Regulation of Hazardous Air Pollutants Under the Clean Air Act: A Primer

Regulation of Hazardous Air Pollutants Under

the Clean Air Act: A Primer

August 4, 2026

Congressional Research Service

https://crsreports.congress.gov

R49070

SUMMARY

Regulation of Hazardous Air Pollutants Under

the Clean Air Act: A Primer

R49070

August 4, 2026

Jonathan D. Haskett

Analyst in Environmental

Under provisions in the Clean Air Act (CAA), the U.S. Environmental Protection Agency (EPA)

Policy

regulates two main classes of air pollutants: criteria air pollutants and hazardous air pollutants

(HAPs). HAPs, which are “air pollutants that can cause serious illnesses, such as cancer, or

death,” are regulated under Section 112 of the CAA. Within Section 112 there are two classes of

HAPs: (1) those that are emitted regularly through ordinary activities such as manufacturing or

the provision of goods and services, and (2) those that, in case of accidental release, may cause

serious injury or death. Each of these classes has its own regulatory framework. HAPs emitted in the course of ordinary

activities are regulated through the setting of National Emission Standards for Hazardous Air Pollutants (NESHAPs).

Regulation of HAPs emissions has a three-phase framework that includes (1) HAPs identification, (2) HAPs regulation, and

(3) review of HAPs emissions standards. In the identification phase, EPA identifies and lists HAPs and their sources. The

CAA includes mechanisms for both listing and delisting HAPs and their sources. In the regulation phase, EPA sets standards

to regulate the emissions of the identified HAPs from the identified sources. These standards are based on technological

capabilities for HAPs emissions control. The standards distinguish between major sources of HAPs (those that emit at or

above the threshold of 10 tons per year of a HAP or 25 tons per year of any combination of HAPs) and area sources (which

emit less than the threshold amounts of HAPs). Major sources are subject to regulation based on the Maximum Achievable

Control Technology (MACT) standard. Area sources are generally subject to regulation based on a less stringent Generally

Available Control Technology (GACT) standard. MACT and GACT are technological standards informed by the availability

and efficacy of emissions control technologies. In developing the standards, EPA considers cost, health, and environmental

effects. In the review phase, the standards are examined to determine their efficacy at controlling emissions of HAPs. For

major sources this includes a technology-based review to determine if emissions control is adequate, followed by a healthbased determination of residual risk, if necessary. Area sources regulated under the GACT standard are subject to

technology-based review but not residual risk review.

Section 112’s prevention of accidental release provisions are intended to prevent or mitigate the accidental and potentially

catastrophic release of hazardous substances that may be lethal or seriously adverse to human health or the environment. EPA

addresses the prevention of catastrophic releases of hazardous substances through a three-part program that includes (1)

listing regulated substances and threshold quantities of the more dangerous hazardous substances; (2) requiring source

facilities to create and register a risk management plan (RMP) that includes a hazard assessment, a program for preventing

accidental releases, and a response program in the event a release occurs; and (3) under a general duty clause, requiring

owners of sources to identify hazards, maintain a safe facility, and prevent or minimize accidental releases.

In the 119th Congress, Members have taken action to modify or oversee EPA’s HAPs regulations under Section 112,

including the regulatory standards that apply to different source types and the associated review processes. For example, in

2024, EPA promulgated a rule that a major source of HAPs subject to CAA Section 112(c)(6) could not transition to

regulation under the GACT standard even if its HAPs emissions fell below the major source emissions threshold. In 2025

Congress passed legislation repealing this rule. Members have also introduced legislation to address EPA’s setting of

NESHAPs for HAPs sources. In 2026 EPA repealed the Mercury and Air Toxics Standards (MATS) NESHAP rule. In 2026,

Congress considered a resolution under the Congressional Review Act (S.J.Res. 188) disapproving the repeal. The motion to

proceed with consideration of S.J.Res. 188 was rejected by the Senate. In the 119th Congress Members have also introduced

legislation to make changes to Section 112 of the CAA, including the addition of substances to the list of regulated HAPs, the

removal of some exemptions for oil and gas sources of HAPs, and the removal of the possibility for citizen suits under

Section 112 of the CAA.

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Regulation of Hazardous Air Pollutants Under the Clean Air Act: A Primer

Contents

Introduction ..................................................................................................................................... 1

Historical Background ..................................................................................................................... 1

Hazardous Air Pollutants: Regulatory Framework .......................................................................... 3

Overview of the Regulatory Framework ................................................................................... 3

Phases of the Regulatory Framework........................................................................................ 5

Identification ....................................................................................................................... 5

Regulation ........................................................................................................................... 9

Review .............................................................................................................................. 13

Preventing the Catastrophic Release of Hazardous Substances .................................................... 14

HAPs Activities in the 119th Congress........................................................................................... 17

Contacts

Author Information........................................................................................................................ 20

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Regulation of Hazardous Air Pollutants Under the Clean Air Act: A Primer

Introduction

Hazardous air pollutants (HAPs) are toxic pollutants that can cause serious health effects such as

cancer and birth defects, as well as harmful environmental effects.1 The effects of HAPs are

generally localized, with their highest concentrations occurring near their source.2 In 1970,

Congress recognized HAPs as a distinct class of air pollutants and, in order to protect public

health and welfare, enacted legislation to regulate them as such in Section 112 of the Clean Air

Act (CAA). Congress amended the CAA in 1990, making changes to the regulatory framework

for HAPs. Specifically, these changes were to facilitate the control of HAPs emissions by

identifying HAPs and sources of HAPs, setting emissions standards for HAPs, and reviewing

standards using technology-based and health-risk-based criteria. In addition to these changes,

Congress sought to prevent the catastrophic release of the most dangerous, and potentially lethal,

hazardous substances by the addition of a specific regulatory framework for this purpose.

Since 1990, the U.S. Environmental Protection Agency (EPA) has changed the lists of HAPs and

their source categories, including adding some pollutants and source categories and removing

others, and has changed their regulation.3 In addition to the changes EPA has made, Congress has

taken an active role in the evaluation of Section 112 and the changes EPA has made to the way

the agency regulates HAPs. Congress has taken an interest in recent EPA rulemaking related to

the regulatory impact of HAPs standards. Some Members of Congress introduced challenges to

EPA’s rulemaking under the Congressional Review Act (CRA).4

This report provides an overview of CAA Section 112. It provides a chronology of the

development of Section 112, including the ways in which the 1990 CAA amendments changed

the approach and structure of HAPs regulation. The report next describes the regulation of HAPs

emissions from stationary sources that occur in the course of normal activities, such as

manufacturing or providing goods and services. The report also describes the distinct regulatory

framework that aims to prevent the catastrophic, accidental release of highly dangerous and

potentially lethal hazardous substances. The report concludes with a discussion of congressional

action in the 119th Congress regarding the regulation of HAPs. These actions include the

introduction of legislation to address EPA’s rulemaking on the classification of HAPs sources

within the regulatory framework and the setting of standards for source categories of HAPs. The

concluding section also provides information on legislation Members have introduced to change

some CAA provisions governing the regulation of HAPs.

Historical Background

The CAA designated two categories of air pollutants: criteria pollutants and hazardous air

pollutants.5 Provisions to regulate criteria air pollutants are set out in Section 109 of the CAA,

1 U.S. Environmental Protection Agency (EPA), “Managing Air Quality—Air Pollutant Types,” https://www.epa.gov/

air-quality-management-process/managing-air-quality-air-pollutant-types.

2 Iowa Department of Natural Resources, “Air Pollutants,” https://www.iowadnr.gov/environmental-protection/airquality/air-pollutants.

3 Categories of industrial and other sources that emit hazardous air pollutants (HAPs) are referred to as source

categories. See EPA, “Regulatory and Guidance Information by Topic: Air,” https://www.epa.gov/regulatoryinformation-topic/regulatory-and-guidance-information-topic-air.

4 5 U.S.C. Ch. 8: Congressional Review of Agency Rulemaking.

5 Leslie S. Ritts and Ben Snowden, “The Regulation of Hazardous Air Pollutants,” in The Clean Air Act Handbook, 4th

edition, ed. Julie R. Domike and Alec C. Zacaroli (American Bar Association, 2016), p. 249 (hereinafter Ritts and

Snowden, Clean Air Act Handbook, 2016).

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which requires EPA to establish National Ambient Air Quality Standards (NAAQS) for air

pollutants that endanger public health or welfare.6 Provisions to regulate HAPs are set out in

Section 112 of the CAA.7 CAA Section 112(a)(6) defines a hazardous air pollutant as any air

pollutant listed in CAA Section 112(b). A 1990 House report states that HAPs are “air pollutants

that can cause serious illnesses, such as cancer, or death.”8 HAPs are distinguished from criteria

air pollutants as being “associated with more serious health impacts, such as cancer, neurological

disorders, and reproductive dysfunctions.”9

For each HAP, the 1970 amendments to the CAA required EPA to establish health-based National

Emission Standards for Hazardous Air Pollutants (NESHAPs) that provide “an ample margin of

safety to protect public health.”10 NESHAPs are standards for stationary sources that are intended

to control emissions of HAPs and to protect the public from pollutants that EPA has stated “are

known or suspected to cause cancer or other serious health effects, such as reproductive effects or

birth defects, or adverse environmental effects.”11 From the passage of the 1970 CAA

amendments until the 1990 CAA amendments, EPA would first list a substance as a HAP and

then would set a health-based standard for the substance. Using this health-based standard, from

1970 through 1990, EPA listed and set standards for eight HAPs: beryllium, mercury, vinyl

chloride, asbestos, benzene, radionuclides, arsenic, and coke oven emissions.12 EPA had finalized

standards for seven of these listed HAPs—all those other than coke oven emissions—by 1990

when the CAA was amended. EPA finalized standards for coke oven emissions after 1990.13

The health-based standards of the 1970 CAA amendments required EPA to perform a risk

assessment in order to set emissions control standards for a HAP.14 Such a risk assessment was

required to determine an emissions limit that would “further the protection of public health by

minimizing the health risks.”15 From 1970 until the promulgation of the 1990 amendments, this

risk assessment process included setting limits for so-called nonthreshold carcinogens—those

6 42 U.S.C. §7409. EPA has promulgated National Ambient Air Quality Standards (NAAQS) for six criteria air

pollutants or groups of pollutants: sulfur dioxide (SO2), particulate matter (PM), nitrogen dioxide (NO2), carbon

monoxide (CO), ozone (O3), and lead (Pb). See also EPA, “Criteria Air Pollutants,” https://www.epa.gov/criteria-airpollutants.

