-2. Trends in anthropogenic emissions of NOX and VOC (2002-2014). [Source: 

5 EPA National Emissions Inventory 2014 version 2]

6 The chemistry that leads to O3 formation is complex and can vary depending upon the 

7 relative proportions of different types of precursor pollutants as well as external conditions such 

8 as temperature and sunlight. Over most areas of the U.S., daytime O3 production typically 

9 increases as NOX concentrations increase (2013 ISA, section 3.2.4). Formation of O3 in this 

10 regime is described as “NOX-limited.” At other times and locations, where NOX concentrations 

11 are higher, O3 formation may be only weakly dependent on NOx emissions, or even inversely 
October 2018 2-13 Draft – Do Not Quote or Cite

correlated (i.e., NOX emissions actually deplete O3 locally36 1 ). O3 formation in these regimes 

2 increases as VOC concentrations increase and is described as “VOC-limited.” Once formed, O3

3 near the Earth’s surface can be transported by the prevailing winds before eventually being 

4 removed from the atmosphere over the course of hours to weeks via chemical reactions or 

5 deposition to surfaces. 

6 As described in section 2.2.1, to assess O3 concentrations across the U.S., state and local 

7 environmental agencies operate O3 monitors at various locations and subsequently submit the 

8 data to the EPA for analyses and storage. As shown in Figure 2-3, several locations across the 

9 U.S. have design values that exceeded the O3 standards in 2015-2017. California contains 

10 numerous monitoring sites where design values exceeded 70 ppb in 2015-2017, but high O3 was 

11 also measured in Texas, the Northeast Corridor, along the Lake Michigan shoreline, and certain 

12 urban areas in the western U.S. These locations include some of the most densely populated 

13 areas in the country that also experience conducive meteorology for O3 formation. The highest 

14 daily peak 8-hour average O3 concentrations most commonly occur during the afternoon within

15 the warmer months due to higher solar radiation and other conducive meteorological conditions 

16 during these times. However, there can be exceptions such as the Uintah Basin in Utah where the 

17 highest O3 concentrations occur during the winter on sunny days with strong temperature 

18 inversions and ample snow cover. 

 

36 In these cases, NOx generally results in eventual net ozone production downwind of the emissions sources over 

longer time scales. 
October 2018 2-14 Draft – Do Not Quote or Cite

1 

2 Figure 2-3. 2015-2017 O3 design values across the U.S. Red and orange circles indicate 

3 locations exceeding the standard. Design value data available at: 

4 https://www.epa.gov/air-trends/air-quality-design-values. 

5 Concentrations of O3 in the U.S. have trended downward over the past several decades 

6 due to reductions in precursor emissions noted above. The average downward trend in annual 

7 fourth highest 8-hour daily maximum O3 concentration has been 17% between 2000 and 2017 

8 (U.S. EPA, 2018c), as shown in Figure 2-4 (based on 809 monitoring sites that operated for the 

9 full 18-year period). Downward trends in this metric have been even more substantial in the 

10 Eastern U.S. and in California. Air quality model simulations estimate that O3 air quality will 

11 continue to improve over the next decade as additional reductions in O3 precursors from mobile 

12 sources, industrial processes, and other sources are realized as a result of “on-the-books” EPA 

13 regulations (U.S. EPA, 2015b) and other technological changes. In addition to being affected by 

14 changing emissions, future O3 concentrations may also be affected by climate change (Nolte et 

15 al., 2018) as well as any changes in the amount of O3 transported into the U.S. from other 

16 countries. 

17 
October 2018 2-15 Draft – Do Not Quote or Cite

1 

Figure 2-4. Trends in annual 4th 2 highest daily maximum 8-hour average O3 at all sites 

3 across the U.S. with complete data (2000-2017). The dotted line indicates the 

4 level of the current standard (0.070 ppm).

5 
October 2018 3-1 Draft – Do Not Quote or Cite

1 3 KEY POLICY-RELEVANT ISSUES FOR THE 

2 CURRENT REVIEW 

3 The overarching question in each NAAQS review is: 

4  Do the currently available scientific evidence and exposure/risk-based information 

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

6 standard(s)? 

7 As appropriate, a review also addresses a second overarching question: 

8  What alternative standards, if any, are supported by the currently available 

9 scientific evidence and exposure/risk-based information and are appropriate for 

10 consideration? 

11 In considering these overarching questions, a series of key policy-relevant issues 

12 particular to a given review are addressed. 

13 The policy-relevant issues thus far identified for this review of the O3 standards are 

14 presented in sections 3.1.1 and 3.2.1 below as series of questions intended to frame our approach 

15 to considering the information available in this review of the current primary and secondary 

standards for O3. The ISA and PA developed in this new review37 16 will provide the basis for 

17 addressing these questions and will inform the Administrator’s judgment as to whether the 

18 current primary and secondary standards for O3 provide the requisite protection of public health 

19 and public welfare, and his decisions as to whether to retain or revise these standards. These 

20 assessments focus on policy-relevant scientific information and analyses that address key 

questions related to the adequacy of the O3 standards.38 21 In this chapter, the primary standard is 

22 discussed in section 3.1 and the secondary standard in section 3.2. 

 

37 As summarized in sections 1.2 and 1.3 above, stand-alone REA documents will not be developed for this review. 

Rather, any exposure and risk analyses performed for this review will be presented in the PA along with any such 

information from the last review that remains informative in this review, taking into account the newly available 

evidence presented in the ISA and any other technical documents prepared for the review. 

38 Several examples of policy-relevant analyses in NAAQS reviews, generally, are noted in Pruitt (2018): “EPA’s 

Integrated Science Assessments (ISA), Risk and Exposure Assessments (REA), and Policy Assessments (PA) 

should focus on policy-relevant science and on studies, causal determinations, or analyses that address key 

questions related to the adequacy of primary and secondary NAAQS, including levels near – both above and 

below—the current standard(s). Policy-relevant science may also include information that directly relates to the 

indicator, averaging time, form and level of a NAAQS as well as alternative policy approaches.”; “In developing 

additional analyses in the REA or elsewhere, EPA should focus on policy-relevant including consideration of 

issues such as thresholds or background levels, as appropriate for context.” 
October 2018 3-2 Draft – Do Not Quote or Cite

1 3.1 THE PRIMARY STANDARD 

2 The approach planned for this review of the primary standard is most fundamentally 

3 based on using the Agency’s assessment of the current scientific evidence, quantitative 

4 assessments of exposures and/or risks, and other associated analyses (e.g., air quality analyses) to 

5 inform the Administrator’s judgments regarding a primary standard that is requisite to protect 

6 public health with an adequate margin of safety. This approach involves translating scientific and 

7 technical information into the basis for addressing a series of key policy-relevant questions using 

8 both evidence- and exposure-/risk-based considerations. This series of key questions related to 

9 the primary standard is presented in section 3.1.1, along with a summary of the general approach 

10 for the review. Additionally, to provide context for this review of the current primary O3

11 standard, section 3.1.2 summarizes key aspects of the decisions made in the last review, 

12 including the Agency’s consideration of important policy judgments concerning the scientific 

13 evidence and exposure/risk information, and associated uncertainties and limitations, as well as 

14 the Administrator’s public health policy judgments regarding an adequate margin of safety. 

15 3.1.1 Key Issues Related to the Primary Standard 

16 The approach planned for this review of the primary O3 standard will build on the 

17 substantial body of work developed during the course of the last review, taking into account the 

18 more recent scientific information and air quality data now available to inform our understanding 

19 of the key-policy relevant issues in this review. The ISA, risk and exposure analyses (as 

20 warranted), and PA developed in this review will provide the basis for addressing the key policy21 relevant questions in the review and these documents will inform the Administrator’s decisions 

22 as to whether to retain or revise the primary O3 standard. As summarized in section 1.2, and also 

23 described in chapter 6, evaluations in the PA are intended to inform the Administrator’s public 

24 health policy judgments and decisions. In so doing, the PA considers the potential implications 

25 of various aspects of the scientific evidence, the exposure/risk-based information, and the 

26 associated uncertainties and limitations. 

27 In building upon the conclusions from the last review, the current review takes into 

28 account the updated evidence and information that has become available since that review. The 

29 Agency’s consideration of the full set of evidence and information available in this review will 

30 inform the answer to the following initial overarching question for the review: 

31  Do the currently available scientific evidence and exposure-/risk-based information 

32 support or call into question the adequacy of the public health protection afforded by 

33 the current primary O3 standard? 

34 In reflecting on this question, we will consider the available body of scientific evidence, 

35 assessed in the ISA, and used as a basis for developing or interpreting risk/exposure analyses, 
October 2018 3-3 Draft – Do Not Quote or Cite

1 including whether it supports or calls into question the scientific conclusions reached in the last 

2 review regarding health effects related to exposure to ambient air-related O3. Information 

3 available in this review that may be informative to public health judgments regarding 

4 significance or adversity of key effects will also be considered. Additionally, the currently 

5 available exposure and risk information, whether newly developed in this review or 

6 predominantly developed in the past and interpreted in light of current information, will be 

7 considered, including with regard to the extent to which it may continue to support judgments 

8 made in the last review. Further, in considering this question with regard to the primary O3

9 standard, as in all NAAQS reviews, we give particular attention to exposures and health risks to 

at-risk populations.39 10 

11 Evaluation of the available scientific evidence and risk/exposure information with regard 

12 to this consideration of the current standard will focus on key policy-relevant issues by 

13 addressing a series of questions including the following: 

14  Is there newly available evidence that indicates the importance of photochemical oxidants 

15 other than O3 with regard to abundance in ambient air, and potential for human exposures 

16 and health effects? 

17  Does the currently available scientific evidence alter our conclusions from the last review 

18 regarding the nature of health effects attributable to human exposure to O3 from ambient air? 

19 Is there new evidence on health effects beyond respiratory effects that suggest additional 

20 endpoints should be given increased focus in this review? Are previously identified 

21 uncertainties in the health effects evidence reduced or do important uncertainties remain? 

22  Does the current evidence alter our understanding of populations that are particularly at risk 

23 from O3 exposures? Is there new evidence that suggests additional at-risk populations should 

24 be given increased focus in this review? And what are important uncertainties in that 

25 evidence? 

26  Does the current evidence alter our conclusions from the previous review regarding the 

27 exposure duration and concentrations associated with health effects? To what extent does the 

28 currently available scientific evidence indicate health effects attributable to exposures to O3

29 concentrations lower than previously reported and what are important uncertainties in that 

30 evidence? 

31  To what extent have important uncertainties identified in the last review been reduced and/or 

32 have new uncertainties emerged? 

 

39 As used here and similarly throughout this document, the term population refers to persons having a quality or 

characteristic in common, such as a specific pre-existing illness or a specific age or life stage. Identifying at-risk 

populations involves consideration of susceptibility and vulnerability. Susceptibility refers to innate (e.g., genetic 

or developmental aspects) or acquired (e.g., disease or smoking status) sensitivity that increases the risk of health 

effects occurring with exposure to O3. Vulnerability refers to an increased risk of O3-related health effects due to 

factors such as those related to socioeconomic status, reduced access to health care or exposure. 
October 2018 3-4 Draft – Do Not Quote or Cite

1  What are the nature and magnitude of O3 exposures and associated health risks associated 

2 with air quality conditions just meeting the current standard? 

3  To what extent are the estimates of exposures and risks to at-risk populations associated with 

4 air quality conditions just meeting the current standard reasonably judged important from a 

5 public health perspective? 

6  What are the important uncertainties associated with any risk/exposure estimates? 

7 If the information available in this review suggests that revision of the current primary 

8 standard would be appropriate to consider, the PA will evaluate how the standard might be 

9 revised based on the available scientific information, air quality assessments, and exposure/risk 

10 information, and also considering what the available information indicates as to the health 

11 protection expected to be afforded by the current or potential alternative standards. Such an 

12 evaluation may consider the effect of revision of one or more elements of the standard (indicator, 

13 averaging time, level and form), with the effect being evaluated based on the resulting potential 

14 standard and all of its elements collectively. Based on such evaluations, the PA would then 

15 identify potential alternative standards (specified in terms of indicator, averaging time, level, and 

16 form) intended to reflect a range of alternative policy judgments as to the degree of protection 

17 that is requisite to protect public health with an adequate margin of safety, and options for 

18 standards expected to achieve it. The specific policy-relevant questions that frame such 

19 evaluation of what revision of the standard might be appropriate to consider include: 

20  Does the currently available information call into question the identification of ozone as the 

21 indicator for photochemical oxidants? Is support provided for considering a different 

22 indicator? 

23  Does the currently available information call into question the current averaging time? Is 

24 support provided for considering different averaging times for the standard? 

25  What does the currently available information indicate with regard to a range of levels and 

26 forms of alternative standards that may be supported and what are the uncertainties and 

27 limitations in that information? 

28  What do the available analyses indicate with regard to exposure and risk associated with 

29 specific alternative standards? What are the associated uncertainties? To what extent might 

30 such alternatives be expected to reduce adverse impacts attributable to O3, and what are the 

31 uncertainties in the estimated reductions? 

32 
October 2018 3-5 Draft – Do Not Quote or Cite

1 The approach to reaching conclusions on the current primary standard and, as 

2 appropriate, on potential alternative standards is summarized in general terms in Figure 3-1.

3 

4 Figure 3-1. Overview of general approach for review of the primary O3 standard. 
October 2018 3-6 Draft – Do Not Quote or Cite

1 The Agency’s approach in reviewing primary standards is consistent with requirements 

2 of the provisions of the CAA related to the review of the NAAQS and with how the EPA and the 

3 courts have historically interpreted the CAA. As discussed in section 1.1 above, these provisions 

4 require the Administrator to establish primary standards that, in the Administrator’s judgment, 

5 are requisite (i.e., neither more nor less stringent than necessary) to protect public health with an 

6 adequate margin of safety. The CAA does not require the Administrator to establish a primary 

7 standard at a zero-risk level or at background concentration levels, but rather at a level that 

8 reduces risk sufficiently so as to protect public health with an adequate margin of safety. The 

9 decisions on the adequacy of the current primary standard and, on any alternative standards 

10 considered in a review, are largely public health policy judgments made by the Administrator. 

11 The four basic elements of the NAAQS (i.e., indicator, averaging time, form, and level) are 

12 generally considered collectively in evaluating the health protection afforded by the current 

13 standard, and by any alternatives considered. The Administrator’s final decisions in a review 

14 draw upon the scientific evidence for health effects, quantitative analyses of populations 

15 exposures and/or health risks, as available, and judgments about how to consider the 

16 uncertainties and limitations that are inherent in the scientific evidence and quantitative analyses. 

17 3.1.2 Background on the Current Primary Standard (Considerations and Conclusions in 

18 the Last Review) 

19 The 2015 decision to strengthen the primary standard was based on the scientific 

20 evidence and quantitative exposure and risk analyses available at the time of the last review, the 

21 Administrator’s judgments regarding the available scientific evidence, the appropriate degree of 

22 public health protection for the revised standard, and the available exposure and risk information 

23 regarding the exposures and risk that may be allowed by such a standard (80 FR 65292, October 

24 26, 2015). With the 2015 decision, the EPA revised the level of the primary standard to 0.070 

ppm,40 25 in conjunction with retaining the then-current indicator (O3), averaging time (eight 

26 hours), and form (fourth-highest daily maximum 8-hour average concentration, averaged across 

27 three consecutive years). The 2015 decision drew upon the available scientific evidence assessed 

28 in the 2013 ISA, the exposure and risk information presented and assessed in the 2014 health 

29 REA (HREA), the consideration of that evidence and information in the 2014 PA, the advice and 

30 recommendations of the CASAC, and public comments on the proposed decision (79 FR 75234, 

31 December 17, 2014). 

 

40 Although ppm are the units in which the level of the standard is defined, the units ppb are more commonly used 

throughout the next three chapters for greater consistency with their use in the more recent literature. The level of 

the current primary standard, 0.070 ppm, is equivalent to 70 ppb. 
October 2018 3-7 Draft – Do Not Quote or Cite

1 The health effects evidence base available in the 2015 review included extensive 

2 longstanding evidence from previous reviews as well as the evidence that had emerged since the 

3 previous review had been completed in 2008. This evidence base, spanning several decades, 

4 documents the causal relationship between exposure to O3 and a broad range of respiratory 

5 effects (2013 ISA, p. 1-14). Such effects range from small, reversible changes in pulmonary 

6 function and pulmonary inflammation (documented in controlled human exposure studies 

7 involving exposures ranging from 1 to 8 hours) to more serious effects such as emergency 

8 department visits and hospital admissions and even premature mortality, which have been 

9 associated with ambient air concentrations of O3 in epidemiologic studies (2013 ISA, section 

10 6.2). In addition to extensive controlled human exposure and epidemiologic studies, the evidence 

11 base includes experimental animal studies that provide insight into potential modes of action for 

12 these effects, contributing to the coherence and robust nature of the evidence. Based on this 

13 evidence base, the 2013 ISA concluded there to be a causal relationship between short-term O3

14 exposures and respiratory effects and a likely causal relationship between longer-term exposure 

15 and respiratory effects, and also between short-term exposure and mortality (2013 ISA, p. 1-

14).41 16 

17 With regard to the short-term respiratory effects, the focus of the 2015 decision, the 

18 controlled human exposure studies were recognized to provide the most certain evidence 

19 indicating the occurrence of health effects in humans following specific O3 exposures (80 FR 

20 65343, October 26, 2015; 2014 PA, section 3.4). These studies additionally illustrate the role of 

21 ventilation rate in responses to O3 exposure at the lowest studied concentrations. Generally, for

22 study subjects at rest, the exposure concentrations eliciting a given level of response are higher 

23 than for subjects exposed while at elevated ventilation, such as while exercising (2013 ISA, 

section 6.2.1.1).42 24 Further, while the study subjects in the vast majority of the controlled human 

25 exposure studies (and in all of these studies conducted at the lowest exposures) are healthy 

 

41 The 2013 ISA also concluded that there is likely to be a causal relationship between short-term exposure and 

cardiovascular effects, including related mortality, and that the evidence is suggestive of causal relationships 

between long-term O3 exposures and total mortality, cardiovascular effects and reproductive and developmental 

effects, and between O3 exposure and central nervous system effects (2013 ISA, section 2.5.2). 

42 In the controlled human exposure studies, the magnitude of respiratory effects (e.g., severity of lung function 

decrements and prevalence in symptomatic responses) is influenced by ventilation rate and exposure duration as 

well as exposure concentration, with physical activity increasing ventilation and potential for effects. For 

example, in studies of healthy young adults exposed while at rest for 2 hours, 500 ppb is the lowest concentration 

eliciting a statistically significant O3-induced group mean lung function decrement, while a 1- to 2-hour exposure 

to 120 ppb produces a statistically significant response in lung function when the ventilation rate of the group of 

study subjects is sufficiently increased with exercise (2013 ISA, section 6.2.1.1). 
October 2018 3-8 Draft – Do Not Quote or Cite

adults, people with asthma and children have been identified as at increased risk.43 1 Accordingly, 

2 the HREA exposure-based analyses included these population groups among those modeled 

3 (2014 HREA, p. 3-14). 

4 The exposure and risk information available in the 2015 review included exposure and 

5 risk estimates for air quality conditions just meeting the then-existing standard, and also for air 

6 quality conditions just meeting potential alternative standards. Estimates were derived for two 

7 exposure-based analyses, the first of which involved comparison of population exposure 

estimates at elevated exertion to exposure benchmarks (exposures of concern)44 8 based on 

9 exposure concentrations from controlled human exposure studies in which lung function changes 

10 and other effects were measured in healthy, young adult volunteers exposed to O3 while 

11 engaging in quasi-continuous moderate physical activity for a defined period (generally 6.6 

hours).45 12 The second exposure-based analysis provided population risk estimates of the 

occurrence of days with O3-attributable lung function decrements of varying magnitudes.46 13 Risk 

14 estimates were also derived from ambient air concentrations based on concentration-response 

15 functions from epidemiologic studies but were given less weight by the Administrator in her 

16 decision on the standard, given conclusions reached in the PA and the HREA which reflected 

17 lower confidence in these estimates (80 FR 65316-17, October 26, 2015). 

18 The 2014 HREA developed the exposure-based estimates for several population groups 

19 including all children and all adults. The estimates involving comparison of exposures to 

20 benchmarks were also derived for children with asthma and adults with asthma. The estimates of 

21 percentages of children with exposures above benchmarks were virtually indistinguishable from 

22 the corresponding estimates of percentages of children with asthma. When considered in terms of 

23 the number of children, the estimates for all children were much higher than those for children 

24 with asthma, with the magnitude of the differences varying based on asthma prevalence in each 

 

43 Population groups identified in the 2015 review as being at increased risk of O3-related health effects are people 

with asthma, children, older adults, outdoor workers, individuals with reduced intake of vitamin C and E, and 

individuals with specific genetic susceptibility (2013 ISA, p. 1-8). 

44 The benchmark concentrations to which exposure concentrations experienced while at moderate or greater 

exertion were compared were 60, 70 and 80 ppb. This comparison-to-benchmarks analysis, performed in the 2015 

review, is summarized in section 5.1.1.1 below. 

45 The studies given primary focus were those for which O3 exposures occurred in an exposure chamber and for 

which there were comparisons involving clean air exposures. In both cases, over the course of 6.6 hours, the 

subjects engaged in six 50-minute exercise periods separated by 10-minute rest periods, with a 35-minute lunch 

period occurring after the third hour (e.g., Follinsbee et al., 1988 and Schelegle et al., 2009). 

46 Both exposure-based analyses are described further in section 5.1 below. 
October 2018 3-9 Draft – Do Not Quote or Cite

study area (2014 HREA, sections 5.3.2, 5.4.1.5 and section 5F-1).47 1 The estimates for percent of 

2 children above the benchmarks were higher than percent of adults due to the greater time that 

3 children spend outdoors and engaged in exertion (2014 HREA, section 5.3.2). Thus, 

4 consideration of the exposure-based results in the 2015 decision focused on the results for all 

5 children and children with asthma. 

6 In weighing the 2013 ISA conclusions with regard to the health effects evidence and 

7 making judgments regarding the public health significance of the quantitative estimates of 

8 exposures and risks allowed by the then-existing and alternative standards, as well as judgments 

9 regarding margin of safety, the Administrator considered the currently available information and 

10 commonly accepted guidelines or criteria within the public health community, including the 

American Thoracic Society (ATS), an organization of respiratory disease specialists,48 11 advice 

12 from CASAC and public comments. In so doing, she recognized that the determination of what 

13 constitutes an adequate margin of safety is expressly left to the judgment of the EPA 

14 Administrator (Lead Industries Association v. EPA, 647 F.2d at 1161-62; Mississippi, 744 F. 3d 

15 at 1353). In NAAQS reviews generally, evaluations of how particular primary standards address 

16 the requirement to provide an adequate margin of safety include consideration of such factors as 

17 the nature and severity of the health effects, the size of the sensitive population(s) at risk, and the 

18 kind and degree of the uncertainties present. Consistent with past practice and long-standing 

19 judicial precedent, the Administrator took the need for an adequate margin of safety into account 

20 as an integral part of her decision-making. 

21 The Administrator’s initial decision in the last review was with regard to the adequacy of 

22 protection provided by the then-existing primary standard. Considerations related to that decision 

23 are summarized in section 3.1.2.1 below. The considerations and decisions on revisions to the 

24 then-existing standard in order to provide the requisite protection under the Act, including an 

25 adequate margin of safety, is summarized in section 3.1.2.2. 

26 3.1.2.1 Considering the Need for Revision 

27 The approach to considering the adequacy of the then-current primary standard in the last 

28 review involved the careful consideration of the available evidence, analyses and conclusions 

29 contained in the 2013 ISA, including information newly available in the review; the quantitative 

30 exposure and risk analyses in the 2014 HREA; the information, evaluations, considerations and 

31 conclusions presented in the 2014 PA; advice from the CASAC; and public comment. Key 

 

47 This reflects use of the same time-location-activity diary pool to construct each simulated individual’s timeactivity series, which is based on the similarities observed in the available diary data with regard to time spent 

outdoors and exertion levels (2014 HREA, sections 5.3.2 and 5.4.1.5). 

48 With regard to commonly accepted guidelines or criteria within the public health community, the PA considered 

statements issued by the ATS that had also been considered in prior reviews (ATS, 2000; ATS, 1985). 
October 2018 3-10 Draft – Do Not Quote or Cite

1 considerations informing the Administrator’s decision on the need for revision of the then2 current standard are summarized below. 

3 The Administrator gave primary consideration to the evidence of respiratory effects from 

4 controlled human exposure studies, for which the exposure concentrations were lower than was 

5 the case in the prior review (80 FR 65343, October 26, 2015). This emphasis was consistent with 

6 CASAC comments on the strength of this evidence (Frey, 2014, p. 5). In placing weight on these 

7 studies, the Administrator took note of the variety of respiratory effects reported from the studies 

8 of healthy adults engaged in six 50-minute periods of moderate exertion within a 6.6-hour 

exposure to O3 concentrations of 60,49 63,50 72,51 9 or 80 ppb, and higher. The array of effects 

10 reported include lung function decrements, respiratory symptoms, airway inflammation, airway 

11 hyperresponsiveness, and impaired lung host defense. The largest respiratory effects have been 

12 reported, and the broadest range of effects have been studied and reported following exposures to 

13 80 ppb O3 or higher, with most exposure studies conducted at these higher concentrations. The 

14 combination of lung function decrements and respiratory symptoms was reported in studies of 

15 6.6-hour exposures during quasi-continuous exercise to concentrations as low as 72 ppb, and 

16 lung function decrements and pulmonary inflammation were reported following exposures to O3

17 concentrations as low as 60 ppb. The 2013 ISA indicated that this pattern of effects, increasing 

18 with severity at higher concentrations, is coherent with (i.e., reasonably related to) the health 

19 outcomes reported to be associated with ambient air concentrations in epidemiologic studies 

20 (e.g., respiratory-related hospital admissions, emergency department visits). 

21 In considering the controlled human exposure study findings, the Administrator noted 

22 that the combination of lung function decrements and respiratory symptoms reported following 

exposures to 72 ppb O3 meets ATS criteria for an adverse response,52 23 recognizing the CASAC 

conclusion in this regard.53 24 The CASAC additionally noted that these study findings were for 

25 healthy adults indicating the potential for such effects in some people, such as people with 

 

49 The study by Adams (2006) provided evidence of effects resulting from a 6.6-hour exposure involving quasicontinuous moderate exertion to a mean concentration of 60 ppb.

50 For a 60 ppb target exposure concentration, Schelegle et al. (2009) reported that the actual 6.6-hour mean 

exposure concentration was 63 ppb. 

51 For a 70 ppb target exposure concentration, Schelegle et al. (2009) reported that the actual 6.6-hour mean 

exposure concentration was 72 ppb. 

52 The most recent statement from the ATS available at the time of the 2015 decision stated that “[i]n drawing the 

distinction between adverse and nonadverse reversible effects, this committee recommended that reversible loss 

of lung function in combination with the presence of symptoms should be considered as adverse” (ATS, 2000). 

53 In considering the 72 ppb exposure concentration, the CASAC noted that “the combination of decrements in FEV1

together with the statistically significant alterations in symptoms in human subjects exposed to 72 ppb ozone 

meets the American Thoracic Society’s definition of an adverse health effect” (Frey, 2014c, p. 5). 
October 2018 3-11 Draft – Do Not Quote or Cite

1 asthma, at lower exposures (Frey, 2014c, p. 6). Thus, based on the controlled human exposure 

2 study evidence, the Administrator concluded that “adverse effects are likely to occur following 

3 exposures to O3 concentrations below the level of the [then-current] standard” (80 FR 65343, 

4 October 26, 2015). 

5 With regard to the available epidemiologic evidence, the Administrator noted analyses of 

6 O3 air quality in the 2014 PA indicating that, while most O3 epidemiologic studies reported 

7 health effect associations with O3 concentrations in ambient air that violated the then-current 

8 standard, a small number of single-city U.S. studies indicate the occurrence of asthma-related 

9 hospital admissions and emergency department visits at ambient air O3 concentrations below the 

10 level of the then-current standard. In particular, the Administrator took note of a study that 

11 reported associations between short-term O3 concentrations and asthma emergency department 

12 visits in children and adults in a U.S. location that would have met the then-current standard over 

the entire 5-year study period (80 FR 65344, October 26, 2015; Mar and Koenig, 2009).54 55 13 

While uncertainties56 14 limited the extent to which the Administrator based her conclusions on air 

15 quality in locations of multicity epidemiologic studies, she additionally noted some support from 

16 several multicity studies of morbidity or mortality in which the majority of study locations would 

17 have met the then-current standard (80 FR 65344, October 26, 2015; 2014 PA, section 3.1.4.2). 

18 Accordingly, looking across the body of epidemiologic evidence, the Administrator reached the 

19 conclusion that analyses of air quality in some study locations supported the occurrence of 

20 adverse O3-associated effects at O3 concentrations in ambient air that met, or are likely to have 

21 met, the then-current standard (80 FR 65344, October 26, 2016). Taken together, the 

22 Administrator concluded that the scientific evidence from controlled human exposure and 

23 epidemiologic studies called into question the adequacy of the public health protection provided 

24 by the then-current standard. 

 

54 The design values in this location over the study period were at or somewhat below 75 ppb (Wells et al., 2012). 

55 The Administrator also took note of analyses in the PA for some single-city study locations where the then-current 

standard was not met during the study period (i.e., those evaluated in Silverman and Ito, 2010; Strickland et al., 

2010), finding support for the association of hospital admissions and emergency department visits with short-term 

O3 on subsets of days with virtually all ambient air O3 concentrations below the level of the then-current standard. 

These analyses generally focused on the range of short-term concentrations for which the confidence intervals for 

the concentration-response relationship were tightest, finding these to be represented by many days with O3

concentrations below the level of the standard (80 FR 65344, October 26, 2015). 

56 Compared to the single-city epidemiologic studies the Administrator noted additional uncertainty in interpreting 

the relationships between short-term O3 air quality in individual study cities and reported O3 multicity effect 

estimates. This uncertainty applied specifically to interpreting air quality analyses within the context of multicity 

effect estimates for short-term O3 concentrations, where effect estimates for individual study cities are not 

presented (as is the case for the key O3 studies analyzed in the PA, with the exception of the study by Stieb et al. 

(2009) where none of the city-specific effect estimates for asthma emergency department visits were statistically 

significant) (80 FR 65344; October 26, 2015). 
October 2018 3-12 Draft – Do Not Quote or Cite

1 In considering the exposure and risk information, the Administrator gave particular 

2 attention to the estimates of exposures of concern, focusing on the estimates for children, in 15 

3 urban areas for air quality conditions just meeting the then-current standard. Consistent with the 

4 finding that larger percentages of children than adults were estimated to experience exposures 

5 above benchmarks, the Administrator focused on the results for all children and for children with 

6 asthma, noting that the results in terms of percent of the population group are virtually 

7 indistinguishable (2014 HREA, sections 5.3.2, 5.4.1.5 and section 5F-1). In considering these 

8 estimates, she placed greatest weight on estimates of two or more days with occurrences of 

9 exposures above benchmarks, in light of her increased concern about the potential for adverse 

10 responses with repeated occurrences. In particular, she noted that the types of effects shown to 

11 occur following exposures to O3 concentrations from 60 ppb to 80 ppb, such as inflammation, if

12 occurring repeatedly from repeated exposure, could potentially result in more severe effects 

13 based on the ISA conclusions regarding mode of action (80 FR 65343, 65345, October 26, 2015; 

14 2013 ISA, section 6.2.3). The Administrator also placed great weight on estimates for single 

15 exposures above the higher benchmarks of 70 and 80 ppb (80 FR 65345, October 26, 2015). 

16 With regard to multiple exposures, the HREA found that under conditions just meeting 

17 the then-current standard, fewer than 1% of children in the 15 study areas would be estimated to 

18 experience multiple days in a year with 8-hour exposures at or above 70 ppb while at elevated 

19 ventilation, while the percentage was as high as approximately 2% in the year and location with 

20 the highest exposure estimates (80 FR 65345 and Table 1, October 26, 2015). Although she 

21 expressed less concern with single occurrences, the Administrator noted that the then-current 

22 standard could allow just over 3% of children to experience one or more days, averaged over the 

23 years of analysis, with an 8-hour exposure at or above 70 ppb (while at moderate or greater 

24 exertion), based on the worst-case location, and up to 8% in the worst-case year and location (80 

25 FR 65345, October 26, 2015). She additionally noted that, that in the worst-case year and 

26 location across the 15 study areas, the then-current standard could allow up to about 1% of 

27 children to experience at least one day per year with 8-hour exposures at elevated ventilation at 

or above 80 ppb, the highest benchmark evaluated (80 FR 65345, October 26, 2015).57 28 

29 In considering the HREA estimates of days with exposures at or above 60 ppb, while 

30 expressing less confidence in the adversity of effects observed following exposures as low as 60 

31 ppb, particularly single exposures, she judged the potential for adverse effects to increase with 

32 repeated exposures, as noted above (80 FR 65345, October 26, 2015). In that light, she noted that 

33 the HREA found that under air quality conditions just meeting the then-current standard, 

 

57 The Administrator additionally noted that the then-current standard could allow up to about 3% of children to 

experience one or more days with 8-hour exposures at elevated ventilation at or above 70 ppb, averaged over the 

years of analysis across the 15 study areas (80 FR 65313, Table 1, October 26, 2015). 
October 2018 3-13 Draft – Do Not Quote or Cite

1 approximately 3 to 8% of children in the 15 urban study areas (including approximately 3 to 8% 

2 of asthmatic children), on average across the years of analysis, were estimated to experience two 

3 or more days per year with 8-hour exposures at or above 60 ppb, while at elevated ventilation (80 

4 FR 65345; October 26, 2015). 

5 In considering these exposure estimates with regard to public health implications, the 

6 Administrator concluded that the exposures and risks projected to remain upon meeting the then7 current standard could reasonably be judged to be important from a public health perspective. In 

8 particular, this conclusion was based on her judgment that it is appropriate to set a standard that 

9 would be expected to eliminate, or almost eliminate, the occurrence of exposures, while at 

10 moderate exertion, at or above 70 and 80 ppb. In addition, given that the average percent of 

11 children estimated to experience two or more days with exposures at or above the 60 ppb 

12 benchmark approaches 10% in some urban study areas (on average across the analysis years), the 

13 Administrator concluded that the then-current standard does not incorporate an adequate margin 

14 of safety against the potentially adverse effects that could occur following repeated exposures at 

15 or above 60 ppb (80 FR 65345-46; October 26, 2015). 

16 With regard to the HREA estimates of lung function risk in terms of decrements in forced 

17 expiratory volume in one second (FEV1), the Administrator also gave greatest weight to 

18 estimates of multiple occurrences, while additionally noting CASAC advice that “estimation of 

19 FEV1 decrements of ≥ 15% is appropriate as a scientifically relevant surrogate for adverse health 

20 outcomes in active healthy adults, whereas an FEV1 decrement of ≥ 10% is a scientifically 

21 relevant surrogate for adverse health outcomes for people with asthma and lung disease” (Frey, 

22 2014c, p. 3). The Administrator noted that, when averaged over the years of evaluation, the then23 current standard was estimated to allow about 1 to 3% of children in the 15 urban study areas to 

24 experience two or more O3-induced lung function decrements >15%, and to allow about 8 to

25 12% of children to experience two or more O3-induced lung function decrements >10% (80 FR 

26 65346, October 26, 2015). The Administrator concluded that these HREA estimates for lung 

27 function risk, as well as the epidemiologic-study-based risk estimates (although she recognized 

28 increased uncertainty in and placed less weight on both types of estimates) further support a 

29 conclusion that the O3-associated health effects estimated to remain upon just meeting the then30 current standard are an issue of public health importance on a broad national scale. Thus, she 

31 concluded that O3 exposure and risk estimates, when taken together, support a conclusion that 

32 the exposures and health risks associated with just meeting the then-current standard can 

33 reasonably be judged to be of public health significance, such that the then-current standard was 

34 not sufficiently protective and did not incorporate an adequate margin of safety. 

35 In addition to the evidence and exposure/risk information, the Administrator also took 

36 note of CASAC advice, which included the finding that “the current NAAQS for ozone is not 
October 2018 3-14 Draft – Do Not Quote or Cite

1 protective of human health” and the unanimous recommendation “that the Administrator revise 

2 the current primary ozone standard to protect public health” (Frey, 2014c, p. 5). She further 

noted similar CASAC advice in the prior 2008 review.58 3 

4 In consideration of all of the above, the Administrator concluded that the then-current 

5 primary O3 standard was not requisite to protect public health with an adequate margin of safety, 

6 and that it should be revised to provide increased public health protection. This decision was 

7 based on the Administrator’s conclusions that the available evidence and exposure and risk 

8 information clearly called into question the adequacy of public health protection provided by the 

9 then-current primary standard such that it was “not appropriate, within the meaning of section 

10 109(d)(1) of the CAA, to retain the current standard” (80 FR 65346, October 26, 2015). 

11 3.1.2.2 Considering Revisions to the Standard 

12 The following subsections summarize the Administrator’s key considerations and 

13 conclusions in considering revisions to the indicator, averaging time, form and level of the 

14 primary standard in the 2015 review. 

15 3.1.2.2.1 Indicator 

16 In considering whether O3 continued to be the most appropriate indicator for a standard 

17 meant to provide protection against photochemical oxidants in ambient air, the Administrator 

18 considered findings and assessments in the 2013 ISA and 2014 PA, as well as advice from the 

19 CASAC and public comment. The 2013 ISA specifically noted that O3 is the only photochemical 

20 oxidant (other than nitrogen dioxide) that is routinely monitored and for which a comprehensive 

21 database exists (2013 ISA, section 3.6; 80 FR 65347, October 26, 2015). The PA additionally 

22 noted that, since the precursor emissions that lead to the formation of O3 also generally lead to 

23 the formation of other photochemical oxidants, measures leading to reductions in population 

24 exposures to O3 can generally be expected to lead to reductions in other photochemical oxidants. 

