Contents

• Introduction
• Overview of the NAAQS
• Factors Considered When Establishing NAAQS
• Key Milestones for a NAAQS Revision
• New Source Review (NSR) Air Permitting
• Timeline of a New or Revised NAAQS
• Selected Issues for Congress
• Issues Related to Regulation Timing
• NAAQS Review Cycle
• Effective Date of NAAQS
• Issues Related to External Sources of Air Pollution
• PM_2.5 NAAQS Rulemaking

Summary

Congress passed the Clean Air Act Amendments of 1970 to provide "a more effective program to improve the quality of the Nation's air." This legislation included provisions that helped regulate criteria air pollutants present nationwide in the ambient (outdoor) air, based on the criteria that they were a danger to public health or welfare and were emitted from "numerous diverse mobile or stationary sources." The U.S. Environmental Protection Agency (EPA) has designated six criteria air pollutants: particulate matter (PM), ozone (O₃), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), carbon monoxide (CO), and lead (Pb). For each of these pollutants EPA has established primary and secondary National Ambient Air Quality Standards (NAAQS), uniform nationwide standards that define EPA air quality goals.

EPA sets standards at a level requisite to prevent adverse effects, including for sensitive populations, with an adequate margin of safety. EPA is precluded from considering cost when establishing or revising a NAAQS, but NAAQS determinations can affect pollution control costs; State Implementation Plans (SIPs), which outline states' control strategies to achieve or maintain compliance with air quality standards; and individual facility emissions permits—as well as morbidity and mortality.

To ensure that air pollution from new or expanded stationary criteria-air-pollution emissions sources is sufficiently controlled so an area can attain and maintain the new or revised NAAQS, EPA and the states administer the New Source Review (NSR) permitting program. The NSR program requires preconstruction air permits for new or modified sources such as petroleum refineries and chemical plants. The NSR program includes a minor NSR program required for minor new sources or minor modifications at existing sources that do not exceed certain emissions thresholds, a Prevention of Significant Deterioration (PSD) NSR program required for new major sources or major modifications at existing sources in attainment areas, and a nonattainment NSR program for new major sources or major modifications at existing sources in nonattainment areas.

When EPA establishes a new or revised NAAQS, it triggers a sequential regulatory process requiring

• air permitting agencies to base new PSD permits on the new or revised NAAQS;
• states and tribes to submit recommendations on whether areas should be designated attainment/unclassifiable or nonattainment with respect to achieving air quality goals;
• EPA to evaluate these recommendations and notify states and tribes of agreement or disagreement;
• EPA to designate areas as attainment/unclassifiable or nonattainment;
• states to submit infrastructure SIPs showing they have the necessary programs and authorities to meet the new or revised NAAQS;
• areas in nonattainment to submit SIPs demonstrating attainment of the new or revised NAAQS within a certain time frame; and
• a demonstration of transportation conformity for nonattainment areas indicating that federally funded transportation projects in the areas are consistent with SIP air pollution control goals.

Some Members of Congress have taken an interest in several issues related to NAAQS. These include the timing of the NAAQS regulatory stages, the influence of external pollution sources on the NAAQS-based designation process, and the promulgation of more stringent PM_2.5 NAAQS. Legislation has been introduced to lengthen the NAAQS review cycle and change the timing of preconstruction permitting. Legislation has also been introduced to ensure that pollution from outside an area, either natural or anthropogenic (i.e., caused by humans), cannot affect an area's attainment status. In addition, EPA promulgated more stringent primary annual PM_2.5 NAAQS in 2024. Some Members have opposed this change and have introduced legislation to disapprove of it under the Congressional Review Act, while other Members have approved of it and have sent a letter to EPA expressing this support.

