Introduction

Under Sections 108-109 of the Clean Air Act (CAA),¹ the U.S. Environmental Protection Agency (EPA) is to issue national ambient (outdoor) air quality standards (NAAQS) for certain listed pollutants (1) whose emissions "may reasonably be anticipated to endanger public health or welfare" and (2) whose presence in ambient air "results from numerous or diverse mobile or stationary sources."² EPA has identified and promulgated NAAQS for six principal pollutants, commonly referred to as criteria pollutants:

• 1. particulate matter (PM),
• 2. ozone (O₃),
• 3. nitrogen dioxide (NO₂),³
• 4. sulfur dioxide (SO₂),
• 5. carbon monoxide (CO), and
• 6. lead (Pb).

PM refers to a mixture of solid particles and liquid droplets in the atmosphere. PM components may include acids, organic chemicals, metals, and soil or dust particles. The size of PM varies, ranging from tiny particles that can be seen only through a high-power microscope to larger particles (e.g., soot). Exposure to PM has been associated with adverse health effects (e.g., aggravated asthma, chronic bronchitis, and premature death).⁴ PM has also been linked with haze formation and other ecological effects.⁵

For PM, EPA established short-term (24-hour) and long-term (annual) standards. Additionally, EPA established separate standards for two categories of PM based on size: "fine" PM 2.5 micrometers or less in diameter (referred to as PM_2.5) and slightly larger, but still inhalable, particles less than or equal to 10 micrometers in diameter (referred to as PM₁₀).

The statute requires that EPA review the latest scientific studies and either reaffirm or modify previously established NAAQS every five years. EPA last revised the PM NAAQS in 2012. On December 18, 2020, EPA reviewed and decided to retain the 2012 PM_2.5 and PM₁₀ standards, without revision. While EPA is meant to review the NAAQS every five years, it has rarely met its statutory obligation.⁶ On January 27, 2023, EPA proposed a reconsideration of the 2020 decision to retain the PM NAAQS and would lower the annual PM_2.5 allowable limit. (See PM NAAQS chronology presented in Table A-1.)

This report focuses on the NAAQS implementation process and timelines, primarily as they relate to PM_2.5 and the proposed reconsideration of the PM_2.5 NAAQS. Also included in this report is an overview of the exceptional events process by which EPA excludes air monitoring data influenced by situations such as wildfires and prescribed burns, known as wildland fires, when taking regulatory actions with regard to the air quality standards.⁷ The report concludes with a discussion of issues for potential consideration by Congress.

NAAQS Requirements and Designation Process

The CAA directs EPA to establish two types of NAAQS:

• 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";⁸ and
• 2. secondary standards,⁹ which are necessary to protect public welfare,¹⁰ a broad term that includes visibility impairment as well as damage to crops and vegetation, and effects on soil and nutrient cycling, water, wildlife, property, and building materials, among other things.

Establishing NAAQS does not directly limit emissions or compel specific emissions controls; rather, it represents the EPA Administrator's formal judgment regarding the level of ambient pollution that will protect public health with an adequate margin of safety. In setting the NAAQS, the EPA may not consider the costs of implementing the standards.¹¹ Promulgation of NAAQS sets in motion a process under which the states and tribes first identify geographic nonattainment areas (i.e., those areas failing to meet the NAAQS) based on monitoring and analysis of relevant air quality data.¹² Ambient air monitoring is "the systematic, long-term assessment of pollutant levels by measuring the quantity and types of certain pollutants in the surrounding, outdoor air."¹³ EPA then establishes nonattainment areas based on the data and recommendations. States with nonattainment areas then submit State Implementation Plans (SIPs) to EPA, which identify specific state and federal regulations and emissions control requirements that are to bring areas into compliance, as well as actions for maintaining compliance.¹⁴

The CAA requires EPA to review the scientific data upon which the NAAQS are based every five years, and revise the standards, if necessary. More often than not, EPA has taken more than five years in reviewing the standards, but the establishment of a deadline has allowed interested parties to force review of the standards by filing suit.¹⁵ The CAA also requires EPA to appoint an independent scientific review committee composed of seven members, known as the Clean Air Scientific Advisory Committee (CASAC), that assist in the review process.¹⁶ The CAA directs CASAC to review the NAAQS every five years and recommend to the EPA Administrator "any new national ambient air quality standards and revisions of existing criteria and standards as may be appropriate."¹⁷

PM_2.5 NAAQS Designations and Implementation Timelines

The CAA directs EPA and states to take the following actions following promulgation of a new or revised PM_2.5 NAAQS:

