## Facts

- The Clean Air Amendments of 1970 required EPA to set national ambient air quality standards (NAAQS) and required each state to submit a State Implementation Plan (SIP) to attain and maintain those standards.
- Georgia submitted a SIP containing generally applicable emission limitations and a variance procedure allowing certain sources source-specific relief while maintaining overall compliance with NAAQS.
- EPA approved Georgia’s SIP, treating source-specific variances as permissible SIP “revisions” under Clean Air Act § 110(a)(3), so long as the plan continued to satisfy § 110(a)(2).
- NRDC sought review in the Fifth Circuit, arguing that relief for individual sources could be granted only through the stricter “postponement” mechanism in § 110(f), not through § 110(a)(3) revisions.
- The Fifth Circuit agreed with NRDC and ordered EPA to disapprove Georgia’s variance provision.
- The Supreme Court granted certiorari to review whether EPA’s approval was authorized under the Clean Air Act.

## Issues

1. Whether EPA may approve state-created, source-specific variances from SIP emission limitations as “revisions” under § 110(a)(3), rather than only as “postponements” under § 110(f).
2. Whether a court may reject EPA’s interpretation of the Clean Air Act when the agency’s reading is reasonable.

## Decision

- The Supreme Court reversed the Fifth Circuit.
- The Court held that § 110(a)(3) permits EPA to approve SIP revisions, including source-specific variances, if the revised plan continues to meet § 110(a)(2) and ensures timely attainment and maintenance of NAAQS.
- The Court held that § 110(f) postponements are not the exclusive means for accommodating individual sources.
- The Court concluded EPA’s interpretation was sufficiently reasonable that the court of appeals could not substitute its own construction for the agency’s.

## Legal Principles

- The Clean Air Act establishes national air-quality goals while leaving states discretion to select the mix of emission limitations and enforcement tools, so long as the ultimate effect is compliance with NAAQS.
- EPA must approve SIPs (and SIP revisions) that satisfy the statutory criteria in § 110(a)(2).
- Under § 110(a)(3), EPA may approve SIP “revisions” broadly, including source-specific adjustments, if the revised SIP continues to satisfy § 110(a)(2).
- Section 110(f) creates a separate, limited postponement mechanism with strict prerequisites and does not displace § 110(a)(3)’s revision authority.
- Reviewing courts should defer to an administering agency’s reasonable interpretation of an ambiguous statutory scheme and may not replace it with their own preferred reading.

## Conclusion

The Court upheld EPA’s authority to approve Georgia’s SIP variance mechanism as a § 110(a)(3) revision, rejecting the view that § 110(f) postponements are the sole route for source-specific relief, and it required judicial deference to EPA’s reasonable statutory interpretation within the Clean Air Act’s cooperative federal–state framework.


Train v. Nat. Res. Def. Council, Inc., 421 U.S. 60 (1975)