7 42 U.S.C. §7412.

8 U.S. Congress, House Committee on Energy and Commerce, Clean Air Act Amendments of 1990, report on H.R. 3030

including additional, supplemental, and dissenting views, 101st Cong,, 2nd sess., H. Rept. 101-490, part 1, May 17,

1990, p. 315. See also Robert J. Martineau Jr. and Ben Snowden, “Hazardous Air Pollutants,” in The Clean Air Act

Handbook, 3rd edition, ed. Julie R. Domike and Alec C. Zacaroli (American Bar Association, 2011), pp. 231-232

(hereinafter Martineau and Snowden, Clean Air Act Handbook, 2011).

9 42 U.S.C. §7412(a). See also House Committee on Energy and Commerce, Clean Air Act Amendments of 1990, report

on H.R. 3030, H. Rept. 101-490, May 17, 1990, p. 315.

10 See also P.L. 91-604, §112(b)(1)(B).

11 EPA, “National Emission Standards for Hazardous Air Pollutants Compliance Monitoring,”

https://www.epa.gov/compliance/national-emission-standards-hazardous-air-pollutants-compliance-monitoring. See

also House Committee on Energy and Commerce, Clean Air Act Amendments of 1990, report on H.R. 3030, H. Rept.

101-490, May 17, 1990, p. 315.

12 Martineau and Snowden, Clean Air Act Handbook, 2011, p. 232.

13 Martineau and Snowden, Clean Air Act Handbook, 2011, p. 232. See also 40 C.F.R. Part 61. National Emissions

Standards for Hazardous Air Pollutants.

14 Martineau and Snowden, Clean Air Act Handbook, 2011, p. 232.

15 EPA, “National Emissions Standards for Hazardous Air Pollutants: Proposed Standard for Vinyl Chloride,” 40

Federal Register 59534, December 24, 1975.

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carcinogens for which EPA could not establish an “atmospheric concentration which poses

absolutely no public health risk.”16

Some commentators have stated that Congress regarded the pace of listing and regulating HAPs

under the framework of the 1970 amendments as excessively slow and as a failure to implement

an air toxics program.17 In 1989, a report by the Senate Environment and Public Works

Committee (EPW) noted the slow pace of the air toxics program under Section 112 in listing

HAPs under the process EPA was following.18 The EPW report proposed restructuring the

regulation of HAPs under Section 112 to be based on source categories of air pollutants rather

than the pollutants themselves.19 The restructuring of Section 112 after this report changed the

basis of regulation from pollutants to source categories of pollutants. Health-based standards were

still preserved; as the report stated, “Authority to issue health-based standards is preserved in

modified form to be used for especially serious pollution problems.”20 In the 1990 amendments to

the CAA, Congress restructured Section 112 by including an initial list of 189 HAPs, as well as a

definition and a regulatory framework for stationary sources of HAPs. In addition to this

regulatory restructuring, these amendments also contained provisions regarding the accidental

release of hazardous substances.21

In the 1990 amendments Congress directed EPA to list major sources and area sources of these

189 HAPs and to regulate emissions of these HAPs from these sources by setting NESHAPs.22

Major sources were defined as those that emit HAPs above a threshold level, while sources of

HAPs that emit below this level were defined as area sources. Detailed descriptions of major

sources and area sources appear later in the “Listing of Source Categories” section of this

report.23

Hazardous Air Pollutants: Regulatory Framework

Overview of the Regulatory Framework

As part of its standard-setting process for developing NESHAPs, EPA includes cost, health,

environmental, and energy considerations.24 In assessing the costs of controlling HAPs, EPA

estimates the cost of pollution control equipment, labor, monitoring, recordkeeping, and

reporting.25 In addition to assessing these costs, the EPA guidance documents also require the

16 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 251. See also 40 Federal Register 59534, December 24, 1975.

17 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 252. See also Robert J. Martineau Jr., “Development of

Emissions Standards for Hazardous Air Pollutants,” in The Clean Air Act Handbook, ed. Robert J. Martineau Jr. and

David P. Novello (American Bar Association, 1998), p. 198.

18 U.S. Congress, Senate Committee on Environment and Public Works, Clean Air Act Amendments of 1989, report to

accompany S. 1630, 101st Cong., 1st sess., S.Rept. 101-228, December 20, 1989, pp. 131-132.

19 Senate Committee on Environment and Public Works, Clean Air Act Amendments of 1989, S.Rept. 101-228, p. 133.

20 Senate Committee on Environment and Public Works, Clean Air Act Amendments of 1989, S.Rept. 101-228, p. 133.

21 42 U.S.C. §7412(r).

22 Clean Air Act (CAA) §112(b)(1). See also Ritts and Snowden, Clean Air Act Handbook, 2016, p. 254.

23 CAA §112(a)(1)-(2).

24 CAA §112(d)(2): “Taking into consideration the cost of achieving such emission reduction, and any non-air quality

health and environmental impacts and energy requirements.” See also CAA §112(f)(2): “Promulgate standards for such

category or subcategory if promulgation of such standards is required in order to provide an ample margin of safety to

protect public health.”

25 John L. Sorrels and Thomas G. Walton, “Chapter 2—Cost Estimation: Concepts and Methodology,” in Economic

(continued...)

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agency to analyze impacts on consumers, small businesses, government entities (including

administrative costs and savings), and international trade, as well as energy and employment

effects.26

In assessing the health and environmental effects of HAPs, EPA takes into account the increased

dangers to human health, including cancer, neurological effects, reproductive effects, and other

health problems.27 In addition, EPA takes into account the potential for environmental dangers to

plants and animals from HAPs, such as HAPs that are ingested and concentrated through the food

chain.28 EPA also takes into account the energy requirements of the pollution control devices and

emissions from the electricity generation for meeting their energy needs.29

Under the amended regulatory structure for limiting the risks of HAPs, EPA distinguishes

between major sources and area sources in the standard-setting development of NESHAPs.30 For

major sources of HAPs, the NESHAPs include both cost and health assessments.31 As an example

of the consideration of health and cost effects, for the major source NESHAP for ferroalloys

production, EPA considered “whether the Ferroalloys Production NESHAP provides an ample

margin of safety to protect public health and whether more stringent standards are necessary to

prevent an adverse environmental effect, taking into consideration costs, energy, safety, and other

relevant factors.”32 For area sources of HAPs, the NESHAPs include cost assessments. As some

commentators have noted, “In setting GACT [Generally Available Control Technology] standards

for area sources EPA can consider costs and technical feasibility.”33 As an example of area source

cost analysis, the Clay Ceramics Manufacturing, Glass Manufacturing, and Secondary

Nonferrous Metals Processing area source NESHAP included estimates of capital costs and

and Cost Analysis for Air Pollution Regulations, EPA, 2017, p. 8, https://www.epa.gov/economic-and-cost-analysisair-pollution-regulations/chapter-2-cost-estimation-concepts-and. See also Larry Sorrels, Regulatory Impact Analysis

for the Final Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air Act, EPA, EPA452/R-20-005, 2020, p. 9. See also EPA, “Guidelines for Preparing Economic Analyses—Third Edition,” EPA-240-R24-001, 2024.

26 EPA uses a variety of methods, models, and tools to estimate economic impacts. See EPA, “Guidelines for Preparing

Economic Analyses—Third Edition,” EPA-240-R-24-001, 2024. In this source, p. 1-6 references industry sectors,

small businesses, state or local governments, consumers, and workers; p. 8-31 references international trade; p. 5-10

references energy prices; and p. 8-18 references employment effects. See also EPA, “Economic and Cost Analysis for

Air Pollution Regulations,” https://www.epa.gov/economic-and-cost-analysis-air-pollution-regulations.

27 CAA §112(b)(2): “Substances which are known to be, or may reasonably be anticipated to be, carcinogenic,

mutagenic, teratogenic, neurotoxic, which cause reproductive dysfunction, or which are acutely or chronically toxic.”

28 CAA §112(b)(2): “Adverse environmental effects whether through ambient concentrations, bioaccumulation,

deposition, or otherwise.” See also EPA, “Health and Environmental Effects of Hazardous Air Pollutants,”

https://www.epa.gov/haps/health-and-environmental-effects-hazardous-air-pollutants.

29 EPA, “National Emission Standards for Hazardous Air Pollutants: Lime Manufacturing Plants Technology Review,”

89 Federal Register 57738, 57746, July 16, 2024, https://www.govinfo.gov/content/pkg/FR-2024-07-16/pdf/202414692.pdf: “These secondary impacts typically include the energy needed to power the control devices, solid waste and

wastewater generated from operation of the control devices, and air emissions that result from the generation of

electricity used to operate the control devices.”