• Within a year after a final NAAQS is promulgated, state and tribal area designation recommendations must be submitted to EPA. Areas are designated as meeting (attainment/unclassifiable areas) or not meeting (nonattainment areas) the final NAAQS.¹⁸
• No later than 120 days before promulgating final designations, EPA notifies states and tribes regarding any modifications to their recommendations.¹⁹
• Within two years after a final NAAQS is promulgated, EPA must designate areas with available information as attainment/unclassifiable areas or nonattainment areas, considering the most recent air quality monitoring data and input from states and tribes. All PM_2.5 nonattainment areas are initially designated as Moderate (i.e., the lesser of the two NAAQS PM nonattainment classifications).²⁰
• Final area designations are typically effective 90 days after Federal Register publication.²¹
• Within three years after a final NAAQS is promulgated, CAA Section 110 requires all states to submit SIP revisions to show they have the basic air quality management program components in place to implement the final NAAQS.²²
• States with new transportation projects must submit a conformity determination within one year of the effective date of nonattainment designation.²³
• Within 18 months after the effective date of designations, nonattainment area PM_2.5 SIPs are due.
• The end of the sixth calendar year after the effective date of designations is the attainment date for areas classified as Moderate for PM_2.5 nonattainment designations.²⁴
• The end of the 10^th calendar year after the effective date of designations is the attainment date for areas classified as Serious for PM_2.5 nonattainment designations.²⁵
• Designated nonattainment areas, as well as those designated unclassifiable or attainment/unclassifiable for the NAAQS, are also subject to New Source Review (NSR) requirements. As such, states must update stationary source permitting requirements.²⁶

New Source Review (NSR)

The fundamental framework of the Clean Air Act provides for different treatment of new versus existing sources of air pollution, and "modification" of existing sources. The distinction was based in part on the expectation that older sources would be replaced by newer ones over time. Accordingly, these newer sources would be subject to more stringent control requirements, and requiring existing sources to adopt stringent controls would be needlessly expensive and unwarranted given the expected replacement with controls applicable to new sources. New Source Review (NSR) aims to ensure that newly constructed facilities or major modifications to existing facilities do not result in violation of applicable air quality standards.²⁷ NSR provisions outline permitting requirements both for construction of new major pollution sources and for modifications to existing major pollution sources. The specific NSR requirements for affected sources depend on whether the sources are subject to Prevention of Significant Deterioration (PSD) or nonattainment provisions.²⁸

PSD (attainment area permitting) applies with respect to a new standard in all areas of the United States designated attainment or unclassifiable for the pollutant upon the effective date of the new standard. Any new or modified major emitting facility must undergo preconstruction review and permitting, including the installation of Best Available Control Technology (BACT).²⁹ State permitting agencies determine BACT on a case-by-case basis, taking into account energy, environmental, and economic impacts. BACT cannot be less stringent than the federal New Source Performance Standards (NSPS), but it can be more so.³⁰ More stringent controls can be required if modeling indicates that BACT is insufficient to avoid violating PSD emission limitations, or the NAAQS itself.³¹

Nonattainment New Source Review (nonattainment NSR, or NNSR) applies in areas designated nonattainment for the pollutant, which includes any areas newly designated nonattainment at or after the effective date of designations.³²

PM_2.5 Proposed Reconsideration

On January 27, 2023, EPA proposed reconsideration of the PM_2.5 annual NAAQS, with a comment period that closed on March 28, 2023.³³ The proposal would lower the PM_2.5 annual NAAQS limit from 12.0 micrograms per cubic meter (μg/m³) to a range between 9.0 to 10.0 μg/m³. The proposal also took comment on alternative annual standard levels from 8.0 μg/m³ to 11.0 μg/m³. See Table A-1 for a review of previous PM NAAQS and the current proposal. If EPA adopts the proposed revision of the PM_2.5 NAAQS, the agency would then be required to go through the designation process upon promulgations of the revised NAAQS.³⁴

Designations are based on the most recently available design values, a statistic that describes the air quality status of a given location relative to the level of the NAAQS.³⁵ Design values are computed using Federal Reference Method or equivalent data reported by state, tribal, and local air monitoring agencies to EPA's Air Quality System (AQS).³⁶ Federal Reference Methods are methods developed and evaluated by EPA for accurately and reliably measuring pollutants in outdoor air. If concentrations are flagged by state, tribal, or local monitoring agencies as having been affected by an exceptional event (e.g., wildfire, volcanic eruption) and the associated EPA Regional Office concurs, then the associated data are not included in the design value calculations.³⁷ States and tribes can consider exceptional events data in their designation recommendations.³⁸ EPA is not proposing to revise the PM_2.5 Implementation Rule (i.e., 40 C.F.R. §51, Subpart Z).³⁹ (For more on the Implementation Rule, see section "EPA SIP Requirements and Implementation Rules" of this report.)

The notice also proposed to maintain the current primary and secondary 24-hour PM_2.5 standards, primary and secondary 24-hour standards for PM₁₀, and the secondary annual PM_2.5 standard.