Introduction

Prior to 1970, there was no major federal regulatory role in controlling air pollution; efforts to control air pollution were regional.¹ In 1970, Congress passed the Clean Air Act (CAA) Amendments of 1970 (P.L. 91-604) to provide "for a more effective program to improve the quality of the Nation's air."² This act included provisions to regulate air pollutants that are present at the national level in the ambient (outdoor) air and "can harm human health, harm the environment, and cause property damage."³ These pollutants, such as sulfur dioxide and ozone, are referred to as criteria air pollutants, as their regulation is based on health and environmental criteria.⁴

The regulation of these air pollutants rests on the principle of cooperative federalism, "whereby states share authority with the federal government by regulating within a federally established legal framework."⁵ Under this regulatory framework, the U.S. Environmental Protection Agency (EPA) sets nationwide standards for the maximum allowable concentrations of criteria air pollutants, known as National Ambient Air Quality Standards (NAAQS).⁶ NAAQS apply to ambient air pollutants that exhibit two characteristics: (1) they may reasonably be anticipated to endanger public health or welfare, and (2) their presence in the air results from numerous or diverse mobile or stationary sources.⁷

The CAA provides for two types of NAAQS: primary standards, "the attainment and maintenance of which in the judgment of the [EPA] Administrator ... are requisite to protect the public health" with "an adequate margin of safety"; and secondary standards, which are necessary to protect public welfare.⁸ In setting primary standards, EPA examines information on health effects, including the results of epidemiological studies and information about the effects of air pollution on sensitive populations.⁹ In setting secondary standards, EPA examines whether the primary standards are also sufficiently protective of public welfare.¹⁰ NAAQS are at the core of the CAA, even though they do not directly regulate emissions. In essence, they are consistent nationwide air quality goals that are part of EPA's regulatory framework for addressing air pollution.¹¹

While the CAA authorizes EPA to set NAAQS, states are responsible for establishing procedures to attain and maintain the standards. States adopt plans, known as State Implementation Plans (SIPs), and submit these plans to EPA to ensure that they are adequate to meet statutory requirements.¹² In this framework, the states have the primary role in ensuring that the NAAQS are reached and sustained through the creation and promulgation of these SIPs and associated regulatory actions.¹³ SIPs are based on emissions inventories and computer models to determine whether air quality standards will be met or whether they will be exceeded. If these emissions and computer model data show that air quality standards would be exceeded with the pollution program and state regulations currently in place, the state must impose additional controls on existing mobile or stationary sources of air pollution to ensure that emissions do not cause "exceedances" of the standards.¹⁴ Proposed new and modified sources must obtain state construction permits in which the applicants show how the anticipated emissions will not exceed allowable limits.¹⁵ In areas where the NAAQS are exceeded, known as nonattainment areas, emissions from new or modified sources must also be offset by reductions in emissions from existing sources.¹⁶

EPA uses data from air pollution monitors around the country to determine trends in air quality and to assess air quality against the standards. With the NAAQS-based regulatory framework in place and functioning, according to EPA, emissions of all criteria "air pollutants and their precursors have been reduced substantially since 1980." EPA also notes that "despite great progress in air quality improvement, approximately 109 million people nationwide lived in counties with pollution levels above the primary NAAQS in 2024 [for at least one criteria air pollutant]."¹⁷

Some Members of Congress have taken an interest in various aspects of the NAAQS regulatory framework. These include the regulatory timeline of the standards; the effects of pollution external to a regulated area in determining whether the area does or does not exceed the NAAQS, known as the area's attainment status; and the effects of setting a new or revised NAAQS for particulate matter, one of the six criteria air pollutants.¹⁸

This report begins by providing an overview of the NAAQS, including a listing and definition of the criteria air pollutants. The report next describes some considerations regarding NAAQS, including the role of cost in the NAAQS regulatory framework and the regulatory process set in motion when a new or revised NAAQS is issued. The report describes key milestones of the NAAQS regulatory process, including the designation of areas as attaining or not attaining a new or revised standard; the subsequent development of SIPs to bring areas into compliance with the new or revised standard; and the regulation of new sources of emissions that may affect the areas' ability to attain or maintain the new or revised standard. The report then describes the New Source Review (NSR) air permitting program established to ensure that new or expanded stationary sources of criteria air pollutants meet the new or revised NAAQS. This is followed by a description of the implementation timelines for the NAAQS. The report concludes with a description of selected issues for Congress related to this regulatory framework and examples of associated congressional action, illustrating potential policy options.