30 Ritts and Snowden, Clean Air Act Handbook, 2016, pp. 257-258.

31 40 C.F.R. §63.41.

32 EPA, “National Emissions Standards for Hazardous Air Pollutants: Ferroalloys Production,” 80 Federal Register

37366, 37372, June 30, 2015, https://www.govinfo.gov/content/pkg/FR-2015-06-30/pdf/2015-15038.pdf.

33 Roy S. Belden and Angela R. Morrison, “Chapter 8: Control of Hazardous Air Pollutants,” in Clean Air Act

Essentials, 3rd edition (American Bar Association, 2021), p. 172 (hereinafter Belden and Morrison, Clean Air Act

Essentials, 2021). Generally Available Control Technology is described in the “Generally Achievable Control

Technology (GACT) Standards” section of this report.

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annualized costs that included “costs of the control and monitoring equipment, operation and

maintenance expenses, performance testing, and recordkeeping costs.”34

Since the 1990 amendments to the CAA, EPA has taken action to regulate HAPs under the

amended regulatory structure. EPA has revised the list of HAPs by removing four substances

from the list and by adding one substance.35 In addition to modifying the list of HAPs, EPA has

issued final NESHAPS for over 230 source categories of HAPs, including chemical plants,

petroleum refineries, aerospace manufacturers, and steel mills.36 EPA has changed the lists of

source categories—for example, adding marine loading operations and removing petroleum dry

cleaners.37

Phases of the Regulatory Framework

There are three phases to the regulatory framework for limiting emissions of HAPs from

stationary sources: identification, regulation, and review.38 In the identification phase, EPA

identifies and lists HAPs and the major sources and area sources that emit these listed HAPs. In

the regulation phase, EPA sets control standards to regulate the emissions of the listed HAPs from

the listed sources. In the review phase, EPA subjects major source standards to an initial

technology and residual health risk review to determine if the standards are adequate for

controlling emissions of HAPs to protect public health and the environment. Subsequent reviews

do not include the health-based residual risk assessment. For area sources, all reviews are

technology-based.

Identification

The identification phase of the regulatory framework involves listing HAPs and listing source

categories that emit those HAPs.

Listing of HAPs

The first main part of the identification phase is listing HAPs. As part of the 1990 amendments to

the CAA, Congress included an initial list of 189 HAPs in statute.39 The list included pollutants

believed to be harmful to public health or the environment, those “air pollutants that can cause

serious illnesses, ... or death,”40 and as noted earlier, those “often associated with more serious

34 EPA, “National Emission Standards for Hazardous Air Pollutants for Area Sources: Clay Ceramics Manufacturing,

Glass Manufacturing, and Secondary Nonferrous Metals Processing,” 72 Federal Register 73180, 73194, December 26,

2007, https://www.govinfo.gov/content/pkg/FR-2007-12-26/pdf/E7-24720.pdf. See also EPA, “Clay Ceramics

Manufacturing Area Sources: National Emission Standards for Hazardous Air Pollutants (NESHAP)—Rule History:

12/26/2007 Final Rule,” https://www.epa.gov/stationary-sources-air-pollution/clay-ceramics-manufacturing-areasources-national-emission.

35 EPA, “Amendments to the List of Hazardous Air Pollutants (HAP) of Section 112(b) of the Clean Air Act (CAA),”

https://www.epa.gov/haps/amendments-list-hazardous-air-pollutants-hap-section-112b-clean-air-act-caa.

36 EPA, “National Emission Standards for Hazardous Air Pollutants (NESHAP),” https://www.epa.gov/stationarysources-air-pollution/national-emission-standards-hazardous-air-pollutants-neshap-8.

37 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 263.

38 Air Knowledge: A Partnership Program of U.S. EPA, “TOXC102-CI: Regulation of Hazardous Air Pollutants

(HAPs) from Stationary Sources,” https://airknowledge.gov/ILT/TOXC102/Current/CI/

01TOXC102_CI_Course_PowerPoint.pdf (hereinafter Air Knowledge EPA, “TOXC102”).

39 CAA §112(b)(1). See also Belden and Morrison, Clean Air Act Essentials, 2021, p. 160.

40 House Committee on Energy and Commerce, Clean Air Act Amendments of 1990, report on H.R. 3030, H. Rept. 101490, May 17, 1990, p. 315.

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health impacts, such as cancer, neurological disorders, and reproductive dysfunctions.”41 The

listed HAPs have a variety of chemical and physical characteristics. Some are gases under

ambient conditions while others occur as vapors or mineral fibers. The list of HAPs includes both

individual chemical compounds (such as acetaldehyde) and groups of chemical compounds (such

as arsenic compounds).42 In addition to the variety of chemical and physical characteristics, listed

HAPs are associated with a multitude of industrial processes that include fiber production,

pharmaceutical production, and the production of organic and inorganic chemicals, among

others.43

The CAA includes provisions for adding or removing pollutants from the list through a

rulemaking process. This process is triggered by a periodic review of the list by the EPA

Administrator.44 The Administrator may add pollutants that

present, or may present, through inhalation or other routes of exposure, a threat of adverse

human health effects (including, but not limited to, substances which are known to be, or

may reasonably be anticipated to be, carcinogenic, mutagenic, teratogenic, neurotoxic,

which cause reproductive dysfunction, or which are acutely or chronically toxic) or adverse

environmental effects whether through ambient concentrations, bioaccumulation,

deposition, or otherwise.45

The Administrator may remove a pollutant from the list if it is determined that “there is adequate

data on the health and environmental effects of the substance to determine that emissions,

ambient concentrations, bioaccumulation or deposition of the substance may not reasonably be

anticipated to cause any adverse effects to the human health or adverse environmental effects.”46

Modifications to the list, either additions or deletions, may occur through a determination by the

EPA Administrator, and alternatively, any person may petition the EPA Administrator to modify

the list.47

Since 1990, EPA has made five modifications to the list. Specifically, EPA delisted four

substances and added one substance.48 The substances that have been delisted since 1990 are

caprolactam (1996), surfactant alcohol ethoxylates and their derivatives (2000), ethylene glycol

monobutyl ether (2004), and methyl ethyl ketone (2005).49 One substance, 1-bromopropane, was

added in 2022.50

41 House Committee on Energy and Commerce, Clean Air Act Amendments of 1990, report on H.R. 3030, H. Rept. 101-

490, May 17, 1990, p. 315. See also Martineau and Snowden, Clean Air Act Handbook, 2011, p. 236.

42 42 U.S.C. §7412(b)(1).

43 EPA, “Initial List of Categories of Sources Under Section 112(c)(1) of the Clean Air Act Amendments of 1990,” 57

Federal Register 31576, 31592, July 16, 1992, https://archives.federalregister.gov/issue_slice/1992/7/16/3157431611.pdf.

44 42 U.S.C. §7412(b)(2).

45 42 U.S.C. §7412(b)(2).

46 42 U.S.C. §7412(b)(3)(C).

47 42 U.S.C. §7412(b)(3).

48 EPA, “Amendments to the List of Hazardous Air Pollutants (HAP) of Section 112(b) of the Clean Air Act (CAA),”

https://www.epa.gov/haps/amendments-list-hazardous-air-pollutants-hap-section-112b-clean-air-act-caa (hereinafter

EPA, “Amendments to the List of HAPs”).

49 EPA, “Amendments to the List of HAPs.”

50 EPA, “Amendments to the List of HAPs.”

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Listing of Source Categories

The second main part of the identification phase is listing stationary sources of these HAPs and

classifying them as major sources and area sources.51 As noted above, individual facilities that

emit these listed HAPs are classified as either major sources or as area sources for regulation

based on their level of HAPs emissions.52 Major sources are those that, per year, emit or have the

potential to emit 10 tons or more of any HAP and 25 tons or more of a combination of HAPs.53

Area sources are those that emit HAPs below the threshold level defined for major sources.54 EPA

establishes different NESHAPs for major sources and for area sources.

Sources that typically have emissions at or above the threshold level, and can therefore be

considered major sources, include petroleum refineries, hazardous waste combustors, and

integrated iron and steel manufacturing facilities.55 Examples of sources that typically have

emissions below the threshold level, and can therefore be considered area sources, include glass

manufacturing plants, paints and allied products manufacturing plants, and gasoline dispensing

facilities.56 Some source categories, such as boilers, have both major source and area source

standards.57 EPA is required to list all source categories of major sources that emit HAPs listed in

Section 112(b).58 EPA is required to list all categories of area sources “which the Administrator

finds present a threat of adverse effects to human health or the environment (by such sources

individually or in the aggregate) warranting regulation under this section.”59 EPA used a variety

of emissions data, including “reports, studies, memoranda and other emissions data sources,” to

identify sources of listed HAPs that were then added to the list of categories.60

In addition to identifying major source categories and area source categories, EPA has

responsibilities for addressing hazardous air pollution in urban areas. Specifically, EPA must list

“sufficient categories or subcategories of area sources to ensure that area sources representing 90

percent of the area source emissions of the 30 hazardous air pollutants that present the greatest

threat to public health in the largest number of urban areas are subject to regulation under this

section.”61 To meet this listing requirement, EPA identified a subset of 30 HAPs from area sources

51 42 U.S.C. §§7412(a)(1), 7412(a)(2), and 7412(d)(1). See also U.S. Department of Energy, “National Emission

Standards for Hazardous Air Pollutants (NESHAP) Compliance Monitoring,” https://www.energy.gov/ehss/articles/

national-emission-standards-hazardous-air-pollutants-neshap-compliance-monitoring. Listed HAPs are those listed in

42 U.S.C. §7412(b).

52 42 U.S.C. §§7412(a)(1) and 7412(a)(2).

