## Facts

- The Clean Air Act requires EPA to set and periodically review primary national ambient air quality standards (NAAQS) at levels “requisite to protect the public health” with an “adequate margin of safety,” based on scientific air quality criteria.
- In reviewing the primary sulfur dioxide (SO₂) standards, EPA considered whether to address short-term, high-concentration SO₂ “bursts,” including a proposed five-minute standard of 0.60 ppm.
- The administrative record included exposure evidence that a subset of asthmatics experiences substantial adverse effects from short-term SO₂ bursts, including chest tightness, wheezing, shortness of breath, and decreased pulmonary function, sometimes prompting increased medication, altered activity, or medical care.
- EPA’s analysis implied that thousands of asthmatics could be expected to have atypical reactions to SO₂ bursts each year.
- The record indicated at least six communities with repeated five-minute peaks above 0.60 ppm, and repeated exposure was not limited to those locations.
- EPA ultimately declined to revise the primary SO₂ NAAQS to add a five-minute standard or otherwise tighten the standards to address short-term peaks.
- EPA justified its decision largely by stating that SO₂ bursts were “localized, infrequent, and site-specific,” and therefore did not amount to a “public health problem.”
- American Lung Association, two asthmatic members, and Environmental Defense Fund petitioned for review; industry intervenors supported EPA.

## Issues

1. Whether EPA’s refusal to revise the primary SO₂ NAAQS to address short-term SO₂ bursts was arbitrary and capricious under the Administrative Procedure Act as applied through the Clean Air Act.
2. Whether EPA adequately explained, in light of the record evidence, why short-term SO₂ bursts causing adverse effects in asthmatics were not a “public health problem” under the Clean Air Act’s primary NAAQS standard.

## Decision

- The court upheld EPA’s technical interpretation of the exposure studies as within the agency’s discretion and entitled to deference.
- The court held that EPA failed to provide an adequate explanation connecting the record evidence (including repeated peaks and effects on thousands of asthmatics) to EPA’s conclusion that the bursts did not present a public health problem.
- The court found EPA’s “localized, infrequent, and site-specific” characterization insufficient without a reasoned account of why such conditions could not constitute a public health problem under the Clean Air Act.
- The court remanded to EPA for further explanation; it did not order EPA to promulgate a five-minute SO₂ standard.

## Legal Principles

- Courts defer to agency scientific and technical judgments, but deference does not excuse the agency from reasoned decision-making.
- An agency must articulate a discernible standard and provide a plausible, record-based explanation that permits judicial review under the arbitrary-and-capricious standard.
- Under the Clean Air Act, EPA has discretion in making public-health judgments and selecting an “adequate margin of safety,” but must explain how it applies those judgments to evidence of identifiable health effects, including effects on sensitive subpopulations.
- A remand may be required where the agency’s stated rationale is too conclusory to allow a reviewing court to evaluate whether the action is arbitrary, capricious, an abuse of discretion, or otherwise unlawful.

## Conclusion

The D.C. Circuit remanded EPA’s decision to retain the existing primary SO₂ NAAQS because EPA did not adequately explain why short-term SO₂ bursts causing documented adverse effects for asthmatics—including repeated peaks in multiple communities and impacts on thousands of individuals—did not amount to a public health problem under the Clean Air Act.


Am. Lung Ass’n v. EPA, 134 F.3d 388 (D.C. Cir. 1998)