Regulatory Tracker

National Ambient Air Quality Standards (NAAQS) for Ozone

Click here to return to our Regulatory Tracker or here to sign up for our monthly Tracker email updates. If you’re a reporter and would like to speak with an expert on this rule, please email us.

On this page we track EPA’s review and implementation of revised ozone NAAQS. To learn about EPA’s implementation of the 2015 ozone standards, EPA’s attainment designations, and challenges to those determinations, click here. 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 particulate matter (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

EPA is currently reviewing the science underlying the 2020 Ozone NAAQS with the aid of the Clean Air Science Advisory Committee (CASAC), an independent expert committee that assists EPA in reviewing the NAAQS. EPA is considering whether to retain the current ozone NAAQS or implement more stringent standards.

Why it Matters

Ground-level ozone, or smog, is formed when pollution from vehicles, power plants, and other industrial sources reacts with sunlight. It can aggravate asthma and cause other respiratory problems, especially in children who are playing outdoors and people with existing lung problems.

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.

Current Status

In December 2020, the Trump EPA issued a final rule refusing to update the NAAQS for ground-level ozone from 2015 standards, despite significant scientific evidence that the current standards are inadequate to protect public health. Multiple coalitions of environmental organizations and states have now sued EPA challenging this rule. On his first day in office, President Biden issued an Executive Order requiring EPA to review the Trump rule. On Oct. 28, 2021, EPA announced it will reconsider the ground-level ozone standard.

EPA Staff recommended retaining the standards for the 2020 Ozone NAAQS, which rely heavily on the Trump-administration’s science. On May 13, 2022, CASAC paused review of the ozone NAAQS to determine whether further discussion of the science underlying this recommendation was needed and is currently considering whether to recommend more stringent ozone standards. In its November 2022 final letter, CASAC formally approves the 2020 Integrated Science Assessment (ISA) under the Trump Administration, as constituting a sufficient scientific basis for reconsideration of the Ozone NAAQS. Importantly, this letter recommends that EPA not reopen or revise the 2020 ISA, and dropped language from the previous draft about considering CASAC’s recommendations on the ISA in addition to the ISA.

Read more

Oct. 25, 2015 EPA concludes its review of the ozone NAAQS and determines that the existing standards inadequately protect public health and welfare. EPA tightens the health-based and welfare-based standards to 0.070 parts per million.

Sep. 14, 2016 Murray Energy sues EPA challenging the new standards. Murray Energy Corp. v. EPA, Case No. 15-1385 (D.C. Cir.).

Read more

April 7, 2017 EPA asks the court to delay the oral argument scheduled for April 19, 2017. EPA states it “intends to closely review the 2015 Rule, and the prior positions taken by the Agency with respect to the 2015 Rule may not necessarily reflect its ultimate conclusions after that review is complete.”

April 11, 2017 The court removes the argument from its calendar and orders EPA to provide status reports every 90 days.

June 28, 2017 EPA publishes a formal notice that it will delay implementation of the ozone standards by one year to October 1, 2018.

July 12, 2017 Environmental groups sue EPA over the delay of the ozone rule. American Lung Association et al v. EPA, Case No. 17-1172 (D.C. Cir.).

Aug. 1, 2017 Fifteen states and the District of Columbia sue EPA over the delay of the rule. New York v. EPA, No. 17-1185 (D.C. Cir.).

Aug. 2, 2017 EPA announces it will walk back the national delay and proceed as originally scheduled.

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.

June 26, 2018 EPA announces the beginning of its next review of the ozone NAAQS by issuing a call for information to assist EPA in developing the review plan and science assessment.

July 3, 2018 The Court of Appeals for the District of Columbia returns the Murray Energy Corp. v. EPA litigation to its active docket. Murray Energy Corp. v. EPA, No. 15-1385 (D.C. Cir.).