53 42 U.S.C. §7412(a)(1).

54 42 U.S.C. §7412(a)(2). See also 42 U.S.C. §7412(k)(1-3).

55 EPA, “National Emission Standards for Hazardous Air Pollutants (NESHAP).”

56 EPA, “National Emission Standards for Hazardous Air Pollutants (NESHAP).”

57 EPA, “Industrial, Commercial, and Institutional Boilers and Process Heaters: National Emission Standards for

Hazardous Air Pollutants (NESHAP) for Major Sources,” https://www.epa.gov/stationary-sources-airpollution/industrial-commercial-and-institutional-boilers-and-process-0. See also EPA, “Industrial, Commercial, and

Institutional Area Source Boilers: National Emission Standards for Hazardous Air Pollutants (NESHAP),”

https://www.epa.gov/stationary-sources-air-pollution/industrial-commercial-and-institutional-area-source-boilers.

58 42 U.S.C. §7412(c). EPA is required to list sources that emit one or more of the HAPs included in the CAA in

Section 112(b). EPA does not use the Standard Industrial Code (SIC) system to identify categories of emissions

sources; sources are instead identified by their subpart identified in the Code of Federal Regulations. For example, Iron

and Steel Foundries (major sources) have the identifier 40 C.F.R. §63 Subpart EEEEE. See EPA, “National Emission

Standards for Hazardous Air Pollutants (NESHAP).”

59 42 U.S.C. §7412(c)(3).

60 EPA, “Documentation for Developing the Initial Source Category List,” EPA-450/3-91-030, 1992, p. 2-2.

61 42 U.S.C. §7412(c)(3). See also EPA, “Urban Air Toxic Pollutants,” https://www.epa.gov/haps/urban-air-toxicpollutants; and EPA, “Area Sources of Urban Air Toxics,” https://www.epa.gov/haps/area-sources-urban-air-toxics.

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that posed the greatest threat to public health in urban areas from the HAPs listed under Section

112(b)(1).62 In evaluating the relative hazards of the HAPs EPA considered the “physical

properties, sources of exposure, [and] health hazards”63 of the HAPs and estimated emissions to

develop this list.64

Under Section 112(c)(6), EPA is required to list source categories and subcategories of seven

“specific persistent and bioaccumulative pollutants.”65 These pollutants—alkylated lead

compounds, polycyclic organic matter, hexachlorobenzene, mercury, polychlorinated biphenyls,

2,3,7,8-tetrachlorodibenzofurans, and 2,3,7,8-tetrachlorodibenzo-p-dioxin—are specified in

statute. EPA has stated that these specific pollutants have been identified for their persistence in

the environment and their tendency to bioaccumlate.66 For these substances EPA is required to

“list categories and subcategories of sources assuring that sources accounting for not less than 90

per centum of the aggregate emissions of each such pollutant are subject to standards under

subsection (d)(2) or (d)(4).”67

Major source and area source categories and subcategories can be added to or deleted from the

list of HAPs emissions sources.68 The statute does not define differences between categories and

subcategories, but EPA may consider “whether there were differences in processes, sizes, or other

factors affecting emissions and control technologies that would warrant subcategorization.”69

Under the 1990 CAA amendments, the EPA Administrator can add categories and subcategories

to the list at any time using the same criteria regarding emissions of HAPs used in the formation

of the initial list.70 These are the criteria used in listing major and area sources stated in Sections

112(c)(1) and 112(c)(3). As an example of listing, marine loading operations were added as a

major source category in 1993.71 If a source category is listed due to emissions of a unique

chemical substance that can be shown not to “reasonably be anticipated to cause any adverse

effects to the human health or adverse environmental effects,” the EPA Administrator can delete

the source category.72 In the case of a cancer risk, the source category can be delisted if the

lifetime risk from emissions from this category is 1 in 1 million or less.73 In the case of other,

noncancer health and environmental risks where no source emissions in the category exceed a

level needed to protect public health and the environment, the source category can be delisted.74

Deletions can be made by the EPA Administrator based on the Administrator’s own decision or

62 42 U.S.C. §7412(c)(3)(i-ii). See also EPA, National Air Toxics Program: The Integrated Urban Strategy Report to

Congress, EPA-453/R-99-007, 2000, p. 3-1 (hereinafter EPA, Urban Strategy Report). See also EPA, “About Urban

Air Toxics,” https://www.epa.gov/haps/about-urban-air-toxics.

63 EPA, Urban Strategy Report, p. 3-1.

64 EPA, Urban Strategy Report, p. 3-1.

65 EPA, “Review of Final Rule Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air

Act,” 89 Federal Register 73293, September 10, 2024, https://www.govinfo.gov/content/pkg/FR-2024-09-10/pdf/202420074.pdf. See also 42 U.S.C. §7412(c)(6).

66 42 U.S.C. §7412(c)(6). See also Air Knowledge EPA, “TOXC102.”

67 42 U.S.C. §7412(c)(6).

68 42 U.S.C. §7412(c)(5) and 42 U.S.C. §7412(c)(9).

69 72 Federal Register 53825, September 20, 2007.

70 42 U.S.C. §7412(c)(5).

71 EPA, “Listing of Marine Vessel Loading Operations Under Section 112(c) of the Clean Air Act,” 58 Federal

Register 60021, November 12, 1993, https://archives.federalregister.gov/issue_slice/1993/11/12/6001760022.pdf#page=5.

72

42 U.S.C. §7412(b)(3)(C), 42 U.S.C. §7412(b)(3)(D), and 42 U.S.C. §7412(c)(9).

73 42 U.S.C. §7412(c)(9)(B)(i).

74 42 U.S.C. §7412(c)(9)(B)(ii).

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based on a petition by a member of the public.75 As an example of delisting, the source category

of uranium hexafluoride production was removed in 2002.76

Regulation

The second phase of the regulatory framework is the establishment of standards to limit and

control emissions of HAPs. The framework provides both technology-based and health-based

standards intended to reduce the adverse effects of HAPs.77 For HAPs for which a health

threshold is in place, “The Administrator may consider such threshold level, with an ample

margin of safety, when establishing emission standards under this subsection.”78

The framework distinguishes between major sources and area sources of HAPs, and provides

mechanisms for regulating and controlling emissions of HAPs from these sources.79 The

framework also specifically regulates the subcategory of area sources “representing 90 percent of

the area source emissions of the 30 hazardous air pollutants that present the greatest threat to

public health in the largest number of urban areas,” the listing of which is described in the

previous section.80

The establishment of standards described in this section of the report and the review process

described in a subsequent section are the components of an iterative regulatory process to develop

standards for emissions of HAPs from stationary sources. The first stage involves establishing

technology-based standards. The second stage requires the EPA to review the standards every

eight years to determine if there are “developments in practices, processes, or control

technologies” that may be appropriate to incorporate into the standards.81 The second stage also

involves a one-time health-based evaluation of the standards that are based on Maximum

Achievable Control Technology (MACT) to determine whether additional standards are needed to

address any remaining residual risk associated with HAPs emissions.82 Based on the results of the

review, EPA may be directed to revise the standards.83

The standards developed by EPA are promulgated as NESHAPs for the source categories and

subcategories of listed HAPs.84 NESHAPs for major and area source categories use different

75 42 U.S.C. §7412(c)(9)(B).

76 EPA, “National Emission Standards for Hazardous Air Pollutants: Revision of Source Category List Under Section

112 of the Clean Air Act,” 67 Federal Register 6521, February 12, 2002, https://www.govinfo.gov/content/pkg/FR2002-02-12/pdf/02-3348.pdf.

77 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 253.

78 42 U.S.C. §7412(d)(4). The term health threshold appears in the statute but is not defined in the statute. EPA uses

values from the EPA’s Integrated Risk Information System (IRIS) as part of the regulatory process promulgated under

CAA Section 112. See EPA, “Integrated Risk Information System,” https://www.epa.gov/iris. According to Mills and

Foureman, IRIS contains “[a]gency consensus scientific positions and quantitative values on cancer and noncancer

health effects that may result from lifetime oral or inhalation exposure to specific chemical substances in the

environment.” Amy Mills and Gary L. Foureman, “US EPA’s IRIS Pilot Program: Establishing IRIS as a Centralized,

Peer-Reviewed Data Base with Agency Consensus1,” Toxicology, vol. 127, no. 1 (1998), p. 85.

79 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 254.

80 42 U.S.C. §7412(c)(3). See also EPA, “Urban Air Toxic Pollutants,” https://www.epa.gov/haps/urban-air-toxicpollutants”; and Ritts and Snowden, Clean Air Act Handbook, 2016, p. 300.

81 42 U.S.C. §7412(d)(6).

82

42 U.S.C. §7412(f)(2). See also Belden and Morrison, Clean Air Act Essentials, 2021, p. 174.

83 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 292.

84 42 U.S.C. §7412(c)(2). See also 42 U.S.C. §7412(d).

