Regulatory Tracker

National Ambient Air Quality Standards (NAAQS) for Particulate Matter (PM)

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On this page we track EPA’s review and implementation of revised PM NAAQS. For information on EPA’s National Ambient Air Quality Standards (NAAQS) scientific review and approval process read our NAAQS Regulatory Tracker page here. To follow EPA’s work on the Ozone NAAQS, see our Ozone NAAQS Regulatory Tracker page here. Click here for our Air Transport – Cross-State Air Pollution Rule / Good Neighbor Rule, Section 126 Petitions, and Section 184 Ozone Transportation Commission Petition page.

Quick Take

On February 7, 2024, EPA finalized a rule for particulate matter (PM) NAAQS. The final rule lowers the primary PM 2.5 annual NAAQS from 12.0 µg/m3 to 9.0 µg/m3 but retains the 24-hour standard. The Clean Air Science Advisory Committee (CASAC), an independent expert committee that assists EPA in reviewing the NAAQS, had recommended tightening all PM standards based on its review of the science underlying the 2020 PM NAAQS, for both annual and 24-hour standards. Twenty-six states and industry groups challenged the rule; initial briefs are due June 6, 2024.

Why it Matters

EPA sets NAAQS for six common and harmful pollutants: carbon monoxide, lead, PM, ozone, nitrogen dioxide, and sulfur dioxide. The NAAQS are based solely on public health and welfare protection, meaning that the agency must not consider the cost of revising a standard if the current science demands a standard be tightened to protect public health or welfare. The NAAQS program is highly successful and is the cornerstone of EPA’s work to protect public health and the environment. EPA estimates that between 1980 and 2021, total emissions of these six pollutants dropped by 71% while the economy grew by 182%.

PM can vary in size but is so small that it can be inhaled and enter the bloodstream, which can increase overall mortality rates and is often linked to asthma and other respiratory impacts, cardiovascular disease, and cancer. It can be emitted by a wide range of sources such as vehicles, industrial sources (including power plants), construction sites, and fires.

The Clean Air Act requires that EPA review the NAAQS every five years to ensure their adequacy. The review process is a multi-stage, robust review of the current science that requires significant expert input. If a standard is tightened, there is a cascading effect on air quality policies and programs across the country. States and local regions must ensure that the sources of pollution in their jurisdiction decrease their emissions, so that the region can meet the new, more stringent national standard.

Current Status

In June 2021, the Biden EPA announced that it was beginning the process of reconsidering the 2020 PM NAAQS rule, following an Executive Order requiring  EPA to review rules from the Trump EPA Administration. On February 7, 2024, EPA finalized a rule for PM NAAQS, lowering the primary PM 2.5 annual NAAQS from 12.0 µg/m3 to 9.0 µg/m3. The rule retains the primary and secondary PM 2.5 24-hour standard, the secondary PM 2.5 annual standard, and the primary and secondary PM 10 standards. The rule will become effective 60 days after it is published in the federal register.

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Jan. 15, 2013 EPA concludes its 2012 review of the PM NAAQS and determines that the existing standards inadequately protect public health. EPA tightens annual health-based standards for fine particles (PM2.5) to 12.0 micrograms per cubic meter. EPA estimates that the benefits of the revised standards will be between $3.6 and $9 billion.

Dec. 3, 2014 EPA announces the beginning of its next review of the PM NAAQS by issuing a call for information to assist EPA in developing the Integrated Science Assessment.

Dec. 2016 EPA releases the final review plan for the PM NAAQS review. EPA projects that the scientific review will be completed in Fall 2020 and a final decision made on the NAAQS in 2022.

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Oct. 31, 2017 Administrator Pruitt issues a memo changing the membership requirements for CASAC. Under the new guidelines, receiving an EPA grant is now deemed a “conflict of interest.”  Scientists who have received grants from EPA are no longer eligible to serve on the committee. Most industry-affiliated scientists are unaffected, but this change disqualifies many academic scientists, including then-serving members of CASAC.

March 18, 2020 EPA publishes a supplemental notice of proposed rulemaking to amend its 2018 proposed rule, Strengthening Transparency in Regulatory Science.

