Clean Air Act Issues in the 117th Congress
February 17, 2021
Congress may examine wide-ranging air pollution issues as it deliberates legislation and conducts
oversight of the U.S. Environmental Protection Agency (EPA). Oversight of EPA’s air pollution
Kate C. Shouse
work has historically received significant attention in Congress. Such oversight has often
Analyst in Environmental
examined whether EPA’s programs meet the statutory objective to protect human health and the
Policy
environment from air pollution, how evolving scientific understanding informs Clean Air Act

(CAA) decisionmaking, and the degree to which potential tradeoffs exist between public health
benefits and compliance costs. Some Members of Congress have also raised concerns about

potential disproportionate impacts on communities located near sources of emissions, and related
concerns about whether CAA standards are protective of vulnerable individuals (e.g., children, pregnant women, the elderly,
and persons with preexisting health conditions).
Congress may also consider legislation to address air pollution issues and EPA’s CAA authority. Recent Administrations
have interpreted CAA authority in different, and sometimes conflicting, ways. Historically, many of EPA’s CAA regulations
have been challenged in court, both by industry and by public health and environmental groups, with various states
supporting each side.
Some prominent air topics of potential interest in the 117th Congress are air quality standards, wildfire smoke, hazardous air
pollutant standards, and permits. Related issues, such as greenhouse gas (GHG) emissions or other climate change topics
relevant to the CAA, are not addressed herein.
Air Quality Standards
The CAA requires EPA to review national ambient air quality standards (NAAQS) for common pollutants every five years.
The NAAQS review process has evolved over time. In recent years, EPA restructured the NAAQS review process by
compressing the schedule and disbanding a pollutant-specific scientific review panel that has historically advised agency staff
during their reviews. In December 2020, EPA completed its particulate matter and ozone reviews, and finalized rules to retain
the current PM and ozone standards. While some supported EPA’s efforts to streamline NAAQS reviews, others voiced
concerns that procedural changes compromised the agency’s review of the latest science. Congress may consider whether
EPA’s review process meets the CAA objectives to review in a timely manner the NAAQS and the science upon which they
are based.
Wildfire Smoke
Wildfire smoke can temporarily increase ambient levels of particulate matter and other air pollutants. These increases may be
measured by the national network of stationary air monitors. Given the need for real-time air quality information during
wildfire events, monitoring strategies may include some combination of stationary monitors, mobile sensors, or computer
modeling to estimate pollution levels. Congress may consider which monitoring strategies effectively inform smoke
management and public health responses. Congress may also examine how CAA requirements factor into forest management
and fire prevention strategies.
Hazardous Air Pollutants
The CAA requires EPA to set and periodically review standards limiting hazardous air pollutants (HAPs). Congress may
conduct oversight of EPA’s statutorily mandated reviews of regulations to limit HAPs such as ethylene oxide. Another CAA
issue involves the potential tension between incentivizing pollution prevention and limiting cumulative emissions. Under a
2020 EPA rulemaking, major sources of HAPs can reclassify as “area sources,” which are typically subject to lesser controls,
after meeting conditions to limit emissions below major source thresholds. Congress may consider the health and
environmental implications of this rule, including how potential HAP increases may contribute to cumulative exposures in
communities with relatively high environmental burdens.
CAA Permitting
Congress may conduct oversight of New Source Review (NSR), a CAA preconstruction permitting program intended to
ensure that new and modified stationary sources of air pollution do not significantly degrade air quality. The NSR program
generally requires emission limits based on modern pollution controls when new facilities are built or when existing facilities
make a change that increases emissions above specified thresholds. Historically, NSR applicability determinations have been
contentious and extensively litigated. Congress may consider legislative proposals addressing questions about NSR
applicability and enforcement.
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Clean Air Act Issues in the 117th Congress

Introduction
The 117th Congress is likely to face wide-ranging air pollution issues as it conducts oversight and
deliberates on legislation related to air quality standards, smoke from wildfires, air quality
monitoring, air toxics, and permitting requirements. Congress may factor multiple issues into
these deliberations, including environmental and public health objectives, economic impacts, and
how the U.S. Environmental Protection Agency (EPA) accounts for distributional effects1 in
benefit-cost analysis for Clean Air Act (CAA) rulemakings.
Diverse sources of air emissions—including power plants, industrial facilities, small commercial
facilities, motor vehicles, and wildfires—contribute to the various gaseous and particle pollutants
in ambient, or outdoor, air, which is the purview of the Clean Air Act. Air quality has improved
substantially since Congress enacted the CAA in 1970. Annual emissions of the six “criteria” air
pollutants for which the EPA has set air quality standards—ozone, particulate matter, sulfur
dioxide, carbon monoxide, nitrogen dioxide, and lead—have since declined by more than 70%,
despite increases in population, motor vehicle miles traveled, and economic activity.2
At the same time, EPA has tightened air quality standards as the scientific understanding of the
health effects of air pollution has evolved. The goal of clean air continues to elude many areas.3
For example, approximately 122 million people live in 50 areas of the United States that do not
meet the ozone standards issued by EPA in 2015.4 Additionally, some Members of Congress have
expressed concern about wildfire smoke and air quality, including potential interactions with the
Coronavirus Disease 2019 (COVID-19).5 Exposure to smoke can increase the risk and severity of
respiratory infections such as COVID-19.6
This report begins with background about the CAA framework, including federal and state roles,
and summarizes congressional actions related to the CAA in the 116th and earlier Congresses. The
report also discusses some prominent air topics of potential interest in the 117th Congress: EPA’s
review of particulate matter and ozone standards; management of wildfire smoke; EPA’s review
of hazardous air pollutant standards; classification of hazardous air pollutant sources; and
revisions to applicability determinations under the preconstruction permitting program, known as
New Source Review (NSR). This report does not discuss greenhouse gas (GHG) emissions or
other climate change topics relevant to the CAA, which are addressed in other CRS reports.7

1 The Office of Management and Budget refers to distributional effects as “the impact of a regulatory action across the
population and economy, divided up in various ways (e.g., income groups, race, sex, industrial sector, geography).” See
OMB Circular A-4, “Regulatory Analysis,” September 17, 2003, p. 14.
2 EPA, Our Nation’s Air, https://gispub.epa.gov/air/trendsreport/2020/#home.
3 For example, see EPA’s Green Book for a list of areas that do not meet one or more of the NAAQS. EPA, Green
Book National Area and County-Level Multi-Pollutant Information, https://www.epa.gov/green-book/green-book-
national-area-and-county-level-multi-pollutant-information.
4 Data reported by EPA as of December 31, 2020, based on 2010 population. EPA, 8-Hour Ozone (2015)
Nonattainment Area Summary, December 31, 2020, https://www3.epa.gov/airquality/greenbook/jnsum.html.
5 Letter from Jeffrey A. Merkley, U.S. Senator, et al. to Alex M. Azar II, Secretary, U.S. Department of Health and
Human Services, September 14, 2020.
6 Centers for Disease Control and Prevention, FAQs for Wildland Firefighters, https://www.cdc.gov/coronavirus/2019-
ncov/community/wildland-firefighters-faq.html.
7 For example, see (1) CRS In Focus IF11696, Aviation and Climate Change, by Richard K. Lattanzio; (2) CRS In
Focus IF10752, Methane Emissions: A Primer, by Richard K. Lattanzio; (3) CRS In Focus IF10871, Vehicle Fuel
Economy and Greenhouse Gas Standards, by Richard K. Lattanzio, Linda Tsang, and Bill Canis; (4) CRS Report
R46568, EPA’s Affordable Clean Energy Rule: In Brief, coordinated by Kate C. Shouse; and (5) CRS In Focus
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Overview of the Clean Air Act
The CAA, codified as 42 U.S.C. 7401 et seq., seeks to protect human health and the environment
from emissions that pollute ambient, or outdoor, air.8 It establishes roles for federal and state
agencies and “expressly delegates a large number and variety of regulatory and policymaking
functions.”9 For example, the act requires EPA to establish minimum national standards for air
quality, known as national ambient air quality standards (NAAQS). States have primary
responsibility for assuring compliance with these standards, and for establishing and
incorporating procedures in state implementation plans to attain and maintain the NAAQS. The
CAA also requires that areas not meeting the standards, referred to as “nonattainment areas,”
implement specified air pollution control measures.
Under the CAA, Congress mandated that EPA establish two types of NAAQS for each criteria
pollutant—a primary NAAQS, which must protect public health with an “adequate margin of
safety,” and a secondary NAAQS, which must “protect public welfare from any known or
anticipated adverse effects.”10 Public welfare includes damage to crops, vegetation, property,
building materials, and climate.11
The CAA also requires EPA to review the NAAQS and the science upon which they are based
every five years and then revise the NAAQS if necessary. In addition, the CAA requires EPA to
appoint an independent scientific review committee composed of seven members, which has
become the Clean Air Scientific Advisory Committee (CASAC). The act directs CASAC to
review the NAAQS every five years and recommend to the EPA Administrator “any new national
ambient air quality standards and revisions … as may be appropriate.”12 In practice, CASAC has
evaluated the agency’s work during NAAQS-setting and NAAQS-revision, rather than
conducting its own independent review of the standards. Beyond these CAA requirements,
procedural aspects of the NAAQS review are generally at the discretion of the EPA Administrator.
The CAA also includes provisions related to ambient air quality monitoring. The act requires EPA
to develop standards for the design and operation of an air quality monitoring network. It also
requires state implementation plans to “provide for establishment and operation” of the
monitors.13 States, local agencies, and tribes typically design and operate air quality monitoring
networks based on federal standards.
The CAA contains air pollution provisions beyond the NAAQS requirements. The act establishes
federal standards for certain new and modified stationary sources of air pollution, for mobile
sources of air pollution and their fuels, and for sources of 187 hazardous air pollutants. It
establishes a cap-and-trade program for the emissions that contribute to acid rain. It also
addresses protection of the stratospheric ozone layer.