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technological emissions control standards, described below. EPA has promulgated NESHAPs for

over 100 source categories.85

Maximum Achievable Control Technology (MACT) Standards

NESHAPs for major sources are promulgated under MACT standards.86 For MACT standards,

EPA is required to set standards for sources of the listed pollutants that achieve “the maximum

degree of reduction in emissions” taking into account cost and “non-air-quality health and

environmental impacts and energy requirements.”87 The Senate Committee on Environment and

Public Works has stated, “In setting MACT standards, EPA is to give priority to pollution

prevention measures like process changes and materials substitution.”88

EPA established MACT standards for both new and existing sources of HAPs emissions. The

MACT standards for new sources are based on “emissions levels that are already being achieved

by the best-controlled and lowest-emitting sources in an industry.”89 The MACT standards for

existing sources, where there are more than 30 sources, are based on “the level of performance

already achieved by the average of the top-performing 12 percent of similar sources.”90 The

MACT standards for existing sources where there are fewer than 30 sources are set based on “the

best average emission limitation achieved by the best performing 5 sources.”91 These MACT

standards set minimum levels of emissions control known as the “MACT floor.”92 States may set

emissions standards for HAPs that are more stringent than the MACT floor, but such standards

may not be less stringent.93 As discussed in the next section, MACT standards are subject to a

review and revision process by EPA every eight years that includes a technology assessment.94

MACT standards are also subject to a subsequent, one-time, health-based risk assessment.95

For some source categories the NESHAP includes standards for both major and area sources. For

example, the NESHAP for perchloroethylene (PCE) dry cleaning facilities includes MACT

standards for major sources of dry cleaning using PCE, and for area sources of dry cleaning using

PCE includes Generally Achievable Control Technology (GACT) standards, which are described

in the following section. 96 In some cases the MACT and GACT standards are equally stringent.

For example, EPA determined that the major source (MACT) and area source (GACT) standards

for hazardous waste combustors should be equally stringent.97

85 EPA, “National Emission Standards for Hazardous Air Pollutants (NESHAP).”

86 42 U.S.C. §7412(d)(2).

87 42 U.S.C. §7412(d)(2).

88 Senate Committee on Environment and Public Works, Clean Air Act Amendments of 1989, S. Rept. 101-228, p. 148.

89 EPA, “Risk and Technology Review of the National Emissions Standards for Hazardous Air Pollutants,”

https://www.epa.gov/stationary-sources-air-pollution/risk-and-technology-review-national-emissions-standardshazardous. See also 42 U.S.C. §7412(d)(3).

90 EPA, “Setting Emissions Standards for Major Sources of Toxic Air Pollutants,” https://www.epa.gov/clean-air-actoverview/setting-emissions-standards-major-sources-toxic-air-pollutants. See also 42 U.S.C. §7412(d)(3)(A).

91 42 U.S.C. §7412(d)(3)(B).

92 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 280.

93 42 U.S.C. 85 Subchapter I, Part A, §7416. Retention of State Authority.

94 42 U.S.C. §7412(d)(6). See also 42 U.S.C. §7412(f)(2).

95 42 U.S.C. §7412(d)(6). See also 42 U.S.C. §7412(f)(2).

96 EPA, “National Emission Standards for Hazardous Air Pollutants: National Perchloroethylene Air Emission

Standards for Dry Cleaning Facilities Technology Review,” 90 Federal Register 1041, 1043, January 7, 2025,

https://www.govinfo.gov/content/pkg/FR-2025-01-07/pdf/2024-31223.pdf.

97 Belden and Morrison, Clean Air Act Essentials, 2021, p. 172.

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Generally Achievable Control Technology (GACT) Standards

The EPA Administrator can promulgate MACT standards for area sources of HAPs, but the CAA

also provides the Administrator with the option of promulgating GACT standards for area

sources.98 As GACT standards are less strict than MACT standards, some commentators have

stated that they may facilitate the development of standards for area sources.99 EPA has a degree

of flexibility in determining GACT standards based on cost and technical feasibility.100 The

GACT standards reflect the application of “methods, practices and techniques which are

commercially available and appropriate for application by the sources in the category considering

economic impacts and the technical capabilities of the firms to operate and maintain the

emissions control systems.”101

A GACT standard does not have the same requirements as a MACT standard.102 For example, a

GACT standard does not have a minimum emissions control level or “floor” as a MACT standard

has.103 Another difference is that while area source NESHAPs under the GACT standard are

subject to subsequent technology-based review, they are not subject to the subsequent healthbased risk assessment to which major source NESHAPs are subject (as described in the “Review”

section).104

The regulation of area sources includes specific provisions to address HAPs in urban areas, as the

statute states that “emissions of hazardous air pollutants from area sources may individually, or in

the aggregate, present significant risks to public health in urban areas.”105 As noted in the

previous section, EPA is required to “identify not less than 30 hazardous air pollutants which, as

the result of emissions from area sources, present the greatest threat to public health in the largest

number of urban areas.”106 Having identified the 30 HAPs that present the greatest danger to

public health in urban areas, EPA is directed to regulate the area sources responsible for the

emission of 90% of these 30 identified HAPs.107 To address these pollutants, in 1999, EPA

developed and implemented the Integrated Urban Air Toxics Strategy. The strategy includes

•

•

•

source-specific and sector-based standards, which include regulatory activities

designed to address air toxics on a national level;

national, regional, and community-based initiatives focusing on multimedia and

cumulative risks to address and resolve issues at the local level through

partnerships with state, tribal, and local governments and community

stakeholders;

national-level air toxics assessments using analytical tools such as emissions

inventories, monitoring networks, and analytical assessments to identify risks,

track progress, and help prioritize efforts; and

98 42 U.S.C. §7412(d)(2). See also 42 U.S.C. §7412(f) and 42 U.S.C. §7412(d)(5).

99 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 300.

100 Belden and Morrison, Clean Air Act Essentials, 2021, p. 172.

101 Senate Committee on Environment and Public Works, Clean Air Act Amendments of 1989, S.Rept. 101-228, p. 171.

102 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 300.

103 Belden and Morrison, Clean Air Act Essentials, 2021, p. 172.

104 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 300. See also 42 U.S.C. §7412(d)(5).

105 42 U.S.C. §7412(k)(1).

106 42 U.S.C. §7412(k)(3)(B)(i). See also EPA, “Urban Air Toxic Pollutants,” https://www.epa.gov/haps/urban-air-

toxic-pollutants; and EPA, “Area Sources of Urban Air Toxics,” https://www.epa.gov/haps/area-sources-urban-airtoxics.

107 42 U.S.C. §7412(c)(3).

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•

outreach and education consisting of activities involving state, tribal, and local

agencies, cities, communities, and other groups and organizations that help EPA

implement its program to reduce air toxics emissions.108

“Once-In Always-In” MACT Guidance

The “once-in always-in” issue centers on whether a source that has been classified as a major

source subject to MACT standards can become an area source not subject to MACT standards by

reducing HAPs emissions below the major source threshold.109 In 1995, EPA issued a guidance

memorandum to clarify when a major source of HAPs could become an area source rather than

comply with major source requirements by obtaining federally enforceable limits on its potential

to emit HAPs.110 This memorandum stated that, once a source was subject to MACT standards as

a major source, it remained subject to MACT standards and could not become an area source not

subject to MACT standards, even if it reduced emissions below the major source threshold.111

In 2018 EPA issued a guidance memorandum withdrawing its “once-in always-in policy,” stating

that “sources of hazardous air pollutants previously classified as ‘major sources’ may be

reclassified as ‘area sources’ at any time, provided the facility limits its potential to emit below

major source thresholds.”112 In 2020 EPA promulgated a rule that allowed for reclassification

from major source to area source if the source emissions fell below the major source threshold.113

In 2024, however, EPA promulgated a rule that stated that sources of the seven toxic air pollutants

listed in Section 112(c)(6) of the CAA, once having been listed as major sources subject to

MACT standards, would have to continue to comply with MACT standards even if their

emissions fell below threshold levels and they were reclassified as area sources.114 In 2025

Congress enacted legislation (P.L. 119-20) disapproving the rule. This statute repealed the “oncein always-in” rule for sources of these seven HAPs, making it possible for them to be reclassified

as area sources if their emissions were below threshold amounts, and therefore to be no longer

subject to MACT standards.115

108 EPA, “Integrated Urban Air Toxics Strategy,” https://www.epa.gov/haps/integrated-urban-air-toxics-strategy.

109 Harvard Law School Environmental and Energy Law Program, “Once In Always In Guidance for Major Sources

Under the Clean Air Act,” https://eelp.law.harvard.edu/tracker/once-in-always-in-guidance-for-major-sources-underthe-clean-air-act/.

110 Memorandum from J. Seitz, Director, EPA Office of Air Quality Planning and Standards, “Potential to Emit for

MACT Standards—Guidance on Timing Issues,” 1995, https://www.epa.gov/sites/default/files/201802/documents/pteguid.pdf (hereinafter EPA, MACT Guidance, 1995).

111 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 261.

112 EPA, “Guidance for Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air Act,”

https://www.epa.gov/stationary-sources-air-pollution/guidance-reclassification-major-sources-area-sources-undersection. See also EPA, “Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air Act,”

Guidance Memorandum, 2018.

113 EPA, “Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air Act,” 85 Federal

Register 73854, November 19, 2020, https://www.govinfo.gov/content/pkg/FR-2020-11-19/pdf/2020-22044.pdf.

114 EPA, “Review of Final Rule Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air

Act,” 89 Federal Register 73293, September 10, 2024, https://www.govinfo.gov/content/pkg/FR-2024-09-10/pdf/202420074.pdf. EPA, “Review of Final Rule Reclassification of Major Sources as Area Sources Under Section 112 of the

Clean Air Act,” 89 Federal Register 73293, 73295. See also EPA, “EPA Proposes to Strengthen 2020 Air Toxics

Regulation to Prevent Emissions Increases and Protect Public Health,” press release, September 22, 2023,

https://www.epa.gov/newsreleases/epa-proposes-strengthen-2020-air-toxics-regulation-prevent-emissions-increasesand.