In addition, the EPA notice proposed revisions to other key aspects related to the PM NAAQS, including to the Air Quality Index (AQI) and monitoring requirements for the PM NAAQS.⁴⁰ These revisions include

• addressing updates in data calculations,
• approval of reference and equivalent methods,
• updates in quality assurance statistical calculations to account for lower concentration measurements,
• updates to support improvements in PM monitoring methods,
• changes to the PM_2.5 network design to account for at-risk populations,
• updates to the probe and monitoring path siting criteria for NAAQS pollutants,
• changes to the PM_2.5 AQI value of 50 to correspond to a concentration range between 9.0 and 10.0 μg/m³,⁴¹
• updates to the PM_2.5 AQI values of 200, 300, and 500 to correspond to a concentration of 125.4 μg/m³, 225.4 μg/m³, and 325.4 μg/m³, respectively,⁴² and
• changes to the AQI daily reporting requirement from five days per week to seven days per week.

The proposal also included language to modify the PM_2.5 monitoring network design criteria to include an environmental justice factor. This factor aims to account for proximity of populations at increased risk of adverse health effects from PM_2.5 exposures to sources of concern.

Prior PM NAAQS Regulations

Prior to the 2023 proposal, EPA took several actions with respect to the PM NAAQS, some of which were controversial and subject to legal challenge. Table A-1 of Appendix A is a chronology of the PM NAAQS and its revisions. The agency promulgated separate standards for both coarse and fine particulate matter in 1997.⁴³ EPA's review and establishment of the 1997 PM NAAQS was the subject of litigation and challenges. Industry groups argued that EPA's regulations were overly stringent. Environmental groups charged that EPA had not regulated stringently enough. Others made claims regarding the constitutionality of the CAA and the scope of EPA's regulatory authority. The agency's action was the subject of a Supreme Court decision, and the standards themselves were ultimately upheld by the U.S. Court of Appeals for the District of Columbia Circuit (D.C. Circuit).⁴⁴

In 2005, EPA completed a statutorily required review of the PM NAAQS. Based on that review, in 2006, EPA revised the standards.⁴⁵ Several states and industry, agriculture, business, and environmental and public health advocacy groups petitioned the U.S. Court of Appeals for the District of Columbia Circuit, challenging certain aspects of EPA's revisions of the PM NAAQS as promulgated in December 2006. A February 24, 2009, decision by the D.C. Circuit granted the petitions in part, denying other challenges, and remanded the standards to EPA for further consideration, but did not vacate the 2006 standards.⁴⁶

In 2012, EPA completed a statutorily required review of the PM NAAQS, concluded the 2006 standards were inadequately protective of public health, and revised the standards. In December 2012, EPA announced its final decisions to revise the primary NAAQS for PM.⁴⁷ Petitioners challenged EPA's final rule and argued that EPA acted unreasonably in revising the level and form of the annual standard and in amending the monitoring network provisions. On judicial review, the D.C. Circuit upheld the revised standards and monitoring requirements in all respects.⁴⁸

In May 2018, the EPA Administrator issued a memorandum describing a "back-to-basics" process for reviewing the NAAQS.⁴⁹ The memorandum announced EPA would ensure any needed revisions to the PM NAAQS would be finalized by December 2020. In October 2018, the EPA Administrator announced that the role of reviewing the key science assessments developed as part of the ongoing review of the PM NAAQS would be performed by the seven-member chartered CASAC—an independent expert committee tasked with assisting EPA in reviewing the NAAQS.⁵⁰ On December 18, 2020, EPA finalized the retention of the primary and secondary PM NAAQS, without revision.⁵¹

On January 20, 2021, the Biden Administration issued an Executive Order titled "Protecting Public Health and the Environment and Restoring Science to Tackle the Climate Crisis," which identified several environmental and climate policy goals of the incoming Administration.⁵² An accompanying fact sheet provided a "non-exclusive list of agency actions that heads of the relevant agencies will review in accordance with the Executive Order," including the 2020 PM NAAQS action.⁵³

Regulatory Impact Analysis for a Lower PM_2.5 NAAQS

EPA has prepared cost estimates for all economically significant rules since the Carter Administration as the result of executive orders.⁵⁴ Under Executive Order (E.O.) 12866, each economically significant regulatory action taken by executive branch agencies (under any statutory authority) must include estimates of the cost and benefits of the action in a Regulatory Impact Analysis (RIA) before it is proposed, and again before it is promulgated.⁵⁵ RIAs are part of the interagency review process overseen by the Office of Management and Budget, which precedes the publication of significant rules in the Federal Register.⁵⁶

EPA conducted an RIA for the proposed reconsideration of the PM NAAQS.⁵⁷ In the RIA, EPA evaluated the proposed PM_2.5 annual standards of 9.0 and 10.0 μg/m³, represented as 9/35 and 10/35, respectively, to illustrate the proposed annual and 24-hour standards. The RIA also reviewed an alternative PM_2.5 annual standard of 8.0 μg/m³ (8/35), and an alternative annual standard of 10.0 μg/m³ in combination with a 24-hour PM_2.5 standard of 30 μg/m³ (10/30).⁵⁸