IF11541, Hydrofluorocarbons (HFCs): EPA and State Actions, by Kate C. Shouse.
8 For example, one of the stated purposes of Title I of the CAA is to “to protect and enhance the quality of the Nation’s
air resources so as to promote the public health and welfare and the productive capacity of its population” (42 U.S.C.
§7401(b)(1)).
9 Michael R. Barr, “Introduction to the Clean Air Act,” in The Clean Air Act Handbook, ed. Julie R. Domike and Alec
C. Zacaroli, 4th ed. (Chicago: American Bar Association, 2016).
10 42 U.S.C. §7409(b).
11 42 U.S.C. §7602(h).
12 42 U.S.C. §7409(d)(2).
13 42 U.S.C. §§7410(a)(2)(B) and 7619.
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In addition, the CAA establishes two types of permits. One is New Source Review (NSR), a
preconstruction permit issued before construction of a new facility or major modification to an
existing facility. The NSR program generally requires emission limits based on modern pollution
controls when new facilities are built or when existing facilities make a change that increases
emissions above specified thresholds. Historically, NSR applicability determinations have been
contentious and extensively litigated.14
The second type of CAA permit is an operating permit required under Title V. The operating
permit is federally enforceable and specifies each source’s emission limits, compliance schedule,
reporting requirements, and other conditions. States typically administer operating permit
programs and issue the permits. Sources subject to the permit requirements include major sources
that emit or have the potential to emit 100 tons per year of any regulated pollutant, as well as
sources that emit or have potential to emit lesser specified amounts of hazardous air pollutants.15
For more information about the CAA’s major requirements, see CRS Report RL30853, Clean Air
Act: A Summary of the Act and Its Major Requirements, by Kate C. Shouse and Richard K.
Lattanzio.
Air Pollution Issues in Prior Sessions of Congress
Oversight of EPA’s air pollution work has historically received significant attention from
Congress. Such oversight has often examined whether EPA’s programs meet the statutory
objective to protect human health and the environment from air pollution, whether EPA
regulatory efforts are consistent with, or exceed, the agency’s statutory authority, how evolving
scientific understanding informs CAA decisionmaking, and potential tradeoffs between public
health benefits and compliance costs. Some Members of Congress have also raised concerns
about distributional impacts—for example, whether CAA permitting accounts for potential
disproportionate impacts on air quality in communities located closer to sources of emissions, and
relatedly, whether CAA standard-setting accounts for greater health risks among vulnerable
individuals (e.g., children, pregnant women, the elderly, and persons with preexisting health
conditions).
Historical Context: Congressional Oversight of EPA’s CAA Rules
Over the past decade, Congress has deliberated on EPA’s authority to promulgate or implement
new emission control requirements under the CAA. Often under court order, the Obama
Administration’s EPA used authorities Congress gave EPA in the CAA amendments of 1970,
1977, and 1990 to address long-standing issues posed by emissions from various sources. During
that time, EPA’s regulations on greenhouse gas emissions from both mobile and stationary
sources and on a variety of emissions from electric power plants were of particular interest to
Congress, as were the agency’s efforts to revise the ozone and particulate matter NAAQS. Some

14 For discussion of key legal decisions on NSR, see CRS Report R43699, Key Historical Court Decisions Shaping
EPA’s Program Under the Clean Air Act, by Linda Tsang.
15 42 U.S.C. §7661a and 40 CFR §70.3. Permit applicability depends on the type of air pollutant, whether the source is
located in a nonattainment area, and other criteria. For example, some nonmajor sources of hazardous air pollutants,
such as hazardous waste combusters, may be subject to operating permits. Regardless of size, some sources subject to
the CAA Acid Rain requirements as well as certain solid waste incineration units may be subject to operating permits.
In nonattainment areas, the permit requirements include sources that emit as little as 50, 25, or 10 tons per year of
VOCs, depending on the severity of the region’s ozone nonattainment status (serious, severe, or extreme). See also
EPA, “Who Has to Obtain a Title V Permit?” https://www.epa.gov/title-v-operating-permits/who-has-obtain-title-v-
permit.
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Members of Congress raised concerns that some of EPA’s regulatory activities exceeded the
authority that Congress had provided through the CAA, that the agency ignored or
underestimated the costs and economic impacts, and that EPA overestimated the benefits of
proposed and promulgated rules. Other Members of Congress disagreed that EPA exceeded its
CAA authority, concluding that the agency had a statutory obligation to take action on significant
air pollution issues, such as limiting mercury emissions from power plants, and emphasized the
health and environmental benefits of CAA regulations.
Deliberations on air pollution issues continued in the 115th and 116th Congresses through
oversight hearings and the introduction of some CAA-related bills. At the same time, the Trump
Administration reviewed many CAA regulations. The CAA mandated some of these reviews,
such as the NAAQS reviews and residual risk and technology reviews of hazardous air pollutant
standards. Judicial actions, such as the remand of a 2016 rule addressing interstate transport of
ground-level ozone, prompted other reviews.16 The Trump Administration’s regulatory reform
initiative also led EPA to evaluate existing regulations and some of the accompanying benefit-cost
analyses, and to identify those regulations that should be considered for replacement, repeal, or
modification.17 Examples of rules evaluated under this initiative include those for NSR and the
EPA findings from 2000, 2012, and 2016 that limits on hazardous air pollutants from coal- and
oil-fired power plants are “appropriate and necessary” under CAA Section 112(n)(1).
EPA’s reviews resulted in some regulatory revisions. Some Members of Congress and
stakeholders disagreed with EPA’s rationale—e.g., statutory interpretations and revised benefit-
cost analyses—for various regulatory decisions and maintained that EPA’s decisions would
worsen air quality.18 Other Members of Congress and stakeholders agreed with EPA’s revised
statutory interpretations and updated accounting of benefits and costs.19
Legislative Proposals in the 116th Congress
At least 47 CAA-related bills were introduced in the 116th Congress. Many of the bills addressed
either renewable fuel standards or hazardous air emissions. For example, some bills would have
revised CAA provisions for renewable fuel standards, and others would have authorized EPA to
take various actions with respect to hazardous air pollutants. Additional proposals would have
modified CAA permitting provisions: some would have modified NSR provisions, and others

16 On September 13, 2019, the U.S. Court of Appeals for the District of Columbia Circuit remanded EPA’s 2016 Cross-
State Air Pollution Rule Update in Wisconsin v. EPA. For more information, see EPA, “Revised Cross-State Air
Pollution Rule Update,” https://www.epa.gov/csapr/revised-cross-state-air-pollution-rule-update.
17 Executive Order 13777 of February 24, 2017, “Enforcing the Regulatory Reform Agenda,” 82 Federal
Register 12285, March 1, 2017, https://www.federalregister.gov/documents/2017/03/01/2017-04107/enforcing-the-
regulatory-reform-agenda. In addition, E.O. 13771 directed agencies, among other actions, to eliminate two regulations
for each new regulatory action. Executive Order 13771, “Reducing Regulation and Controlling Regulatory Costs,” 82
Federal Register 9339, January 30, 2017, https://www.federalregister.gov/documents/2017/02/03/2017-02451/
reducing-regulation-and-controlling-regulatory-costs. E.O. 13771 and E.O. 13777 were revoked by Executive Order
13992 of January 20, 2021, “Revocation of Certain Executive Orders Concerning Federal Regulation,” 86 Federal
Register 7049, January 25, 2021, https://www.federalregister.gov/documents/2021/01/25/2021-01767/revocation-of-
certain-executive-orders-concerning-federal-regulation.
18 For example, see Senator Tom Carper, A Pandemic of Pollution, U.S. Senate Environment and Public Works
Committee, Staff Report, May 2020, https://www.epw.senate.gov/public/index.cfm/2020/5/carper-releases-new-staff-
report-on-epa-s-pandemic-of-pollution.
19 For example, see quotes from Members of Congress reported in EPA, “EPA Finalizes MATS Supplemental Cost
Finding and ‘Risk and Technology Review,’” press release, April 16, 2020, https://www.epa.gov/newsreleases/epa-
finalizes-mats-supplemental-cost-finding-and-risk-and-technology-review.
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would have restricted issuance of CAA operating permits for new sources based on
environmental justice considerations20 among multiple other criteria.
Two additional air quality topics of interest in the 116th Congress were (1) wildfire smoke and (2)
potential interactions between air pollution and the COVID-19 pandemic. The 116th Congress
considered options for federal support and assistance to address wildfires and introduced
legislation centered on wildfire smoke.21 For example, S. 1812 and H.R. 4924 would have
authorized EPA to research and mitigate the impacts of smoke emissions from wildfires.
Some Members of Congress raised concerns about whether poor air quality makes individuals
more vulnerable to COVID-19. For many years, scientific studies found links between air
pollution, illness, and premature mortality.22 For example, research has linked exposure to fine
particles with a range of health effects, including aggravated asthma, chronic bronchitis, and
increased premature mortality. More recently, researchers have begun to examine whether there is
a link between long-term exposure to fine particles and COVID-19 mortality.23
Air Pollution Issues in the 117th Congress
Congress may continue to debate questions about air quality and emissions control through its
EPA oversight or legislation. Congressional oversight also may consider how the Biden
Administration could shape CAA implementation through various administrative tools, such as
executive orders and policy memoranda. In particular, the Biden Administration has directed
federal agencies to review regulations and other agency actions issued by the Trump
Administration, and to consider taking actions based on the review. For example, a White House
memorandum directed EPA and other agencies to, among other things, consider postponing for 60
days (from January 20, 2021) the effective dates of regulations that had been published in the
Federal Register but had not yet taken effect.24 A second example is Executive Order (E.O.)
13990.25 E.O. 13990 requires federal agencies to review “all existing regulations, orders,
guidance documents, policies, and any other similar agency actions” taken during the Trump
Administration and to consider “suspending, revising, or rescinding” agency actions that are
deemed inconsistent with the order’s stated policy concerning protection of public health and the
environment and addressing climate change.26 E.O. 13990 also specifies timelines for agency
heads to consider taking action—suspend, revise, or rescind—on certain rulemakings through a
notice-and-comment proposed rulemaking. One of these rules is EPA’s 2020 review of the

