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PRO^
Overview of the Environmental Protection
Agency's Process for Reviewing the
National Ambient Air Quality Standards
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EPA-452/R-24-019
December 2024
Overview of the Environmental Protection Agency's
Process for Reviewing the National Ambient Air Quality Standards
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Health and Environmental Impacts Division
Research Triangle Park, NC
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Table of Contents
1 Introduction 1-1
2 Reviewing the NAAQS: An Overview 2-1
2.1 Phases of the Review and Roles of Key EPA Offices 2-1
2.1.1 The Planning Phase 2-3
2.1.2 The Assessment Phase 2-3
2.1.3 The Regulatory Decision-Making Phase 2-4
2.2 Role of the CASAC 2-7
2.3 Public Involvement 2-12
2.4 Fundamental Aspects of NAAQS Reviews 2-13
2.4.1 Elements of a Standard, Design Values, and Air Quality Characterization.. 2-13
2.4.2 The Primary Standard 2-15
2.4.3 The Secondary Standard 2-16
2.4.4 Role of Policy Judgment in Standards Decisions 2-16
3 Review Planning 3-1
3.1 Initial Planning 3-1
3.2 Integrated Review Plan 3-2
3.2.1 Volume 1: Background Document 3-2
3.2.2 Volume 2: Planning for the Review and the Integrated Science Assessment.3-3
3.2.3 Volume 3: Planning Document for Quantitative Exposure/Risk Analyses 3-5
4 Assessments 4-1
4.1 Science Assessment 4-1
4.2 Risk and Exposure Assessment 4-5
4.3 Policy Assessment 4-12
5 Regulatory Decision-Making 5-1
5.1 Proposed Decision 5-2
5.2 Final Decision 5-3
6 References 6-1
Appendix A. Legislative Requirements
Appendix B. Examples of Some Analytical Approaches Used in NAAQS Risk and
Exposure Assessments
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1 INTRODUCTION
This document is intended to describe the general process followed by the U.S.
Environmental Protection Agency (EPA) in reviewing the national ambient air quality
standards (NAAQS).1 It describes the phases of a review, the documents associated with
each phase, the general timing and sequence of document development, the role of the
Clean Air Scientific Advisory Committee (CASAC), and public involvement opportunities.
An overview and key concepts are presented in Chapter 2. Chapters 3, 4 and 5 provide a
more in-depth description of each phase of the review, including the Planning Phase,
the Assessment Phase, and the Regulatory Decision-Making Phase, respectively. While
not exhaustive, this document serves as a reference to provide clarity regarding the
different steps in a NAAQS review. The EPA intends to maintain this document as a
reference for the public and the CASAC, with periodic updates as appropriate.
There are two types of NAAQS: "primary" (health-based) and "secondary"
(welfare-based).2 The Clean Air Act (CAA) governs the establishment, review, and
revision of these standards.3 It specifies that the NAAQS are to be based on air quality
criteria that "accurately reflect the latest scientific knowledge useful in indicating the
kind and extent of all identifiable effects on public health or welfare which may be
expected from the presence of the pollutant in ambient air." The pollutants for which
NAAQS are established are therefore termed "criteria" pollutants, of which there are
currently six: carbon monoxide (CO), lead (Pb), oxides of nitrogen, ozone (O3) and
1 EPA's process for reviewing the NAAQS is subject to revision and adaptation as appropriate for
individual reviews and based on experience, the issues arising in a particular review, and stakeholder
input. This document is intended to provide a general description, rather than prescribe specific
requirements that would be adhered to each review, and to be updated on an ongoing basis.
2 Under the Clean Air Act, welfare effects 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" (CAA 302(h)).
3 Appendix A summarizes these legislative requirements.
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related photochemical oxidants, particulate matter (PM), and sulfur oxides (SOx).45 The
CAA requires periodic reviews of the air quality criteria and the standards themselves.6
The EPA's process for NAAQS reviews includes assessing the current scientific
information (i.e., the air quality criteria); conducting quantitative analyses of air quality,
exposure and risk, as warranted; and evaluating policy options regarding the standards,
all of which contribute to the Agency's decisions in each review. As part of this process
and in accordance with the CAA, the Administrator receives advice from an independent
scientific review committee, the CASAC. The EPA also invites the public to participate
throughout the process. The public may provide comments on planning for the review,
the assessments that inform the Administrator's decisions, and on the Administrator's
proposed decisions. Together, the scientific and technical assessments, CASAC advice
and public input inform the Administrator's judgments, as required under the CAA,
whether to retain or revise the current standards.
The chapters that follow provide more details regarding the various elements of
the NAAQS review process and the roles of various partners contributing to it. For each
review, the EPA will provide additional pertinent detail in the documents accompanying
that review. However, this overview document can serve as a general reference for
CASAC and the public regarding typical procedures in each review.
4 The CAA describes criteria pollutants as those pollutants "emissions of which, in [the Administrator's]
judgment, cause or contribute to air pollution which may reasonably be anticipated to endanger public
health or welfare"; "the presence of which in the ambient air results from numerous or diverse mobile
or stationary sources"; and for which the Administrator "plans to issue air quality criteria...." (42 U.S.C. §
7408(a)(1)).
5 The current NAAQS are listed at: httDs://www.eDa.aov/criteria-air-Dottutants/naaas-table.
5 The Act requires the Administrator to complete a review, making revisions as appropriate, "at five-year
intervals" and also authorizes review and revision, as appropriate, "more frequently."
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2 REVIEWING THE NAAQS: AN OVERVIEW
2.1 PHASES OF THE REVIEW AND ROLES OF KEY EPA OFFICES
Each review of the air quality criteria and NAAQS has three phases, a Planning
Phase, an Assessment Phase and a Regulatory Decision-Making Phase (Figure 2-1).
The planning phase begins with a Call for Information for the Agency to consider in the
review followed by a workshop when warranted. In the assessment phase, the Agency
generally develops three types of assessments: a science assessment, one or more7
risk/exposure assessment(s) and a policy assessment. In the regulatory decision-making
phase, the Agency publishes a proposed notice for public comment and then publishes
a final decision notice, as well as the EPA's responses to public comments on the
proposed decision. The documents prepared in all three phases, summarized below, are
announced in the Federal Register and made available to the public on an Agency web
site maintained for this purpose (http://www.epo.aov/nooas).
7 The EPA may develop assessments for human exposure and health risk and/or for environmental
exposures and welfare risk. Thus, in some reviews there may be multiple risk and/or exposure
assessments. Alternatively, in some reviews, the new scientific evidence, technical information, and
available methods/tools do not provide support for updating exposure/risk analyses to address
previously identified limitations or uncertainties or to provide additional insight beyond those provided
by the assessment in prior review. In such cases, the EPA may conclude that development of a new
assessment for the review is not warranted, and the PA will consider the exposure and risk information
from the assessment conducted in the prior review.
8 The Federal Register is a daily gazette and system for publishing government documents and ensuring
the American public access to government information. In NAAQS reviews, notices of document
availability, opportunities for public comment and proposed and final decisions are published in the
Federal Register. The Federal Register is available online at: www.federalreaister.gov.
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0)
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Figure 2-1. Overview of the NAAQS review process and its phases.
Two main offices of the EPA guide the review: the Office of Air and Radiation
(OAR) and the Office of Research and Development (ORD). The critical offices within
OAR and ORD include the Office of Air Quality Planning and Standards (OAQPS) and the
Center for Public Health and Environmental Assessment (CPHEA), respectively (Figure 2-
2). The OAQPS leads the overall NAAQS review and development of the risk/exposure
and policy assessments (REA and PA), while the CPHEA leads the review of the criteria
with development of the science assessment.
Office of Air and Radiation (OAR)
[—Office of Air Quality Planning and Standards (OAQPS)
[—• Health and Environmental Impacts Division (HEID)
I— /
¦ Air Quality Assessment Division (AQAD)
Office of Research and Development (ORD)
h Center for Public Health & Environmental Assessment (CPHEA)
I— Health & Environmental Effects Assessment Division (HEEAD)
|— Integrated Climate Sciences Division (ICSD)
Figure 2-2. The EPA offices with primary roles in NAAQS review activities.
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2.1.1 The Planning Phase
Following the Call for Information, the EPA develops detailed planning
documents for each review. Currently, the EPA's plans for each review are presented to
the public in an Integrated Review Plan (IRP).9 The IRP is prepared jointly by the
CPHEA and OAQPS. In general, the IRP contains background material, such as the
history of the existing criteria and standards and the projected timeline for the review, as
well as key scientific, technical or policy aspects of plans for the new review.
For efficiency and to facilitate timely input from the CASAC and the public, the
IRP is composed of three volumes. Volume 1 provides background information and
serves as a reference for the public and the CASAC. Volume 2 addresses the general
approach for the review, identifying key policy-re levant issues that will guide the review,
and also addresses planning for the science assessment, including key considerations in
its development. Volume 3 is the planning document for quantitative risk and exposure
analyses to be considered in the policy assessment. All three volumes of the IRP are
publicly available, and the key aspects of plans for the assessments in the new review
(Volumes 2 and 3) are the subject of CASAC consultation and public comment.
2.1.2 The Assessment Phase
In the assessment phase, the EPA prepares the science assessment, risk and
exposure assessment(s), and the policy assessment. A list of these assessments is shown
here, followed by a fuller description of each in section 2.2.
- Integrated Science Assessment (ISA)10—The ISA and its associated materials
provide a comprehensive assessment of the current scientific literature on
health and welfare effects associated with the presence of the pollutant in the
ambient air, emphasizing information that has become available since the last
such assessment to reflect the current state of knowledge. The ISA forms the
scientific foundation for each NAAQS review.
- Risk/Exposure Assessment (REA)—Based on the updated scientific
information presented in the ISA, the EPA develops quantitative assessments
9 Development of the IRP for some NAAQS reviews may be informed by a science policy workshop to help
the Agency identify issues and questions to frame the review.
10 The ISA and its associated materials function in the NAAQS review process today as the Air Quality
Criteria Document (AQCD) did in reviews of the past.
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of exposure and/or risk of health and/or welfare effects, which may vary
significantly in scope and content, as warranted. The results and their
implications are discussed in the policy assessment, with the full detailed REA
presented as an appendix to the policy assessment, as in recent reviews, or in a
companion document, as appropriate.
- Policy Assessment (PA)11—The PA evaluates implications of the current
scientific evidence, air quality information, and exposure and risk analyses for
the current standard(s) and potential alternative standards, and frames policy
options for consideration by the Administrator. Review of the PA by the CASAC
facilitates CASAC advice to the Administrator, as provided for in the CAA, on
the existing standards and any revisions that may be appropriate to consider.
The EPA makes drafts of these assessments available for CASAC review and public
comment and prepares final documents with consideration of the CASAC reviews and
public comments.
2.1.3 The Regulatory Decision-Making Phase
The regulatory decision-making phase of the review process generally follows
issuance of the final PA and consideration of conclusions presented therein. In this
phase, the Agency develops and publishes a notice of proposed decision to
communicate to the public the Administrator's proposed decisions regarding the
standards review and considerations underlying the proposed decisions. When the
proposed decision is to revise the existing NAAQS or establish new NAAQS, the notice
presents the proposed regulatory changes. Regardless of whether a change to the
NAAQS is proposed, the notice outlines the supporting rationale for the decision, based
on the underlying scientific evidence and quantitative exposure and risk information as
well as associated uncertainties. A public comment period, during which one or more
public hearings are generally held, follows publication of the proposed decision. Taking
into account comments received on the proposed decision,12 the Agency develops a
notice of final decision, including any regulatory revision. Publication of the final
decision generally completes the review.