In the RIA, EPA quantified the number and economic value of the estimated avoided premature deaths and illnesses attributable to applying hypothetical national control strategies for more stringent annual PM_2.5 standards with a sensitivity analysis for a more stringent 24-hour standard that reduces PM_2.5 concentrations in 2032.⁵⁹ The RIA estimates that the 2032 avoided premature mortalities for adults (ages 18-99) for the proposed annual standard of 9/35 would be 4,200 adults, with an annual monetized benefit ranging from $19 billion to $43 billion, and for the proposed annual standard of 10/35, the 2032 avoided premature mortalities for adults would be 1,700 adults, with an annual monetized benefit ranging from $7.6 billion to $17 billion. For the alternative annual standard of 8/35, the avoided premature mortality would be 9,200 adults, with an annual monetized benefit ranging from $41 billion to $95 billion, and for the alternative standard of 10/30, the avoided premature mortality would be 1,900 adults, with an annual monetized benefit ranging from $8.6 billion to $20 billion.⁶⁰

In the RIA, EPA also estimated the annualized control costs attributable to applying hypothetical national control strategies for more stringent annual PM_2.5 standards. The RIA estimates that the 2032 annualized control costs for the proposed annual standard of 9/35 would be $393.3 million, and for the proposed annual standard of 10/35 it would be $94.5 million. For the alternative annual standard of 8/35, the 2032 annualized control costs would be $1.8 billion, and for the alternative standard of 10/30, it would be $257.2 million.⁶¹

Possible Impacts of a Lower Annual PM_2.5 NAAQS

Air agencies input monitoring data into the AQS and certify the data on an annual basis. Certification of air monitoring data and accompanying reports are due to EPA May 1 of each year.⁶² If the proposed revision is finalized, the designation process will not be based on past or current monitoring data; rather, the process will partly use monitoring data that either have not yet been certified or have not yet been collected. EPA estimates that the data likely to be used for the designation process will be 2021-2024 monitoring data.

For the current existing PM_2.5 NAAQS (1997, 2006, and 2012 standards), several areas remain nonattainment. Figure 1 illustrates the areas that are nonattainment for one or more of the current PM_2.5 standards.

Figure 1. Areas Currently (2023) Designated Nonattainment for a PM2.5 Standard Existing PM2.5 include the 1997, 2006, and 2012 standards

Source: U.S. Environmental Protection Agency (EPA), Green Book Area Maps for Counties Designated Nonattainment for PM-2.5 (1997), PM-2.5 (2006), and/or PM-2.5 (2012), at https://www3.epa.gov/airquality/greenbook/mappm25both.html. Notes: Nonattainment areas are indicated by color and current as of July 31, 2023. When only a portion of a county is shown in color, it indicates that only part of the county is within a nonattainment area boundary. Hawaii is not included, because there are no areas designated nonattainment in Hawaii.

Available complete county-level PM_2.5 three-year design values (collected over 2019-2021) can be used to identify the counties that could possibly be affected by a reconsidered PM_2.5 annual NAAQS.⁶³ For example, Figure 2 illustrates the location of counties listed in Table B-1, which lists the counties that exceed the proposed PM2.5 standard of 9.0 μg/m³ based on their 2019-2021 design values.

These counties are highlighted for illustrative purposes only, and the figure is not intended to project or predict the outcome of any forthcoming designation process. Future area designations (attainment/nonattainment) will not be based on these data, but likely on monitoring data collected between 2021 and 2024.

Figure 2. Areas That Would Likely Not Meet the Proposed PM2.5 NAAQS Based on Their 2019-2021 Design Values

Source: U.S. Environmental Protection Agency (EPA), Proposed Decision for the Reconsideration of the National Ambient Air Quality Standards for Particulate Matter (PM), at https://www.epa.gov/system/files/documents/2023-01/PM%20Maps%20-%202022%20proposal%20%282%29.pdf. Notes: Map reflects monitored counties with complete monitoring data. Future area designations will not be based on these data, but on future monitoring data. Of the 112 counties with 2019-2021 design values above 9.0 μg/m3, 24 counties are totally or partially contained in nonattainment areas for the current 2012 PM2.5 standards. These counties are highlighted for illustrative purposes only, and the figure is not intended to project or predict the outcome of any forthcoming designation process. Future area designations will not be based on these data, but likely on monitoring data collected between 2021 and 2024.

Counties meeting the current standard likely would not meet a lowered standard, based on current measurements. The number of counties not meeting a lowered standard would depend on the new standard, with more stringent standards having a higher number of nonattainment counties. For example, 88 counties meeting the current standard would not meet the proposed 9.0 μg/m³ level based on their 2019-2021 PM_2.5 design values.