20 For more on environmental justice, see CRS In Focus IF10529, Role of the U.S. Environmental Protection Agency in
Environmental Justice, by David M. Bearden and Angela C. Jones.
21 CRS In Focus IF10732, Federal Assistance for Wildfire Response and Recovery, by Katie Hoover.
22 For example, see EPA, Integrated Science Assessment (ISA) for Particulate Matter, December 2019,
https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=347534.
23 X. Wu et al., “Air Pollution and COVID-19 Mortality in the United States: Strengths and Limitations of an
Ecological Regression Analysis,” Science Advances, vol. 6, no. 45 (November 4, 2020),
https://advances.sciencemag.org/content/6/45/eabd4049.
24 Presidential Actions, Memorandum for the Heads of Executive Departments and Agencies, “Regulatory Freeze
Pending Review,” January 20, 2021, https://www.whitehouse.gov/briefing-room/presidential-actions/2021/01/20/
regulatory-freeze-pending-review/. See also CRS Legal Sidebar LSB10566, Responses to Midnight Rulemaking: Legal
Issues, by Daniel J. Sheffner and Kate R. Bowers.
25 Executive Order 13990 of January 20, 2021, “Protecting Public Health and the Environment and Restoring Science
to Tackle the Climate Crisis,” 86 Federal Register 7037, January 25, 2021.
26 The policy is presented in E.O. 13990, Section 1.
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benefits and costs of reducing mercury and other hazardous air pollutants from coal- and oil-fired
power plants.27 EPA is to consider taking action on this rule by August 2021.28
Congress may consider legislation to address continued concerns about air pollution and
disagreements about EPA’s CAA authority. Recent Administrations have interpreted CAA
authority in different, and sometimes conflicting, ways. Historically, many of EPA’s CAA
regulations have been challenged in court, both by industry and by public health and
environmental groups, with various states supporting each side. Industry stakeholders have
maintained that under some Administrations, EPA has overreached its CAA authority.
Environmental and public health groups have often asserted that EPA standards are not
sufficiently protective of public health or do not meet statutory requirements, or that the agency
has disregarded its science advisors.
The remainder of this report discusses CAA topics that will likely garner attention in the 117th
Congress.
Congressional Review
Congress may also use the Congressional Review Act (CRA) to review some rules that were
promulgated by the Trump Administration and submitted late in the 116th Congress.29 The most
likely timing for using the CRA is at the outset of the 117th Congress.
The CRA provides one special legislative mechanism through which Congress may review and
disapprove of an agency rule through a joint resolution, which cannot be filibustered in the
Senate, provided it meets certain conditions.30 Under the CRA, if Congress passes a joint
resolution disapproving a rule and the resolution becomes law, the rule cannot take effect or
continue in effect.31 The agency may not reissue either that rule or any substantially similar one,
except under authority of a subsequently enacted law. For a CRA resolution to become law,
however, the President must sign it or allow it to become law without signature, or Congress
overrides a presidential veto.
The CRA specifies deadlines that determine which final rules could be considered using this
legislative mechanism. If Congress adjourns before the period authorized by the CRA to
introduce and act on a disapproval resolution for a rule, the CRA’s “lookback” provision allows
the next Congress to consider doing so. The lookback provision is intended to ensure that
Congress will have the full periods contemplated by the act to disapprove a rule regardless of
when it is received.32 Under Section 801(d) of the CRA, final rules submitted to Congress on or
after the 60th meeting day before adjournment in either the Senate or the House are subject to

27 EPA, “National Emission Standards for Hazardous Air Pollutants: Coal- and Oil-Fired Electric Utility Steam
Generating Units—Subcategory of Certain Existing Electric Utility Steam Generating Units Firing Eastern Bituminous
Coal Refuse for Emissions of Acid Gas Hazardous Air Pollutants,” 85 Federal Register 20838, April 15, 2020.
28 E.O. 13990, Section 2(iv). The 2020 rule is discussed later in this report; see “Mercury from Power Plants: EPA’s
Consideration of Co-Benefits.”
29 Title II, Subtitle E, P.L. 104-121, 5 U.S.C. §§801-808.
30 CRS Report R43992, The Congressional Review Act (CRA): Frequently Asked Questions, by Maeve P. Carey and
Christopher M. Davis. For questions about the CRA, congressional clients may contact Maeve Carey, Specialist in
Government Organization and Management.
31 The CRA adopts a broad definition of rule; for more information, see CRS Report R43992, The Congressional
Review Act (CRA): Frequently Asked Questions, by Maeve P. Carey and Christopher M. Davis.
32 CRS In Focus IF10023, The Congressional Review Act (CRA), by Maeve P. Carey and Christopher M. Davis.
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renewed periods for congressional review.33 That is, for regulations submitted to the 116th
Congress within 60 meeting days before adjournment in either chamber, the 117th Congress may
consider a joint resolution of disapproval of such regulations using the special CRA procedures.
CRS unofficially estimates that final rules received during the 116th Congress on or after August
21, 2020, are potentially eligible for additional periods of CRA review in the 117th Congress. The
Senate and House Parliamentarians are the sole definitive arbiters of the CRA mechanism,
however, and should be consulted for guidance on any specific question.
EPA has finalized numerous rules that may be considered under the CRA, and at least 90 of these
rules were promulgated in the last six months of the Trump Administration. The vast majority of
the rules promulgated in the last six months of the Trump Administration were actions on state
implementation plans, based on EPA’s review of the plans. Of the remaining CAA rules
promulgated during this period, the following four are discussed in greater detail in this report:
1. Air Quality Standards: EPA, “Review of the National Ambient Air Quality
Standards for Ozone,” final rule, 85 Federal Register 87256, December 31, 2020.
(See “Particulate Matter and Ozone NAAQS Reviews.”)
2. Air Quality Standards: EPA, “Review of the National Ambient Air Quality
Standards for Particulate Matter,” final rule, 85 Federal Register 82684,
December 18, 2020. (See “Particulate Matter and Ozone NAAQS Reviews.”)
3. Preconstruction Permits: EPA, “Prevention of Significant Deterioration (PSD)
and Nonattainment New Source Review (NNSR): Project Emissions
Accounting,” 85 Federal Register 74890, November 24, 2020. (See “New Source
Review Permits.”)
4. Hazardous Air Pollutants: EPA, “Reclassification of Major Sources as Area
Sources Under Section 112 of the Clean Air Act,” final rule, 85 Federal Register
73854, November 19, 2020. (See “Source Classification: 2018 EPA Withdrawal
of “Once In, Always In” Policy.”)
In addition to considering rules pursuant to the CRA, the 117th Congress may also address rules
through other types of legislation or through the appropriations process. The courts will continue
to play an important role, and the 117th Congress and EPA itself may act to make important
revisions to CAA regulations.

33 §801(d) of the CRA provides that, if a final rule is submitted to Congress less than 60 session days in the Senate or
less than 60 legislative days in the House of Representatives before Congress adjourns a session sine die, a new period
for congressional review of that rule becomes available in the next session of Congress. For more information about
this “lookback” period and other CRA elements, see CRS Report R43992, The Congressional Review Act (CRA):
Frequently Asked Questions, by Maeve P. Carey and Christopher M. Davis.
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Particulate Matter and Ozone NAAQS Reviews
In December 2020, EPA completed its statutorily mandated reviews of the particulate matter
NAAQS and the ozone NAAQS. EPA finalized two rules, one to retain the current particulate
matter standards34 and another to retain the current ozone standards.35
Stakeholder opinion regarding whether EPA should revise the particulate matter and ozone
standards varies. For example, some Members of Congress concurred with EPA’s rulemaking to
retain particulate matter standards, describing the current standards as “among the strictest
safeguards” globally and observing that U.S. air quality has improved over time.36 Other
Members of Congress have urged EPA to tighten the particulate matter standards. For example, in
a letter to the EPA Administrator dated June 29, 2020, some Members noted concerns about how
air quality may disproportionately affect certain communities—such as low-income communities,
communities of color, and Tribal and indigenous communities—and potentially interact with
COVID-19.37
NAAQS Review Process
The NAAQS review process has evolved over time, with multiple Administrations introducing
procedural modifications intended to streamline the process, improve transparency, or strengthen
the scientific basis. Beyond the CAA requirements discussed earlier in this report, the procedural
aspects of the NAAQS review are generally at the discretion of the EPA Administrator.38 Most
recently, the Trump Administration has sought to streamline NAAQS reviews by restructuring the
review process.39 For example, EPA structured the most recent ozone review to last roughly two-
and-a-half years. The previous ozone review lasted about seven years. EPA compressed the ozone
NAAQS review schedule by releasing two draft analyses—the Integrated Science Assessment
and the Policy Assessment—nearly concurrently for simultaneous review by CASAC. This
approach differed from previously completed reviews in which EPA considered CASAC input
and public comments on the Integrated Science Assessment as EPA developed the Policy
Assessment.

