## Facts

- Clean Air Act (CAA) §112 requires EPA to set hazardous air pollutant standards for major sources based on maximum achievable control technology (MACT) and the “maximum degree of reduction in emissions,” subject to a minimum “MACT floor.”
- CAA §302(k) defines an “emission limitation” or “emission standard” as a requirement that limits the quantity, rate, or concentration of emissions “on a continuous basis.”
- EPA issued National Emission Standards for Hazardous Air Pollutants (NESHAP) General Provisions that, while establishing MACT-based standards, also included provisions addressing periods of startup, shutdown, and malfunction (SSM).
- Under the challenged SSM provisions, sources were exempt from otherwise applicable MACT emission limits during SSM events if they met an alternative requirement: a general duty to minimize emissions and use proper air pollution control practices during those periods.
- Sierra Club petitioned for review in the D.C. Circuit, arguing the SSM exemption conflicted with the Act because it created periods when enforceable MACT limits did not apply.
- Industry groups intervened in support of EPA, defending the SSM approach as a reasonable implementation of the statutory term “continuous.”

## Issues

1. Whether EPA may exempt major sources from otherwise applicable CAA §112 MACT emission standards during startup, shutdown, and malfunction events and replace those standards with only a general duty to minimize emissions, consistent with CAA §302(k)’s requirement that emission standards apply “on a continuous basis.”
2. Whether the SSM general-duty obligation qualifies as an “emission standard” under the CAA sufficient to satisfy §112’s MACT framework and the MACT floor during SSM periods.

## Decision

- The court granted the petitions for review and vacated the SSM exemption provisions in the NESHAP General Provisions.
- The court held that the CAA requires some §112 emission standard to apply at all times, including during SSM events.
- The court concluded that EPA’s generalized SSM duty was not a valid substitute for a §112 MACT emission standard because it did not operate as a continuous, enforceable emissions limit consistent with §112(d).
- Because the statutory text required continuous application of §112 standards, the court rejected EPA’s interpretation and did not defer to the agency.

## Legal Principles

- CAA §112 standards must function continuously; EPA cannot create categorical time-based carve-outs that leave sources without a §112-compliant emission standard during defined operating periods.
- “Continuous basis” in CAA §302(k) allows standards to account for operational conditions, but it does not allow gaps in the applicability of §112 emission standards.
- A general obligation to minimize emissions and operate control equipment properly, without a definite limitation tied to MACT and the MACT floor, is not an “emission standard” that can replace MACT limits during SSM.
- When statutory text is clear, a court will invalidate agency rules that conflict with the statute rather than defer to agency policy choices.

## Conclusion

In *Sierra Club v. Environmental Protection Agency*, the D.C. Circuit vacated EPA’s NESHAP General Provisions that exempted major sources from §112 MACT limits during startup, shutdown, and malfunction. The court held that the Clean Air Act requires §112 emission standards to apply on a continuous basis and that EPA could not substitute a non-MACT, generalized duty to minimize emissions for enforceable MACT-based limits during SSM periods.


Sierra Club v. Environmental Protection Agency, 551 F.3d 1019 (D.C. Cir. 2008)