EPA SIP Requirements and Implementation Rules

After EPA finalizes a new or revised NAAQS, Section 110(a)(1) of the CAA requires states to submit new or revised SIPs that provide for implementation, maintenance, and enforcement of that NAAQS. All states are required to submit SIPs that include the basic program requirements for managing air quality required in Section 110(a)(2) of the CAA showing that they have the capacity to attain, maintain, and enforce the revisions associated with a new or revised NAAQS.⁶⁴ These infrastructure SIP (iSIP) submissions must address several basic elements, including

• ambient air quality monitoring and data systems,
• programs for enforcement of control measures,
• adequate authority and resources to implement the plan, and
• prohibition of interstate pollution transport.

In the event EPA promulgates a new PM_2.5 annual standard, revised iSIPs must also demonstrate provisions adequate to ensure the states will not adversely affect other states' ability to attain the new NAAQS (known as the "good neighbor" provisions). EPA's implementation rule describes the requirements that states and tribes must meet in their implementation plans to achieve and maintain attainment. EPA typically issues a revised implementation rule after promulgating a new NAAQS.

Framework Used to Address the "Good Neighbor" Provisions

Section 110(a)(2)(D)(i) of the CAA contains elements that revised iSIPs must address. The first two elements require each state in its iSIP to demonstrate adequate provisions for the ability to prohibit air emissions within the state that (1) contribute significantly to another state's nonattainment of the NAAQS, or (2) interfere with another state's maintenance of the NAAQS.⁶⁵ These are often referred to as the good neighbor provisions.⁶⁶

For the 2012 PM NAAQS, in March 2016, EPA issued a memorandum to provide guidance for states and tribes as they developed their PM iSIPs and addressed the required good neighbor provisions.⁶⁷ The memorandum was based on EPA's established framework previously used to address the good neighbor provisions and provided modeled potential future year PM_2.5 design values for PM_2.5 ambient monitors throughout the United States.⁶⁸ The memorandum also described how these projected potential design values could be used to help determine which air quality monitors should be further evaluated to potentially address situations where emissions from other states significantly contribute to nonattainment or interfere with maintenance of the 2012 PM_2.5 NAAQS at those monitoring sites.

EPA has developed a four-step framework to address states' good neighbor obligations. The four steps include

• 1. identifying downwind areas with monitors that are expected to have problems attaining or maintaining the NAAQS;
• 2. identifying which upwind states contribute to these identified problems in amounts sufficient to warrant further review and analysis;
• 3. for states identified as contributing to downwind air quality problems, identifying upwind emissions reductions necessary to prevent an upwind state from significantly contributing to nonattainment or interfering with maintenance of the NAAQS downwind; and
• 4. for states that are found to have emissions that significantly contribute to nonattainment or interfere with maintenance of the NAAQS downwind, reducing the identified upwind emissions through adoption of permanent and enforceable measures.

For the 2012 PM_2.5 NAAQS, EPA noted that "with support from the modeling…, most states will be able to develop good neighbor SIPs that demonstrate that they do not contribute significantly to nonattainment or interfere with maintenance of the 2012 PM_2.5 NAAQS in any downwind state."⁶⁹

Current PM NAAQS Requirements and Implementation Rule

EPA typically publishes an implementation rule that describes the requirements that states and tribes must meet in their implementation plans to achieve and maintain attainment. The rule also provides guidance and procedures for establishing controls to achieve and maintain attainment. In addition, the implementation rule generally includes guidance for submitting a SIP when reaching attainment within the required time frame is impractical. The implementation rule considers existing (and often pending) federal regulations that contribute to controlling criteria pollutants and their precursors.

The current PM implementation rule establishes planning requirements for states with areas that do not meet the NAAQS for the PM_2.5 standards. These states must develop plans that demonstrate how they will meet the standards.⁷⁰ Nonattainment areas are initially classified as Moderate. See Appendix C for a list of the Moderate plan requirements. EPA is not proposing to revise the PM_2.5 Implementation Rule (i.e., 40 C.F.R. §51, Subpart Z) at this time.

If a Moderate nonattainment area fails to attain by the applicable date or the extension date, or if EPA determines the area cannot practicably attain by that date, it is then likely reclassified as a Serious nonattainment area. See Appendix C for a list of the Serious plan requirements.

If an area is reclassified from Moderate nonattainment to Serious nonattainment, its major stationary source threshold, for sources subject to permitting and controls, is reduced from 100 tons per year (tpy) to 70 tpy.⁷¹ In addition, Moderate nonattainment areas require Reasonably Available Control Measures and Technology (RACM/RACT) to be implemented as expeditiously as possible. When an area is reclassified to Serious, in addition to the threshold being lowered, the area implements Best Available Control Measures and Technology (BACM/BACT). BACM is more stringent than RACM, and it may achieve greater reductions in PM_2.5 and its precursor pollutants than do the RACM control measures. Finally, if the state requests an extension for an area's Serious nonattainment date, it must demonstrate that the SIP for the area includes the Most Stringent Measures (MSM). See Appendix C for further information on RACM, BACM, and MSM.