Aug. 1, 2018 EPA files a status report with the DC Circuit indicating that “the appropriate EPA officials have reviewed the 2015 [Ozone NAAQS] Rule and have determined that at this time, EPA does not intend to revisit the 2015 Rule.” In this filing, EPA indicates it intends to address background ozone and the “relative contribution of natural and anthropogenic ozone to design values” in its new review of the Ozone NAAQS. “EPA anticipates revisiting both the question of when background concentrations interfere with attainment of the NAAQS and the question of how to consider potential interference with attainment in deciding whether or how to revise the NAAQS.” Murray Energy Corp. v. EPA, No. 15-1385 (D.C. Cir.).

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.

Aug. 23, 2019 The DC Circuit upholds the 2015 primary ozone NAAQS in Murray Energy Corp. v. EPA, No. 15-1385. The panel denies most challenges, sends back secondary standards to the agency for reconsideration, and vacates a provision that grandfathered in permit applications submitted by industry before the 2015 revision that would not comply with the newly revised NAAQS.

Sep. 26, 2019 EPA publishes the draft Integrated Science Assessment for Ozone, which will be discussed at the December 2019 CASAC meeting.

Oct. 31, 2019 EPA releases the draft Policy Assessment for the Ozone NAAQS before CASAC has reviewed and sought revisions to the science assessment. The Policy Assessment finds that the newly available evidence continues to support the current ozone NAAQS. The Policy Assessment is also slated for discussion at the December 2019 CASAC meeting.

Dec. 2, 2019 18 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.

Dec. 6, 2019 CASAC fails to reach consensus regarding the adequacy of the existing ozone NAAQS. Six members endorse retaining the current standards while one member would recommend tightening the standards. CASAC later announces a teleconference on Feb. 11-12 to continue discussing the committee’s recommendations for ozone.

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.

May 31, 2020 EPA publishes the final Policy Assessment for ozone, affirming that the most recent scientific evidence continues to support retaining the current ozone NAAQS. The Policy Assessment projects that EPA will publish a proposed decision on retaining or changing the ozone NAAQS in mid-2020.

July 13, 2020 EPA announces it is proposing to retain the ozone NAAQS. The proposed decision will be open for comment once it is published in the Federal Register.

Dec. 31, 2020 EPA publishes a final rule retaining the current NAAQS for ozone, finding “that the current primary standard is requisite to protect public health, including the health of at-risk populations, with an adequate margin of safety, and should be retained, without revision.” The rule is effective immediately.

Jan. 19, 2021 A coalition of fifteen states, the District of Columbia, and New York City challenge EPA’s decision not to update the ground-level ozone NAAQS. The group alleges that the decision violates EPA’s CAA obligations by conducting a flawed review process that imperils public health. New York et al. v. EPA, No. 21-1028 (D.C. Cir.).

Read more

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.

Feb. 11, 2021 Fourteen public health and environmental groups challenge EPA’s refusal to update the ozone NAAQS, arguing that EPA rushed the scientific review process, and endangered public health. American Academy of Pediatrics, et al., v. EPA, No. 21-01060 (D.C. Cir.).

Feb. 25, 2021 The Center for Biological Diversity files a petition for review in the DC Circuit challenging the ozone NAAQS, claiming EPA failed to consult wildlife experts regarding the rule’s impact on endangered species. The suit is similar to a suit CBD filed regarding the PM NAAQS. Center for Biological Diversity v. EPA, No. 21-1073 (DC Cir.). Read more about the PM NAAQS here.

Feb. 26, 2021 The D.C. Circuit consolidates the three cases challenging EPA’s decision to maintain the ozone NAAQS under Docket No. 21-01060, and holds them in abeyance. The three cases are: American Academy of Pediatrics, et al v. EPA, No. 21-01060 (D.C. Cir.), Center for Biological Div v. EPA, et al, No. 21-01073 (D.C. Cir.), and State of New York, et al v. EPA, et al, No. 21-01028 (D.C. Cir.).

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.

