## Facts

- EPA issued regulations under Clean Air Act § 202 setting emission standards for particulate matter and oxides of nitrogen (NOx) from diesel-powered light-duty vehicles and light-duty trucks for the 1982 and 1985 model years.
- The particulate standards included limits of 0.60 grams per mile for 1982 models and tighter 1985 limits (including 0.20 gpm for light-duty vehicles and 0.26 gpm for light-duty trucks).
- EPA’s feasibility determination relied substantially on expected development and deployment of “trap-oxidizer” aftertreatment devices, along with engine modifications, to control diesel particulate emissions.
- Environmental petitioners (NRDC) challenged the standards as insufficiently protective of public health.
- Industry petitioners (including General Motors and other manufacturers) challenged the standards as overly stringent and not technologically feasible on the required timeline, and argued EPA gave inadequate consideration to safety and reliability concerns associated with trap-oxidizers.
- Multiple petitions for review were consolidated in the D.C. Circuit.

## Issues

1. Whether EPA’s diesel particulate and NOx standards were supported by a rational, adequately explained determination of technological feasibility under Clean Air Act § 202 and were not arbitrary or capricious.
2. Whether EPA unlawfully set particulate standards that were too lenient in light of Clean Air Act objectives to protect public health and welfare.
3. Whether EPA inadequately evaluated safety and engineering concerns (including reliability and fire-risk issues) related to trap-oxidizer technology, making the standards unlawful.

## Decision

- The court denied the petitions for review and upheld the challenged regulations in their entirety.
- The court held that EPA provided adequate reasons for its choices and acted consistently with Clean Air Act § 202.
- The court accepted EPA’s predictive judgment that compliance technology (including trap-oxidizers) would be feasible by the relevant model years, given EPA’s explanation of remaining development steps and the basis for expecting timely solutions.
- The court rejected NRDC’s claim that the standards had to be more stringent, concluding EPA permissibly grounded the standards in technology-based feasibility judgments.
- The court rejected industry claims that safety and engineering concerns were insufficiently considered, finding EPA’s discussion and approach adequate under deferential review of technical judgments.

## Legal Principles

- Emission standards set under Clean Air Act § 202(a)(1) are technology-based and must be premised on a reasoned finding of technological feasibility.
- In reviewing EPA’s predictive technical determinations, a court asks whether EPA identified the major remaining development steps and gave plausible reasons for expecting industry to solve them within the available time, rather than requiring EPA to supply detailed engineering solutions.
- Under arbitrary-and-capricious review, EPA must address significant safety and technical concerns and provide a rational explanation, but need not eliminate all uncertainty before setting forward-looking standards.
- Courts will not substitute their judgment for EPA’s on complex technical matters when the agency’s explanation is adequate and consistent with statutory criteria.

## Conclusion

The D.C. Circuit upheld EPA’s diesel particulate and NOx standards against both environmental and industry challenges, holding that EPA reasonably explained its technology-feasibility and safety assessments and acted within Clean Air Act § 202’s technology-based framework.


Nat. Res. Def. Council, Inc. v. EPA, 655 F.2d 318 (D.C. Cir. 1981)