The three-judge panel upheld the 2024 National Ambient Air Quality Standards (NAAQS) for fine particulate matter (PM2.5), which tightened the annual limit from 12 to 9 micrograms per cubic meter. The Biden administration finalized that rule in February 2024. After taking office, the Trump administration reversed course and asked the court to strike down the standard on procedural grounds, arguing the EPA had not conducted a thorough review as required by the Clean Air Act before revising the limits. The court rejected that argument. EPA said it is reviewing the decision.
The ruling keeps a contested regulatory standard in place, but implementation remains unresolved. Under the Clean Air Act, EPA was required to designate areas not in compliance with the 2024 standard by February 6, 2026. The agency missed that deadline. A coalition of health, environmental, and community groups filed suit in April 2026 to compel the designations, as did a separate state coalition led by California. Until nonattainment areas are formally identified, the compliance obligations that flow from that designation, including state implementation plans and targeted pollution reduction requirements for sources in those areas, cannot begin.
For EHS and compliance teams at facilities that emit PM2.5, the ruling clears one source of uncertainty. The 9 microgram standard is now legally intact, and further challenges to the standard itself are unlikely to succeed on the same grounds the Trump EPA raised. The more immediate compliance question is when EPA will complete the nonattainment designations and what geographic areas will be included. A recent white paper cited by Earthjustice estimated that 75 million people live in counties whose air quality violates the 2024 standard, a figure that, if reflected in formal designations, would create compliance obligations for a substantial number of industrial sources.
Industry groups were quick to flag concern. The American Iron and Steel Institute, whose president Kevin Dempsey called the ruling a setback, argued that much of U.S. steel production could face nonattainment exposure under the 9 microgram threshold and that EPA should develop a more targeted standard. By EPA's own estimate, implementing the 2024 rule would prevent 4,500 premature deaths, 800,000 asthma symptom cases, and 2,000 hospitalizations by 2032, with net public health benefits potentially reaching $46 billion. The agency said Friday it is reviewing the decision and has not indicated whether it will seek further review or accelerate nonattainment designations.
