EPA scraps ozone plans


Aug. 22, 2023

The Environmental Protection Agency has gone back to the drawing board with its plans for the National Ambient Air Quality Standards for Ozone. In a letter to the Clean Air Scientific Advisory Committee, Administrator Michael Regan stated that he was announcing a new review of the ozone standards.

NAAQS and ozone

  • Under the Clean Air Act (CAA), the Environmental Protection Agency (EPA) is required to set National Ambient Air Quality Standards (NAAQS) for six different “criteria air pollutants.” These common pollutants are carbon monoxide, lead, particulate matter (PM), ozone, nitrogen dioxide, and sulfur dioxide. The CAA requires that the EPA periodically review these standards to ensure that each one provides adequate protection for public health and the environment.
  • The Clean Air Scientific Advisory Committee (CASAC) is an independent group of science advisors that provides advice to the EPA Administrator on the technical bases for NAAQS. CASAC also provides advice on other areas including research on air quality and pollution, options to maintain air quality standards, and more. CASAC’s work is generally performed by ad hoc panels that are convened for the specific air pollutants under review but are chaired by chartered members of CASAC.
  • Ozone (O3) is a gas that can be found both in the Earth’s atmosphere (stratospheric ozone) as well as at ground level (tropospheric ozone). Tropospheric ozone is generally created by reactions between nitrogen oxides (NOx) and various volatile organic compounds (VOCs). Tropospheric ozone can be created from emissions from cars, industrial plants, refineries, and other chemicals or sources that react to ultraviolet light or sunlight. Ozone can cause several health effects, including aggravating existing lung diseases and increasing the frequency of asthma attacks. Ozone can also damage the environment by reducing photosynthesis in plants and increasing the risk of disease in certain plant species.
  • On December 31, 2020, the EPA announced that it planned to retain the ozone NAAQS set in 2015. The primary and secondary standards for NAAQS were both set at 70 parts per billion (ppb) for the fourth-highest daily maximum eight-hour concentration, as averaged across three consecutive years (a W126 value).
  • However, the agency reversed course on October 29, 2021, and announced that they would reconsider the agency’s decision to retain those 2015 standards. The agency explained in its decision that tropospheric ozone levels have fallen significantly in the past 20 years, but about 100 counties still measure ozone levels above the 2015 standards. The agency also received numerous legal challenges over the 2020 decision, and the 2020 review was completed without a CASAC Ozone Panel review.
  • CASAC issued a draft review of the EPA’s ozone PA on May 2, 2023. This policy assessment (PA) agreed with some of the decisions the EPA had made in the 2020 decision, but noted significant issues based on potential public health risks from ozone that may have been overlooked in that decision. The Ozone Panel met on May 23 and 24, 2023, to discuss the draft review.
  • On June 9, 2023, CASAC issued its official review of the EPA’s draft ozone PA (note: clicking the link will begin an automatic download of the review). In its response, the panel stated that the PA clearly presented background and historical information on the development of the 2020 decision, which provided context for the reconsideration. CASAC requested that the agency provide additional detail on how CASAC’s comments on the 2020 Integrated Science Assessment (ISA) factored into the 2023 PA, specifically regarding the health effects of ozone. In addition, CASAC recommends that future risk and exposure assessments (REAS) be developed as separate, standalone documents that should be reviewed by CASAC before the development of PAs.

Letter from Administrator Regan

  • On August 18, 2023, EPA Administrator Michael Regan sent a letter (link is a direct download) to CASAC announcing that the EPA would not continue with the reconsideration, and would instead initiate a new statutory review of the ozone NAAQS and the underlying air quality criteria and officially end the 2020 ozone NAAQS review.
  • The letter keys in on some of the administrator’s reasoning for restarting the process. The letter cites CASAC’s comments explaining that during the reconsideration, the committee identified several issues arising in the reconsideration that warranted additional review. Administrator Regan opined that restarting the process was the correct way forward as opposed to continuing with the reconsideration. Regan stated that he remains “committed to upholding the integrity and rigor of the NAAQS review process and to addressing the issues that CASAC has raised.”
  • The new approach will focus on wrapping the reconsideration process into the future review and add additional research and work to address advice provided by CASAC on previous reviews. The agency plans to consider new studies and updated analyses as part of a larger “full and systematic” update of the air quality criteria and standards.

Next steps

  • The agency will now conduct a review of the ozone NAAQS, as well as the underlying air quality criteria. Realistically, this will take several years and will stretch far beyond the next election cycle. If the administration changes in the next election cycle, this new review could potentially face issues like the 2020 review if administration priorities change. The agency will host a public science and policy workshop for the NAAQS in Spring 2024 and will summarize those proceedings in Summer 2024. In fall 2024, the EPA plans to release its Integrated Review Plan, Volume 2 to help guide CASAC feedback and the future of the ISA.
  • The agency will issue a call for information on the new version of the ozone ISA in the Federal Register. In a pre-publication notice, the agency announced that the ISA will review the air quality criteria and the primary (health-based) and secondary (welfare-based) NAAQS for O3 and related photochemical oxidants. The agency is specifically interested in newly available information, particularly concerning toxicological studies of effects of controlled exposure to O3 in laboratory animals, humans, and in vitro systems; epidemiologic (observational) studies of health effects associated with ambient O3 exposures in human populations; studies examining populations and life stages that may be at increased risk of O3-related health effects.

Featuring previous research by Walker Livingston.

To contact the author of this analysis, please email Walker Livingston.
To contact the editor of this analysis, please email Patricia Iscaro.

Key Documents and Dates


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