The implementation rule identifies sulfur dioxide (SO₂), oxides of nitrogen (NO_x), volatile organic compounds (VOC), and ammonia as PM_2.5 precursors that may be addressed in attainment planning and NNSR permitting. It also authorizes a state not to adopt control requirements to reduce emissions of a particular PM_2.5 precursor if the state adequately demonstrates that the precursor does not contribute significantly to PM_2.5 levels that exceed the NAAQS in a PM_2.5 nonattainment area.

Exceptional Events and the NAAQS

EPA's NAAQS regulations rely on ambient air quality data. Section 319(a) authorizes the establishment of a national ambient air quality network to provide data that EPA then uses in developing NAAQS. Congress has recognized that some data collected through that network may not be appropriate for consideration in developing the NAAQS. In 2005, Congress amended CAA Section 319 to include the exceptional events provision.⁷² Section 319(b) defines an exceptional event, directs the EPA Administrator to establish exceptional events regulations, and lists the principles and requirements for establishing the exceptional events guidance. Section 319(b) also authorizes EPA to exclude air monitoring data influenced by exceptional events when making certain regulatory determinations, such as establishing area designations under a NAAQS.⁷³

Exceptional events may include wildfires, prescribed fires, high-wind dust events, stratospheric ozone intrusions, and volcanic and seismic activities. If a state wishes to have EPA exclude air monitoring data, it must submit to EPA an "exceptional events demonstration," which documents how the exceptional event caused an exceedance of the NAAQS for any of the criteria pollutants.⁷⁴

The 2016 Exceptional Events Rule (40 C.F.R. §50.14(c)(3)) states that an exceptional event demonstration must include the following elements:

• a narrative conceptual model that describes the event(s) causing the exceedance or violation and a discussion of how emissions from the event(s) led to the exceedance or violation at the affected monitor(s);
• a demonstration that the event affected air quality in such a way that there exists a clear causal relationship between the specific event and the monitored exceedance or violation;
• analyses comparing the claimed event-influenced concentration(s) to concentrations at the same monitoring site at other times (the EPA Administrator shall not require a state to prove a specific percentile point in the distribution of data);
• a demonstration that the event was both not reasonably controllable and not reasonably preventable;
• a demonstration that the event was caused by human activity that is unlikely to recur at a particular location or was a natural event; and
• documentation that the submitting air agency followed the public comment process.

If the current annual PM_2.5 NAAQS is revised, certain exceptional events, which would otherwise not have regulatory significance, would require an exceptional events demonstration and concurrence by EPA.⁷⁵ In establishing area designations, EPA concurrence on an exceptional events demonstration means the air quality data affected by the event will be excluded from the NAAQS designation process.

Prescribed Fires and Wildfire Exceptional Events

In addition to the 2016 Exceptional Events Rule, EPA published 2019 exceptional events guidance for prescribed fires on wildlands that may influence PM concentrations.⁷⁶ In that guidance, EPA explained when it is appropriate to develop an exceptional events demonstration for prescribed fires. The air quality impacts of prescribed fires are often less severe than uncontrolled wildfires, because prescribed fires are conducted under specific conditions on specific days.⁷⁷

Figure 3. Prescribed Fire Exceptional Events Process

Source: Congressional Research Service, based on data adapted from the U.S. Environmental Protection Agency (EPA), "Exceptional Events Guidance for Prescribed Fires," August 2019, at https://www.epa.gov/sites/default/files/2019-08/documents/ee_prescribed_fire_final_guidance_-_august_2019.pdf. Notes: FLM = Federal Land Manager. AQS = Air Quality System.

Figure 3 illustrates that the process begins if there is an exceedance or violation of the PM NAAQS and a nearby prescribed fire. Because the PM_2.5 NAAQS annual standard is an annual average, air agencies and/or Federal Land Managers (FLMs) must track these prescribed burns and determine if they had any regulatory significance.⁷⁸ If the prescribed fire did not occur on wildlands, the air agency consults with its EPA regional office to complete the exceptional events process.⁷⁹ If prescribed burn is on wildlands, the air agency and/or FLM provide initial notification to the EPA regional office and work collaboratively to determine the scope of the exceptional event. This includes the relevant dates, monitors that have regulatory significance, and EPA approvability considerations. Next, the air agency flags the data in AQS and completes the demonstration in accordance with the exceptional events guidance. The demonstration is submitted to EPA, and EPA responds either with an "on-hold letter" within 60 days or an initial review within 120 days for demonstrations that are deemed to have regulatory significance. If there is regulatory significance, EPA issues its decision on the demonstration within 12 months, either concurring with the AQS flagged data or not concurring.