April 30, 2018 EPA publishes a proposed rule, Strengthening Transparency in Regulatory Science. The proposal would require that EPA consider only scientific studies for which the underlying data can be made public. The proposal would significantly limit the studies available to EPA in reviewing the NAAQS, because many epidemiological studies use confidential health information that cannot be made public. EPA receives nearly 600,000 comments on the proposed rule before the comment period closes.

May 9, 2018 Administrator Scott Pruitt signs a Memorandum, Back-to-Basics Process for Reviewing National Ambient Air Quality Standards, which modifies and accelerates the process for reviewing and setting the NAAQS. The memo directs the agency to complete the current reviews of the PM NAAQS by December 2020. Many experts argue that this schedule is incompatible with the thorough scientific review required by the Clean Air Act.

Oct. 10, 2018 EPA issues a press release tasking CASAC with reviewing the PM NAAQS, notably eliminating the role of independent review panels that were historically convened to assist CASAC. The PM panelists report receiving a dismissal email from EPA. These expert panels are essential to a robust scientific review, given that CASAC has – by consequence of being a seven-member panel – limited expertise.

Oct. 23, 2018 EPA announces the public comment period for the draft Integrated Science Assessment for PM.

April 11, 2019 CASAC writes a letter to Administrator Wheeler critiquing the Draft Integrated Science Assessment for PM. CASAC advises EPA to develop a Second Draft Integrated Science Assessment for the committee to review and to reappoint the disbanded PM panel. CASAC specifically seeks additional expertise in diverse scientific fields to ensure that “meaningful independent scientific review” can occur.

Sep. 5, 2019 EPA releases the draft Policy Assessment for PM NAAQS. The Policy Assessment states that the available evidence and analyses “can reasonably be viewed as calling into question the adequacy of the public health protection afforded by” the PM2.5 standards.

Sep. 13, 2019 EPA announces the selection of 12 consultants to answer individual questions passed through the CASAC chair from CASAC members regarding the review of the NAAQS for ozone and PM. The use of consultants, rather than an independent panel, severely reduces transparency and effectiveness of the review process by not allowing for an open debate among and deliberation by experts meeting together.

Oct. 22, 2019 The Independent Particulate PM Panel, which comprises eighteen former members of the now-disbanded PM Review Panel, submit consensus recommendations to CASAC ahead of its October meeting. The independent panel recommends tightening the PM NAAQS in order to meet the Clean Air Act standard of adequately protecting public health. This report follows a two-day meeting by the group, which sought to closely mirror the meeting they would have had if EPA had not disbanded the panel.

Nov. 13, 2019 CASAC releases a draft report following its October meetings. The report recommends EPA retain the current PM NAAQS but acknowledges that CASAC did not reach a consensus on this recommendation.

Dec. 3, 2019 CASAC votes to approve an updated version of the Nov. 13 report that recommends EPA retain the current PM NAAQS. The report makes clear that this recommendation does not reflect a consensus among CASAC members, meaning that not all members agree that the current PM NAAQS adequately protect public health.

Dec. 16, 2019 CASAC finalizes its report to Administrator Wheeler on the PM NAAQS. The report confirms that CASAC is divided as to whether the current science calls into question the adequacy of the existing standards, but ultimately, CASAC recommends EPA retain the existing NAAQS.

Jan. 27, 2020 EPA publishes the final 2019 Integrated Science Assessment for PM.

Jan. 29, 2020 EPA releases the final Policy Assessment for PM. The final assessment states that “the available scientific evidence, air quality analyses, and the risk assessment…can reasonably be viewed as calling into question the adequacy of the public health protection afforded by the combination of the current annual and 24-hour primary PM2.5 standards. In contrast to this conclusion, a conclusion that the current primary PM2.5 standards do provide adequate public health protection would place little weight on the broad body of epidemiologic evidence reporting generally positive and statistically significant health effect associations, particularly for PM2.5 air quality distributions likely to have been allowed by the current primary standards.” This conclusion calls into question CASAC’s recommendation to retain the existing PM NAAQS.

Feb. 10, 2020 The Southern District of New York holds that EPA violated the law when issuing the conflicts of interest directive and vacates the relevant language. As a result, until EPA modifies its decision or fixes the procedural defects the court identified, EPA cannot categorically prohibit grant recipients from serving on advisory panels. Natural Resources Defense Council, Inc. v. EPA, No. 1:19-cv-05174-DLC.