34 EPA, “Review of the National Ambient Air Quality Standards for Particulate Matter,” Final action, 85 Federal
Register 82684, December 18, 2020. EPA completed its prior review of the particulate matter NAAQS in late 2012 and
promulgated revisions to strengthen the standards. EPA published the final rule in the Federal Register in 2013; see
EPA, “National Ambient Air Quality Standards for Particulate Matter, Final Rule,” 78 Federal Register 3086, January
15, 2013.
35 EPA, “Review of the Ozone National Ambient Air Quality Standards,” Final action, 85 Federal Register 87256,
December 31, 2020. EPA completed its prior review of the ozone NAAQS in 2015, at which time it strengthened the
ozone standards. See EPA, “National Ambient Air Quality Standards for Ozone, Final Rule,” 80 Federal Register
65292, October 26, 2015.
36 EPA, “What They Are Saying: EPA Proposes to Retain NAAQS for Particulate Matter,” press release, April 14,
2020, https://www.epa.gov/newsreleases/what-they-are-saying-epa-proposes-retain-naaqs-particulate-matter.
37 Some Members of Congress stated that such communities are disproportionately affected by particulate matter air
pollution and that studies show that “Americans living in areas with higher levels of particulate matter pollution are
more likely to die from COVID-19 than those living in areas with cleaner air.” Letter from Thomas R. Carper, U.S.
Senator, et al. to Andrew Wheeler, EPA Administrator, June 29, 2020.
38 For summary of relevant statutory requirements, see the “Overview of the Clean Air Act” section of this report.
39 Letter from the Honorable E. Scott Pruitt, EPA Administrator, to EPA Assistant Administrators, May 9, 2018,
https://www.epa.gov/criteria-air-pollutants/back-basics-process-reviewing-national-ambient-air-quality-standards.
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Historically, pollutant-specific scientific review panels have advised agency staff during their
reviews. EPA took a different approach in the multiyear NAAQS reviews it completed in 2020.40
For example, in 2018, EPA disbanded the CASAC Particulate Matter Review Panel formed in
2015. The agency did not convene an ozone panel to review the 2015 ozone NAAQS. Instead,
EPA directed the seven-member CASAC to assist EPA with reviews for the particulate matter and
ozone NAAQS on an expedited timeline. CASAC and others have expressed concerns about the
lack of pollutant-specific panels. In 2019, CASAC recommended EPA either reappoint the
CASAC particulate matter panel or appoint a new panel with similar expertise. CASAC stated
that the “breadth and diversity of evidence to be considered exceeds the expertise of the statutory
CASAC members.”41 EPA subsequently announced the availability of 12 subject matter experts to
assist CASAC with technical questions.42 CASAC continued to urge EPA to “consider restoring a
traditional interactive discussion process in which the CASAC can interact directly with external
expert panels, while also keeping the option of obtaining written responses from external experts
to specific questions.”43
In its oversight role, Congress may consider if EPA’s revised approach meets the CAA objectives
to review the NAAQS and the science upon which they are based in a timely manner. EPA’s
modifications to the NAAQS review process underscore the tension between competing concerns.
Some stakeholders, interest groups, and Members of Congress have criticized the timeliness of
past NAAQS reviews, noting that delayed reviews contribute to regulatory uncertainty. Others
question whether expedited NAAQS decisions are able to reflect the latest science and if the
scientific basis is sufficiently rigorous.
Wildfire Smoke
Congress may continue to deliberate legislation related to wildfires and smoke management in the
117th Congress. Wildfire smoke is a complex mixture of air pollutants that can temporarily
degrade air quality and harm human health. The chemical composition of smoke depends on
various factors including burn conditions (e.g., fire temperature), type of biomass burned (e.g.,
vegetation), and weather-related influences (e.g., wind).44
Wildfire smoke can temporarily increase ambient levels of particulate matter and other criteria
pollutants regulated under the CAA.45 These increases may be measured by air monitoring
stations comprising a national network, which informs determinations about NAAQS compliance.

40 EPA initiated this particulate matter NAAQS review in December 2014 and initiated this ozone NAAQS review in
May 2018. EPA, “Review of the National Ambient Air Quality Standards for Particulate Matter,” Final action, 85
Federal Register 87261, December 18, 2020.
41 Letter from Dr. Louis Anthony Cox, Jr., Chair, CASAC, to the Honorable Andrew R. Wheeler, EPA Administrator,
April 11, 2019.
42 EPA, “Administrator Wheeler Announces New CASAC Member, Pool of NAAQS Subject Matter Experts,” press
release, September 13, 2019, https://www.epa.gov/newsreleases/administrator-wheeler-announces-new-casac-member-
pool-naaqs-subject-matter-experts.
43 Letter from Dr. Louis Anthony Cox, Jr., Chair, CASAC, to the Honorable Andrew R. Wheeler, EPA Administrator,
February 19, 2020.
44 EPA, U.S. Forest Service, U.S. Centers for Disease Control and Prevention, and California Air Resources Board,
California Office of Environmental Health Hazard Assessment, Wildfire Smoke: A Guide for Public Health Officials,
Revised 2019, https://www.airnow.gov/publications/wildfire-smoke-guide/wildfire-smoke-a-guide-for-public-health-
officials/.
45 For more information, see CRS Insight IN11528, Wildfire Smoke: Air Quality Concerns and Management, by Kate
C. Shouse.
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Congress authorized EPA to treat emissions from certain natural events differently than those
from anthropogenic sources. The CAA allows EPA to exclude air quality data from regulatory
decisions if such data were demonstratively influenced by “exceptional events” such as certain
natural events (42 U.S.C. §7619(b)). EPA regulations specify conditions under which states and
tribes can demonstrate that air quality impacts from wildfires should be excluded from NAAQS
compliance determinations.46 Such exclusions are neither required nor guaranteed. That is, air
quality data influenced by exceptional events are not excluded unless a state, tribe, federal land
manager, or other federal agency submits a demonstration to EPA and the agency approves the
demonstration.47 EPA’s review may qualitatively weigh the evidence presented in the exceptional
events demonstration “based on its relevance to the Exceptional Events Rule criterion being
addressed, the degree of certainty, its persuasiveness, and other considerations appropriate to the
individual pollutant and the nature and type of event.”48
Wildfire response strategies rely on air quality monitoring, smoke forecasting, and timely
communication of air quality conditions and related health risks to the public.49 Various federal,
tribal, state, and local agencies contribute to these tasks.50 To measure ambient air pollutant
levels, they rely on established monitors from the national network and deploy temporary
monitors. Specialists also use computer modeling to estimate pollution levels. In addition, EPA
manages AirNow, a multiagency website that reports air quality based on monitoring data
received on a regular basis from state, local, and federal agencies.51 AirNow compiles the data in
a consistent format and displays it through interactive maps, such as the Fire and Smoke Map.52
The need for real-time air quality information is critical during wildfire events.
EPA, the U.S. Forest Service, other agencies, and stakeholders are exploring emerging
technologies to improve air quality monitoring during wildfire events. For example, the AirNow
Sensor Data Pilot adds air pollution data from “low-cost sensors” to the Fire and Smoke Map.
Federal agencies caution that such data should be considered supplemental to existing resources,
given uncertainties about the “precision, accuracy, and reliability” of sensors.53

46 For example, states and tribes may submit a demonstration that addresses the technical criterion that “the event
affected air quality in such a way that there exists a clear causal relationship between the specific event and the
monitored exceedance or violation.” 40 C.F.R. §50.14(c)(3)(iv)(B)-(C). See also EPA, “Treatment of Air Quality Data
Influenced by Exceptional Events,” https://www.epa.gov/air-quality-analysis/treatment-air-quality-data-influenced-
exceptional-events-homepage-exceptional.
47 42 U.S.C. §7619(b)(1). See also 40 C.F.R. §50.14 and EPA, “2016 Revisions to the Exceptional Events Rule: Update
to Frequently Asked Questions,” February 2020, https://www.epa.gov/air-quality-analysis/updated-exceptional-events-
rule-faqs.
48 EPA, “Exceptional Events Guidance: Prescribed Fire on Wildland that May Influence Ozone and Particulate Matter
Concentrations,” August 19, 2019, https://www.epa.gov/air-quality-analysis/exceptional-events-guidance-prescribed-
fire-wildland-may-influence-ozone-and.
49 For example, see California Air Resources Board, California Office of Environmental Health Hazard Assessment,
EPA, U.S. Centers for Disease Control and Prevention, and U.S. Forest Service, Wildfire Smoke: A Guide for Public
Health Officials, Revised 2019, https://www.airnow.gov/publications/wildfire-smoke-guide/wildfire-smoke-a-guide-
for-public-health-officials/. Hereinafter, “2019 Wildfire Smoke Guide.”
50 For example, see (1) Interagency Wildland Fire Air Quality Response Program, “Air Resource Advisors,”
https://sites.google.com/firenet.gov/wfaqrp-external/air-resource-advisors; and (2) AirNow, “Smoke Advisories,”
https://www.airnow.gov/air-quality-and-health/fires/smoke-advisories/.
51 AirNow, About AirNow, https://www.airnow.gov/about-airnow/.
52 AirNow, Fire and Smoke Map, https://fire.airnow.gov/.
53 2019 Wildfire Smoke Guide, p. 3.
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CAA requirements may also factor into forest management and fire prevention strategies,
specifically the use of prescribed fires. Prescribed fires—the deliberate use of fire in specific
areas within prescribed fuel and weather conditions—are one option often considered to reduce
fuel levels (e.g., dead wood) and thereby reduce risk of larger wildfires.54 Various factors
influence the use of prescribed fires, including available capacity and funding, air quality and
other health and safety concerns, compliance with air quality requirements, and landscape
conditions and conservation priorities.55 Many states have developed programs to manage and
control smoke from prescribed fires.56 Smoke management plans seek to minimize smoke
entering populated areas, prevent public safety hazards, and maintain CAA compliance.
Notwithstanding the potential for longer-term air quality and fire protection benefits, prescribed
fires may have near-term implications for compliance with federal air quality standards. EPA’s
exceptional events guidance describes conditions under which states and tribes can demonstrate
that air quality impacts from prescribed fires57 should be excluded from NAAQS compliance
determinations.58 In particular, the guidance discusses how to demonstrate that a prescribed fire
“caused the event-related exceedance(s) or violation(s), was not reasonably controllable or
preventable, and is unlikely to recur at a particular location.”59
As Congress deliberates on wildfire legislation, it may consider which monitoring strategies
effectively inform smoke management and public health responses. Monitoring strategies may
include some combination of stationary monitors, mobile sensors, or models. Congress may also
consider monitoring costs, which may vary by location, along with public health benefits.
In addition, Congress may continue to consider legislation related to prescribed fires. In the 116th
Congress, some Members introduced legislation to increase the frequency and scale of prescribed
burns as part of a broader strategy to mitigate future wildfire risk.60 Deliberations around such
proposals may consider the potential barriers to prescribed burns. Some have expressed concern
that resource limitations and, in some cases, regulatory requirements, may deter the use of
prescribed fires.61 Among other things, Congress may consider whether exceptional event
demonstrations—which are determined on a case-by-case basis and may be resource intensive—
balance air quality objectives with regulatory certainty and resource constraints.

