Regulatory Tracker

National Ambient Air Quality Standards (NAAQS)

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On this page we follow EPA’s process of reviewing and deciding whether to strengthen the national ambient air quality standards (NAAQS) for ozone and particulate matter (PM); we monitor litigation and regulatory actions that impact the review and development all NAAQS rules. To follow EPA’s work on the Ozone NAAQS, see our Ozone NAAQS Regulatory Tracker page here and our page tracking 2015 Ozone NAAQS Implementation. For information on the PM NAAQS, see our PM 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

Under the Biden administration, EPA revised the membership requirements of the Clean Air Science Advisory Committee (CASAC), which is an independent expert committee that assists EPA in reviewing the NAAQS. Former CASAC members that were not rehired to the reconstituted CASAC have challenged the new committee in the DC District Court.

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 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%.

The Clean Air Act (CAA) 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

Under the Trump administration, EPA weakened the five-year review of the NAAQS, by eroding the science-based review process and altering the membership requirements of  CASAC. The Biden administration’s EPA has reinstated the CASAC membership requirements and is reviewing the science underlying the 2020 ozone and particulate matter (PM) NAAQS.

On October 7, 2021, former Trump-era CASAC members that were not rehired filed a complaint against EPA in the DC District Court, alleging that the agency violated the Administrative Procedure Act (APA) and Federal Advisory Committee Act (FACA) and that CASAC members have conflicts of interest. The DC District Court, on November 2, 2022, entered partial final judgment in EPA’s favor on the claims related to the validity of the current composition of CASAC. On November 18, 2022, the former CASAC members appealed this partial final judgment to the US Court of Appeals for the DC Circuit, where it is currently docketed (Docket No. 22-05305 (D.C. Cir. Nov 22, 2022)).

Trump Administration
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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 ozone NAAQS by October 2020. Many experts argue that this schedule is incompatible with the thorough scientific review required by the CAA.

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

Dec. 10, 2018 Three current CASAC members urge EPA to reconvene the Ozone Review Panel in their comments on the draft Integrated Review Plan for Ozone NAAQS.

Aug. 22, 2019 EPA publishes the Integrated Review Plan for the Review of the Ozone NAAQS, which includes a timeline projecting that EPA will finalize the Ozone NAAQS in “Winter 2020/2021.” EPA does not include a plan to develop a Risk and Exposure Assessment and instead plans to include the relevant analysis in the Policy Assessment. The Risk and Exposure Assessment is a separate scientific assessment that builds upon the conclusions of the Integrated Science Assessment. EPA has historically made the Risk and Exposure Assessment available for public comment and sought CASAC’s review of the document independently from other assessments.

Dec. 2, 2019 Eighteen former members of the CASAC Ozone Review Panel send a letter to EPA stating that the changes EPA has made to the NAAQS review process  “are collectively harmful to the quality, credibility, and integrity of EPA’s scientific review process and to CASAC as an advisory body.” The letter goes on to state that “[t]he NAAQS review for ozone should be suspended until these deficiencies are corrected.”

Dec. 3-6, 2019 CASAC meets to discuss the Integrated Science Assessment and Policy Assessment for Ozone.  Reviewing the ozone science and policy assessments together decreases the level of review possible by CASAC and departs from the previous long-standing NAAQS process of determining the science before considering policy.

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.

Feb. 12, 2020 CASAC agrees on a final report to Administrator Wheeler recommending no change to the existing ground-level ozone standard. The report also urges EPA to restore the independent ozone review panel and in-person meetings between CASAC and members of the panel.

April 1, 2020 Administrator Wheeler sends a letter to CASAC Chair Tony Cox stating that EPA will stick to its 2020 deadline for the Ozone NAAQS. Wheeler acknowledges that this deadline means many of CASAC’s comments on the Integrated Science Assessment that are “more substantial or cross-cutting” will not be addressed in this review cycle.

Biden Administration
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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 ozone NAAQS rule.

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.

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 APA and 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. 2, 2021 Six states intervening in the New York, et al. v. EPA suit in support of the Trump ozone NAAQS oppose EPA’s request to hold the case in abeyance while it reconsiders the standards. New York et al. v. EPA, No. 21-1028 (D.C. Cir.)

Nov. 5, 2021 Plaintiffs, including fifteen Democratic Attorneys General, agree to hold in abeyance their suit challenging the Trump EPA’s decision not to update the ground-level ozone NAAQS pending the Biden EPA’s revision of the standards. The case will be held in abeyance until the end of 2023 when EPA plans to release the new rule. New York et al. v. EPA, No. 21-1028 (D.C. Cir.) and consolidated cases.