115 EPA, “Congressional Review Act Revocation of 2024 Review of Final Rule Reclassification of Major Sources as

Area Sources Under Section 112 of the Clean Air Act,” 91 Federal Register 58, January 2, 2026.

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Review

The third phase of the regulatory framework for limiting HAPs emissions from stationary sources

is the iterative process of review to determine if there is sufficient control of risk from HAPs.116 If

EPA determines during the review process that controls are insufficient, the Administrator may

choose to revise the standards through the process described in the previous section. This review

process has both technology-based and health-based review elements.117 In the 1990 CAA

amendments Congress included an iterative review process that included health and

environmental risk assessments to determine if the technology-based standards were sufficient to

control the risks from HAPs.118

Eight years after promulgating a NESHAP, EPA is required to review the standard and determine

if revision is necessary.119 There are two elements of this review: a technology review and a

health-based residual risk review.120 The technology review is required for both MACT- and

GACT-based NESHAPs for major and area sources.121 The technology review is the first stage of

the review process and examines whether there are “developments in practices, processes, or

control technologies” that may be appropriate to incorporate into the standards.122 This review

includes an evaluation of whether more efficient, cost-effective technologies have become

available, indicating a need for tighter standards.123 The technology review is required to be

repeated no less often than every eight years after the NESHAP is promulgated.124

The health-based residual risk review is required within eight years after the NESHAP is

promulgated and occurs only once.125 The residual risk review is required for all sources for

which MACT standards have been established.126 No residual risk review is required for area

sources subject only to GACT standards.127 In the residual risk analysis, EPA is required to review

any “residual risk” to public health and the environment that remains after implementation of

MACT standards.128 EPA must promulgate emissions standards necessary to provide “an ample

116 Air Knowledge EPA, “TOXC102.”

117 EPA, “Risk and Technology Review of the National Emissions Standards for Hazardous Air Pollutants,”

https://www.epa.gov/stationary-sources-air-pollution/risk-and-technology-review-national-emissions-standardshazardous.

118 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 253. See also, EPA, “Risk and Technology Review of the

National Emissions Standards for Hazardous Air Pollutants.”

119 42 U.S.C. §7412(d)(6).

120 EPA, “Risk and Technology Review of the National Emissions Standards for Hazardous Air Pollutants.”

121

42 U.S.C. §7412(f)(2) and 42 U.S.C. § 7412(d)(6). See also EPA, “National Emission Standards for Hazardous Air

Pollutants: Iron and Steel Foundries Major Source Residual Risk and Technology Review and Area Source Technology

Review,” 85 Federal Register 56080, 56082, September 10, 2020, https://www.govinfo.gov/content/pkg/FR-2020-0910/pdf/2020-14143.pdf.

122 42 U.S.C. §7412(d)(6).

123 EPA, “Setting Emissions Standards for Major Sources of Toxic Air Pollutants,” https://www.epa.gov/clean-air-actoverview/setting-emissions-standards-major-sources-toxic-air-pollutants.

124 42 U.S.C. §7412(d)(6).

125 42 U.S.C. §7412(f)(2). See also Ritts and Snowden, Clean Air Act Handbook, 2016, p. 253. See also EPA, “Risk

and Technology Review of the National Emissions Standards for Hazardous Air Pollutants.”

126 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 292.

127 EPA, “Review of Final Rule Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air

Act,” 88 Federal Register 66336, 66339, September 27, 2023, https://www.govinfo.gov/content/pkg/FR-2023-09-27/

pdf/2023-21041.pdf: “Unlike MACT standards required for major sources, GACT standards are not required to be

updated pursuant to residual risk reviews.”

128 Belden and Morrison, Clean Air Act Essentials, 2021, p. 174.

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margin of safety” to protect public health based on the residual risk review or determine that the

standards being reviewed already provide such a margin without any revision.129

EPA’s interpretation of the requirement to “provide an ample margin of safety to protect public

health” has effectively established a range of permissible risk levels within which it may set

residual risk standards for carcinogenic HAPs.130 EPA generally uses the same approach that the

agency used in the 1989 benzene NESHAP.131 Following the benzene NESHAP, EPA has

established a three-tiered regulatory framework for setting standards for carcinogens, known as

the “fuzzy bright line.”132 The approach to addressing residual risk by EPA is determined by the

level of risk:

EPA must eliminate risks above one hundred in one million (1 in 10,000), does not have

to address risks below one in one million (1 in 1,000,000), and has discretion to set a

residual risk standard somewhere in between.... In a second step, EPA can consider whether

providing the public with “an ample margin of safety” requires risks to be reduced further

than this “safe” level, based on EPA’s consideration of health information and other factors

such as cost, economic impact, and technological feasibility.133

For MACT standards EPA combines the two types of review—that is, the technology assessment

and the residual risk review—into a joint Risk and Technology Review (RTR), in the review at

the end of the first eight-year period for MACT standards.134 As noted above, subsequent reviews

do include the technological assessments after the first review but do not include the residual risk

assessment in subsequent reviews.135

Preventing the Catastrophic Release of Hazardous

Substances

Section 112(r) of the CAA includes distinct provisions to prevent and minimize the consequences

of the accidental catastrophic release of hazardous substances.136 Such releases occurred at a

chemical plant in Bhopal, India, in 1984, killing thousands, and at a chemical plant in Institute,

129 42 U.S.C. §7412(f)(2)(A).

130 42 U.S.C. §7412(f)(2)(A).

131 EPA, Residual Risk: Report to Congress, EPA-453/R-99-001, 1999, p. ES-6.

132 EPA, “Technical and Regulatory Support to Develop a Rulemaking to Potentially Modify the NESHAP Subpart W

Standard for Radon Emissions from Operating Uranium Mills (40 CFR 61.250),” 2014, Table 2, p. 10. See also Alex

Jackson, “EPA’s Fuzzy Bright Line Approach to Residual Risk,” Ecology Law Quarterly, vol. 36 (2009), pp. 439-466.

133 EPA, “Technical and Regulatory Support to Develop a Rulemaking to Potentially Modify the NESHAP Subpart W

Standard for Radon Emissions from Operating Uranium Mills (40 CFR 61.250),” 2014, Table 2, p. 10. See also EPA,

Residual Risk: Report to Congress, EPA-453/R-99-001, 1999.

134 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 291.

135 EPA, “Risk and Technology Review of the National Emissions Standards for Hazardous Air Pollutants.” See also

Air Knowledge EPA, “TOXC102.”

136 42 U.S.C. §7412(r).

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WV, in 1985, injuring hundreds.137 These accidents prompted Congress to include Section 112(r)

in the 1990 CAA amendments.138

Section 112(r) includes the establishment of an independent Chemical Safety and Hazard

Investigation Board (CSB).139 The CSB is responsible for investigating accidents involving

releases of hazardous substances “resulting in a fatality, serious injury or substantial property

damages”;140 conducting studies, including incident investigations; and preparing reports on the

handling of toxic materials and measures to reduce the risk of accidents.141

EPA Program for Preventing Catastrophic Releases of Hazardous Substances

EPA’s program for preventing the catastrophic releases of hazardous substances includes the following three

components:

1. Promulgation of a list of regulated substances and threshold quantities that “are known to cause or may

reasonably be anticipated to cause death, injury, or serious adverse effects to human health or the environment.”

42 U.S.C. §7412(r)(3)-(5).

2. The implementation of a risk management plan (RMP) by owners and operators “to detect and prevent or

minimize accidental releases of such substances from the stationary source, and to provide a prompt emergency

response to any such releases in order to protect human health and the environment.” 42 U.S.C.

§7412(r)(7)(B)(ii).

3. The implementation of a general duty clause that requires owners and operators of stationary sources “to

identify hazards which may result from such releases using appropriate hazard assessment techniques, to design

and maintain a safe facility taking such steps as are necessary to prevent releases, and to minimize the

consequences of accidental releases which do occur.” 42 U.S.C. §7412(r)(1).

Section 112(r)(3) requires EPA to promulgate a list of hazardous substances to be regulated with

respect to accidental release and “threshold quantities” or levels at which death, injury, and

serious adverse effects are reasonably likely to occur.142 This list is distinct from the list of HAPs

in Section 112(b)(1), although some substances, such as phosgene and hydrogen fluoride, appear

on both lists.143 The statute states that substances on the list of substances regulated under Section

112(r) are evaluated for inclusion in the list based on “(i) the severity of any acute adverse health

effects associated with accidental releases of the substance; (ii) the likelihood of accidental

releases of the substance; and (iii) the potential magnitude of human exposure to accidental

releases of the substance.”144

As set out in the Code of Federal Regulations, the list of substances regulated under Section

112(r) may be revised, with substances being added or removed from the list by the action of the

137

42 U.S.C. §7412(r)(3): These are hazardous substances, the release of which “may reasonably be anticipated to

cause death, injury, or serious adverse effects to human health or the environment.” See also Senate Committee on

Environment and Public Works, Clean Air Act Amendments of 1989, S.Rept. 101-228, p. 134. See also E. Broughton,

“The Bhopal Disaster and Its Aftermath: A Review,” Environmental Health, vol. 4, no. 6 (2005). See also R. C. Baron

et al., “Surveillance for Adverse Health Effects Following a Chemical Release in West Virginia,” Disasters, vol. 12,

no. 4 (1988), p. 356. See also Martineau and Snowden, Clean Air Act Handbook, 2011, p. 285.

