## Facts

- The Clean Air Act (CAA) requires EPA to set primary national ambient air quality standards (NAAQS) “requisite to protect the public health” with an “adequate margin of safety,” and secondary NAAQS “requisite to protect the public welfare.”
- In 1997, EPA revised the NAAQS for particulate matter (PM) (including adopting a fine-particle PM₂.₅ standard) and for ozone (replacing a 1-hour standard with an 8-hour standard at 0.08 ppm).
- EPA based the revisions on a scientific record including epidemiological and toxicological studies and reviews by the Clean Air Scientific Advisory Committee (CASAC).
- Industry and state petitioners challenged the revised PM and ozone standards under the Administrative Procedure Act (APA), asserting that EPA’s scientific and policy judgments were inadequately explained and unsupported by the record.
- The Supreme Court previously rejected a nondelegation challenge to CAA § 109(b)(1), held EPA may not consider costs when setting NAAQS, and remanded for further proceedings on remaining challenges and certain implementation questions.

## Issues

1. Whether EPA’s revised PM NAAQS were arbitrary and capricious because EPA did not identify a no-risk “threshold” concentration and allegedly failed to account for interactions with other pollutants and climate conditions.
2. Whether EPA’s revised primary ozone NAAQS (8-hour, 0.08 ppm) were arbitrary and capricious because EPA allegedly failed to show greater health protection than the prior 1-hour standard and selected 0.08 ppm as an unexplained compromise.
3. Whether EPA’s secondary (welfare-based) ozone standard was arbitrary in addressing vegetation and crop effects in light of confounding non-ozone variables.
4. What further action, if any, EPA had to take on ozone implementation provisions (including the relationship between subpart 1 and subpart 2 of Part D) consistent with controlling decisions.

## Decision

- The court denied the petitions for review challenging the revised PM NAAQS and the revised primary ozone NAAQS, holding the standards were not arbitrary or capricious under the APA.
- The court upheld EPA’s approach of setting health-based standards without first identifying a pollutant concentration at which there is no risk, where the science did not establish a clear threshold.
- The court upheld EPA’s selection of an 8-hour ozone standard at 0.08 ppm as adequately explained and supported by the record, including CASAC input and studies of longer-duration exposure.
- The court upheld EPA’s secondary ozone standard against challenges that EPA did not fully isolate ozone-caused welfare effects from other variables, concluding EPA reasonably explained its conclusions from the record.
- The court required only limited further EPA action to the extent compelled by prior controlling rulings on implementation, without disturbing the core numerical NAAQS.

## Legal Principles

- Under APA arbitrary-and-capricious review, courts generally defer to EPA’s scientific judgments when the agency explains its reasoning and connects it to the record and statutory criteria.
- CAA § 109(b)(1) permits EPA to set primary NAAQS based on public-health protection with an “adequate margin of safety,” even when science does not identify a zero-risk threshold.
- EPA may choose among scientifically supportable options within the statutory health-based framework and is not required to prove a single “most protective” metric so long as the agency provides a reasoned explanation.
- For secondary (welfare) NAAQS, EPA may rely on studies of anticipated adverse effects on welfare interests (including vegetation and crops) without perfectly disentangling every confounding factor, provided its inferences are reasonably explained.
- After Supreme Court guidance, remaining implementation questions must be addressed consistently with controlling interpretations of the CAA’s implementation provisions, but such issues do not necessarily undermine the validity of the NAAQS levels themselves.

## Conclusion

The D.C. Circuit upheld EPA’s 1997 PM and ozone NAAQS against remaining APA challenges on remand, concluding EPA reasonably exercised its health- and welfare-based authority under CAA § 109 and adequately explained its scientific judgments, while leaving only limited implementation-related follow-up for EPA as required by prior controlling rulings.


Am. Trucking Ass’ns, Inc. v. EPA, 283 F.3d 355 (D.C. Cir. 2002)