The U.S. Court of Appeals for the D.C. Circuit has issued a significant ruling in ongoing litigation over the national ambient air quality standard (NAAQS) for fine particulate matter (PM2.5), denying challenges to the Biden administration’s revised standard and rejecting EPA’s request to vacate the rule.
Background
The Biden EPA tightened the annual PM2.5 NAAQS from 12 μg/m³ to 9 μg/m³ in early 2024, the first revision to the standard in over a decade. The rule faced legal challenges from industry groups and states almost immediately, arguing that EPA had overstepped its authority or failed to justify the more stringent limit based on available scientific evidence.
Separately, EPA itself had sought vacatur of the standard after the change in administration, creating a procedurally unusual situation in which the agency was asking the court to undo its own rule.
The Court’s Decision
The D.C. Circuit denied both the industry challenges to the rule and EPA’s request for vacatur, leaving the Biden-era 9 μg/m³ standard intact. On the merits of the underlying rule, the court found that the former EPA Administrator had adequately reasoned through the scientific evidence supporting the revised standard. In upholding the agency’s decision, the court stated that the Administrator “offered reasoned explanations for how [he] approached and weighed the evidence, and why the scientific evidence supported [his] revision of the NAAQS,” which the court found sufficient to uphold the rule on its merits.
By denying EPA’s vacatur request, the court rejected the current administration’s effort to roll back the tighter standard through litigation rather than the standard notice-and-comment rulemaking process.
What Comes Next
The ruling is widely expected to be appealed. With the D.C. Circuit having now ruled against both industry challengers and EPA on the vacatur question, the likely next step is an appeal to the U.S. Supreme Court, making this the final court of review for this litigation.
In the meantime, the 9 μg/m³ annual PM2.5 standard remains in effect. Regulated entities, state and local air agencies, and permitting authorities should continue to plan accordingly, as implementation timelines for nonattainment designations and associated regulatory obligations continue to move forward absent further court action.
Implications for Industry
The decision has broad implications for industries subject to air quality permitting and attainment planning requirements, including steel, manufacturing, energy, and transportation. Areas currently designated nonattainment under the 9 μg/m³ standard, or at risk of future designation, face continued pressure to reduce emissions to meet the tighter limit.
Those with ongoing permitting actions, state implementation plan obligations, or nonattainment area compliance timelines should monitor further developments closely, particularly any Supreme Court appeal.
If you have any questions about the ruling, please reach out to your local Trinity Consultants office.