Overview of the NAAQS

As introduced above, Section 108 of the CAA (42 U.S.C. §§7401 et seq.) authorizes EPA to issue NAAQS for air pollutants

• whose emissions "cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare" and
• whose presence in ambient air "results from numerous or diverse mobile or stationary sources."¹⁹

EPA has identified and promulgated NAAQS for six common air pollutants, referred to as criteria air pollutants.²⁰ Each NAAQS is a numerical standard that specifies the maximum permissible concentration for a particular criteria air pollutant in the ambient air as evaluated over a given averaging time and structured around specific statistics known as design values.²¹ NAAQS can be considered "ceilings for concentrations."²²

The health effects of criteria air pollutants are complex and can include "detrimental effects to health and increased disease, especially in vulnerable populations," such as children and the elderly.²³ EPA provides information on criteria air pollutants including their management, relevant air quality standards, and health effects.²⁴ The six criteria air pollutants, along with certain health effects that EPA has identified, are as follows:

• 1. particulate matter (PM), which is linked to premature death in association with existing heart or lung disease; ²⁵
• 2. ozone (O₃), which may worsen lung diseases such as asthma and emphysema;²⁶
• 3. nitrogen dioxide (NO₂), which may worsen respiratory diseases including asthma;²⁷
• 4. sulfur dioxide (SO₂), which can adversely affect the respiratory system;²⁸
• 5. carbon monoxide (CO), which can reduce oxygen flow to the heart and brain;²⁹ and
• 6. lead (Pb), which can harm the nervous and cardiovascular systems.³⁰

As noted previously, under CAA Section 109, EPA must establish two types of NAAQS:³¹

• 1. primary standards, "the attainment and maintenance of which in the judgment of the [EPA] Administrator ... are requisite to protect the public health" with "an adequate margin of safety";³² and
• 2. secondary standards, which are necessary to protect public welfare, a broad term that includes threats of visibility impairment; damage to crops and vegetation (such as slowing plant growth and increasing plants' risk of disease and insect damage);³³ and effects on soil and nutrient cycling, water, wildlife, property, and building materials, among other things.

The act requires that every five years EPA review the scientific data upon which the standards are based and revise the standards, if necessary. Once a NAAQS has been set, EPA uses monitoring data and other information submitted by the states to identify areas that exceed the standard and must therefore reduce pollutant concentrations to meet the standard. After these nonattainment areas are identified and designated, state and local governments must produce SIPs outlining the measures they will implement to reduce pollution levels and attain the standards.

Since the passage of the CAA Amendments of 1990 (P.L. 101-549), EPA has retained existing NAAQS, added new additional standards to NAAQS, or promulgated more stringent NAAQS, and since the passage of P.L. 101-549 EPA has not promulgated NAAQS that were less stringent than previously existing NAAQS.³⁴ Section 172(e) of the CAA provides "anti-backsliding" provisions that state the following:

If the Administrator relaxes a national primary ambient air quality standard after November 15, 1990, the Administrator shall, within 12 months after the relaxation, promulgate requirements applicable to all areas which have not attained that standard as of the date of such relaxation. Such requirements shall provide for controls which are not less stringent than the controls applicable to areas designated nonattainment before such relaxation.³⁵

Factors Considered When Establishing NAAQS

As a foundational part of the regulatory framework for controlling criteria air pollutants, setting NAAQS can have wide-ranging economic, public health, and public welfare effects. In setting NAAQS, EPA gathers and assesses a wide range of detailed medical, scientific, and technical information.³⁶ EPA sets primary standards at levels requisite to protect public health, including the health of sensitive populations, with an adequate margin of safety.

According to the Supreme Court holding in the case of Whitman v. American Trucking Associations, the CAA directive to set primary NAAQS precludes EPA from considering implementation costs.³⁷ Nonetheless, although EPA's determinations for primary NAAQS are based on health and environmental considerations, they can have effects on compliance requirements and associated costs for pollution control equipment, SIPs, and individual facility and emissions unit permits. In addition, EPA's determinations for primary NAAQS may have effects on morbidity and mortality and their associated costs, resulting from decreased ambient concentrations of certain air pollutants.³⁸

