## Facts

- EPA regulations for new source performance standards and hazardous air pollutant standards specify “reference test” methods for measuring emissions and assessing compliance.
- EPA adopted the “credible evidence” rule, providing that “any credible evidence,” including non-reference-test data (e.g., calculations, indirect estimates, other measurements), may be used to establish compliance or noncompliance.
- Industry groups and regulated entities petitioned for review, arguing the rule exceeded EPA’s Clean Air Act authority, was adopted without adequate notice-and-comment, and improperly forced changes to State Implementation Plans (SIPs) and Title V permitting programs.
- The court reviewed the rule under the Administrative Procedure Act and Clean Air Act judicial review provisions.

## Issues

1. Whether the Clean Air Act authorizes EPA to allow proof of violations through “any credible evidence,” rather than limiting enforcement to specified reference test methods.
2. Whether the final rule satisfied APA notice-and-comment requirements, including whether it was a logical outgrowth of the proposal.
3. Whether the rule unlawfully operated as a de facto amendment of approved SIPs or state Title V permitting programs without following statutory revision procedures.

## Decision

- The court denied the petition in part and upheld EPA’s authority to adopt a credible evidence approach for enforcement.
- The court rejected the claim that reference test methods are the exclusive means to prove noncompliance.
- The court held that the final rule was a logical outgrowth of the proposal and that notice-and-comment procedures were adequate.
- The court granted the petition in part and vacated (or limited) the rule to the extent it purported to rewrite, override, or require revisions to SIPs or state Title V programs outside the Clean Air Act’s prescribed processes.

## Legal Principles

- The Clean Air Act does not unambiguously restrict enforcement proof to results from specified reference test methods; EPA may permissibly allow “any credible evidence” to establish violations.
- A rule that expands permissible evidentiary bases for proving violations may be treated as an enforcement/evidentiary measure rather than a change to substantive emission limits.
- APA notice is satisfied when the final rule is a logical outgrowth of the proposal and interested parties could reasonably anticipate the final approach.
- EPA may not use a generally applicable federal enforcement rule to self-execute amendments to approved SIP provisions or to compel changes to state implementation or Title V permitting programs without following the Act’s SIP/permit revision and approval procedures.

## Conclusion

The D.C. Circuit upheld EPA’s ability to rely on non-reference-test information as “credible evidence” in Clean Air Act enforcement, but it prevented EPA from using that evidentiary rule as a back-door mechanism to alter state SIPs or Title V programs without the statute’s required revision procedures.


Clean Air Implementation Project v. EPA, 150 F.3d 1200 (D.C. Cir. 1998)