25 The CASAC indicated its view that O3 is the appropriate indicator “based on its causal or likely 

26 causal associations with multiple adverse health outcomes and its representation of a class of 

27 pollutants known as photochemical oxidants” (Frey, 2014c, p. ii). Based on all of these 

28 considerations and public comments, the Administrator concluded that O3 remains the most 

29 appropriate indicator for a standard meant to provide protection against photochemical oxidants 

30 in ambient air, and she retained O3 as the indicator for the primary standard (80 FR 65347, 

31 October 26, 2015). 

 

58 The CASAC O3 Panel for the 2008 review likewise recommended revision of the standard to one with a level 

below 75 ppb. This earlier recommendation was based entirely on the evidence and information in the record for 

the decision on the 2008 standard, which had been extended in the 2015 review (Samet, 2011; Frey and Samet, 

2012). 
October 2018 3-15 Draft – Do Not Quote or Cite

1 3.1.2.2.2 Averaging time 

2 The 8-hour averaging time for the primary O3 standard was established in 1997 with the 

3 decision to replace the then-existing 1-hour standard with an 8-hour standard (62 FR 38856, July 

4 18, 1997). The decision in that review was based on evidence from numerous controlled human 

5 exposure studies reporting associations between adverse respiratory effects and 6- to 8-hour 

6 exposures, as well as quantitative analyses indicating the control provided by an 8-hour 

7 averaging time of both 8-hour and 1-hour peak exposures and associated health risk (62 FR 

8 38861, July 18, 1997; U.S. EPA, 1996b). The decision at that time was also consistent with 

9 advice from the CASAC (62 FR 38861, July 18, 1997; 61 FR 65727; December 13, 1996). The 

10 EPA reached similar conclusions in the subsequent 2008 review in which the 8-hour averaging 

11 time was retained (73 FR 16436, March 27, 2008). 

12 In the review completed in 2015, the Administrator considered the averaging time for the 

13 standard in light of both the strong evidence for O3-associated respiratory effects following 

14 short-term exposures and the available evidence related to effects following longer-term 

15 exposures (80 FR 65347-50, October 26, 2015). In so doing, the Administrator noted the 

16 substantial health effects evidence from controlled human exposure studies that demonstrate that 

17 a wide range of respiratory effects (e.g., pulmonary function decrements, increases in respiratory 

18 symptoms, lung inflammation, lung permeability, decreased lung host defense, and airway 

19 hyperresponsiveness) occur in healthy adults following exposures ranging from 1 to 8 hours (80 

20 FR 65348, October 26, 2015; 2013 ISA, section 6.2.1.1). The Administrator also noted the 

21 strength of evidence from epidemiologic studies that evaluated a wide variety of populations 

22 (e.g., including at-risk lifestages and populations, such as children and people with asthma, 

23 respectively) using a number of different short-term averaging times, including the maximum 1-

24 hour concentration within a 24-hour period (1-hour max), the maximum 8-hour average 

25 concentration within a 24-hour period (8-hour max), and the 24-hour average (80 FR 65348, 

26 October 26, 2015; 2013 ISA, chapter 6). It was recognized that an 8-hour averaging time is 

27 similar to the exposure periods evaluated in the more recent controlled human exposure studies 

28 conducted at the lowest concentrations, and the Administrator noted that the epidemiologic 

29 evidence alone did not provide a strong basis for distinguishing between the appropriateness of 

30 1-hour, 8-hour and 24-hour averaging times. Thus, in consideration of the then-available health 

31 effects information, the Administrator concluded that an 8-hour averaging time remained 

32 appropriate for addressing health effects associated with short-term exposures to ambient air O3

33 (80 FR 65348, October 26, 2015). 

34 In considering the evidence related to longer-term exposures, the Administrator initially 

35 considered the extent to which currently available evidence and exposure/risk information 

36 suggested that a standard with an 8-hour averaging time can provide protection against 
October 2018 3-16 Draft – Do Not Quote or Cite

1 respiratory effects associated with longer-term exposures to ambient air O3. As in previous 

2 reviews, the review completed in 2015 recognized and further evaluated changes in long-term air 

3 quality patterns in response to attaining an 8-hour standard and the reduction in potential risk of 

4 health effects associated with long-term exposures in areas meeting an 8-hour standard (80 FR 

5 65348, October 26, 2015). Analyses described in detail in the HREA suggested that reductions in 

6 O3 precursors emissions in order to meet a standard with an 8-hour averaging time, coupled with 

7 the appropriate form and level, would be expected to reduce long-term O3 concentrations 

8 reported in epidemiologic studies to be associated with respiratory morbidity and mortality (80 

9 FR 65348, October 26, 2015). 

10 In summary, based on the then-available evidence and information discussed in detail in 

11 the 2013 ISA, 2014 HREA, and 2014 PA, along with CASAC advice and public comments, the 

12 Administrator concluded that a standard with an 8-hour averaging time could effectively limit 

13 health effects attributable to both short- and long-term O3 exposures. Furthermore, the 

14 Administrator observed that the CASAC agreed with the choice of averaging time (Frey, 2014c, 

15 p. ii). Therefore, the Administrator concluded it to be appropriate to retain the 8-hour averaging 

16 time and to not set a separate standard with a different averaging time (80 FR 65350, October 26, 

17 2015). 

18 3.1.2.2.3 Form 

19 While giving foremost consideration to the adequacy of public health protection provided 

20 by the combination of all elements of the standard, including the form, the Administrator placed 

21 considerable weight on the findings from prior reviews with regard to the use of the nth-high 

22 metric, as described below (80 FR 65350-65352, October 26, 2015). Based on these findings and 

23 consideration of CASAC advice, the Administrator judged it appropriate to retain the fourth-high 

24 form, more specifically the fourth-highest daily maximum 8-hour O3 average concentration, 

25 averaged over 3 years (80 FR 65352, October 26, 2015). 

26 The concentration-based form was established in the 1997 review when it was recognized 

27 that such a form better reflects the continuum of health effects associated with increasing O3

28 concentrations than an expected exceedance form, which had been the form of the standard prior 

29 to 1997. Unlike an expected exceedance form, a concentration-based form gives proportionally 

30 more weight to years when 8-hour O3 concentrations are well above the level of the standard 

31 than years when 8-hour O3 concentrations are just above the level of the standard. More weight 

32 was given to high O3 concentrations, in light of the available health evidence that indicated a 

33 continuum of effects associated with exposures to varying concentrations of O3, and because the 

34 extent to which public health is affected by exposure to O3 in ambient air is related to the actual 

35 magnitude of the O3 concentration, not just whether the concentration is above a specified level. 
October 2018 3-17 Draft – Do Not Quote or Cite

1 With regard to a specific concentration-based form, the fourth-highest daily maximum was 

2 selected in 1997, recognizing that a less restrictive form (e.g., fifth highest) would allow a larger 

3 percentage of sites to experience O3 peaks above the level of the standard, and would allow more 

4 days on which the level of the standard may be exceeded when the site attains the standard (62 

5 FR 38868-38873, July 18, 1997). 

6 In the subsequent 2008 review, the EPA considered the potential value of a percentile7 based form, recognizing that such a statistic is useful for comparing datasets of varying length 

8 because it samples approximately the same place in the distribution of air quality values, whether 

9 the dataset is several months or several years long (73 FR 16474, March 27, 2008). However, the 

10 EPA concluded that, because of the differing lengths of the monitoring season for O3 across the 

11 U.S., a percentile-based statistic would not be effective in ensuring the same degree of public 

12 health protection across the country. Specifically, a percentile-based form would allow more 

13 days with higher air quality values (i.e., higher O3 concentrations) in locations with longer O3

14 seasons relative to locations with shorter O3 seasons. Thus, the EPA concluded in the 2008 

15 review that a form based on the nth-highest maximum O3 concentration would more effectively 

16 ensure that people who live in areas with different length O3 seasons received the same degree of 

17 public health protection (73 FR 16474-75, March 27, 2008). At that time, it was also recognized 

18 that it is important to have a form that provides stability with regard to implementation of the 

19 standard. In the case of O3, for example, it was noted that it was important to have a form that 

20 provides stability and insulation from the impacts of extreme meteorological events that are 

21 conducive to O3 formation. Such events could have the effect of reducing public health 

22 protection, to the extent they result in frequent shifts in and out of attainment due to 

23 meteorological conditions because such frequent shifting could disrupt an area’s ongoing 

24 implementation plans and associated control programs (73 FR 16475, March 27, 2008). 

25 In the 2015 review, the Administrator continued to recognize the considerations 

26 supporting the decisions in 1997 and 2008, and additionally noted recent CASAC advice in 

27 which the CASAC indicated that the O3 standard should be based on the fourth-highest, daily 

28 maximum 8-hour average value (averaged over 3 years), by stating that this form “provides 

29 health protection while allowing for atypical meteorological conditions that can lead to 

30 abnormally high ambient ozone concentrations which, in turn, provides programmatic stability” 

31 (Frey, 2014c, p. 6; 80 FR 65352, October 26, 2015). 

32 3.1.2.2.4 Level 

33 The Administrator’s decision to revise the level of the primary O3 standard to 70 ppb 

34 built upon her conclusion (summarized in section 3.1.2.1 above) that the overall body of 

35 scientific evidence and exposure/risk information called into question the adequacy of the public 
October 2018 3-18 Draft – Do Not Quote or Cite

1 health protection afforded by the then-current standard, particularly for at-risk populations and 

2 lifestages (80 FR 65362, October 26, 2015). In her decision on level, the Administrator placed 

3 the greatest weight on the results of controlled human exposure studies and on quantitative 

4 analyses based on information from these studies, particularly analyses of O3 exposures of 

5 concern. The Administrator viewed the results of the lung function risk assessment, analyses of 

6 O3 air quality in locations of epidemiologic studies, and epidemiology-based quantitative health 

7 risk assessment as providing information in support of her decision to revise the then-current 

8 standard, but of less utility for selecting a particular standard level among a range of options (80 

9 FR 65362, October 26, 2015). In placing weight on information from controlled human exposure 

10 studies and analyses based on information from these studies, the Administrator noted that 

11 controlled human exposure studies provide the most certain evidence indicating the occurrence 

12 of health effects in humans following specific O3 exposures, noting in particular that the effects 

13 reported in the controlled human exposure studies are due solely to O3 exposures, and are not 

14 complicated by the presence of co-occurring pollutants or pollutant mixtures (as is the case in 

15 epidemiologic studies). The Administrator’s emphasis on the information from the controlled 

16 human exposure studies was consistent with the CASAC’s advice and interpretation of the 

17 scientific evidence (80 FR 65362, October 26, 2015; Frey, 2014c). 

18 With regard to the effects shown in controlled human exposure studies following specific 

19 O3 exposures, the Administrator noted that (1) the largest respiratory effects, and the broadest 

20 range of effects, have been studied and reported following exposures to 80 ppb O3 or higher (i.e., 

21 decreased lung function, increased airway inflammation, increased respiratory symptoms, airway 

22 hyperresponsiveness, and decreased lung host defense); (2) exposures to O3 concentrations as 

23 low as 72 ppb have been shown to both decrease lung function and to result in respiratory 

24 symptoms; and (3) exposures to O3 concentrations as low as 60 ppb have been shown to decrease 

25 lung function and to increase airway inflammation (80 FR 65363, October 26, 2015). The 

26 Administrator considered ATS recommendations and CASAC advice to inform her judgments 

27 on the potential adversity to public health of effects reported in controlled human exposure 

28 studies (80 FR 65363, October 26, 2015). In doing so, the Administrator concluded that the 

29 evidence from controlled human exposure studies provided strong support for the conclusion that 

30 a revised standard with a level of 70 ppb is requisite to protect public health with an adequate 

31 margin of safety. This conclusion was based, in part, on the fact that such a standard level would 

32 be well below the O3 exposure concentration shown to result in the widest range of respiratory 

33 effects (i.e., 80 ppb), and below the lowest O3 exposure concentration shown to result in the 

34 adverse combination of lung function decrements and respiratory symptoms, i.e., 72 ppb (80 FR 

35 65363, October 26, 2015). 
October 2018 3-19 Draft – Do Not Quote or Cite

1 In considering the degree of protection provided by a revised primary O3 standard, the 

2 Administrator considered the extent to which that standard would be expected to limit population 

3 exposures to the broad range of O3 exposures shown to result in health effects (80 FR 65363, 

4 October 26, 2015). In considering the exposure estimates from the HREA, the Administrator 

5 focused on the estimates of two or more exposures of concern in order to provide a health6 protective approach to considering the potential for repeated occurrences of exposures that could 

7 result in adverse effects. In so doing, she placed the most emphasis on setting a standard that 

8 appropriately limits repeated occurrences of exposures while at elevated ventilation at or above 

9 the 70 and 80 ppb benchmarks. She noted that a revised standard with a level of 70 ppb was 

10 estimated to eliminate the occurrence of two or more days with exposures at or above 80 ppb and 

11 to virtually eliminate the occurrence of two or more days with exposures at or above 70 ppb for 

all children and children with asthma, even in the worst-case year and location evaluated.59 12 

13 Given the considerable protection provided against repeated exposures of concern for all 

14 benchmarks evaluated in the HREA, the Administrator judged that a standard with a level of 70 

15 ppb incorporated a margin of safety against the adverse O3-induced effects shown to occur 

16 following exposures at or above 72 ppb, and judged likely to occur following exposures 

17 somewhat below 72 ppb (80 FR 65364, October 26, 2015). 

18 While she was less confident that adverse effects would occur following exposures to O3

concentrations as low as 60 ppb,60 19 as discussed above, the Administrator judged it to also be 

20 appropriate to consider estimates of exposures for the 60 ppb benchmark (80 FR 65363-64, 

21 October 26, 2015). In so doing, she recognized that while CASAC advice regarding the potential 

22 adversity of effects at 60 ppb was less definitive than for effects at 72 ppb, the CASAC did 

23 clearly advise the EPA to consider the extent to which a revised standard is estimated to limit the 

24 effects observed in studies of 60 ppb exposures (80 FR 65364, October 26, 2015; Frey, 2014c). 

25 The Administrator’s consideration of exposures at or above the 60 ppb benchmark was primarily 

26 in the context of considering the extent to which the health protection provided by a revised 

27 standard included a margin of safety against the occurrence of adverse O3-induced effects. In this 

28 context, the Administrator noted that a revised standard with a level of 70 ppb was estimated to 

 

59 Under conditions just meeting an alternative standard with a level of 70 ppb across the 15 urban study areas, the 

estimate for two or more days with exposures at or above 70 ppb was 0.4% of children, in the worst year and 

worst area (80 FR 65313, Table 1, October 26, 2015). 

60 The Administrator was “notably less confident in the adversity to public health of the respiratory effects that have 

been observed following exposures to O3 concentrations as low as 60 ppb, given her consideration of the 

following: (1) ATS recommendations indicating uncertainty in judging adversity based on lung function 

decrements alone; (2) uncertainty in the extent to which a short-term, transient population-level decrease in FEV1

would increase the risk of other, more serious respiratory effects in that population (i.e., per ATS 

recommendations on population-level risk); and (3) compared to 72 ppb, CASAC advice is less clear regarding 

the potential adversity of effects at 60 ppb” (80 FR 65363, October 26, 2015). 
October 2018 3-20 Draft – Do Not Quote or Cite

1 protect the vast majority of children in urban study areas (i.e., about 96% to more than 99% of 

2 children in individual areas) from experiencing two or more days with exposures at or above 60 

3 ppb (while at moderate or greater exertion). Compared to the estimates for the then-current 

4 standard, this represented a reduction of more than 60%. Given the considerable protection 

5 provided against repeated exposures of concern for all of the benchmarks evaluated, including 

6 the 60 ppb benchmark, the Administrator judged that a standard with a level of 70 ppb would 

7 incorporate a margin of safety against the adverse O3-induced effects shown to occur following 

8 exposures at or above 72 ppb, and that she judged likely to occur following exposures somewhat 

9 below 72 ppb. The Administrator also judged the HREA results for one or more exposures at or 

10 above 60 ppb to provide further support for her somewhat broader conclusion that “a standard 

11 with a level of 70 ppb would incorporate an adequate margin of safety against the occurrence of 

12 O3 exposures that can result in effects that are adverse to public health” (80 FR 65364, October 

26, 2015).61 13 

While placing limited weight on the lung function risk estimates,62 14 epidemiologic 

evidence63 15 and quantitative estimates based on information from the epidemiologic studies, the 

16 Administrator additionally considered that information in the context of her consideration of a 

17 standard with a level of 70 ppb. For example, she judged that a standard with a level of 70 ppb 

18 would be expected to result in important reductions in the population-level risk of O3-induced 

19 lung function decrements in children, including children with asthma (80 FR 65364, October 26, 

 

61 While the Administrator was less concerned about single occurrences of O3 exposures of concern, especially for 

the 60 ppb benchmark, she judged that estimates of one or more exposures of concern can provide further insight 

into the margin of safety provided by a revised standard. In this regard, she noted that “a standard with a level of 

70 ppb is estimated to (1) virtually eliminate all occurrences of exposures of concern at or above 80 ppb; (2) 

protect the vast majority of children in urban study areas from experiencing any exposures of concern at or above 

70 ppb (i.e., ≥ about 99%, based on mean estimates; Table 1); and (3) to achieve substantial reductions, compared 

to the then-current standard, in the occurrence of one or more exposures of concern at or above 60 ppb (i.e., about 

a 50% reduction; Table 1)” (80 FR 65364, October 26, 2015). 

62 The Administrator noted important uncertainties in using lung function risk estimates as a basis for considering 

the occurrence of adverse effects in the population (also recognized in the prior review) that limited her reliance 

on these estimates to distinguish between the appropriateness of the health protection afforded by a standard level 

of 70 ppb versus lower levels (80 FR 65364, October 26, 2015). These uncertainties related to (1) the ATS 

recommendation that “a small, transient loss of lung function, by itself, should not automatically be designated as 

adverse” (ATS, 2000); (2) uncertainty in the extent to which a transient population-level decrease in FEV1 would 

increase the risk of other, more serious respiratory effects in that population (i.e., per ATS recommendations on 

population-level risk); and (3) that CASAC did not advise considering a standard that would be estimated to 

eliminate O3-induced lung function decrements ≥10 or 15% (Frey, 2014c); 80 FR 65364, October 26, 2015). 

63 While the Administrator concluded that analyses of air quality in single-city epidemiologic studies support a level 

at least as low as 70 ppb, based on a study (Mar and Koenig, 2009) reporting health effect associations in a 

location that met the then-current standard over the entire study period but that would have violated a revised 

standard with a level of 70 ppb, she further judged that they are of more limited utility for distinguishing between 

the appropriateness of the health protection estimated for a standard level of 70 ppb and the protection estimated 

for lower levels (80 FR 65364, October 26, 2015). 
October 2018 3-21 Draft – Do Not Quote or Cite

1 2015). With regard to the epidemiologic evidence, the Administrator noted that a revised 

2 standard with a level of 70 ppb would provide additional public health protection, beyond that 

3 provided by the then-current standard, against the clearly adverse effects analyzed in 

4 epidemiologic studies (80 FR 65364, October 26, 2015). With regard to the epidemiology-based 

5 risk estimates, the Administrator judged that a revised standard with a level of 70 ppb will result 

6 in meaningful reductions in the mortality and respiratory morbidity risk that is associated with 

7 short- or long-term concentrations of O3 in ambient air (80 FR 65365, October 26, 2015). 

8 In summary, given her consideration of the evidence, exposure and risk information, 

9 advice from CASAC, and public comments, the Administrator judged a primary standard of 70 

10 ppb in terms of the 3-year average of fourth-highest daily maximum 8-hour average O3

11 concentrations to be requisite to protect public health, including the health of at-risk populations, 

12 with an adequate margin of safety (80 FR 65365, October 26, 2015). 

13 3.2 THE SECONDARY STANDARD 

14 The approach planned for this review of the secondary standard is most fundamentally 

15 based on using the Agency’s assessment of the current scientific evidence and associated 

16 quantitative analyses to inform the Administrator’s judgments regarding a secondary standard 

17 that is requisite to protect the public welfare from known or anticipated adverse effects. This 

18 approach involves translating scientific and technical information into the basis for addressing a 

19 series of key policy-relevant questions using both evidence- and exposure/risk-based 

20 considerations. This series of key questions related to the secondary standard is presented in 

21 section 3.2.1, along with a summary of the general approach for the review. Additionally, to 

22 provide context for this review of the current secondary standard, section 3.2.2 below 

23 summarizes key aspects of the decisions made in the last review, including the Agency’s 

24 consideration of important policy judgments on effects that may be adverse to the public welfare, 

25 as well as uncertainties and limitations in the scientific evidence and in the air quality and 

26 exposure/risk information. 

27 3.2.1 Key Issues Related to the Secondary Standard 

28 The approach planned for this review of the secondary O3 standard will build on the 

29 substantial body of work developed during the course of the last review, taking into account the 

30 more recent scientific information and air quality data now available to inform our understanding 

31 of the key policy-relevant issues in this review. The ISA, risk and exposure analyses (as 

32 warranted), and PA developed in this new review will provide the basis for addressing the key 

33 policy-relevant questions and these documents will inform the Administrator’s decisions as to 

34 whether to retain or revise this standard. As summarized in section 1.2, and also described in 
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1 chapter 6, evaluations in the PA are intended to inform the Administrator’s public welfare policy 

2 judgments and decisions. In so doing, the PA considers the potential implications of various 

3 aspects of the scientific evidence, the exposure/risk-based information, and the associated 

4 uncertainties and limitations. 

5 In building upon the conclusions from the last review, the current review of the 

6 secondary standard, as with the review of the primary standard, takes into account the updated 

7 evidence and information that has become available since the last review. The Agency’s 

8 consideration of the full set of evidence and information available in this review will inform the 

9 answer to the following initial overarching question for the review: 

10  Do the currently available scientific evidence and exposure-/risk-based information 

11 support or call into question the adequacy of the public welfare protection afforded by 

12 the current secondary O3 standard? 

13 In reflecting on this question, we will consider the available body of scientific evidence, 

14 assessed in the ISA, and considered as a basis for developing or interpreting risk and exposure 

15 analyses, including whether it supports or calls into question the scientific conclusions reached in 

16 the last review regarding welfare effects related to exposure to O3 in ambient air. Information 

17 available in this review that may be informative to public policy judgments regarding 

18 significance or adversity of key effects on the public welfare will also be considered. 

19 Additionally, the currently available exposure and risk information, whether newly developed in 

20 this review or predominantly developed in the past and interpreted in light of current 

21 information, will be considered, including with regard to the extent to which it may continue to 

22 support judgments made in the last review. Further, in considering this question with regard to 

23 the secondary O3 standard, we give particular attention to exposures and risks for effects with the 

24 greatest potential for public welfare significance. 

25 Evaluation of the available scientific evidence and risk/exposure information with regard 

26 to consideration of the current standard will focus on key policy-relevant issues by addressing a 

27 series of questions including the following: 

28  Is there newly available evidence that indicates the importance of photochemical oxidants 

29 other than O3 with regard to abundance in ambient air, and potential for welfare effects? 

30  Does the current evidence alter our conclusions from the last review regarding the nature of 

31 welfare effects attributable to O3 in ambient air? Is there new evidence on welfare effects 

32 beyond those identified in the last review that suggest additional endpoints should be given 

33 increased focus in this review? 

34  To what extent have important uncertainties in the evidence identified in the last review been 

35 reduced and/or have new uncertainties been recognized? 
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1  Does the current evidence continue to support a cumulative, seasonal exposure index as a 

2 biologically-relevant and appropriate metric for assessment of the evidence of exposure/risk 

3 information for vegetation? 

4  To what extent does the currently available evidence suggest locations or ecosystems where 

5 the vulnerability of sensitive species to O3-related effects would have special significance to 

6 the public welfare? 

7  To what extent does the available evidence indicate the occurrence of O3-related effects 

8 attributable to cumulative O3 exposures lower than previously established or that might be 

9 expected to occur under the current standard? 

10  Is there new evidence on factors that influence relationships between O3 concentrations and 

11 vegetation-related or other welfare effects? 

12  What are the nature and magnitude of exposure- and risk-related estimates for vegetation 

13 associated with conditions just meeting the current standard? To what extent does risk or 

14 exposure information considered in this review suggest that ecosystem exposures of concern 

15 for welfare effects are likely to occur with O3 concentrations that just meet the standard? To 

16 what extent are such exposures and risks important from a public welfare perspective? 

17  What are the important uncertainties associated with any exposure/risk analyses? 

18 If the information available in this review suggests that revision of the current secondary 

19 standard would be appropriate to consider, the PA will include evaluation of how the standard 

20 might be revised, based on the currently available scientific information, air quality assessments 

21 and exposure/risk information, and also considering what the available information indicates as 

22 to public welfare protection expected to be afforded by the current or potential alternative 

23 standards. In such an evaluation, the PA may consider the effect of revision of one or more 

24 elements of the standard (indicator, averaging time, level and form), with the effect being 

25 evaluated based on the resulting potential standard and all of its elements collectively. Based on 

26 such evaluations, the PA would then identify potential alternative standards (in terms of 

27 indicator, averaging time, level, and form) that would reflect a range of alternative policy 

28 judgments as to the degree of protection that is requisite to protect public welfare from known or 

29 anticipated adverse effects, and options for standards expected to achieve it. The specific policy30 relevant questions that frame such evaluation of what revision of the standard might be 

31 appropriate to consider include: 

32  Does the currently available information call into question the identification of ozone as the 

33 indicator for photochemical oxidants? Is support provided for considering a different 

34 indicator? 

35  To what extent does the currently available information call into question the current 

36 averaging time? Is support provided for considering different averaging times for the 

37 standard? 
October 2018 3-24 Draft – Do Not Quote or Cite

1  What does the currently available information indicate with regard to a range of levels and 

2 forms of alternative standards that may be supported and what are the uncertainties and 

3 limitations in that information? 

4  What do the available analyses indicate with regard to exposure and risk associated with 

5 specific alternative standards? What are the associated uncertainties? To what extent might 

6 such alternatives be expected to reduce adverse impacts attributable to O3, and what are the 

7 uncertainties in the estimated reductions? 

8 
October 2018 3-25 Draft – Do Not Quote or Cite

1 The approach to reaching conclusions on the current secondary O3 standard and, as 

2 appropriate, on potential alternative standards, including consideration of the policy-relevant 

3 questions which will frame the current review, is illustrated in Figure 3-2. 

4 

5 Figure 3-2. Overview of general approach for review of the secondary O3 standard. 
October 2018 3-26 Draft – Do Not Quote or Cite

1 The Agency’s approach in review secondary standards is consistent with the 

2 requirements of the provisions of the CAA related to the review of NAAQS and with how the 

3 EPA and the courts have historically interpreted the CAA. As discussed in section 1.1 above, 

4 these provisions require the Administrator to establish secondary standards that, in the 

5 Administrator’s judgment, are requisite (i.e., neither more nor less stringent than necessary) to 

6 protect the public welfare from known or anticipated adverse effects. The CAA does not require 

7 that standards be set at a zero-risk level, but rather at a level that reduces risk sufficiently so as to 

8 protect the public welfare from known or anticipated adverse effects. The Agency’s decisions on 

9 the adequacy of the current secondary standard and, as appropriate, on any potential alternative 

10 standards considered in a review, are largely public welfare policy judgments made by the 

11 Administrator. The four basic elements of the NAAQS (i.e., indicator, averaging time, form, and 

12 level) will be considered collectively in evaluating the protection afforded by the current 

13 standard, or any alternative standards considered. The Administrator’s final decisions in a review 

14 draw upon the scientific information and analyses about welfare effects, environmental 

15 exposures and risks, and associated public welfare significance, as well as judgments about how 

16 to consider the range and magnitude of uncertainties that are inherent in the scientific evidence 

17 and analyses. 

18 3.2.2 Background on the Current Secondary Standard (Considerations and Conclusions 

19 in the Last Review) 

20 The 2015 decision to revise the secondary O3 standard was based on the scientific and 

21 technical information available at that time, as well as the Administrator’s judgments regarding 

22 the available welfare effects evidence, the appropriate degree of public welfare protection for the 

23 revised standard, and available air quality information on seasonal cumulative exposures that 

24 may be allowed by such a standard (80 FR 65292, October 26, 2015). With the 2015 decision, 

25 the Administrator revised the level of the secondary standard to 0.070 ppm, in conjunction with 

26 retaining the then-current indicator, averaging time (8 hours) and form (fourth-highest daily 

27 maximum 8-hour average concentration, averaged across three years). 

28 The welfare effects evidence base available in the 2015 review includes more than fifty 

29 years of extensive research on O3’s phytotoxic effects, conducted both in and outside of the U.S. 

30 that documents the impacts of O3 on plants and their associated ecosystems (U.S. EPA, 1978, 

31 1986, 1996, 2006, 2013). As was established in prior reviews, O3 can interfere with carbon gain 

32 (photosynthesis) and allocation of carbon within the plant, making fewer carbohydrates available 

33 for plant growth, reproduction, and/or yield. For seed-bearing plants, these reproductive effects 

34 will culminate in reduced seed production or yield (U.S. EPA, 1996, pp. 5-28 and 5-29). The 

35 strongest evidence for effects from O3 exposure on vegetation is from controlled exposure 
October 2018 3-27 Draft – Do Not Quote or Cite

1 studies, which “have clearly shown that exposure to O3 is causally linked to visible foliar injury, 

2 decreased photosynthesis, changes in reproduction, and decreased growth” in many species of 

3 vegetation (2013 ISA, p. 1-15). Such effects at the plant scale can also be linked to an array of 

4 effects at larger spatial scales, with the evidence available in the last review indicating that 

5 “ambient O3 exposures can affect ecosystem productivity, crop yield, water cycling, and 

6 ecosystem community composition” (2013 ISA, p. 1-15, Chapter 9, section 9.4). 

7 In light of this robust evidence base, the 2013 ISA concluded there to be causal 

8 relationships between O3 and visible foliar injury, reduced vegetation growth, reduced 

9 productivity in terrestrial ecosystems, reduced yield and quality of agricultural crops and 

10 alteration of below-ground biogeochemical cycles. The 2013 ISA additionally found there to 

11 likely be a causal relationship between O3 and reduced carbon sequestration in terrestrial 

12 ecosystems, alteration of terrestrial ecosystem water cycling and alteration of terrestrial 

13 community composition (2013 ISA, Table 9-19). Further, based on the then-available evidence 

14 with regard to O3 effects on climate, the 2013 ISA also found there to be a causal relationship 

15 between changes in tropospheric O3 concentrations and radiative forcing, found there likely to be

16 a causal relationship between tropospheric O3 concentrations and effects on climate as quantified 

17 through surface temperature response, and found the evidence to be inadequate to determine if a 

18 causal relationship exists between tropospheric O3 concentrations and health and welfare effects 

19 related to UV-B shielding (2013 ISA, section 10.5). 

20 The 2015 decision was a public welfare policy judgment made by the Administrator, 

21 which drew upon the available scientific evidence for O3-attributable welfare effects and on 

22 analyses of exposures and public welfare risks based on impacts to vegetation, ecosystems and 

23 their associated services, as well as judgments about the appropriate weight to place on the range 

24 of uncertainties inherent in the evidence and analyses. Such judgments in the context of that 

25 review included judgments on the weight to place on the evidence of specific vegetation-related 

26 effects estimated to result across a range of cumulative seasonal concentration-weighted O3

27 exposures; on the weight to give associated uncertainties, including those related to the 

28 variability in occurrence of such effects in areas of the U.S., especially areas of particular public 

29 welfare significance; and on the extent to which such effects in such areas may be considered 

30 adverse to public welfare. 

31 The decision was based on a thorough review, in the 2013 ISA, of the scientific 

32 information on O3-induced environmental effects. The decision also took into account: (1) staff 

33 assessments in the 2014 PA of the most policy-relevant information in the 2013 ISA regarding 

34 evidence of adverse effects of O3 to vegetation and ecosystems, information on biologically35 relevant exposure metrics, 2014 welfare REA (WREA) analyses of air quality, exposure, and 

36 ecological risks and associated ecosystem services, and staff analyses of relationships between 
October 2018 3-28 Draft – Do Not Quote or Cite

levels of a W126-based exposure index64 1 and potential alternative standard levels in combination 

2 with the form and averaging time of the then-current standard; (2) additional air quality analyses 

3 of the W126 index and design values based on the form and averaging time of the then-current 

4 standard (3) CASAC advice and recommendations; and (4) public comments received during the 

5 development of these documents and on the proposal notice. In addition to reviewing the most 

6 recent scientific information as required by the CAA, the 2015 rulemaking also incorporated the 

7 EPA’s response to the judicial remand of the 2008 secondary O3 standard in Mississippi v. EPA, 

8 744 F.3d 1334 (D.C. Cir. 2013) and, in accordance with the court’s decision in that case, fully 

9 explained the Administrator’s conclusions as to the level of air quality that provides the requisite 

10 protection of public welfare from known or anticipated adverse effects. 

11 Consistent with the general approach routinely employed in NAAQS reviews, the initial 

12 consideration in the last review of the secondary standard was with regard to the adequacy of 

13 protection provided by the then-existing standard. Key aspects of that consideration are 

14 summarized in section 3.2.2.1 below. The subsequent selection of a standard concluded by the 

15 Administrator to provide the requisite protection under the Act is summarized in section 3.2.2.2. 

16 3.2.2.1 Considering the Need for Revision 

17 The approach to considering the adequacy of the secondary O3 standard in the 2015 

18 review involved the careful consideration of the available evidence, analyses and conclusions 

19 contained in the 2013 ISA, including information newly available in the review; the information, 

20 quantitative assessments, considerations and conclusions presented in the 2014 WREA and 2014 

21 PA; additionally available air quality analyses; the advice and recommendations from CASAC; 

22 and public comments. The Administrator gave primary consideration to the evidence of growth 

23 effects in well-studied tree species and information on cumulative seasonal O3 exposures 

occurring in Class I areas65 24 when the then-current standard was met (80 FR 65385-65386, 

25 October 26, 2015). The exposure information for Class I areas evaluated in terms of the W126 

26 cumulative seasonal exposure index, an index recognized by the 2013 ISA as a mathematical 

27 approach “for summarizing ambient air quality information in [a] biologically meaningful form[] 

28 for O3 vegetation effects assessment purposes’’ (2013 ISA, section 9.5.3). The EPA focused on 

 

64 The W126 index is a cumulative seasonal metric described as the sigmoidally weighted sum of all hourly O3

concentrations observed during a specified daily and seasonal time window, where each hourly O3 concentration 

is given a weight that increases from zero to one with increasing concentration (80 FR 65373-74, October 26, 

2015). Accordingly, W126 index values are in the units of ppm-hours (ppm-hrs). 

65 Areas designated as Class I include all international parks, national wilderness areas which exceed 5,000 acres in 

size, national memorial parks which exceed 5,000 acres in size, and national parks which exceed six thousand 

acres in size, provided the park or wilderness area was in existence on August 7, 1977. Other areas may also be 

Class I if designated as Class I consistent with the CAA. 
October 2018 3-29 Draft – Do Not Quote or Cite

1 the W126 index for this purpose consistent with the evidence in the 2013 ISA and advice from 

2 the CASAC (80 FR 65375, October 26, 2015). 

3 In her decision making, the Administrator considered the effects of O3 on tree seedling 

4 growth, as suggested by the CASAC, as a surrogate or proxy for the full array of vegetation5 related effects of O3, ranging from effects on sensitive species to broader ecosystem-level effects 

6 (80 FR 65369, 65406, October 26, 2015). The metric used for quantifying effects on tree 

7 seedling growth in the review was relative biomass loss (RBL), with the evidence base providing 

8 robust and established exposure-response (E-R) functions for seedlings of 11 tree species (80 FR 

65391-92, October 26, 2015; 2014 PA, Appendix 5C).66 9 The Administrator used this proxy in 

10 making her judgments on O3 effects to the public welfare. 

11 In considering the public welfare protection provided by the then-current standard, the 

12 Administrator gave primary consideration to an analysis of cumulative seasonal exposures in or 

13 near Class I areas during periods when the then-current standard was met and the associated 

14 estimates of growth effects, in terms of the O3 attributable reductions in RBL in the median 

15 species for which exposure-response (E-R) functions have been established (80 FR 65389-

65390, October 26, 2015). 67 16 The Administrator noted the occurrence of exposures for which the 

17 associated estimates of growth effects in the median species extend above a magnitude 

considered to be “unacceptably high” by CASAC.68 18 This analysis estimated such cumulative 

19 exposures occurring under the then-current standard for nearly a dozen areas, distributed across 

20 two NOAA climatic regions of the U.S (80 FR 65385-86, October 26, 2015). The Administrator 

21 gave particular weight to this analysis because of its focus in Class I areas, lands that Congress 

22 set aside for specific uses intended to provide benefits to the public welfare, including lands that 

23 are to be protected so as to conserve the scenic value and the natural vegetation and wildlife 

24 within such areas, and to leave them unimpaired for the enjoyment of future generations. Such an 

25 emphasis on lands afforded special government protections, such as national parks and forests, 

 

66 These functions for RBL estimate the reduction in a year’s growth as a percentage of that expected in the absence 

of O3 (2013 ISA, section 9.6.2; 2014 WREA, section 6.2). 

67 In specifically evaluating exposure levels in terms of the W126 index as to potential for impacts on vegetation, the 

Administrator focused on RBL estimates for the median across the eleven tree species for which robust E-R 

functions were available. The presentation of robust established E-R functions for growth effects on tree seedlings 

(and crops) included estimates of RBL (and RYL) at a range of W126-based exposure levels (2014 PA, Tables 

5C-1 and 5C-2). The median tree species RBL or crop RYL was presented for each W126 level (2014 PA, Table 

5C-3; 80 FR 65391 [Table 4], October 26, 2015). The Administrator focused on RBL as a surrogate or proxy for 

the broader array of vegetation-related effects of potential public welfare significance, which include effects on 

growth of individual sensitive species and extend to ecosystem-level effects, such as community composition in 

natural forests, particularly in protected public lands, as well as forest productivity (80 FR 65406, October 26, 

2015). 