Some Members of Congress and some stakeholders have at times raised concerns over EPA and state applications of new or revised NAAQS.³⁹ These concerns have involved the process of designating areas regarding their attainment status with respect to meeting the NAAQS, timelines,⁴⁰ and implementation of a final standard.⁴¹ These concerns include whether the standard provides appropriate health protections and whether the standard creates an undue financial burden for the associated public and private sectors.⁴² When EPA proposes a new or revised, more stringent NAAQS, it means analyses by the agency have indicated that the more stringent standard would result in fewer adverse health effects among the general population—particularly within sensitive populations such as children, asthmatics, and the elderly—as well as improved welfare effects.⁴³ Some stakeholders have raised concerns about new and revised NAAQS, such as the associated public- and private-sector compliance costs, while other stakeholders are supportive due to the health benefits associated with reductions in air pollution that accompany implementation of a more stringent NAAQS.⁴⁴

Key Milestones for a NAAQS Revision

Under Title I of the CAA, EPA must periodically review NAAQS for criteria air pollutants and determine whether they need to be revised. Specifically, CAA Section 109(d) requires EPA to review the scientific data upon which NAAQS are based every five years and to revise the standards, if necessary, to protect public health and public welfare.⁴⁵ This scientific review includes a synthesis of policy-relevant scientific information, an evaluation of environmental exposures and air-quality-associated health and welfare risks, as well as quantitative exposure and risk analyses.⁴⁶ This process is a wide-ranging review of the scientific and technical literature.⁴⁷ During the process, draft documents are prepared and subject to peer and public review.⁴⁸

As part of this process, the CAA requires EPA to appoint a seven-member independent scientific review committee, known as the Clean Air Scientific Advisory Committee (CASAC). The CASAC assists EPA in the review process and recommends to the EPA Administrator "any new national ambient air quality standards and revisions of existing criteria and standards as may be appropriate."⁴⁹ If, as a result of this review process, EPA determines that the current NAAQS is not sufficient to protect public health and welfare, EPA is required to revise the NAAQS. When EPA establishes a new or revised NAAQS, it must determine which areas of the country meet the standard and which do not. EPA makes these decisions based on ambient air monitoring data and recommendations from states and tribes.⁵⁰ Under Section 107(d) of the CAA, EPA must designate all geographic areas as nonattainment, attainment, or unclassifiable for each standard as follows:⁵¹

• Nonattainment: "any area that does not meet (or that contributes to ambient air quality in a nearby area that does not meet) the national primary or secondary ambient air quality standard for the pollutant."⁵²
• Attainment: "any area (other than an area identified in clause (i) [that is identified as nonattainment]) that meets the national primary or secondary ambient air quality standard for the pollutant."⁵³
• Unclassifiable: "any area that cannot be classified on the basis of available information as meeting or not meeting the national primary or secondary ambient air quality standard for the pollutant."⁵⁴

After EPA determines which areas comply with the new or revised NAAQS, states develop and implement SIPs to, among other things, assure maintenance or attainment of the NAAQS.⁵⁵ According to EPA, a SIP is "a collection of regulations and documents used by a state, territory, or local air district to implement, maintain, and enforce the National Ambient Air Quality Standards, or NAAQS, and to fulfill other requirements of the Clean Air Act."⁵⁶ SIPs evolve and can include "statutes, rules, transportation control measures, emissions inventories, local ordinances, permits and agency [EPA] orders."⁵⁷ EPA notes that SIPs serve three main purposes:

• 1. Demonstrate that the state has the basic air quality management program components in place to implement a new or revised NAAQS.
• 2. Identify the emissions control requirements the state will rely upon to attain and/or maintain the primary and secondary NAAQS.
• 3. Prevent air quality deterioration for areas that are in attainment with the NAAQS and reduce common or criteria air pollutants emitted in nonattainment areas.⁵⁸

Under the CAA, the requirements for SIPs vary depending on an area's status as being in attainment/unclassifiable or nonattainment.⁵⁹ Generally speaking, the CAA requires that plans for higher-classified nonattainment areas—that is, areas with greater concentrations of ambient air pollution—impose more stringent pollution control measures.⁶⁰ The CAA also includes a "Good Neighbor" provision that requires states to "address the interstate transport of air pollution" that may move across state lines and affect the ability of a downwind state to "attain and maintain" a NAAQS.⁶¹ Specifically, CAA Section 110(a)(2)(D)(i)(I) requires each state's SIP to prohibit anthropogenic (i.e., human-caused) emissions that will "contribute significantly to nonattainment in, or interfere with maintenance" of, a NAAQS in a downwind state.⁶²

