Under the Clean Air Act, EPA regulates six criteria pollutants through the National Ambient Air Quality Standards (“NAAQS”) program. EPA sets primary and secondary NAAQS that states must meet through regulation of stationary and mobile sources. The Clean Air Act requires EPA to review the NAAQS at least every five years and revise the standards as appropriate. Reviewing and revising the NAAQS has become politicized, as the level at which NAAQS are set has significant economic impacts and affects, for example, if and how new sources can be constructed and whether and to what extent existing sources can be expanded or modified.

            The non-discretionary duty of the EPA Administrator to review the NAAQS is frequently the subject of citizen suits seeking to compel the Administrator do so on the five-year interval set forth in the Clean Air Act, which has often slipped behind schedule. Under the current Trump Administration, however, this typical trend has not played out. Although EPA is behind on its five-year review of several NAAQS, citizen suits seeking to compel the Agency to complete that review are noticeably absent from the courts, as environmental groups seek to avoid an outcome of that review that they consider unfavorable. Below, we provide an update on EPA’s review of the NAAQS and ongoing litigation regarding the NAAQS, including changes in EPA’s position on its defense of Biden-era standards.

Particulate Matter NAAQS

            In February 2024, EPA lowered the fine particulate matter (“PM_2.5”) NAAQS to 9.0 micrograms per cubic meter (“µg/m³”). After setting the new standard, EPA had a non-discretionary duty under the Clean Air Act to make attainment designations within two years, which the Agency has not yet done. On April 13, 2026, several groups filed suit against Administrator Zeldin in the United States District Court for the Northern District of California in Alliance of Nurses for Healthy Environments et al v. Zeldin, seeking a court order for EPA to make attainment designations within 150 days from the date of the suit.

            Shortly after the 2024 PM_2.5 NAAQS were promulgated, they were challenged by a group of states lead by Kentucky, and oral argument was held in the U.S. Court of Appeals for the D.C. Circuit in December 2024. Following the change in administration, EPA changed its position and is no longer defending the rule. EPA is now seeking judicial vacatur of the rule, which would bring the 2012 PM_2.5 NAAQS (which were retained in 2020) back into effect. If the D.C. Circuit grants EPA’s motion to vacate the rule, the suit seeking to compel EPA to make attainment designations will become moot. Therefore, the Commonwealth of Kentucky vs. EPA case remains the one to watch.

Ozone NAAQS

            The ozone NAAQS were last revised in 2015 during the Obama Administration. In 2020, EPA reviewed and retained the 2015 ozone NAAQS. EPA’s review of the ozone NAAQS was due in 2025, but the Agency has not yet issued an Integrated Review Plan that details the scope and schedule of its review, indicating that it remains at a nascent stage. Notwithstanding that the ozone NAAQS review is overdue, to date, no case has been filed that seeks to compel the Administrator to undertake his non-discretionary duty under the Clean Air Act to review the 2015 ozone NAAQS and revise them if appropriate.

Sulfur Dioxide NAAQS

            EPA last reviewed the sulfur dioxide (“SO₂”) primary NAAQS in 2019, when the 2010 standard was retained without revision. EPA’s review of the SO₂ NAAQS is overdue, but no suit is currently pending that would compel the EPA Administrator to undertake such review.

Nitrogen Oxides NAAQS

            EPA last reviewed the nitrogen oxides (“NO_x”) primary NAAQS in 2018, when the 2010 standard was retained without revision. Although EPA’s review of the NO_x NAAQS is overdue, no suit is currently pending that would compel the EPA Administrator to undertake such review.

Lead NAAQS

            EPA last reviewed the lead NAAQS in 2016, when the 2008 standard was retained without revision. EPA’s review of the lead NAAQS is overdue, but no suit is currently pending that would compel the EPA Administrator to undertake such review.

Carbon Monoxide NAAQS

            EPA set NAAQS for carbon monoxide in 1971, which have been reviewed and retained three times, most recently in 2011. EPA’s review of the carbon monoxide NAAQS is well overdue, but again, no suit is currently pending that would compel the EPA Administrator to undertake such review.

            Continue to follow our blog for updates on NAAQS litigation and regulatory actions.