11 The PA functions like the Staff Paper in past reviews.
12 The Agency responds to all timely significant comments on the proposal at the time of the final action.
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The relative order of document development in the three phases is illustrated by
a generic NAAQS review timeline in Figure 2-3 below.13 The following sections address
the role of CASAC (section 2.2) and public input (section 2.3), and key concepts in the
review and establishment of NAAQS (section 2.4). Subsequent chapters provide a more
in-depth discussion of each phase of the review.
13 Figure 2-3 is a generic timeline and may not reflect the specific schedule for activities in a given review.
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Call for
Information
I
i
IRP Vol
1 & 2 t
IH
IRP Vol
Science Assessment
i
Draft |
ISA |
: f
i
Risk,1 Exposure Assessment
~ Planning Phase
~ Assessment Phase
~ Regulatory Decision-Making Phase
~ CASAC
~ Public
Initiate literature
search and
screening
H Proposed
Final
H] Decision
B Decision
Time
Notes: Orange-yellow cross-hatching indicates co-occurring public and CASAC involvement. Although not shown, aspects of the regulatory decision-making phase also begin with the Call for
Information (see Chapter 5). The full REA may be presented in PA appendices or a standalone document. For simplicity, the potential for multiple draft assessments document is not shown.
Figure 2-3. Generic timeline and sequencing of phases for a NAAQS review and points of public and CASAC
involvement.
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2.2 ROLEOFTHECASAC
The CASAC provides advice to the EPA Administrator in each NAAQS review in
fulfillment of the CAA requirement for an independent scientific review committee, as
summarized in Appendix A.14 The Administrator appoints the members of the CASAC, as
required by the CAA, which specifies that the independent scientific review committee
be composed of "seven members including at least one member of the National
Academy of Sciences, one physician, and one person representing State air pollution
control agencies" (CAA, section 109(d)(2)(A)). Under the CAA, the Committee is required
to review the air quality criteria and national primary and secondary ambient air quality
standards and make recommendations to the Administrator regarding any new
standards or revisions to the criteria and standards as appropriate. The nomination
process and appointment of the CASAC is managed through the EPA's Science Advisory
Board (SAB) Staff Office.15 For each review, the SAB Staff Office typically selects a
pollutant-specific panel of experts to assist the seven-member CASAC. These individuals
are nationally and/or internationally recognized for their expertise and research in the
field of air pollution related to the criteria pollutant under review, and generally include
one or more members of the CASAC. Together, these CASAC members and the
pollutant-specific experts form the "CASAC Panel" for that review. However, while the
CASAC Panel provides input to the CASAC, ultimately it is the seven-member CASAC
that officially advises the EPA Administrator on the air quality criteria and standards.
During each review, typically the CASAC and CASAC Panel review the key
planning and assessment documents, including the IRP, the ISA, the REA, and the PA.
The role of the CASAC and the public in the planning and assessment phases is
summarized in Table 2-1, and described separately in further detail in this section and
14 Since the early 1980s, this independent review function has been performed by the Clean Air Scientific
Advisory Committee (CASAC) of the EPA's Science Advisory Board (SAB). The CASAC charter (available
on the "About the CASAC" webpage, accessible from httD$://eDa.aov/ca$ac) provides more information
on the objectives and scope of the CASAC's activities and a description of duties, as well as other
administrative guidance. The CASAC charter is renewed every two years in accordance with the
provisions of the Federal Advisory Committee Act (FACA).
15 Information about this process is available on the EPA's CASAC membership web page:
httDs://casac.epa.aov/ords/sab/r/sab apex/casac/mnp?session=666951693277.
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section 2.3, respectively. With the exception of Volume 1 of the IRP, which is a reference
document for the review, all of the planning documents and assessments undergo
consideration by the CASAC and are available for public comment.
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Table 2-1. Role of CASAC review and public comment during planning and assessment phases.
Phase
Contents of each Document or Assessment
CASAC Review and Public Input
Planning
Integrated Review Plan
Volume 1, Background Document, includes a summary of the
review history for the criteria pollutant(s) under review, review
milestones, and the anticipated timeline.
Serves as a reference to the CASAC and the public regarding the pollutant and
NAAQS under review.
Volume 2, Planning for the Review and the Integrated Science
Assessment, describes planning considerations for the overall review,
including policy-relevant issues or questions, and for development of
the ISA, including key scientific questions.
Subject of a consultation with the CASAC Panel and public comment. Panel
member input (at the public meeting and in post-meeting comments) and public
comments are considered in developing the drafts of the ISA and PA,
respectively.
Volume 3, Planning Document for Quantitative Exposure/Risk
Analyses, describes key planning considerations for REA, including
information or tools that might support new/updated analyses.
Subject of a consultation with the CASAC Panel and public comment. Panel
member input (at the public meeting and in post-meeting comments) and public
comments are considered in developing the draft REA(s).
Assessment
Integrated Science Assessment.1 The ISA describes a
comprehensive review, synthesis, and evaluation of the most policy-
relevant scientific information on the pollutant(s) under review,
including key science judgments important to the design and scope of
air quality, exposure and risk analyses, as well as other aspects of the
NAAQS review.
Subject to review by the CASAC and public comment. Advice and comments
from the CASAC and public comments on the draft ISA are considered by the
EPA in developing the final ISA.
Risk and Exposure Assessments2 The REA evaluates human or
environmental exposures and health or welfare risks associated with
air quality conditions meeting the existing and potential alternative
standards, as appropriate.
Subject to review by the CASAC and public comment. Advice and comments
from the CASAC on the draft REA are considered in developing the final REA.
Policy Assessment^ The PA evaluates the policy implications of
information from the ISA and quantitative exposure/risk analyses to
frame policy options for consideration by the Administrator.
Subject to review by the CASAC and public comment. In its review, the CASAC
also provides its advice to the Administrator on the existing standards or
revisions that may be appropriate to consider, per the CAA. Advice and
comments from the CASAC on the draft PA and its conclusions are considered
by OAQPS staff in developing the final PA, which also includes a summary of
CASAC advice.
1 More than one draft of a document may be developed, as warranted by the available scientific and quantitative information and CASAC advice in its review.
2 The REA may be presented as appendices to the PA, as in recent reviews, or in a separate document depending on the review.
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During the planning phase, the EPA receives input from the CASAC Panel on key
aspects of the review plans via a consultation on information in the IRP, which includes
the policy-relevant questions that will guide the review, the scope of ISA, and planning
of quantitative analyses for the REA, among other aspects. As recognized earlier in this
Chapter, the EPA engages in such consultations on Volumes 2 and 3 of the IRP. In these
consultations, the CASAC Panel convenes at public meetings that are announced in the
Federal Register. Members of the public are also invited to provide comments (orally
and/or in writing) to the CASAC Panel at the consultation meeting. At the public
meeting, the Panel members discuss the volumes and individual Panel member
comments are also conveyed to the Agency after the meeting.16 The Panel members'
views provided in these consultations inform the EPA's planning for the scientific,
technical and policy assessments in each review.
During the assessment phase, the CASAC Panel reviews drafts of the ISA, REA and
PA and discusses their comments in one or more public meetings. Simultaneous with
releasing a draft document to the public, the EPA provides the document to the SAB
Staff Office for distribution to the CASAC Panel. With transmittal of the document, the
EPA also provides a series of charge questions to guide review of each assessment. In
some cases, these charge questions are similar from review to review (e.g., "Is the
information presented in the draft PA technically sound, clearly communicated and
appropriately characterized?"). In other cases, charge questions may be specific to the
document and criteria pollutant under review and thus vary across reviews. The CASAC
Panel convenes at public meetings that are announced in the Federal Register and that
offer the opportunity for the public to provide oral and/or written comments.
In fulfilling their role in review of the draft assessment documents, the CASAC
Panel prepares draft advisory reports, summarizing comments and recommendations to
the EPA in the context of addressing the charge questions on the draft documents. The
draft reports include a draft letter for transmitting the report that also highlights the
most important recommendations and includes specific responses to the charge
questions. A draft of each report is made available to the public and to the seven-
15 There are not consensus comments of the Panel (or of the CASAC) developed for consultations.
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member CASAC. The draft report is discussed, deliberated on, and finalized by the Panel
at a public meeting. The seven-member CASAC joins this meeting and seeks
clarifications and revisions on the draft report from the Panel. At the end of the meeting,
the seven-member CASAC votes on adopting the Panel's draft report. Once adopted,
the final report, which includes the letter from the CASAC, responses to the Agency
charge questions, and the individual Panel member comments, is transmitted to the
EPA. In this way, the final report conveys CASAC advice on the draft documents to the
Administrator.17
In completing its review of the ISA, which provides the scientific evidence base for
the NAAQS review, the CASAC generally considers whether the document (once revised
to address comments) will be suitable for the EPA to rely on in NAAQS decision-making.
The CASAC letter on the draft PA additionally conveys the CASAC recommendations
regarding the standards being reviewed. Together, the letters on the various assessment
documents reflect the CASAC's statutory mandate to advise the Agency on both the
criteria and the standards.
Section 109(d)(2)(C) of the CAA identifies several other advisory functions for the
CASAC, including advising the EPA on information gaps in NAAQS reviews and relevant
related research, and advising on relative contributions to air pollution concentrations of
natural as well as anthropogenic activity. These areas are generally addressed in the PA
and considered by the CASAC in its review of a draft PA. Another function also identified
for the Committee is advising on "adverse public health, welfare, social, economic, or
energy effects" associated with strategies for NAAQS attainment and maintenance (CAA
109(d)(2)(C)(iv). In situations where strategies for attaining the NAAQS could result in
adverse public health or welfare effects (e.g., because control of one pollutant could
lead to increases in another), that possibility can be addressed as part of the CASAC's
advice on the criteria and standards. However, as noted in Appendix A, costs of
implementing the standards cannot be considered in the NAAQS-setting process.
Accordingly, any advice on "social, economic and energy effects" of achieving the
17 The letter is addressed to the EPA Administrator from the CASAC Chair, who may or may not also chair
the pollutant-specific Panel. If the CASAC Chair is not the chair of the pollutant-specific panel, the chair
of that panel also signs the letter.
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standard cannot be considered by the Administrator during a NAAQS review. Consistent
with this requirement, the EPA's practice is not to solicit and the CASAC does not
provide advice on these topics during NAAQS reviews.18
2.3 PUBLIC INVOLVEMENT
The EPA provides multiple opportunities for public involvement during a NAAQS
review. The EPA begins each review with a broad Call for Information from the public.
When announcing the availability of Volumes 2 and 3 of the IRP and drafts of the ISA,
REA and the PA in the Federal Register, the EPA also solicits public comment on these
documents. The Federal Register notices describe the process by which the public can
submit comments on each document to the relevant docket19 and specifies the period
during which comments are to be submitted. The Agency considers these comments in
completing the final documents for the review. Further, the Federal Register
announcements of CASAC and CASAC Panel meetings on these documents also invite
the public to share views for consideration in the deliberations over the documents.
During the regulatory decision-making phase, the Agency solicits public
comment on the proposed decision(s) and also provides an opportunity for delivery of
oral comments at a public hearing. The comment period during which the public may
submit comments to the regulatory docket on a proposed NAAQS decision generally
ranges from 45 to 90 days, although variations may occur. As described in Chapter 5,
the Administrator considers comments received from the public in reaching decisions in
the review.