68 In the CASAC’s consideration of RBL estimates presented in the draft PA, it characterized an estimate of 6% 

RBL in the median studied species as “unacceptably high” (Frey, 2014c). 
October 2018 3-30 Draft – Do Not Quote or Cite

1 wildlife refuges, and wilderness areas, some of which are designated Class I areas under the 

2 CAA, was consistent with a similar emphasis in the 2008 review of the standard (73 FR 16485, 

3 March 27, 2008). The Administrator additionally recognized that states, tribes and public interest 

4 groups also set aside areas that are intended to provide similar benefits to the public welfare for 

5 residents on those lands, as well as for visitors to those areas (80 FR 65390, October 26, 2015). 

6 As noted across reviews of O3 secondary standards, the Administrator’s judgments 

7 regarding effects that are adverse to public welfare consider the intended use of the ecological 

8 receptors, resources and ecosystems affected (80 FR 65389, October 26, 2015). Thus, in the 

9 2015 review, the Administrator utilized the median RBL estimate for the studied species as a 

10 quantitative tool within a larger framework of considerations pertaining to the public welfare 

11 significance of O3 effects. She recognized such considerations to include effects that are 

12 associated with effects on growth and that the 2013 ISA determined to be causally or likely 

13 causally related to O3 in ambient air, yet for which there are greater uncertainties affecting our 

14 estimates of impacts on public welfare. These other effects included reduced productivity in 

15 terrestrial ecosystems, reduced carbon sequestration in terrestrial ecosystems, alteration of 

16 terrestrial community composition, alteration of below-grown biogeochemical cycles, and 

17 alteration of terrestrial ecosystem water cycles. Thus, in giving attention to the CASAC’s 

18 characterization of a 6% estimate for tree seedling RBL in the median studied species as 

19 “unacceptably high”, the Administrator, while mindful of uncertainties with regard to the 

20 magnitude of growth impact that might be expected in mature trees, was also mindful of related, 

21 broader, ecosystem-level effects for which the available tools for quantitative estimates are more 

22 uncertain and those for which the policy foundation for consideration of public welfare impacts 

23 is less well established. As a result, the Administrator considered tree growth effects of O3, in 

24 terms of RBL as a surrogate for the broader array of O3 effects at the plant and ecosystem levels 

25 (80 FR 65389, October 26, 2015). 

26 Based on all of these considerations, and taking into consideration CASAC advice, the 

27 Administrator concluded that the protection afforded by the then-current standard was not 

28 sufficient and that the standard needed to be revised to provide additional protection from known 

29 and anticipated adverse effects to public welfare, related to effects on sensitive vegetation and 

30 ecosystems, most particularly those occurring in Class I areas, and also in other areas set aside by 

31 states, tribes and public interest groups to provide similar benefits to the public welfare for 

32 residents on those lands, as well as for visitors to those areas. In so doing, she further noted that a 

33 revised standard would provide increased protection for other growth-related effects, including 

34 for crop yield loss, reduced carbon storage and for areas for which it is more difficult to 

35 determine public welfare significance, as well as for other welfare effects of O3, such as visible 

36 foliar injury (80 FR 65390, October 26, 2015).
October 2018 3-31 Draft – Do Not Quote or Cite

1 3.2.2.2 Considering Revisions to the Standard 

2 Consistent with the approach employed for considering the adequacy of the then-current 

3 secondary standard, the approach for considering revisions that would result in a standard 

4 providing the requisite protection under the Act also focused on growth-related effects of O3,

5 using RBL as a surrogate for the broad array of vegetation-related effects and included 

6 judgments on the magnitude of such effects that would contribute to public welfare impacts of 

7 concern. In considering the adequacy of potential alternative standards to provide protection 

8 from such effects, the approach also focused on considering the cumulative seasonal O3

9 exposures likely to occur with different alternative standards. 

10 In light of the judicial remand of the 2008 secondary O3 standard referenced above, the 

11 2015 decision on selection of a revised secondary standard first considered the available 

12 evidence and quantitative analyses in the context of an approach for considering and identifying 

13 public welfare objectives for such a standard (80 FR 65403-65408, October 26, 2015). The 

14 robust and longstanding evidence of O3 effects on vegetation and associated terrestrial 

15 ecosystems, including evidence newly available in the 2015 review, provided the foundation for 

16 the Administrator’s consideration of O3 effects, associated public welfare protection objectives, 

17 and the revisions to the standard needed to achieve those objectives. In light of the extensive 

18 evidence base in this regard, the Administrator focused on protection against adverse public 

19 welfare effects of O3 related effects on vegetation. In so doing, she took note of effects that 

20 compromise plant function and productivity, with associated effects on ecosystems. She had 

21 particular concern about such effects in natural ecosystems, such as those in areas with 

22 protection designated by Congress for current and future generations, as well as areas similarly 

23 set aside by states, tribes and public interest groups with the intention of providing similar 

24 benefits to the public welfare. The Administrator additionally recognized that providing 

25 protection for this purpose will also provide a level of protection for other vegetation that is used 

26 by the public and potentially affected by O3 including timber, produce grown for consumption 

27 and horticultural plants used for landscaping (80 FR 65403, October 26, 2015). 

28 As an initial matter, the Administrator considered the use of a cumulative seasonal 

29 exposure index for purposes of assessing potential public welfare risks, and similarly, for 

30 assessing potential protection achieved against such risks on a national scale. In consideration of 

31 conclusions of the 2013 ISA and 2014 PA, as well as advice from CASAC and public comments, 

32 the focus was on a W126 index described as a maximum 3-month, 12-hour index, defined by the 

33 3-consecutive-month period within the O3 season with the maximum sum of W126-weighted 

34 hourly O3 concentrations during the period from 8:00 a.m. to 8:00 p.m. each day (80 FR 65404, 

35 October 26, 2015). While recognizing that no one definition of an exposure metric used for the 

36 assessment of protection for multiple effects at a national scale will be exactly tailored to every 
October 2018 3-32 Draft – Do Not Quote or Cite

1 species or each vegetation type, ecosystem and region of the country, the Administrator judged 

2 that on balance, a W126 index derived in this way, and averaged over three years would be 

3 appropriate for such purposes. Thus, in considering revisions to the secondary standard that 

4 would specify a level of air quality to provide the necessary public welfare protection, the 

5 Administrator focused on use of a cumulative seasonal concentration-weighted exposure index, 

6 including specifically the W126 index, for assessing exposure, both for making judgments with 

7 regard to the potential harm to public welfare posed by conditions allowed by various levels of 

8 air quality and for making the associated judgments regarding the appropriate degree of 

9 protection against such potential harm (80 FR 65403, October 26, 2015). 

10 Based on a number of considerations, the Administrator recognized greater confidence in 

11 judgments related to public welfare impacts based on a 3-year average metric than a single year 

12 metric, and consequently concluded it to be appropriate to use an index averaged across three 

13 years for judging public welfare protection afforded by a revised secondary standard (80 FR 

14 65404, October 26, 2015). For example, while recognizing that the scientific evidence 

15 documents the effects on vegetation resulting from individual growing season exposures of 

16 specific magnitude, including those that can affect the vegetation in subsequent years, the 

17 Administrator was also mindful of both the strengths and limitations of the evidence and of the 

18 information on which to base her judgments with regard to adversity of effects on the public 

19 welfare. In this regard, she recognized uncertainties associated with interpretation of the public 

20 welfare significance of effects resulting from a single-year exposure, and that the public welfare 

21 significance of effects associated with multiple years of critical exposures are potentially greater 

22 than those associated with a single year of such exposure. While recognizing the potential for 

23 effects on vegetation associated with a single-year exposure, the Administrator concluded that 

24 use of a 3-year average metric can address the potential for adverse effects to public welfare that 

25 may relate to shorter exposure periods, including a single year (80 FR 65404, October 26, 

2015).69 26 

27 In reaching a conclusion on the amount of public welfare protection from the presence of 

28 O3 in ambient air that is appropriate to be afforded by a revised secondary standard, the 

29 Administrator gave particular consideration to the following: (1) the nature and degree of effects 

 

69 While the Administrator recognized the scientific information and interpretations, as well as CASAC advice, with 

regard to a single-year exposure index, she also took note of uncertainties associated with judging the degree of 

vegetation impacts for annual effects that would be adverse to public welfare. It was noted that even in the case of 

annual crops, the assessment of public welfare significance is unclear due to the role of crop management and 

related agricultural practices. The Administrator was also mindful of the variability in ambient air O3

concentrations from year to year, as well as year-to-year variability in environmental factors, including rainfall 

and other meteorological factors, that influence the occurrence and magnitude of O3-related effects in any year, 

and contribute uncertainties to interpretation of the potential for harm to public welfare over the longer term (80 

FR 65404, October 26, 2015). 
October 2018 3-33 Draft – Do Not Quote or Cite

1 of O3 on vegetation, including her judgments as to what constitutes an adverse effect to the 

2 public welfare; (2) the strengths and limitations of the available and relevant information; (3) 

3 comments from the public on the Administrator’s proposed decision, including comments related 

4 to identification of a target level of protection; and (4) CASAC’s views regarding the strength of 

5 the evidence and its adequacy to inform judgments on public welfare protection. The 

6 Administrator recognized that such judgments include judgments about the interpretation of the 

7 evidence and other information, such as the quantitative analyses of air quality monitoring, 

8 exposure and risk. She also recognized that such judgments should neither overstate nor 

9 understate the strengths and limitations of the evidence and information nor the appropriate 

10 inferences to be drawn as to risks to public welfare. It was also noted that the CAA does not 

11 require that a secondary standard be protective of all effects associated with a pollutant in the 

12 ambient air but rather those known or anticipated effects judged adverse to the public welfare. 

13 She additionally recognized that the choice of the appropriate level of protection is a public 

14 welfare policy judgment entrusted to the Administrator under the CAA taking into account both 

15 the available evidence and the uncertainties (80 FR 65404-05, October 26, 2015). 

16 With regard to the extensive evidence of welfare effects of O3, including the established 

17 evidence base regarding O3 and visible foliar injury, in addition to the long-standing evidence 

18 base on O3-attributable crop yield loss, the information available for forest tree species was 

19 judged to be more useful in informing judgments regarding the nature and severity of effects 

20 associated with different air quality conditions and associated public welfare significance. 

21 Accordingly, the Administrator gave particular attention to the effects related to native tree 

22 growth and productivity, recognizing their relationship to a range of ecosystem services, 

23 including forest and forest community composition (80 FR 65405-06, October 26, 2015). 

24 In so doing, the Administrator recognized that the robust evidence base documented a 

25 broad array of O3-induced vegetation effects, among which were the occurrence of visible foliar 

26 injury and growth and/or yield loss in O3-sensitive annual and perennial species, including crops 

27 and other commercial species, such as timber, horticultural and landscaping plants, as well as 

28 native species in unmanaged natural areas (80 FR 65405, October 26, 2015). In regard to visible 

29 foliar injury, as stated in the 2013 ISA, “[e]xperimental evidence has clearly established a 

30 consistent association of visible injury with O3 exposure, with greater exposure often resulting in 

31 greater and more prevalent injury” (2013 ISA, p. 9–41). The Administrator recognized the 

32 potential for this effect to affect the public welfare in the context of affecting values pertaining to 

33 natural forests, particularly those afforded special government protection, with the significance 

34 of O3-induced visible foliar injury depending on the extent and severity of the injury (80 FR 

35 65407, October 26, 2015). In so doing, however, the Administrator also took note of limitations 

36 in the available visible foliar injury information, including the lack of robust E-R functions that 
October 2018 3-34 Draft – Do Not Quote or Cite

1 would allow prediction of visible foliar injury severity and incidence under varying air quality 

2 and environmental conditions, a lack of clear quantitative relationships linking visible foliar 

3 injury with other O3-induced vegetation effects, such as growth or related ecosystem effects, and 

4 a lack of established criteria or objectives that might inform consideration of potential public 

5 welfare impacts related to this vegetation effect (80 FR 65407, October 26, 2015). Similarly, 

6 while O3-related growth effects on agricultural and commodity crops had been extensively 

7 studied and robust E-R functions developed for a number of species, the Administrator found 

8 this information less useful in informing her judgments regarding an appropriate level of public 

welfare protection (80 FR 65405, October 26, 2015).70 9 

10 Thus, and in light of the extensive evidence base in this regard, the Administrator focused 

11 on trees and associated ecosystems in identifying the appropriate level of protection for the 

12 secondary standard. Accordingly, the Administrator found the estimates of tree seedling growth 

13 impacts (in terms of RBL) associated with a range of W126-based index values developed from 

14 the robust E-R functions for 11 tree species to be appropriate and useful for considering the 

15 appropriate public welfare protection objective for a revised standard (80 FR 65391-92, Table 4, 

16 October 26, 2015). The Administrator also incorporated into her considerations the broader 

17 evidence base associated with forest tree seedling biomass loss, including other less quantifiable 

18 effects of potentially greater public welfare significance. That is, in drawing on these RBL 

19 estimates, the Administrator recognized she was not simply making judgments about a specific 

20 magnitude of growth effect in seedlings that would be acceptable or unacceptable in the natural 

21 environment. Rather, though mindful of associated uncertainties, the Administrator used the 

22 RBL estimates as a surrogate or proxy for consideration of the broader array of related 

23 vegetation and ecosystem effects of potential public welfare significance that include effects on 

24 growth of individual sensitive species and extend to ecosystem-level effects, such as community 

25 composition in natural forests, particularly in protected public lands, as well as forest 

26 productivity (80 FR 65406, October 26, 2015). 

27 Thus, the Administrator used the RBL estimates as a proxy for the array of vegetation28 related effects, including those for which public welfare implications are more significant but for 

29 which the tools for quantitative estimates were more uncertain. In so doing, the Administrator 

 

70 With respect to commercial production of commodities, the Administrator noted that judgments about the extent 

to which O3-related effects on commercially managed vegetation are adverse from a public welfare perspective 

are particularly difficult to reach, given that the extensive management of such vegetation (which, as the CASAC 

noted, may reduce yield variability) may also to some degree mitigate potential O3-related effects. The 

management practices used on these lands are highly variable and are designed to achieve optimal yields, taking 

into consideration various environmental conditions. In addition, changes in yield of commercial crops and 

commercial commodities, such as timber, may affect producers and consumers differently, further complicating 

the question of assessing overall public welfare impacts (80 FR 65405, October 26, 2015). 
October 2018 3-35 Draft – Do Not Quote or Cite

1 recognized that the CASAC gave weight to these relationships in formulating its advice and she 

2 took particular note of the characterization by the CASAC of the 6% RBL level in the median 

3 studied species as “unacceptably high,” as this comment was provided in the context of the 

4 CASAC’s consideration of the significance of effects associated with a range of alternatives for 

5 the secondary standard (Frey, 2014c, pp. iii, 13, 14; 80 FR 65406, October 26, 2015). Moreover, 

6 the range recommended by the CASAC excluded W126 index values for which the median 

7 species was estimated to have a 6% RBL in the draft PA (which was the context for the CASAC 

8 advice) (Frey, 2014c, p. 12-13; 80 FR 65406, October 26, 2015). In consideration of CASAC 

9 advice; strengths, limitations and uncertainties in the evidence; and the linkages of growth effects 

10 to larger population, community and ecosystem impacts, the Administrator considered it 

11 appropriate to focus on a standard that would generally limit cumulative exposures to those for 

12 which the median RBL estimate would be somewhat below 6% (80 FR 65406-07, October 26, 

13 2015). 

14 In focusing on cumulative exposures associated with a median RBL estimate somewhat 

15 below 6%, the Administrator considered the relationships between W126-based exposure and 

16 RBL in the studied species (presented in the final PA and proposal notice), noting that the 

17 median RBL estimate was 6% for a cumulative seasonal W126 exposure index of 19 ppm-hrs 

(80 FR 65391-92, Table 4, October 26, 2015).71 18 Given the information on median RBL at 

19 different W126 exposure levels, using a 3-year cumulative exposure index for assessing 

20 vegetation effects, the potential for single-season effects of concern, and CASAC comments on 

21 the appropriateness of a lower value for a 3-year average W126 index, the Administrator 

22 concluded it was appropriate to identify a standard that would restrict cumulative seasonal 

23 exposures to 17 ppm-hrs or lower, in terms of a 3-year W126 index, in nearly all instances (80 

24 FR 65407, October 26, 2015). Based on such then-current information to inform consideration of 

25 vegetation effects and their potential adversity to public welfare, the Administrator additionally 

26 judged that the RBL estimates associated with marginally higher exposures in isolated, rare 

27 instances are not indicative of effects that would be adverse to the public welfare, particularly in 

28 light of variability in the array of environmental factors that can influence O3 effects in different 

29 systems and uncertainties associated with estimates of effects associated with this magnitude of 

30 cumulative exposure in the natural environment (80 FR 65407, October 26, 2015). 

31 The Administrator’s decisions regarding the revisions to the then-current standard that 

32 would appropriately achieve these public welfare protection objectives were based on extensive 

33 air quality analyses that extended from the then most recently available data (monitoring year 

 

71 The median RBL estimate was 5.7% (which rounds to 6%) for a cumulative seasonal W126 exposure index of 18 

ppm-hrs and the median RBL estimate was 5.3% (which rounds to 5%) for 17 ppm-hrs (80 FR 65407, October 

26, 2015). 
October 2018 3-36 Draft – Do Not Quote or Cite

1 2013) back more than a decade (80 FR 65408, October 26, 2015; Wells, 2015). These analyses 

2 evaluated the cumulative seasonal exposure levels in locations meeting different alternative 

3 levels for a standard of the then-current form and averaging time, indicating reductions in 

4 cumulative exposures associated with air quality meeting lower levels of a standard of the 

5 existing form and averaging time. Based on these analyses, the Administrator judged that the 

6 desired level of public welfare protection could be achieved with a secondary standard having a 

7 revised level in combination with the existing form and averaging time (80 FR 65408, October 

8 26, 2015). 

9 The air quality analyses described the occurrences of 3-year W126 index values of 

10 various magnitudes at monitor locations where O3 concentrations met potential alternative 

11 standards defined by different levels combined with the current form and averaging time (Wells, 

12 2015). In the then-most recent period, 2011-2013, across the monitor locations meeting the then13 current standard (with a level of 75 ppb), the 3-year W126 index values were above 17 ppm-hrs 

14 in 25 sites distributed across different NOAA climatic regions, and above 19 ppm-hr at nearly 

15 half of these sites, with some well above. In comparison, among sites meeting an alternative 

16 standard of 70 ppb, there were no occurrences of a W126 value above 17 ppm-hrs and fewer than 

a handful of occurrences that equaled 17 ppm-hrs.72 17 For the longer time period (extending back 

18 to 2001), among the nearly 4000 locations meeting a standard level of 70 ppb, there was only a 

19 handful of isolated occurrences of 3-year W126 index values above 17 ppm-hrs, all but one of 

20 which were below 19 ppm-hrs. 73 The Administrator concluded that that single higher value of

21 19.1 ppm-hrs, observed at a monitor for the 3-year period of 2006-2008, was reasonably 

22 regarded as an extremely rare and isolated occurrence, and, as such, it was unclear whether it 

23 would recur, particularly as areas across U.S. took further steps to reduce O3 to meet revised 

24 primary and secondary standards. Further, based on all of the then available information, as 

25 noted above, the Administrator did not judge RBL estimates associated with marginally higher 

26 exposures in isolated, rare instances to be indicative of adverse effects to the public welfare. The 

27 Administrator concluded that a standard with a level of 70 ppb and the current form and 

28 averaging time may be expected to limit cumulative exposures, in terms of a 3-year average 

29 W126 exposure index, to values at or below 17 ppm-hrs, in nearly all instances, and accordingly, 

30 to eliminate or virtually eliminate cumulative exposures associated with a median RBL of 6% or 

 

72 The more than 500 monitors that would meet an alternative standard of 70 ppb during the 2011-2013 period were 

distributed across all nine NOAA climatic regions and 46 of the 50 states (Wells, 2015 and associated dataset in 

the docket [document identifier, EPA-HQ-OAR-2008-0699-4325]). 

73 Among sites meeting a level of 65 ppb, there were no occurrences above 11 ppm-hrs, well below the objectives 

identified for affording public welfare protection. For this level, the appreciably smaller and less geographically 

extensive database contributes uncertainty to conclusions based on such analysis (80 FR 65409, October 26, 

2015). 
October 2018 3-37 Draft – Do Not Quote or Cite

1 greater (80 FR 65409, October 26, 2015). Thus, using RBL as a proxy in judging effects to 

2 public welfare, the Administrator judged that a standard with a level of 70 ppb would provide the 

3 requisite protection from adverse effects to public welfare by limiting cumulative seasonal 

4 exposures to 17 ppm-hrs or lower, in terms of a 3-year W126 index, in nearly all instances. 

5 In summary, the Administrator judged that the revised standard would protect natural 

6 forests in Class I and other similarly protected areas against an array of adverse vegetation 

7 effects, most notably including those related to effects on growth and productivity in sensitive 

8 tree species. The Administrator additionally judged that a revised standard set at a level of 70 

9 ppb would be sufficient to protect public welfare from known or anticipated adverse effects. This 

10 judgment by the Administrator appropriately recognized that the CAA does not require that 

11 standards be set at a zero-risk level, but rather at a level that reduces risk sufficiently so as to 

12 protect the public welfare from known or anticipated adverse effects. Thus, based on the 

13 conclusions drawn from the air quality analyses which demonstrated a strong, positive 

14 relationship between the 8-hour and W126 metrics and the findings that indicated the significant 

15 amount of control provided by the fourth-high metric, the evidence base of O3 effects on 

16 vegetation and her public welfare policy judgments, as well as public comments and CASAC 

17 advice, the Administrator decided to retain the existing form and averaging time and revise the 

18 level to 0.070 ppm, judging that such a standard would provide the requisite protection to the 

19 public welfare from any known or anticipated adverse effects associated with the presence of O3

20 in ambient air (80 FR 65409-10, October 26, 2015). 
October 2018 4-1 Draft – Do Not Quote or Cite

1 4 SCIENCE ASSESSMENT 

2 Integrated Science Assessments serve as the scientific foundation of the NAAQS review 

3 process and are developed by the EPA’s NCEA. This assessment focuses on reviewing and 

4 updating the air quality criteria associated with primary (health-based) and secondary (welfarebased74 5 ) effects evidence to inform science policy judgments about the primary and secondary 

6 standards for O3 and other photochemical oxidants. This chapter provides an overview of the ISA 

7 development process and discusses key aspects of the EPA’s planned approach for the ISA in 

8 this review. 

9 4.1 PURPOSE OF THE ISA 

10 The purpose of the ISA is to draw upon the existing body of evidence to synthesize and 

11 provide a critical evaluation of the current state of scientific knowledge on the most relevant 

12 issues pertinent to the review of the NAAQS for O3 and other photochemical oxidants, to 

13 identify changes in the scientific evidence bases since the previous review, and to describe 

14 remaining or newly identified uncertainties. The ISA will identify, critically evaluate and 

15 synthesize the most policy-relevant current scientific literature (e.g., epidemiology, controlled 

16 human exposure, animal toxicology, atmospheric science, exposure science, ecology and 

17 climate-related science), including key science judgments that are important to inform the 

18 development of risk and exposure analyses (as warranted) and the PA, as well as other aspects of 

19 the NAAQS review process (summarized in section 1.2 above). The ISA will provide a focused 

20 assessment of the scientific evidence to address specific scientific questions (section 4.4) and 

21 inform the overall policy-relevant questions for the PA (as described in Chapter 3). 

22 4.2 ORGANIZATION OF THE ISA 

23 In planning for the ISA, the general organization of the ISA for the current review will be 

consistent with that being used in the 2nd 24 External Review Draft ISA for Oxides of Nitrogen, 

25 Oxides of Sulfur, and Particulate Matter-Ecological Criteria (U.S. EPA, 2018d). Accordingly, 

26 the ISA will begin with a Preface discussing major legal and historical aspects of prior O3

27 NAAQS reviews. An executive summary targeted to a wide range of audiences will succinctly 

28 summarize the conclusions of the ISA. An integrated synthesis will serve as the main body of the 

29 ISA and provide a detailed summary of the key information for each topic area, including 

 

74 Under Clean Air Act, section 302(h) (42 U.S.C. § 7602(h)), effects on welfare 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.” 
October 2018 4-2 Draft – Do Not Quote or Cite

1 background concentrations of O3 in the U.S., conclusions regarding the nature of health and 

2 welfare effects associated with O3 exposure (including causality determinations for relationships 

3 between exposure to O3 and specific types of health and welfare effects), and identification of the 

4 human lifestages and populations at increased risk of the effects of O3. The integrated synthesis 

5 will discuss additional policy-relevant issues, such as the exposure durations, metrics, and 

6 concentrations eliciting health and welfare effects; the concentration-response relationships for 

7 specific effects, including the overall shape and whether or not there is evidence of a discernible 

8 threshold below which effects are not likely to occur; and the public health and welfare impact of 

9 effects associated with exposure to O3. The synthesis will also discuss important issues for 

10 different types of studies, such as the air quality metrics and the lag structure of epidemiologic 

11 associations with health effects. Subsequent appendices will be organized by subject area, with 

12 the detailed assessment of atmospheric science, exposure, health, and welfare evidence presented 

13 in separate appendices. Thus, the integrated synthesis focus will make the ISA more concise than 

14 in the past, improve its clarify and also its focus on policy-relevant scientific information and 

15 analysis; the ISA scope, as addressed in section 4.3.2 is also more focused than in past ISAs 

16 (e.g., as discussed in Pruitt [2018]). Each of the appendices will contain an evaluation of results 

17 from recent studies integrated with previous findings (see section 4.4 for specific issues to be 

18 addressed). Appendices for each broad health effect category (e.g., respiratory effects) will 

19 conclude with a causal determination describing the strength of the evidence between exposure 

20 to O3 and the health effect(s) [more detail on the types of causal determinations applied in the 

21 ISA is given in the Preamble to the ISAs (U.S. EPA, 2015c) and in section 4.3.6 and figure 4-1 

22 of this chapter]. Likewise, the appendices devoted to ecology and climate evidence for welfare 

23 effects will conclude with causality determinations for multiple effects on ecosystems and 

24 climate, respectively. 

25 4.3 ASSESSMENT APPROACH 

26 4.3.1 Introduction 

27 In developing ISAs, the EPA employs systematic review methodologies to identify and 

28 evaluate relevant scientific information and produces summary text and figures to communicate 

29 the state of the science to varied audiences. The process begins with a “Call for Information” 

30 published in the Federal Register that announces the start of a NAAQS review and invites the 

31 public to assist in this process through the submission of research studies in identified subject 

32 areas. For the O3 NAAQS review, this notice was published on June 26, 2018 (83 FR 29785). 

33 The subsequent ISA development steps are generally presented in Figure 4-1 and are described 

34 in greater detail in the Preamble to the Integrated Science Assessments (U.S. EPA, 2015c), 

35 which provides a general overview of the ISA development process. The plan for developing the 
October 2018 4-3 Draft – Do Not Quote or Cite

1 ISA for the current review is described in detail in the following sections. The process for 

2 review of the draft ISA is described in Section 4.5. 

3 

4 Source: Modified from Figure II of the Preamble to the Integrated Science Assessments (U.S. EPA, 2015c). 

5 Figure 4-1. General process for development of Integrated Science Assessments. 
October 2018 4-4 Draft – Do Not Quote or Cite

1 The ISA is developed by authors who are EPA staff in NCEA with extensive knowledge 

2 in their respective fields and extramural scientists who are solicited by the EPA for their subject 

3 matter expertise. The ISA authors apply systematic review methodologies to identify relevant 

4 scientific findings that have emerged since the previous assessment. The process is further 

5 described in sections below, including clear definition of the scope (Section 4.3.2), literature 

6 search and identification of relevant studies (Section 4.3.3), evaluation of individual study 

7 quality (Section 4.3.5), evaluation of relevant studies (Section 4.3.6) and evidence integration 

8 and determination of causality (Section 4.3.7). 

9 4.3.2 Scope of the ISA 

10 Through iterative reviews, ISAs build on the data and conclusions of previous 

11 assessments. The previous O3 ISA was published in 2013 (U.S. EPA, 2013) and included peer12 reviewed literature published through July 2011. The ISA for the current review will identify and 

13 evaluate studies published since 2011, synthesizing and integrating the new evidence in the 

14 context of the conclusions from the previous review. Key findings, conclusions, and 

15 uncertainties from the 2013 ISA will be briefly summarized at the beginning of individual 

16 sections. Important older studies may be discussed to reinforce key concepts and conclusions. 

17 Older studies also may be the primary focus in some subject areas or scientific disciplines where 

18 research efforts have subsided, and these older studies remain the definitive works available in 

19 the literature. 

20 Scientific information will be identified and evaluated in order to provide a better 

21 understanding of the following issues: (1) the natural and anthropogenic sources of O3 precursors 

22 in the ambient air; (2) formation, transport, and fate of O3 in the environment; (3) measurement 

23 methods and ambient concentrations of O3; (4) how exposure assessment methods used in 

24 epidemiologic studies can influence inferences drawn about O3 health effects; (5) the 

25 independent effect of O3 exposure on health and welfare; (6) the potential influence of other 

26 factors (e.g., other pollutants in the ambient air, ambient air temperature) shown to be correlated 

27 with O3 and health or welfare effects; (7) the shape of the concentration-response relationship at 

28 O3 concentrations at the low end of the distribution; and (8) populations and lifestages at 

increased risk of O3-related health effects75 29 . 

 

75 The focus of the ISA is on O3 since it is the NAAQS indicator for all photochemical oxidants. Ozone is the only 

photochemical oxidant other than nitrogen dioxide (NO2) that is routinely monitored and for which a 

comprehensive database exists. Data for other photochemical oxidants (e.g., PAN, H2O2, etc.) typically have been 

obtained only as part of special field studies. Consequently, no data on nationwide patterns of occurrence are 

available for these other oxidants; nor are extensive data available on the relationships of concentrations and 

patterns of these oxidants to those of O3. As a result, this review focuses on O3, the NAAQS indicator for 

photochemical oxidants. 
October 2018 4-5 Draft – Do Not Quote or Cite

1 In the 2013 ISA, evidence from across scientific disciplines for related health and welfare 

2 effects was evaluated, synthesized, and integrated to develop conclusions and causality 

3 determinations. As described in the Preamble to the ISAs (U.S. EPA, 2015c) and in section 4.3.6 

4 and figure 4-1 of this chapter, the EPA uses a structured framework to provide a consistent and 

5 transparent basis for classifying the weight of available evidence for health and welfare effects 

6 according to a five-level hierarchy: (1) causal relationship; (2) likely to be a causal relationship; 

7 (3) suggestive of, but not sufficient to infer, a causal relationship; (4) inadequate to infer the 

8 presence or absence of a causal relationship; and (5) not likely to be a causal relationship. This 

9 framework will be applied in the ISA for the current review. 

10 In this review, the EPA will more fully evaluate those health and welfare effects for 

11 which the evidence in the 2013 ISA was less certain (i.e., effects where the causality 

12 determination was “likely to be causal”, “suggestive”, or “inadequate” as described in section 

13 4.4.1) and where there is now a larger body of evidence. In doing so, the EPA is addressing 

14 uncertainties and limitations in the evidence identified in the prior review. 

15 For those health and welfare effects for which the 2013 ISA concluded that the evidence 

16 was sufficient to infer a causal relationship (i.e., for the health evidence: short-term O3

17 exposures [i.e., days to weeks] and respiratory effects; and for the welfare evidence: O3

18 exposures and ecological effects and effects on climate), the ISA for the current review will 

19 integrate and synthesize the new evidence, placing emphasis on policy-relevant considerations, 

20 such as the concentrations at which effects are observed, and characterizing the extent to which 

21 new studies address key uncertainties and limitations identified in the previous review or provide 

22 insight on new issues. 

23 The scope of the health and welfare portions of the ISA is further refined by scoping 

24 statements that generally define the relevant Population, Exposure, Comparison, Outcome, and 

25 Study Design (PECOS) (Complete PECOS statements provided in Section 4.3.3). The PECOS 

26 statements characterize the parameters and provide a framework to aid in identifying the relevant 

27 evidence in the literature to inform the ISA. There are discipline-specific PECOS statements for 

28 experimental studies, epidemiology, ecology and effects of tropospheric ozone on climate, which 

29 differ depending on the types of questions to be answered and are influenced by a priori 

30 knowledge related to that question. The use of PECOS statements is a widely accepted and 

31 rapidly growing approach to systematic review in risk assessment, and consistent with 

32 recommendations by the National Academy of Sciences for improving the design of risk 

33 assessment through planning, scoping, and problem formulation to better meet the needs of 

34 decision-makers (National Research Council 2009). The PECOS statements serve as guides for 

35 several aspects of the ISA process, including the literature search strategy, criteria for the 
October 2018 4-6 Draft – Do Not Quote or Cite

1 inclusion or exclusion of studies in the ISA, the types of data extracted from studies, and the 

2 integration and synthesis of the results. 

3 4.3.3 Literature Search and Identification of Relevant Studies 

4 4.3.3.1 Systematic Literature Search 

5 The EPA uses a structured approach to identify relevant studies for consideration and 

6 inclusion in the ISAs. The search for relevant literature in this review began with publication of 

7 the Federal Register notice announcing the initiation of this O3 review and requesting 

8 information from the public including relevant literature (83 FR 29785). In addition, the EPA 

9 identifies publications by conducting a multi-tiered systematic literature search that includes 

10 extensive mining of literature databases on specific topics in a variety of disciplines. The search 

11 strategies are designed a priori to optimize identification of pertinent published papers. Studies 

12 identified in the literature search are documented in the Health and Environmental Research 

13 Online (HERO) database. The HERO project page for this ISA 

14 (https://hero.epa.gov/hero/index.cfm/project/page/project_id/2737) will contain the references 

15 that will be considered for inclusion in the ISA and electronic links to bibliographic information 

16 and abstracts. It is accessible to the public. 

17 For this ISA, discipline-specific approaches will be used to identify literature. In each 

18 case, careful consideration will be given to literature search strategies used in the development of 

19 previous assessments and the methods that resulted in the best precision and recall for each of the 

20 disciplines, including atmospheric science (section 4.3.4.1), exposure assessment (section 

21 4.3.4.2), experimental health studies (section 4.3.4.3), epidemiology (section 4.3.4.4), ecology 

22 (section 4.3.4.5), and climate (section 4.3.4.6). The literature identification approaches include 

23 broad keyword searches in routinely used databases with Automatic Topic Classification, and 

24 citation mapping (see section 4.3.4 for specific approaches used for each discipline). 

25 As has been done for past ISAs, a broad keyword search was developed as a starting 

26 point to capture literature pertinent to the pollutant of interest. In this case, the main keyword 

27 string to be used is “ozone OR O3”, which is sufficiently broad to capture O3-relevant literature 

28 in each database (i.e., PubMed, Web of Science, TOXLINE). Following the broad keyword 

29 search for O3, automatic topic classification will be used to categorize references by discipline 

30 (e.g., epidemiology, toxicology, etc.). This step employs machine learning where positive and 

negative seed references76 31 for a particular discipline are used to train an algorithm to identify 

 

76 Positive seed references are those that are examples of references that are relevant, i.e., the references would be 

selected for full-text screening. Negative seed references are those that are examples of references that are not 

relevant, i.e., they would not be selected for full-text screening. For ISAs, the positive seed set includes references 

from the prior ISA for the discipline of interest. The negative seed set includes the references from all of the other 

disciplines in the prior ISA. 
October 2018 4-7 Draft – Do Not Quote or Cite

1 discipline specific references based on word use and frequency in titles and abstracts. This 

2 method varies in effectiveness across disciplines due to the broad range of topics and variability 

3 in term usage in some evidence bases. However, it is invaluable when effective, and has been 

4 used in several prior ISAs. 

5 Another approach used in past ISAs that will be employed in this review is citation 

6 mapping, or relational reference searching. In this approach, a set of relevant published 

7 references are identified as a seed set and then more recent literature that has cited any of the 

8 references in the seed set are collected. References from the previous ISA for the respective 

9 pollutant comprise the seed set for the new ISA. Because the seed set is highly relevant to the 

10 topic of interest, this targeted approach to reference identification is more precise than keyword 

11 searches, and it further allows for relevance ranking based on the number of references in a 

12 bibliography that match references in the seed set. 

13 References may be identified for inclusion in several additional ways including: 

14 identification of relevant literature by NCEA expert scientists; recommendations received in 

15 response to the call for information and the external review process for the ISA; and review of 

16 citations included in previous assessments. 

17 All these search methods will be used to identify recent research published or accepted 

18 for publication starting January 1, 2011, providing some overlap with the July 2011 cutoff date 

19 from the last review. Studies published after the literature cutoff date for this review (i.e., March 

20 30, 2018) may also be considered in subsequent phases of the NAAQS review (e.g., studies 

21 identified by CASAC members during review of the draft ISA), after assessing whether they 

22 provide new information that impacts key scientific conclusions. 

23 4.3.3.2 Initial Screening (Level 1) of Studies from Literature Search 

24 Once studies are identified, ISA authors (EPA staff and extramural scientists) will review 

25 the studies for relevance. For the primary O3 NAAQS, relevant studies include epidemiologic, 

26 toxicological, and controlled human exposure studies, including studies of dosimetry and mode 

27 of action, or those that examine ambient O3 exposure assessment, atmospheric chemistry, 

28 sources and emissions. For the review of the secondary O3 NAAQS, relevant studies are those 

29 that examine ecological effects and the effects of O3 on climate. Specific information detailing 

30 the scope of the ISA for the current review, and subsequently those studies that will be evaluated 

31 within it are detailed above in section 4.3.2. 

