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

March 12, 2025 EPA announced a reconsideration of NAAQS.

Feb. 25, 2025 The DC Circuit granted a motion to hold the case in abeyance for 60 days to allow new agency leadership to review the underlying rule. Kentucky et al., v. EPA et al., No. 24-01050 (D.C. Cir. Mar. 6, 2024).

Dec. 16, 2024 Oral arguments were held before the DC Circuit on the challenge to the final PM NAAQS rule. Kentucky et al., v. EPA et al., No. 24-01050 (D.C. Cir. Mar. 6, 2024).

April 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). 

March 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).

March 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). 

March 6, 2024 EPA finalized stronger air quality standards to protect against fine particulate matter (PM) exposure, both an annual standard and a 24-hour standard. States and industry groups have challenged the new standards and oral arguments before the DC Circuit were held on Dec. 16, 2024. 

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.

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.

June 27, 2023 WHEJAC sent 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 held 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.

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

May 31, 2023 EPA released a draft Policy Assessment (PA) for the secondary NAAQS for PM, NOx, and SOx. In developing this PA, EPA relied on the 2020 Integrated Science Assessment (ISA) for the air pollutants along with additional quantitative air quality, exposure, and risk analyses. The draft PA included 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 accepted public comment on the draft PA till July 31, 2023.

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

Jan. 6, 2023 EPA proposed tightening the PM 2.5 annual NAAQS, but retaining the primary 24-hour NAAQS without revision. EPA’s PM 2.5 annual standard is 12 μg/m3 and CASAC recommended a range of 8.0 to 10.0 μg/m3. The agency proposed 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 proposed 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.

Aug. 17, 2022 Young filed 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.).

July 5, 2022 EPA published 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.).

June 3, 2022 The Environmental Defense Fund released 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.

May 31, 2022 EPA staff released 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 recommended retaining the short-term exposure threshold with its level of 35 ug/m3.

May 12, 2022 EPA added 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 established the process for adding new areas required to submit plans.  

April 21, 2022 The Association of Air Pollution Control Agencies released its annual State Air Trends and Successes report and the American Lung Association released 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.

April 13, 2022 Environmental groups sued to require EPA to set a deadline to assess secondary standards, which were 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.)

March 18, 2022 The CASAC’s PM Review Panel sent a letter to EPA Administrator Regan stating that the panel agreed that the current annual standard for PM2.5 did 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.

Feb. 16, 2022 The court denied 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.).

Feb. 4, 2022 EPA’s CASAC panel on PM released a draft report recommending tougher limits on fine PM (PM2.5), bolstering Administrator Regan’s efforts to tighten the PM NAAQS. Specifically, the CASAC recommended 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.

Dec. 23, 2021 EPA revived 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.

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.

Nov. 15, 2021 Members of the CASAC PM Panel posted 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.

Oct. 7, 2021 S. Stanley Young, a former member of the SAB who was not rehired when the panel was reconstituted, filed 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 alleged 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.).

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

Sep. 30, 2021 EPA released 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.

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

July 22, 2021 EPA’s Clean Air Act Advisory Committee (CAAAC) reviewed draft recommendations issued as part of a report commemorating the Clean Air Act’s 50th Anniversary. The report included 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.

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

March 31, 2021 Administrator Regan announced 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.

Feb. 16, 2021 A coalition of states petitioned 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 submited a similar petition.

Feb. 9, 2021 The Center for Biological Diversity (CBD) sued 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.).

Jan. 20, 2021 President Biden issued 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 planned to review the PM NAAQS rule.

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

Oct. 10, 2018 EPA issued 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 reported 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.

May 9, 2018 Administrator Scott Pruitt signed a Memorandum, Back-to-Basics Process for Reviewing National Ambient Air Quality Standards, which modified and accelerated 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.

April 30, 2018 EPA published 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 received nearly 600,000 comments on the proposed rule before the comment period closes.

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

Oct. 31, 2017 Administrator Pruitt issued a memo changing the membership requirements for CASAC. Under the new guidelines, receiving an EPA grant was deemed a “conflict of interest.”  Scientists who have received grants from EPA were 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.

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

Dec. 3, 2014 EPA announced 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.

Jan. 15, 2013 EPA concluded 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.