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Why it Matters
EPA sets National Ambient Air Quality Standards (NAAQS) for six common and harmful pollutants: carbon monoxide, lead, particulate matter (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 in the past 40 years, total emissions of these six pollutants dropped by 71% while the economy grew by 182%.
Particulate matter 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.
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 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.
For more information on the implementation of the 2015 ozone standards, EPA’s attainment designations, and challenges to those determinations, see our Regulatory Tracker page, 2015 Ozone National Ambient Air Quality Standards.
In December 2020, the Trump EPA issued two final rules refusing to update the NAAQS for PM and ground-level ozone, 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 both rules. On his first day in office, President Biden issued an Executive Order requiring EPA to review the Trump rules. On June 10, 2021, EPA announced it was beginning the process of reconsidering the Trump EPA’s decision not to strengthen PM standards and expects to issue a proposed rule by Summer 2022. On Oct. 28, 2021, EPA announced it will also reconsider the ground-level ozone standard.
OBAMA AdministrationRead more
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.
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.)
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.
Trump AdministrationRead 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 6, 2017 Administrator Pruitt announces EPA will delay implementation of the new ozone standards by one year.Oct. 31, 2017 Administrator Pruitt issues a memo changing the membership requirements for the Clean Air Science Advisory Committee (CASAC). CASAC is an independent expert committee that assists EPA in reviewing the NAAQS. Under the new guidelines, scientists who have received grants from EPA are no longer eligible to serve on the committee.
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.
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 and 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.
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 and 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.
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. 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.
Sep. 26, 2019 EPA publishes the draft Integrated Science Assessment for Ozone, which will be discussed at the December 2019 CASAC meeting.
Oct. 22, 2019 The Independent Particulate Matter Review Panel, which comprises 18 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.
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.
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. 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 as well as the Policy Assessment for PM. 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. 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. 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.
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.
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.
March 4, 2020 EPA sends its proposed decision on whether to maintain the Particulate Matter NAAQS to the Office of Management and Budget for review. This is a required step prior to publishing the guidance.
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’s published in the Federal Register.
Nov. 4, 2020 EPA submits its final rule to maintain the PM NAAQS to the Office of Management and Budget (OMB) for final review. This is the last step in the review process before EPA publishes the rule in the Federal Register and it becomes effective.
Dec. 7, 2020 EPA issues a final rule refusing to update the primary and secondary NAAQS for particulate matter (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.”
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. 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 coalition of 15 states, the District of Columbia, and New York City challenges EPA’s decision not to update the ground-level ozone NAAQS. The group alleges that the decision violates EPA’s Clean Air Act obligations by conducting a flawed review process that imperils public health. New York et al. v. EPA, No. 21-1028 (D.C. Cir.).
Jan. 19, 2021 A group of 11 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.).
Biden AdministrationRead more
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, [Docket No. Pending] (D.C. Cir.).
Feb. 11, 2021 14 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, [Docket No. Pending] (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.
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.).
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 ozone and PM NAAQS reviews.
Aug. 31, 2021 EPA announces the 22 members of the reconstituted CASAC PM Panel.
Sep. 9, 2021 In a court filing, EPA says it will decide whether to pursue reconsideration of the Trump administration’s ozone NAAQS decision by Oct. 22.
Sep. 22, 2021 EPA announces two public meetings of the CASAC’s PM Panel to review updated research on the health and environmental effects of PM exposure.
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. 28, 2021 EPA announces it will also 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 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.
Nov. 15, 2021 EPA issues a public request for nominations for scientific experts to join the Clean Air Scientific Advisory Committee (CASAC) ozone panel as part of the agency’s reconsideration of the ozone NAAQS.
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.