32 As described above, the literature search methods will be targeted for discipline-relevant 

33 references to the extent possible, and the subsequent screening will result in a further refined list 

34 of references to be included in the ISA. References for each discipline will first undergo title and 

35 abstract screening using SWIFT-ActiveScreener (SWIFT-AS), which is referred to as Level 1 
October 2018 4-8 Draft – Do Not Quote or Cite

1 screening. Level 1 screening criteria for inclusion will be broad and err on the side of inclusion. 

2 For each discipline, title and abstracts will be selected for inclusion if there is indication of O3 

3 and a quantifiable effect relevant to that discipline. SWIFT-AS is a software application that 

4 employs machine learning in real-time to identify relevant literature. The machine learning 

5 feature builds a model to predict relevant references based on inclusion/exclusion screening 

6 decisions in real-time as scientists screen each reference. As title/abstract screening is conducted, 

7 references are queued based on the predicted relevance and SWIFT-AS further predicts when a 

95% recall threshold has been reached77 8 , a level often used to evaluate the performance of 

9 machine learning applications and considered comparable to human error rates (Cohen et al. 

10 2006, Howard et al. 2016). 

11 The application of SWIFT-AS will be tailored for each discipline. This will include using 

12 a specific seed set of 50-100 relevant references from the 2013 ISA to train the SWIFT-AS 

13 algorithm and developing specific screening questions for each discipline to allow for the 

14 categorization of references based on the information available in the title and abstract. 

15 Understanding the volume and topics of the recent literature on ambient O3 will be important 

16 information to consider in refining the scope of the ISA. Specific details about 

17 inclusion/exclusion criteria and the screening questions for each discipline are described in more 

18 detail below. 

19 Following Level 1 screening, references identified for inclusion will be acquired and 

20 compiled in HERO for full-text Level 2 screening conducted by NCEA subject matter experts. 

21 The Level 2 screening decisions for each discipline will be based on the scoping decisions (see 

22 section 4.3.4). References will be tracked for both relevance to the broad ISA and for the defined 

23 scope for each topic area (e.g., outcome category). 

24 4.3.3.3 Criteria of In-Scope Studies 

25 To be included in the ISA, relevant studies and reports must have undergone scientific 

26 peer review and have been published or accepted for publication before the cutoff date. Some 

27 publications retrieved from the literature search will be excluded as not being relevant in Level 1 

28 screening based on the title/abstract (e.g., not about air pollution, conference abstract, review 

29 articles, commentaries). For other publications, decisions about relevance will be made in Level 

30 2 screening as they require reading beyond the title. These publications will be labeled as 

31 “considered” for inclusion in the ISA. Inclusion and exclusion decisions will be documented in 

32 the HERO database (https://hero.epa.gov/hero/index.cfm/project/page/project_id/2737). 

 

77 A 95% recall threshold represents the point at which 95% of the potentially relevant references have been 

identified. 
October 2018 4-9 Draft – Do Not Quote or Cite

1 4.3.4 Discipline-Specific Scoping, Searching and Screening 

2 4.3.4.1 Atmospheric Science 

3 4.3.4.1.1 Scope 

4 The ISA will present and evaluate relevant data and summarize scientific understanding 

5 from previous reviews and new evidence that has emerged since the 2013 ISA concerning the 

6 sources and ambient concentrations of O3 in the lower troposphere and surface boundary 

7 layer. O3 present in the lower troposphere is predominantly formed through photochemical 

8 reaction between oxides of nitrogen (NOx) and volatile organic precursor gases. This ISA 

9 discussion will be divided into two sections: one discussing O3 that would be present in the 

10 lower atmosphere in the absence of any manmade emissions in the U.S. (i.e., O3 that has been 

11 transported across international boundaries, produced by natural processes such as lightning or 

12 drawn down from the stratosphere, or forms from natural or transported precursors), referred to 

13 as "U.S. background" O3; the other discussing O3 that results from U.S. anthropogenic 

14 emissions. 

15 4.3.4.1.2 Search and Screen 

16 Literature related to atmospheric science topics will be identified by citation mapping 

17 methods that will rely upon references cited in the 2013 ISA. More specifically, references will 

18 be collected from the atmospheric science sections of the 2013 ISA, including sub-topics on 

19 physical and chemical processes, atmospheric modeling, monitoring, and background O3

20 concentrations. Citation mapping will be conducted in Web of Science and will be done 

21 separately for each sub-topic to retain flexibility in the references to be to be screened and 

22 included in the ISA. While the focus for evaluation of the recent literature will be on background 

23 concentration of O3 in ambient air, other literature identified from citation mapping will be 

24 considered for inclusion. 

25 4.3.4.2 Exposure Assessment 

26 4.3.4.2.1 Scope 

27 The ISA will describe the commonly employed exposure assessment methods in the 

28 epidemiologic evidence, including strengths and limitations of the methods, study designs in 

29 which those methods are used, and how errors and uncertainties inherent in those methods 

30 influence the bias and precision of health effect estimates for short-term and long-term O3

31 exposure studies. The exposure assessment appendix includes a summary table that describes 

32 each method, how it is used in epidemiologic studies, and how strengths and limitations of each 

33 method may impact interpretation of the epidemiologic results. 
October 2018 4-10 Draft – Do Not Quote or Cite

1 4.3.4.2.2 Search and Screen 

2 Exposure literature relevant to O3 will be identified using the broad keyword search 

3 described in Section 4.3.2 and Automatic Topic Classification. Automatic Topic Classification 

4 for exposure references will include a sufficiently large set of positive and negative seeds from 

5 previous ISAs. More specifically, positive seeds will include references from the exposure 

chapter from the 2016 NOx ISA78 6 and the 2013 ISA; the negative seeds will include non-relevant

7 references (i.e., those from other disciplines in these two ISAs). Following identification and 

8 binning of the literature, SWIFT-AS will be used for Level 1 screening. Positive seeds to train 

9 the SWIFT-AS algorithm will include a subset of the exposure references cited in the 2013 ISA. 

10 Additionally, references will be categorized in Level 1 screening in SWIFT-AS by study type, 

11 study location, and exposure duration. The references identified for inclusion in Level 1 will then 

12 undergo Level 2 full-text screening. 

13 4.3.4.3 Health – Experimental Studies 

14 4.3.4.3.1 Scope 

15 For experimental studies, specifically controlled human or animal exposure studies, the 

16 evaluation will focus on those studies that also address key uncertainties and limitations in the 

17 evidence identified in the previous review. For example, does the new evidence advance 

18 understanding of the biological mechanisms by which O3 elicits a health effect or provide 

19 coherence for the effects assessed in epidemiologic studies? The scope of the experimental 

20 evidence encompasses studies of short-term (i.e., hours to weeks) and long-term (i.e., months to 

21 years) exposures conducted at concentrations of O3 that are relevant to the range of human 

22 exposures to ambient air (up to 2 ppm, which is one to two orders of magnitude above ambient 

23 concentrations) (Table 4-1). 

24 

 

78 The 2016 NOX ISA is the most recent ISA that had the appropriate level of detail comparative to what is needed 

for this current review. 
October 2018 4-11 Draft – Do Not Quote or Cite

1 Table 4-1. PECOS statements for experimental studies. 

Exposure Duration and Health 

Effect 

Population, Exposure, Comparison, Outcome, Study Design (PECOS)

Statements 

Short-term exposure and 

respiratory, cardiovascular, 

metabolic, nervous system, 

reproductive or developmental 

effects 

Among the study population of any controlled human exposure or animal 

toxicological study of mammals at any lifestage (P), of interest are the 

studies of the relationship between short-term (in the order of minutes to 

weeks) inhalation exposure to relevant O3 concentrations (i.e., 0.4 ppm or 

below for humans, 2 ppm or below for other mammals) (E) and respiratory, 

cardiovascular, metabolic, nervous system, reproductive or developmental 

effects (O) when human subjects serve as their own controls with an 

appropriate washout period or when comparison to a reference population 

exposed to lower levels is available, or, in toxicological studies of mammals, 

an appropriate comparison group is exposed to a negative control (i.e., clean 

air or filtered air control) (C).

Long-term exposure and 

respiratory, cardiovascular, 

metabolic, nervous system, 

carcinogenic, reproductive or 

developmental effects 

Among the study population of any animal toxicological study of mammals 

at any lifestage (P), of interest are the studies of the relationship between

long-term (in the order of months to years) inhalation exposure to relevant 

O3 concentrations (i.e., 2 ppm or below) (E) and respiratory, cardiovascular, 

metabolic or nervous system, carcinogenic, reproductive or developmental 

effects (O) when an appropriate comparison group is exposed to a negative 

control (i.e., clean air or filtered air control) (C). 

2 

3 4.3.4.3.2 Search and Screen 

4 Identification of experimental (i.e., controlled human exposure and animal toxicology) 

5 studies examining the health effects of O3 exposure will be identified using the broad keyword 

6 search described in Section 4.3.2 and Automatic Topic Classification. The Automatic Topic 

7 Classification for experimental references will include a sufficiently large set of positive seeds, 

8 including controlled human exposure and animal toxicology references cited in the 2016 NOx

9 ISA and the 2013 ISA, and a sufficiently large set of negative seeds, including nonexperimental 

10 references cited in these two ISAs. Following identification of the literature, SWIFT-AS will be 

11 used for Level 1 screening. The SWIFT-AS algorithm will be trained using a set of positive seed 

12 references from a selection of controlled human exposure and animal toxicology studies cited in 

13 the 2013 ISA. Additionally, references will be categorized in Level 1 screening in SWIFT-AS by 

14 health outcome category (e.g., respiratory, cardiovascular, metabolic, etc.), exposure duration 

15 (e.g., short-term, long-term), and study type (e.g., controlled human exposure, animal toxicology, 

16 etc.). The references identified for inclusion at Level 1 will then undergo Level 2 full-text 

17 screening, for each health outcome category, for relevance to the defined scope as described 

18 above. 

19 4.3.4.4 Health – Observational (Epidemiologic) Studies 

20 4.3.4.4.1 Scope 

21 The evaluation of epidemiologic studies will focus on the associations between short- and 

22 long-term exposure to O3 and a range of health effects, including respiratory, cardiovascular, 
October 2018 4-12 Draft – Do Not Quote or Cite

1 reproductive and developmental, metabolic, and nervous system outcomes (Table 4-2). The 

2 discussion of epidemiologic results will emphasize the impact of exposure assessment techniques 

3 on associations observed; evaluating potential copollutant confounding; examining heterogeneity 

4 in O3 associations; and the shape of the concentration-response relationship. 

5 
October 2018 4-13 Draft – Do Not Quote or Cite

1 Table 4-2. PECOS statements for epidemiologic studies. 

Exposure Duration and Health Effect Population, Exposure, Comparison, Outcome, Study Design 

(PECOS) Statements 

Short-term exposure and respiratory 

effects 

In any U.S. or Canadian population, including populations or 

lifestages that might be at increased risk (P), of interest is the change 

in risk (incidence/prevalence) of respiratory effects (O) per unit 

increase (C) in ppb of short-term ambient concentration of O3 (E), 

observed in studies relevant for the health outcome and exposure 

duration of interest (S). Also of interest is the lowest concentration 

that produces a measurable change in risk. 

Short-term exposure and mortality 

In any U.S. or Canadian population, including populations or 

lifestages that might be at increased risk (P), of interest is the change 

in risk (incidence) of mortality (O) per unit increase (C) in ppb of 

short-term ambient concentration of O3 (E), observed in studies 

relevant for the health outcome and exposure duration of interest (S).

Also of interest is the lowest concentration that produces a measurable 

change in risk. 

Long-term exposure and respiratory 

effects 

In any U.S. or Canadian population, including populations or 

lifestages that might be at increased risk (P), of interest is the change 

in risk (incidence/prevalence) of respiratory effects (O) per unit 

increase (C) in ppb of long-term ambient concentration of O3 (E), 

observed in studies relevant for the health outcome and exposure 

duration of interest (S). Also of interest is the lowest concentration 

that produces a measurable change in risk. 

Short-term exposure and cardiovascular 

effects 

In any U.S., Canadian, European or Australian population, including 

populations or lifestages that might be at increased risk (P), of interest 

is the change in risk (incidence/prevalence) of cardiovascular effects 

(O) per unit increase (C) in ppb of short-term ambient concentration 

of O3 (E), observed in studies relevant for the health outcome and 

exposure duration of interest (S). Also of interest is the lowest 

concentration that produces a measurable change in risk. 

Short-term exposure and nervous 

system effects; long-term exposure and 

cardiovascular, nervous system, 

reproductive or developmental effects, 

cancer, or mortality 

In any population, including populations or lifestages that might be at 

increased risk (P), of interest is the change in risk 

(incidence/prevalence) of a health effect (O) per unit increase (C) in 

ppb of short- or long-term ambient concentration of O3 (E), observed 

in studies relevant for the health outcome and exposure duration of 

interest (S). Also of interest is the lowest concentration that produces a 

measurable change in risk. 

Population: the general population, all age groups, living both in urban and in rural areas exposed on a daily 

basis to O3 through outdoor (ambient) air, and not exclusively in occupational settings or as a result of indoor 

exposure. Populations and lifestages at increased risk are included, such as those with specific pre-existing health 

conditions (e.g. respiratory or cardiovascular diseases), children, or older adults. 

Exposure: ambient air O3 from any source measured as short-term (minutes to weeks) or long-term (months to 

years). 

Comparator: the health effect observed by unit increase in concentration of O3 in the same or in a control 

population. 

Outcome: clearly measurable health endpoint. 

Study design: epidemiologic studies on health effects of O3 consisting of cross-sectional, case-control, casecrossover, cohort, panel and time-series studies. 

2 
October 2018 4-14 Draft – Do Not Quote or Cite

1 4.3.4.4.2 Search and Screen 

2 Identification of recent epidemiologic studies examining a health effect and ambient 

3 exposure to O3 will be identified using the broad keyword search described in Section 4.3.3 and 

4 Automatic Topic Classification. The approach for Automatic Topic Classification to identify 

5 epidemiologic studies from the broad literature search results parallels the approach described in 

6 Section 4.3.4.3.2 for the experimental studies. A sufficiently large set of seed references cited in 

7 the 2016 NOx and 2013 ISAs) will be used, with positive seeds comprised of epidemiologic 

8 references in those ISAs and negative seeds comprised of all references other than epidemiologic 

9 references. Following identification of the literature, SWIFT-AS will be used for Level 1 

10 screening. Positive seeds will also be used to train the SWIFT-AS algorithm, and will include 

11 select epidemiologic references cited in the 2013 ISA. Additionally, references will be 

12 categorized in Level 1 SWIFT-AS screening by health outcome category (e.g., mortality, 

13 respiratory, cardiovascular, etc.), exposure duration (e.g., short-term, long-term), and study 

14 location (e.g., U.S., Canada, Europe, etc.). The references identified for inclusion in Level 1 

15 screening will then undergo Level 2 full-text screening, for each health effect category, for 

16 relevance to the defined scope. 

17 4.3.4.5 Welfare Effects – Ecological Studies 

18 4.3.4.5.1 Scope 

19 With respect to ecological effects, this ISA will build on information available during the 

20 last review describing the effect of O3 exposure on vegetation and ecosystems. For research 

21 evaluating ecological effects, emphasis will be placed on recent studies that: (1) evaluate effects 

22 at realistic ambient air concentrations and (2) investigate effects on cultivated and noncultivated 

23 vegetation and ecosystems that occur in the U.S. (Table 4-3). 

24 4.3.4.5.2 Search and Screen 

25 Studies relevant to the ecological effects of O3 exposure will be identified by citation 

26 mapping. The broad keyword searches and Automatic Topic Classification have not resulted in a 

27 well-targeted set of references for Level 1 screening in past ISAs for ecological endpoints. 

28 Citation mapping in Web of Science based on ecological studies cited in the 2013 ISA is 

29 expected to yield a more refined set of references. Following citation mapping, Level 1 screening 

30 of the identified references will be conducted in SWIFT-AS, including the use of a seed set of 

31 ecological references from the 2013 ISA. Screening questions to facilitate organization of the 

32 literature will include effect category (e.g., foliar injury, plant growth, biodiversity, etc.), 

33 exposure conditions, location, and ecosystem type (e.g., wetland, crop, etc.). As will be the case 

34 for the other disciplines, Level 2 full-text screening will be conducted for references included in 

35 Level 1 screening, and full-text inclusion criteria will be defined by the scope. 
October 2018 4-15 Draft – Do Not Quote or Cite

1 Table 4-3. PECOS statements for ecological studies. 

Organizing Principle  PECOS Statements 

Visible Foliar Injury; Vegetation Growth; 

Reduced Yield/Quality of Agricultural 

Crops; Reduced Productivity; Alteration 

of Below‐ground Biogeochemical Cycles  

For any individual, population (in the sense of a group of individuals 

of the same species), species, community, or ecosystem in North

America (P); of interest are the effects of ambient ozone exposures, 

or experimentally elevated ozone concentrations (within an order of 

magnitude of study‐specific ambient or control concentrations) (E);

when compared to relevant control sites, treatments, or parameters 

(C); on ecological endpoints (O); in laboratory, greenhouse, OTC, 

FACE, field, gradient, or modelling studies (S)

Alteration of Terrestrial Water Cycling; 

Reduced Carbon Sequestration; 

Alteration of Terrestrial Community 

Composition; Plant reproduction, 

phenology or mortality; Insects and 

other wildlife; Plant‐animal signaling  

For any individual, population (in the sense of a group of individuals 

of the same species), species, community, or ecosystem on any 

continent (P) of interest are the effects of ambient ozone exposures, 

or experimentally elevated ozone concentrations (within an order of 

magnitude of study‐specific ambient or control concentrations) (E); 

when compared to relevant control sites, treatments, or parameters 

(C); on ecological endpoints (O); in laboratory, greenhouse, OTC, 

FACE, field, gradient, or modelling studies (S)

Exposure indices; Ozone interactions 

with abiotic or biotic environmental 

factors 

For any individual, population, species, community, or ecosystem 

(P) of interest  are the effects of ozone in dose‐response 

relationships, exposure indices, or in interaction with other 

environmental factors (e.g., carbon dioxide, nitrogen, temperature, 

water availability) (E); when compared to relevant control sites, 

treatments, or parameters (C); on endpoints in North America (yield 

and quality of agricultural crops, foliar injury, vegetation growth and 

productivity, or below‐ground biogeochemical cycling), or global‐

scope endpoints  (terrestrial water cycling, carbon sequestration, 

terrestrial community composition, plant reproduction, phenology 

and mortality, plant‐animal signaling, and invertebrate and 

vertebrate responses) (O); in laboratory, greenhouse, OTC, FACE, 

field, gradient, or modelling studies (S)

Sub‐organismal effects of ozone 

Ozone effects on sub‐organismal endpoints such as gene expression, 

molecular or chemical composition, or physiological processes for 

any multi‐cellular organism (plant, invertebrate, vertebrate); as well 

as on plant volatile chemical emissions and plant‐plant signaling 

were not specifically reviewed. 

Population: unit of study *note this definition of population is for the purpose of applying PECOS to ecology. 

Ecological populations are defined as a group of individuals of the same species. 

Exposure: environmental variable to which population is exposed 

Comparator: change in endpoint observed by unit increase in concentration of ozone in the same or in a control 

population. 

Outcome: relevant outcomes resulting from exposure 

Study design: field, gradient, open top chamber (OTC), Free Air Carbon Enrichment (FACE), greenhouse, other 

2 

3 
October 2018 4-16 Draft – Do Not Quote or Cite

1 4.3.4.6 Welfare – Effects on Climate 

2 4.3.4.6.1 Scope 

3 For effects on climate, the ISA will focus on effects of tropospheric O3 on climate, 

4 consistent with the inclusion of “climate” in the list of effects on welfare in section 302(h) of the 

5 Clean Air Act. The ISA will not focus on downstream ecosystem effects, human health effects, 

6 or future air quality projections resulting from changes in climate. Studies that inform the 

7 independent role of O3 in climate forcing as well as effects on U.S. national and regional climate 

8 are within the scope of the literature to be considered in the review (Table 4-4). 

9 Table 4-4. PECOS statements for effects of tropospheric O3 on climate. 

Effect on Climate Population, Exposure, Comparison, Outcome, Study Design (PECOS) Statements

Changes in radiative 

forcing (RF) 

Among evaluations of radiative forcing at the regional, continental, and/or global scale 

(P), of interest is the radiative forcing (O) resulting from a given change in tropospheric 

O3 concentration (E) compared to relevant baseline or unperturbed scenarios/conditions 

(C) in observational or modelling studies (S). 

Changes in climate 

(e.g., surface 

temperature, 

hydrological cycle) 

Among evaluations of climate effects at the regional, continental, and/or global scale 

(P), of interest are the subsequent climate effects (via radiative forcing) (O) resulting 

from a given change in tropospheric O3 concentration (E) compared to relevant baseline 

or unperturbed scenarios/conditions (C) in observational or modelling studies (S).

Changes in climate 

mechanisms and 

feedbacks 

Among evaluations of related mechanisms or feedbacks at the regional, continental, 

and/or global scale (P), of interest are the mechanisms, feedbacks, and/or copollutants 

that influence the radiative forcing and/or climate effects (O) resulting from a given 

change in tropospheric O3 concentration (E) compared to relevant baseline or 

unperturbed scenarios/conditions (C) in observational or modelling studies (S). 

Population/Geographical scope: spatial extent of study *note this definition is for applying PECOS to climate. 

Exposure: environmental variable (tropospheric O3 concentrations) 

Comparator: radiative forcing or climate effects observed from unit change in tropospheric O3 concentration. 

Outcome: relevant radiative forcing or climate outcomes resulting from change in tropospheric O3. 

Study design: observations/satellite, modelling 

10 

11 4.3.4.6.2 Search and Screen 

12 Studies examining the effect of tropospheric O3 on climate will be identified in two ways. 

13 First, references will be identified by citation mapping in Web of Science using references cited 

14 in the 2013 ISA. In addition, relevant references will be identified from recent national and 

15 international climate assessments, such as the National Climate Assessment (USGCRP, 2017) 

16 and Intergovernmental Panel on Climate Change (IPCC, 2013), and other recent, more focused 

17 reports relevant to O3 climate forcing. Level 1 screening of the identified references will be 

18 conducted in SWIFT-AS aided by a seed set of select references from the climate section of the 

19 2013 ISA and screening questions to facilitate organization of the literature. The screening 
October 2018 4-17 Draft – Do Not Quote or Cite

1 questions will pertain to the following topics: radiative forcing, climate impacts, precursor and 

2 copollutant effects, and factors and feedbacks. Level 2 screening will be conducted for 

3 references included in Level 1 and full-text inclusion criteria will be defined by the scope. 

4 4.3.5 Identification of Policy-Relevant Studies 

5 From the group of “considered” references (see section 4.3.4), studies and reports will be 

6 selected for inclusion in the ISA based on review of the full text. The selection process will be 

7 based on the extent to which the study is potentially policy-relevant and informative. Potentially 

8 policy-relevant and informative studies will include those that provide a basis for or describe the 

9 relationship between exposure to O3 and effects, particularly, those studies that reduce 

10 uncertainty or address limitations of critical issues. Also pertinent are studies that offer 

11 innovation in method or design or present novel information on effects or issues previously not 

12 identified. Uncertainty can be addressed to some extent, for example, by analyses informing the 

13 independent effect of O3 on health and welfare effects, analyses of potential confounding or 

14 effect modification by co-pollutants or other factors, analyses of concentration-response or dose15 response relationships, or analyses related to time between exposure and response. In keeping 

16 with the ISA’s intent to accurately reflect the latest scientific knowledge, the focus of the 

17 discussion in the ISA will be on studies published since July 2011 (i.e., the literature cutoff date 

18 for the 2013 ISA). Building on the last review, the EPA plans to evaluate the recent evidence in 

19 the context of the conclusions from the 2013 ISA. In some cases, evidence from older studies 

20 may be the key policy-relevant information in a particular subject area or scientific discipline and 

21 will be included. Analyses conducted by the EPA using publicly available data—for example, air 

22 quality and emissions data—will also be considered for inclusion in the ISA. Informative studies 

23 will not be limited to specific study designs, model systems, or outcomes. 

24 While study quality is important, it is not the sole criteria for study inclusion. The 

25 combination of approaches described above are intended to produce a comprehensive collection 

26 of pertinent studies needed to address the key scientific issues that form the basis of the ISA. 

27 References for the included studies will be cited in the ISA with a hyperlink to the HERO 

28 database. 

29 4.3.6 Evaluation of Individual Study Quality 

30 After selecting studies for inclusion, individual study quality is evaluated by considering 

31 the design, methods, conduct, and documentation of each study, but not the study results. In the 

32 ISA for the current review, conclusions about the strength of inference from study results will be 

33 made by independently evaluating the overall quality of each study (U.S. EPA, 2015c). This 

34 uniform approach aims to consider the strengths, limitations, and possible roles of chance, 
October 2018 4-18 Draft – Do Not Quote or Cite

1 confounding, and other biases that may affect the interpretation of individual studies and the 

2 strength of inference from the results of the study. 

3 In general, in assessing the scientific quality of studies on health and welfare effects, the 

4 following questions are considered. 

5  Were the study design, study groups, methods, data, and results clearly presented in 

6 relation to the study objectives to allow for study evaluation? Were limitations and any 

7 underlying assumptions of the design and other aspects of the study stated? 

8  Were the ecosystems, study site(s), study populations, subjects, or organism models 

9 adequately selected, and are they sufficiently well-defined to allow for meaningful 

10 comparisons between study or exposure groups? 

11  Are the air quality, exposure, or dose metrics of adequate quality and are they sufficiently 

12 representative of or pertinent to ambient air? 

13  Are the health or welfare effect measurements meaningful, valid, and reliable? 

14  Were likely covariates or modifying factors adequately controlled or taken into account in 

15 the study design and statistical analysis? 

16  Do the analytical methods provide adequate sensitivity and precision to support 

17 conclusions? 

18  Were the statistical analyses appropriate, properly performed, and properly interpreted? 

19 Additional considerations in evaluating individual study quality specific to particular 

20 scientific disciplines are discussed in detail in the Preamble to the ISAs (U.S. EPA, 2015c). 

21 4.3.7 Integration of Evidence and Determination of Causality 

22 As described in the Preamble to the ISAs (U.S. EPA, 2015c), the EPA uses a structured 

23 framework to provide a consistent and transparent basis for classifying the weight of available 

24 evidence for health and welfare effects according to a five-level hierarchy: (1) causal 

25 relationship; (2) likely to be a causal relationship; (3) suggestive of, but not sufficient to infer, a 

26 causal relationship; (4) inadequate to infer a causal relationship; and (5) not likely to be a causal 

27 relationship (Table 4-5). 
October 2018 4-19 Draft – Do Not Quote or Cite

1 Table 4-5. Weight of evidence for causality determinations. 

 Health Effects Welfare Effects 

Causal 

relationship 

Evidence is sufficient to conclude that there is a 

causal relationship with relevant pollutant 

exposures (e.g., doses or exposures generally 

within one to two orders of magnitude of recent 

concentrations). That is, the pollutant has been 

shown to result in health effects in studies in 

which chance, confounding, and other biases 

could be ruled out with reasonable confidence. 

For example: (1) controlled human exposure 

studies that demonstrate consistent effects, or 

(2) observational studies that cannot be 

explained by plausible alternatives or that are 

supported by other lines of evidence (e.g., animal 

studies or mode of action information). Generally, 

the determination is based on multiple 

high-quality studies conducted by multiple 

research groups. 

Evidence is sufficient to conclude that there is a 

causal relationship with relevant pollutant 

exposures. That is, the pollutant has been 

shown to result in effects in studies in which 

chance, confounding, and other biases could be 

ruled out with reasonable confidence. Controlled 

exposure studies (laboratory or small- to 

medium-scale field studies) provide the 

strongest evidence for causality, but the scope 

of inference may be limited. Generally, the 

determination is based on multiple studies 

conducted by multiple research groups, and 

evidence that is considered sufficient to infer a 

causal relationship is usually obtained from the 

joint consideration of many lines of evidence 

that reinforce each other. 

Likely to be a 

causal 

relationship 

Evidence is sufficient to conclude that a causal 

relationship is likely to exist with relevant 

pollutant exposures. That is, the pollutant has 

been shown to result in health effects in studies 

where results are not explained by chance, 

confounding, and other biases, but uncertainties 

remain in the evidence overall. For example: 

(1) observational studies show an association, 

but copollutant exposures are difficult to address 

and/or other lines of evidence (controlled human 

exposure, animal, or mode of action information) 

are limited or inconsistent, or (2) animal 

toxicological evidence from multiple studies from 

different laboratories demonstrate effects, but 

limited or no human data are available. 

Generally, the determination is based on multiple 

high-quality studies. 

Evidence is sufficient to conclude that there is a 

likely causal association with relevant pollutant 

exposures. That is, an association has been 

observed between the pollutant and the 

outcome in studies in which chance, 

confounding, and other biases are minimized 

but uncertainties remain. For example, field 

studies show a relationship, but suspected 

interacting factors cannot be controlled, and 

other lines of evidence are limited or 

inconsistent. Generally, the determination is 

based on multiple studies by multiple research 

groups. 

Suggestive of, 

but not 

sufficient to 

infer, a causal 

relationship 

Evidence is suggestive of a causal relationship 

with relevant pollutant exposures but is limited, 

and chance, confounding, and other biases 

cannot be ruled out. For example: (1) when the 

body of evidence is relatively small, at least one 

high-quality epidemiologic study shows an 

association with a given health outcome and/or at 

least one high-quality toxicological study shows 

effects relevant to humans in animal species, or 

(2) when the body of evidence is relatively large, 

evidence from studies of varying quality is 

generally supportive but not entirely consistent, 

and there may be coherence across lines of 

evidence (e.g., animal studies or mode of action 

information) to support the determination. 

Evidence is suggestive of a causal relationship 

with relevant pollutant exposures, but chance, 

confounding, and other biases cannot be ruled 

out. For example, at least one high-quality study 

shows an effect, but the results of other studies 

are inconsistent. 
October 2018 4-20 Draft – Do Not Quote or Cite

 Health Effects Welfare Effects 

Inadequate to 

infer a causal 

relationship 

Evidence is inadequate to determine that a 

causal relationship exists with relevant pollutant 

exposures. The available studies are of 

insufficient quantity, quality, consistency, or 

statistical power to permit a conclusion regarding 

the presence or absence of an effect. 

Evidence is inadequate to determine that a 

causal relationship exists with relevant pollutant 

exposures. The available studies are of 

insufficient quality, consistency, or statistical 

power to permit a conclusion regarding the 

presence or absence of an effect. 

Not likely to be 

a causal 

relationship 

Evidence indicates there is no causal relationship with 

relevant pollutant exposures. Several adequate studies, 

covering the full range of levels of exposure that human 

beings are known to encounter and considering at-risk 

populations and lifestages, are mutually consistent in 

not showing an effect at any level of exposure. 

Evidence indicates there is no causal relationship with 

relevant pollutant exposures. Several adequate 

studies examining relationships with relevant 

exposures are consistent in failing to show an effect at 

any level of exposure. 

1 Source: U.S. EPA (2015c) 

2 

3 Determination of causality involves evaluating and integrating evidence for different 

4 types of health or welfare effects associated with short- and long-term exposure periods. Key 

5 considerations in drawing conclusions about causality include consistency of findings for an 

6 endpoint across studies, coherence of the evidence across disciplines and across related 

7 endpoints, and biological plausibility. As judged by these parameters, studies in which chance, 

8 confounding, and other biases could be ruled out with reasonable confidence are sufficient to 

9 infer a causal relationship. Increasing uncertainty due to limited available information, 

10 inconsistency across the body of evidence, and/or limited coherence and biological plausibility 

11 may lead to conclusions lower in the causality hierarchy. Causality determinations are based on 

12 the confidence in the integrated body of evidence, considering study design and quality and 

13 strengths and weaknesses in the overall collection of previous and recent studies across 

14 disciplines. In discussing each determination of causality, the EPA characterizes the evidence 

15 upon which the judgment is based, including the extent of and weight of evidence for individual 

16 endpoints within the health or welfare effect category or group of related endpoints. 

17 For evaluation of human health effects, determinations of causality are made for major 

18 health effect categories or groups of related endpoints (e.g., respiratory effects) and for the range 

19 of exposure concentrations of O3 defined to be relevant to ambient concentrations (e.g., up to 2 

20 ppm). The main lines of evidence for use in causality determinations for human health are 

21 controlled human exposure, epidemiologic, and animal toxicological studies. Evidence is 

22 integrated from previous and recent studies. Other information including mechanistic evidence, 

23 toxicokinetics, and exposure assessment may be drawn upon if relevant to the evaluation of 

24 health effects and if of sufficient importance to affect the overall evaluation. The relative 

25 importance of different sources of evidence to the conclusions varies by pollutant or assessment, 

26 as does the availability of different sources of evidence when making a causality determination. 

27 In forming judgments of causality, NCEA scientists will also evaluate uncertainty in the 
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1 scientific evidence, considering issues such as generalizing results from a small number of 

2 controlled human exposure subjects to the larger population; extrapolations of observed 

3 pollutant-induced pathophysiological alterations from laboratory animals to humans; 

4 confounding by co-exposure to other ambient pollutants, meteorological factors, or other factors; 

5 the potential for effects to be due to exposure to air pollution mixtures; and the influence of 

6 exposure measurement error on epidemiologic study findings. Judgments of causality also are 

7 informed by the extent to which uncertainty in one line of evidence (e.g., potential copollutant 

8 confounding in epidemiologic results) is addressed by another line of evidence (e.g., coherence 

9 of effects observed in epidemiologic studies with experimental findings, mode of action 

10 information). Thus, evidence integration is not a unidirectional process but occurs iteratively 

11 within and across scientific disciplines and related outcomes. 

12 A similar process is used for the integration of evidence and determination of causality 

13 for welfare-related effects. For ecological effects this includes evaluating evidence relevant to 

14 quantitative relationships between pollutant exposures and ecological effects. This also includes 

15 reviewing concentration-response relationships and, to the extent possible, drawing conclusions 

16 on the levels at which effects are observed. Also evaluated are O3 effects on biological levels of 

17 organization from species to populations to biological communities and ecosystems. Both 

18 laboratory and field studies (including field experiments and observational studies) can provide 

19 useful data for causality determination. Integration of evidence for effects on climate draws 

20 upon modeling and monitoring data as well as experimental approaches designed to characterize 

21 the role of O3 in atmospheric processes. Generally, a causality determination is made based on 

22 many lines of evidence that reinforce each other and are based on integrating evidence from both 

23 previous and recent studies. 

24 4.3.8 Peer Input Workshop 

25 During the development of the ISA, EPA will be holding a preliminary peer input meeting. This 

26 meeting brings together subject matter experts from a variety of disciplines to review initial draft 

27 materials for the ISA. This workshop has been scheduled to span multiple days (October 29 & 

28 31, November 1 & 5, 2018), covering a different topic area each day. This workshop occurs prior 

29 to the integration of evidence across scientific disciplines and the consideration of the collective 

30 body of evidence for the purposes of making causality determinations. Therefore, the peer input 

31 review is different than what will be provided by the Clean Air Scientific Advisory Committee 

(CASAC) and the public following the release of the completed 1st 32 draft ISA., During the peer 

33 input meeting, expert panelists are asked to address the following overarching questions: 
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1 a. Do the initial draft materials capture the key new studies from the peer-reviewed 

2 literature that have been published since the completion of the 2013 O3 ISA? Are 

3 there additional studies published since the 2013 O3 ISA that should be included? 

4 

5 b. Are there specific issues that should be considered or highlighted that will be 

6 important for integrating evidence across disciplines? 

7 

8 4.3.9 Quality Management 

9 Within the EPA, Quality Management Plans (QMP) are developed to ensure that all 

10 Agency materials meet a high standard for quality. NCEA participates in the Agency-wide 

11 Quality Management System, which requires the development of a QMP. Implementation of the 

12 NCEA QMP ensures that all data generated or used by NCEA scientists are “of the type and 

13 quality needed and expected for their intended use” and that all information disseminated by 

14 NCEA adheres to a high standard for quality including objectivity, utility, and integrity. Quality 

15 assurance (QA) measures detailed in the QMP will be employed for the development of the ISA. 

16 NCEA QA staff will be responsible for the review and approval of quality-related 

17 documentation. NCEA scientists will be responsible for the evaluation of all inputs to the ISA, 

18 including primary (new) and secondary (existing) data, to ensure their quality is appropriate for 

19 their intended purpose. NCEA adheres to Data Quality Objectives, which identify the most 

20 appropriate inputs to the science assessment and provide QA instruction for researchers citing 

21 secondary information. The approaches utilized to search the literature and criteria applied to 

22 select and evaluate studies were detailed in the two preceding subsections. Generally, NCEA 

23 scientists rely on scientific information found in peer-reviewed journal articles, books, and 

24 government reports. The ISA also can include information that is integrated or summarized from 

25 multiple sources to create new figures, tables, or summation, which is subject to rigorous quality 

26 assurance measures to ensure their accuracy. 

27 4.4 SPECIFIC SCIENCE ISSUES TO BE ADDRESSED IN THE ISA 

28 The ISA will provide the scientific foundation for this NAAQS review process and 

29 inform the consideration of whether it is appropriate to retain or revise the current primary and 

30 secondary O3 NAAQS. Decisions on the specific content of the ISA will be guided by policy31 relevant questions that frame the entire NAAQS review as outlined in Chapter 3. Policy-relevant 

32 questions for the ISA are related to two overarching issues: (1) the adequacy of the standard to 

33 protect public health, and (2) reductions in uncertainties identified in the previous review or new 

34 sources of uncertainties. The initial overarching policy-relevant question for the primary and 

35 secondary standards concerns the adequacy of public health or public welfare protection afforded 
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1 by the standard. In considering this overarching question, the PA addresses a series of more 

2 specific questions (sections 3.1.2 and 3.2.2). The more specific questions relate to the nature of 

3 health and welfare effects attributable to O3; the populations, ecosystems or species particularly 

4 at risk of such effects and the exposure concentrations of O3 associated with health and welfare 

5 effects. Another question concerns whether uncertainties from the last review have been reduced 

6 and/or whether new uncertainties have emerged. In the integrated synthesis and each of the 

7 health and welfare effects appendices, the current ISA will evaluate uncertainties and limitations 

8 in the scientific data, as described below. 

