## Facts

- EPA regulates hazardous air pollutants under Clean Air Act (CAA) § 112 through technology-based MACT standards, followed by (1) a residual risk review under § 112(f) and (2) a periodic technology review under § 112(d)(6).
- EPA issued MACT standards for the synthetic organic chemical manufacturing industry (SOCMI), relying on controls such as recovery devices and oxidizers.
- In its § 112(f) residual risk assessment for SOCMI, EPA found that, under existing MACT limits, no individual would face an excess lifetime cancer risk greater than 100-in-one-million, which EPA treated as “presumptively acceptable” under its risk policy.
- In its § 112(d)(6) technology review, EPA concluded there were no relevant technological developments warranting revision and issued a rule leaving MACT standards unchanged.
- NRDC and LEAN petitioned for review, arguing that § 112(f) required tightening limits to ensure maximum individual lifetime cancer risk no greater than one-in-one-million and that EPA failed to satisfy § 112(d)(6)’s review-and-revise obligation.

## Issues

1. Whether CAA § 112(f)’s requirement to protect public health with an “ample margin of safety” compelled EPA to reduce maximum individual residual cancer risk to one-in-one-million rather than permitting EPA to treat 100-in-one-million as “presumptively acceptable.”
2. Whether EPA satisfied CAA § 112(d)(6) by conducting a review and issuing a rule that made no revisions based on a finding of no relevant technological developments.
3. Whether EPA’s interpretations and determinations under § 112(f) and § 112(d)(6) were reasonable under Chevron and adequately explained under the APA’s arbitrary-and-capricious standard.

## Decision

- The court denied the petition for review.
- The court upheld EPA’s § 112(f) residual risk rule, including EPA’s use of a 100-in-one-million maximum individual cancer risk as “presumptively acceptable” under its policy.
- The court upheld EPA’s § 112(d)(6) technology review and EPA’s decision to leave existing MACT standards unchanged.
- The court concluded EPA’s statutory interpretations were permissible and EPA’s technical and policy judgments were sufficiently supported and explained.

## Legal Principles

- Under CAA § 112(f), “ample margin of safety” does not impose a fixed numeric residual-risk ceiling; EPA may apply a reasoned risk framework when deciding whether additional standards are “necessary.”
- Where statutory text does not unambiguously require a specific outcome, EPA’s reasonable construction of CAA § 112(f) is entitled to Chevron deference.
- Under CAA § 112(d)(6), EPA must periodically review MACT standards and revise them only “as necessary”; a reasoned decision not to revise can satisfy the statute.
- Under the APA, EPA must provide a rational explanation grounded in the record for its residual risk and technology findings; courts defer to EPA’s technical judgments when adequately justified.

## Conclusion

The D.C. Circuit sustained EPA’s discretion to apply a non-mandatory risk benchmark in § 112(f) residual risk determinations and held that § 112(d)(6) can be satisfied through a documented review culminating in a supported decision not to revise existing MACT standards.


Nat. Res. Def. Council v. EPA, 529 F.3d 1077 (D.C. Cir. 2008)