New Source Review (NSR) Air Permitting

The CAA established the New Source Review (NSR) permitting program, administered by EPA and the states, to ensure that air pollution from new or modified stationary sources (such as chemical manufacturing facilities, refineries, utility power plants, and paper mills) is sufficiently controlled so that an area can attain and maintain a new or revised NAAQS. State NSR programs are evaluated by EPA and, when approved, are included in the SIP.⁶³ States are primarily the designated authority to issue these permits within their jurisdictions.⁶⁴ The NSR program applies when construction is proposed for a new stationary source of air pollution or when an existing source of air pollution is to undergo major or minor modification.⁶⁵

The NSR program consists of three different preconstruction permit and review programs: Prevention of Significant Deterioration (PSD), nonattainment NSR, and minor NSR.⁶⁶ The PSD and nonattainment NSR programs are typically referred to as the major NSR programs because those permit programs regulate the construction of new major stationary sources or major modifications to existing major stationary sources of a criteria air pollutant.⁶⁷ The PSD program applies to new major stationary sources or modifications of existing major stationary sources of a criteria air pollutant proposed in areas designated as being in attainment or as unclassifiable for that NAAQS.⁶⁸ By contrast, the nonattainment NSR program applies to new major stationary sources or major modifications of existing major stationary sources of a criteria air pollutant located in an area that is designated as being in nonattainment for that NAAQS.⁶⁹ Minor NSR refers to the preconstruction permitting program for stationary sources with potential to emit pollutants below major source thresholds or minor modifications to major sources.⁷⁰ Each state develops its own minor NSR program as part of its SIP.⁷¹

Timeline of a New or Revised NAAQS

Once a substance has been listed as a criteria air pollutant under Section 108 of the CAA, Section 109 requires the EPA Administrator to propose a NAAQS for that criteria air pollutant within 12 months.⁷² Once a NAAQS has been promulgated, the NAAQS must be reviewed by EPA every five years, and EPA may revise the NAAQS as a result of that review.⁷³

After EPA promulgates a new or revised NAAQS, the CAA requires that a series of steps be implemented that apply to EPA, states, and tribes. These steps, and the timelines associated with them, are shown in Figure 1 and described below.

• When EPA promulgates a new or revised NAAQS, all applicants for air permits to construct a new major stationary source or to undertake major modification of an existing major stationary source need to conduct an air quality analysis that considers the new or revised NAAQS. This analysis is required under the PSD portion of a state's NSR permitting program, as described in the "New Source Review (NSR) Air Permitting" section of this report. This occurs at the effective date stated in the promulgation of the NAAQS final rule, which is typically 60 days after its publication in the Federal Register as part of the promulgation process.
• States and tribes submit their recommendations as to whether specific areas should be designated as being in nonattainment or in attainment/unclassifiable status for the new or revised NAAQS. This must occur no sooner than 120 days and no later than one year after the promulgation of a new or revised NAAQS.⁷⁴
• EPA notifies states and tribes of deviations between its determinations and their recommendations. This must occur no later than 120 days prior to the promulgation of designations and provide such state or tribe an opportunity to demonstrate why any proposed modification is inappropriate.⁷⁵
• EPA designates areas as attainment/unclassifiable or nonattainment. This occurs "in no case later than 2 years from the date of promulgation of the new or revised" NAAQS. The EPA Administrator has the authority to extend the two-year designation deadline by an additional year "in the event the Administrator has insufficient information to promulgate the designations."⁷⁶ In practice, EPA frequently invokes this statutory extension, making three years a typical time frame for final designations.⁷⁷
• Three years after the promulgation of a new or revised NAAQS, states must submit "infrastructure" SIPs, including "Good Neighbor" provisions, which demonstrate that they have the necessary clean air programs and authorities to meet the revised standard. This occurs within three years after the new or revised NAAQS is established.⁷⁸ Unlike nonattainment planning requirements, infrastructure SIP obligations are triggered directly by promulgating a new or revised NAAQS and are independent of designations.⁷⁹ Areas designated as being in nonattainment have planning obligations to demonstrate attainment and to meet the new standard within a certain time frame following the nonattainment designations. This process differs based on the criteria air pollutant for which the NAAQS is established. Nonattainment NSR programs are due with the nonattainment area SIPs 18 months after designation for all but the ozone NAAQS, which is due three years after designation.
• Nonattainment areas submit transportation conformity determinations. If an area is designated as being in nonattainment, then transportation "conformity" is required one year after the nonattainment designation.⁸⁰ Transportation conformity ensures that federal approval and funding are given to highway and transit projects that are consistent with ("conform to") the air quality goals established by a SIP.⁸¹