9 In order to evaluate potential confounding by other ambient air pollutants, the ISA will 

10 examine whether epidemiologic associations with O3 are observed in copollutant models. 

11 Copollutant models are the predominant method used in air pollution epidemiology to estimate 

12 the effect of one pollutant controlling for a given concentration of a copollutant. The ISA also 

13 will evaluate whether O3 has either interactions with copollutants or joint effects in associations 

14 with health outcomes. The assessment of potential confounding, interactions, or joint effects will 

15 draw upon results from health effects studies, available information on copollutant interactions in 

16 the atmosphere that influence the spatial distributions of O3 and copollutants, as well as 

17 information from experimental studies that examine the health effects of O3 exposures alone and 

18 O3 in combination with other pollutants. In the absence of these methods, the ISA will examine 

19 whether single-pollutant epidemiologic associations with health effects in a given study differ 

20 between O3 and copollutants, and if insights regarding potential copollutant confounding can be 

21 gained by examining the magnitude of correlation between pollutants. 

22 The ISA will consider the strengths and limitations of various exposure assessment 

23 methods. Monitoring data and model output will be used to characterize ambient O3

24 concentrations and provide surrogates for human exposures. Additionally, the ISA will evaluate 

25 the strength of inference in epidemiologic studies by considering information such as the 

26 exposure duration being examined, the extent of temporal and/or spatial variability in O3 in the 

27 study area, the distribution of monitoring sites in the study area, the performance of exposure 

28 models used, and time-activity patterns of the study population. The adequacy of exposure 

29 assessment in epidemiologic studies will be considered in weighing the quality of evidence, and 

30 in turn, forming causality determinations. 

31 Epidemiologic evidence is unlikely to completely address the uncertainties mentioned 

32 above. Any individual study is unlikely to evaluate all potentially correlated copollutants, and the 

33 limitations of epidemiologic methods in separating effects of highly correlated pollutants or 

34 separating the effects of more than two pollutants in the same model are well recognized. Thus, 

35 coherence with other lines of evidence may strengthen inferences when there are uncertainties in 

36 epidemiologic evidence due to copollutant confounding. Controlled human exposure and 
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1 toxicological studies that demonstrate similar effects at relevant O3 exposures may demonstrate 

2 an independent effect of O3 exposure, provide coherence with epidemiologic evidence. Further, 

3 experimental results may provide biological plausibility. 

4 In the previous O3 review, a number of uncertainties were identified with respect to 

5 quantitative relationships between O3 and effects on public welfare. Variation in O3 effects on 

6 vegetation arises from the influence of co-occurring environmental stressors (e.g., drought, 

7 nitrogen deposition), as well as from variation in O3 sensitivity at different vegetative growth 

8 stages or between genotypes. The 2013 ISA identified uncertainties in the magnitude of O3

9 effects on climate, including the net radiative forcing due to changes in O3 concentrations and the 

10 resulting surface temperature response. The ISA will evaluate the status of these uncertainties 

11 and limitations in each of the welfare effects sections and this information will be used in the 

12 development of causality determinations. 

13 The ISA also will address a set of more specific policy-relevant questions related to the 

14 available scientific evidence, as described in the following sections. These questions were 

15 derived from the last O3 NAAQS review. 

16 4.4.1 Causality Determinations from 2013 ISA 

17 The causality determinations in the 2013 ISA, based on the causal framework and 

18 integration of available evidence from previous and recent studies, were presented with a 

19 summary of the available evidence at the end of the sections for each broad health and welfare 

20 effect category and in the integrative synthesis chapter at the beginning of the ISA (U.S. EPA, 

21 2013). 

22 In the 2013 ISA, for human health effects, the EPA concluded that the findings of 

23 epidemiologic, controlled human exposure, and animal toxicological studies collectively 

24 provided evidence of a “causal relationship” for short-term O3 exposures and respiratory effects. 

25 In evaluating a broader range of health effects for O3, the 2013 ISA concluded there was 

26 evidence of a “likely to be causal relationship” for long-term O3 exposures and respiratory 

27 effects and for short-term O3 exposures and cardiovascular effects and mortality. Additionally, 

28 there was evidence “suggestive of a causal relationship” for O3 exposures and other health 

29 effects, including developmental and reproductive effects (e.g., low birth weight, infant 

30 mortality) and central nervous system effects (e.g., cognitive development). 

31 In the 2013 ISA, for welfare effects, the evidence indicated a “causal relationship” 

32 between O3 exposure and visible foliar injury effects on vegetation, reduced vegetation growth, 

33 reduced productivity in terrestrial ecosystems, reduced yield and quality of agricultural crops, 

34 and alteration of below-ground biogeochemical cycles. The evidence indicated a “likely to be 

35 causal relationship” for reduced carbon sequestration in terrestrial ecosystems, alteration of 
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1 terrestrial ecosystem water cycling and alteration of terrestrial community composition. For 

2 climate there was a causal relationship between changes in tropospheric O3 concentration and 

3 radiative forcing and likely to be a causal relationship between changes in tropospheric O3

4 concentration and effects on climate. 

5 In the current review, specific science questions related to the causality determinations 

6 that we plan to address include: 

7  Does the evidence base from recent studies contain new information to support or call into 

8 question the causality determinations made for relationships between O3 exposure and 

9 various health and welfare effects in the 2013 ISA? 

10  Is there new information to extend causality determinations to other ecological endpoints? 

11  Does new evidence confirm or extend biological plausibility of O3-related health 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 O3

15 in the study area? 

16 o Assessed potential confounding by other pollutants and factors?

17  What does the available information indicate with regard to changes in population health 

18 status that may be associated with a decrease in ambient air O3 concentrations that might 

19 inform causality determinations? 

20 4.4.2 Ambient Concentrations of O3

21 The ISA will present and evaluate relevant data, and summarize the current scientific 

22 understanding concerning the sources and ambient air concentrations of O3 in the U.S. lower 

23 troposphere and surface boundary layer. O3 present in the lower troposphere is predominantly 

24 formed through photochemical reaction involving reactive volatile organic compounds and/or 

25 NOX as precursor gases. The discussion divides ambient O3 into two classes: U.S. background 

26 O3 and non-background O3 (see section 4.3.4.1.1). Specific science questions that we plan to 

27 address in the ISA include: 

28 

29  What are the origins of U.S. background O3 concentrations, especially related to 

30 international transport into the U.S., stratospheric exchange, and natural emissions from 

31 biogenic sources, wildfires, and lightning? How well quantified are contributions from 

32 these sources on overall tropospheric O3 concentrations? 

33  What modeling strategies have been used to estimate U.S. background O3 concentrations? 

34 What are the sources of bias and uncertainty associated with the models used to estimate 

35 U.S. background O3 concentrations? What observations or alternative estimates are 

36 available that quantify U.S. background O3 concentrations and characterize its 

37 spatiotemporal patterns? 
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1  What data are available to characterize precursor emissions of non-background O3? How 

2 does recent evidence contribute to what is known about the photochemical production of 

3 non-background O3? How has modeling non-background O3 evolved since the last ISA? 

4 Are there new models, or recent studies that have evaluated the validity of existing 

5 models? 

6  Have methods for measuring non-background O3 substantively changed since the last 

7 ISA? What are recent ambient O3 concentrations and longitudinal trends in O3

8 concentrations? 

9 4.4.3 Human Exposure 

10 The ISA will evaluate methods for estimating exposure to ambient O3, as well as the 

11 ability to make inferences about personal exposure to ambient O3 when extrapolating from 

12 ambient concentration data, particularly in the context of interpreting results from epidemiologic 

13 studies. The issues surrounding the ability to make inferences about personal exposure differ by 

14 the exposure period of interest. Short-term exposure studies (i.e., exposures ranging from hours 

15 up to weeks) examine how temporal variation in exposure is associated with temporal variation 

16 in a health outcome while long-term exposure studies (i.e., exposures ranging from months to 

17 years) typically examine how spatial variability of exposure is associated with spatial variation in 

18 a health outcome averaged over time. Specific science questions related to human exposure that 

19 we plan to address in the ISA include: 

20  What new developments have occurred with respect to chemical transport modeling of 

21 short-term and long-term O3 concentrations for use in exposure assessment? How might 

22 modeling and satellite data supplement monitoring data for understanding human 

23 exposures? What are the limitations of using modeling or satellite data in lieu of 

24 monitoring data? What advancements have been made with respect to techniques for 

25 fusing modeling, monitoring, and/or satellite data for assessing exposures to ambient O3? 

26 What are the uncertainties in data from chemical transport models and satellites at the 

27 extremes of the concentration distribution, such as in high and low concentration areas 

28 (e.g., near roadways, rural areas) and times? 

29  What are the errors and uncertainties associated with extrapolating from stationary O3

30 monitoring instruments to personal exposure to O3 of ambient origin? Issues may arise 

31 from instrument error in outdoor ambient air monitors, the use of fixed-site monitors for 

32 estimating community concentrations across different spatial scales (e.g., neighborhood 

33 scale, urban scale), spatial misalignment from using fixed-site monitors as a surrogate for 

34 personal exposure to O3 of ambient origin, and uncertainty in the time-activity patterns of 

35 exposed individuals whose exposure is represented by fixed-site monitors. 

36  What new developments have been made in assessing and/or correcting the influence of 

37 exposure measurement error on health effect estimates for epidemiologic studies of short38 term and long-term exposure? How do these methods reduce the uncertainty and/or bias 

39 in the health effect estimates for O3 exposure? 
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1 4.4.4 Health Effects 

2 In the 2013 ISA, the health effects evidence indicated that a “causal relationship exists” 

3 for short-term exposures to O3 and respiratory effects, and a “likely to be causal relationship 

4 exists” for long-term O3 exposures and respiratory effects and short-term O3 exposures and 

5 cardiovascular effects and mortality. More limited evidence with a larger degree of uncertainty 

6 formed the basis for the determinations for other health effects. The EPA will build on the 

7 conclusions of the 2013 ISA by evaluating the newly available literature related to O3 exposures 

8 and health effects, including, but not limited to respiratory, cardiovascular, nervous system, 

9 reproductive and developmental effects, mortality, and cancer. Depending on data availability 

10 and resources, other health effects may be evaluated. 

11 The ISA will evaluate health effects that occur following both short- and long-term 

12 exposures as examined in epidemiologic, controlled human exposure, and animal toxicological 

13 studies. Efforts will be directed towards identifying the concentrations at which effects are 

14 observed, particularly in potential at-risk lifestages and populations, and assessing the role of O3

15 within the broader mixture of ambient air pollutants. The discussion of health effects will be 

16 integrated with relevant information on exposure, dosimetry and biological plausibility. 

17 In the current review, specific science questions that we plan to address in consideration 

18 of health effects associated with short- and long-term exposure to O3, include the following: 

19 Short-Term Exposure 

20  What recent evidence is available to inform policy-relevant considerations of the O3

21 NAAQS (summarized in Chapter 3) for short-term O3 exposures and respiratory effects? 

22 Do recent controlled human exposure and toxicological studies continue to provide support 

23 for biologically plausible relationships between short-term O3 exposures and respiratory 

24 health effects? Do recent studies report O3-attributable effects at lower O3 exposure 

25 concentrations or for different durations or patterns of exposure than indicated by studies 

26 available in the last review? 

27  How do results of recent studies expand understanding of the relationship between short28 term exposure to O3 and cardiovascular effects, such as ischemic heart disease, heart 

29 failure, or vascular effects? Does recent evidence improve coherence across disciplines for 

30 heart rate variability, blood pressure, and outcomes such as cardiovascular hospital 

31 admissions or emergency department visits? 

32  To what extent is short-term exposure to O3 related to or associated with the progression of 

33 diabetes, other metabolic diseases, and/or to other endocrine system effects? To what 

34 extent are new health outcomes related to or associated with O3 exposures? 

35  Across the evaluated health effects, what new evidence is available on effects occurring 

36 from exposures of different durations than indicated by the previously available evidence? 
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1 Long-Term Exposure 

2  What new evidence is available to inform policy-relevant considerations of the O3 NAAQS 

3 (summarized in Chapter 3) for long-term O3 exposures and respiratory effects? Do new 

4 epidemiologic and toxicological studies continue to provide support for biologically 

5 plausible relationships between long-term O3 exposures and respiratory health effects? Do 

6 new studies report O3-attributable effects at lower O3 concentrations than indicated by 

7 studies available in the last review? 

8  To what extent do recent studies improve understanding of the relationships between long9 term O3 exposure and the development of asthma or to the impairment of lung 

10 development? Do recent studies improve coherence across disciplines for respiratory 

11 disease incidence, pulmonary inflammation and oxidative stress, and allergic responses? 

12  To what extent do recent studies improve understanding of the relationship between O3

13 exposure and reproductive and developmental health outcomes, such as adverse birth 

14 outcomes, fertility and pregnancy outcomes (e.g., infertility, sperm quality, preeclampsia, 

15 gestational hypertension), or developmental outcomes (e.g., neurocognitive effects)? Are 

16 there new studies linking exposures during critical windows of development to increased 

17 risk of O3-related health effects later in life? 

18  To what extent does new literature support a biologically plausible relationship between 

19 long-term O3 exposures and nervous system effects (e.g., cognitive decline and autism)? 

20  How do results of recent studies expand our understanding of the relationship between 

21 long-term O3 exposure and mortality? To what extent does the evidence indicate that long22 term exposure to O3 can increase the risk of respiratory-related mortality or other cause23 specific mortality? 

24  To what extent is long-term exposure to O3 related to or associated with the development 

25 of diabetes and other metabolic diseases, as well as to health effects in the endocrine 

26 system or other organ systems? To what extent are new health outcomes related to or 

27 associated with O3 exposures? 

28 Additional Science Considerations 

29  Do epidemiologic studies of mortality, hospital admissions, or emergency department 

30 visits provide new information to improve our understanding of the potential heterogeneity 

31 in effects assessed in U.S. multicity studies? 

32  How do the results of recent studies inform the shape of the concentration-response 

33 relationship for O3 and various health outcomes (e.g., mortality, hospital admissions, etc.), 

34 especially for concentrations near or below the levels of the current O3 NAAQS? 

35  What new evidence adds to the understanding of which lifestages and populations are at 

36 increased risk of O3-related health effects? 

37  What new evidence supports evaluation of inter-individual variability in response to O3

38 exposures? 

39  What is the relationship between short- and long-term exposures and O3-related health 

40 effects? More specifically, across health effects, what new information is available to 
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1 delineate the effects of chronic exposure to lower concentrations versus acute, repeated 

2 exposures to higher concentrations of O3? 

3  What is the nature of health effects in persons exposed to multipollutant mixtures that 

4 contain O3 in comparison to exposure to O3 alone? 

5 4.4.5 At-Risk Lifestages and Populations and Public Health Impact 

6 The NAAQS are intended to protect public health with an adequate margin of safety, 

7 including protection for the for populations or lifestages potentially at increased risk for O3-

8 related health effects. Thus, the ISA will evaluate evidence for an array of factors that may 

9 contribute to increased risk of O3-related health effects for various lifestages or populations (e.g., 

10 populations with preexisting disease). The evaluation of recent evidence will build on the 

conclusions from the 2013 ISA, where application of the at-risk framework79 11 to classify evidence 

12 demonstrated that there was adequate evidence that children, older adults, people with pre13 existing asthma, people with certain genetic variants, people with nutritional deficiencies, and 

14 outdoor workers are at increased risk of O3-related health effects. The ISA will evaluate recent 

15 evidence that informs the identification of at-risk factors (e.g., lifestage, preexisting disease) in 

16 each of the health appendices. Key considerations in characterizing the evidence include 

17 consistency of findings for a factor within a discipline and, where available, coherence of the 

18 evidence across disciplines as well as biological plausibility. When evaluating evidence to 

19 inform the identification of at-risk lifestages or populations, emphasis will be placed on the 

20 health effects for which there is a causal or likely to be a causal relationship with exposure to O3. 

21 Specific questions we plan to address include: 

22  What new evidence is available to further support or call into question the at-risk 

23 determination made for lifestages or populations in the 2013 ISA? 

24  What new evidence is available regarding additional lifestages or populations (e.g., pre25 existing diseases such as diabetes) potentially at increased risk of an O3-related health 

26 effect? 

27  Is there new information that identifies a combination of factors (i.e., co-occurring) that 

28 can lead to one lifestage or population being at greater risk compared to another? 

29 4.4.6 Welfare Effects 

30 In the 2013 ISA, the welfare effects evidence for O3 focused on the effects of O3 on 

31 vegetation and ecosystems and the role of tropospheric O3 in climate change. The EPA will build 

 

79 In recent reviews, the term “at-risk” has been used to define populations and lifestages potentially at increased risk 

of an air pollutant-related health effect (e.g., see 2013 O3 ISA and 2016 NOX ISA; U.S. EPA, 2013; U.S. EPA, 

2016). At-risk populations can include those with intrinsic factors that make them more susceptible to pollutantrelated effects (e.g., pre-existing disease, genetic characteristics) or that increase pollutant dose (e.g., breathing 

patterns), and extrinsic factors that could increase pollutant exposures (e.g., activity patterns; proximity to 

sources) (U.S. EPA, 2016, pp. 1xiii to 1xiv). 
October 2018 4-30 Draft – Do Not Quote or Cite

1 on the 2013 ISA by evaluating the newly available literature related to O3 exposures and welfare 

2 effects, specifically ecological effects and effects on climate

3 4.4.6.1 Ecological Effects 

4 The ISA will evaluate the literature related to O3 exposures at levels of biological 

5 organization from the organism to the ecosystem including effects on biodiversity. Evidence 

6 from experimental (e.g. laboratory, greenhouse, OTC, FACE) and field, gradient or modeling 

7 studies that address effects of O3 on ecological endpoints will be considered to identify 

8 concentrations at which effects are observed (Table 4-5). 

9 4.4.6.1.1 Plant-level Effects 

10 Ambient O3 concentrations have long been known to cause foliar injury and decreased 

11 growth and biomass accumulation in annual, perennial and woody plants, including agronomic 

12 crops, annuals, shrubs grasses, and trees. In the 2013 ISA the evidence was sufficient to infer a 

13 causal relationship between O3 exposure and endpoints on vegetation including, visible foliar

14 injury, reduced growth, and reduced yield and quality from individual plants that are agricultural 

15 crop species. Evidence for foliar injury includes data from field, lab and chamber studies dating 

16 back to the 1960’s. Decreased growth at the plant scale has been well established for several 

17 decades and may translate to damages the stand and then ecosystem scales. In the current review 

18 specific policy-relevant questions related to O3 effects on plant-level effects include the 

19 following: 

20  Is there any additional information on foliar injury or biomass growth in U.S. species 

21 attributable to O3 in ambient air? 

22  Is there additional information on the factors influencing the relationship between O3 and 

23 visible foliar injury? 

24  Is there additional information regarding a relationship between visible foliar injury and 

25 growth? 

26  Is there any additional information on interspecies differences in responses to O3? 

27 4.4.6.1.2 Ecosystem-level Effects 

28 Effects at the individual plant level can result in changes in ecosystems such as 

29 productivity, below-ground processes, carbon storage, water cycling and nutrient cycling. The 

30 2013 ISA determined there was a causal relationship between O3 exposure and reduced 

31 productivity. Results of long-term experiments provided evidence of the association of O3

32 exposure and reduced productivity at the ecosystem level of organization which were supported 

33 by decreased plant growth and modeling studies. The 2013 ISA also determined there was a 

34 causal relationship between O3 exposure and alteration of below-ground biogeochemical cycles 

35 including altered carbon allocation to below-ground tissues; and altered rates of leaf and root 
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1 production, turnover, and decomposition. These shifts can affect overall carbon loss and nitrogen 

2 loss from the ecosystem. Studies from the leaf and plant level provided biologically plausible 

3 mechanisms and results from experimental studies consistently showed responses of below4 ground processes to O3 exposure. The 2013 ISA determined there was a likely causal 

5 relationship between O3 exposure and reduced carbon sequestration. Evidence for that 

6 conclusion was primarily from global and regional modeling simulations. The 2013 ISA 

7 determined there was a likely causal relationship between O3 and alteration of terrestrial water 

8 cycling. Alteration of stomatal functioning may affect water use in leaves, whole plants, and at 

9 the watershed level based on field and modeling studies. In the current review specific policy10 relevant questions related to O3 effects on ecosystem processes include the following: 

11  What new information is available, including that for O3-related effects on ecosystem 

12 services, on alteration of below-ground biogeochemical cycles, decreased productivity, 

13 reduced carbon sequestration, and alteration of terrestrial ecosystem water cycling? 

14  Are there newly identified ecological endpoints or processes affected by O3? 

15 4.4.6.1.3 Biodiversity 

16 O3 exposure can lead to loss of sensitive species and alter community composition of 

17 plants and microorganisms in some ecosystems. In the 2013 ISA the evidence was sufficient to 

18 infer a likely causal relationship between O3 and alteration of terrestrial community composition. 

19 Studies of the impact of O3 on species competition and community composition showed declines 

20 in community composition of above-ground and below-ground communities. In the current 

21 review specific policy-relevant questions related to O3 effects on biodiversity include the 

22 following: 

23  Is there additional evidence with respect to O3 effects on ecosystem structure and 

24 terrestrial community composition 

25  Is there additional evidence with respect to O3 effects on other organisms such as insects 

26 or other wildlife? 

27 4.4.6.1.4 Air Quality Indices and Exposure-Response Relationships 

28 Exposure indices are metrics that quantify exposure as it relates to measured plant 

29 response (e.g., reduced growth). In the 2013 ISA, exposure indices that cumulated and 

30 differentially weighted the higher hourly average concentrations and included the mid-level 

31 values offered the most reliable approach for use in developing response functions and 

32 comparing studies, as well as for defining future indices for vegetation protection. Exposure33 response relationships were available for several tree and crop species from a variety of 

34 experiments. In the current review specific policy-relevant questions related to air quality indices 

35 and exposure-response include the following: 
October 2018 4-32 Draft – Do Not Quote or Cite

1  Are there new U.S. studies which use various O3 metrics to further characterize O3 effects 

2 on plant foliar injury and/or growth? 

3  Are there new studies which improve the characterization of O3 exposure-response on 

4 vegetation at the local, regional and/or national scale? 

5 4.4.6.2 Effects on Climate 

6 The ISA will present information on radiative forcing resulting from changes in 

7 tropospheric O3 concentration and subsequent effects on climate endpoints such as surface air 

8 temperature. The focus will be on information necessary for interpretation of effects and on 

9 newly available information since the last ISA. Specific questions include: 

10  What new information is available to decrease uncertainties in the magnitude of the 

11 radiative forcing and climate response attributed to tropospheric O3? 

12  To what extent do we understand the independent effects of O3 on climate in the broader 

13 context of other climate forcers, including copollutants and O3 precursors? 

14  What feedbacks affect the climate response to radiative perturbations from tropospheric O3

15 concentration changes? 

16  What recent advancements have been made in understanding O3 effects on regional climate 

17 in the U.S.? 

18 4.5 SCIENTIFIC AND PUBLIC REVIEW 

19 The EPA’s Peer Review Handbook dictates the process for scientific peer review of all 

20 EPA products (U.S. EPA, 2015d). Accordingly, a draft of the ISA will be made available for 

21 review by the CASAC, as well as by the public. Availability of the draft document will be 

22 announced in the Federal Register. The CASAC will review the draft ISA at a public meeting 

23 that will be announced in the Federal Register. The EPA will consider comments, advice, and 

24 recommendations received from the CASAC and from the public in revising the draft ISA 

document. The EPA has established a public docket for the development of the ISA.80 25 After 

26 appropriate revision based on comments received from the CASAC and the public, the final 

27 document will be made available on the EPA website. A notice announcing the availability of the 

28 final ISA will be published in the Federal Register. 

 

80 The ISA docket can be accessed at www.regulations.gov using Docket ID number EPA-HQ-ORD-2018-0274. 
October 2018 5-1 Draft – Do Not Quote or Cite

1 5 QUANTITATIVE RISK AND EXPOSURE 

2 ASSESSMENTS 

In NAAQS reviews, quantitative REAs81 3 are generally designed to assess human 

4 exposure and health risk, as well as ecological exposures and risks to public welfare, for air 

5 quality conditions associated with the existing standards, and as appropriate, for conditions 

6 associated with potential alternative standards. The objective for such assessments is generally to 

7 provide quantitative estimates of impacts that inform judgments on the public health and public 

8 welfare significance of exposures likely to occur under air quality conditions reflective of the 

9 current NAAQS, and, as appropriate, any alternative standards under consideration. Accordingly, 

10 the assessments are also intended to provide a basis for judgments as to the extent of public 

11 health and public welfare protection afforded by such standards. 

12 In developing REAs in each NAAQS review, we draw upon the currently available health 

13 effects evidence that is characterized in the ISA. This includes information on atmospheric 

14 chemistry, air quality, human and environmental exposures, dosimetry and mode of action, and 

15 information on health and welfare effects associated with exposures considered likely to occur 

16 because of pollutant concentrations in ambient air. We additionally employ current methods and 

17 tools to support the quantitative modeling and assessment. 

18 The REAs commonly rely on a case study approach which involves quantitative analyses 

19 focused on populations and pollutant concentrations in one or more specific geographic areas 

20 under air quality conditions that just meet the existing standards (and alternatives as appropriate). 

21 Reliance on this approach is intended to provide assessments of the air quality scenario(s) of 

22 interest for a set of study areas and associated exposed at-risk populations and ecosystems that 

23 will be informative to the EPA’s consideration of potential exposures and risks that may be 

24 associated with the stated air quality conditions. For example, we are interested in the exposure 

25 and risk associated with air quality conditions that just meet the current standard(s); such 

26 information is useful in interpreting the degree of protectiveness given by the current standard(s), 

27 the adequacy of such standard(s), and the need to consider alternatives. Further, the REA 

28 analyses employ a case study approach that addresses practical considerations, such as 

29 employing a tractable scale and considering resource constraints, while providing estimates for 

30 populations and geographic areas of interest and also having broader applicability (e.g., offering 

31 risk perspective for similar study areas that were not assessed). Thus, REA analyses are not 

 

81 While the term REA has in the past several NAAQS reviews referred to assessments presented in a stand-alone 

REA document, in this review, we are also using this term, or the phrase “REA analyses” to simply refer to the 

analyses which we intend to present in appendices or as supplemental materials to the PA. 
October 2018 5-2 Draft – Do Not Quote or Cite

1 generally intended to provide a comprehensive national assessment of such conditions, nor are 

2 they necessarily intended to provide such an assessment of existing air quality. Rather, the 

3 purpose is to assess population exposure and risk for particular air quality conditions based on 

4 currently available scientific information, modeling tools, and other technical information. 

5 In planning any REA analyses that may be appropriate for a new NAAQS review, we 

6 first consider the analyses conducted in the last review and the extent to which they provided 

7 important insights that were informative to the Agency’s decision on the current standard. 

8 Conclusions in this regard are generally influenced by an assessment of the uncertainties 

9 associated with each type of analysis and the corresponding consideration of each type’s relative 

10 strength, as documented in the notice of the decision for the prior review and associated 

11 assessment documents such as the PA and REA. In considering whether new analyses are 

12 warranted for particular types of assessments, we evaluate the availability of new scientific 

13 evidence and technical information in this review, as well as improved methods and tools, that 

14 may provide support for conducting updates to address key limitations or uncertainties of 

15 analyses from the last review, or to provide additional insight beyond those provided by the prior 

16 REA. Thus, we focus on identifying the new analyses that are warranted in consideration of 

17 factors such as those raised here, while also bearing in mind practical and logistical 

18 considerations such as available resources and timeline for the review. 

19 The purpose of this chapter is to briefly summarize the comprehensive, complex, and 

20 resource-intensive quantitative health and welfare assessments completed in the last review of 

21 the O3 NAAQS, giving attention to those analyses concluded to be most informative to the 

22 decisions reached on the standards in that review. In considering the issues raised above, we 

23 additionally summarize key uncertainties and limitations of the analyses conducted for the last 

24 review and consider the extent to which newly available information, tools or methodologies 

25 might address those areas. For example, the scope of any analyses for this review would be 

26 informed by the new scientific information characterized in the upcoming ISA; recent air quality 

27 data; the availability of improved data, methods, tools, and models that can be used to address 

28 limitations and uncertainties from the last review; and any constraints on resources and the 

29 review timeline. The goal is to focus on those analyses that may be particularly policy relevant 

30 and informative to decision-making in this review and to identify the types of analyses for which 

31 updates are warranted and will be conducted in this review (in contrast to, for example, other 

32 types of analyses for which the assessments presented in the 2014 REAs may remain 

33 appropriately informative). 

34 We are planning that the quantitative exposure and risk analyses newly developed in this 

35 review will be presented in the draft PA, and to consider them along with any previously 

36 conducted analyses that remain pertinent and informative to consideration of the adequacy of the 
October 2018 5-3 Draft – Do Not Quote or Cite

1 current standards (and alternative standards, as appropriate). We intend to provide associated 

2 technical details for any new exposure and risk analyses in appendices or supplemental materials 

3 for the PA, while analyses from the last review are documented in the 2014 REAs, 2014 PA, and 

4 technical memos available in the O3 docket for the last review. Any quantitative assessments 

5 newly developed in this review would then be made available for public comment and reviewed 

6 by the CASAC in the context of the draft PA. Public comments and CASAC advice on such 

7 REA-related analyses in the draft PA would be considered in finalizing analyses for presentation 

8 in the final PA. 

9 In this chapter, quantitative exposure and risk assessments for informing the primary 

10 standard are discussed in section 5.1 and those pertaining to the secondary standard are discussed 

11 in section 5.2. Both of those sections present overviews of the types of analyses performed in the 

12 last review, and highlight some considerations for analyses in this review. 

13 5.1 ASSESSMENTS INFORMING REVIEW OF THE PRIMARY 

14 STANDARD 

15 In reviews of primary NAAQS, quantitative exposure and health risk assessments are 

16 generally intended to inform consideration of key policy relevant questions (see section 3.1), 

17 such as the following: 

18  What are the nature and magnitude of exposures and health risks associated with air 

19 quality conditions just meeting the current standard? 

20  To what extent are the estimates of exposures and risks to at-risk populations associated 

21 with air quality conditions just meeting the current standard reasonably judged important 

22 from a public health perspective? 

23 In considering exposure and risk estimates in this context, an accompanying consideration is: 

24  What are the important uncertainties associated with any risk/exposure estimates? 

25 The types of analyses performed generally reflect the nature and strength of the evidence 

26 in various aspects. For example, for the health effects pertaining to exposures associated with the 

27 presence of the pollutant in ambient air, the availability and type of information from the health 

28 effects literature on relationships between internal dose, exposure or ambient air concentration 

29 and health response influences the types of exposure assessment and risk characterization that 

30 are performed. The health assessments focus on exposure metrics that are appropriate for effects 

31 of concern for the subject pollutant, and along with available ambient air concentration 

32 measurements and model estimates, where appropriate, are used to generate estimates of 

33 exposure. Consistent with the health risk approaches that have been used in NAAQS reviews 

34 (illustrated in Figure 5-1), assessments of O3-related health risks have been conducted in past 

35 reviews (including the last review) based on two different types of risk approaches. The first 
October 2018 5-4 Draft – Do Not Quote or Cite

1 approach is based on relating areawide average ambient air concentrations to results from air 

2 quality epidemiologic studies by linking ambient air quality concentrations with concentration3 response functions. The second approach is based on relating population exposure estimates to 

4 results from controlled human exposure studies and employing either a benchmark 

5 concentration- or exposure-response function-based approach to estimate risk. 

6 

7 Figure 5-1. Summary of health risk assessment approaches that have been employed in 

8 NAAQS reviews. 

9 In the review of the primary O3 standard completed in 2015, the different types of 

10 analyses that were performed varied in the extent to which they informed consideration of the 

11 policy-relevant questions posed above. Accordingly, they also varied in the extent to which they 

12 informed conclusions and judgments related to revision of the then-existing primary standard. 

13 For example, the EPA generally expressed higher confidence in the 2014 HREA results for 

14 exposure-based analyses, with their basis in the results from controlled human exposure studies, 

15 as compared to HREA estimates derived from the ambient air concentrations and epidemiologic 

study associations (2014 HREA, section 9.6; 80 FR 65316).82 16 These two types of analyses are 

17 described below in sections 5.1.1.1 and 5.1.1.2, respectively. The roles of the analyses in 

18 conclusions reached and judgments made in the 2015 review are summarized in section 5.1.2, as 

 

82 The 2015 decision notice recognized a number of key uncertainties in utilizing the estimated air concentrations 

and epidemiologic study relationships (often called epidemiologic-based risk estimates) with potentially 

important implications for the Administrator’s consideration of epidemiology-based risk estimates (80 FR 65316; 

79 FR 75277-75279; 2014 HREA, sections 3.2.3.2 and 9.6). These included the heterogeneity in effect estimates 

between locations, the potential for exposure measurement errors, and uncertainty in the interpretation of the 

shape of concentration-response functions at lower O3 concentrations, as well as uncertainties related to the public

health importance of increases in relatively low O3 concentrations following air quality adjustment. Additionally,

as noted in section 5.1.1.2 below, lower confidence was placed in the results of the epidemiologic-based 

assessment of respiratory mortality risks associated with long-term O3 exposures in consideration of several 

factors. 
October 2018 5-5 Draft – Do Not Quote or Cite

1 are key uncertainties and limitations of the analyses, along with considerations related to the 

2 availability of information, methods or tools in this review that may address them. 

3 5.1.1 Overview of Assessments in Last Review 

4 The HREA completed for the last review included two types of analyses. The first type 

5 was based on assessment of population exposure using exposure modeling (section 5.1.1.1), 

6 while the second relied on relating ambient air concentrations to adverse health outcomes using 

7 concentration-response relationships drawn from epidemiologic studies (section 5.1.1.2). Figure 

8 5-2 illustrates the conceptual model for these types of assessments in the framework of the 

9 traditional source to dose to health effects model. 

10 

11 Figure 5-2. Conceptual model for 2014 O3 health risk assessment. Solid lines indicate 

12 processes included in the 2014 assessment. 

Emissions of O3 precursors to ambient air

Indoor Air

Exposures of Concern:

Number & percent of 

people experiencing a 

day with an exposure (at 

elevated breathing rates) 

above benchmarks

Lung Function Risk:

Number & percent of 

people experiencing a 

day with an O3-induced 

FEV1 reduction

(≥10%, 15%, 20%)

Incidence of Respiratory 

Health Outcomes: 

(e.g., hospital emergency 

department [ED] visits, 

hospital admissions [HA], 

mortality)

Respiratory system

Lung function decrements (FEV1, sRaw), inflammation, respiratory 

symptoms, etc.

O3 in ambient air

RISK METRICS POPULATIONS ROUTES PATHWAYS SOURCES ENDPOINTS

ORGAN 

SYSTEMS

EXPOSURE EFFECTS RISK

O3 originating 

indoors

Children and adults (all and subgroups with asthma)

Inhalation while at moderate or greater exertion
October 2018 5-6 Draft – Do Not Quote or Cite

1 The long-standing evidence base for O3-related adverse health effects is built from a large 

2 assemblage of controlled human exposure studies, laboratory animal research studies, and air 

3 quality epidemiological studies. Together, these health effect studies lead to the strongly 

4 supported conclusion that O3-related exposure causes respiratory effects (2013 ISA, section

5 6.2.9; 80 FR 65302). The controlled human exposure studies document the occurrence of an 

6 array of respiratory effects in humans in a variety of exposure circumstances, and additionally, in 

7 combination with the laboratory animal research studies, inform our understanding of the mode 

8 of action for O3-attributable effects. The air quality epidemiological studies provide additional 

9 support for the causal conclusion regarding effects of O3 in ambient air (2013 ISA, section 

10 6.2.9). 

11 For developing quantitative characterizations of health risk or potential risk, the support 

12 has been strongest for the exposure-based risk analyses, including the analysis used in the last 

13 three O3 NAAQS reviews that involves the comparison of estimated population-based O3

exposures experienced while at elevated exertion83 14 to benchmark concentrations drawn from the 

15 controlled human exposure studies. A second set of exposure-based risk analyses performed for 

16 the last three O3 reviews, has been those that employ a lung function risk estimation approach 

17 that also draws on results of the controlled human exposure studies. Another type of analysis that 

18 has been used is a risk approach based on ambient air concentration-response relationships from 

19 air quality epidemiological studies. This approach was also employed in the last two O3 NAAQS 

20 reviews (e.g., to estimate risk for various health outcomes, such as hospital admissions), with a 

21 recognition of the uncertainties associated with the quantitative concentration-response 

22 relationships used in that approach. In initial planning for the current review, we consider 

23 support for both types of health risk approaches (i.e., exposure- and air quality epidemiologic24 based), evaluating the extent to which the information newly available in this review provides 

25 support for developing updated or enhanced analyses that would substantially improve the utility 

26 of risk estimates for informing the current review. 

27 In the 2014 HREA, the two exposure-based risk analyses were performed in a set of 15 

28 urban study areas and the air quality epidemiologic-based risk analyses were performed for a 

subset of those areas.84 29 Both approaches were performed for five different air quality scenarios: 

 

83 As summarized in section 3.1 above, the focus on exposures while at elevated exertion reflects the evidence from 

controlled human exposure studies in which exposures to O3 concentrations of a magnitude relevant to those 

occurring in ambient air have only been shown to result in respiratory effects if the ventilation rates of people in 

the exposed populations are raised to a sufficient degree, such as through physical exertion (2013 ISA, section 

6.2.1.1). 

