## Facts

- The Clean Air Act (CAA) directs the Environmental Protection Agency (EPA) to develop air quality criteria for listed pollutants and to set National Ambient Air Quality Standards (NAAQS) through regulations.  
- For each pollutant, EPA sets a primary NAAQS to protect public health “with an adequate margin of safety” and a secondary NAAQS to protect public welfare.  
- In 1997, EPA issued final rules revising the NAAQS for ozone and particulate matter (PM), including an updated ozone standard and new or revised PM standards.  
- EPA’s rules reflected the view that exposure to ozone and PM at any nonzero level could pose some health risk, and that risk generally decreases as ambient concentrations decrease.  
- Petitioners—led by American Trucking Associations and joined by other industry groups and some state and local interests—filed consolidated petitions for review in the D.C. Circuit, challenging the 1997 ozone and PM NAAQS.  
- Petitioners argued that EPA failed to articulate a meaningful standard for choosing *how stringent* the NAAQS must be once EPA concluded some risk existed, leaving EPA with effectively unlimited discretion.  
- The dispute also raised how the CAA’s ozone nonattainment provisions applied to implementation of the revised ozone standard, specifically whether EPA could proceed under the general nonattainment provisions (Subpart 1) rather than the ozone-specific scheme (Subpart 2).  
- Petitioners further argued that EPA’s criteria and standards-setting process could not ignore positive, identifiable effects of pollutants in the ambient air.

## Issues

1. Whether EPA’s construction and application of CAA §§ 108–109 in setting the 1997 ozone and PM NAAQS lacked a constitutionally sufficient limiting principle under the nondelegation doctrine.  
2. Whether, in formulating air quality criteria under § 108 and setting NAAQS under § 109, EPA must consider positive, identifiable effects of a pollutant’s presence in ambient air (and not only adverse effects).  
3. Whether areas designated nonattainment under a revised primary ozone NAAQS must be classified and subject to attainment dates under Part D’s ozone-specific Subpart 2, rather than the general provisions of Subpart 1.

## Decision

- The court concluded that EPA’s approach to selecting the level of the ozone and PM NAAQS did not supply an “intelligible principle” that meaningfully limited EPA’s discretion, creating a nondelegation problem as applied.  
- Instead of invalidating the statute on its face, the court remanded the ozone and PM NAAQS to EPA for further explanation or a limiting construction that would channel EPA’s standards-setting judgment.  
- The court held that EPA must consider positive, identifiable effects of a pollutant’s presence in the ambient air when developing air quality criteria and setting NAAQS.  
- On ozone implementation, the court held that the ozone-specific nonattainment regime in Subpart 2 (including classifications and attainment dates) governs areas designated nonattainment under a revised primary ozone NAAQS; EPA could not default to Subpart 1 to obtain greater flexibility.  
- The petitions were granted in part and denied in part, and the matter was remanded for further proceedings consistent with the opinion.

## Legal Principles

- Congress may delegate regulatory authority to an agency only if the statute, as construed and applied, supplies a sufficiently definite standard—an “intelligible principle”—to guide and limit the agency’s discretion.  
- When an agency interprets and applies a broad statutory mandate in a way that leaves no meaningful stopping point for regulatory stringency, the agency must supply a limiting construction or reasoned standard that cabins its own discretion.  
- Under the CAA’s criteria-and-standards structure, EPA’s development of air quality criteria and NAAQS must address relevant effects of pollutants; where there are positive, identifiable effects associated with ambient presence, EPA may not ignore them in its criteria and standards-setting analysis.  
- Where Congress has enacted pollutant-specific implementation requirements within Part D (such as Subpart 2 for ozone), EPA must implement the NAAQS through that specific scheme rather than substituting the general provisions of Subpart 1.

## Conclusion

In reviewing EPA’s 1997 ozone and particulate matter NAAQS, the D.C. Circuit held that EPA’s method for selecting the standards’ stringency did not provide a sufficient limiting principle, remanded the rules for EPA to supply a constraining approach, required EPA to consider positive, identifiable effects of pollutants in developing criteria and NAAQS, and directed that implementation of a revised primary ozone NAAQS proceed under the ozone-specific Subpart 2 nonattainment framework rather than the general Subpart 1 provisions.


American Trucking Associations, Inc. v. Environmental Protection Agency, 175 F.3d 1027 (1999)