In addition to the dates noted in Figure 1, the CAA includes a pathway for states to seek additional time for attainment of a new or revised NAAQS for areas designated as nonattainment. States may apply for, and EPA may grant upon application, a one-year extension of the attainment dates;⁸² however, no more than two one-year extensions may be granted. Additionally, EPA typically issues an implementation rule or guidance document following the promulgation of a new or revised NAAQS. While the issuance of this implementation plan is not statutorily mandated as a formal, independent procedural step under the CAA, it serves as an integral component of the regulatory framework. These implementation plans provide technical and policy guidance necessary to inform and facilitate states' development of their required SIPs.

Figure 1. Timeline of NAAQS Process

Source: CRS, based on Clean Air Act (42 U.S.C. §7401 et seq). Notes: EPA = U.S. Environmental Protection Agency, NAAQS = National Ambient Air Quality Standards, PSD = Prevention of Significant Deterioration, SIP = State Implementation Plan. Area designation requirements pursuant to 42 U.S.C. § 7407(d)(1). In accordance with Part D of Title I of the Clean Air Act (42 U.S.C. §§ 171-193 and §§ 7501-7515), areas designated as being in nonattainment have planning obligations to demonstrate attainment and meet the new standard within a certain time frame following the designation. Infrastructure SIP requirements are pursuant to 42 U.S.C. § 7410(a)(2). Transportation conformity requirements are pursuant to 42 U.S.C. §7506(c)(6).

Selected Issues for Congress

Options for congressional action related to NAAQS include legislation to address identified issues or concerns, oversight of EPA's administration and execution of NAAQS and the CAA more broadly, and evaluating resources and technology development for local and regional air monitoring. Congress also may opt to take no action on NAAQS and defer to EPA's authorities over the standards. The following examples highlight specific, recent areas of interest and activity in Congress related to NAAQS.

Some Members of Congress have taken an interest in several issues related to NAAQS. These areas of interest include the statutory timelines governing regulations, standard reviews, the effects of external sources of pollution (such as transported, upwind, or international air pollution) on local attainment determinations, and the regulatory implications and application of a more stringent PM_2.5 standard within the NAAQS regulatory framework.⁸³ Some Members of Congress have responded to these issues by introducing legislation and by stating support or opposition to EPA regulatory actions related to NAAQS. These selected issues are discussed below.

Issues Related to Regulation Timing

Some Members of Congress have considered the issue of the timing of administrative actions in establishing, reviewing, and implementing NAAQS. Issues of timing include the period of time between NAAQS review cycles as well as changes to the date of NAAQS compliance. EPA's regulatory timing can have impacts on human health and on the costs to public- and private-sector stakeholders of controlling emissions. The longer it takes to promulgate a more stringent standard, the greater the number of people who may experience health effects. A longer time period would also extend the period that businesses could defer compliance costs.

NAAQS Review Cycle

The period of time between NAAQS review cycles may affect the timing of the associated regulations. Some Members of Congress have introduced legislation to extend the timeline of the NAAQS review cycle beyond five years. The Clean Air and Economic Advancement Reform Act (CLEAR Act; H.R. 4218), introduced in the 119^th Congress, includes provisions that would change the required NAAQS review cycle from every 5 years to every 10 years. This would double the period between mandatory reviews of NAAQS and, as a result, may extend the period between the promulgation of new or revised NAAQS.