Nov. 15, 2021 EPA issues a public request for nominations for scientific experts to join the CASAC ozone panel as part of the agency’s reconsideration of the ozone NAAQS.

Jan. 21, 2022 EPA seeks public comment on the list of 32 candidates for the CASAC’s ozone review panel. Comments are due by February 11, 2022.

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.).

Feb. 25, 2022 EPA names 18 members to a reconstituted ozone review panel charged with assisting EPA in evaluating whether to tighten the current 70 parts per billion (ppb) standard for ground-level ozone.

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.).

Sept. 30, 2022 The DC District Court denied the plaintiffs’ motion for summary judgment and entered partial judgment for EPA. The Court held that the appointed panel satisfies FACA’s fair balance requirement in meeting the panel’s mandate of being a scientific and technical committee, due to the current membership’s “varied technical backgrounds”. This allows CASAC and the Science Advisory Board, with its current composition, to continue advising EPA on the technical background for NAAQS. This includes the 2020 Ozone Integrated Science Assessment which will form the basis for forthcoming 2020 Ozone NAAQS.

Nov. 2, 2022 The DC District Court enters a partial final judgment, affirming the Sept. 30, 2022 partial summary judgment order in favor of EPA. The Court entered final judgment on the plaintiff’s claims challenging CASAC, and requested the court to stay proceedings for the rest of the claims related to the Science Advisory Board (SAB), which the Court granted. Young et al., v. EPA, et al., No. 21-2623 (D.D.C.)

Nov. 18, 2022 Former Trump-era CASAC members challenge the DC District Court’s partial final judgment in the US Court of Appeals for the DC Circuit. This challenge pertains specifically to the claims of the validity of the current composition of CASAC. S. Young, et al., v. EPA, et al., No. 22-05305 (D.C. Cir.). Dissolution of the current CASAC as a result of this appeal could delay EPA’s timeline for reviewing ozone and PM NAAQS.

Jan. 27, 2023 EPA issues a notice seeking nominations of experts “representing state air pollution control agencies” to serve on CASAC. The Agency  is seeking candidates with expertise related to criteria pollutants. Nominations can be made here, and are due Feb. 27, 2023. EPA will publish the list of nominees for a 21-day comment period before selecting a new Committee member.

Mar. 17, 2023 EPA proposes Draft Guidance that would assist state air agencies with the Clean Air Act requirement to include Contingency Measures (CMs) and control requirements in state SIPs for areas that are in nonattainment with ozone or PM NAAQS. This draft includes the following three additions and revisions to EPA’s CM guidance: i) the method state air agencies should use to calculate the emission reductions from CMs, ii) recommended approach to develop a state air agency’s “infeasibility justification” if an area lacks sufficient CMs to achieve 1-years’ worth of reductions, and iii) an extension  from 1 to 2 years as the recommended time period within which the emission reductions due to CMs should occur, when there are insufficient CMs available to reduce the emissions within 1 year. Comments on the draft guidance are due by Apr. 24, 2023.

Mar. 31, 2023 EPA files its response brief in the US Court of Appeals for the DC Circuit, alleging that former Trump-era CASAC members lack legal standing and that the case is not judicially reviewable. S. Young, et al., v. EPA, et al., No. 22-05305 (D.C. Cir.).

Oct. 23, 2023 EPA proposed to revise its Guideline on Air Quality Models to better assess the background cumulative impacts of multiple sources of air pollution when implementing the NAAQS program.

Oct. 30, 2023. The Center for Biological Diversity,, the San Carlo Apache Tribe, the State of Oregon, and the State of Minnesota issued a notice of intent to sue the EPA pursuant to the CAA for unreasonable delay in responding to the Dec. 2, 2009 petition requesting that the EPA establish national pollution limits for greenhouse gases.

Oct. 30, 2023. The Center for Biological Diversity,, the San Carlo Apache Tribe, the State of Oregon, and the State of Minnesota issued a notice of intent to sue the EPA pursuant to the CAA for unreasonable delay in responding to the Dec. 2, 2009 petition requesting that the EPA establish national pollution limits for greenhouse gases. 

Nov. 3, 2023. EPA issued a notice seeking nominations of scientific experts to serve on the CASAC nitrous oxide panel. The Agency  is seeking candidates with expertise related to criteria pollutants. Nominations can be made here, and are due Nov. 24, 2023. EPA will publish the list of nominees for a 21-day comment period before selecting a new Committee member.

Mar. 19, 2024 CASAC publishes a draft letter requesting a more robust review of the NAAQS. Later CASAC meetings discussing the letter also raise questions about adding additional criteria pollutants to the NAAQS.