April 14, 2020 EPA announces that it is proposing to retain the PM NAAQS. The proposed action is open for comment until June 29, 2020.

Dec. 7, 2020 EPA issues a final rule refusing to update the primary and secondary NAAQS for PM due to “important uncertainties” in the evidence regarding adverse health effects of PM below current standards. The rule bypasses a report published by EPA’s Office of Air Quality Planning and Standards finding that the current primary PM standards fail to prevent “a substantial number” of premature deaths each year.

Dec. 18, 2020 EPA issues a response to significant comments received on the PM NAAQS proposed rule. EPA defends its refusal to consider evidence linking soot exposure and increased COVID-19 mortality arguing that “research in this area is new and emerging.”

Jan. 13, 2021 A coalition of 17 states and New York City files a lawsuit against EPA for refusing to update the PM NAAQS. The coalition alleges that EPA conducted a flawed review of the current NAAQS and disregarded the scientific consensus on the need for stronger standards. California et al. v. EPA, No. 21-01014 (D.C. Cir.).

Jan. 19, 2021 A group of eleven public health and environmental organizations challenges EPA’s decision not to update the PM NAAQS. The group argues that EPA rushed its decision to not update the NAAQS, despite the scientific consensus of the danger of the current PM standards. American Lung Association v. EPA, No. 21-01027 (D.C. Cir.).

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Jan. 20, 2021 President Biden issues an Executive Order revoking a Trump-era memo that required EPA to reevaluate its process for setting the NAAQS. Administrator Pruitt relied on the President’s memo to issue the Back-to-Basics memo that accelerated EPA’s process for reviewing and setting the NAAQS. Under the same order, EPA will review the PM NAAQS rule.

Feb. 9, 2021 The Center for Biological Diversity (CBD) sues EPA alleging the agency failed to consider the effects of soot pollution on endangered plants and animals when it refused to update the PM NAAQS. Center for Biological Diversity v. EPA, No. 21-1073 (D.C. Cir.).

Feb. 16, 2021 A coalition of states petitions EPA to reconsider the PM NAAQS, pointing to new studies demonstrating significant long-term health risks from PM exposure, including increased mortality from COVID-19. The same day, a coalition of public health and environmental groups submits a similar petition.

Mar. 31, 2021 Administrator Regan announces that the CASAC, as well as the Scientific Advisory Board (SAB), will be dissolved and reconstituted. EPA says resetting these committees “seeks to reverse deficiencies” from the previous administration, including Pruitt’s October 2017 directive, the elimination of the ozone review panel, and failure to follow standard processes for appointing committee members. Current members are invited to reapply.

June 10, 2021 EPA announces it will reconsider the Trump administration decision not to strengthen PM standards. It plans to issue a proposed rulemaking in Summer 2022.

July 22, 2021 EPA’s Clean Air Act Advisory Committee (CAAAC) reviews draft recommendations issued as part of a report commemorating the Clean Air Act’s 50th Anniversary. The report includes recommendations to strengthen limits on criteria pollutants, including revisiting EPA’s system of classifying nonattainment areas, expanding the national air quality monitoring network, and reviewing averaging times for the next PM NAAQS review.

Aug. 31, 2021 EPA announces the 22 members of the reconstituted CASAC PM Panel.

Sep. 30, 2021 EPA releases an update to the Trump EPA’s 2019 integrated science assessment (ISA), arguing that the science demonstrates significant health impacts of PM exposure and thus supports EPA’s decision to tighten the PM standards.

Oct 8, 2021 EPA releases a draft supplemental policy assessment to support the reconsideration of the PM NAAQS.

Oct. 7, 2021 S. Stanley Young, a former member of the SAB who was not rehired when the panel was reconstituted, files a complaint against EPA claiming the agency violated the Administrative Procedure Act (APA) and Federal Advisory Committee Act (FACA) because the reconstituted CASAC and SAB are not “fairly balanced” as required under FACA. He also alleges that several CASAC members have conflicts of interest in violation of FACA and that the decision to reconstitute both bodies was arbitrary and capricious. Louis Anthony Cox, Jr., a former member of both CASAC and the SAB who was not rehired for either body, joined the suit as a plaintiff. Young et al., v. EPA, et al., No. 21-2623 (D.D.C.).