18 The means by which the CASAC, supplemented by a panel with appropriate expertise, could provide
advice on these topics is beyond the scope of NAAQS reviews and, thus, of this document.
19 A docket is a collection of documents made available by an agency for public viewing. Often associated
with an opportunity for public comment, EPA dockets consist of materials used in a rulemaking or other
agency action. These may include documents specifically referenced in the Federal Register, public
comments received, and other information used by the Agency to explain or support its decisions. In
each NAAQS review, the EPA maintains two dockets, one for the review of the air quality criteria (the
ISA docket) and one for the review of the NAAQS (the regulatory docket).
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2.4 FUNDAMENTAL ASPECTS OF NAAQS REVIEWS
There are several fundamental aspects of NAAQS reviews that are integral to the
Administrator's policy judgments and decisions in any review and also play a role in the
assessments that inform the Administrator's decision-making in a review.
2.4.1 Elements of a Standard, Design Values, and Air Quality Characterization
Each NAAQS is defined by four elements: indicator, averaging time, form and
level. The indicator of a standard is the chemical species or mixture that is to be
measured in determining whether a standard is met.20 For example, the indicator of the
primary standard for sulfur oxides is sulfur dioxide, and the NAAQS for PM include
standards with two different indicators, particles with nominal mass median diameter
less than or equal to 2.5 and 10 micrometers (PM2.5 and PM10, respectively). The
averaging time is the time period over which measurements of the chemical are
averaged (e.g., one hour in the case of the primary sulfur dioxide standard). The form
describes the relevant mathematical or statistical treatment of the dataset of ambient air
measurements, such that together with the averaging time, it defines the air quality
statistic that is to be compared to the level of the standard in determining whether a
standard is met. For example, the form of the primary standard for sulfur oxides is the
annual 99th percentile daily maximum 1-hour concentrations averaged over three
years.21 Lastly, the level is the value of the standard in terms of the chemical
concentration, averaged and handled as specified by the form and averaging time. For
example, the level of the 1 -hour primary standard for sulfur oxides is 75 parts per billion.
Thus, the primary standard for sulfur oxides is met at an ambient air quality monitoring
site when the three-year average of the annual 99th percentile of the daily maximum 1 -
20 The sampling and analysis methods for measurement of the pollutant consistent with the indicator for
the standard are specified in regulation.
21 As other examples, the form of the annual PM2.5 NAAQS is the 3-year average of the weighted annual
mean PM2.5 concentrations, while the form of the current 3-month Pb NAAQS is a 3-month average
concentration not to be exceeded during a 3-year period.
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hour average concentrations is less than or equal to 75 parts per billion (as determined
in accordance with relevant data handling regulations).22
The public health or welfare protection provided by NAAQS is evaluated based
on consideration of the four elements of each NAAQS collectively. A metric called a
design value facilitates such consideration. The design value is an air quality statistic
calculated in terms of the indicator, averaging time, and form of the standard, as
specified by any data handling regulations specific to the standard. The EPA calculates
and annually publishes design values for each ambient air monitoring site with data
meeting relevant regulatory requirements (https://www.epa.gov/air-trends/air-auality-
designvalues). When a design value is greater than the level of the NAAQS, the monitor
is described as violating the NAAQS; when the design value is at or below the level of
the NAAQS, the NAAQS is met.
Pollutant concentrations vary spatially with location, reflecting factors such as the
influence of source types, emissions magnitude and pollutant chemistry and transport.
Therefore, the design values will also vary with location. This variation can contribute to
a given urban area having a location with a design value above a given standard level
and other locations well below it.
As recognized in section 4.2 below, the REA in a review focuses on characterizing
exposures and risks estimated to be associated with air quality conditions that meet the
existing or potential alternative standards. For example, when evaluating health
protection or risk, such as in a health REA focusing on specific urban areas, the EPA
generally characterizes the exposure and risk associated with the patterns of air quality
across the area that meet different standards, as determined by pollutant concentrations
at their highest point in the area yielding a design value equal to the level of the
standard being considered. In these air quality scenarios (characterized by a design
value equal to the standard level of interest), there is not a uniform level of air quality at
that concentration across a geographic area. Rather, there is a pattern of air quality for
which the location of the highest concentration just meets the standard, and
22 The primary standard for sulfur oxides is specified in the code of federal regulations at 40 CFR 50.17
(httD$://www.ecfr.aov/current/title-4Q/chaDter-l/suhchaDter-C/Dart-5Q/section-50.17).
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concentrations at surrounding locations will be lower.23 This air quality pattern includes
temporally and spatially varying pollutant concentrations to which people may be
exposed in their daily activities.24 Based on the exposure and risk estimates for such air
quality scenarios, the Administrator can consider the extent of protection for public
health or welfare associated with the standards in reaching decisions on standards that
provide the requisite protection (as further discussed in sections 2.4.2 and 2.4.3).
2.4.2 The Primary Standard
In accordance with the CAA,25 primary (health-based) standards must "specify a
level of air quality the attainment and maintenance of which, in the judgment of the
Administrator [...] and allowing an adequate margin of safety, are requisite to protect
the public health." Further, consistent with the legislative history, primary standards are
established to protect the public health, with an adequate margin of safety, including
the health of at-risk populations.26 The requirement in the Act for public health
protection with "an adequate margin of safety" is intended to address uncertainties
associated with inconclusive evidence and to provide a reasonable degree of protection
against hazards that research has not yet identified. In addressing the requirement to
protect the public health with an adequate margin of safety, the EPA considers such
factors as the nature and severity of the health effects involved, the size of the sensitive
population(s),27 and the kind and degree of uncertainties. Selecting any particular
23 How much lower the lower concentrations may be and the spatial distribution of these concentrations
depend on many factors specific to the meteorology and pollutant source characteristics in an area.
24 These exposure concentrations are generally evaluated in an REA in terms of an exposure metric
identified in consideration of the health effects evidence for that pollutant. Such an exposure metric for
an REA may have some general similarity to the averaging time and form of the pollutant standard
(e.g., short-term or long-term in nature); but it generally differs in any of a number of ways.
25 Further detail on legislative requirements and history, as well as relevant court decisions, is provided in
Appendix A.
25 In this context, at-risk populations refers to persons comprising the sensitive group or lifestage rather
than to a single person in such a group or lifestage.
27 As used here and similarly throughout this document, the term population (or group) refers to persons
having a quality or characteristic in common, such as a specific pre-existing illness or a specific age or
life stage. Identification of such sensitive groups (called at-risk groups or at-risk populations) involves
consideration of susceptibility and vulnerability.
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approach to providing an adequate margin of safety is a policy choice left specifically to
the Administrator's judgment.
2.4.3 The Secondary Standard
In accordance with the CAA,28 secondary (welfare-based) standards must "specify
a level of air quality the attainment and maintenance of which, in the judgment of the
Administrator [...] is requisite to protect the public welfare from any known or
anticipated adverse effects associated with the presence of [the] pollutant in the
ambient air" (CAA section 109). Although the CAA defines "welfare effects," it does not
define "public welfare." In addressing effects that are adverse to the public welfare, the
EPA considers not just whether there is a welfare effect caused by the pollutant under
review (and under what air quality conditions), but what implications it might have for
the public welfare. Determining which effects are "adverse" to the public welfare, and
under what conditions, requires policy judgments about the societal impacts of the
various welfare effects (e.g., effects on soils, waterbodies, crops, vegetation, visibility)
and the kind and degree of associated uncertainties. Aspects of the evidence and
quantitative exposure/risk information important to this consideration include the
extent and severity of the effect under relevant air quality conditions, among other
factors. Such factors inform the Administrator's judgments regarding the level of
protection necessary to address the potential for adversity to the public welfare.
Altogether, the Administrator's decisions on secondary standards must provide the
requisite protection of the public welfare from known or anticipated adverse effects.
2.4.4 Role of Policy Judgment in Standards Decisions
As governed by the CAA, the Administrator's final decisions in reviews of NAAQS
are largely public health and public welfare policy judgments. Such judgments are
informed by the understanding that the health and welfare effects evidence generally
reflect a continuum consisting of exposure concentrations at which scientists generally
agree that health or welfare effects are likely to occur, through lower exposure
concentrations at which the likelihood and magnitude of the response become
28 Further detail on legislative requirements and history, as well as relevant court decisions, is provided in
Appendix A.
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increasingly uncertain. Decisions on particular elements of the NAAQS may also involve
other judgments, such as about the weight to be placed on extreme or unusual data and
the benefits of stability for air quality management. This approach is consistent with the
CAA requirements and how the courts have interpreted the Act. The CAA provisions
require the Administrator to establish standards that are requisite. The term "requisite"
means sufficient but not more than necessary. The CAA does not require that standards
be established at zero-risk levels or at background concentrations. Thus, in light of all of
the relevant considerations, and in accordance with the CAA, the Administrator
establishes the NAAQS such that, in the Administrator's judgment, the primary NAAQS
are sufficient to protect public health with an adequate margin of safety and secondary
NAAQS sufficient to protect the public welfare against any known or anticipated adverse
effects associated with the presence of such air pollutant in the ambient air.
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3 REVIEW PLANNING
Planning for the review begins with the EPA's Federal Register announcement
concerning the new review and extends through development of Volume 3 of the IRP
(Figure 2-3). As recognized in Chapter 2, planning for the review is a joint responsibility
of the CPHEA and the OAQPS. The initial stages of planning in a review are summarized
in section 3.1, and section 3.2 describes key aspects of the 3-volume IRP.
3.1 INITIAL PLANNING
Each NAAQS review is initiated with a Call for Information published in the
Federal Register. The Call for Information announces initiation of the review and invites
the public to submit scientific studies and related information pertinent to the review of
the air quality criteria and standards review; it may also solicit comments from the public
on policy-relevant issues important to address in the review. The public is directed on
how to submit any comments on the review and the specified time period within which
to do so.
As a part of the early planning in a review, the CPHEA and OAQPS staff look to
the last review to identify issues and information particularly relevant to the new review
and areas of particular uncertainty.29 Together, these help focus the new review. The
CPHEA and OAQPS staff also review key uncertainties and data gaps identified in the
previous review (e.g., in the PA and decision notices).30 Particular attention is given to
uncertainties and data gaps documented in the PA for the last review, including those
identified by both staff and by the CASAC.
As warranted, the EPA may also seek initial input from experts representing a
variety of scientific disciplines relevant to a specific review (e.g., via a workshop to
discuss key scientific- and policy-re levant issues being and/or to be considered in the
29 The PA for each review identifies key uncertainties, data gaps and areas for future research, and the
notices of proposed and final decisions in the review note uncertainties particularly relevant to the
NAAQS decisions.
30 As they identify studies for the new review, the staff also considers studies identified in the last review
after completion of the ISA. These include studies that were submitted by public commenters on the
proposed decision and which the EPA provisionally considered at that time.
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review.31 Workshop discussions may then inform planning for the review and
development of the ISA.
3.2 INTEGRATED REVIEW PLAN
The three volumes of the IRP are: Volume 1, which is a background document;
Volume 2, which addresses the general approach for the review and the plan for
developing the ISA; and, Volume 3, the REA planning document. An overview of each of
these documents is provided below.