84 The 15 urban study areas assessed were Atlanta, Baltimore, Boston, Chicago, Cleveland, Dallas, Denver, Detroit, 

Houston, Los Angeles, New York, Philadelphia, Sacramento, St. Louis, and Washington, DC. The three not 

included in the epidemiologic-based assessment were Chicago, Dallas and Washington, DC. 
October 2018 5-7 Draft – Do Not Quote or Cite

1 unadjusted air quality conditions, air quality adjusted to just meet the then-existing standard (75 

2 ppb O3 as a 3-year average of annual fourth highest daily maximum 8-hour average 

3 concentrations), and air quality adjusted to just meet potential alternative standards with levels of 

70, 65 and 60 ppb.85 4 The scenarios were based on air quality representing two 3-year periods: 

5 2006-2008 and 2008-2010. 

6 For the air quality scenarios with adjusted air quality, ambient air O3 concentrations that 

7 would just meet the then-current and potential alternative standards were estimated using a 

8 photochemical model-based adjustment approach (2014 HREA, Chapter 4). This approach 

9 employed the Community Multiscale Air Quality Model version 4.7.1 (CMAQv4.7.1) 

instrumented with the higher order decoupled direct method (CMAQ-HDDM).86 10 The CMAQHDDM was used to estimate sensitivities87 11 of O3 concentrations to changes in precursor 

12 emissions; using this approach, we estimated hourly O3 concentrations at each monitor location 

resulting from reductions in U.S. anthropogenic precursor emissions (i.e., NOX, VOC).88 13 This 

14 approach to adjusting air quality reflects the physical and chemical atmospheric processes that 

influence O3 concentrations in ambient air (2014 HREA, Chapter 4).89,90 15 

 

85 These scenarios reflect air quality with design values – 8-hour values using the existing form of the NAAQS – 

that meet the level of the current or potential alternative standards. These simulations are illustrative and do not 

reflect any consideration of specific control programs designed to meet the specified standards. Further, these 

simulations do not represent predictions of when, whether, or how areas might meet the specified standards. 

86 Details on model set-up, configuration, and input data are provided in 2014 HREA, Appendix 4B. 

87 Sensitivities of O3 refer to predicted incremental changes in O3 concentrations in response to incremental changes 

in emissions. The “higher order” aspect of the HDDM tool refers to the capability of capturing nonlinear response 

curves. 

88 Exposure and risk analyses for most of the urban study areas focus on reducing U.S. anthropogenic NOX

emissions alone. The exceptions are Chicago and Denver. Exposure and risk analyses for Chicago and Denver are 

based on reductions in emissions of both NOX and VOC (2014 HREA, section 4.3.3.1; Appendix 4D). 

89 Compared to the statistical approaches that have been used in the past (e.g., a quadratic equation used in the 2007 

REA to adjust high concentrations downwards at a greater rate than lower concentrations), the photochemical 

model adjustment approach provides more realistic estimates of the spatial and temporal responses of O3 to 

reductions in precursor emissions. Because NOX in ambient air can contribute to both the formation and the 

destruction of O3 (2014 HREA, Chapter 4), the response of ambient air O3 concentrations to reductions in NOX

emissions is more variable than indicated by the previously used quadratic adjustment. This improved approach to 

adjusting O3 air quality is consistent with recommendations from the National Research Council of the National 

Academies of Sciences (NRC, 2008). In addition, the CASAC strongly supported the new approach as an 

improvement and endorsed the way it was utilized in the HREA, stating that “the quadratic rollback approach has 

been replaced by a scientifically more valid Higher-order Decoupled Direct Method (HDDM)” and that “[t]he 

replacement of the quadratic rollback procedure by the HDDM procedure is important and supported by the 

CASAC” (Frey, 2014a, pp. 1 and 3). 

90 Within urban study areas, the model-based air quality adjustments show reductions in the O3 levels at the upper 

ends of ambient air concentrations and increases in the O3 levels at the lower ends of those distributions (2014 

HREA, section 4.3.3.2, Figures 4-9 and 4-10). It is important to note that sensitivity analyses in the HREA 

indicate that the increases in low O3 concentrations are smaller when NOX and VOC emissions are reduced 

together than when only NOX emissions are reduced (2014 HREA, Appendix 4-D, section 4.7). Seasonal means 

of daily O3 concentrations generally exhibit only modest changes upon model adjustment, reflecting the seasonal 
October 2018 5-8 Draft – Do Not Quote or Cite

1 5.1.1.1 Exposure-based Risk Analyses 

2 As noted above, two exposure-based risk analyses were performed for the 2014 HREA in 

3 the last review: one involving comparison of population exposures, while at elevated exertion, to 

4 benchmark concentrations, and the second involving estimated population occurrences of O3-

5 associated lung function decrements (Figure 5-3). The exposure-to-benchmark comparison 

6 characterizes the extent to which individuals in at-risk populations could experience exposures of 

7 concern (i.e., concentrations at or above specific benchmarks while at moderate or greater 

8 exertion levels) while engaging in their daily activities in study areas with air quality just 

9 meeting different O3 standards. The lung function risk analysis provides estimates of the extent 

10 to which populations in such areas could experience decrements in lung function. For the former, 

11 results were characterized using three benchmark concentrations (60, 70, and 80 ppb O3), 

12 exposures to which in controlled human exposure studies yielded different occurrences and 

13 severity of respiratory effects in the human subjects (2014 HREA, section 3.2). Similarly, based 

14 on the range of health effects considered clinically relevant and the potential for varied responses 

15 in healthy individuals versus people with asthma, the lung function risk analysis reported results 

16 for three different magnitudes of lung function decrement (FEV1 reductions of at least 10%, 

17 15%, and 20%) (2014 HREA, section 6.2.1). 

18 The risk analysis involving comparison of 8-hour average exposure concentrations to 

19 benchmark concentrations (section 5.1.1.1.1) provides perspective on the extent to which air 

20 quality adjusted to just meet different standards could be associated with discrete exposures to O3

21 concentrations reported to result in respiratory effects. For example, estimates of such exposures 

22 can provide a sense of the potential for O3-related effects in the exposed population, including 

23 effects for which we do not have exposure-response relationships that could be used in 

24 quantitative risk analyses (e.g., airway inflammation). The exposure benchmark analysis differs 

25 from the second exposure-based risk analysis which estimates the population incidence of days 

26 with lung function decrements of magnitudes of interest. In the lung function risk analysis 

27 (section 5.1.1.1.2), the time-series of exposures (rather than 8-hour average exposures) for each 

28 modeled individual is used to estimate the associated occurrence of lung function decrements in 

29 that individual. 

30 

 

balance between daily decreases in relatively higher concentrations and increases in relatively lower 

concentrations (2014 HREA, Figures 4-9 and 4-10). 
October 2018 5-9 Draft – Do Not Quote or Cite

1 

2 Figure 5-3. Analytical approach for exposure-based risk analyses. Dashed lines and gray 

3 box indicates the sole lung function risk approach used prior to 2014 HREA.

4 The 2014 HREA derived both exposure-based analysis results for a set of populations in 

5 the 15 study areas under the conditions for each of the air quality scenarios. Population-based 

6 exposures used for analyses in the 2014 HREA were estimated using the Air Pollutants Exposure 

(APEX) model.91 7 The APEX model is a probabilistic model that simulates a large number of 

 

91 Exposure modeling has been employed in the past several reviews of the O3 NAAQS, as well as reviews of the 

primary NAAQS for sulfur oxides, oxides of nitrogen, and carbon monoxide (U.S. EPA, 2008, 2009, 2010, 

2018). In the absence of large scale exposure studies that encompass the general population, as well as at-risk 

Exposure Modeling (APEX)

(exposure concentrations and ventilation rate for each individual’s exposure events)

MSS-FEV1

Lung Function 

Risk Model

Exposures (at elevated exertion) 

at or above Benchmarks

Output: Number and percent of at-risk populations 

at moderate or greater exertion estimated to be 

exposed to daily maximum O3 concentrations that 

exceed benchmark concentrations

Lung Function Risk

Output: Number and percent of simulated at-risk 

population groups estimated to experience 1 or 

more days with lung function responses

(FEV1 >10%, 15% and 20%)

Hourly concentrations at monitoring site locations for different 

AQ scenarios (just meeting existing and alternative standards)

Exposure Risk Air Quality

Adjustment factors

Controlled Human 

Exposure Data

(exposures involving 

moderate/greater 

exertion)

Health-Based 

Benchmark 

Concentrations

Photochemical 

Air Quality 

Modeling

Ambient Air Monitoring Data (hourly concentrations)

Hourly concentrations at monitoring sites

Voronoi Neighbor Averaging (VNA) Interpolation

Time series of O3

exposure events 

(concentrations and 

ventilation rates) for 

each individual

Population counts of 8-

hour daily maximum O3

exposures at elevated 

ventilation

Hourly concentrations at census tracts

ExposureResponse 

(E-R) 

Function

Population counts 

of 8-hour daily 

maximum O3

exposures at 

elevated ventilation
October 2018 5-10 Draft – Do Not Quote or Cite

1 randomly sampled individuals residing within a given study area (i.e., 50,000 to 200,000 people, 

2 depending on the simulated study group). U.S. Census demographic data are used by APEX, 

3 typically at a census tract level, to weight the population distribution within the geographic area 

4 and best represent area-wide population exposures. The APEX model simulates the movement of 

5 individuals through time and space by accounting for the places they may visit and the activities 

6 they may perform, and then estimates their time-series of O3 exposures occurring within indoor, 

7 outdoor, and in-vehicle microenvironments (2014 HREA, section 5.1.3). By incorporating 

8 individual activity patterns, the model estimates physical exertion associated with each exposure 

event.92 9 This aspect of the exposure modeling is critical in assessing exposure, intake dose, and 

10 associated health risk for ambient air concentrations of O3. 

11 The APEX model accounts for the most important factors that contribute to human 

12 exposure to O3 from ambient air, including the temporal and spatial distributions of people and 

13 O3 concentrations throughout a study area, the variation of O3 concentrations within various 

14 microenvironments in which people conduct their daily activities, and the effects of activities 

15 involving different levels of exertion on breathing rate (or ventilation rate) for the exposed 

16 individuals of different sex, age and body mass in the area simulated (2014 HREA, section 

17 5.1.3). To the extent spatial and/or temporal patterns of ambient air O3 concentrations are 

18 modified upon air quality adjustment approaches, as discussed above, exposure estimates reflect 

19 population exposures to those modified patterns of ambient air concentrations. 

20 In representing personal time-location-activity patterns of simulated individuals, the 

21 APEX model draws from the consolidated human activity database (CHAD) developed and 

22 maintained by the EPA, Office of Research and Development (McCurdy et al., 2000; U.S. EPA, 

2017).93 23 The activity patterns of individuals are an important determinant of their exposure due 

24 to the influence of exposure concentration, event duration, and ventilation rate (2013 ISA, 

25 section 4.4.1). Because of variation in O3 concentrations among the various microenvironments 

26 in which individuals are active, the amount of time spent in each location, as well as the exertion 

27 level of the activity performed, will influence an individual’s exposure to O3 from ambient air 

 

populations, modeling is the preferred approach to estimating exposures to O3. Additional information on APEX 

can be found at: https://www.epa.gov/fera/human-exposure-modeling-air-pollutants-exposure-model. 

92 An exposure event occurs when a simulated individual inhabits a microenvironment for a specified time, while 

engaged at a constant exertion level and experiencing a particular pollutant concentration. If the 

microenvironmental concentration and/or activity/activity level changes, a new exposure event occurs (McCurdy 

and Graham, 2003). 

93 The CHAD is comprised of data from several surveys that collected activity pattern data at city, state, and national 

levels. Included are personal attributes of survey participants (e.g., age, sex), the locations visited and activities 

performed by survey participants throughout a day, and the time-of-day activities occurred and their duration. 

Additional information is available at: https://www.epa.gov/healthresearch/consolidated-human-activitydatabase-chad-use-human-exposure-and-health-studies-and 
October 2018 5-11 Draft – Do Not Quote or Cite

1 and potential for adverse health effects. Activity patterns vary both among and within 

2 individuals, resulting in corresponding variations in exposure across a population and over time 

3 (2013 ISA, section 4.4.1). For each exposure event, APEX tracks activity, ventilation rate, 

4 exposure concentration, and duration. The time-series of exposure events serve as the basis for 

5 exposure metrics of interest, such as the daily maximum 8-hour exposure. Development of the 

6 two exposure-based metrics derived for the 2014 HREA (comparison to benchmarks and lung 

7 function risk) is summarized in the subsections below. 

8 5.1.1.1.1 Benchmark Comparison 

9 In the comparison-to-benchmarks analysis for the last review, the percent and number of 

10 individuals in the study area populations expected to experience one or more days with an 

11 exposure at or above benchmark concentrations, while at specified exertion levels, were 

12 estimated (2014 HREA, chapter 5). As summarized in section 3.1 above, the benchmark 

13 concentrations for this analysis (60, 70, and 80 ppb O3) were established based on a set of 

14 controlled human exposure studies of healthy adults engaged in moderate or greater exertion, 

15 while exposed to those concentrations (2013 ISA, section 6.2; 2014 PA, section 3.1.2.1). These 

16 studies employed a 6.6-hour continuous exposure during which subjects participated in five 50-

17 minute exercise periods, each followed by 10-minute rest periods, with a 35-minute lunch period 

18 after the third hour (e.g., Folinsbee et al., 1998 and Schelegle et al., 2009). The lowest 

19 benchmark, 60 ppb, represents the lowest O3 exposure concentration for which these controlled 

20 human exposure studies have reported respiratory effects. At this concentration, there is evidence 

21 of a statistically significant decrease in lung function and increase in airway inflammation 

(Brown et al., 2008; Adams, 2006). Exposure to approximately 70 ppb94 22 resulted in larger lung 

23 function decrements (and greater prevalence of decrements) than was observed for 60 ppb, as 

24 well as an increase in prevalence of respiratory symptoms. Exposures at 80 ppb O3 resulted in 

25 larger lung function decrements than following exposures to 60 or 70 ppb, in addition to an 

26 increase in airway inflammation, increased respiratory symptoms, increased airway 

27 responsiveness, and decreased resistance to other respiratory (section 3.1.2.1, above). 

28 For the 2014 REA, exposures were estimated for four study groups: all school-age 

29 children (ages 5 to 18), school-age children with asthma, adults with asthma (ages 19 to 95), and 

30 all older adults (ages 65 to 95) in each of the 15 urban study areas (2014 HREA, section 5.2.5). 

31 The results given primary attention in the review were those for school-age children (ages 5-18), 

including school-age children with asthma,95 32 both of which were identified as key at-risk 

 

94 The study on which the 70 ppb benchmark concentration is based, Schelegle et al. (2009), reported that the actual 

mean exposure concentration was 72 ppb. 

95 In terms of the percentage of the exposed population experiencing days at or above the benchmark concentrations, 

the estimates for all children and children with asthma are virtually indistinguishable (2014 HREA, Chapter 5). 
October 2018 5-12 Draft – Do Not Quote or Cite

1 populations in the ISA (2014 PA, section 3.1.5). The percentages of children estimated to 

2 experience exposures above benchmarks are considerably larger than the percentages estimated 

3 for adult populations (2014 HREA, section 5.3.2 and Figures 5-5 to 5-8). The larger exposure 

4 estimates for children are due primarily to the larger percentage of children estimated to spend an 

5 extended period of time being physically active outdoors during times of day when O3

6 concentrations are elevated compared to other population study groups (2014 HREA, sections 

7 5.3.2 and 5.4.1). 

8 In estimating the exposures used for comparison to benchmark concentrations, the APEX 

9 model averages exposures over a duration of interest. In addition, the model averages the 

ventilation rate (V̇ 10 E) for the exposed individual (based on the activities performed) over that 

exact same period. This can be done because APEX simultaneously estimates V̇ 11 E and exposure 

12 concentration for every individual’s time-series of exposure events. For the exposure duration of 

13 interest (e.g., 5 minutes, 1 hour or 8 hours), the model then derives and outputs the daily 

maximum average V̇ E (and hence an equivalent ventilation rate or EVR)96 14 and exposure 

15 concentration for the specified duration for each simulated individual. The model produces 

16 summary tables based on comparison to the specified benchmark concentrations. The averaging 

time and EVR used in the 2014 HREA – 8-hour average and 13 L/min-m2 17 – reflect parameter 

18 values for the exposure assessments performed for the last three O3 NAAQs reviews (2014 

19 HREA; U.S. EPA, 2007; Whitfield, 1996). Additional details on this analysis are provided in 

20 Chapter 5 and the associated appendices of the 2014 HREA. 

21 5.1.1.1.2 Lung Function Risk Assessment 

22 In the 2014 HREA, risk of lung function decrements in terms of FEV1 reductions of at 

least 10%, 15% and 20% was estimated using two different approaches.97 23 The primary estimates 

24 were based on a new approach that estimates FEV1 responses for simulated individuals 

25 associated with short-term exposures to O3 (McDonnell, Stewart, and Smith, 2007, 2010; 

26 McDonnell et al., 2012). This approach (termed here, the McDonnell-Stewart-Smith [MSS]-

27 FEV1 model) uses the time-series of O3 exposure, corresponding ventilation rates, and a few 

 

This is because HREA analyses indicate that activity data (i.e., time spent outdoors, exertion level) for people 

with asthma are generally similar to that for the population not having asthma (2014 HREA, Appendix 5G, 

Tables 5G-2 to 5G-5). 

96 To reasonably extrapolate the ventilation rate of the controlled human study subjects (i.e., adults having a 

specified body size and related lung capacity), who were engaging in quasi-continuous exercise during the study 

period, to individuals having varying body sizes (e.g., children with smaller size and related lung capacity), an 

equivalent ventilation rate (EVR) was calculated by normalizing the ventilation rate (L/min) by body surface area 

(m2

). 

97 Both approaches to estimating lung function risk have been implemented in the air pollution exposure model 

APEX (U.S. EPA, 2012a,b). 
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1 other influential personal attributes (e.g., age, body surface area) for each APEX simulated 

2 individual to estimate their personal time-series of O3-associated FEV1 reductions, effectively 

3 utilizing an individual-based approach to estimate lung function risk. When selecting for the 

4 daily maximum FEV1 reduction for each person and aggregating across individuals, APEX 

5 estimates the percent and number of people at risk, i.e., those experiencing FEV1 reductions of at 

6 least 10%, 15% and 20%, in a study area. 

7 The 2014 HREA also provided lung function risk estimates following the methodology 

8 used in the previous reviews which employs a simpler, population-based E-R approach to 

9 estimate the percent and number of people at risk in a study area. More specifically, a Bayesian 

10 Markov Chain Monte Carlo approach was used to develop probabilistic E-R functions to 

11 estimate the probability of O3-attributable lung function decrements (U.S. EPA, 2007). These 

12 functions were then combined with the APEX estimated population distribution of 8-hr 

maximum exposures for people at or above moderate exertion (≥ 13 L/min-m2 13 body surface area) 

14 to estimate the number of people expected to experience lung function decrements. The 2014 

15 HREA referred to this model as the population E-R model used in previous reviews. A key 

16 difference between the population-based E-R approach and the MSS-FEV1 model is that the 

17 previous method estimates a population distribution of FEV1 reductions by using the population18 based distribution of daily maximum 8-hour average concentrations while at moderate or greater 

19 exertion, where the MSS-FEV1 model estimates maximum FEV1 reductions at the individual 

20 level using their continuous time series of exposures and associated breathing rates. The 

21 individual estimates are then aggregated to a population level (2014 HREA, section 6.2.2). 

22 The MSS-FEV1 model was used in the 2014 HREA to estimate exposure-based lung 

23 function risk for three population groups: school-age children (5-18 years), young adults (19-35 

24 years), and adults (aged 36-55 years) in all 15 urban study areas (2014 HREA, section 6.3). This 

25 model (along with an age adjustment term) was developed based on data from study subjects 

26 aged 18 to 35 years and was used in the 2014 HREA to estimate lung function risk for 

27 individuals as young as 5 years and as old as 55 years based on the 2013 ISA’s interpretation of 
October 2018 5-14 Draft – Do Not Quote or Cite

1 the available information for these age groups (2014 HREA, section 6.2.4 and Appendix 6E).

98,99 2 

3 Additional details on this analysis are provided in Chapter 6 and the associated 

4 appendices of the 2014 HREA. 

5 5.1.1.2 Air Quality Epidemiologic Study-based Risk Analyses 

6 Ozone-associated risk of various respiratory health outcomes and mortality were 

7 estimated in twelve urban study areas using concentration-response (C-R) relationships drawn 

8 from the epidemiologic studies and “area-wide” average O3 concentrations, primarily in terms of 

several daily air quality metrics (HREA, Table 7-2, Appendix 7A).100 9 

10 The health outcomes for which O3-associated risk was estimated using the daily air 

11 quality metrics were: hospital admissions (HAs) for any respiratory outcome (Katsouyanni et al., 

12 2009; Linn et al., 2000); HAs for chronic lung disease, except asthma (Medina-Ramon et al., 

13 2006); emergency department (ED) visits for any respiratory outcome (Strickland et al., 2010, 

14 Tolbert et al., 2017, Darrow et al., 2011); ED visits for asthma (Ito et al., 2007), incidence of 

 

98 Assumptions made for extending the MSS-FEV1 model to children younger than 18 years old were in part based

on a McDonnell et al. (1985) study of children aged 8 to 11 years old who experienced FEV1 responses similar to 

those observed in adults aged 18 to 35 years old when both groups were exposed to 120 ppb O3 at an EVR of 32-

35 L/min/m2

. In addition, summer camp studies of school-aged children exposed outdoors in the Northeast also 

showed O3-induced lung function changes similar in magnitude to those observed in controlled human exposure 

studies using adults (e.g., Spektor et al., 1988; Spektor and Lippmann, 1991; see ISA section 6.2.1.2). Thus, for 

children younger than 18 years old, we set the MSS-FEV1 model age term to its highest value, the value used for 

age 18. 

99 Assumptions made for extending the MSS-FEV1 model to adults older than 35 years old were based on evidence

indicating lung function responses to O3 exposure for adults older than 18 decrease with age until around age 55, 

when responses are minimal. “Children, adolescents, and young adults appear, on average, to have nearly 

equivalent spirometric responses to O3, but have greater responses than middle-aged and older adults when 

similarly exposed to O3” (2013 ISA p. 6-21). “In healthy individuals, the fastest rate of decline in O3

responsiveness appears between the ages of 18 and 35 years (Passannante et al., 1998; Seal et al., 1996), more so 

for females than males (Hazucha et al., 2003). During the middle age period (35-55 years), O3 sensitivity 

continues to decline, but at a much lower rate. Beyond this age (>55 years), acute O3 exposure elicits minimal 

spirometric changes” (2013 ISA p. 6-23). Based on the effect age has on responses observed for middle aged 

adults, the model was set with a linearly decreasing response with increasing age for individuals between ages 36 

to 55. For adults older than 55 years, the MSS-FEV1 model age term was nullified (2014 HREA, sections 6.2.3.1 

and 6.2.4; 2013 ISA, pp. 6-21 and 6-23). Simulations were still performed for adults older than 55 years; 

however, there was minimal O3-induced lung function risk estimated for any of the air quality scenarios (HREA, 

section 6.6). 

100 The air quality metrics analyzed in the epidemiologic studies from which concentration-response relationships 

were taken are daily maximum 1-hour, daily maximum 8-hour average and daily 24-hour average concentrations, 

each averaged across multiple monitors within study areas (2014 HREA, Appendix 7A, Table 7-2). The 

epidemiologic studies use these ambient air quality metrics as surrogates for the spatial and temporal patterns of 

exposures in study populations. Accordingly, the HREA applied the C-R functions obtained from the 

epidemiologic studies to O3 concentrations in terms of these same ambient air metrics, as averaged across 

ambient air monitor locations in each study area (2014 HREA, section 4.3.2.2). In the last review, we referred to 

these area-averaged concentrations as “composite monitor” or “area-wide” O3 concentrations (e.g., 2014 PA, 

section 3.1.4; 2014 HREA, section 4.3.2.2). 
October 2018 5-15 Draft – Do Not Quote or Cite

1 asthma exacerbation-related chest tightness, shortness of breath or wheeze (Gent et al., 2003); 

and mortality (Smith et al., 2009; Zanobetti and Schwartz, 2008; Jerrett et al., 2009101 2 ). Risk 

estimates were derived for each health outcome for 12 urban study areas,102 3 or a subset thereof, 

4 depending on the array of study areas included in the epidemiologic studies from which each C5 R function was drawn (HREA, Table 3-1). 

6 These risk estimates were derived for air quality scenarios involving unadjusted air 

7 quality from five years encompassing two 3-year periods (2006-2008, 2008-2010), model8 adjusted air quality just meeting the then-current standard (75 ppb), and three potential 

9 alternative standards with alternative levels of 70, 65 and 60 ppb (2014 HREA, section 7.1.1). 

10 The risk estimates were derived using the EPA’s Environmental Benefits Mapping and Analysis 

Program (BenMAP, version 4.0)103 11 for the specified health outcomes and locations with the C-R 

12 information from the studies cited for those outcomes and other relevant information for the 

13 analysis. In presenting the results for the two 3-year periods assessed for each air quality 

14 scenario, the HREA presented the annual risk estimates for one year with generally higher O3

15 concentrations (2007) and one year with generally lower O3 concentrations (2009). Additional 

16 detail on these analyses is provided in section 3.7, Chapter 7 and the associated appendices of the 

17 2014 HREA. 

 

101 The C-R relationships from Jerrett et al. (2009) related O3-associated respiratory mortality to seasonal averages 

of daily max 1-hour O3. 

102 The 12 urban areas were Atlanta, Baltimore, Boston, Cleveland, Denver, Detroit, Houston, Los Angeles, New 

York, Philadelphia, Sacramento, and St. Louis. 

103 BenMAP is a GIS-based computer program that draws upon a database of population, baseline 

incidence/prevalence rates and effect coefficients to automate the calculation of health impacts (2014 HREA, 

Chapter 7; U.S. EPA, 2013b). Additonal information available at: https://www.epa.gov/benmap. 
October 2018 5-16 Draft – Do Not Quote or Cite

1 

2 

3 Figure 5-4. Analytical approach for epidemiologic-based analyses. 

4 5.1.2 Consideration of Assessments for this Review 

5 Our planning for assessments in this review will consider the uncertainties and 

limitations that were highlighted during the last review104 6 in order to direct new analyses (if any) 

7 toward reducing such uncertainties. This could potentially improve the utility of risk estimates in 

8 informing the current review. As in any review, key considerations in planning risk and exposure 

9 analyses that may be appropriate include: 

10  Availability of new information, models and tools since completion of the prior 

11 assessment that have potential to better characterize key areas of uncertainty; 

 

104 The 2014 HREA included a characterization of uncertainty in which some assessment elements were judged 

regarding the potential for associated uncertainty to influence the risk estimates (2014 HREA, sections 5.5, 6.5.7, 

7.4.2). This was one of the characterizations of uncertainties that Appendix 5-A is drawn from, with particular 

attention given to those elements described to have the potential for a “moderate” or greater influence on risk 

estimates. Other sources for this section and for Appendix 5-A are discussions in the 2014 PA, the proposed and 

final rulemaking notices in the last review and consideration of public comments in the 2015 response to 

comments document. 

Output: Incidence of hospital admissions, 

emergency room visits, mortality

Ambient Air Monitoring Data (hourly concentrations)

Hourly concentrations at monitoring sites for different AQ 

scenarios (just meeting existing and alternative standards)

Risk Air Quality (Exposure)

Photochemical 

Air Quality 

Modeling

Adjustment factors

Hourly concentrations at monitoring sites

Area-averaged daily O3 ambient air concentration metrics for 

each study area and the different AQ scenarios 

(just meeting existing and alternative standards)

Environmental Benefits Mapping and Analysis 

Program (BenMAP)
October 2018 5-17 Draft – Do Not Quote or Cite

1  Identification of model/assessment aspects for which updates may reduce uncertainty or 

2 address limitations, thus improving appropriateness of model outputs for their intended 

3 purposes. 

4 With regard to the exposure-based analyses, a number of important uncertainties were 

5 identified in the last review, largely related to estimation of ambient air concentrations, 

6 estimating exposure concentrations (and associated exertion levels), and modeling lung function 

7 decrements. In this review, there are newly available ambient air quality data that better reflect 

8 concentrations at or near the current standard, updated emissions data and air quality models, and 

9 updates to the exposure model to better estimate exposure-based risk. Regarding the 

10 epidemiological-based risk approach, there were also a number of important uncertainties 

11 identified in the 2014 HREA; however, it is expected that, for most if not all of the recognized 

12 uncertainties outside of those related to the estimation of ambient air quality, there is unlikely to 

13 be newly available information, models, or tools that would result in substantially improved risk 

estimates with appreciably less uncertainty than those in the 2014 HREA.105 14 

15 Accordingly, we expect that any new quantitative analyses in this review will likely focus 

16 on exposure-based analyses that can benefit from updated information or methods, ensuring that 

17 the new exposure and risk estimates are both improved and appropriately targeted. Estimates 

18 from the exposure-based analyses, particularly the comparison to benchmark concentrations, 

19 were most informative to the Administrator’s decision in the last review (as summarized in 

20 section 3.1.2 above). This largely reflected the EPA conclusion that “controlled human exposure 

21 studies provide the most certain evidence indicating the occurrence of health effects in humans 

22 following specific O3 exposures,” and recognition that “effects reported in controlled human 

23 exposure studies are due solely to O3 exposures, and interpretation of study results is not 

24 complicated by the presence of co-occurring pollutants or pollutant mixtures (as is the case in 

25 epidemiologic studies)” (80 FR 65343, October 26, 2015). In the last review, the Administrator 

26 placed relatively less weight on the air quality epidemiologic-based risk estimates, in recognition 

27 of an array of uncertainties, including, for example, those related to exposure measurement error 

28 (80 FR 65346, October 26, 2015). 

29 Therefore, based on preliminary consideration of the information cited here, including 

30 consideration of the complex and extensive exposure and risk analyses performed for the 2014 

 

105 There are a number of important uncertainties associated with aspects of the O3 epidemiologic study-based 

approach used in the last review for which information available in this review is not expected to appreciably 

affect, such that they are expected to still have a moderate or greater impact on risk estimates. Such uncertainties 

include those in the relationship between population O3 exposures and ambient air monitor concentrations 

(including the use of area wide average O3 concentrations) and uncertainties in the concentration-response 

functions (e.g., the shape of concentration response curves at the lowest O3 concentrations). See Appendix 5A for 

details. 
October 2018 5-18 Draft – Do Not Quote or Cite

1 REA, and given the expedited nature of this review, we are preliminarily planning to focus new 

2 analyses in this review on exposure-based risk analyses. This would reflect the emphasis given to 

3 these analyses and the characterization of their uncertainties in the last review, along with the 

4 expectation of having newly available information, models, and tools that could increase our 

5 confidence in risks estimated in the last review. Any updates to these analyses would build upon 

6 more recent air quality and updated photochemical modeling in estimating ambient air 

7 concentrations for the air quality scenarios to be assessed. Further, a number of updates to the 

8 exposure modeling approach can be explored in consideration of uncertainties recognized in the 

last review (e.g., exposure duration and target V̇ 9 E for benchmark comparisons and lung function 

10 risk based on the population E-R approach) (Appendix 5-A). Given the rapid timeline for this 

11 review, we would expect to focus on a streamlined set of study areas and air quality scenarios 

12 compared to the expansive set assessed in the last review. 

13 We expect to consider in the PA other types of analyses from the last review that we do 

14 not update in this review but that remain informative to this review when viewed in the context 

15 of the currently available evidence as characterized in the ISA and of updated air quality and 

16 other analyses performed for this review. Accordingly, the PA for this review will describe and 

17 discuss all risk and exposure analyses being considered in this review. This would include risk 

18 and exposure analyses newly developed in this review, as well as analyses performed for the last 

19 review for which updated analyses were not performed. The draft PA will be released for public 

20 comment and provided to the CASAC for their review. Advice and comments received on this 

21 information will be considered in completing any updated risk and exposure analyses and 

22 drawing on them in the policy evaluations presented in the final PA. 

23 5.2 ASSESSMENTS INFORMING REVIEW OF THE SECONDARY 

24 STANDARD 

25 In reviews of secondary standards, quantitative exposure and risk assessments for welfare 

26 effects are generally intended to inform consideration of key policy relevant questions (see 

27 section 3.2), such as the following: 

28  What are the nature and magnitude of exposure- and risk-related estimates for welfare 

29 effects associated with air quality conditions just meeting the current standard? 

30  To what extent are the estimates of exposures and risks associated with air quality 

31 conditions just meeting the current standard reasonably judged important from a public 

32 welfare perspective? 

33 In considering exposure and risk estimates in this context, an accompanying important 

34 consideration is: 

35  What are the important uncertainties associated with any risk/exposure estimates? 
October 2018 5-19 Draft – Do Not Quote or Cite

1 The types of analyses performed generally reflect the nature and strength of the evidence 

2 in various aspects. For example, for the welfare effects pertaining to exposures associated with 

3 the presence of the pollutant in ambient air, the availability of concentration-response or dose4 response data from the welfare or ecological effects literature influences the types of exposure 

5 assessment and risk characterization that are performed. The assessments focus on exposure 

6 metrics that are appropriate for effects of concern for the subject pollutant, with available 

7 measurements and model estimates, where appropriate, used to generate estimates of exposure. 

8 A number of different exposure/risk analyses were conducted in the last review of the 

9 secondary O3 standard. They included extensive air quality-based analyses, E-R analyses and 

10 some monitoring-based analyses in the 2014 WREA, as well as monitoring-based analyses in the 

11 2014 PA and in technical memoranda developed for the rulemaking notices. Some types of these 

12 quantitative analyses were more informative to the 2015 decision on the standard than others. 

13 With regard to the questions above, the uncertainties associated with results for some analyses 

14 limited their use in the Administrator’s decision-making, while uncertainties regarding public 

15 welfare significance of the findings for other analyses also limited such use of those analyses. In 

16 general, decision-making in the last review placed greatest weight on estimates of cumulative 

17 exposures to vegetation based on ambient air monitoring data and consideration of those 

18 estimates in light of E-R relationships for O3-related reduction in tree growth (summarized in 

19 section 3.2 above). These analyses supported the Administrator’s consideration of the potential 

20 for O3 effects on tree growth and productivity, as well as its associated impacts on a range of 

21 ecosystem services, including forest ecosystem productivity and community composition (80 FR 

22 65292, October 26, 2015). 

23 In the first section below (section 5.2.1), we provide an overview of the set of 

24 assessments performed in the 2015 review. In the subsequent section (section 5.2.2), the relative 

25 roles of the analyses in judgments made and conclusions reached in the 2015 review are 

26 indicated, along with some key uncertainties and limitations of the analyses. In this section we 

27 additionally consider information, methods or tools that may be newly available in this review 

28 and that may address these uncertainties or limitations and thus provide for the development of 

29 appreciably improved analyses that might be considered in this review, in combination with the 

30 comprehensive analyses developed in the last review that remain informative to this review. 

31 5.2.1 Overview of Assessments in Last Review 

32 Quantitative analyses performed in the last review included both the extensive analyses 

33 presented in the 2014 WREA and also a smaller set of additional analyses, which were presented 

34 in the 2014 PA or in technical memoranda to the rulemaking docket and that were described in 

35 the notices of proposed and final rulemaking for the 2015 decision. 
October 2018 5-20 Draft – Do Not Quote or Cite

1 The full set of analyses presented in the 2014 WREA were generally related to two types 

2 of effects on vegetation: (1) reduced growth in both trees (relative biomass loss or RBL) and 

3 agricultural crops (relative yield loss or RYL), and (2) visible foliar injury (2014 PA; 2014 

4 WREA; 80 FR 65374-65376, October 26, 2015; 79 FR 75324-75329, December 17, 2014). 

5 Assessments of O3-associated reduced growth in native trees and crops were based on E-R 

6 functions described in the 2013 ISA for a set of tree and crop species and estimates of O3

7 exposures. These were done nationally, as well as in a small set of study areas. The foliar injury 

8 related analyses were based on information from the U.S. Forest Service (USFS) that included 

estimates of W126-based cumulative exposure106 9 and foliar injury scores at established 

biosites107 in 41 states in the contiguous U.S.108 10 Analyses of reduced growth, in both trees and 

11 agricultural crops, are described in section 5.2.1.1 and assessments regarding visible foliar injury 

are described in section 5.2.1.2.109 12 The additional analyses, which, in combination with E-R 

13 functions described in the 2014 WREA and summarized in the 2014 PA, proved to be more 

14 informative to the 2015 decision than the WREA analyses, are summarized in section 5.2.1.3 

15 (2014 WREA, section 6.2; 2014 PA, section 5.2.1; 80 FR 65382-65410, October 26, 2015). 

16 5.2.1.1 Growth-related Assessments 

17 The growth-related assessments performed in the WREA included national-scale 

18 analyses of tree growth (in terms of RBL) and crop yield (in terms of RYL), and also estimation, 

19 at national or smaller scales, of associated changes in related ecosystem services, including 

20 pollution removal, carbon sequestration or storage, and hydrology, as well as impacts on the 

21 forestry and agriculture sectors of the economy. These were conducted for several air quality 

22 scenarios developed by adjusting air quality data using factors derived from regional 

23 photochemical modeling to achieve reduced concentrations of O3 that just met the different 

24 scenario objectives. 

25 The air quality scenarios included one in which the then-current standard was just met 

26 and additional scenarios in which the maximum 3-year W126-based exposure equaled 15, 11, 

 

106 The W126 index is described above in section 3.2.2. 

107 Sampling sites in the National Forest Service’s Forest Inventory and Analysis Forest Health Monitoring O3

biomonitoring program, called “biosites”, are plots of land on which data are collected regarding the incidence 

and severity of visible foliar injury on a variety of O3-sensitive plant species. Biosite index scores are derived 

from these data (2014 WREA, section 7.2.1). 

108 Data were not available for several western states (Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona, 

New Mexico, Oklahoma, and portions of Texas). 