138 Ritts and Snowden, Clean Air Act Handbook, 2016, p. 315.

139 42 U.S.C. §7412(r)(6).

140 42 U.S.C. §7412(r)(6(C)(i).

141 42 U.S.C. §7412(r)(6). See also U.S. Chemical Safety Board (CSB), “U.S. Chemical Safety Board Issues Final

Report on Toxic Hydrogen Fluoride Incidents,” press release, May 27, 2025, https://www.csb.gov/us-chemical-safetyboard-issues-final-report-on-toxic-hydrogen-fluoride-incidents/.

142 42 U.S.C. §7412(r)(3). See also 42 U.S.C. §7412(r)(5).

143 42 U.S.C. §7412(b)(1). See also EPA, “How Did EPA Select the List of Regulated Substances?”

https://www.epa.gov/rmp/how-did-epa-select-list-regulated-substances.

144 42 U.S.C. §7412(r)(4)(A).

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EPA Administrator or by petition. The list is to be reviewed every five years.145 A substance may

be added to the list “if, in the case of an accidental release, it is known to cause or may be

reasonably anticipated to cause death, injury, or serious adverse effects to human health or the

environment.”146 A substance may be deleted from the list “if adequate data on the health and

environmental effects of the substance are available to determine that the substance, in the case of

an accidental release, is not known to cause and may not be reasonably anticipated to cause death,

injury, or serious adverse effects to human health or the environment.”147

Section 112(r)(7)(ii) requires preparation of a risk management plan (RMP) by an owner or

operator of a stationary source of a regulated or hazardous substance above a threshold level.148

RMPs have three components: (1) a hazard assessment, (2) a program for preventing accidental

releases, and (3) a response program in the event that a release occurs.149 The hazard assessment

requires estimates of potential regulated substance release quantities, downwind effects, a fiveyear release history, and a worst-case scenario.150 The program for preventing accidental releases

entails provisions for preventing accidental releases that include maintenance, safety precautions,

monitoring, and employee training at the source location.151 The response program is to include

specific actions to be taken in the event of an accidental release, including informing the public

and notifying local emergency health care responders.152 Facilities must submit their RMPs to

EPA, to the CSB, to the state where the source is located, and to the local agency tasked with

planning for and responding to accidental releases.153

Section 112(r) includes a general duty clause that applies to owners and operators of stationary

sources that store or use listed substances or other hazardous materials, including those owners

and operators not required to submit an RMP.154 The CAA states the general duty clause in

Section 112(r)(1):

The owners and operators of stationary sources producing, processing, handling or storing

such substances have a general duty ... to identify hazards which may result from such

releases using appropriate hazard assessment techniques, to design and maintain a safe

facility taking such steps as are necessary to prevent releases, and to minimize the

consequences of accidental releases which do occur.155

The requirements under the general duty clause are not limited to the substances listed under

Section 112(r)(3); they also include “any other extremely hazardous substance.”156 The

requirements of the general duty clause in Section 112(r)(1) are the same as those for employers

and employees under the Occupational Safety and Health Act: “To identify hazards which may

result from such releases using appropriate hazard assessment techniques, to design and maintain

145 42 U.S.C. §7412(r)(3). See also 40 C.F.R. §68.130.

146 40 C.F.R. §68.120 Petition process (b).

147 40 C.F.R. §68.120 Petition process (c).

148 42 U.S.C. §7412(r)(7)(B)(ii).

149 42 U.S.C. §7412(r)(7)(B)(ii).

150 42 U.S.C. §7412(r)(7)(B)(ii)(I).

151 42 U.S.C. §7412(r)(7)(B)(ii)(II).

152 42 U.S.C. §7412(r)(7)(B)(ii)(III).

153 42 U.S.C. §7412(r)(7)(B)(iii).

154

EPA, “The General Duty Clause,” EPA 550-F-20-002, 2020, p. 2.

155 42 U.S.C. §7412(r)(1).

156 42 U.S.C. §7412(r)(1).

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a safe facility taking such steps as are necessary to prevent releases, and to minimize the

consequences of accidental releases which do occur.”157

EPA has provided guidance to owners and operators of facilities to comply with the general duty

clause, stating that these owners and operators should

(1) Adopt or follow any relevant industry codes, practices or consensus standards (for the

process or facility as a whole as well as for particular chemicals or pieces of equipment),

(2) Be aware of unique circumstances of your facility which may require a tailored accident

prevention program, and

(3) Be aware of accidents and other incidents in your industry that indicate potential

hazards.158

The general duty clause does not require a risk management plan but does provide authority for

enforcement actions through administrative penalties and civil actions.159

HAPs Activities in the 119th Congress

In the 119th Congress, Members have introduced legislation and conducted oversight to address

EPA’s regulation of HAPs under Section 112 of the CAA. This legislative activity and oversight

has included action by Members on EPA’s rulemaking on the classification of major and area

HAPs sources and EPA’s rulemaking on the NESHAPs of specific HAPs sources.160 Congress has

options to examine rules promulgated by EPA under Section 112. If Members of Congress

disagree with the rules, they can introduce legislation through the use of the CRA, within the

statutory time frame, to repeal rules.161 Members used the CRA to challenge EPA’s rulemaking on

the regulation of HAPs. Some of these challenges were successful, while other challenges were

not successful.162

Members of Congress have also introduced legislation to change provisions of the CAA

governing the regulation of HAPs. Bills have included proposed legislation to add substances to

the list of HAPs in Section 112(b) of the CAA, legislation to remove the exemption for

aggregation of oil and gas sources of HAPs, and legislation to remove the possibility for citizen

suits under Section 112 of the CAA. Key examples and discussion of related congressional

actions in the 119th Congress regarding Section 112 of the CAA are presented below. These

examples do not comprise a comprehensive review of legislation or action that may have

provisions relevant to Section 112, which would be beyond the scope of this report.

The first example of recent congressional action on EPA regulation of HAPs is the legislative

activity in response to the “once-in always-in” rule. In 2024, as noted above, EPA promulgated a

rule that stated that sources of the seven toxic air pollutants listed in Section 112(c)(6), once

having been listed as major sources subject to the relevant NESHAP, would have to continue to

157 42 U.S.C. §7412(r)(1). See also 29 U.S.C. §654.

158 EPA, “Fact Sheet: The General Duty Clause,” EPA 550-F-09-002, March 2009, https://archive.epa.gov/

epa/sites/production/files/2013-10/documents/gdc-fact.pdf. See also EPA, “Guidance for Implementation of the

General Duty Clause Clean Air Act Section 112(r)(1),” EPA 550-B00-002, 2000.

159 Belden and Morrison, Clean Air Act Essentials, 2021, p. 184.

160 CRS In Focus IF10015, Congressional Oversight and Investigations, by Todd Garvey, Mark J. Oleszek, and Ben

Wilhelm.

161 5 U.S.C. Chapter 8: Congressional Review of Agency Rulemaking.

162 For example, Congress has disapproved rulemaking on standards for tire manufacturing under Section 112 using the

CRA (see H.J.Res. 61).

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comply with those standards even if their emissions fell below threshold levels and they were

reclassified as area sources.163 This applied a new “once-in always-in” rule to existing major

sources of these seven HAPs whereby they could be reclassified as area sources but would still be

subject to the major source NESHAP.164 On March 5, 2025, S.J.Res. 31 was introduced, which

disapproved of the rule and which subsequently repealed the rule under the CRA, also as noted

above.165

On June 30, 2025, S.J.Res. 31 became law (P.L. 119-20), repealing the new “once-in always-in”

rule for sources of these seven HAPs. This repeal made it possible for sources of these HAPs to

be removed from regulation under MACT standards if their emissions were below threshold

amounts.166 The transition from major source to area source is sometimes referred to as “MM2A.”

Sources that are reclassified as area sources will now become subject to area source NESHAPs

for their source category.167

Some Members supported the repeal of this rule, saying that the repeal would provide an

incentive to reduce emissions and that the rule was harmful to the manufacturing and energy

sectors.168 Other Members opposed the repeal, saying that the rule prevented regression to higher

levels of dangerous air pollution and ensured air pollution control by industrial facilities such as

oil refineries.169

Another key example of recent congressional action on EPA regulation of HAPs is the legislative

activity in response to the Mercury Air Toxics Standards rule. In 2024 EPA promulgated a rule,

commonly referred to as the Mercury and Air Toxics Standards (MATS) rule, that required certain

coal-fired electricity generating units to meet a more stringent standard for filterable particulate

163 EPA, “Review of Final Rule Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air

Act,” 89 Federal Register 73293, 73295. The seven HAPs covered by this rule as stated in CAA Section 112(c)(6) are

alkylated lead compounds, polycyclic organic matter, hexachlorobenzene, mercury, polychlorinated biphenyls, 2,3,7,8tetrachlorodibenzofurans, and 2,3,7,8-tetrachlorodibenzo-p-dioxin.

164 EPA, “Review of Final Rule Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air

Act,” 89 Federal Register 73293. See also EPA, “EPA Proposes to Strengthen 2020 Air Toxics Regulation to Prevent

Emissions Increases and Protect Public Health,” press release, September 22, 2023, https://www.epa.gov/

newsreleases/epa-proposes-strengthen-2020-air-toxics-regulation-prevent-emissions-increases-and. See also EPA,

“Fact Sheet: Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air Act (MM2A)—

Final Action,” https://www.epa.gov/system/files/documents/2024-09/mm2a-final_fact-sheet.pdf. A history of the

“once-in, always-in policy” is provided in this fact sheet.