54 Prescribed fuel and weather conditions may include fuel moisture content, relative humidity, and wind speed. For
more information about prescribed burning and other measures to reduce fuel levels (e.g., dead wood) for fire
protection, see CRS Report R40811, Wildfire Fuels and Fuel Reduction, by Katie Hoover.
55 Courtney A. Schultz, Sarah M. McCaffrey, and Heidi R. Huber-Stearns, “Policy Barriers and Opportunities for
Prescribed Fire Application in the Western United States,” International Journal of Wildland Fire, vol. 28 (2019).
56 National Wildlife Coordinating Group, NWCG Smoke Management Guide for Prescribed Fire, PMS 420-3,
November 2020, https://www.nwcg.gov/sites/default/files/publications/pms420-3.pdf.
57 For purposes of exceptional event demonstrations, EPA has defined prescribed fire as “any fire intentionally ignited
by management actions in accordance with applicable laws, policies, and regulations to meet specific land or resource
management objectives” (40 C.F.R. §50.1).
58 EPA, “Exceptional Events Guidance: Prescribed Fire on Wildland that May Influence Ozone and Particulate Matter
Concentrations,” August 19, 2019, https://www.epa.gov/air-quality-analysis/exceptional-events-guidance-prescribed-
fire-wildland-may-influence-ozone-and. Hereinafter, “2019 Exceptional Events Guidance.”
59 2019 Exceptional Events Guidance. See also EPA, “EPA Releases Additional Resource on Prescribed Fires to
Support Air Agencies,” press release, August 14, 2019, https://www.epa.gov/newsreleases/epa-releases-additional-
resource-prescribed-fires-support-air-agencies.
60 S. 4626, National Prescribed Fire Act of 2020. Examples of proposed legislation related to prescribed fires from the
115th Congress include H.R. 7042, H.R. 4208, S. 1991, and S. 2068.
61 Courtney A. Schultz, Sarah M. McCaffrey, and Heidi R. Huber-Stearns, “Policy Barriers and Opportunities for
Prescribed Fire Application in the Western United States,” International Journal of Wildland Fire, vol. 28 (2019).
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For more information about air quality requirements related to wildfires, see CRS Insight
IN11528, Wildfire Smoke: Air Quality Concerns and Management, by Kate C. Shouse.
Review of Air Toxics Standards
The CAA directs EPA to promulgate emission standards for sources of the 187 hazardous air
pollutants (HAPs), informally referred to as “air toxics,” that are listed in Section 112(b).62 In
general, these standards, known as National Emission Standards for Hazardous Air Pollutants
(NESHAPs), require major sources63 to meet numeric emission limits that have been achieved in
practice by the best-performing similar sources. These standards are generally referred to as
Maximum Achievable Control Technology (MACT) standards. The CAA also requires EPA to
conduct certain reviews of the MACT standards.64
The remainder of this section highlights some of the air toxics issues that may garner interest in
the 117th Congress.
Mercury from Power Plants: EPA’s Consideration of Co-Benefits
In 2020, EPA promulgated the latest in a series of rulemakings on mercury from power plants
based on its review of the benefits and costs of reducing mercury and other HAPs from coal- and
oil-fired power plants. As explained below, the 2020 rulemaking revealed a change in EPA’s
interpretation of a statutory provision—Section 112(n)(1)—that was expected to set a precedent
for EPA’s consideration of benefits under other CAA authorities. It remains uncertain whether the
Biden Administration will retain that interpretation. Congress may conduct oversight and consider
how the agency factors benefits and costs into regulatory decisions.
EPA concluded in 2020 that limits on HAPs from coal- and oil-fired power plants are not
“appropriate and necessary” (A&N) under CAA Section 112(n)(1).65 The 2020 A&N rule
reversed prior A&N determinations that led to the 2000 listing of coal- and oil-fired power plants
as a major source of HAPs and the 2012 Mercury and Air Toxics Standards (MATS) limiting
those HAPs. EPA’s accompanying analysis, published in 2011, projected annual benefits between
$37 billion and $90 billion in 2016. Nearly all of the monetized benefits were from the rule’s
particulate matter co-benefits. EPA monetized one of the expected mercury impacts—intelligence
quotient loss to children exposed to mercury from recreationally caught freshwater fish—but
could not monetize other mercury impacts. Such nonmonetized impacts may include other
neurologic effects (e.g., memory and behavior), cardiovascular effects, and effects on wildlife.
Factors that precluded comprehensively monetizing mercury and other HAP benefits from the
MATS rule included gaps in toxicological data, uncertainties in estimating human effects based

62 42 U.S.C. §7412. The 1990 CAA amendments specified 189 pollutants, but P.L. 102-187, enacted on December 4,
1991, deleted hydrogen sulfide from the list of toxic pollutants, leaving only 188. On December 19, 2005, EPA
removed methyl ethyl ketone (MEK) from the list of toxic air pollutants. The total number of listed air toxics is now
187.
63 CAA §112(a) defines a major source as “any stationary source or group of stationary sources located within a
contiguous area and under common control that emits or has the potential to emit considering controls, in the aggregate,
10 tons per year or more of any hazardous air pollutant or 25 tons per year or more of any combination of hazardous air
pollutants” 42 U.S.C. §7412(a)(1).
64 CAA §112(f)(2), codified at 42 U.S.C. §7412(f)(2).
65 EPA, “National Emission Standards for Hazardous Air Pollutants: Coal- and Oil-Fired Electric Utility Steam
Generating Units—Reconsideration of Supplemental Finding and Residual Risk and Technology Review,” 85 Federal
Register 31286, May 22, 2020.
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on animal experiments, and insufficient economic research to translate the health and
environmental effects to dollar value terms.
EPA stated that the 2020 A&N rule corrects errors in the agency’s consideration of benefits in a
prior A&N finding. In its determination for the 2020 A&N rule, EPA excluded from consideration
any co-benefits to human health from reductions in pollutants not targeted by MATS. While EPA
acknowledged that estimation of all benefits and costs, including ancillary impacts, is consistent
with federal guidance, the agency concluded that it had erred when it gave equal consideration to
benefits (HAP reductions) and co-benefits (non-HAP reductions) when making its prior A&N
finding under Section 112(n).66 The 2020 A&N rule concluded that an A&N finding under
Section 112(n)(1) must instead be justified “overwhelmingly” by HAP reduction benefits.
Notwithstanding the 2020 A&N rule, the 2012 MATS limits remain in effect for power plants
because EPA determined that it could not meet the criteria under CAA Section 112(c)(9) to delist
them. Furthermore, the A&N finding does not change the regulatory status of other pollution
sources because CAA Section 112(n)(1) applies only to power plants.
Some have raised questions about why EPA reversed the A&N finding and how it might affect
regulated entities. For example, some power plant owners are concerned the A&N reversal may
compromise their ability to recover from ratepayers the costs of installing MATS pollution
controls. Others find this unlikely, but legal challenges to the 2020 A&N rule are under way. For
more information about the legal issues, see CRS In Focus IF11622, Judicial Review of Mercury
and Air Toxics Regulations, by Kate R. Bowers and Linda Tsang.
The May 2020 A&N rule was a final action. The Biden Administration has the option to modify
or repeal the 2020 A&N rule, but it would need to follow the same process used to promulgate
new rules. The Biden Administration has directed the EPA Administrator to review the 2020 A&N
rule in accordance with E.O. 13990.67 Specifically, EPA is to consider taking action—suspending,
revising, or rescinding—on the 2020 A&N rule through a notice-and-comment proposed
rulemaking by August 2021.68
Congress may consider development of legislation that addresses how EPA and other federal
agencies factor benefits and costs into rulemaking decisions. For example, Congress may explore
opportunities to clarify how much weight an agency gives to benefits and ancillary impacts. Such
legislation may involve consideration of the tension between providing more specific direction to
the agencies and allowing an agency sufficient discretion to tailor its approach as warranted.
While legislative direction may provide greater consistency across Administrations, it may also
limit an agency’s discretion to consider case-specific factors and apply its evolving understanding
of the science and economics.

66 In 2012, EPA reaffirmed the 2000 A&N finding and promulgated the Mercury and Air Toxics Standards
Rule. Numerous parties petitioned the courts to review MATS. Among other things, some petitioners disagreed with
EPA’s conclusion that it was not appropriate to consider costs when making an A&N finding under CAA §112. In
2015, the Supreme Court agreed with the petitioners and remanded the rule for further consideration, but it did not
address whether EPA has authority to consider monetized co-benefits in evaluating the cost of MATS (Michigan v.
EPA, 135 S. Ct. 2699 (2015)). In 2016, EPA finalized a supplemental A&N finding based on its review of the 2012
rule’s estimated costs. EPA concluded that it was appropriate and necessary to regulate mercury and other HAPs from
power plants after considering regulatory costs.
67 E.O. 13990 requires federal agencies to review “all existing regulations, orders, guidance documents, policies, and
any other similar agency actions” taken during the Trump Administration and to consider “suspending, revising, or
rescinding” agency actions that are deemed inconsistent with the order’s stated policy concerning protection of public
health and the environment and addressing climate change.
68 E.O. 13990, §2(iv).
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Ethylene Oxide: EPA’s Review of Emission Standards
Ethylene oxide is a flammable, colorless gas used to sterilize medical equipment, fumigate spices,
and make products such as antifreeze, textiles, and plastics. It has also long been considered
mutagenic69 and has been associated with certain risks to human health.70 Congress listed
ethylene oxide as a hazardous air pollutant under the 1990 CAA amendments.71 EPA has
promulgated various regulations under CAA Section 112 to limit ethylene oxide emissions from
the following sources: chemical manufacturers (synthetic organic chemical manufacturing,
miscellaneous organic chemical manufacturers, polyether polyols production) and sterilizers
(commercial sterilizers and hospital ethylene oxide sterilizers).72
2018 National Air Toxics Assessment
EPA’s 2018 National Air Toxics Assessment (NATA) estimated that ethylene oxide significantly
contributes to potential elevated cancer risks in some areas.73 The 2018 NATA was based on
emissions data from 2014 and computer modeling as well as information about health effects,
including the ethylene oxide carcinogenicity assessment that EPA had updated in 2016.74 EPA’s
2016 carcinogenicity assessment for ethylene oxide concluded that it is carcinogenic to humans.75
As noted by EPA, NATA is intended as a screening tool designed to help identify which
pollutants, emission sources, and geographic areas to focus further evaluation of risks and
determine whether additional regulatory action may be warranted.
Stakeholders have expressed varying concerns. Some, such as the American Chemistry Council
and the Texas Commission on Environmental Quality, disagree with the NATA findings for
ethylene oxide, and recommend that EPA withdraw the ethylene oxide risk estimates.76 Others,
including environmental groups and some states and local communities, have expressed concerns
about exposure to ethylene oxide emissions based on NATA findings and other relevant