3.2.1 Volume 1: Background Document
Volume 1 of the IRP provides background on the criteria and standards for the
subject pollutant in the review. More specifically, it includes a history of past reviews and
decisions, including key aspects of the Administrator's decisions and judgments
concerning the specific NAAQS being reviewed. This volume also covers the general
milestones for the review, including release of the various review documents, and the
anticipated timeline for the review, noting any factors that may influence this timeline
(e.g., a court ordered deadline or consent decree).
Volume 1 generally includes appendices with additional information for reference
by the CASAC and the public. For example, these may include information on ambient
air quality monitoring and data handling requirements for the criteria pollutant under
review,32 and a summary of the development process for the ISA, consistent with the
process presented in the Preamble to the ISAs (U.S. EPA, 2015). Volume 1 serves as a
reference document for the CASAC and is released by the EPA with Volume 2.
31 The decision on whether to hold a workshop has typically depended on the criteria pollutant under
review, as well as the resources and timeline associated with the review. As an example, expert input
may be more important in reviews with a substantial volume of new scientific evidence since the last
review, in a review with complex quantitative analyses, and/or in a review where new information has
emerged that may alter the approach for the review.
32 In addition to information on monitoring and data handling regulations, the appendices may
summarize recent air quality information, drawing on documents presenting an overview of recent air
quality for each criteria pollutant that are updated on an annual basis (e.g., Overview of Ozone (O3) Air
Quality in the United States) at httDs://www.eDa.qov/air-aualitv-analvsis/naaas-review-analysis.
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3.2.2 Volume 2: Planning for the Review and the Integrated Science Assessment
Volume 2 of the IRP summarizes the general approach for the review, including
policy-relevant topics, and describes key planning considerations for developing the ISA.
The policy-relevant topics are used to frame consideration of the scientific evidence and
technical information in the PA. The key ISA planning considerations include
identification of scientific questions to guide the development of the ISA.
In describing the general approach for the review, Volume 2 recognizes that a
critical aspect of the assessments in the review is to provide the Administrator with the
information needed to make decisions, as required under the CAA, regarding whether
the standards under review should be retained or revised. In this context, Volume 2
generally presents an initial set of the "policy-relevant" questions the OAQPS staff
addresses in its consideration of the available scientific evidence, quantitative risk and
exposure analyses, and any associated uncertainties in the PA. Policy-relevant questions
in each review generally revolve around consideration of the extent to which air quality
conditions associated with the current and/or potential alternative standards have the
potential to be associated with exposures and risks of concern for the public health or
welfare. Questions may also relate to the individual elements of the standard (i.e.,
indicator, form, averaging time, level). Generic examples of some of these policy-
relevant questions, which may evolve with the initial stages of the PA development, are
presented in Table 3-1.
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Table 3-1. Generic policy-relevant questions considered in a PA for a primary
and/or secondary NAAQS review.
• At what exposure levels do health/welfare effects occur? Is there evidence of effects at
exposure levels lower than those previously observed, and what are the important
uncertainties associated with that evidence?
• Does the currently available information alter or strengthen our understanding of the
health/welfare effects associated with exposure to the criteria pollutant?
• What populations/ecosystems/ecosystem components are most at risk for this criteria
pollutant?
• Is there new evidence on factors that influence relationships between criteria pollutant
concentrations, exposures and health/welfare effects?
• Does the current risk and exposure information indicate exposures of concern
associated with air quality conditions that just meet the NAAQS? Are these risks and
exposures of sufficient magnitude such that the health effects might reasonably be
judged to be important from a public health/welfare perspective? What are the
important uncertainties associated with these risk and exposure estimates?
• To what extent have important uncertainties identified in the last review been reduced
and/or have new uncertainties emerged?
• To what extent does the newly available information call into question support for any of
the basic elements of the standard (i.e., indicator, form, averaging time, level)?
With regard to planning for the ISA, Volume 2 describes plans for the ISA
organization and scope. It also identifies specific scientific questions to focus on in
considering the available scientific information, in light of the overarching policy-
relevant questions for the review. These scientific questions may cover topics such as:
pollutant sources, atmospheric chemistry, and ambient air concentrations; human
exposure, toxicokinetics, and biomarkers; health effects; at-risk lifestages and
populations; and welfare effects, including ecological effects, materials damage, and
visibility impairment, among other topics.
Consultation with the CASAC Panel and comments from the public on Volume 2
(see sections 2.1, 2.2 and 2.3 above) provide early input to the EPA in its planning for a
review. Such input may pertain to policy-re levant issues important to consider in the
review (e.g., beyond those identified by the EPA or that may warrant further
investigation or focus), as well as to the scope and specific scientific questions to focus
the evaluation of the scientific evidence. Together, input in these areas at the planning
stages informs development of the draft ISA and PA.
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3.2.3 Volume 3: Planning Document for Quantitative Exposure/Risk Analyses
Volume 3 of the IRP serves as the planning document for quantitative risk and
exposure analyses considered for the review. These analyses are generally intended to
provide quantitative estimates of human exposure and health risk, and/or of
environmental exposures and welfare risk, for air quality conditions of interest in the
review. Decisions on analyses to be undertaken in each review are informed by the
scientific evidence for the subject pollutant, as characterized by the ISA, the extent to
which there is newly available evidence indicating different effects or effects under
different exposure conditions than was known in the last review, and the availability of
relevant data and modeling tools.
Volume 3 presents a critical evaluation of the extent to which newly available
scientific evidence, tools, or methodologies provide support for and warrant the conduct
of quantitative risk and exposure analyses that would inform the review. As warranted,
this document discusses initial planning for such analyses. Aspects of this discussion
may include how the analysis approaches from prior NAAQS review(s) may be refined,
based on newly available evidence, and/or consideration of new analyses appropriate to
be conducted in the review.
With regard to both health and welfare assessments, Volume 3 generally begins
with a comprehensive review of the analyses conducted in the last review, a summary of
key uncertainties and limitations associated with the analyses, and a description of how
the analyses were considered by the Administrator in reaching decisions in the last
review. Additionally, the EPA staff considers scientific evidence newly available in the
review and how it may inform risk and exposure analyses, including information related
to inputs for risk and exposure assessments and/or methods or tools for modeling
exposure and/or risk. Where the available information indicates it is appropriate to
consider a new type of analysis and/or analysis focused on a newly identified health or
welfare effect or endpoint, the document discusses relevant approaches. As a result of
these considerations, several tasks may be identified as appropriate to consider. These
include:
- Updating quantitative analyses from the previous review. This may be
appropriate to reflect more recent advances in methodologies or tools and any
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new scientific information that together may yield important updates to the
exposure/risk information from the prior review.
- Conducting new quantitative analyses. This may arise from advancements in
the scientific literature and/or evolution of the modeling tools, or health or
welfare effects newly identified by the current evidence (or for which
quantitative assessment is newly supported), that would result in new or
different quantitative analyses from those in the prior review.
- Excluding quantitative analyses conducted in the prior review. At times, the EPA
mayjudge it unnecessary to repeat analyses or carry forward approaches from
previous reviews. This may be when significant uncertainties have been
identified in a prior analysis, affecting its usefulness in decisions of the prior
review, and newly available information has not been identified that might
address these uncertainties.33
- Relying on quantitative analyses conducted in the previous review. This may be
the appropriate approach when new information or improved approaches are
not available to support appreciably updated analyses (e.g., when there are no
or very limited advancements in the scientific evidence, methodologies, or
analysis tools), such that new or updated analyses would not be expected to
provide appreciably different information from prior assessments.
Consultation with the CASAC Panel and public comments on Volume 3 assist the
EPA in its planning decisions for quantitative analyses that may be appropriate to
conduct in a review. The timing of Volume 3, generally coinciding with the public
availability of the draft ISA (and the associated CASAC review), also ensures that the REA
planning takes into account the current scientific evidence, including availability of any
new evidence. Comments received at this planning stage are taken into account in
development of any quantitative analyses and draft REA.
33 When there are numerous analyses that might be conducted in a criteria pollutant or review, the EPA
recognizes the need to prioritize with regard to scientific support and potential influence on decision-
making. By focusing the quantitative analyses performed in a review on those that are most policy-
relevant and for which the scientific support is strong, they can be of greatest utility to the decision
making by the Administrator. Accordingly, by such a focus, the EPA can ensure the analyses conducted
in a review are appropriately comprehensive and technically sound, while recognizing the potential
constraints of time and resources.
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4 ASSESSMENTS
The subsections below describe the three types of assessments that inform
NAAQS reviews. Section 4.1 briefly summarizes the science assessment presented in the
ISA. Assessments of exposure and risk, which are based on the scientific information
presented in the ISA, are described in section 4.2. Lastly, the PA is described in section
4.3.
4.1 SCIENCE ASSESSMENT
The Integrated Science Assessment provides the scientific evidence base for the
Agency's decision-making in each NAAQS review, and fulfills the EPA's responsibilities
under the CAA for review of the air quality criteria. The ISA characterizes the currently
available scientific evidence of the health and welfare effects associated with the
presence of the pollutant in ambient air, including any recent advances in scientific
knowledge in these areas. The general process for developing an ISA is described in the
Preamble to the Integrated Science Assessments (U.S. EPA, 2015).34 Individual ISAs (e.g.,
U.S. EPA, 2024a) build on that general process to reflect advances in assessment
methods; advice from the CASAC; and comments from outside scientific organizations,
stakeholder groups, and other members of the public. The reader is referred to those
documents for greater detail on the ISA process. Here, the EPA provides only a broad
overview of the science assessment, along with key aspects of its role in NAAQS reviews.
The ISA is a comprehensive review, synthesis, and evaluation of the most policy-
relevant science (e.g., epidemiology, controlled human exposure, animal toxicology,
atmospheric science, exposure science, environmental science, and ecology). The
fundamental process for developing an ISA includes several elements: literature
searches; study selection; evaluation of individual study quality; evaluation, synthesis,
and integration of the evidence; and development of causality determinations and other
scientific conclusions. The ISAs build on the data and conclusions of previous NAAQS
34 The Preamble to the ISA, as well as more general information about the ISAs, are available to the public
on the Integrated Science Assessment webpage (httos://www.epo.aov/isa).
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reviews through review of the available scientific evidence, with a focus on studies
published since the literature search publication cutoff date for studies included in the
prior ISA. Important older studies may be discussed to reinforce key concepts and
conclusions. Older studies may also be the primary focus in some subject areas or
scientific disciplines where research efforts have subsided and/or where these older
studies remain the definitive works available in the literature.
Scientific judgments made in the ISA on topics such as causality, populations at
higher risk than the general population (at-risk populations) and quantitative exposure-
response relationships, are important to the design and scope of air quality, exposure
and risk analyses, as well as other aspects of the NAAQS review. For example, the ISA's
findings related to three general questions are particularly relevant: What are the effects?
Who {or What} is affected? Under what exposure conditions? Answers to these questions
are conveyed by the scientific findings in the ISA, including its characterization of
limitations and associated uncertainties in the evidence. As described below, these
findings inform the development of other aspects of the NAAQS review process and
provide the scientific foundation for the Agency's decisions in each review.