Nov. 15, 2021 Members of the CASAC PM Panel post their preliminary comments on whether EPA should tighten the PM NAAQS. Several members agree that, based on the current science, there is sufficient evidence showing the current standards are not adequate and should be revised.

Dec. 2, 2021 During the second day of CASAC Meetings considering federal limits for PM2.5, at least 17 of the 21 CASAC members voiced support for setting the annual primary standard between 8 ug/m3 and 10 ug/m3. A smaller majority of CASAC members discussed the daily standard and found it to be inadequate to protect public health.

Dec. 23, 2021 EPA revives two CASAC panels to review the “ecological” NAAQS for nitrogen oxides (NOx), sulfur oxides (SOx), and PM, and a new review for lead (the last lead review was in October 2016). EPA selected members for the panels earlier in December.

Feb. 4, 2022 EPA’s CASAC panel on PM releases a draft report recommending tougher limits on fine PM (PM2.5), bolstering Administrator Regan’s efforts to tighten the PM NAAQS. Specifically, the CASAC recommends decreasing annual limits from 12 ug/m3 to 8-10 ug/m3, and daily limits from 35 ug/m3 to 25-30 ug/m3. Some PM panelists also expressed concern that EPA’s risk assessment failed to capture the effects of certain climatic events and wildfires on short-term exposure.

Feb. 16, 2022 The court denies Young’s motion for a preliminary injunction to prevent CASAC from continuing its activities, finding that Young failed to show he would suffer irreparable harm if the Committee continues to meet allowing the panel to continue to meet. Young v. EPA, No. 21-2623 (D.D.C.).

Mar. 18, 2022 The CASAC’s PM Review Panel sends a letter to EPA Administrator Regan stating that the panel agreed that the current annual standard for PM2.5 does not adequately protect public health and should be lowered. The majority of CASAC members said the annual standard should be lowered to 8-10 μg/m3. A majority of members also agreed that the 24-hour PM2.5 standard is inadequate to protect public health and should be lowered to 25-30 μg/m3.

Apr. 13, 2022 Environmental groups sue to require EPA to set a deadline to assess secondary standards, which are aimed at protecting ecosystems rather than human health, for sulfur dioxide, nitrogen oxides, and PM. Center For Biological Diversity et al v. Regan, No. 4:22-cv-02285 (N.D. Cal.)

Apr. 21, 2022 The Association of Air Pollution Control Agencies releases its annual State Air Trends and Successes report and the American Lung Association releases its annual State of the Air report showing improvement in air pollution, with the exception of PM levels, which are driven by increasing smoke from wildfires.

May 12, 2022 EPA adds fifteen additional areas to the 29 areas that EPA has notified as required to submit mitigation plans to reduce incidence of fires under the 2016 Exceptional Events Rule. This notice also establishes the process for adding new areas required to submit plans.  

May 31, 2022 EPA staff releases the final PA for the reconsideration of the 2020 PM NAAQS, recommending that the EPA consider tightening its primary PM standards for the yearly exposure threshold (from 12 micrograms per cubic meter (ug/m3) to between 8 ug/m3 and 12 ug/m3) but recommends retaining the short-term exposure threshold with its level of 35 ug/m3.

June 3, 2022 The Environmental Defense Fund releases a report analyzing disparate impacts of PM pollution, specifically finding that Black Americans 65 and older are three times more likely to die from exposure to PM than white Americans over 65 years old.

July 5, 2022 EPA publishes a notice of a proposed consent decree with the Center for Biological Diversity, agreeing to issue a proposed rule to set secondary NAAQS for NOx, SOx, and PM by February 9, 2024 and a final rule by December 10, 2024. Center For Biological Diversity et al v. Regan, No. 4:22-cv-02285 (N.D. Cal.).

Aug. 17, 2022 Young files a motion for notifying the DC District Court that EPA has sent the proposed PM rule to OMB, and requests an expedited ruling on their pending Motion for Partial Summary Judgment. Young et al., v. EPA, et al., No. 21-2623 (D.D.C.).

