June 15, 2022

Ozone Data Available to Reconsider NAAQS

In April 2022, EPA released its Draft Policy Assessment for the Reconsideration of Ozone NAAQS. [87 FR 25485] The document is part of a reconsideration of the 2020 decision on the ozone NAAQS and will “bridge the gap” between the scientific and technical information assessed in the 2020 Integrated Science Assessment for Ozone and Related Photochemical Oxidants (2020 ISA) and any air quality, exposure, and risk analyses available. In essence, it reassesses the policy implications of the 2020 ISA and provides the necessary information to EPA policymakers to decide whether to retain the existing ozone NAAQS. More information on the NAAQS review of ozone is available at Docket ID No. EPA-HQ-OAR-2018-0279.

The 2022 draft policy assessment is over 1,000 pages, much of which is analysis of the relevant scientific data. Four chapters provide this information:

Chapter 1 reviews the CAA statutory requirements for establishing and reviewing NAAQS. The history of the ozone NAAQS is particularly complicated, with a long series of petitions and court challenges. Beginning with the very first ozone review in 1979, the regulated community, interest groups, and courts have raised issues related to the standard being “arbitrary and capricious,” an unconstitutional delegation of legislative authority, considering the positive effects of ozone by blocking ultraviolet radiation, and the standard simply being too stringent.
Chapter 2 provides an overview of ozone and photochemical oxidant chemistry, updated information on emission sources of precursor chemicals, and model-based estimates of ozone resulting from natural and anthropogenic sources. Ozone occurs naturally and is an important part of stratospheric chemistry. But in the troposphere, ozone, the main ingredient of smog, results in numerous respiratory ailments. Though there are few sources of ozone, there are many sources of nitrogen oxides (NO_X), volatile organic compounds (VOCs), methane (CH₄), and carbon monoxide (CO), the main precursors of tropospheric ozone.
Chapter 3 focuses on exposure/risk information and the policy-relevant aspects of the primary, health-based standard. One of the notable changes between the 2020 and previous reviews is the consideration of data regarding at-risk populations, such as people with asthma. Despite the volume of data, uncertainties and areas for future research remain. Included topics are the need for improved modeling, better consideration of long-term ozone exposure, how co-pollutants such as PM and SO₂ contribute to the correlation between ozone concentrations and reported health outcomes, and establishing a more robust ambient modeling network.
Chapter 4 is similar to Chapter 3 but focuses on the secondary, welfare-based standard. Whereas the primary NAAQS focus on public health protection, the secondary NAAQS protect against decreased visibility and damage to animals, crops, vegetation, and buildings. Significant uncertainty remains in how ozone impacts various plant and animal species, let alone how those impacts depend on season and climate.

The draft policy assessment is also being provided to the Clean Air Scientific Advisory Committee (CASAC), which provides independent advice to EPA on the technical aspects of NAAQS. Though the draft policy assessment states it is appropriate to retain the existing primary and secondary standards, the CASAC previously recommended a lower primary limit. The agency is targeting the end of 2023 to complete decision-making in this ozone NAAQS reconsideration.

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