With regard to the first question: What are the effects?, the ISA presents a
comprehensive characterization of the evidence to identify what health or welfare
effects are associated with the presence of the criteria pollutant in ambient air. In
reaching conclusions in this regard, the ISA uses a weight-of-evidence framework for
characterizing the strength of the available scientific evidence supporting causal
relationships between criteria pollutant exposures and specific health and welfare
effects. The EPA weighs the array of scientific evidence and reaches conclusions about
the extent of scientific support for a causal relationship between exposures to the air
pollutant and specific effects. Application of this framework provides for one of five
causality determinations, as summarized in Table 4-1.35 These determinations reflect the
35 Rationale and details associated with these determinations are presented in the Preamble to the ISA
(U.S. EPA, 2015). The EPA sponsored a study with the National Academies of Science, Engineering, and
Medicine (NASEM) to evaluate the current ISA framework for reaching causality determinations
(NASEM, 2022). NASEM was generally supportive of the framework used in the ISA, with several
recommendations for improvement subsequently reflected in Appendix A of Volume 2 of the IRP for
nitrogen oxides (U.S. EPA, 2024b). The NASEM recommendations, together with CASAC Nitrogen
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EPA's conclusions on the health and welfare effects of the criteria pollutant as supported
by the scientific evidence. These conclusions inform the consideration of health and
welfare effects evidence in subsequent steps of the review, and thus are important to
REA planning considerations and policy evaluations in the PA, as well as to judgments of
the Administrator regarding the protection provided by the NAAQS.
Table 4-1. The five types of causality determinations that may be made in the
science assessment of health and welfare effects.
Descriptor
Meaning
Causal relationship
Evidence is sufficient to conclude that there is a causal relationship {of health
or welfare effect} with relevant pollutant exposures.
Likely to be a causal
relationship
Evidence is sufficient to conclude that a causal relationship is likely to exist {for
health or welfare effect} with relevant pollutant exposures.
Suggestive of, but not sufficient
to infer, a causal relationship
Evidence is suggestive of, but not sufficient to infer, a causal relationship {of
health or welfare effect} with relevant pollutant exposures; chance,
confounding, and bias cannot be ruled out with confidence.
Inadequate to infer a causal
relationship
Evidence is inadequate to determine that a causal relationship exists {of health
or welfare effect} with relevant pollutant exposures.
Not likely to be a causal
relationship
Evidence indicates there is no causal relationship {of health or welfare effect}
with relevant pollutant exposures.
In the context of welfare effects, the causality determinations that address the
first question also address the second question, What is affected?, through identification
of the affected ecological receptor. For example, the 2024 Pb ISA identified terrestrial
plants as an affected entity with regard to Pb by concluding there to be a causal
relationship between Pb and terrestrial plant growth. Causality determinations reached
on the welfare effects evidence include additional dimensions, compared to those for
health effects evidence, in light of the multiple flora and fauna and organizational units
(e.g., species, communities and ecosystems).
With regard to health effects, the ISA addresses the second question, Who is
affected?, by reaching conclusions on characteristics that may result in populations or
lifestages being at higher risk of air pollutant-related health effects than the general
population. In its identification of potential risk factors and populations/lifestages at
Oxides Review Panel comments on Appendix A of oxides of nitrogen IRP, Volume 2, were then reflected
in the Appendix of Volume 2 of the IRP for ozone and related photochemical oxidants (U.S. EPA, 2024c).
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increased risk (i.e., at-risk populations and lifestages), the ISA characterizes the strength
of the evidence for such identification (e.g., adequate, suggestive or inadequate) based
on a structured framework as described in detail in the Preamble. This identification
informs REA planning considerations, policy evaluations in the PA, and the
Administrator's decisions on adequacy of protection provided by primary standards,
which are intended to protect public health, including the health of at-risk populations.
For example, to the extent there is support for such analysis, the REA may focus on
characterizing risk to an at-risk population, as has been the case for the O3 health REAs
that have characterized risks for children with asthma.
In its comprehensive characterization of the current evidence, the ISA also
provides information important to the third question, Under what exposure conditions?.
This information, which includes identification of limitations and associated
uncertainties, is essential to exposure/risk analyses, policy evaluations and the
associated judgments of the Administrator in each review. For example, in the case of
human respiratory effects for SO2, the strong evidence base for respiratory effects, as
characterized in the ISA, identified people with asthma (an important at-risk population
for SO2) as particularly sensitive to very short exposures, with studies of 5-minute
exposures providing the basis for identifying health-based benchmark concentrations
for use in the REA. Thus, the ISA characterization of the evidence regarding exposure
conditions eliciting effects was a critical foundation for risk characterization in the REA,
which has been important to the Administrator's judgments in establishing and
retaining the current primary NAAQS for SOx.36 For welfare effects, this question can
relate to air quality conditions. An example is visibility impairment associated with PM,
for which the evidence base indicates visibility can vary with humidity and PM
composition. Analyses based on this information have been included in the REA and
have informed the Administrator's judgments regarding the current 24-hour secondary
PM2.5 standard.37
35 The most recent decision on this standard is described in 84 FR 9866, March 18, 2019.
37 The most recent decision on this standard is described in 89 FR 16202, March 6, 2024.
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In its review of a draft ISA, areas in which the CASAC usually provides advice to
the EPA include the causality determinations for health effects and conclusions on at-
risk populations/ lifestages, as well as the causality determinations for welfare effects,
including associated ecosystem components. The CASAC may also identify additional
studies that it believes meet the ISA's scoping and study quality criteria and should be
considered for inclusion in the ISA. The CASAC also generally advises the EPA when the
document is (or will be, following revisions) adequate for the EPA's purposes in the
review. The EPA carefully considers advice received from the CASAC and comments from
the public in developing the final ISA. The final ISA is posted on the EPA website, with its
public availability announced in the Federal Register.
4.2 RISK AND EXPOSURE ASSESSMENT
In NAAQS reviews, quantitative exposure/risk information generally provides a
basis for considering the potential for health or welfare effects of concern to occur
under air quality conditions that meet the existing standards, or potential alternative
standards contemplated, and associated estimates of the magnitude of risk of such
effects. This information may come from REAs newly developed in a review or, when a
new REA is not conducted in a review, from previously available information. The PA
considers the REA and its results, together with the ISA, as part of the evidence that may
support a range of policy options for consideration by the Administrator in reaching
decisions, as required under section 109 of the CAA, on what standards are requisite.
The REAs are generally designed to assess human exposure and health risk, as
well as ecological exposure and welfare risk, for air quality conditions associated with
the existing standards and potential alternative standards. For example, a policy-relevant
question posed of a health or welfare-based REA is:
- What are the nature and magnitude of exposures and associated health/welfare
risks for air quality conditions just meeting the current standard?
A related question concerns the strengths and limitations of the analyses conducted:
- What are the important uncertainties associated with these risk and exposure
estimates?
The types of analyses that comprise each REA are based on the nature and
strength of the scientific evidence available in the review, as well as the available
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assessment and modeling tools and associated data. In identifying health and welfare
effects or endpoints for quantitative assessment (at the planning stage, as summarized
in section 3.2.3), the EPA focuses on considering endpoints for which there is the
strongest support in the scientific evidence, as presented in the ISA. Generally, such
endpoints are those for which the ISA has determined pollutant exposures to be causally
or likely to be causally related to occurrence of the health or welfare endpoint. For
analysis in the REA, the extent of support for quantitative assessment, including strong
evidence describing a relationship between exposures and the health or welfare
endpoint of interest, is then also considered. Identification of the exposure metric or the
surrogate metric for the assessment depends on the extent to which the evidence
provides support for a quantitative relationship between the endpoint and a given
metric.
With regard to health REAs, the evidence bases for the different criteria pollutants
provide support for quantitative health risk assessment of varying levels of refinement,
with the exposure metric used ranging from a surrogate for personal exposure (e.g., PM)
to exposure concentrations (e.g., O3) to internal dose metrics (e.g., CO and Pb), as
illustrated in Figure 4-1 below. In identifying the assessment approach for a particular
criteria pollutant, the EPA evaluates the extent of support in the scientific evidence for
relationships between exposure metrics and health effects of interest. This approach
results in development of an assessment based on the relationships most well founded
in the biological evidence base for each pollutant.
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Figure 4-1. Array of concentration/exposure/dose metrics used in health REAs.
Based on the type of evidence available for a particular pollutant, any of a variety
of quantitative human exposure and health risk analyses may be conducted, as
illustrated by the array of examples in Table 4-2 below. As one example, in recent O3
REAs, the evidence from controlled human exposure studies has supported the
derivation of exposure-response (E-R) models for relating exposures to lung function
response and the identification of health-based benchmark concentrations for risk
characterization across an array of respiratory effects (U.S. EPA, 2020a, Appendix 3D).
More detail presentations of analysis approaches employed in O3 and Pb REAs are
provided in Appendix B.
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Table 4-2. Examples of quantitative health risk assessments performed for past
NAAQS reviews, and the models and datasets on which they rely.
Pollutant: Analysis
Key Elements of Approach
Modeling, Tools, Datasets
03: Characterization
of potential risk to
children with asthma
of experiencing days
with O3 exposures
above health-based
benchmark
concentrations for an
array of respiratory
effects, and of
experiencing lung
function decrements.
Estimated spatial and temporal
pattern of study area O3
concentrations for conditions
meeting existing and potential
alternative standards
Ambient air monitoring data with application of spatial
interpolation technique; air quality modeling (CMAQ) to
derive air quality scenario adjustment; emissions
estimates, meteorological data
Estimates of human exposure
concentrations at elevated
exertion
Population exposure modeling (APEX); U.S. Census,
National Health Interview Survey data on disease status,
human activity data (CHAD); other inputs
Health-based exposure
concentration benchmarks
Controlled human exposure studies, at exertion, for 6.6-
hour average exposure concentrations eliciting array of
respiratory effects
03-related lung function risk
function
Database of controlled human exposure study lung
function decrements at varying exertion and duration
PM: Quantification of
premature mortality
risk associated with
PM2.5 concentrations
in ambient air
Estimated spatial pattern of
annual average PM2.5
concentrations for conditions
meeting existing standard and
potential alternatives
Spatial fields of PM2.5 concentrations in each study area
based on photochemical grid model (e.g. CMAQ)
estimates combined with measured PM2.5 concentrations;
emissions estimates, meteorological data, PM2.5
monitoring data
PM-related premature mortality
risk function and premature
mortality risk estimates
BenMAP; concentration-response relationships from U.S.
multicity studies of PM and mortality; National Center for
Health Statistics mortality data; U.S. Census demographic
information
CO: Characterization
of potential risk to
adults with coronary
heart disease of
experiencing days
with internal COHB
levels above health-
based benchmark
concentrations for
angina-related
response.
Estimated spatial and temporal
pattern of CO concentrations
across study area for conditions
meeting existing and potential
alternative standards
Ambient air quality monitoring data; studies of indoor and
outdoor CO concentrations Proportional adjustment used
for different air quality scenarios; microenvironmental
proximity factors to derive concentrations for indoor
locations
Estimates of human exposure
concentrations
Population exposure modeling (APEX); U.S. Census data;
human activity data (CHAD); NHIS data on disease status
Estimates of internal COHB
levels
Physiological model (Coburn-Forester-Kane equation) for
COHB
Health-based COHB
benchmarks
Controlled human exposure studies of COHB and reduced
time to chest pain while exercising
Pb: Assessment of
IQ decrement risk to
young children
associated with multi-
pathway, multi-route
exposure to air-
related Pb
Estimates of ambient air, indoor
air and dust Pb concentrations
Ambient air quality monitoring data; micro-
environmental/indoor factor; regressions and hybrid
(mechanistic compartmental-2-stage regression) models
Estimates of blood Pb
concentrations
IEUBK model; national data for soil, drinking water and
dietary Pb concentrations; other inputs; U.S. Census data
Blood Pb concentration-IQ
response functions
Epidemiological studies of young children blood Pb
associations with IQ decrements
Abbreviations: APEX = Air Pollutants Exposure Model; BenMAP = Benefits Mapping and Analysis Program; CHAD =
Comprehensive Human Activity Database; CMAQ = Community Multiscale Air Quality model; COHB = carboxyhemoglobin;
IEUBK = Integrated Exposure Uptake Biokinetic Model NHIS = National Health Interview Survey
References: O3 REA - U.S. EPA, 2020a (section 3.4 and Appendices 3C and 3D); PM REA - U.S. EPA, 2020b (section 3.3,
Appendix C); CO REA - U.S. EPA, 2010; Pb REA - U.S. EPA, 2007a,b,c.
