D.C. Circuit Upholds Annual PM2.5 NAAQS Revision to 9.0 µg/m³

Posted: July 2nd, 2026

Author: Dan Dix

On June 26, 2026, the U.S. Court of Appeals for the District of Columbia Circuit issued its decision upholding the U.S. Environmental Protection Agency’s (EPA’s) February 2024 rule revising the primary annual PM_2.5 National Ambient Air Quality Standard (NAAQS) from 12.0 micrograms per cubic meter (µg/m³) to 9.0 µg/m³. The court rejected challenges from a coalition of states and industry groups, as well as the U.S. EPA’s own motion to vacate the rule filed in November 2025. In a significant development, U.S. EPA had reversed course midway through the litigation, arguing the rule was unlawful, but the court found those arguments lacked merit.

The Litigation

Industry petitioners and 23 states challenged the 2024 Final Rule on three main grounds:

(1) The U.S. EPA lacked statutory authority to conduct an off-cycle revision,

(2) Environmental justice and climate change considerations improperly influenced the decision, and

(3) The rule was arbitrary and capricious for failing to consider costs and attainability.

After initial briefing and oral argument on December 16, 2024, a new U.S. EPA Administrator was appointed in 2025. In November 2025, that Administrator filed a motion to vacate, joining the Industry Petitioners in arguing the U.S. EPA had exceeded its statutory authority and failed to consider economic costs.

Court’s Holding on Statutory Authority

Senior Circuit Judge Douglas Ginsburg, writing for the panel, held that the U.S. EPA had clear statutory authority to revise the PM_2.5 NAAQS off-cycle without first performing a mandatory “thorough review.” The court’s statutory interpretation turned on the following two Clean Air Act (CAA) requirements:

The Administrator must complete a thorough review of air quality criteria and NAAQS every five years and revise them as appropriate.
The Administrator is permitted to “review and revise criteria or promulgate new standards earlier or more frequently than required under this paragraph.”

The U.S. EPA and petitioners argued that the second requirement implicitly requires a “thorough review” whenever the U.S. EPA revises a standard off-cycle, since both requirements use the word “review.” The court rejected this reading, finding the deliberate omission of the word “thorough” in the second requirement controlling. Under standard canons of statutory construction, when Congress includes language in one provision but omits it in an adjacent provision, courts presume Congress intended a difference in meaning.

In the end this decision means that U.S. EPA may revise a NAAQS off-cycle when new scientific evidence warrants it, without waiting for the five-year cycle and without conducting a comprehensive review of all available information.

Economic Costs and Attainability

The court held that the U.S. EPA was not required to and correctly did not consider economic costs or attainability when deciding to revise the NAAQS. The petitioners attempted to distinguish between (1) the decision whether to revise a NAAQS, to which costs might apply, and (2) setting the numerical level of a NAAQS, to which costs do not apply. The court rejected this two-step approach, finding it would impermissibly inject economic considerations into the NAAQS-setting process regardless of the timing within the cycle.

Impacts to Air Permitting

The ruling leaves the 9.0 µg/m³ annual PM_2.5 NAAQS in effect and forecloses the U.S. EPA’s effort to vacate it. However, area designations under the revised standard have not been issued (the February 7, 2026 deadline was missed). The CAA requires U.S. EPA to finalize initial area designations within two years of promulgating a revised NAAQS, but it also allows U.S. EPA to take up to one additional year if the Administrator determines there is insufficient information to make the designation decisions, which moves the outer deadline from February 7, 2026 to February 7, 2027. U.S. EPA is expected to rely on ambient monitoring data adequacy concerns as the basis for the extension, a rationale that is already being challenged in federal court by states and environmental groups who argue the 2022-2024 design values and the state recommendations submitted back in February 2025 gave U.S. EPA everything it needed to act on time.

It remains to be seen whether a court will give U.S. EPA a deadline to make designations prior to February 2027. Although the 2023-2025 PM_2.5 monitoring data are available, there may not be time to rely upon that data to make designations, especially if some of the 2025 data should be excluded as exceptional events. It will be interesting to see if that data would change any decisions based on 2022-2024 data. The most recent U.S. EPA tools for exceptional events may make the analysis easier.

Until designations are formally issued, projects subject to PSD permitting are in a state of regulatory ambiguity: the 9.0 µg/m³ standard is legally effective, but the nonattainment areas designated under it do not yet exist. This creates both compliance and business risk for projects in areas that will ultimately be nonattainment under the revised standard. Likewise, in areas where the background is very close to the 9.0 µg/m³ annual PM_2.5 NAAQS, it will be difficult to conduct an air dispersion modeling demonstration for major projects, so economic activity could be stifled in attainment areas as well. At a minimum, it could be time to sharpen the pencil on your PM_2.5 emissions inventory and collect some site-specific data.

Bottom Line

The D.C. Circuit’s decision removes a major legal cloud that had hung over the PM_2.5 NAAQS revision since the U.S. EPA stated its desire to reconsider the 2024 rule in March 2025. U.S. EPA now has two options, assuming they do not appeal the decision: initiate a reconsideration process and try to justify why the standard should be higher or initiate its required full 5-year review and use all information available to determine the appropriate level of the NAAQS. It is not likely that either process can be completed prior to the date by which it will be required to make designations under the 2024 standard.

For up-to-date information on the PM_2.5 NAAQS designation process, check out ALL4’s PM_2.5 NAAQS Resource webpage or reach out to Dan Dix at 610.422.1118 or ddix@all4inc.com to discuss how ALL4 can help you with air quality permitting and compliance.