109 The 2014 WREA also presented several more descriptive exposure analyses where W126-based cumulative O3

exposure was estimated for different modeled air quality scenarios in areas of high fire or beetle infestation threat 

(2014 WREA, sections 5.2.3 and 5.4). 
October 2018 5-21 Draft – Do Not Quote or Cite

and 7 ppm-hrs.110 1 These scenarios were developed from monitoring data for 2006 to 2008 and 

2 adjustments based on model-predicted relationships between the response of O3 concentrations at 

3 each monitor location to reductions in NOx emissions for the associated NOAA climate region. 

4 The adjustments were applied independently for each of the nine NOAA climate regions in the 

5 U.S., such that the highest monitor in the region was adjusted to just meet the target for the air 

6 quality scenario, and other monitor sites not already at/below the target for the scenario were 

adjusted by the same factor.111 7 Based on the adjusted concentrations at all monitor sites, 

8 concentrations were derived for each 12km by 12 km grid cell in a national-scale spatial surface 

9 by applying the Voronoi Neighbor Averaging (VNA) spatial interpolation technique to the 

10 monitor-location values. This step resulted in further reduction of the highest values in each 

modeling region.112 11 

12 Because the W126 estimates generated for the different air quality scenarios assessed are 

13 inputs to the vegetation risk analyses for tree biomass and crop yield loss, and also used in some 

14 components of the visible foliar injury assessments, limitations and uncertainties in the air 

15 quality analyses, which are discussed in detail in the WREA and some of which are mentioned 

16 here, were propagated into those analyses (2014 WREA, chapters 4 and 8, including section 8.5, 

17 and Table 4-5). An important uncertainty in the analyses is the application of adjustments at the 

18 regional-scale based on modeled emissions reductions in NOx that characterize only one 

19 potential distribution of air quality across a region for situations when all monitor locations in a 

20 region meet the then-current standard or the W126 cumulative exposure targets (2014 WREA, 

 

110 The target for each scenario was judged to have been met when the O3 concentrations at the monitor location 

with the highest concentrations equaled the target. For example, for the then-current standard scenario, the highest 

monitor location had a fourth highest daily maximum 8-hour O3 concentration averaged over three years equal to 

75 ppb. For the W126 scenario of 15 ppm-hrs, the target was met when the 3-year average W126 index value at 

the monitor with the highest 3-year W126 value equaled 15 ppm-hrs. The development of the air quality scenarios 

is further summarized in the final decision notice (80 FR 65374-65375, October 26, 2015) and Table 5-4 of the 

2014 PA, and described in detail in Chapter 4 and Appendix 4A of the 2014 WREA. 

111 The adjustment was based on the minimum percentage reduction in NOx emissions necessary to reduce O3

concentrations at all monitors within a region sufficiently to meet the target. This adjustment results in broad 

regional reductions in O3 and includes reductions in O3 at some monitors that were already at or below the target 

level (2014 WREA, sections 4.3.4.2 and 4.4). 

112 This is seen when comparing the W126 index values from before and after the application of the VNA approach 

to the then-existing standard scenario. After the adjustment of the monitor location concentrations such that the 

highest location in each NOAA region just met the then-existing standard (using the model-based relationships), 

the maximum 3-year average W126 values in the nine regions ranged from 18.9 ppm-hrs in the West region to 

2.6 ppm-hrs in the Northeast region (2014 WREA, Table 4-3). After application of the VNA technique, however, 

the highest 3-year average W126 values across the national surface grid cells, which were in the Southwest 

region, were below 15 ppm-hrs (2014 WREA, Figure 4-7). Thus, it can be seen that application of the VNA 

interpolation method to estimate W126 index values at the centroid of every 12 km x 12 km grid cell compared to 

only at each monitor location results in a lowering of the highest values in each region (80 FR 65374, October 26, 

2018). 
October 2018 5-22 Draft – Do Not Quote or Cite

1 section 4.3.4.2). The impact of the approach’s broad regional reductions on O3 concentrations at 

2 monitor locations that were already well below the target indicated an uncertainty with regard to 

3 air quality expected from specific control strategies that might be implemented to meet a 

4 particular target level (80 FR 65375, October 26, 2015). 

5 An additional uncertainty related to the W126 index estimates in the national surfaces for 

6 each air quality scenario, and to the estimates for the single-year surfaces used in the visible 

7 foliar injury cumulative analysis, is associated with the creation of the national-scale spatial 

surfaces of grid cells from the monitor-location O3 data.113 8 In general, spatial interpolation 

9 techniques perform better in areas where the O3 monitoring network is denser. Therefore, the 

10 W126 index values estimated using this technique in those rural areas within the West, 

11 Northwest, Southwest, and West North Central regions where there are few or no monitors (2014 

12 WREA, Figure 2-1) are more uncertain than those estimated for areas with denser monitoring. 

13 Further, as noted above, this interpolation method may underpredict the highest W126 exposure 

14 index values in a region. Due to the important influence of higher exposures in determining risks 

15 to plants and the potential for the interpolation step to dampen peak W126 index values, some 

16 risk underestimation could have resulted. 

17 The assessments related to tree growth relied on the species-specific E-R functions 

18 referenced in section 3.2.2 above. For the air quality scenarios assessed, the species-specific E-R 

19 functions were used to develop estimates of O3-associated RBL and associated effects on 

20 productivity, carbon storage and associated ecosystem services (2014 WREA, Chapter 6). More 

21 specifically, the WREA derived species-specific and weighted RBL estimates for grid cells 

22 across the continental U.S. and summarized the estimates by counties, regions and Class I areas 

23 and national parks (2014 WREA, section 6.2.1 and 6.8). Potential impacts on commercial timber 

24 were also estimated (2014 WREA, section 6.3). Additional case study analyses estimated 

25 impacts on carbon removal and pollutant removal in selected urban areas (2014 WREA, sections 

26 6.6.2 and 6.7). 

27 Relative biomass loss nationally (across all of the air quality surface grid cells) was 

estimated for each of eleven studied species114 28 from the composite E-R functions for each 

29 species and information on the distribution of those species across the U.S. (2014 WREA, 

30 section 6.2.1.3 and Appendix 6A). These analyses provided estimates of per-species RBL, as 

 

113 Some uncertainty is inherent in any approach to characterizing O3 air quality over broad geographic areas based 

on concentrations at monitor locations. 

114 In consideration of CASAC advice regarding uncertainties associated with the E-R function derived for a twelfth 

species, the eastern cottonwood, the WREA derived RBL and weighted RBL estimates separately, both with and 

without the eastern cottonwood, with primary focus given to analyses that excluded cottonwood (Frey, 2014c, p. 

10; 2014 WREA; 2014 PA; 79 FR 75234, December 17, 2014; 80 FR 65292, October 26, 2015). 
October 2018 5-23 Draft – Do Not Quote or Cite

1 well as median and total RBL across resident species in the different air quality scenarios. The 

2 WREA also used the E-R functions to estimate RBL across tree lifespans and the resulting 

3 changes in consumer and producer/farmer economic surplus in the forestry and agriculture 

4 sectors of the economy. Case studies in five urban areas provided comparisons across air quality 

5 scenarios of estimates for urban tree pollutant removal and carbon storage or sequestration (2014 

6 WREA, sections 6.6.2 and 6.7). The array of uncertainties associated with estimates from these 

7 tree RBL analyses, including those associated with the air quality adjustment approach which 

8 contributed to a potential for the air quality scenarios to underestimate the higher W126 index 

9 values and the associated implications for the RBL-related estimates, are summarized in section 

10 5.2.2 below. 

11 The assessments of O3 impacts on agricultural crops relied on the robust E-R functions 

12 established prior to the last review. For the different air quality scenarios, the WREA applied the 

13 species-specific E-R functions to develop estimates of O3 impacts related to crop yield, including 

14 annual yield losses, for 10 commodity crops grown in the U.S. and estimates of how these losses 

15 might be expected to affect producer and consumer economic surpluses (2014 WREA, sections 

16 6.2 and 6.5). The WREA derived estimates of crop RYL nationally and in a county-specific 

17 analysis, relying on information regarding crop distribution (2014 WREA, section 6.5). As with 

18 the tree analyses described above, the county analysis included estimates based on the median O3

19 response across the studied crop species (2014 WREA, section 6.5.1, Appendix 6B). 

20 Overall effects on agricultural yields and producer and consumer surplus depend on the 

21 ability of producers/farmers to substitute other crops that are less O3 sensitive, and the 

22 responsiveness, or elasticity, of demand and supply (2014 WREA, section 6.5). The WREA 

23 discusses multiple areas of uncertainty associated with the crop yield loss estimates, including 

24 those associated with the model-based adjustment methodology as well as those associated with 

25 the projection of yield loss using the Forest and Agriculture Sector Optimization Model (with 

26 greenhouse gases) at the estimated O3 concentrations (2014 WREA, Table 6-27, section 8.5) and 

27 the lack of a role in the assessment for agricultural crop management practices which have 

28 substantial influence on crop yield. Because the W126 index estimates generated in the air 

29 quality scenarios are inputs to the vegetation risk analyses for crop yield loss, any uncertainties 

30 in the air quality scenario estimation of W126 index values are propagated into those analyses 

31 (2014 WREA, Table 6-27, section 8.5). Therefore, the air quality scenarios in the crop yield 

32 analyses have the same uncertainties and limitations as in the biomass loss analyses (summarized 

33 above), including those associated with the model-based adjustment approach (2014 WREA, 

34 section 8.5). 
October 2018 5-24 Draft – Do Not Quote or Cite

1 5.2.1.2 Foliar Injury Assessments 

2 The foliar injury assessments involved analysis of W126 cumulative exposure estimates 

3 and foliar injury scores at USFS biosites for five years (2006-2010), and consideration of the 

4 implications of the analysis with regard to risk of O3-related foliar injury in nationally protected 

5 areas such as national parks (2014 WREA, Chapter 7; Smith and Murphy, 2015; 80 FR 65376, 

6 65395-65396, October 26, 2015). In the biosite data analysis, the WREA used the biomonitoring 

7 site data from the USFS FHM/FIA Network (USFS, 2011), associated soil moisture data during 

8 the sample years, and national surfaces of ambient air O3 concentrations based on spatial 

interpolation of monitoring data from 2006 to 2010115 9 in a cumulative analysis of the proportion 

10 of biosite records with any visible foliar injury, as indicated by a nonzero biosite index score 

11 (2014 WREA, section 7.2). This analysis was done for all records together, and also for subsets 

12 based on soil moisture conditions (normal, wet or dry). 

13 In each cumulative analysis, the biosite records were ordered by W126 index and then, 

14 moving from low to high W126 index, the records were cumulated into a progressively larger 

15 dataset. With the addition of each new data point (composed of biosite index score and W126 

16 index value for a biosite and year combination) to the cumulative dataset, the percentage of sites 

17 with a nonzero biosite index score was derived and plotted versus the W126 index estimate for 

18 the just added data point. This analysis was found to be appreciably affected by the larger 

19 representation within the subset of the lower W126 conditions which are associated with a lower 

occurrence or extent of foliar injury.116 20 Nearly two thirds of the dataset included records for 

21 which the W126 index estimates are at or below 11 ppm-hrs (Smith and Murphy, 2015, Table 1). 

22 In a technical memorandum prepared subsequent to the WREA, the same dataset was re23 presented in a different format to more directly consider what the data indicate with regard to a 

24 relationship between O3 exposure in terms of W126 and foliar injury. This presentation indicated 

25 the reduction in the occurrence (and severity) of visible foliar injury with decreasing exposures 

 

115 Estimates of W126 were drawn from national-scale spatial surfaces of single-year, unadjusted W126 index values 

created for each year from 2006 through 2010 using the VNA interpolation technique applied to the monitor 

location index values for these years (2014 WREA, section 4.3.2, Appendix 4A). 

116 The cumulative analysis for all sites indicated that (1) as the cumulative set of sites grows with addition of sites 

with progressively higher W126 index values, the proportion of the dataset for which no foliar injury was 

recorded changes (increases) noticeably prior to about 10 ppm-hrs (10.46 ppm-hrs), and (2) as the cumulative 

dataset grows still larger with the addition of records for higher W126 index estimates, the proportion of the 

cumulative dataset with no foliar injury remains relatively constant (2014 WREA, Figure 7-10). This “leveling 

off” (e.g., observed above ~10 ppm-hrs in the “all sites” analysis) likely reflects the counterbalancing of visible 

foliar injury occurrence at the relatively fewer higher O3 sites by the larger representation within the subset of the 

lower W126 conditions associated with which there is lower occurrence or extent of foliar injury (Smith and 

Murphy, 2015). 
October 2018 5-25 Draft – Do Not Quote or Cite

1 across a range that extended from above 19 ppm-hrs to below 7 ppm-hrs (Smith and Murphy, 

2015, Table 2).117 2 

3 5.2.1.3 Additional Air Quality/Exposure and E-R Analyses 

4 Additional analyses developed in the last review included two air quality and exposure 

5 analyses, summarized below, and a separate tabular presentation involving tree and crop E-R 

6 functions. The tabular presentation was based on the robust established E-R functions for growth 

7 effects on tree seedlings and crops was developed for the 2014 PA (2014 PA, Appendix 5C). 

This analysis presented the estimates of RBL118 8 (and RYL) at a range of W126-based exposure 

9 levels for 11 tree species and 10 crop species, respectively (2014 PA, Tables 5C-1 and 5C-2). 

10 Additionally, the median tree species RBL (or crop RYL) was presented for each W126 level 

11 (2014 PA, Table 5C-3; 80 FR 65391 [Table 4], October 26, 2015). As summarized in section 

12 3.2.2 above, the 2015 decision on the secondary standard included a focus on RBL as a surrogate 

13 or proxy for the broader array of vegetation-related effects of potential public welfare 

14 significance, which include effects on growth of individual sensitive species and extend to 

15 ecosystem-level effects, such as community composition in natural forests, particularly in 

16 protected public lands, as well as forest productivity (80 FR 65406, October 26, 2015). 

17 The first of the two sets of air quality/exposure analyses included the development of 

18 W126-based cumulative exposure estimates in Class I areas during 3-year periods that met the 

19 then-current standard (75 ppb, in terms of the 3-year average of consecutive year fourth highest 

20 daily maximum 8-hour averages). The second set of air quality/exposure analyses investigated 

21 the W126-based cumulative exposure estimates for locations and time periods that met the then22 current and several potential alternative standards, in terms of 3-year averages of the fourth 

23 highest daily maximum 8-hour average concentration. The former analysis was particularly 

24 informative to the decision regarding the need to revise the then-current standard of 75 ppb (80 

 

117 Criteria derived from the WREA cumulative analyses were used in two additional WREA analyses. The nationalscale screening-level assessment compared W126 index values estimated within 214 national parks using the 

VNA technique described above for the individual years from 2006 to 2010 with benchmark criteria developed 

from the biosite data analysis (2014 WREA, Appendix 7A and section 7.3). Separate case study analyses 

described visits, as well as visitor uses and expenditures for three national parks, and the 3-year W126 index 

estimates in those parks for the four air quality scenarios (2014 WREA, section 7.4). Uncertainties associated 

with these analyses, included those associated with the W126 index estimates, are discussed in the WREA, 

sections 7.5 and 8.5.3, and in WREA Table 7-24, and also summarized in the PA (2014 PA, section 6.3). 

118 These functions for RBL estimate the reduction in a year’s growth as a percentage of that expected in the absence 

of O3 (2013 ISA, section 9.6.2; 2014 WREA, section 6.2). In specifically evaluating exposure levels, in terms of 

the W126 index the 2014 PA focused particularly on RBL estimates for the median across the 11 tree species for 

which robust E-R functions are available (80 FR 65391-65392 [Table 4], October 26, 2015; 2014 WREA, 

Appendix 5C, Table 5C-3). 
October 2018 5-26 Draft – Do Not Quote or Cite

1 FR 65389-65390, October 26, 2015), while the second set of analyses informed the 

2 Administrator’s decision on the appropriate revision (80 FR 65403-65410, October 26, 2015). 

3 The first set of air quality/exposure analyses, as presented and relied upon in the final 

4 decision, was an update of an analysis initially presented in the 2014 PA (2014 PA, pp. 5-27 to 

5 5-29). Based on air quality data for the period from 1998 to 2013, the analysis focused 

consideration on 17 Class I areas,119 6 in which during one or more three-year periods the air 

7 quality met the current standard and the three-year average W126 index value was at or above 15 

8 ppm-hrs. The analysis that informed the 2015 decision was restricted to data for monitors sited in 

or within 15 kilometers of a Class I area.120 9 

10 This analysis considered cumulative exposure estimates in Class I areas during times that 

11 met the then-current standard in the context of such estimates associated with varying RBL 

12 values for the median species derived using the robust E-R functions for RBL in seedlings of 11 

13 species. The analysis gave particular weight to the W126 index values at or above 19 ppm-hrs, 

14 which were associated with a 6% median RBL, described as “unacceptably high” by the CASAC 

15 (80 FR 65391-92, October 26, 2015; Frey, 2014c). In the analysis, the numbers of areas, states 

16 and NOAA climatic regions, for which the 3-year W126 exposure index values ranged at or 

17 above 19 ppm-hrs were tallied and characterized as to magnitude and variation across the three 

18 years. 

19 The second set of air quality/exposure analyses were focused on air quality monitoring 

20 for O3 monitoring sites with complete data for the most recent 3-year period and also for periods 

extending back to 2001.121 21 This set was comprised of several analyses of air quality that

22 considered relationships between 3-year W126 index based exposure estimates and the design 

23 value for the then current standard (referred to as the “fourth-high” metric) (2014 PA, Chapter 2, 

24 Appendix 2B and section 6.4; Wells, 2015). These analyses indicated that, depending on the 

25 level, a standard of the then-current averaging time and form could be expected to control 

26 cumulative seasonal O3 exposures to such that they may meet specific 3-year average W126 

27 index values. The fourth-high and W126 metrics, and changes in the two metrics over the past 

 

119 For the four modeled air quality scenarios in the WREA, the WREA also derived detailed estimates of 3-year 

W126-based exposures in a screening-level national park assessment and in three individual national parks. (2014 

WREA, section 4.3.2, Appendix 4A). Limitations and uncertainties associated with the WREA air quality 

adjustment approach limited their usefulness in the EPA’s final decision-making. 

120 The 15 km distance was selected as a natural breakpoint in distance of O3 monitoring sites from Class I areas and 

as still providing similar surroundings to those occurring in the Class I area. We note that given the strict 

restrictions on structures and access within some of these areas, it is common for monitors intended to collect data 

pertaining to air quality in these types of areas to be sited outside their boundaries. 

121 These analyses are summarized and discussed in sections IV.C.1.c, IV.C.2.d and IV.C.3 of the 2015 decision 

notice and presented in detail in a technical memorandum to the rulemaking docket (80 FR 65292, 65400-65401, 

65408-65409, October 26, 2015; Wells, 2015). 
October 2018 5-27 Draft – Do Not Quote or Cite

1 decade, were found to be highly correlated (2014 PA, section 6.4 and Appendix 2B; Wells, 

2 2015). 

3 These analyses were performed for two recent periods (2009-2011 and 2011-2013), as 

4 well as extending back to 2001 (2014 PA, section 6.4; Wells, 2015). All NOAA climatic regions 

5 in the contiguous U.S. were represented. These analyses illustrated the extent and magnitude of 

6 W126-based exposures at monitoring sites meeting the then existing standard and alternate 

7 standards, including the now-current standard of 70 ppb (2014 PA, section 6.4 and Appendix 2B; 

8 Wells, 2015). 

9 5.2.2 Consideration of Assessments for this Review 

10 In the preceding section we have briefly summarized air quality, exposure and risk 

11 analyses developed in the last review, noting key uncertainties or limitations associated with the 

12 various assessments. In considering what types of assessments may be warranted for this review, 

13 we give particular attention to those types of analyses that formed the main foundation for 

14 conclusions in the last review due to their relatively lesser uncertainty. In so doing, we consider 

15 the availability at this time of any new information that may address limitations or uncertainties 

16 in any of the analyses from the last review. We also consider the availability of more recent 

17 information on air quality patterns that may affect patterns of vegetation exposure under 

18 conditions just meeting the now-current standard. It is those analyses with relatively lesser 

19 uncertainty or fewer limitations regarding their interpretation, which include those most 

20 informative in the last review, that we plan to emphasize in considering analyses that may be 

21 appropriate to conduct for the current review. 

22 Our planning for assessments in this review will consider uncertainties and limitations of 

23 the analyses developed in the last review in order to focus new analyses (if any) on those for 

24 which updated analyses would have appreciably reduced uncertainty with regard to such 

25 previously recognized aspects, and thus would have the potential to substantially improve the 

26 utility of risk estimates in informing the current review. We expect that considering the 

27 information in this way, we will focus any new quantitative analyses in this review on the 

28 analyses that can benefit from updated information or methods, with the goal of ensuring that the 

29 exposure and risk estimates for this review reflect consideration of newly available information 

30 or methods. Accordingly, we expect that in this new review we will develop updated analyses for 

31 types of assessments for which new/updated information, methods or tools provide a basis for 

32 producing appreciably improved or more targeted exposure and risk information. Accordingly, 

33 we do not expect to develop updated analyses for types of assessments for which associated 

34 uncertainties limited their usefulness in the 2015 decision and are unlikely to be addressed by 

35 information available in this review. 
October 2018 5-28 Draft – Do Not Quote or Cite

1 As in any review, key considerations in planning risk and exposure analyses that may be 

2 appropriate include: 

3  Availability of new information (including more recent air quality patterns), models 

4 and tools since completion of the prior assessment that have potential to address key 

5 areas of uncertainty; 

6  Identification of model/assessment aspects for which updates are available and 

7 feasible within the constraints of the timeline for the review may reduce uncertainty 

8 or address limitations, thus improving appropriateness of model outputs for their 

9 intended purposes. 

10 Based on preliminary consideration of the information cited here, including consideration 

11 of the complex and extensive exposure and risk analyses performed for the 2014 WREA, and 

12 given the expedited nature of this review, we are preliminarily planning to focus any new 

13 analyses in this review on the two sets of air quality monitoring analyses (that for Class I areas 

14 and also the analysis for monitoring sites nationally). This would reflect the relatively lesser 

15 uncertainty associated with these types of analyses which made them more informative in the last 

16 review. Updates to these analyses will be able to build upon more recent air quality monitoring 

17 data and any newly available information on tree seedling E-R functions for RBL. Preliminary 

18 consideration of the other analyses, described in the 2014 WREA and based on model-adjusted 

air quality scenarios, does not indicate a potential for appreciably addressing key uncertainties.122 19 

20 We expect to also consider in the PA any other types of analyses from the last review that 

21 we do not update in this review but that are still informative to this review when viewed in the 

22 context of the currently available evidence as characterized in the ISA and of updated air quality 

23 and other analyses performed for this review. Accordingly, the PA will include description and 

24 discussion of all risk and exposure analyses being considered in this review, both those newly 

25 developed in this review as well as analyses performed for the last review for which an updated 

26 assessment was not performed but that are still informative for this review. The draft PA will be 

27 released for public comment and provided to the CASAC for their review. Advice and comments 

28 received will be considered in completing the final version of the risk and exposure analyses and 

29 drawing on all of the analyses considered in the policy evaluations presented in the final PA. 

 

122 Note that the approach for the WREA differed from that used in the HREA, with the latter focused on urban areas 

(as summarized in section 5.1 above) as compared to the large regions that were the focus of the adjustment 

approach in the WREA (2014 WREA, section 4.3; 2014 HREA, section 2.2). 
October 2018 6-1 Draft – Do Not Quote or Cite

1 6 POLICY ASSESSMENT 

2 As described in section 1.2 above, the PA is a document that provides an evaluation of 

3 the currently available information with regard to the adequacy of the current standards and 

4 potential alternatives, if any are appropriate to consider in the current review. The PA integrates 

5 and interprets the information from the ISA and available information from quantitative 

6 exposure/risk analyses to frame policy options for consideration by the Administrator. This 

7 evaluation of policy implications is intended to “bridge the gap” between the Agency’s scientific 

8 assessments and the judgments required of the EPA Administrator in determining whether it is 

9 appropriate to retain or revise the NAAQS. 

10 The discussion in the O3 PA in this review will be framed by consideration of a series of 

11 the policy-relevant questions drawn from those outlined in chapter 3, including the fundamental 

12 questions associated with the adequacy of the current standards and, as appropriate, 

13 consideration of alternative standards that involve revision to any of the specific elements of the 

14 standards: indicator, averaging time, level, and form. The PA conclusions will be based on the 

15 assessment of the scientific information contained in the ISA, and, as available, any updated 

16 exposure/risk assessments, and any additional evaluations and assessments discussed in the PA. 

17 Thus, the PA will address the implications of the science and quantitative assessments for the 

18 adequacy of the current standards, and, as appropriate, for any potential alternative standards. To 

19 the extent it is concluded to be appropriate to consider potential alternative standards, the PA will 

20 also describe a range of policy options for such revisions that is supported by the available 

21 information. In so doing, the PA will describe the underlying interpretations of the scientific 

22 evidence, risk/exposure information and any other quantitative analyses that might support such 

23 alternative policy options and that could be considered by the Administrator in making decisions 

24 for the O3 standards. Additionally, the PA will identify key uncertainties in this policy evaluation 

25 and areas for future research and data collection. 

26 With regard to the primary standard, it is recognized that the final decision will be largely 

27 a public health policy judgment by the Administrator. A final decision must draw upon scientific 

28 information and analyses about health effects and risks, as well as judgments about how to deal 

29 with the range of uncertainties that are inherent in the scientific evidence and analyses. 

30 Consistent with the Agency’s approach across all NAAQS reviews, the approach of the PA to 

31 informing these judgments is based on a recognition that the available health effects evidence 

32 generally reflects continuums that include ambient air exposures for which scientists generally 

33 agree that health effects are likely to occur through lower levels at which the likelihood and 

34 magnitude of response become increasingly uncertain. This approach is consistent with the 
October 2018 6-2 Draft – Do Not Quote or Cite

1 requirements of the NAAQS provisions of the Act and with how the EPA and the courts have 

2 historically interpreted the Act. These provisions require the Administrator to establish primary 

3 standards that are requisite to protect public health with an adequate margin of safety. In so 

4 doing, the Administrator seeks to establish standards that are neither more nor less stringent than 

5 necessary for this purpose. The provisions do not require that standards be set at a zero-risk level, 

6 but rather at a level that avoids unacceptable risks to public health, including the health of 

sensitive groups.123 7 

8 With regard to the secondary standard, it is recognized that the final decision will be 

9 largely a public policy judgment by the Administrator. A final decision must draw upon 

10 scientific evidence and analyses about effects on public welfare, as well as judgments about how 

11 to deal with the range of uncertainties that are inherent in the relevant information. This approach 

12 is consistent with the requirements of the NAAQS provisions of the Act and with how the EPA 

13 and the courts have historically interpreted the Act. These provisions require the Administrator to 

14 establish secondary standards that are requisite to protect public welfare from any known or 

15 anticipated adverse effects associated with the presence of the pollutant in the ambient air. In so 

16 doing, the Administrator seeks to establish standards that are neither more nor less stringent than 

17 necessary for this purpose. The provisions do not require that secondary standards be set to 

18 eliminate all welfare effects, but rather to protect public welfare from those effects that are 

19 judged to be adverse. 

20 The PA, in draft form, will be distributed to the CASAC for its consideration and 

21 provided to the public for review and comment. Review of the draft PA by the CASAC also 

22 facilitates CASAC’s advice to the Agency and recommendations to the Administrator on the 

23 adequacy of the existing standards or revisions that may be appropriate to consider, as provided 

24 for in the Clean Air Act. The CASAC will discuss its review of the PA at public meetings that 

25 will be announced in the Federal Register. Based on past practice by the CASAC, the EPA 

26 expects that key advice and recommendations for revision of the document would be 

27 summarized by the CASAC in a letter to the EPA Administrator. In revising the draft PA 

28 document, any such advice and recommendations will be taken into account, and comments 

29 received from the public will also be considered. The final document will be made available on 

30 an EPA website, with its public availability announced in the Federal Register.

 

123 A number of different population groups may be identified in a NAAQS review. The decision will reflect 

consideration of the degree to which protection is provided for these sensitive population groups. To the extent 

that any particular population group is not among the sensitive groups, a decision that provides protection for the 

sensitive groups would be expected to provide protection for the other groups (as well as any other less sensitive 

population groups). 
October 2018 7-1 Draft – Do Not Quote or Cite

1 7 PROPOSED AND FINAL DECISIONS 

2 Following issuance of the final PA and consideration of analyses and conclusions 

3 presented therein, and taking into consideration CASAC advice and recommendations, the 

4 Agency will develop a notice of proposed decisions. This notice will convey the Administrator’s 

5 proposed conclusions, reached in consideration of the analyses and conclusions in the documents 

6 developed in the review (e.g., as described in the preceding chapters) and advice and 

7 recommendations from the CASAC, regarding the adequacy of the current standards and any 

8 revision(s) that may be appropriate. Development of the notice of the proposed (and final) 

9 decisions will take into account issues related to the NAAQS process (e.g., Pruitt, 2018), as 

10 appropriate in this review. As appropriate, a draft notice of proposed decision will be submitted 

11 to the Office of Management and Budget (OMB) for its review and comment. In this interagency 

12 review step, the OMB also provides to other federal agencies the opportunity for review and 

13 comment. After the completion of interagency review, the notice of proposed action is published 

14 in the Federal Register. 

15 At the time of publication of the notice of the proposed action, all materials on which the 

proposal is based are made available in the public docket for the review.124 16 Publication of the 

17 proposal notice is followed by a public comment period, generally lasting 60 to 90 days, during 

18 which the public is invited to submit comments on the proposal to the docket. Taking into 

19 account comments received on the proposed action, the Agency will then develop a notice of 

20 final action, which communicates the Administrator’s decisions regarding this review and which 

21 may again undergo OMB-coordinated interagency review prior to issuance by the EPA. At the 

time of the final action, the Agency responds to all significant comments on the proposal.125 22 

23 Publication of the notice of the final action in the Federal Register will complete the review 

24 process. 

 

124 The docket for the current O3 NAAQS review is identified as EPA-HQ-OAR-2018-0279. This docket has 

incorporated the ISA docket (EPA-HQ-ORD-2018-0274) by reference. Both dockets are publicly accessible at 

www.regulations.gov. 

125 For example, Agency responses to all significant comments on the 2014 notice of proposed rulemaking in the last 

review were provided in the preamble to the final rule and in a document titled “Response to Significant 

Comments on the 2014 Proposed Rule on the National Ambient Air Quality Standards for Ozone (December 17, 

2014; 79 FR 75234)”, which is available at: https://www.epa.gov/naaqs/responses-significant-comments-2014-

proposed-rule-national-ambient-air-quality-standards-ozone. 
October 2018 8-1 Draft – Do Not Quote or Cite

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28 Research Triangle Park, NC, EPA-452/R-09-007, July 2009. Available at: 

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31 Amended. Office of Air Quality Planning and Standards, Research Triangle Park, NC, 

32 EPA-452/R-10-006, July 2010. Available at: https://www.epa.gov/naaqs/carbon33 monoxide-co-standards-risk-and-exposure-assessments-current-review 

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35 Related Photochemical Oxidants, U.S. Environmental Protection Agency, Research 

36 Triangle Park, NC, EPA 600/R-10/076F. Available: 

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1 U.S. Environmental Protection Agency. (2014a). Health Risk and Exposure Assessment for 

2 Ozone. Office of Air Quality Planning and Standards, Research Triangle Park, NC. EPA3 452/P-14-004a. http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html. 

4 U.S. Environmental Protection Agency. (2014b). Welfare Risk and Exposure Assessment for 

5 Ozone. Office of Air Quality Planning and Standards, Research Triangle Park, NC. EPA6 452/P-14-005a. http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html. 

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10 U.S. Environmental Protection Agency, 2014c. Policy Assessment for the Review of the Ozone 

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12 Research Triangle Park, NC, EPA-452/R-14-006. 

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14 Proposed Rule on the National Ambient Air Quality Standards for Ozone (December 17, 

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18 Environmental Protection Agency, Research Triangle Park, NC, EPA-452/R-15-007. 

19 U.S. EPA (2015c). Preamble to the Integrated Science Assessments. U.S. Environmental 

20 Protection Agency, Washington, DC, EPA/600/R-15/067. Available:

21 https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=310244. 

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29 Exposure Research Laboratory, Research Triangle Park, NC, 27711. In preparation. 

30 Previous version (09/15/2014). Available at: 

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33 U.S. Environmental Protection Agency (2018a). 2014 National Emissions Inventory, version 2 

34 Technical Support Document, U.S. Environmental Protection Agency, Emissions 

35 Inventory and Analysis Group, Research Triangle Park, NC. 
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1 U.S. Environmental Protection Agency, 2018b. Air Quality National Summary, U.S. 

2 Environmental Protection Agency, https://www.epa.gov/air-trends/air-quality-national3 summary. 

4 U.S. Environmental Protection Agency, 2018c. Ozone Trends, U.S. Environmental Protection 

5 Agency, https://www.epa.gov/air-trends/ozone-trends. 

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7 Particulate Matter-Ecological Criteria (2nd External Review Draft). EPA/60/R-18/097. 

8 Available: https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=340671 

9 U.S. EPA. (2018e). Risk and Exposure Assessment for the Review of the Primary National 

10 Ambient Air Quality Standard for Sulfur Oxides. Office of Air Quality Planning and 

11 Standards, Research Triangle Park, NC, EPA-452/R-18-003, May 2018. Available at: 

12 https://www.epa.gov/sites/production/files/2018-05/documents/primary_so2_naaqs_-

13 _final_rea_-_may_2018.pdf 

14 USGCRP, 2017: Climate Science Special Report: Fourth National Climate Assessment, Volume 

15 I. Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. 

16 Maycock [eds.] U.S. Global Change Research Program, Washington, DC, USA, 470 pp, 

17 doi: 10.7930/J0J964J6 

18 Wells, B, Wesson, K, Jenkins, S. (2012). Analysis of Recent U.S. Ozone Air Quality Data to 

19 Support the O3 NAAQS Review and Quadratic Rollback Simulations to Support the First 

20 Draft of the Risk and Exposure Assessment. Memorandum to the EPA Docket # EPA21 HQ-EPA-2008-0699. Document Identifier EPA-HQ-EPA-2008-0699-4253. 

22 Wells, B. (2015). Expanded Comparison of Ozone Metrics Considered in Current NAAQS 

23 Review. Memorandum to the Ozone NAAQS Review Docket, EPA-HQ-OAR-2008-

24 0699. Document Identifier EPA-HQ-OAR-2008-0699-4325. 

25 Whitfield, R, W Biller, M Jusko and J Keisler. (1996). A Probabilistic Assessment of Health 

26 Risks Associated with Short- and Long-Term Exposure to Tropospheric Ozone. Argonne, 

27 IL: Argonne National Laboratory. Available at: 

28 https://www3.epa.gov/ttn/naaqs/standards/ozone/data/riskrep.pdf 

29 Zanobetti, A, J Schwartz. (2008). Mortality displacement in the association of ozone with 

30 mortality: an analysis of 48 cities in the United States. American Journal of Respiratory 

31 and Critical Care Medicine. 177:184-189. 
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Appendix 5-A 

 
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Appendix 5-A. Limitations and uncertainties of exposure and risk analyses developed in the last review of standard and consideration of related newly available information and tools. Drawn from the 2014 HREA, 

20, 7-4, notice of final decision and response to comments document for the review.

Analysis Element Limitations/Uncertainty identified in 2014 HREA 2014 Uncertainty Characteriz

Information for Current Revie

Ambient Air ConcentrationsAmbient air monitoring data The monitoring datasets used for the 2014 HREA were for the period from 2006 

through 2010. 

Overall, O3 measurement data 

low uncertainty. Newly availabl3-year period (2015-2017). 

Approach used to derive factors to adjust air quality to just meet then-existing and potential alternate standards 

Modeling Platforms and Approaches: Model predictions from the Community 

Multiscale Air Quality (CMAQ) model, like all deterministic photochemical models, have both parametric and structural uncertainty associated with them. Higher Order Decoupled Direct Method (HDDM) allows for the efficient approximation of O3 concentrations under alternate emissions scenarios. This approximation is less 

accurate for larger emissions perturbations, especially under nonlinear chemistry conditions. 

Low to medium magnitude of imFEV1 risk estimates potentially over-estimation of ambient conUpdated modeling platforms arof the 2014 HREA. We could aCAMxv6.5 photochemical modechemical mechanisms reflectinimportant chemical pathways fodestruction that have been exteavailable during the last reviewinputs that reflect emissions, mtransport representing a more rBased on results from modeling HREA and time constraints for primarily on NOX reductions alo

To reduce uncertainty in analysselect a subset of study areas bCMAQ/HDDM model performaas well as occurrence of any at

the modeled period. 

Application of HDDM sensitivities to ambient data: there is uncertainty in the statistical regressions used to relate O3 response to emissions perturbations with 

ambient O3 concentrations for every season, hour-of-the-day, and monitor 

location. Further, relationships between O3 response and hourly O3 concentration 

were developed based on 8 months of modeling: January and April-October 2007 and applied to ambient data from 2006-2010. Some locations monitor for months not included in this modeling (February, March, November, and December) while others do not. 

Emissions Reduction Assumptions: In cases where VOC reductions were modeled, equal percentage NOx and VOC reductions were applied in the adjustment methodology. Assumption of across-the-board emissions reductions: Ozone response is modeled for across-the-board reductions in U.S. anthropogenic NOX (and VOC). These across-the-board cuts do not reflect actual emissions 

control strategies. 

Approach used to spatially interpolate ambient air monitor concentrations to census tracts 

Voronoi Neighbor Averaging (VNA) is a spatial interpolation technique used to estimate O3 concentrations in unmonitored areas, which has inherent uncertainty. 

The relative influence on exposure and risk estimates range from low to moderate, with greatest uncertainties when interpolating large distances between monitors. 