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Given the broad array of effects included in the definition of welfare effects, the
air quality, exposure, and risk analysis approaches commonly employed for welfare REAs
vary widely. The types of information that might support different assessments may
include databases of ecosystem pollutant loading that have been associated with
different levels of specific acidification metrics, as in the case of the assessment of
deposition-related acidification effects of sulfur oxides, or E-R functions derived from
controlled exposure studies of plants of interest, as in the case of E-R functions that
relate O3 exposures to tree seedling growth. The type of evidence available will influence
what quantitative analyses are developed for the REA. Table 4-3 presents examples of
quantitative welfare effects analyses developed in past NAAQS reviews.
Table 4-3. Examples of quantitative assessments performed for past reviews of
secondary standards, and the models and datasets on which they rely.
Pollutant: Analysis
Key Elements of Approach
Modeling, Tools and Datasets
PM: Potential for
visibility-related
effects
Light extinction coefficient-based
visibility metric
Three derivation approaches relying on: PM2.5 and
PM10 monitoring data, PM2.5 and PM10 composition
data; meteorological data (relative humidity)
Potential visibility protection targets
Human preference study findings
S0X:
Characterization of
aquatic acidification
risk in acid-sensitive
ecoregions
Estimates of annual sulfur and
nitrogen deposition during five time
periods in U.S. waterbodies
TDep (i.e., Spatial fields of total sulfur and nitrogen
annual deposition based on CMAQ estimates of wet
and dry deposition combined with NADP wet
deposition measurements), SOx emissions,
meteorological data, PM2.5 monitoring data
Deposition estimates for achieving
three acid neutralizing capacity
targets in individual waterbodies
Critical loads database (NCLD), water chemistry
models (e.g., MAGIC)
Characterization of ecoregion risk
from waterbody-specific results
Ecoregion-specific summarization of portion of
waterbodies achieving ANC targets for ecoregion
deposition at/below values and for different time
periods
O3: Tree seedling
growth-related risk
and effects on
carbon
sequestration
Estimated spatial pattern of W126
index across tree species' ranges for
multiple air quality scenarios
Ambient air monitoring data with spatial interpolation
technique; air quality modeling (CMAQ) for air quality
adjustment; emissions estimates, meteorological data,
Species-specific exposure-tree
growth response functions
Controlled tree seedling exposure studies of annual
growth; W126 cumulative exposure index function
Estimates of carbon storage in multi-
species tree and crop communities
Forest- and agriculture-related carbon sequestration
model (FASOMGHG), urban tree-related carbon
sequestration model (iTREE)
Abbreviations: CMAQ = Community Multiscale Air Quality model; MAGIC = Model of Acidification of Groundwater In
Catchments; NADP = National Atmospheric Deposition Program; NCLD = National Critical Load Database; TDep = estimates
developed by NADP, TDep Science Committee (https://nadp.slh.wisc.edu/committees/tdep/)
References: PM REA - U.S. EPA, 2020b (section 3.3, Appendix C); SOx REA - U.S. EPA, 2024d (section 5.1, Appendix 5A);
O3 REA - U.S. EPA, 2014a,b (section 6.2, Appendices 4A, 6A, 6B, 6F)
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As noted above, the air quality conditions evaluated in each REA generally
include those associated with meeting the existing standard(s), as well as air quality
conditions that might be associated with potential alternative standards. Depending on
the pollutant and type of assessment (e.g., air pathway only or multimedia), the air
quality conditions may be characterized using ambient air measurements from fixed site
monitors, satellite measurements, air quality modeling, or a combination of these or
other approaches. Determining the best approach for using available air quality
information to generate air quality scenarios that simulate just meeting the current or
alternative standards may depend on the chemistry and transport of the pollutant, the
spatial resolution needed, and the exposure time frame of interest.
In the exposure assessment step, as noted above, depending on the scientific
evidence for a pollutant (e.g., support for internal dose-, personal exposure-, or ambient
air concentration-response relationships in a health risk assessment), the ambient air
concentration estimates may sometimes be used as surrogates for exposure. In deriving
human exposure and also dose estimates, however, the EPA generally utilizes exposure
and internal dosimetry modeling.38 In addition to influencing whether the exposure
metric used in the assessment is personal (or population) exposure, an internal dose
metric, or an ambient air concentration estimate, the scientific evidence base also
influences the type of mathematical metric used. For example, the health REA focus
could be on 5-minute exposure concentrations or the frequency of 5-minute exposure
concentrations above specific benchmarks, or the focus could be on annual average
38 For example, the established Air Pollutant Exposure (APEX) and Integrated Exposure, Uptake and
Biokinetic (IEUBK) models are routinely used in human exposure modeling for O3 and Pb reviews,
respectively.
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ambient air concentrations.39 As noted above (e.g., Figure 4-1), the metrics used in a
welfare effects assessment also reflect the underlying evidence base.40
The REAs for NAAQS reviews generally utilize a case-study design such that
exposure and risk estimates are derived for one or more case studies. The set of case
studies employed are intended to illustrate differences in the variables that influence risk
and that may occur across the U.S., particularly in areas where ambient air
concentrations are near the existing NAAQS. While the same conceptual air quality
scenarios are simulated in all study areas (e.g., conditions that just meet an existing or
potential alternative standard), variability in factors such as pollutant or precursor
emissions patterns, meteorological conditions, and population characteristics in the
study areas contribute to variability in the estimated magnitude of exposure and
associated risk across study areas. The case studies illustrate a variety of exposure
patterns that may be associated with air quality conditions occurring under an existing
or potential alternative standards.
In the most recent NAAQS reviews, the complete REA is published in association
with the PA and discussed within the body of the PA.41 In addition to providing a time-
saving efficiency in a review, this approach helps to explicitly convey the role of the REA
in informing policy-re levant considerations in NAAQS reviews. The quantitative
39 For example, for an assessment focused on risk associated with short-term exposure (e.g., lung function
decrements following inhalation of SO2), the analyses may generate hourly air quality concentrations
across an urban case study area. In other situations, an annual average may be more appropriate for
assessment of a longer-term exposure health endpoint (e.g., mortality risk from PM2.5). Additionally,
other air quality approaches may be used in ecological assessments that focus on air quality
information linking the transformation and deposition of the criteria pollutant.
40 While the averaging time and form of a pollutant standard may be established based on some similar
considerations as the exposure metric for the REA, there are a number of additional considerations in
specifying the elements of a standard. Accordingly, the exposure metric for an REA may have some
similarity to the averaging time and form of the pollutant standard (e.g., short-term or long-term in
nature); but it generally differs in any of a number of ways. For example, in the health REA for recent
reviews of the primary NAAQS for SOx, the exposure metric was a 5-minute average S02 exposure
concentration, and the standard is the 3-year average of 99th percentile annual 1 -hour ambient air
concentrations.
41 With this approach, the details of the REA (e.g., design, methods, results, uncertainty characterization)
are presented in one or more appendices to the PA. There may also be situations where the REA is
presented in a separate volume accompanying the PA.
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estimates from the REA are considered in the policy evaluations of the PA along with the
health and/or welfare effects evidence from the ISA. Together, these evaluations inform
the Administrator's decision-making on the public health and/or welfare protection
provided by the current or potential alternative standards.
The draft REA (e.g., in association with the draft PA) is made available to the
public and transmitted to the CASAC for its review, along with a set of charge questions
(as noted in section 2.2). The charge questions are intended to elicit review of the
soundness of the quantitative analyses for their purpose in informing an understanding
of exposure and risk associated with the existing standards and potential alternatives, as
well as an understanding of their limitations and associated uncertainties. The EPA
carefully considers the comments from the CASAC and the public in developing final
analyses. The final REA generally includes a summary of changes made to analyses and
their presentation in response to the CASAC review and public comments. The final REA
is posted on the EPA website, with its public availability announced in the Federal
Register.
4.3 POLICY ASSESSMENT
The PA presents analyses and staff conclusions regarding a range of policy
options supported by the current scientific information and exposure/risk information. It
considers the policy implications of the key scientific and technical information, along
with its limitations and associated uncertainties. The PA integrates and interprets the
current scientific evidence from the ISA and the analyses from the REA to address a set
of policy-relevant questions. The PA focuses on information that is most pertinent to
evaluating each standard (i.e., is "policy-relevant"). This includes both evidence-based
(i.e., drawn from the ISA and the studies contained therein) and exposure/risk-based
(i.e., drawn from REA analyses) considerations. In focusing on the policy implications of
the current information, the PA is intended to "bridge the gap" between the Agency's
scientific and quantitative risk and exposure assessments (in the ISA and REA) and the
judgments required of the Administrator in determining whether it is appropriate to
retain or revise the NAAQS.
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In addition to the evaluations of policy implications of the current information,
the PA includes background material that informs the evaluations. The background
material includes an overview of the legislative requirements for the NAAQS and a
history of past NAAQS reviews for the pollutant, as well as the progress and future steps
in the current review. The air quality information presented generally includes recent
information on pollutant or precursor emissions and ambient air concentrations, as well
as a summary of the NAAQS sampling and monitoring network regulations, and data
handling conventions for comparisons to the NAAQS. For some pollutants (e.g., O3), a
characterization of concentrations associated with background sources may also be
presented.42 The PA also summarizes the general approach for the review, including
fundamental aspects of NAAQS reviews (summarized in section 2.4). As a point of
reference for the policy evaluations, the PA also includes a summary of the current
NAAQS and key aspects of their basis, including important judgments made by the
Administrator in the last review.
As part of the evaluation, the PA summarizes the policy-re levant aspects of the
current evidence and quantitative exposure/risk information. This includes an overview
of key conclusions from the ISA, including identification of the health and welfare effects
associated with the pollutant (e.g., causal determinations), at-risk populations (for
primary standard reviews), as well as discussion regarding public health and welfare
implications and uncertainties associated with the evidence. In summarizing this
information, the PA addresses questions such as the following: Does the currently
available scientific evidence alter conclusions from the last review regarding the
health/welfare effects attributable to the criteria pollutant in ambient air and at-risk
populations? What are important limitations of the evidence and associated uncertainties?
The PA also describes the quantitative analyses for the review, including the risk and
exposure assessments, and summarizes and discusses the key results and associated
uncertainties and limitations. The PA evaluates policy implications of the exposure/risk
estimates, including by addressing the question: Are these exposures and risks of
42 The air quality information presented in the PA includes analyses and information relevant to the policy
assessment.
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sufficient magnitude such that the heaith or welfare effects might reasonably be judged to
be important from a pubiic heaith or public welfare perspective? Further, the PA
addresses the question: What are the important uncertainties associated with these risk
and exposure estimates?