The need for an exceptional events demonstration is dependent on the time, duration, location, and impact of the prescribed fire, in addition to the regulatory significance and the status of the affected monitor(s). EPA has issued updates to its "Exceptional Events Rule: Frequently Asked Questions" document,⁸⁰ "Exceptional Events Guidance: Prescribed Fire on Wildland,"⁸¹ and its "Guidance on the Preparation of Exceptional Events Demonstrations for Wildfire Events."⁸²

Issues for Consideration

Because the health and economic consequences of PM standards are potentially significant, the PM NAAQS are likely to remain an issue of interest during the 118^th Congress.

Issues Regarding the PM NAAQS

EPA actions to establish or modify NAAQS have no direct impact on air pollution control or costs. EPA is mandated to issue primary NAAQS based solely on scientific considerations "allowing an adequate margin of safety…requisite to protect the public health." Nonetheless, the values established for NAAQS have a cascading series of indirect impacts, including requirements and associated costs for pollution control equipment, State Implementation Plans, individual facility and emissions unit permits, and morbidity and mortality impacts resulting from decreased ambient concentrations of certain air pollutants.

Analyses indicate a revised PM NAAQS could 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.⁸³ However, concerns remain regarding the associated compliance costs of a more stringent standard.

The evolution and development of the PM NAAQS have been the subject of extensive congressional oversight.⁸⁴ EPA not performing timely review and revision of the NAAQS as required under the CAA has also been an area of concern to some in Congress and others.

Commenters had a wide range of feedback on the proposed reconsideration of the PM_2.5 NAAQS. Some commenters called for the annual standard to remain unchanged, citing "harm to businesses and workers up and down the supply chain."⁸⁵ Some commenters called for the annual standard to be set at 11.0 μg/m³, arguing that "uncertainties remain in the current epidemiological evidence."⁸⁶

Some commenters agreed with the proposal, stating that "a move to reduce the burden of fine particulate matter pollution is a move towards justice."⁸⁷ Others called for the annual standard to be set at the higher end of the proposed range of 9.0 to 10.0 μg/m³, stating that "our preference for the 10 μg/m³ standard reflects the concerns expressed by CASAC members about the scientific and technical bases for a more substantial reduction of the current 12 μg/m³ standard. Our views also are influenced by the potential adverse job impacts of creating new or expanded nonattainment areas under a standard of 8 or 9 μg/m³."⁸⁸

Some commenters noted that the proposed reconsideration is "neither stringent enough
to promote ... health and safety ... nor establish clean air," arguing that the annual standard should be set at 8.0 μg/m³ and the 24-hour standards should be no higher than 25.0 μg/m³.⁸⁹ Similarly, a coalition of 18 state attorneys general called for the annual standard to be set at 8.0 μg/m³ and the 24-hour standards to be in a range of 25.0 to 30.0 μg/m³.⁹⁰ Some noted that if the EPA does not lower the annual standard to 8.0 μg/m³ and the 24-hour standards to 25.0 μg/m³, then "the Agency should at least lower the standard for communities of color and communities with low income, i.e., EJ communities."⁹¹ Other commenters noted that the proposed standard "is too high to protect health," and, according to World Health Organization (WHO) guidelines, the annual PM_2.5 NAAQS should be no more than 5.0 μg/m³.⁹²

Changes to the NAAQS have historically triggered litigation alleging that the standards are too stringent or not stringent enough, often resulting in delays in implementation. The agency's final designations of nonattainment areas and the proposed PM_2.5 NAAQS are expected to generate further interest and oversight.

Issues Regarding Exceptional Events

Persistent drought in some areas and changes to the timing, duration, and intensity of wildfire seasons have occurred.⁹³ The number of acres affected annually by wildfires, while variable, generally has increased.⁹⁴ This has raised concerns about the emissions from these fires affecting air quality, and whether EPA air quality protection programs address these emissions effectively. EPA's implementation of the exceptional events provision has received criticism and attention from some stakeholders. For example, states raised several concerns about implementation of the rule, such as administrative burden on state agencies, consistency across EPA regions, complexities of documenting evidence, and the clarity of requirements and definitions.⁹⁵

Some in Congress and some state and local air agencies have expressed concern regarding the resources expected to be needed for documenting exceptional events should EPA promulgate a lower annual standard for PM_2.5. If the PM_2.5 NAAQS is lowered, more wildfires would qualify as regulatorily significant and require exceptional event demonstrations. Some commenters noted the following:

If the annual standard for PM_2.5 is lowered to 10 μg/m³or lower, Maricopa County will spend considerable resources documenting the effect of exceptional events on air quality, to have those events excluded from regulatory consideration. In the past, it has cost tens of thousands of dollars in staff time to complete a single demonstration that an exceedance was caused by wildfire smoke, rather than inadequate local controls. Lowering the annual standard will require more exceptional event demonstrations, resulting in a significant increase in workload for the State of Arizona and Maricopa County, with no benefit to air quality or public health. These resources would be better spent on local programs to reduce PM_2.5 concentrations and to protect human health when the area is impacted by wildfire smoke and dust storms.⁹⁶