July 22, 2021 EPA’s CAA Advisory Committee (CAAAC) reviews draft recommendations issued as part of a report commemorating the CAA’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 ozone NAAQS review.

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.).

Oct. 28, 2021 EPA announces it will reconsider the Trump EPA’s decision not to strengthen the ground-level ozone NAAQS.

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.

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.

Apr. 28, 2022 EPA Staff releases its draft policy assessment (PA) finding that it is appropriate to retain the primary and secondary ozone NAAQS set in 2015 and retained in 2020 by the Trump administration. The draft PA acknowledges that it “draws heavily on information presented in the 2020 PA.” EPA is accepting comments on the draft PA through May 31, 2022.

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 13, 2022 Chair of the CASAC Ozone Panel posts a memo pausing review of the draft PA before finalization, to determine whether further discussion of the science was needed. Specifically, CASAC determines it needs to discuss concerns raised during the April 29, 2022 meeting regarding EPA’s evaluation of newer studies’ impacts on the 2020 ISA and causal determinations made in the 2020 ISA. CASAC will meet on June 8, 2022 to discuss whether they would like further discussion of the science prior to reviewing the draft PA.

June 10, 2022 In a virtual meeting, CASAC Ozone Panel decides to further review the Trump-era science underlying EPA Staff’s recommendation to retain the 2020 ozone NAAQS. The Panel will conduct this assessment through the summer, which will delay the EPA’s goal of  implementing the new federal standards by the end of 2024.

July 12, 2022 EPA announces upcoming CASAC’s Ozone Review Panel meetings to hear a briefing on the science underlying the draft PA on August 29 and again on September 12, 14, and 16 to discuss the additional information.

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.).

Aug. 30, 2022 The CASAC ozone panel resumes its review of the ozone NAAQS. During these hearings, the panel urges EPA to adopt “secondary” NAAQS to protect the environment for the first time, using distinct metrics.

Oct. 25, 2022 A draft of CASAC’s letter addressed to EPA Administrator Michael Regan indicates that the Integrated Science Assessment (ISA) conducted in 2020 to support the 2020 Ozone NAAQS under the Trump Administration (which retained the 2015 standards), along with CASAC’s advice in the letter, constitutes a sufficient scientific foundation for the Biden Administration’s reconsideration of the Ozone NAAQS. This draft letter also contains critiques of the ISA process overall in determination of ozone limits, but reiterates the panel’s support for secondary standards to protect environmental resources. CASAC will be evaluate the draft letter at a virtual meeting set to be held on Nov. 14-15, before issuing a final letter.

Nov. 14, 2022 Chair of CASAC issues proposed changes to the Oct. 25th draft CASAC letter, clarifying that the 2020 ISA is a “final document” and that CASAC is “not recommending… [it] be reopened and revised.” Environmental organizations support not reopening the 2020 ISA so EPA can finalize and implement the rule expeditiously.

Nov. 22, 2022 CASAC submits a finalized letter sent to EPA Administrator Regan formally approving the 2020 ISA conducted by the Trump EPA. CASAC also finds that the evidence in 2020 ISA supports setting a more robust secondary standard for Ozone.

Jan. 3, 2023 EPA notifies CASAC that the agency is rewriting a draft policy assessment (PA), as a preliminary step for the agency’s reconsideration of the 2020 Ozone NAAQS. This is based on CASAC’s earlier recommendation that the scientific evidence in the 2020 ISA provides adequate basis to review the 2020 Ozone standards, which warrants EPA’s re-writing of the current PA that concludes EPA should retain the 2020 Ozone standards.

Feb. 13, 2023 CASAC’s Ozone Panel announces that on Mar. 2, 2023, EPA will conduct a briefing on a forthcoming draft Policy Assessment (PA) for EPA’s Reconsideration of Ozone NAAQS. CASAC will hold a public meeting on Mar. 29-30, 2023 for the panel to peer review the PA.