69 A mutagen can change the genetic code in a cell. Chronic exposure to elevated levels of ethylene oxide has been
associated with the development of certain cancers based on animal studies. EPA, Evaluation of the Inhalation
Carcinogenicity of Ethylene Oxide (CASRN 75-21-8) in Support of Summary Information on the Integrated Risk
Information System (IRIS), EPA/635/R-16/350Fa, December 2016, https://cfpub.epa.gov/ncea/iris/iris_documents/
documents/toxreviews/1025tr.pdf. Hereinafter, “2016 IRIS EtO”.
70 For example, acute exposures to ethylene oxide are associated with respiratory irritation and effects to the nervous
system.
71 Additional statutes apply to various uses of ethylene oxide. For example, the distribution, sale, and use of ethylene
oxide as a sterilant is regulated by EPA under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA); see 7
U.S.C. §136 et seq. This report focuses on CAA requirements.
72 For more information about the source categories, see EPA, Strategy for Reviewing Ethylene Oxide Emissions,
https://www.epa.gov/hazardous-air-pollutants-ethylene-oxide/agency-actions-ethylene-oxide#regulations.
73 EPA, “2014 National Air Toxics Assessment,” https://www.epa.gov/national-air-toxics-assessment/2014-national-
air-toxics-assessment.
74 EPA, “NATA Frequent Questions,” https://www.epa.gov/national-air-toxics-assessment/nata-frequent-questions.
75 EPA’s prior assessment, completed in 1985, concluded that ethylene oxide is “probably carcinogenic to humans”; see
EPA, “Ethylene Oxide Emissions Standards for Sterilization Facilities,” 67 Federal Register 17715, April 7, 2006. In
2016, EPA changed the classification to “carcinogenic to humans” based on occupational epidemiological studies; see
2016 IRIS EtO.
76 The American Chemistry Council, which represents producers and consumers of ethylene oxide, submitted a Request
for Correction under the Information Quality Act asking that the “NATA risk estimates for [ethylene oxide] be
withdrawn and corrected to reflect scientifically supportable risk values.” See EPA, “National Emission Standards for
Hazardous Air Pollutants: Miscellaneous Organic Chemical Manufacturing Residual Risk and Technology Review,”
proposed rule, 84 Federal Register 69218, December 17, 2019.
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information, and have urged EPA to strengthen the emission standards. Some Members of
Congress have questioned whether the current standards for sources of ethylene oxide afford
adequate protection of human health and have introduced legislation in prior sessions of
Congress.77
Residual Risk and Technology Review
The CAA requires EPA to conduct two kinds of reviews of the MACT standards—the residual
risk review and the technology review. For the residual risk review, the CAA requires EPA to
evaluate MACT standards within eight years of promulgation to determine whether revisions are
needed to address any remaining risk associated with HAP emissions from the source category.78
EPA’s residual risk review considers whether revisions to the MACT standards are required to
provide an ample margin of safety to protect public health or to prevent an adverse environmental
effect, taking into consideration factors such as costs.79 For the technology review, the CAA
requires EPA to review the MACT standards and revise as needed, after considering
“developments in practices, processes, and control technologies” every eight years.80 EPA often
combines the two reviews into one, known as the residual risk and technology review.
In 2020, EPA completed the statutorily required residual risk and technology review of the
miscellaneous organic chemical manufacturing (MON) NESHAP, which includes limits on
ethylene oxide and other HAPs.81 EPA revised the MON NESHAP based on this review and
published the final rule on August 12, 2020.82 Among other things, EPA promulgated additional
requirements to limit ethylene oxide from process vents, storage tanks, and equipment leaks at
major sources. The revised requirements to limit ethylene oxide were based on the residual risk
review. EPA used the updated risk values for ethylene oxide in its residual risk review and
concluded that, absent revisions to the MON standards, the “risks are unacceptable.”83
EPA’s review of the standards limiting ethylene oxide from commercial sterilizers is under way.
This source category includes medical equipment suppliers, pharmaceutical suppliers, other
health-related industries, spice manufacturers, and contract sterilizers.84 Some Members of
Congress have urged EPA to “exercise its full authority to regulate” ethylene oxide under the
CAA and to conduct a residual risk review along with the statutorily mandated technology review
for the source category.85 EPA published an advanced notice of proposed rulemaking (ANPR) in
late 2019 that requested information “on potential control measures for reducing ethylene oxide
emissions from commercial sterilization and fumigation operations.”86 The ANPR also discussed

77 Examples of legislative proposals from the 116th Congress include H.R. 1152/S. 458 and H.R. 7822/S. 4369.
78 CAA §112(f)(2), codified at 42 U.S.C. §7412(f)(2).
79 42 U.S.C. §7412(f)(2).
80 CAA §112(d)(6), codified at 42 U.S.C. §7412(d)(6).
81 EPA was under court order to complete the CAA §112 risk and technology review of the MON NESHAP and 32
other NESHAPs. Order, Cal. Communities Against Toxics, et al. v. Pruitt, No. 1:15-cv-00512-TSC, Doc. No. 49
(D.D.C. Mar. 13, 2017).
82 EPA, “National Emission Standards for Hazardous Air Pollutants: Miscellaneous Organic Chemical Manufacturing
Residual Risk and Technology Review,” final rule, 85 Federal Register 49084, August 12, 2020. Hereinafter, “MON
Final Rule.”
83 MON Final Rule, p. 49097.
84 EPA, Strategy for Reviewing Ethylene Oxide Emissions, https://www.epa.gov/hazardous-air-pollutants-ethylene-
oxide/agency-actions-ethylene-oxide#regulations.
85 Letter from Senator Tammy Duckworth et al. to Honorable Andrew Wheeler, EPA Administrator, July 8, 2020.
86 EPA, “National Emission Standards for Hazardous Air Pollutants: Ethylene Oxide Commercial Sterilization and
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small business considerations, noting that EPA has identified about 35 facilities owned by small
businesses within the commercial sterilizer source category.87 According to the ANPR, EPA
intends to convene a Small Business Advocacy Review Panel before it takes “any significant
regulatory action.”88 The Trump Administration published a regulatory agenda in fall 2020, which
estimated that EPA will propose a decision about whether and how to revise the commercial
sterilizer NESHAP in March 2021.89
Congress may conduct oversight as EPA reviews the commercial sterilizer standards and seek
clarity from EPA on its plans for the remaining industrial categories that emit ethylene oxide.90
Beyond oversight, Congress may consider whether the CAA provides adequate authority for EPA
to evaluate or revise NESHAPs based on updated risk information. The U.S. District Court for the
Northern District of California recently ruled on the required frequency of residual risk review,
and specifically on whether the CAA requires EPA to conduct the residual risk review each time it
revises the MACT standards for a source, or only after the initial promulgation of MACT
standards. The court concluded that the CAA does not impose “a mandatory duty on EPA to
revisit its risk-based standards for hazardous pollution sources whenever the agency revises” the
MACT standards.91
Congress may consider the implications of the court’s finding, in particular when updated risk
information would alter the conclusions of EPA’s residual risk reviews. EPA completed a residual
risk review for commercial sterilizers in 2006—a decade before EPA revised the risk value for
ethylene oxide.92 It is unknown whether the updated risk value for ethylene oxide would lead EPA
to a different decision about the stringency of the standard. While CAA Section 112(f)(2)
identifies factors to inform EPA’s consideration of whether revisions are required, the act does
not specify the stringency of the standards that EPA must set. CAA Section 112(f)(2) allows EPA
to base revisions on both health-based and non-health-based factors.93

Fumigation Operations,” advance notice of proposed rulemaking, 84 Federal Register 67889, December 12, 2019.
Hereinafter, “Sterilizer ANPR.”
87 Sterilizer ANPR, p. 67893. EPA identified a total of 135 facilities in the commercial sterilizer source category. The
Regulatory Flexibility Act (RFA) of 1980, 5. U.S.C. §§601-612, requires federal agencies to assess the impact of their
forthcoming regulations on small entities. The RFA also requires agencies to ensure that small entities have an
opportunity to participate in the rulemaking process, and it has special requirements for proposed rules issued by EPA,
the Occupational Safety and Health Administration, and the Consumer Financial Protection Bureau. See CRS Report
RL32240, The Federal Rulemaking Process: An Overview, coordinated by Maeve P. Carey.
88 Sterilizer ANPR, p. 67894.
89 Office of Management and Budget, “Fall 2020 Unified Agenda of Regulatory and Deregulatory Actions,”
EPA/OAR, RIN 2060-AU37, https://www.reginfo.gov/public/do/eAgendaViewRule?pubId=202010&RIN=2060-
AU37.
90 EPA reported plans to “take a closer look at air toxics emissions standards for other industries that emit ethylene
oxide to determine whether a review of those rules is needed. EPA will start this closer look with its air toxics
emissions standards for commercial sterilizers.” See EPA, “Strategy for Reviewing Ethylene Oxide Emissions,”
https://www.epa.gov/hazardous-air-pollutants-ethylene-oxide/agency-actions-ethylene-oxide#regulations, website last
updated on August 19, 2020.
91 Citizens for Pennsylvania’s Future v. Wheeler, 469 F. Supp. 3d 920, 922 (N.D. Cal. 2020). For more information,
congressional clients may contact CRS Legislative Attorney Kate Bowers.
92 In 2006, EPA determined no additional control requirements were warranted and therefore did not revise the ethylene
oxide sterilization NESHAP. EPA, “Ethylene Oxide Emissions Standards for Sterilization Facilities,” 71 Federal
Register 17712, April 7, 2006.
93 42 U.S.C. §7412(f)(2).
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Source Classification: 2018 EPA Withdrawal of “Once In, Always In” Policy
Congress may conduct oversight related to a 2020 EPA rulemaking pertaining to the two kinds of
sources of HAPs defined in CAA Section 112(a)—major sources and area sources. The 2020
rulemaking allows major sources of HAPs to reclassify as an area source after meeting conditions
to limit emissions below major source thresholds.94 Major source refers to
any stationary source or group of stationary sources located within a contiguous area and
under common control that emits or has the potential to emit considering controls, in the
aggregate, 10 tons per year or more of any hazardous air pollutant or 25 tons per year or
more of any combination of hazardous air pollutants.95
Area source refers to “any stationary source of hazardous air pollutants that is not a major
source.”96 Classification as a major or area source has implications for which emission standards
apply to the source. Major sources are subject to the technology-based emission standards—
MACT standards—as well as monitoring, recordkeeping, and reporting requirements. Area
sources are typically subject to lesser controls than major sources. Under the CAA, EPA may
elect to establish more lenient standards for area sources, known as generally available control
technologies, or GACT.97 In some cases, EPA has promulgated requirements for major sources
but not for area sources in some source categories, which means that HAPs from some area
sources are not regulated under the NESHAPs.98
An EPA 1995 guidance memorandum first explained when a major source of HAPs could be
reclassified as an area source.99 At the time, EPA was implementing the 1990 CAA amendments
and various MACT standards were going into effect. EPA’s 1995 memorandum stated that major
sources of HAPs could “switch to area source status at any time until the ‘first compliance date’
of the standard.”100 EPA also discussed its “Once In, Always In” (OIAI) interpretation of the
CAA.101 Specifically, EPA determined that facilities that are major sources for HAPs on the “first
compliance date” would be “required to comply permanently with the MACT standard,” even if
the facility subsequently reduced emissions below the major source threshold.102 The 1995
memorandum concluded that the OIAI policy would ensure that emissions reductions were
maintained over time.103
EPA’s OIAI interpretation remained in effect until 2018, though EPA had previously considered
proposals that would have modified OIAI.104 In 2018, EPA withdrew the 1995 memorandum—