Some have also called for "expanding the EPA Exceptional Events Rule for prescribed fire" to include a wider range of prescribed fires that are currently regulatorily significant under the rule.⁹⁷ Not all prescribed fire exceptional event demonstrations are deemed to have regulatory significance. Some commenters noted the following:

Prescribed fire, which is being used more frequently with the aim of preventing or curbing more dangerous fires, also poses PM_2.5 problems in some areas. It is important that EPA's exceptional events rule and guidance be viewed through the lens of state and local air agencies' struggles and that a concerted collaborative initiative be undertaken right away by EPA and state and local air agencies to identify and discuss issues of concern and approaches for addressing them in a way that facilitates reasonable exceptional events demonstrations and EPA approval of them and, at the same time, ensures protection of public health.⁹⁸

Some commenters noted that the proposed PM_2.5 NAAQS would "impede the use of beneficial fire in fire-dependent ecosystems, countering EPA's own support, by limiting the number of weather-appropriate days available to public, private, and tribal fire practitioners to implement beneficial fire across the United States."⁹⁹ They note that a revised PM_2.5 NAAQS could affect and jeopardize the use of prescribed fires used to maintain tallgrass prairies in the Flint Hills of Kansas and Oklahoma and prescribed fires in Idaho, Iowa, and Georgia where officials rely on the PM_2.5 NAAQS to decide prescribed burn days. They also note that the proposed PM_2.5 NAAQS could "place undue burden upon fire practitioners and state air quality regulatory agencies to submit technically demanding and expensive Exceptional Event Demonstrations."¹⁰⁰

Congress may wish to ask EPA to revisit the 2016 Exceptional Events Rule and subsequent guidance documents to address process-based concerns regarding wildfire- and prescribed-fire-related exceptional events.

Appendix A. PM NAAQS Chronology

Appendix B. PM Design Values

Table B-1 illustrates the counties that exceed the proposed PM_2.5 standard of 9.0 μg/m³ based on their 2019-2021 design values.

Appendix C. PM Implementation Rule Requirements

For any area designated as nonattainment and initially classified as Moderate for a PM_2.5 NAAQS, the state(s) shall submit a Moderate area attainment plan that meets all the following requirements:

• base year emissions inventory requirements;¹⁰¹
• attainment projected emissions inventory requirements;¹⁰²
• Moderate area attainment plan control strategy requirements;¹⁰³
• attainment demonstration and modeling requirements;¹⁰⁴
• reasonable further progress (RFP) requirements;¹⁰⁵
• quantitative milestone reporting;¹⁰⁶
• contingency measure requirements;¹⁰⁷ and
• nonattainment new source review plan requirements.¹⁰⁸

When designated, each PM 2.5 nonattainment area is initially classified as Moderate. The area submits its Moderate area attainment plan to EPA no later than 18 months from the effective date of designation of the area. The Moderate area attainment plan control strategy includes the implementation schedule of reasonably available control measures/reasonably available control technology (RACM/RACT). RACM is any technologically and economically feasible measure that can be implemented in whole or in part within four years after the effective date of designation of a PM_2.5 nonattainment area and that achieves permanent and enforceable reductions in direct PM_2.5 emissions and/or PM_2.5 plan precursor emissions from sources in the area. RACM includes RACT.

Each state in which all or part of a Serious nonattainment area is located shall submit an implementation plan for that area that includes each of the following:

• base year emissions inventory requirements;¹⁰⁹
• attainment projected emissions inventory requirements;¹¹⁰
• Serious area attainment plan control strategy requirements;¹¹¹
• attainment demonstration and modeling requirements;¹¹²
• RFP requirements;¹¹³
• quantitative milestone reporting;¹¹⁴
• contingency measure requirements;¹¹⁵ and
• nonattainment new source review plan requirements.¹¹⁶

Serious area attainment plan control strategies include provisions to assure that best available control measures/best available control technology (BACM/BACT) shall be implemented no later than four years after the date the area is reclassified as a Serious nonattainment area. BACM is any technologically and economically feasible control measure that can be implemented in whole or in part within four years after the date of reclassification of a Moderate PM_2.5 nonattainment area to Serious and that generally can achieve greater permanent and enforceable emissions reductions in direct PM_2.5 emissions and/or emissions of PM_2.5 plan precursors from sources in the area than can be achieved through the implementation of RACM on the same source(s). BACM includes BACT.

If the state requests an extension of an area's Serious nonattainment date, it must demonstrate that the SIP for the area includes the most stringent measures (MSM). MSM is any permanent and enforceable control measure that achieves the most stringent emissions reductions in direct PM_2.5 emissions and/or emissions of PM_2.5 plan precursors from among those control measures that are either included in the SIP for any other NAAQS, or have been achieved in practice in any state, and that can feasibly be implemented in the relevant PM_2.5 NAAQS nonattainment area.