824 F.2d 1146 (D.C. Cir. 1987) (en banc)
Natural Resources Defense Council v. EPA is a foundational administrative and environmental law decision interpreting the pre-1990 Clean Air Act's hazardous air pollutant program.
Does Clean Air Act §112 permit EPA's two-step interpretation of the "ample margin of safety" mandate—first determining an acceptable level of risk and then adding a margin of safety informed by feasibility and other factors—and were EPA's specific benzene NESHAP choices (including risk thresholds, subcategorization, and de minimis exemptions) reasonable and supported by the administrative record?
Under Chevron, when a statute is ambiguous, an agency's reasonable interpretation controls. Section 112 requires EPA to set standards that protect public health with an "ample margin of safety," which the court held does not compel a zero-risk standard or pure cost-benefit balancing. EPA may (1) identify a level of "acceptable risk" grounded primarily in health protection and scientific risk assessment (e.g., the MIR and related risk metrics), and then (2) provide an additional "ample margin of safety" that accounts for uncertainties, the distribution and magnitude of risk, and technological and economic feasibility considerations to the extent consistent with health protection. EPA may adopt narrow de minimis exemptions where regulation would produce trivial or no health benefits, but only if those exemptions are consistent with the statute's purpose and supported by a reasoned explanation and record evidence. Agency action must reflect a rational connection between the facts found and the choices made.
The court upheld EPA's two-step "acceptable risk" and "ample margin of safety" interpretation of §112 as a permissible construction under Chevron. It approved EPA's use of MIR and related risk metrics and permitted consideration of feasibility and limited cost factors at the margin-of-safety step (but not as a substitute for health protection). However, the court remanded portions of the benzene NESHAP where EPA failed adequately to justify specific thresholds, subcategorizations, and de minimis exemptions, and where the record did not sufficiently support the agency's choices or consideration of risk distribution and uncertainties.
This decision is a touchstone for environmental and administrative law. It crystallizes the now-classic two-step "acceptable risk/ample margin of safety" framework under §112 and delineates the permissible role of feasibility and limited cost considerations in a health-based statute. It also reinforces Chevron deference while demanding rigorous, record-based reasoning—illustrating how courts both defer to agency policy choices within statutory bounds and police the quality of agency justification. Even after Congress overhauled §112 in the 1990 Clean Air Act Amendments (adopting technology-based MACT standards followed by a residual-risk review), this case continues to guide EPA's residual risk determinations under §112(f) and remains widely cited for risk assessment, de minimis authority, and the articulation of a margin-of-safety standard.