Jan. 6, 2023 EPA proposes tightening the PM 2.5 annual NAAQS, but retaining the primary 24-hour NAAQS without revision. EPA’s current PM 2.5 annual standard is 12 μg/m3 and CASAC recommended a range of 8.0 to 10.0 μg/m3. The agency proposes revising the annual PM 2.5 standard to a range of 9.0 to 10.0 μg/m3, but is seeking comments on standards ranging from 8.0 to 11.0 μg/m3. For the PM 2.5 primary 24-hour standard, CASAC recommended revising EPA’s current standard of 35 μg/m3 to a range of 25 to 30 μg/m3. EPA proposes to retain the current standard but is seeking comment on the impact of lowering the standard to 25 μg/m3.  Comments will be due 60 days from publication.

Jan. 27, 2023 EPA publishes the proposed rule for PM 2.5 NAAQS in the Federal Register, starting a 60-day public comment period. Comments can be submitted here by Mar. 28, 2023.

May 31, 2023 EPA releases a draft Policy Assessment (PA) for the secondary NAAQS for PM, NOx, and SOx. In developing this PA, EPA relies on the 2020 Integrated Science Assessment (ISA) for the air pollutants along with additional quantitative air quality, exposure, and risk analyses. The draft PA includes several recommendations, including retaining the current secondary NAAQS for PM2.5 at 35 ug/m3, or to lower the 24-hour standard to 25 ug/m3. EPA is accepting public comment on the draft PA till July 31, 2023.

June 13-15, 2023 White House Environmental Justice Advisory Council (WHEJAC) hosts a public meeting to discuss a range of topics, including recommendations to EPA for the reconsideration of PM2.5 and ozone NAAQS.

June 27, 2023 WHEJAC sends a letter to EPA, recommending EPA lower the primary annual PM2.5 NAAQS from 12 ug/m3 to 8 ug/m3, and the  primary 24-hour PM2.5 NAAQS from 25 ug/m3 to 25 ug/m3 for all communities, given the severity of public health impacts. Alternatively, WHEJAC recommends a “novel” approach of lowering the NAAQS for environmental justice communities to address the disparate health impacts of PM2.5 in these communities. WHEJAC also urges EPA to recognize jurisdictions in perpetual non-attainment as violating the civil rights of the residents of those jurisdictions, and take enforcement action accordingly. June 28-29, 2023 CASAC’s NOx, SOx, and PM Secondary NAAQS Panel holds a public meeting for EPA to brief the panel on its draft PA for the review of the secondary NAAQS for PM, NOx, and SOx.

Sept. 22, 2023 EPA sent its final rule reconsidering the PM NAAQS to OMB for interagency review. This is the final step before EPA promulgates the final rule.

Feb. 7, 2024 EPA finalized a rule for PM NAAQS, lowering the primary PM 2.5 annual NAAQS from 12.0 µg/m3 to 9.0 µg/m3. The rule retains the existing primary and secondary PM 2.5 24-hour standard, the secondary PM 2.5 annual standard, and the primary and secondary PM 10 standards. The rule will become effective 60 days after it is published in the federal register.

Mar. 6, 2024 Twenty-six states and industry groups filed petitions asking the District of Columbia to review the final PM NAAQS rule. The court consolidated the cases and directed petitioners to file initial statements of the issues. Kentucky et al., v. EPA et al., No. 24-01050 (D.C. Cir. Mar. 6, 2024); Chamber of Commerce et al., v EPA et al., No. 24-01051 (D.C. Cir. Mar. 6, 2024); Texas et al., v. EPA et al., No. 24-01052 (D.C. Cir. Mar. 6, 2024); Warren Petersen, et al. v. EPA et al., No. 24-01073 (D.C. Cir. Mar. 6, 2024). 

Mar. 27, 2024 Environmental groups filed a motion to intervene in the challenge to the final PM NAAQS rule. Kentucky et al., v. EPA et al., No. 24-01050 (D.C. Cir. Mar. 6, 2024).

Apr. 5, 2024 Sixteen states, the District of Columbia, and New York City filed a motion to intervene in the challenge to the final PM NAAQS rule. Kentucky et al., v. EPA et al., No. 24-01050 (D.C. Cir. Mar. 6, 2024). 

May 14, 2024 The D.C. Circuit issued a briefing schedule, with opening briefs due June 6, 2024 and final briefs due October 15, 2024. Kentucky et al., v. EPA et al., Docket No. 24-01050 (D.C. Cir. Mar. 6, 2024).