The uncertainty in this approacand over-estimation of ambient the magnitude of impact to exp

estimates was estimated to ranFor this review, preferred study 
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Information for Current Revie

coverage of ambient air monitodimensions as a study selectio

Exposure ModelingAPEX general input databases 

There are several general databases used including year 2000 population demographics and commuting, CHAD activity diaries, area-specific meteorological data and 2006-2010 asthma prevalence. 

2014 HREA characterization inwere of high quality and had low exposures. Meteorological and could be updated to appropriateselected study areas and exposnew activity pattern data howevbeen expanded and the associwere revised. The demographic reflect the 2010 census. Howevanalysis in the 2014 HREA usinindicated a small effect, though FEV1 risk estimates (Table 6-18

APEX anthropometric attributes and physiological processes 

There are several databases and algorithms used to estimate body weight (BW), resting metabolic rate (RMR), normalized oxygen consumption rate (nVO2), 

metabolic equivalents of work (METS), and ventilation rates (VE) that may contribute to uncertainty in the estimated exposures. 

The 2014 HREA characterized low to moderate impact on esti(VE and METS) potentially conWe have since updated each ousing either recent data or new nVO2. 

APEX microenvironmental concentrations 

There was uncertainty associated with approaches and factors used to estimate concentrations within indoor, outdoor, and inside vehicle microenvironments including air exchange rates, air conditioning (A/C) prevalence, indoor removal rates, proximity factors to adjust for near road concentrations, and penetration factors. 

Because the highest O3 exposu

environments, these factors welow impact to estimated exposuwould be updated (e.g., A/C preused in 2014 would be reapplie

Representation of time outdoors considering air quality advisories 

Limited availability of data on averting behavior in response to air quality alerts indicates that a small percentage of the population may engage in averting behavior. The lack of representation of this in the exposure modeling may contribute to overestimates of actual exposures in such circumstances (2014 HREA, pp. 5-53 to 5-54; p. 9-11). A sensitivity analysis performed for the 2014 

HREA estimated 1-2 percentage point reductions in the percent of simulated children at or above benchmark levels when accounting for averting by a portion of 

the population and for a particular duration. These results indicate that, depending on benchmark levels, averting could lead to 20% or greater reductions in the 

number of people experiencing exposures of interest. 

While not specifically charactersimulating the averting of high amoderate impact on the estimathe number of people exposed levels may be overestimated. Tpublished literature to support tdevelop the averting scenario oto better reflect current averting characterize the impact to expo
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Estimating repeated exposures for select at-risk populations 

The limited availability of longitudinal activity diary data and the general population modeling approach used may underestimate the correlation in activity patterns for certain potentially at-risk populations (e.g., outdoor workers or the subset of children with systematically high outdoor activity levels). Accordingly, the results may underestimate how often there are repeated exposures to exposures above benchmarks and we are limited in our ability to identify the percent of the 

population with unusually high numbers of multiple exposures (2014 HREA, section 9.5.2). The simulated scenarios were highly dependent on existing activity pattern data and several assumptions made to characterize a particular at-risk population. 

While not specifically charactersimulating potentially at-risk poexposure to high air pollution eimpact on the estimated exposnumber of people exposed at omay have been underestimatednew data are available to enha

Comparison of Simulated Exposures to BenchmarksCut point for moderate or greater ventilation

An equivalent ventilation rate (EVR in L/min--m2) served as a cut point for 

selecting simulated individuals performing moderate or greater exertion activities. The EVR was used to allow for extrapolation of information obtained from adults to children. The value used (13 L/min-m2) was a lower bound based on 

approximating the 5th percentile of the distribution of targeted ventilation rates 

maintained by the study subjects. There is uncertainty in the extrapolation of adult data to simulated children and the use of a lower bound value. 

The 2014 HREA recognized thapeople achieving this level of eoverestimated, affecting the resbenchmarks and the populationto estimate lung function risk. A identifying when individuals maexertion could be explored usindata. 

Exposure duration The exposure duration for the studies from which the benchmark concentrations 

are drawn is 6.6 hours (6 x 50 min exercise periods separated by 10-minute rest 

periods, and with a 35-minute lunch after 3rd hour). Simulated exposures relied on 

a daily maximum 8-hour averaging time. Therefore, health responses observed at a 6.6-hour concentration would directly relate to a lower 8-hour average concentration. Further, there is some indication that the pattern of the exposure may be important to generating the adverse health response (2013 ISA, section 6.2.1.1, pp. 6-10 to 6-11). The approach used to define the exposure benchmark considered average concentration over the exposure period without consideration of exposure pattern or peak concentrations within the exposure averaging time. 

The simulated number of peopabove benchmarks and those efunction decrements via the poapproach could have been 1) uconsidering the different averagunderestimated or overestimateof exposure within the averaginthat better reflect the averaging human exposure study data cohours) 

Benchmark concentrations An important uncertainty is that there is only very limited evidence from controlled 

human exposure studies of population groups potentially at greater risk. Compared to the healthy young adults included in the controlled human exposure studies, members of some populations (e.g., children with asthma) are considered 

more likely to experience adverse effects following exposures to lower O3

concentrations (80 FR 65322, 65346, October 26, 2015; Frey 2014a, p. 7). 

Although not directly characteribenchmark levels derived from exposure studies may not be eeffects likely to be exhibited by and could underestimate the siand/or the magnitude of advers

MSS FEV1 Lung Function Risk Assessment
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Analysis Element Limitations/Uncertainty identified in 2014 HREA 2014 Uncertainty Characteriz

Information for Current Revie

The McDonnell-StewartSmith (MSS) FEV1

model for ages 18 to 35

While there is a good conceptual foundation for the structure of the MSS model, the variability in measurements of FEV1 and estimated parameters of the model 

introduce uncertainty into estimates of FEV1 reductions. For instance, some of the 

estimated parameters have wide confidence intervals (2014 HREA, Table 6-14). Sensitivity analyses in the 2014 HREA additionally addressed how the general pattern of exercise/ventilation of study subjects affects estimated risks, however there were no evaluations of how exposure patterns of study subjects or changes in other influential attributes may affect risk estimates. 

A new MSS model (McDonnellin this review. 

Representation of interindividual variability There is uncertainty in the degree to which the MSS model represents inter- personal variability in FEV1 reductions (i.e., via the MSS model variable Var(U)). 

This is the result of having very few exposure studies with repeated clinical trials using the same individuals, likely yielding an underestimate in the Var(U) parameters. In addition, the method used for adjusting for filtered air (FA) exposures in the data used to fit the MSS model does not use the subject-specific adjustments, rather the mean FA response across a study is used to adjust the O3

responses of each subject in the study. Furthermore, there are few clinical data for population with diseased lungs (i.e., asthma), thus the MSS model may not account for the increase in inter-individual variability that would result from inclusion of exposure-response data from such individuals. A higher Var(U) indicates greater between-individual variability and less within-individual variability, therefore more responsive individuals are more likely to see repeated occurrences of high 

ΔFEV1 (and thus less responsive individuals are more likely to see no 

occurrences of high 

ΔFEV1). 

The 2014 HREA concluded thaexperiencing FEV1 decrements 

overestimated given underestimparameter (absent the influence uncertainty). 

Representation of intraindividual variability There is uncertainty in the degree to which the MSS model represents intra- personal variability in FEV1 reductions (i.e., via the MSS model variable Var(ε)). 

The Var(ε) term is assumed to have a Gaussian distribution {mean=0, standard deviation=4.14} and for our purposes in estimating risk was bounded at ±2 standard deviations (i.e., ±8.3). Extending or restricting these bounds will result in either greater or fewer simulated individuals experiencing lung function decrements, respectively. The assumption that the distribution of this term is Gaussian is convenient for fitting the data, but may not be accurate. 

Sensitivity analyses conducted that how the Var(ε) parameters moderate or greater influence iof the population with FEV1 dec

is not clear how potential missp

distribution shape affects its pavariables in the MSS model, anaffect risk estimates. 

Extrapolation of MSS variable parameters estimated for adults (18-35) to children (ages 5 to 18) 

There are virtually no controlled human exposure data for children (i.e., the youngest age for which controlled human exposure data are generally available is 18 years old). Thus, the 2014 HREA essentially applied the same lung function response following O3 exposures to children as was applied for adults (2014 

HREA, section 6.5.3). This assumption is justified in part by the findings of McDonnell et al. (1985), who reported that children (8-11 years old) experienced FEV1 responses similar to those observed in adults (18-35 years old) (2014 

The 2014 HREA concluded thain moderate over- or underestim

function decrements in simulate
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Analysis Element Limitations/Uncertainty identified in 2014 HREA 2014 Uncertainty Characteriz

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HREA, p. 3-10) and from summer camp studies of school-aged children reported O3-induced lung function decrements similar in magnitude to those observed in 

controlled human exposure studies using adults (2013 ISA, section 6.2.1). To estimate health risk for children, a constant value was used for the MSS model age variable (and derived from 18-year olds, and as a maximum value). There is uncertainty in this approach, depending on how this age term influences overall risk estimates for children compared to adults in controlled human exposure studies (2014 HREA, section 6.5.3). 

Extrapolation of exposure-response data from healthy subjects to simulated people with asthma 

There is uncertainty associated with using exposure-response relationships derived from healthy subjects in the controlled exposure studies to estimate O3-

induced lung function risk in simulated individuals with asthma (2014 HREA, section 6.5.4). Although the evidence is mixed (2013 ISA, section 6.2.1.1), several studies have reported statistically larger, or a tendency toward larger, O3-induced 

lung function decrements in asthmatics than in non-asthmatics (Kreit et al., 1989; Horstman et al., 1995; Jorres et al., 1996; Alexis et al., 2000). On this issue, CASAC noted that “[a]sthmatic subjects appear to be at least as sensitive, if not more sensitive, than non-asthmatic subjects in manifesting O3-induced pulmonary 

function decrements” (Frey, 2014c, p. 4). Furthermore, the response could depend on a variety of factors that have not been well-evaluated, including the severity of asthma and the prevalence of medication use. For instance, responses to O3

increase with severity of asthma (Horstman et al., 1995) and corticosteroid usage does not prevent O3-induced lung function decrements or respiratory symptoms in 

people with asthma (Vagaggini et al., 2001, 2007). 

The 2014 HREA indicated that larger O3-induced lung function 

healthy adults used to develop relationships, the impacts of O3 

in asthmatics, including asthmaunderestimated, albeit to an un

Population-based Exposure-Response modelCut point for moderate or greater ventilation 

See entry for this element under Comparison to Benchmarks section. The approach used could overestimate the number of individuals at moderate or greater exertion. 

While not directly characterized reported number and percent oexperience a lung function decgreater than that estimated usinvalue to estimate elevated exer

Exposure duration See entry for this element under Comparison to Benchmarks section. The duration 

used results in fewer simulated individuals identified as having the exposure of interest than expected for the E-R function. 

While not directly characterized reported number and percent oexperience a lung function decunderestimated given the differ

E-R function shape In both the 2007 O3 Staff Paper and 2014 HREA, an exposure-response model 

was derived using a combination of two functions (90% logistic fit and 10% linearthreshold). The selection of this parameterization was based largely on 1) linearity of E-R relationship for exposures between 0.08 – 0.12 ppm (and used in the 1997 O3 risk assessment), a “very good” logistic model fit (2007 Staff Paper), and 

While not directly characterized reported number and percent oexperience a lung function decwhen using primarily a logit fit tBased on the 2009 and 2018 S
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CASAC advice noting a linear model cannot entirely be ruled out given the are limited data at the two lowest exposure levels (Henderson, 2006). Sensitivity analyses of three different logistic/linear-threshold forms (90/10, 80/20, 50/50) indicated differences in the estimated risks, most notably lower risks estimated with increasing proportion of the linear threshold form and when considering the air quality adjusted to the lowest standard level of 64 ppb (2007 Staff Paper). A key issue of concern regarding each of these model fits is how responses are estimated at concentrations below which we have controlled human exposure study data (i.e., <40 ppb). 

probit form of a logistic model is using a logit form. This is based the distribution of individual thresupporting the use of a probit futhe inverse of the cumulative stfunction, rather than a logistic flogistic distribution, for estimati2018e). It is possible the combmay be more similar to a probit response at lowest concentratioestimates remains uncertain. 

Ambient AQ (epidemiologic study)-based riskAmbient air concentrations Relationship between population exposures and ambient air monitor 

concentrations: One of the assumptions in the use of ambient air concentrationresponse functions drawn from epidemiological studies to estimate risk associated with a pollutant for a modeled air quality scenario and population is that the relationship between ambient air monitor concentrations (usually represented in the studies by an area-wide average) and the exposure of the population is the same in the modeled air quality scenario and population as what existed in the epidemiologic study situation. Listed below are several aspects of that relationship. Use of areawide average concentrations: The use of areawide averages can miss important patterns of exposure within urban study areas introducing uncertainty into the epi study effect estimates and accordingly into the C-R functions applied in the HREA. Monitor locations used for area wide averages: For some of the HREA analyses, the locations of the ambient air monitors used to characterize air quality in the HREA urban study areas do not necessarily match directly with the locations of monitors used in the original epidemiological study. This may be due to differences in the monitors operating during and used in the study and those for which data are available in the years included in the HREA. This may additionally occur due to the use of CBSAs for the HREA study area, given that CBSAs are generally larger areas than the epi study areas. Population Residence and Activity: Differences in the residences and activity patterns of the simulated population and the epi study population can contribute 

It is difficult to quantitatively chamagnitude the uncertainty in mon risk estimates. The issue colarge urban areas which may e

levels compared to small urban sources, topography, and patteaddition, populations living nearoadways may experience differelative to more generalized urb

and co-pollutants such as PM2.

increased uncertainty in the resconcentration levels, it remains whether there are known and qlow concentrations. Regardless, we expect there to appropriately and accurately reambient air conditions used in crelationships previously used afrom newly available epidemiolcurrent review. Differences in population repreassessment compared with the 

epidemiologic studies could hamagnitude of impact on the est
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uncertainty to risk estimates given the relationships between individual activity and exposure to pollutants in ambient air are not accounted for in an epi study. For instance, in one HREA study area, the O3 C-R functions were based on an 

epidemiological study in a region (northern Connecticut and Springfield) that did not encompass the actual urban study area assessed for risk (Boston). Another area of uncertainty relates to the location of exposure events vs location of the ambient air monitors and the relationship of the associated ME concentrations vs ambient air monitors. All of these can contribute to differences between the HREA and the epidemiologic studies in the relationship between ambient air monitor concentration and population exposure, which can contribute uncertainty to the risk estimates. 

lead to instances of over and uTable 7-4). We expect there to population representation when information for the current revie

Population baseline incidence of health outcome being assessed 

At-risk populations: To some extent, differences in risk factors for the outcomes 

being quantified are accounted for by using baseline incidence rates. Uncertainty can be introduced into the characterization of baseline incidence in varying ways (e.g., error in reporting incidence for specific endpoints, mismatch between the spatial scale in which the baseline data were captured and the level of the risk assessment). 

We would anticipate that sourcbaseline incidence (e.g., potentspatial scale of reporting in epidrisk modeling) would still apply completed. 

Concentration-Response (C-R) functions 

Use of effect estimates obtained from epidemiology studies as the basis for C-R 

functions: Exposure measurement error combined with other factors (e.g., magnitude of the effect, sample size, controls for confounding variables, consideration for effect modification) can affect the statistical models and associated effect estimates obtained from O3 epidemiological studies. Uncertainty 

in effect estimates due to these influential factors contributes to uncertainty in the O3 C-R functions used to estimate risk. Consequently, this introduces uncertainty 

to the epidemiological-based risk estimates. See discussion in 2014 HREA (p. 7-43) regarding statistical fit of the O3 C-R functions. 

Shape of the C-R curve at lower concentrations: The shape of the curve at the 

most prevalent ambient air concentrations can have an important influence impact on the risk estimates. Most of the population will experience relatively low ambient air concentrations compared with a lesser proportion of the population experiencing concentrations having a high level of risk. The 2013 ISA indicates reduced certainty in the shape of O3 C-R functions at lower ambient air 

concentrations due to lesser prevalence of these concentrations in the 

The HREA recognized the unceassociated with the O3 C-R fun

impact to risk estimates and, in result in either over- or underesOf importance however, regardestimated risks associated with 

concentrations. The PA recognconcern for adverse O3-attribut

ambient O3 concentrations (wh

concentrations, section 3.2.2 of to risks associated with lower csuggests that shape of the C-R ambient O3 concentrations, comincreased low concentrations readjustment approach (see abovcontribute to over- or under-est
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epidemiological studies (2014 HREA, pp. 7-43 to 7-44; 2013 ISA, section 2.5.4.4). As a result, the HREA provides estimates of epidemiology-based mortality risks using the entire distribution of ambient O3, as well as providing estimates of 

mortality associated with specific ambient O3 concentrations. 

Specifying lag structure (short-term exposure studies): There is uncertainty 

associated with specifying the exact lag structure to use in modeling short-term O3-attributable mortality and respiratory-related morbidity. Most studies examining 

different lag models suggest that O3 effects occur within a few days of exposure 

(see O3 ISA, section 2.5.4.3). While the nature of an ideal lag model remains 

uncertain, we consider this uncertainty to be relatively small in magnitude compared with other the identified uncertainties. C-R function for long-term (seasonal average 1-hr daily max) mortality: There is 

also uncertainty about the extent to which mortality estimates based on the longterm metric in Jerrett et al. (2009) (i.e., seasonal average of 1-hour daily maximum concentrations) reflects associations with long-term average O3 versus repeated 

occurrences of elevated short-term concentrations. For example, the CASAC concluded that “[i]n light of the potential nonlinearity of the C-R function for longterm exposure reflecting a threshold of the mortality response, the estimated number of premature deaths avoidable for long-term exposure reductions for several levels need to be viewed with caution” (Frey, 2014a, p. 3). Lack of C-R functions that have addressed potential for influence of co-pollutants: 

The inclusion or exclusion of co-pollutants in epidemiologic study models may confound, or in other ways, impact the O3 effect estimates reported in the epi 

studies in those instances where other pollutants are causally associated with the endpoint of interest. Regarding PM as one copollutant, the O3 ISA notes that 

across studies where its role was assessed, the potential impact of co-pollutants such as PM on O3-mortality risk estimates tended to be much smaller than the 

variation in O3-mortality risk estimates across epi study cities. 

broader impact of this uncertainlast review is associated with thof the increases in relatively low 

following air quality adjustment 26, 2015). To the extent advers

more strongly supported for higthe impacts on risk estimates o

concentrations (an impact of re

precursors) reflect an importanAQ epidemiologic risk estimate26, 2015). While it is possible that differenconsidered in addition to the prapportioning the contribution of estimates, we expect there to bthe O3 C-R functions when usin

information, approaches, or tooreview. 
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Appendix 5-B
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Appendix 5-B. Limitations and uncertainties of the air quality, exposure and risk analyses developed in th

consideration of related newly available information and tools. Drawn from the 2014 WREA

proposed and final decisions; and, response to comments document for the review.

Analysis Element Limitations/Uncertainty Identified in Last Review Conclusions from Last R

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[Section 5.2.1.3] W126-based Cumulative Exposure Estimates for Class I Areas (based on Air Monitoring Data) Ambient air monitoring data for O3

The monitoring dataset used was for the period from 1998 through 2013 (80 FR 65385, October 26, 2018). The data set included SLAMS monitors as well as CASTNET monitors, thus providing extended representation in rural areas. The monitoring season 

varies across states in length from May to September to year-round, with duration intended to capture the highest concentration periods, thus including highest 3-month period needed for derivation of W126 index values. 

Overall O3 measurements 

have low uncertainty (2014 Ambient air monitoring damore recent years, e.g., thyear. 

Class I area representation by monitoring sites 

This analysis focused on monitors sited in or within 15 km of a Class I area for which any of the years in the time period had a W126 index value above 15 ppm-hrs (80 FR 65385, October 26, 2015). The 15 km distance was selected as a natural breakpoint in distance of O3 monitoring sites from Class I areas and as still providing similar 

surroundings to those occurring in the Class I area. We note that given the strict restrictions on structures and access within some of these areas, it is common for monitors intended to collect data pertaining to air quality in these types of areas to be sited outside their boundaries. The analysis focused on those sites for which at least one 3-year period between 1998 and 2013 included a 3-year W126 value at/above 15 ppm-hrs (80 FR 65385, October 26, 2015). 

The 17 locations in this anof the approximately 70 Care ambient air monitors wapproximately 10% of the areas in the U.S. (80 FR 6There is an O3 monitor wit

Class I areas (somewhat lmonitor in or within 15 km (somewhat fewer than hal26, 2015). More recent moadditional sites. 

[Section 5.2.1.3] W126-based Cumulative Exposure Estimates for O3 Monitoring Sites across the U.S. with Design Values at/below 75, 70, Ambient air monitoring data for O3 

The monitoring dataset used was for the period from 1998 through 2013 (80 FR 65400, October 26, 2015; Wells, 2015). The data set included SLAMS monitors, which are largely focused in urban and suburban areas, as well as CASTNET monitors, which are located in rural areas, thus providing extended representation in rural areas (as summarized in section 2.2 above). The monitoring network in some areas of the Western U.S. is much less dense than in the eastern portions of the U.S. and the west coast states (Wells, 2015, Figures 1 and 2). The monitoring season varies across states in length from May to September to year-round, with duration intended to capture the highest concentration periods, thus including highest 3-month period 

needed for derivation of W126 index. 

Overall O3 measurements have low uncertainty (2014 are now available through monitoring year, e.g., four extending through the 201available. Data are also avadditional sites in Montana above). 

Nationwide representation by monitoring sites 

The analysis included 1430 monitoring sites with sufficient data to derive valid air quality metrics for at least one 3-year period (Wells, 2015). During the then-most recent 

3-year period (2011-2013), there were more than 500 monitoring sites that would meet 

the now-current standard of 70 ppb. These monitors were distributed across all nine 

Given the reductions in O3 

occurred since then (see sthere are more sites that m
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NOAA climatic regions and 46 of the 50 states. Across all 11 3-year periods of data over the complete time period, there were nearly 4000 site-time period instances for which the now-current standard of 70 ppb would have been met. 

standard in an update of smore recent 3-year period

[Section 5.2.1.1] National and Regional/Urban Estimates of O3-attributable Impacts for Model-adjusted O3 Concentrations in Nine NOAA RAmbient Air Concentrations Ambient air monitoring data The monitoring dataset used was for the 3-year period from 2006 through 2008 

(WREA, Table 4-5). 

Overall O3 measurements 

have low uncertainty (2014 are now available for the p

Approach used to derive factors to adjust air quality to just meet then-existing standard 

Modeling Platforms and Approaches: Model predictions from the Community Multiscale 

Air Quality (CMAQ) model, like all deterministic photochemical models, have both parametric and structural uncertainty associated with them. Higher Order Decoupled Direct Method (HDDM) allows for the efficient approximation of O3 concentrations 

under alternate emissions scenarios. This approximation is less accurate for larger emissions perturbations, especially under nonlinear chemistry conditions (WREA, Table 4-5). 

Medium magnitude of impboth under- and over-estimconcentrations. Updated mavailable since the 2014 Wphotochemical model inclumechanisms for O3 format

pathways. Somewhat mormeteorology and internatioavailable (e.g., for 2016). As the adjustment is applieach region, the adjustmereductions in O3, including 

already meeting or below adjustments performed to a target level at the highesresulted in substantial redin some areas of the regiomonitors already well belouncertainty with regard to specific control strategies to meet a particular target October 26, 2015). Adjustareas might reduce this un

Application of HDDM sensitivities to ambient data: there is uncertainty in the statistical regressions used to relate O3 response to emissions perturbations with ambient O3

concentrations for every season, hour-of-the-day, and monitor location (WREA, Table 4-5). 

Emissions Reduction Assumptions: Assumption of across-the-board emissions reductions: Ozone response is modeled for across-the-board reductions in U.S. anthropogenic NOx. These across-the-board cuts do not reflect actual emissions 

control strategies. The form, locations, and timing of emissions reductions that would be undertaken to meet various levels of the O3 standard are unknown. The across-theboard emissions reductions bring levels down uniformly across time and space to show 

how O3 would respond to changes in ambient levels of precursor species but do not 

reflect spatial and temporal heterogeneity that may occur in local and regional emissions reductions (WREA, Table 4-5). 

Concentration Adjustment: Adjustments were applied independently for each of the nine NOAA climate regions in continental U.S. such that the highest monitor location in each region just met the then-existing standard (WREA, Table 4-5). In regions where the air quality adjustment was applied, it was based on emissions reductions determined necessary for the highest monitor in that region to just equal the existing standard or the W126 target for the scenario. Concentrations at all other monitor locations in the region were also adjusted based on the same emissions reductions assumptions. 
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Approach used to derive factors to adjust air quality to just meet the 3-year W126 targets (15, 11, and 7 ppm-hr) 

Model-based adjustments – Beginning with concentrations at monitor locations that had 

been adjusted to just meet the then-existing standard, further adjustments were made at all sites in each NOAA region in which at least one site was not already at/below the target W126 value for that scenario (2014 WREA, section 4.3.4.1). In such regions, the adjustment made at all sites was that determined necessary for the highest monitor in that region to just equal the W126 target. 

See above. 

Approach used to spatially interpolate ambient monitor concentrations to grid cells 

Spatial interpolation technique: Voronoi Neighbor Averaging (VNA) was used to estimate O3 concentrations in unmonitored areas (as summarized in section 5.2.1.1 

above). The uncertainty tends to increase with greater distance from the monitoring sites as the VNA estimates are weighted based on distance from neighboring monitoring sites. Thus, there is less uncertainty in the VNA estimates near urban areas with more dense monitoring networks, and more uncertainty in sparsely populated areas where monitors are further apart, such as in the Western U.S. (2014 WREA, Table 4-5). 

The uncertainty in this appunder- and over-estimatioHowever, the magnitude oexposure and risk estimatmoderate, with greatest uninterpolating large distancWREA, Table 4-5). 

Impacts on Tree Growth at Species- and Ecosystem-level Response estimates for controlled exposures 

Robust and well-established E–R functions for RBL are available for eleven tree 

species in the seedling growth stage: black cherry, Douglas fir, loblolly pine, ponderosa pine, quaking aspen, red alder, red maple, sugar maple, tulip poplar, Virginia pine, and 

white pine (2013 ISA; 2014 PA; 80 FR 65371-73, 65383-65384, 75393-65395, October 26, 2015). The data for these species come from extensive controlled studies in open top chambers (OTCs), with most species studied multiple times under a wide range of exposure and/or growing conditions 

New field-based studies aqualitatively strengthened in the evidence from the Oadditional evidence that O

biomass loss effects obseoccurs in the field (2014 P

Species-specific E-R functions 

Robust composite species-specific E-R functions were developed for each of the 

eleven tree species (above) based on the separate E–R functions for each combination 

of species, exposure condition and growing condition scenario (2013 ISA, section 9.6.1). The species-specific composite E–R functions have been successfully used to predict the biomass loss response from tree seedling species over a range of cumulative exposure conditions (2013 ISA, section 9.6.2). A 12th E-R function was 

considered but not given the same emphasis as the other eleven, as it lacked the robust basis of the others give that its underlying data were from a single gradient study that did not control for O3 and climatic conditions, as contrasted with the more well 

controlled OTC exposure studies (Frey, 2014c, p. 10, 80 FR 65372, October 26, 2015). Shape of E-R function: Relative biomass loss estimates are highly sensitive to the 

parameters in the E-R function. Some species are represented by one study, other species by many studies (WREA, Table 6-27). 

Sensitivity analyses showeinterspecific variability. Amrobust E-R functions are asensitive species and seve

sensitivity. It is unknown hlarger suite of tree species influence on risk estimates magnitude (WREA, Table 
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Absence of functions for many sensitive species: Robust E-R functions are not available for the majority of trees in the modeled urban areas and Class I areas, precluding their representation. Study data for other species do not support E-R development (WREA, Table 6-27). Use of seedling functions for adult trees: E-R functions for trees are based on analyses of tree seedlings, but most biomass impacts are from effects on adult trees (WREA, Table 6-27). National distribution of species with E-R functions: While the available robust E-R functions are for species representing only a small fraction (0.8 percent) of the total number of native tree species in the contiguous U.S. (1,497), this small subset includes eastern and western species, deciduous and coniferous species, and species that grow in a variety of ecosystems and represent a range of tolerance to O3 (2013 ISA, section 

9.6.2; 2014 WREA, section 6.2, Figure 6–2, Table 6–1). The range of each species is based on data from USFS and used to specify presence/absence of each species nationally and, in ecosystem-level analysis were used to scale biomass loss by proportional presence of each species (WREA, Table 6-27). Species distribution in urban case study areas and availability of E-R functions: E-R 

functions are available for only small portion of trees in the urban case study areas. Eighty to 90 percent of the total trees in the urban case study areas are excluded from the analysis as they are species for which we do not have E-R functions; we have some data indicating sensitivity for two of these species. 

Additional sensitive specieTherefore, total tree biomaunderestimated, with medmagnitude of impact (WREknown yet if there would bavailable for additional trereview. Generally, RBL estimates comparable to adult tree eexceptions such as black overestimate and some untrees, with low to medium impact (2014 WREA, TablThe magnitude of the influcommunity composition in potential influence on natioestimated to be low to mefor urban case studies (WIt is unclear whether robusavailable in this review for 

Species distributions Tree basal area estimates used to assess larger scale ecosystem effects: Estimates of 

basal area were modeled by the U.S. Forest Service’s Forest Health Technology Enterprise Team (FHTET) at a scale of 240 m2. These values were aggregated to the 

144 (12 x12) km2 CMAQ grid. 

Assumption of constant forest composition: Forest and Agriculture Sector Optimization 

Model with greenhouse gases (FASOMGHG) modeling (used for the urban case study analyses) does not reflect changes in tree species mixes within a forest type made by natural adaptation and adaptive management by landowners due to O3. Less sensitive 

tree species may gain relative advantage over more sensitive species. The magnitude of potential influence of associated uncertainties on risk estimates is estimated to be low (WREA, Table 6-27, p. 6-70). 

The magnitude of the poteassociated uncertainty on estimates is expected to bTable 6-27). While USDA’on refining its model, the eon risk estimates, though vsmall (WREA Table 6-27)While updates to FASOM may be available, we do nappreciable improvements in incorporation of change
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Crop Yield ImpactsResponse estimates for controlled exposures 

Experimental data: There is strong evidence for established E–R functions for 10 crops 

(barley, field corn, cotton, kidney bean, lettuce, peanut, potato, grain sorghum, soybean and winter wheat). The established E–R functions for relative yield loss (RYL) were developed from OTC-type experiments from the National Crop Loss Assessment Network (NCLAN) (2013 ISA, section 9.6.3; 2014 WREA, section 6.2; 2014 PA, Figure 5–4 and section 6.3; 80 FR 65372, October 26, 2015). These crops were originally selected for study based on their significant role among U.S. commodity crops nationwide (e.g., representing approximately 85% of the commodity crops grown in the U.S. in the 1980s). Data newly available in the 2015 review continued to confirm earlier findings, leading to the ISA conclusion of little new evidence that crops are becoming more tolerant of O3 (U.S. EPA, 2006a; U.S. EPA 2013). 

It is not clear what percengrown today the evaluated is not clear to what degree changed over time due to change in varieties planted

Species-specific E-R functions 

Shape of E-R function: Crop yield loss estimates are highly sensitive to the parameters 

in the E-R function. Some functions are based on one study and others on many studies (WREA, Table 6-27). 

Sensitivity analyses for 10 studies) showed high intravariability It is unknown howith E-R functions reflects the U.S (WREA, Table 6-2

Agricultural and Timber Market ImpactsApproach to estimating impacts on agricultural and timber markets 

Use of median parameters for crop species E-R functions used to assess national 

agricultural impacts (in FASOM): In addition to the robust E-R functions developed for the 10 commodity crops above, this modeling used the median E-R function for oranges, rice, and tomatoes, three species for which E-R functions in terms of W126 are not available (2014 WREA, Table 6-27, p. 6-69). Crop proxy and forest type assumptions: Actual impacts may differ from those of the crop proxy or the forest type as the crops/tree species modeled are only a subset of species present in U.S. agriculture and forestry systems. Further, FASOMGHG modeling used a simple average of tree RYLs for all forest types within a region (2014 WREA, Table 6-27). Omission of agriculture/ forestry on public lands: The model used (FASOMGHG) does not include public lands (2014 WREA, Table 6-27). International trade projections in FASOMGHG: FASOMGHG reflects future international trade projections by USDA based on recent O3 conditions. Soybeans and 

wheat are major crop exports and have relatively large responses to O3, which are not 

reflected in the trade projections (2014 WREA, Table 6-27). 

Using alternative E-R funcor higher O3 impacts on cr

biomass productivity, poteequilibrium outcomes (201The extent to which updatthis uncertainty is yet to be Because public lands are model, the estimates of chproducer surplus would likwere included (2014 WRE
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Overall effects on agricultural yields and producer and consumer surplus depend on the ability of producers/farmers to substitute other crops that are less O3 sensitive, and 

the responsiveness, or elasticity, of demand and supply (U.S. EPA, 2014b, section 6.5). The WREA discusses multiple areas of uncertainty associated with the crop yield loss estimates, including those associated with the model-based adjustment methodology as well as those associated with the projection of yield loss using the FASOMGHG at the estimated O3 concentrations (U.S. EPA, 2014b, Table 6–27, 

section 8.5). Because the W126 index estimates generated in the air quality scenarios are inputs to the vegetation risk analyses for crop yield loss, any uncertainties in the air quality scenario estimation of W126 index values are propagated into those analyses (U.S. EPA, 2014b, Table 6–27, section 8.5). Therefore, the air quality scenarios in the crop yield analyses have the same uncertainties and limitations as in the biomass loss analyses (summarized above), including those associated with the model-based adjustment methodology (U.S. EPA, 2014b, section 8.5). 

While having sufficient crowelfare value, important cheavily managed to produMoreover, based on the eand demand, increases in expected to result in lower and their associated goodbenefit consumers. These producers and consumers these effects in terms of ppublic welfare (U.S. EPA, 5.7). (80 FR 65379, Octob

Carbon SequestrationSpecies-specific estimates Functions for estimating carbon sequestration: The functions applied in the models to 

estimate carbon sequestration are uncertain and vary by species. Pollution removal is calculated based on field, pollution concentration, and meteorological data. The pollution removal functions in iTree are from Nowak et al. (2006). 

This uncertainty was judgemagnitude of potential infl(2014 WREA, Table 6-27)these models have reduce

National-scale estimates Carbon sequestration estimates in small set of urban areas (using iTree model) 

Representation and distribution of trees within assessed urban areas: The base 

inventory of urban trees, including species and distribution, in iTree has uncertainty. The iTree model estimates are based on tree growth and pollution removal functions that are specific to the forest structure in each urban area, including the species composition, number of trees, and diameter distribution of trees. Of the 11 species with E-R functions, only 2-3 species were in each urban area, comprising at most 18.5% of total tree population (2014 WREA, section 6.6). 

The urban tree inventories analyses are based on fiemeasurements of trees in analyzed. Although such dconsidered less uncertain inventories, any associatepropagated into the estimaand pollution removal bas(2014 WREA, Table 6-27)

Pollutant RemovalPollutant removal Estimates in small set urban areas (using iTree model) 

Estimation of pollutant removal: The functions applied in iTree to estimate growing 

trees’ removal of some common air pollutants are uncertain and vary by species. Assumption of zero pollutant emissions: Many tree species are biogenic sources of volatile organic compounds (VOC) that contribute to formation of O3. Additional VOC 

emissions associated with biomass gains are not addressed. 

Magnitude of potential inflestimates estimated to be 27). The availability of updfunctions addressing pote

known. 
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[Section 5.2.1.2] Foliar Injury Analyses Associating foliar injury data with CMAQ-generated O3 exposures by grid cell 

assignments 

Spatial assignment of foliar injury biosite data to 12x12 km grids. Because of privacy 

laws that require the exact location information of sampling sites to not be made public, the data were assigned to the CMAQ grid by the USFS, except for data in California, Oregon, and Washington which were assigned to the CMAQ grid by EPA staff based on publicly available geographic coordinates, rather than coordinates specific to the sites. Thus, these data have greater uncertainty (2014 WREA, Table 7-24). Availability of biosite sampling data: Because sampling was discontinued in some states prior to this analysis, we did not include data for many western states (Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona, New Mexico, Oklahoma, and portions of Texas). 

Magnitude of potential inflrisk estimates was estimatTable 6-27). 

Categorization of biosites by moisture level 

Soil moisture threshold for foliar injury: Low soil moisture reduces the potential for foliar 

injury, but injury could still occur because plants must open their stomata even during periods of drought (2014 WREA, Table 7-24). Spatial resolution of soil moisture data: Some vegetation such as along riverbanks may experience sufficient soil moisture during periods of drought to exhibit foliar injury. In addition, we did not have soil moisture data for Alaska, Hawaii, Puerto Rico, or Guam (2014 WREA, Table 7-24). Time period for soil moisture data: Short-term estimates of soil moisture are highly variable over time, even from month to month within a single year; yet using averages to address variability contributes to a potential temporal mismatch between soil moisture and injury (2014 WREA, Table 7-24). Drought categories: The soil moisture categories used to derive the foliar injury benchmarks (i.e., wet, normal, and dry) are uncertain (2014 WREA, Table 7-24). 

The 2014 WREA estimatemedium magnitude of impWREA, Table 7-24). The 2014 WREA estimatemedium magnitude of impWREA, Table 7-24). More known to be available. The 2014 WREA estimatelow-medium magnitude of (2014 WREA, Table 7-24)The 2014 WREA estimateunknown magnitude of imWREA, Table 7-24). 
Agency 

Environmental Protection United States 

Health and EnvironmentaOffice of Air Quality Planning and Standards 

l Impacts Division 

Research Triangle Park, NC 

Publication No. EPA-452/P-18-001 

October 2018