The U.S. Environmental Protection Agency (EPA) has announced its decision to retain the current National Ambient Air Quality Standards (NAAQS) for fine particulate matter (PM_2.5) and ozone (O₃) under the Clean Air Act. However, the new Biden EPA is all but certain to reevaluate the standards and likely to reach different conclusions.

PM_2.5 is a mixture of small liquid or solid particles found in the air that are less than 2.5 micrometers (μm) in aerodynamic diameter. O₃ is a reactive gas that is formed through chemical reactions of nitrogen oxides and volatile organic compounds in the atmosphere. Under the CAA, EPA must ensure the ambient standards for both pollutants are established at a level “requisite to protect the public health” with “an adequate margin of safety,” and EPA must review the NAAQS every five years to determine whether the standards should be retained or revised.

In its most recent review of the NAAQS, EPA examined a variety of scientific studies, consulted with the Clean Air Scientific Advisory Committee, and considered nearly 60,000 public comments (relating to the PM_2.5 standard) and 50,000 public comments (relating to the O₃ standard). With respect to the PM_2.5 standard, EPA claimed there were “significant uncertainties” with the scientific evidence supporting revising or strengthening the current PM_2.5 standard. With respect to the O₃ standard, EPA concluded that “newly available health effects evidence does not differ substantially from that available in the last review when the standard was set.” As a result, EPA determined that the current standards for both PM_2.5 and O₃ remain protective of public health and welfare and should be retained without revision.

Critics of these decisions claim EPA has ignored important scientific evidence linking health effects to air pollution. In particular, these groups cite studies that have linked air pollution to impacts associated with COVID-19. Eighteen states have already brought suit challenging EPA’s decision to retain the PM_2.5 standard.

Whether the Biden administration will work to reverse these decisions or simply restart the five-year process of reviewing the standards remains unclear. In issuing its decisions to retain the current standards, EPA made the rules effective immediately, which means the Biden administration may not be able to immediately freeze the decisions like other last-minute rules that are not yet effective. Nevertheless, the new Biden EPA is likely to act quickly to address what is certain to be a high priority for the incoming administration.