The evaluation in the PA considers aspects of the scientific evidence and
exposure/risk information that are particularly relevant in the context of the NAAQS. The
basic elements of a standard - indicator, averaging time, form, and level - that together
serve to define each standard, are considered collectively in evaluating the public health
and public welfare protection the standard affords. The PA describes the support in the
evidence and quantitative information for an array of policy options, as appropriate, and
identifies limitations of the information and associated uncertainties. The CASAC, in
fulfilling its responsibility under section 109 of the CAA, reviews the preliminary
conclusions of the draft PA and provides its advice on the standards. Together, the staff
conclusions in the final PA, along with the evidence- and exposure/risk-based
considerations and the CASAC advice, are summarized for consideration by the
Administrator.
In presenting its conclusions, the PA recognizes that the decisions on the NAAQS
are, as noted in section 2.4.4 above, decisions made by the Administrator. It further
recognizes that such decisions are based on public health and public welfare policy
judgments necessary to reach conclusions as to what standards are requisite to protect
public health or public welfare and are neither more nor less stringent than necessary
for this purpose. In this context, the PA, in its evaluation of the health and/or welfare
effects evidence from the ISA and exposure/risk information from the REA, provides the
Administrator with information needed to make decisions, as required under section 109
of the CAA, on standards that are requisite.
The draft PA is made available to the public and transmitted to the CASAC for its
review. As noted in section 2.2, in transmitting the draft PA, the EPA also provides a
series of charge questions. While there may also be pollutant- or NAAQS-specific charge
questions, there is also a commonality to these questions across reviews (e.g., Table 4-
4). The CASAC conveys its comments on the PA and advice regarding the standards, in
accordance with its responsibility under the CAA, in a letter to the Administrator. The
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EPA carefully considers the comments from the CASAC and the public in developing
final PA and its conclusions. The final PA incorporates appropriate changes and includes
a section summarizing the CASAC advice on the standards. The final PA is posted on the
EPA website, with its public availability announced in the Federal Register.
Table 4-4. Examples of generic charge questions for the CASAC in its review of a
draft PA.
• Is the information presented in the draft PA technically sound, clearly communicated and appropriately
characterized? Are key aspects of the available scientific evidence and quantitative analyses, along with
the uncertainties associated with each, accurately presented?
• What are the CASAC's views on the preliminary conclusions presented in the draft PA regarding the
range of policy options identified as appropriate for the Administrator to consider based on the available
scientific evidence and quantitative information, along with associated uncertainties?
• Does the draft PA identify the key uncertainties and areas for additional research and data collection? Are
there additional areas that should be highlighted?
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5 REGULATORY DECISION-MAKING
The regulatory decision-making process in a NAAQS review conforms to the
procedures for such regulatory actions in the federal government and those specified by
the CAA.43 This includes establishment and maintenance of dockets for information
considered in the decision making and publication of proposed and final decisions in
the Federal Register, with opportunity provided for public comment on the proposed
decisions. These aspects are further described below.
At the initiation of a NAAQS review, the EPA establishes two dockets, one for the
review of the air quality criteria (the ISA docket) and one for the review of the NAAQS
(the regulatory docket).44 The ISA docket generally receives the public comments from
the Call for Information and also public comments submitted on the draft ISA. The EPA
also places into the ISA docket any additional information supporting development of
the ISA, including draft ISAs, letters transmitting CASAC advice on draft ISAs and
communications (e.g., emails) with authors of individual scientific studies regarding
salient details of studies. The regulatory docket receives all Federal Register notices for
the review of the NAAQS (e.g., announcements of document availability, public
comment periods, and public hearings; notices of proposed and final rulemakings) and
all public comments in response to these notices. Additionally, the EPA places into the
regulatory docket all documents considered in the decision-making for a review,
including the IRP volumes, and draft and final REA and PA, with associated materials
(e.g., quantitative analyses, scientific studies, datasets), and other relevant documents
(e.g., technical memoranda).
In reaching decisions in the review, the Administrator considers the currently
available information, including the scientific conclusions of the ISA, quantitative air
quality/exposure/risk information, policy evaluations in the PA, public comment, and
43 The administrative process that the EPA follows in NAAQS reviews, including for public participation
and judicial review, adheres to the CAA section 307(d) requirements for administrative proceedings and
judicial review, as well as any additional requirements of the Administrative Procedure Act, which
describes aspects of the process for federal agencies in general to develop and issue regulations.
44 Both dockets are publicly accessible at www.reaulations.gov.
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CASAC advice. As summarized in section 2.4.4, the Administrator is required to exercise
his judgment to set standards that are requisite to protect public health (with an
adequate margin of safety) or public welfare and are neither more nor less stringent
than necessary for this purpose. All of the information supporting the proposed and
final decisions is made available in the docket with those decisions.
In accordance with CAA and other requirements, the Administrator's proposed
decisions in each review are published in the Federal Register with opportunity for public
comment. After consideration of public comments and any related additional analyses,
the final decisions are also published in the Federal Register. The sections below
summarize the process for developing and issuing the notices of proposed and final
decisions in NAAQS reviews.
5.1 PROPOSED DECISION
Following the issuance of the final PA, the Agency develops a notice of proposed
decision based on the available scientific evidence and quantitative information, as well
as the conclusions presented within the final PA, and taking into consideration advice
from the CASAC and public comments received up to that point in the review. This
notice describes the basis for and the Administrator's rationale for the proposed
decision, reached after considering the analyses and conclusions in the documents
developed in the review (e.g., as described in the preceding sections), public comment
and advice from the CASAC.
As appropriate, the draft notice of the proposed decision is submitted to the
Office of Management and Budget (OMB)'s Office of Information and Regulatory Affairs
(OIRA) for its review and for its coordination of review by other federal agencies.45 The
interagency review period generally lasts 90 days. The EPA considers the comments
received from the OIRA and other agencies during this step, making revisions as
appropriate.46
45 Executive Order 12866 describes OIRA's role in the rulemaking process.
45 In accordance with Executive Order 12866 disclosure provisions, the OMB makes available on
www.Realnfo.aov a list of all rules undergoing EO 12866 regulatory review. Consistent with the CAA and
other requirements, the EPA includes in the regulatory docket notice of any changes to the draft notice
made at this stage.
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After completing the interagency review, the notice of proposed action is
published in the Federal Register.47 At the time of publication, all materials on which the
proposed decisions are based are made available in the public docket for the review.
Publication of the proposal notice is followed by a public comment period, generally
lasting 45 to 90 days, during which the public is invited to submit comments on the
proposal to the docket. The EPA also offers the opportunity for one or more public
hearings for the public and stakeholders to provide comments orally. The EPA also
offers the opportunity for consultation with federally recognized Tribal governments to
ensure meaningful and timely input by Tribal officials prior to taking actions or
implementing decisions that may affect Tribes.48
5.2 FINAL DECISION
After consideration of comments received on the proposed decision, the Agency
develops a notice of final action, which communicates the Administrator's final
decision(s) in the NAAQS review. The final decision notice describes the basis for and
the Administrator's rationale for the decision, including consideration of the scientific
and quantitative information, limitations and associated uncertainties, advice from the
CASAC, and comments from the public. The EPA also responds to all significant
comments on the proposal; this may be done wholly within the final decision notice or,
depending on the volume of significant comments received, the notice may be
augmented by a separate response-to-comments document.
In reviewing and considering public comments, the EPA also provisionally
considers any "new" scientific studies cited by public commenters to enable the EPA to
consider them in the context of the associated comments as they relate to the rationale
47 Where implementation of the proposed decision would have an annual effect on the economy of $100
million or more, e.g., by necessitating the implementation of emissions controls, the EPA develops and
releases a draft regulatory impact analysis (RIA) concurrent with the notice of proposed rulemaking.
This activity is conducted under Executive Order 12866 and is completely independent of, and by
statute is not considered in, decisions regarding the review of the NAAQS.
48 The EPA Policy on Consultation with Indian Tribes provides more information on the policy and process
for Tribal consultations in the regulatory decision-making phase
(httDs://www.eDa.aov/svstem/fHes/documents/2023- 12/eDa-Dolicv-on-consultation-with-indian-tribes-
2023.pdf).
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and decisions in the review. As the EPA's provisional consideration of studies at this
stage of the review cannot provide the same kind of in-depth critical review of the
scientific evidence as in the assessment phase, this consideration focuses on
determining whether the "new" studies warrant reopening the air quality criteria.49
Based on this provisional consideration, if the EPA concludes that the studies would not
materially change the scientific conclusions of the ISA in the review, the air quality
criteria are not reopened and the studies are considered in the context of addressing
the comments on the proposed decision.50
The draft notice of final action also generally undergoes OMB-coordinated
interagency review by the process described in section 5.1. When complete, the
Administrator signs the notice and it is published in the Federal Register, completing the
review process.51
49 The EPA's NAAQS decisions are based on studies and related information included in the ISA, REA, and
PA, which have undergone CASAC and public review. The studies assessed in the ISA and the
integration of the scientific evidence in that document have undergone extensive critical review by the
EPA, CASAC, and the public during ISA development. The rigor of that review makes these studies, and
their integrative assessment, the most reliable source of scientific information on which to base NAAQS
decisions. Decisions on the NAAQS can have profound impacts on public health and welfare and should
be based on studies that have been rigorously assessed in an integrative manner not only by the EPA
but also by the statutorily-mandated independent advisory committee, CASAC, and have also been
subject to the public review that accompanies this process. Provisional assessments do not provide that
kind of in-depth critical review.
50 Where the air quality criteria are not reopened, studies provisionally considered in this way are
considered for inclusion in the ISA for the next review (as noted in section 3.1).
51 The notice of final decision specifies the date on which the decision, and any associated regulatory
changes, are effective.
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6 REFERENCES
National Academies of Sciences, Engineering, and Medicine. 2022. Advancing the
Framework for Assessing Causality of Health and Welfare Effects to Inform
National Ambient Air Quality Standard Reviews. Washington, DC: The National
Academies Press, h ttps://doi. org/10.7 7226/26612.
U.S. EPA. 2007a. Lead: Human Exposure and Health Risk Assessments for Selected Case
Studies. Volume I. Human Exposure and Health Risk Assessments - Full-scale.
Office of Air Quality Planning and Standards, Research Triangle Park, NC. EPA-
452/R-07-014a.
U.S. EPA. 2007b. Lead: Human Exposure and Health Risk Assessments for Selected Case
Studies. Volume II. Appendices. Office of Air Quality Planning and Standards,
Research Triangle Park, NC. EPA-452/R-07-014b.
U.S. EPA. 2007c. Review of the National Ambient Air Quality Standards for Lead: Policy
Assessment of Scientific and Technical Information. OAQPS Staff Paper. Office of
Air Quality Planning and Standards, Research Triangle Park, NC. EPA-452/R-07-
013.
U.S. EPA. 2010. Quantitative Risk and Exposure Assessment for Carbon Monoxide -
Amended. Office of Air Quality Planning and Standards, Research Triangle Park,
NC. EPA-452/R-10-009.
U.S. EPA. 2014a. Welfare Risk and Exposure Assessment for Ozone. Final. Office of Air
Quality Planning and Standards, Research Triangle Park, NC. EPA-452/P-14-005a.
U.S. EPA. 2014b. Welfare Risk and Exposure Assessment for Ozone. Final. Appendices.
Office of Air Quality Planning and Standards, Research Triangle Park, NC. EPA-
452/P-14-005b.
U.S. EPA. 2015. Preamble To the Integrated Science Assessments (ISA). U.S.
Environmental Protection Agency, Washington, DC, EPA/600/R-15/067.