Some argue that this change could reduce compliance costs for emissions sources by reducing the frequency with which more stringent NAAQS could be promulgated and therefore reduce the frequency with which more stringent pollution controls may be required. Some Members support the CLEAR Act, arguing it would provide states with sufficient time to comply with air quality standards while avoiding economically burdensome regulations.⁸⁴

Others argue that lengthening the review cycle could affect public health if a more stringent standard promulgated sooner could reduce deaths and illness due to pollution levels. Some Members oppose the CLEAR Act, arguing it would weaken public health protections and base air quality standards on compliance costs rather than health effects.⁸⁵

Effective Date of NAAQS

The regulatory timeline may also be affected by changing EPA's current practice of setting the date for NAAQS compliance. EPA provides guidance on meeting the new or revised NAAQS, including guidance on deadlines, planning requirements, monitoring requirements, NSR program requirements, and other matters.⁸⁶ As mentioned previously, following the promulgation of a new or revised NAAQS, EPA typically issues an implementation rule or guidance document that provides technical and policy guidance necessary to inform and facilitate states' development of their required SIPs.

Some time may elapse between when EPA issues a NAAQS and when EPA issues regulations and guidance for implementing that NAAQS. For example, EPA established the 2008 ozone NAAQS on March 27, 2008, with an effective date of May 27, 2008,⁸⁷ and the implementation plan was issued March 6, 2015.⁸⁸ EPA established the 2015 ozone NAAQS on October 26, 2015, with an effective date of December 28, 2015,⁸⁹ and the implementation plan was issued on December 6, 2018.⁹⁰ For the last two revisions to the PM NAAQS, EPA established the 2012 PM NAAQS on January 15, 2013, with an effective date of March 18, 2013,⁹¹ and the implementation plan was issued August 24, 2016.⁹² EPA established the 2024 PM NAAQS on March 6, 2024, with an effective date of May 6, 2024,⁹³ and, as of September 14, 2026, EPA has yet to publish an implementation rule or guidance updates.⁹⁴

In the 119^th Congress, H.R. 4214, the Clean Air and Building Infrastructure Improvement Act, was introduced. This bill would require that certain regulatory processes initiated by a new or revised NAAQS do not start with the issuance of the NAAQS, but rather when EPA issues regulations and guidance for implementing the NAAQS. The bill provides that NSR preconstruction permits for facilities that emit criteria air pollutants would be reviewed and evaluated under the previous NAAQS if the new or revised NAAQS had been promulgated but EPA had not yet issued the regulations or guidance for implementing it. Requiring the review of preconstruction permits under a previous, less stringent NAAQS, rather than under a new, more stringent NAAQS, could result in higher air pollution levels. This is because some sources may implement less stringent pollution controls or may delay implementing emissions controls. Such higher air pollution levels could delay the attainment of a new or revised NAAQS, as sources in an area continue to be regulated under the previous NAAQS.⁹⁵

Some Members of Congress have supported changing the provisions governing the requirements for preconstruction permits under H.R. 4214, while other Members have opposed them. Some Members have supported the Clean Air and Building Infrastructure Improvement Act as clarifying the NAAQS permitting and compliance requirements.⁹⁶ Other Members of Congress have opposed the act, arguing it would increase the public's exposure to dangerous air pollution by creating a gap in industry regulation.⁹⁷

Issues Related to External Sources of Air Pollution

Under the CAA, the levels of criteria air pollutants in an area determine if the area is in attainment status at the time of designation.⁹⁸ If the air pollution in the area exceeds the NAAQS, then the area is designated nonattainment and, under the CAA, the state is required to take corrective action to reduce the levels of criteria air pollutants and attain the standard.⁹⁹ In some locations, air pollution that is considered external to an area may affect the air quality and nonattainment status of that area. Examples of such external pollution include the following:

• Pollution that originates outside the area either domestically or internationally and flows into the area, contributing to air pollution levels.
• Pollution that originates from exceptional events, which the CAA defines as events that (1) affect air quality, (2) "are not reasonably controllable using techniques that tribal, state or local air agencies may implement in order to attain and maintain the NAAQS," and (3) are caused by human activity or natural events unlikely to recur.¹⁰⁰
• Pollution within the area that originates in the area but over which the state has no control, such as some mobile source pollution.¹⁰¹