94 EPA, “Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air Act,” final rule, 85
Federal Register 73854, November 19, 2020. Hereinafter, “2020 Reclassification Rule.”
95 42 U.S.C. §7412(a)(1).
96 42 U.S.C. §7412(a)(1).
97 42 U.S.C. §7412(d)(5). See also CRS Report R43699, Key Historical Court Decisions Shaping EPA’s Program
Under the Clean Air Act, by Linda Tsang.
98 2020 Reclassification Rule, p. 73859.
99 Memorandum from John S. Seitz, Director, Office of Air Quality Planning and Standards, to EPA Regional
Directors, May 16, 1995, https://www.epa.gov/stationary-sources-air-pollution/guidance-reclassification-major-
sources-area-sources-under-section. Hereinafter, “1995 Memorandum.”
100 1995 Memorandum.
101 1995 Memorandum.
102 1995 Memorandum.
103 1995 Memorandum.
104 See (1) EPA, “National Emission Standards for Hazardous Air Pollutants for Source Categories: General
Provisions,” proposed rule, 68 Federal Register 26249, May 15, 2003; and (2) EPA, “National Emission Standards for
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and the OIAI interpretation—and issued a different CAA interpretation in a new guidance
memorandum. EPA’s 2018 memorandum concluded that “Congress placed no temporal
limitations on the determination of whether a source emits or has the [potential to emit] HAP in
sufficient quantity to qualify as a major source,” and described conditions under which major
sources could be reclassified to area sources.105 The 2020 final rulemaking (2020 Reclassification
Rule) codified the agency’s withdrawal of its 1995 guidance on source classification under
Section 112. Under the 2020 Reclassification Rule, a major source can be reclassified to area
source status at any time upon reducing its potential to emit HAP to below the major source
thresholds—10 tons per year of any single HAP and 25 tons per year of any combination of HAP,
upon approval of the source’s request.106
Members of Congress have expressed varying opinions about the change, with some Members
supporting the rescission of the OIAI policy107 and others urging EPA to retain it.108 Much of the
debate about the 2020 Reclassification Rule has centered on whether and how it will affect HAP
emissions. Supporters of the 2020 Reclassification Rule expect it will incentivize sources to
reduce emissions below major source thresholds.109 For example, facilities may be more likely to
pursue pollution reduction opportunities in exchange for the reduced regulatory obligations
associated with area sources. Others have raised concerns that it could nonetheless lead to an
increase in emissions from sources that stop complying with MACT standards, even if those
sources remain below major source thresholds. For example, if a major source elects to reclassify
as an area source, it would switch to area source standards, which may allow the facility to use
less effective controls or use the pollution controls less often compared to the MACT standards.110
EPA Region 9 raised this concern in 2004, when the agency considered a different proposal to
allow reclassification. At the time, EPA Region 9 commented that in “many instances, the MACT
requirements could lead to greater reductions when compared to sources accepting” enforceable
limits of 24 tons per year for a combination of HAPs and 9 tons per year for a single HAP.111
More recently, EPA reviewed permits and other information for 69 sources that reclassified to
area sources under the 2018 memorandum. EPA found that emissions increased at 1 of the 69
sources, and that 68 of the 69 sources “achieved and maintain area source status by operating the

Hazardous Air Pollutants for Source Categories: General Provisions,” proposed rule, 72 Federal Register 69, January
3, 2007. EPA did not finalize the OIAI-related provisions from either of these proposals.
105 Memorandum from William L. Wehrum, EPA Assistant Administrator, to Regional Air Division Directors, January
25, 2018, https://www.epa.gov/stationary-sources-air-pollution/guidance-reclassification-major-sources-area-sources-
under-section.
106 2020 Reclassification Rule, p. 73865.
107 For example, see U.S. Senate Environment and Public Works Committee, “Senators Call on EPA to Remove
Burdensome ‘Once-In-Always-In’ Policy,” press release, January 9, 2018, https://www.epw.senate.gov/public/
index.cfm/2018/1/senators-call-on-epa-to-remove-burdensome-once-in-always-in-policy.
108 For example, see Letter from Senator Carper et al. to EPA Administrator Pruitt, March 14, 2018,
https://www.epw.senate.gov/public/_cache/files/1/3/131714ba-f373-4b43-bfb8-a3820ac63a50/
6DCFBCE44BF189136EE1F2CC17E66B54.carper-markey-lead-democrats-in-urging-pruitt-to-reinstate-strict-air-
toxics-standards-for-the-country-s-dirtiest-industrial-sources.pdf.
109 For example, see Letter from API to the EPA Docket, September 24, 2019, Docket ID EPA-HQ-OAR-2019-0282-
0298, https://www.regulations.gov/document?D=EPA-HQ-OAR-2019-0282-0298.
110 For example, see Harvard Environmental & Energy Law Program, “Once In Always In Guidance for Major Sources
Under the Clean Air Act,” https://eelp.law.harvard.edu/2018/02/once-in-always-in-guidance-for-major-sources-under-
the-clean-air-act/.
111 Letter from Michael S. Bandrowski, Air Toxics Chief, EPA Region IX, to David Cozzie, Group Leader, EPA Office
of Air Quality Planning and Standards, December 13, 2005, p. 3, see EPA Docket ID EPA-HQ-OAR-2004-0094-0151.
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emission controls or continuing to implement the practices they used to comply with the major
source NESHAP requirements.”112
EPA also identified source categories likely to be affected by the 2020 Reclassification Rule and
estimated potential changes in emissions. EPA concluded that while HAP emissions from 65
source categories would not change as a result of the 2020 Reclassification Rule, “approximately
130 facilities in seven source categories could increase emissions if they were to reclassify and
were allowed to reduce operation of adjustable add-on controls.”113 EPA estimated that
nationwide, the potential HAPs emissions increase could range from about 900 tons per year to
1,260 tons per year.114 EPA considered alternative scenarios, one of which estimated that
emissions could decrease nationwide by about 180 tons per year.115
Congress may consider whether CAA Section 112 provides EPA adequate authority to incentivize
pollution prevention while also limiting cumulative HAP emissions. The act requires EPA to
establish technology-based emission limits (MACT) for major sources of HAPs. Under EPA’s
current interpretation of CAA Section 112, sources may change their classification after meeting
conditions to limit HAPs below major source thresholds. While the 2020 Reclassification Rule
may provide sources an incentive to reduce HAPs below the major source thresholds, it is unclear
whether actual emissions will decrease at each source that reclassifies as an area source. It is
difficult to ascertain how the 2020 Reclassification Rule may affect emissions because the
reclassification is voluntary and due to the structure of MACT emission standards. MACT
standards generally do not limit emissions to a fixed level, which makes it challenging to
determine how emissions from a particular source may change under a different standard.116
Notwithstanding the various uncertainties, EPA estimated that, under certain assumptions, HAP
emissions could increase from about 130 facilities as a result of the 2020 Reclassification Rule.
Congress may consider the health and environmental implications of this potential outcome.117
Among other things, Congress may consider how these potential HAP increases contribute to
cumulative exposures in communities with disproportionate environmental burdens.
In addition, Congress may consider other impacts of the rule, such as the potential compliance
cost savings. EPA considered three categories of potential costs: (1) costs for major sources to
apply for reclassification, (2) costs for air pollution control agencies to process these applications,
and (3) compliance cost savings for a facility to meet area source standards compared to major
source standards. EPA’s illustrative analysis estimated a net annual cost savings across these three
cost categories when compared to a world with the OIAI policy.118 For example, EPA’s primary

112 2020 Reclassification Rule, p. 73880.
113 EPA, “Final Rule: Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air Act,” fact
sheet, updated October 7, 2020, https://www.epa.gov/stationary-sources-air-pollution/documentation-reclassification-
major-sources-area-sources-under. Hereinafter, “Reclassification Fact Sheet.”
114 Reclassification Fact Sheet.
115 Reclassification Fact Sheet.
116 MACT standards are generally expressed in terms of percent reduction, mass of emissions per mass production, or
other similar format. Emissions from major sources may change over time based on various factors, such as changes in
production levels. See Letter from William L. Wehrum, Acting Assistant Administrator, to the Honorable John D.
Dingell, Chairman, House Committee on Energy and Commerce, March 30, 2007; see EPA Docket ID EPA-HQ-OAR-
2004-0094-0106.
117 EPA assessed rulemaking’s potential impact on tribal and environmental justice communities. See 2020
Reclassification Rule, p. 73882.
118 Memorandum from Eastern Research Group, Inc., to EPA, “Documentation of the Compliance Cost Savings
Analysis for the Rule ‘Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air Act,’”
August 2020, https://www.epa.gov/sites/production/files/2020-10/documents/mm2a_final_cost_analysis_tsm.pdf.
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analytical scenario estimated $16.1 million in compliance costs in the first year and $90.6 million
in compliance cost savings in subsequent years.119
New Source Review Permits
Congress may continue to conduct oversight of New Source Review, a CAA preconstruction
permitting program intended to ensure that new and modified stationary sources of air pollution
do not significantly degrade air quality. The NSR program generally requires emission limits
based on modern pollution controls when new facilities are built or when existing facilities make
a change that increases emissions above specified thresholds. Owners or operators must obtain an
NSR permit before the construction or modification begins.120 Historically, NSR applicability
determinations have been contentious and extensively litigated.121
Stakeholder views on NSR are often long-standing and divergent. Some stakeholders view NSR
as essential to maintaining air quality, noting that because of its requirements for pollution control
strategies, the program “yields overall reductions in pollution even as facilities expand
production.”122 Other stakeholders have described NSR as outdated and an impediment to
economic growth, stating that the complexities and costs of the NSR permitting process
discourage pollution-control projects.123
Over time, EPA has issued guidance and explored rulemakings meant to improve or clarify
program implementation. In 2017, EPA announced an NSR reform initiative, which included
revisiting some NSR proposals from prior Administrations that were not finalized.124 Since 2017,
EPA has issued various guidance memoranda and taken regulatory actions under this initiative.125
These actions have centered on NSR applicability and enforcement. For example, EPA guidance
has discussed how to determine what counts as a source for NSR purposes;126 how to account for

