Benzene, a known human carcinogen, was listed by EPA as a hazardous air pollutant under §112 of the Clean Air Act, which required standards that protect public health with an "ample margin of safety." In 1984, EPA promulgated a benzene NESHAP targeting emissions from several industrial source categories (including fugitive equipment leaks and certain production processes). To implement §112's health-protection mandate, EPA adopted a two-step risk policy: (1) identify an "acceptable risk" level using risk assessment tools (including the maximum individual lifetime risk, or MIR, metric) and (2) add an "ample margin of safety" by tightening controls further, taking into account uncertainties, the distribution and magnitude of risks, numbers of people exposed, and the technological and economic feasibility of additional reductions. In the benzene rule, EPA treated an MIR on the order of 1 in 10,000 as within the range of acceptable risk (subject to context) and then adjusted standards to provide additional protection. The rule also contained certain categorical distinctions and limited exemptions EPA characterized as de minimis. Environmental petitioners (including NRDC) argued EPA unlawfully tolerated excessive cancer risk and improperly factored costs, while industry petitioners contended that the statute did not compel such stringent health-based controls and that EPA's risk policy was arbitrary. Multiple petitions for review were consolidated before the D.C. Circuit sitting en banc.
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.
Text and structure: The phrase "ample margin of safety" is inherently indeterminate and does not fix a numeric risk threshold or require absolute zero risk. The statutory focus on public health supports EPA's health-first approach while allowing room to address uncertainties and feasibility in determining the margin of safety. Chevron deference: Because §112 does not dictate a precise risk level or methodology, EPA's two-step framework reasonably fills the statutory gap. The court rejected NRDC's view that §112 mandates the lowest technologically achievable (or zero-risk) standard, and it likewise rejected industry's contention that the statute invites open-ended cost-benefit analysis that could subordinate health protection. Risk methodology: The court approved EPA's use of MIR to anchor the "acceptable risk" inquiry, recognizing MIR's role in protecting the most exposed individual while acknowledging that other considerations—such as the number of people exposed, background exposures, and cumulative risks—are relevant when determining the margin of safety. The court emphasized that EPA must make clear how it balances these considerations consistent with the statute's health-protective purpose. Feasibility and costs: The court held that EPA may consider technological and economic feasibility when providing an "ample margin of safety," but not at the initial step of identifying what risk levels are acceptable from a health perspective. The statute does not authorize pure cost-benefit tradeoffs that would compromise health protection, but it does allow feasibility to inform how far beyond the "acceptable" level the margin of safety should extend. De minimis exemptions and record support: While agencies may craft narrow de minimis exemptions (consistent with the court's de minimis jurisprudence) where regulation would yield trivial benefits, EPA must justify such exemptions with evidence and reasoned explanation. The court found parts of the benzene rule insufficiently explained—e.g., certain thresholds, subcategory lines, or exemptions lacked adequate risk quantification or failed to account for uncertainties or population risk—and therefore remanded those elements for further consideration. In sum, the court sustained EPA's overarching interpretation but required the agency to shore up its reasoning and record for specific regulatory choices.
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.
NRDC v. EPA established the bedrock two-step framework for implementing §112's mandate to protect public health with an ample margin of safety. By endorsing an "acceptable risk" determination followed by a further protective buffer that may consider feasibility and uncertainties, the court reconciled a health-first statute with practical implementation and scientific indeterminacy.