The CAA includes provisions that address some of these situations. Under the CAA, if a state can establish for the EPA Administrator that it would have been in attainment for CO, O₃, or PM₁₀¹⁰² (in particular) but for "emissions emanating from outside the United States," the state would not be subject to the nonattainment area requirements.¹⁰³ The CAA also includes provisions to exclude air quality monitoring data in determining compliance with NAAQS if those data are affected by an exceptional event. In 2016, EPA finalized revisions to the Exceptional Events Rule to establish criteria and procedures for use in determining if air quality monitoring data have been affected by exceptional events, including an analysis showing "that emissions of the pollutant of interest resulting from the event were transported to the monitor(s) recording the elevated concentration measurement(s)."¹⁰⁴ EPA has since issued several guidance documents on the treatment of air quality monitoring data influenced by exceptional events.¹⁰⁵

In 2020, EPA issued a guidance document recommending that when demonstrating a nonattainment area would be compliant with the relevant NAAQS, but for emissions coming from outside the United States, the support for this demonstration should "focus on the contribution to ambient concentrations attributable to international anthropogenic emissions."¹⁰⁶ This type of demonstration is known as a 179B demonstration.¹⁰⁷ Under this guidance, only human-caused pollution originating from outside the United States would be included in a 179B demonstration, while pollution originating from natural causes outside the United States would be excluded. In 2025, EPA rescinded the 2020 179B guidance and stated that "EPA intends to work with state and local air agencies to develop the evidence necessary to grant regulatory relief under CAA Section [179B]."¹⁰⁸ The inclusion or exclusion of naturally occurring emissions from outside the United States in making NAAQS determinations could determine whether an area is designated as a nonattainment area and therefore subject to more stringent regulatory requirements than if it had been designated as an attainment area.

In the 119^th Congress, H.R. 6409, the Foreign Emissions and Nonattainment Clarification for Economic Stability Act, also known as the FENCES Act, was introduced. The bill includes provisions stating that if an area would be in attainment but for emissions from outside the United States, it may not be designated as being a nonattainment area, "regardless of whether such emissions result from human activity."¹⁰⁹

Some Members have supported the FENCES Act, arguing that it would ensure emissions from wildfires and other natural events, as well as from foreign sources, are not included in air monitoring data used for determining attainment status.¹¹⁰ Other Members have opposed the FENCES Act, arguing it would compromise public health protections.¹¹¹

PM_2.5 NAAQS Rulemaking

On March 6, 2024, EPA issued a final rule revising the primary annual PM_2.5 NAAQS from 12 to 9 micrograms per cubic meter (µg/m³).¹¹² EPA prepares a regulatory impact analysis (RIA) to estimate the costs and benefits of its major rulemakings. According to the RIA, health benefits of the revised NAAQS can include reductions in adult and infant mortality, hospital admissions, and emergency department visits.¹¹³ The analysis of costs includes purchases of pollution control equipment, while accounting for equipment lifespan and borrowing costs.¹¹⁴ The RIA for this rule estimated that the total annualized control costs of this change were $593.8 million in 2032.¹¹⁵ The RIA also estimated that the annualized monetized health benefits of the change were in the range of about $20 billion to $46 billion for the year 2032.¹¹⁶

Some Members have opposed the final rule, expressing concerns about the estimated costs of the change, while other Members have supported the change, citing the projected health benefits. For example, on March 14, 2024, S.J.Res. 65, a joint resolution providing for congressional disapproval of the rule under the Congressional Review Act, was introduced.¹¹⁷ In another example of opposition, on February 28, 2024, certain Members of Congress sent a letter to the EPA opposing the revised PM_2.5 NAAQS, stating that "the economic ramifications of the new or revised NAAQS will far exceed any emissions reductions the EPA hopes to obtain from implementation."¹¹⁸ The letter stated that the finalized standards would "cripple domestic manufacturing … weaken U.S. supply chains" and "undermine the affordability and reliability of the electric grid."¹¹⁹

Other Members have supported the change in the primary annual PM_2.5 NAAQS from 12 µg/m³ to 9 µg/m³. For example, on December 16, 2025, three lawmakers sent a letter to EPA supporting the revised NAAQS and calling on EPA to "abandon any efforts, judicial or administrative, to roll back the" standard.¹²⁰

Footnotes