Hereinafter, “EPA Reclassification Cost Analysis.”
119 EPA’s primary illustrative scenario estimated the compliance costs and compliance cost savings for 2,567 facilities
to reclassify. The 2,567 facilities represent roughly one-third of the 7,183 sources with emissions that are already below
the major source threshold. See both EPA Reclassification Cost Analysis and Reclassification Fact Sheet.
120 The NSR permit is a legal document that establishes site-specific requirements for the source, detailing approved
types of construction, emission limits during operation, monitoring and reporting requirements, and other construction
and operating conditions. State and local permitting agencies generally implement NSR and issue the permits. EPA
generally oversees the state’s implementation.
121 For discussion of key legal decisions on NSR, see CRS Report R43699, Key Historical Court Decisions Shaping
EPA’s Program Under the Clean Air Act, by Linda Tsang.
122 Harvard Environmental and Energy Law Program, EPA’s Attack on New Source Review and Other Air Quality
Protection Tools, November 1, 2019, http://eelp.law.harvard.edu/wp-content/uploads/NSR-paper-EELP.pdf.
123 Art Fraas, John Graham, and Jeff Holmstead, “EPA’s New Source Review Program: Time for Reform?”
Environmental Law Reporter, vol. 47, no. 1, (2017). Also available at https://www.rff.org/publications/journal-articles/
epas-new-source-review-program-time-for-reform/.
124 U.S. Environmental Protection Agency, Final Report on Review of Agency Actions that Potentially Burden the Safe,
Efficient Development of Domestic Energy Resources Under Executive Order 13783, October 2017,
https://www.epa.gov/sites/production/files/2017-10/documents/eo-13783-final-report-10-25-2017.pdf. EPA has
previously explored rulemakings intended to improve or clarify NSR; see selection of rulemakings that EPA identifies
under “NRS Reform” at https://www.epa.gov/nsr/nsr-regulatory-actions#nsrreform.
125 For a list of EPA guidance memoranda related to NSR, see U.S. Environmental Protection Agency, New Source
Review Policy and Guidance Document Index, https://www.epa.gov/nsr/new-source-review-policy-and-guidance-
document-index. For regulatory actions that EPA identifies as part of “NSR Reform,” see https://www.epa.gov/nsr/nsr-
regulatory-actions#nsrreform.
126 For example, EPA published a guidance memorandum to clarify when it considers industrial facilities to be under
“common control” and therefore count as one source for NSR purposes. See Memorandum from William L. Wehrum,
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air emissions in determining whether NSR applies;127 and circumstances under which EPA will
not “second-guess” a facility’s projections of emissions that will result from a modification.128
The Biden Administration may review the Trump Administration’s NSR actions if EPA
determines that those actions fall within the scope of its reviews under E.O. 13990.129 In addition,
at least one NSR action may be eligible for review by Congress under the CRA. Known as the
“project emissions accounting” rule, EPA revised the NSR applicability regulations in November
2020.130
Stakeholders that view NSR as essential to maintaining air quality have voiced concern that these
NSR actions would be expected to reduce EPA’s oversight and create a more lenient NSR regime,
thereby allowing increased emissions of harmful pollutants from U.S. industrial facilities.131
Stakeholders critical of NSR generally have expressed support for some of EPA’s recent NSR
actions, citing, for example, greater regulatory certainty and incentives for projects that emit less
pollution.132

Assistant Administrator, EPA, to Patrick McDonnell, Secretary of the Pennsylvania Department of Environmental
Protection, April 30, 2018, https://www.epa.gov/sites/production/files/2018-05/documents/meadowbrook_2018.pdf.
EPA’s 2019 guidance regarding the term adjacent is another example; it discusses whether sources located on
“adjacent” properties should be combined for purposes of NSR; see Memorandum from Anne L. Idsal, Acting
Assistant Administrator, EPA, to EPA Regional Administrators, November 26, 2019, https://www.epa.gov/sites/
production/files/2019-11/documents/adjacent_guidance.pdf.
127 For example, EPA’s March 2018 “Project Emissions Accounting” memorandum states that a facility owner would
consider both emission increases and emission decreases, provided they are from a single project, in the first step of a
multistep process to determine whether emissions from the proposed project would trigger NSR; see Memorandum
from Honorable E. Scott Pruitt, EPA Administrator, to EPA Regional Administrators, March 13, 2018,
https://www.epa.gov/sites/production/files/2018-03/documents/nsr_memo_03-13-2018.pdf. In 2020, EPA finalized a
rule to revise the NSR applicability regulations to “make it clear that both emissions increases and emissions decreases
that result from a given proposed project are to be considered at Step 1 of the NSR major modification applicability
test.” See EPA, “Prevention of Significant Deterioration (PSD) and Nonattainment New Source Review (NNSR):
Project Emissions Accounting,” 85 Federal Register 74890, November 24, 2020.
128 Letter from Honorable E. Scott Pruitt, EPA Administrator, to EPA Regional Administrators, December 7, 2017,
https://www.epa.gov/sites/production/files/2017-12/documents/policy_memo.12.7.17.pdf.
129 E.O. 13990 requires federal agencies to review “all existing regulations, orders, guidance documents, policies, and
any other similar agency actions” taken during the Trump Administration and to consider “suspending, revising, or
rescinding” agency actions that are deemed inconsistent with the order’s stated policy concerning protection of public
health and the environment and addressing climate change.
130 EPA sought to “make it clear that both emissions increases and emissions decreases that result from a given
proposed project are to be considered at Step 1 of the NSR major modification applicability test.” See EPA,
“Prevention of Significant Deterioration (PSD) and Nonattainment New Source Review (NNSR): Project Emissions
Accounting,” 85 Federal Register 74890, November 24, 2020.
131 Harvard Environmental and Energy Law Program, Regulatory Rollback: New Source Review, December 11, 2018
(updated February 2020), https://eelp.law.harvard.edu/2018/12/new-source-review/.
132 For example, a letter signed by 12 industry associations stated that EPA’s Project Emissions Accounting (PEA)
proposal (84 Federal Register 39244, August 9, 2019) to “codify the PEA interpretation is reasonable, consistent with
the Clean Air Act and sound policy, and important to provide regulatory certainty to enable and even incentivize
projects to reduce emissions and drive productive capacity”; see https://www.regulations.gov/document?D=EPA-HQ-
OAR-2018-0048-0081.
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Congress may conduct oversight of the NSR program or consider legislative proposals to modify
it. Congress may also use the appropriations process to support or limit EPA spending on specific
activities. Congress has previously considered legislative proposals to modify NSR.133 For
example, in the 116th Congress, S. 2662 and H.R. 172 would have amended the CAA definition of
modification, a key term in determining NSR applicability. The CAA broadly
defines modification as “any” physical or operational change in a stationary source “that increases
the emissions of any air pollutant or results in the emission of any air pollutant not previously
emitted.”134 S. 2662 and H.R. 172 would have, among other things, allowed emissions increases
from modifications to be calculated on an hourly basis rather than an annual basis. The bills
would also have excluded certain projects, such as those designed to reduce the amount of any air
pollutant emitted or “to restore, maintain, or improve the reliability of operations at, or the safety
of, the source” from the definition of a modification.

Author Information

Kate C. Shouse

Analyst in Environmental Policy

Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan
shared staff to congressional committees and Members of Congress. It operates solely at the behest of and
under the direction of Congress. Information in a CRS Report should not be relied upon for purposes other
than public understanding of information that has been provided by CRS to Members of Congress in
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copy or otherwise use copyrighted material.

133 See U.S. Congress, Senate Committee on Environment and Public Works, Hearing on S. 2662, The Growing
American Innovation (GAIN) Act, 116th Cong., 1st sess., November 6, 2019, https://www.epw.senate.gov/public/
index.cfm/hearings?ID=9D9A2920-4591-4532-9D64-AE9F98437707. See also (1) U.S. Congress, House Committee
on Energy and Commerce, Subcommittee on Environment, New Source Review Permitting Challenges for
Manufacturing and Infrastructure, 115th Cong., 2nd sess., February 14, 2018, https://energycommerce.house.gov/
hearings/new-source-review-permitting-challenges-manufacturing-infrastructure/; and (2) U.S. Congress, House
Committee on Energy and Commerce, Subcommittee on Environment, Legislation Addressing New Source Review
Permitting Reform, 115th Cong., 2nd sess., May 16, 2018, https://energycommerce.house.gov/committee-activity/
hearings/hearing-on-legislation-addressing-new-source-review-permitting-reform.
134 CAA §111(a)(4), 42 U.S.C. §7411(a)(4), defines modification for purposes of the NSPS section of the CAA. CAA
§169(2)(C), 42 U.S.C. §7479(2)(C), specifies that that definition applies as well within the Prevention of Significant
Deterioration portion of the statute. EPA and state air pollution control agencies have interpreted this definition to
implement NSR through regulations and policy guidance. EPA’s interpretation of modification under the NSR program
has been subject to various legal challenges.
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