U.S. EPA. 2020a. Policy Assessment for the Review of the Ozone National Ambient Air
Quality Standards. Office of Air Quality Planning and Standards, Research Triangle
Park, NC. EPA-452/R-20-001.
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U.S. EPA. 2020b. Policy Assessment for the Review of the National Ambient Air Quality
Standards for Particulate Matter. Office of Air Quality Planning and Standards,
Research Triangle Park, NC. EPA-452/R-20-002.
U.S. EPA. 2023. Integrated Review Plan for the National Ambient Air Quality Standards
for Lead. Volume 3: Planning Document for Quantitative Exposure/Risk Analyses.
Office of Air Quality Planning and Standards, Research Triangle Park, NC. EPA-
452/R-22-003c.
U.S. EPA. 2024a. Integrated Science Assessment for Lead (Final Report). Office of
Research and Development, Research Triangle Park, NC. EPA/600/R-23/375.
January 2024. Available online at:
httDs://ossessments.eDa.aov/iso/document/8ideid=359536.
U.S. EPA. 2024b. Integrated Review Plan for the Primary National Ambient Air Quality
Standards for Oxides of Nitrogen. Volume 2: Planning for the Review and the
Integrated Science Assessment. Office of Air Quality Planning and Standards,
Health and Environmental Impacts Division and Center for Public Health and
Environmental Assessment, Office of Research and Development. Research
Triangle Park, NC. EPA-452/R-24-010b.
U.S. EPA. 2024c. Integrated Review Plan for the National Ambient Air Quality Standards
for Ozone and Related Photochemical Oxidants. Volume 2: Planning for the
Review and the Integrated Science Assessment. Office of Air Quality Planning
Standards and Center for Public Health and Environmental Assessment, Research
Triangle Park, NC. EPA-452/R-24-001b.
U.S. EPA. 2024d. Policy Assessment for the Review of the Secondary National Ambient
Air Quality Standards for Oxides of Nitrogen, Oxides of Sulfur and Particulate
Matter. Office of Air Quality Planning and Standards, Research Triangle Park, NC.
EPA-452/R-24-003.
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APPENDIX A
LEGISLATIVE REQUIREMENTS
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The Clean Air Act (CAA) is the comprehensive federal law that regulates air
emissions from stationary and mobile sources. Sections 108 and 109 of the CAA govern
the establishment, review, and revision, as appropriate, of the NAAQS for each criteria
air pollutant. These sections of the CAA are discussed more below, along with key
caselaw (Figure A-1).
Section 108 (42 U.S.C. 7408) describes the
air quality criteria, which are the basis for
reviewing NAAQS.52 Air quality criteria are
intended to "accurately reflect the latest scientific
lead, oxides of nitrogen,
knowledge useful in indicating the kind and
extent of all identifiable effects on public health
i * ^ particulate matter,
or welfare which may be expected from the
presence of [a] pollutant in the ambient air...." ¦
(42 U.S.C. § 7408(a)(2)).
Section 109 (42 U.S.C. 7409) directs the Administrator to propose and promulgate
"primary" and "secondary" NAAQS for pollutants for which air quality criteria are issued
(42 U.S.C. § 7409(a)). Section 109(b)(1) defines primary standards as ones "the
attainment and maintenance of which in the judgment of the Administrator, based on
such criteria and allowing an adequate margin of safety, are requisite to protect the
public health."53 Under section 109(b)(2), a secondary standard must "specify a level of
air quality the attainment and maintenance of which, in the judgment of the
Administrator, based on such criteria, is requisite to protect the public welfare from any
52 The CAA describes criteria pollutants as those pollutants "emissions of which, in [the Administrator's]
judgment, cause or contribute to air pollution which may reasonably be anticipated to endanger public
health or welfare"; "the presence of which in the ambient air results from numerous or diverse mobile
or stationary sources"; and for which the Administrator "plans to issue air quality criteria...." (42 U.S.C. §
7408(a)(1)).
53 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); see also Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 618 (D.C. Cir. 2010),
Am. Lung Ass'n v. EPA, 134 F.3d 388, 389 (D.C. Cir. 1998).
A-1
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known or anticipated adverse effects associated with the presence of [the] pollutant in
the ambient air."54
In setting primary and secondary standards that are "requisite" to protect public
health and welfare, as provided in section 109(b), the EPA's task is to establish standards
that are neither more nor less stringent than necessary. In so doing, the EPA may not
consider the costs of implementing the standards. See, Whitman v. American Trucking
Ass'ns, 531 U.S. 457, 465-472, 475-76 (2001). Likewise, "[attainability and technological
feasibility are not relevant considerations in the promulgation of national ambient air
quality standards." See American Petroieum Institute v. Costle, 665 F.2d 1176, 1185 (D.C.
Cir. 1981); accord Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019).55
The requirement that primary standards protect the public health with an
adequate margin of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time of standard
setting. It was also intended to provide a reasonable degree of protection against
hazards that research still needs to identify. See Lead Industries Ass'n v. EPA, 647 F.2d
1130, 1154 (D.C. Cir. 1980); American Petroleum Institute v. Costle, 665 F.2d at 1186;
Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 (D.C. Cir. 2010);
Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013). Both uncertainties are
components of the risk associated with pollution at levels below those at which human
health effects can occur with reasonable scientific certainty. Thus, in selecting primary
standards that include an adequate margin of safety, the Administrator is seeking not
only to prevent pollution levels that have been demonstrated to be harmful but also to
prevent lower pollutant levels that may pose an unacceptable risk of harm, even if the
risk is not precisely identified as to nature or degree. The CAA does not require the
54 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."
55 At the same time, courts have clarified that the EPA may consider "relative proximity to peak
background ... concentrations" as a factor in deciding how to revise the NAAQS in the context of
considering standard levels within the range of reasonable values supported by the air quality criteria
and judgments of the Administrator. See American Trucking Ass'ns, v. EPA, 283 F.3d 355, 379 (D.C. Cir.
2002).
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Administrator to establish a primary NAAQS at a zero-risk level or at background
concentration levels (see Lead Industries Ass'h v. EPA, 647 F.2d at 1156 n.51, Mississippi v.
EPA, 744 F.3d at 1351), but rather at a level that reduces risk sufficiently to protect public
health with an adequate margin of safety.
In addressing the requirement to protect the public health with an adequate
margin of safety, the EPA considers such factors as the nature and severity of the health
effects involved, the size of the sensitive population(s), and the kind and degree of
uncertainties. Selecting any particular approach to providing an adequate margin of
safety is a policy choice left specifically to the Administrator's judgment. See Lead
Industries Ass'n v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353. In
addressing the requirement to protect the public welfare from known or anticipated
adverse effect, the EPA considers which effects are "adverse" to the public welfare. Such
determinations require policy judgments about the societal impacts of the various
welfare effects (e.g., effects on soils, water, crops, vegetation), and the kind and degree
of associated uncertainties.
Section 109(d)(1) of the Act requires periodic review and, if appropriate, revision
of existing air quality criteria to reflect advances in scientific knowledge concerning the
effects of the pollutant on public health and welfare. Under the same provision, the EPA
is also to review periodically and, if appropriate, revise the NAAQS based on the revised
air quality criteria.56
55 This section of the Act requires the Administrator to complete these reviews and make any revisions
that may be appropriate "at five-year intervals." It further authorizes the Administrator to review and
revise criteria or standards "earlier or more frequently."
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Lead Industries Ass'n v.
EPA
Whitman v. American
Trucking Ass'ns
Coalition of Battery
Recyclers Ass'n v. EPA
Murray Energy Corp. v.
EPA
1981
2002
2013
1980
2001
2010
2019
American Petroleum Institute American Trucking Ass'ns, v. Mississippi v. EPA
v. Costle EPA
Figure A-1. Timeline depicting key caselaw.
Section 109(d)(2) addresses the appointment and advisory functions of an
independent scientific review committee. Section 109(d)(2)(A) requires the Administrator
to appoint this committee, which is to be composed of "seven members including at
least one member of the National Academy of Sciences, one physician, and one person
representing State air pollution control agencies." Section 109(d)(2)(B) provides that the
independent scientific review committee "shall complete a review of the criteria...and the
national primary and secondary ambient air quality standards...and shall recommend to
the Administrator any new...standards and revisions of existing criteria and standards as
may be appropriate ..." Since the early 1980s, this independent review function has been
performed by the Clean Air Scientific Advisory Committee (CASAC) of the EPA's Science
Advisory Board (SAB).57 In each review, the seven-member CASAC is typically assisted by
a pollutant-specific panel of experts that are nationally and internationally recognized
for their expertise and research in the field of air pollution related to the criteria
pollutant under review. The nomination process and appointment of the CASAC and
pollutant specific panels is managed through the EPA's SAB Staff Office.
Several other advisory functions are also identified for the committee by section
109(d)(2)(C), which reads:
Such committee shall also (i) advise the Administrator of areas in which
additional knowledge is required to appraise the adequacy and basis of
57 The CASAC charter (publicly available on the "About the CASAC" webpage at: www,epa.aov/casac)
provides more information on the objectives and scope of the CASAC's activities, a description of
duties, as well as other administrative guidance. The CASAC charter is renewed in accordance with the
provisions of the Federal Advisory Committee Act (FACA) every two years.
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existing, new, or revised national ambient air quality standards, (ii) describe
the research efforts necessary to provide the required information, (iii)
advise the Administrator on the relative contribution to air pollution
concentrations of natural as well as anthropogenic activity, and (iv) advise
the Administrator of any adverse public health, welfare, social, economic,
or energy effects which may result from various strategies for attainment
and maintenance of such national ambient air quality standards.
As previously noted, the Supreme Court has held that section 109(b)
"unambiguously bars cost considerations from the NAAQS-setting process" in Whitman
v. American Trucking Ass'ns, 531 U.S. 457, 471 (2001). Accordingly, while some of the
issues listed in section 109(d)(2)(C), such as those on which Congress has directed the
CASAC to advise the Administrator, are relevant to the standard-setting process, others
are not. Issues that are not relevant to standard setting may be relevant to
implementing the NAAQS once they are established.
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APPENDIX B
EXAMPLES OF SOME ANALYTICAL APPROACHES
USED IN
NAAQS RISK AND EXPOSURE ASSESSMENTS
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Ambient Air Monitoring Data (hourly concentrations)
~co
a
10%, 15% and 20%)
Figure B-1. General analytical approach employed in an ozone NAAQS REA (U.S.
EPA, 2020a).
B-1
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=3
a
<
Ambient Air Monitoring Data (DV=max 3-rnonlli average concentration in 3 years) |
Ratio from recent air quality data for DVs to an annual average for exposure modeling
i
Annual average Pb concentration for AQ scenario(s)
0
01
X
LL)
Factor for
inhalation
exposure vs
outdoor
air Pb cone
Hybrid indoor dust Pb model
Annual ave
indoor dust
Pb loading
Annual
average Pb
inhalation
exposure
concentration
i
- Sources not related to recent air ¦
[updated values] (EUBKinpub)
_ 4-
£
£
V>
o
Q
Annual ave
indoor dust
Pb cone
Ave soil Pt)
concentration
Average OW
Pb
concentration
Ave dietary
Pb
Pb Exposure/Dosimetry Modeling (IEUBK)
(exposure concentrations and physiological inputs for child from birth thru <6 years of age)
<6 yr-
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United States
Environmental Protection
Agency
Office of Air Quality Planning and Standards
Health and Environmental Impacts Division
Research Triangle Park, NC
Publication No. EPA-452/R-24-019
December 2024
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