Mar. 1, 2023 EPA issues a second draft Policy Assessment (PA) for Reconsideration of the 2015 Ozone NAAQS, reassessing the scientific evidence presented in the 2020 ISA, related air quality, exposure, and risk analyses, and the 2020 PA. EPA staff recommends the agency leaves the 2015 standards unchanged from 70 ppb, finding that the current standards provide “adequa[te]” protection from ozone exposure. EPA’s first periodic review of 2015 Ozone NAAQS in Dec. 2020 also retained the 2015 Ozone NAAQS at 70 ppb.

Mar. 2, 2023 EPA briefs CASAC on the draft Policy Assessment (PA) for the Reconsideration of the 2015 Ozone NAAQS.

Mar. 29-30, 2023 CASAC’s Ozone Review Panel holds its public meeting to peer review EPA’s draft Policy Assessment (PA) for the Reconsideration of the 2015 Ozone NAAQS.

May 2, 2023 CASAC releases a draft report that reviews EPA’s Policy Assessment (PA) for the reconsideration of the ozone NAAQS and expresses concern that EPA’s proposed approach of retaining the 70 ppb standard “substantially underestimate[s] public health risk.” A majority of CASAC members recommend a stricter ozone NAAQS range of 55 to 60 ppb. CASAC also recommends that EPA include epidemiological data and data on the impact of ozone on children and outdoor workers to fully capture the health risks posed to at-risk populations before finalizing the revised ozone NAAQS. CASAC’s ozone panel will hold a public meeting to finalize the draft report on May 23-24, 2023.

June 9, 2023 CASAC sends a letter and final report to EPA, reviewing EPA’s Policy Assessment (PA) in which a majority of CASAC members recommend EPA consider a revised NAAQS range of 55 to 60 ppb. CASAC finds that EPA did not fully utilize the “wealth of information” the agency had in developing the PA, instead relying heavily on controlled human exposure (CHE) and quantitative risk and exposure assessment (REA) data, and not adequate incorporating epidemiological and animal studies, or information about impacts on children, at-risk populations, and outdoor workers.

July 17, 2023 Environmental Groups sue EPA over its decision to accept an “exceptional event” that allows Detroit to maintain its attainment designation. Sierra Club v. EPA, Docket No. 23-3581 (6th Cir. July 17, 2023).

Aug. 18, 2023 EPA sends a letter notifying CASAC that it will initiate a new statutory review of the ozone NAAQS and the underlying air-quality criteria.

Aug. 25, 2023 EPA issues a public notice soliciting scientific information for consideration in developing the Integrated Science Assessment (ISA) for its review of the primary and secondary ozone NAAQS. Specifically, EPA is seeking peer-reviewed scientific literature and research studies that have been published since EPA last collected information for the most recent ISA in 2020. Comments are due by Oct. 24, 2023.

Oct. 6, 2023 Petitioners file a motion to lift abeyance with the D.C. Circuit to restart their case challenging EPA’s 2020 decision to not revise the ozone NAAQS, because EPA recently “informed the parties and the Court that it will take no action regarding the 2020 ozone decision at this time.” New York et al. v. EPA, No. 21-1028 (D.C. Cir. Jan. 19, 2021).

Oct. 12, 2023 The DC Circuit granted petitioners’ motion to lift abeyance and return the consolidated cases to the court’s active docket. New York et al. v. EPA, No. 21-1028 (D.C. Cir. Jan. 19, 2021).

Jan. 3, 2024 EPA filed an unopposed motion for voluntary remand without vacatur, arguing that the agency will institute a new review of the Ozone NAAQS. New York et al. v. EPA, No. 21-1028 (D.C. Cir. Jan. 19, 2021).

May 2, 2024 Industry groups file an amicus brief in the Sixth Circuit case, Sierra Club v. EPA, supporting EPA’s exceptional event determination in Detroit. Sierra Club v. EPA, Docket No. 23-3581 (6th Cir. July 17, 2023).