165 S.J.Res. 31. See also 5 U.S.C. Chapter 8: Congressional Review of Agency Rulemaking.

166 EPA, “Guidance for Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air Act,”

https://www.epa.gov/stationary-sources-air-pollution/guidance-reclassification-major-sources-area-sources-undersection.

167 EPA, “Reclassification of Major Sources as Area Sources Under Section 112 of the Clean Air Act,”

https://www.epa.gov/stationary-sources-air-pollution/reclassification-major-sources-area-sources-under-section-112.

See also Belden and Morrison, Clean Air Act Essentials, 2021, p. 172.

168 Rep. Randy Weber, “Providing for Congressional Disapproval of the Rule Submitted by the Environmental

Protection Agency Relating to ‘Review of Final Rule Reclassification of Major Sources as Area Sources Under Section

112 of the Clean Air Act,’” Congressional Record, vol. 171, no. 86, May 21, 2025, p. H2213. See also Rep. Troy

Balderson, “Providing for Congressional Disapproval of the Rule Submitted by the Environmental Protection Agency

Relating to ‘Review of Final Rule Reclassification of Major Sources as Area Sources Under Section 112 of the Clean

Air Act,’” Congressional Record, vol. 171, no. 86, May 21, 2025, p. H2214.

169 Rep. Frank Pallone, “Providing for Congressional Disapproval of the Rule Submitted by the Environmental

Protection Agency Relating to ‘Review of Final Rule Reclassification of Major Sources as Area Sources Under Section

112 of the Clean Air Act,’” Congressional Record, vol. 171, no. 86, May 21, 2025, p. H2213. See also Rep. Paul

Tonko, “Providing for Congressional Disapproval of the Rule Submitted by the Environmental Protection Agency

Relating to ‘Review of Final Rule Reclassification of Major Sources as Area Sources Under Section 112 of the Clean

Air Act,’” Congressional Record, vol. 171, no. 86, May 21, 2025, p. H2214.

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matter (fPM) emissions, among other requirements.170 Some Members of Congress opposed this

rule and introduced legislation in 2025 to repeal it or prevent the rule’s enforcement. The

Combating Overregulation And Limitation of Proven, Operable, Working Energy Resources Act,

or COAL POWER Act (H.R. 3870), included provisions to repeal the MATS rule, and the

Reliable Grid Act (H.R. 3751) included provisions to prohibit the EPA Administrator from

enforcing the MATS rule.

On June 17, 2025, EPA proposed to “Repeal specific amendments to the NESHAP for Coal- and

Oil-Fired Electric Utility Steam Generating Units (EGUs), commonly referred to as the Mercury

and Air Toxics Standards (MATS), that were promulgated on May 7, 2024.”171 On February 19,

2026, EPA finalized “the repeal of certain amendments, finalized in 2024, to the MATS Rule.”172

This action repealed specific amendments to the MATS rule for the coal- and oil-fired EGU

source category, among others. In the 2026 repeal, EPA stated the following:

The EPA is repealing the revised filterable particulate matter (fPM) emission standard,

which serves as a surrogate for non-mercury hazardous air pollutant (HAP) metals for

existing coal-fired EGUs; the revised fPM emission standard compliance demonstration

requirements; and the revised mercury (Hg) emission standard for lignite-fired EGUs.173

The repeal of the MATS rule was challenged under the CRA.174 In April 2026, a Senator

introduced S.J.Res. 188 that disapproved of the rule repealing the MATS rule. The motion to

proceed with consideration of S.J.Res. 188 was rejected by the Senate, and the joint resolution did

not proceed.

EPA’s regulatory impact analysis (RIA) of the repeal of the MATS rule estimated that there would

be compliance cost savings for the electric power industry with the repeal.175 The RIA also

estimated that with the repeal there would be increases in emissions of mercury and nonmercury

heavy metals.176 EPA has identified mercury as a neurotoxin.177 The RIA estimated that the repeal

of the MATS rule would result in hundreds of millions of dollars in compliance cost savings for

the power industry178 while having no effect on retail electricity prices.179

170 EPA, “National Emission Standards for Hazardous Air Pollutants: Coal- and Oil-Fired Electric Utility Steam

Generating Units Review of the Residual Risk and Technology Review,” 89 Federal Register 38508, May 7, 2024,

https://www.govinfo.gov/content/pkg/FR-2024-05-07/pdf/2024-09148.pdf.

171 EPA, “National Emission Standards for Hazardous Air Pollutants: Coal- and Oil-Fired Electric Utility Steam

Generating Units,” 90 Federal Register 25535, June 17, 2025, https://www.govinfo.gov/content/pkg/FR-2025-06-17/

pdf/2025-10992.pdf.

172 EPA, “Mercury and Air Toxics Standards,” https://www.epa.gov/stationary-sources-air-pollution/mercury-and-airtoxics-standards.

173 EPA, “National Emission Standards for Hazardous Air Pollutants: Coal- and Oil-Fired Electric Utility Steam

Generating Units: Final Repeal,” 91 Federal Register 9088, February 24, 2026, https://www.govinfo.gov/content/pkg/

FR-2026-02-24/pdf/2026-03638.pdf.

174 5 U.S.C. Chapter 8: Congressional Review of Agency Rulemaking.

175 EPA, “Regulatory Impact Analysis for the Final Repeal of Amendments to National Emission Standards for

Hazardous Air Pollutants: Coal- and Oil-Fired Electric Utility Steam Generating Units,” EPA-452/R-26-001, 2026, p.

ES-3 (hereinafter EPA, RIA MATS Repeal 2026).

176 EPA, RIA MATS Repeal 2026, p. ES-4.

177

EPA, “Health Effects of Exposures to Mercury,” https://www.epa.gov/mercury/health-effects-exposures-mercury.

178 EPA, RIA MATS Repeal 2026, Table ES-1, p. ES-3.

179 EPA, RIA MATS Repeal 2026, Table ES-3, p. ES-4.

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In addition to these examples of congressional action, in the 119th Congress, Members have

challenged the promulgation of NESHAPS for other HAPs source categories under the CRA. In

2025, a Senator introduced S.J.Res. 66 to challenge the EPA NESHAP for “Coke Ovens: Pushing,

Quenching, and Battery Stacks, and Coke Oven Batteries ” under the CRA.180 In the same year a

Senator also introduced S.J.Res. 67 to challenge the EPA NESHAP for “Integrated Iron and Steel

Manufacturing Facilities” under the CRA.181 In 2025 a Representative introduced H.J.Res. 61 to

challenge the EPA NESHAP for “Rubber Tire Manufacturing” under the CRA.182 This challenge

to the NESHAP for “Rubber Tire Manufacturing” under the CRA was successful, and the rule

was revoked.183

Members have introduced legislation to make changes to Section 112 of the CAA, including the

addition of substances to the list of HAPs in Section 112(b), removing the exemption for

aggregation of oil and gas sources of HAPs, and the removal of the possibility for citizen suits

under Section 112 of the CAA. Representative Haley M. Stevens introduced H.R. 6990, the

Prevent Release Of Toxic Emissions, Contamination, and Transfer Act of 2026, also known as the

PROTECT Act of 2026, that would add “as a class all perfluoroalkyl and polyfluoroalkyl

substances with at least one fully fluorinated carbon atom to the list of hazardous air pollutants

under Section 112(b) of the Clean Air Act.” Representative Yvette D. Clarke introduced H.R.

6081, Closing Loopholes for Oil and other Sources of Emissions Act, also known as the CLOSE

Act, to add hydrogen sulfide to the list of HAPs under Section 112(b) of the CAA. This

legislation also included provisions to repeal the exemption of oil and gas sources under Section

112(n)(4) of the CAA. Senator Lisa Blunt Rochester introduced S. 3529, the Public Health Air

Quality Act of 2025, which would require HAPs fenceline monitoring at some facilities. Senator

Mike Lee introduced S. 3049, the Fair Air Enforcement Act of 2025, to repeal Section 304 of the

CAA that includes provisions for citizen suits; the legislation includes conforming amendments

for Section 112 of the CAA.

Author Information

Jonathan D. Haskett

Analyst in Environmental Policy

180 EPA, “National Emission Standards for Hazardous Air Pollutants for Coke Ovens: Pushing, Quenching, and Battery

Stacks, and Coke Oven Batteries; Residual Risk and Technology Review, and Periodic Technology Review,” 90

Federal Register 29997, July 8, 2025, https://www.govinfo.gov/content/pkg/FR-2025-07-08/pdf/2025-12626.pdf.

181 EPA, “National Emission Standards for Hazardous Air Pollutants: Integrated Iron and Steel Manufacturing

Facilities Technology Review: Interim Final Rule,” 90 Federal Register 29485, July 3, 2025, https://www.govinfo.gov/

content/pkg/FR-2025-07-03/pdf/2025-12407.pdf.

182 EPA, “National Emission Standards for Hazardous Air Pollutants: Rubber Tire Manufacturing,” 89 Federal Register

94886, November 29, 2026, https://www.federalregister.gov/d/2024-26895/page-94886.

183 EPA, “Rubber Tire Manufacturing: National Emission Standards for Hazardous Air Pollutants (NESHAP),”

https://www.epa.gov/stationary-sources-air-pollution/rubber-tire-manufacturing-national-emission-standardshazardous.

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Disclaimer

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