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 (PM2.5) 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.

PM2.5 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.

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