Int. J. Environ. Res. Public Health 2012, 9 2003 “de minimis” level of danger. Agencies often construe statutory safety or health thresholds as levels of risk [47]. EPA’s risk assessment emphasis is shifting from a narrow focus on single stressors, endpoints, sources, pathways, and environmental media to a broad focus on multiples of these factors [3,7,15] , resulting in a continuing, if uneven, transition to the use of cumulative risk assessment as a methodology to determine probable cumulative effects [7]. EPA has publicly embraced and encouraged the transition in Administrator announcements dating back to 1995 ([7], quoting various EPA announcements), and in the 2010 Interim Guidance on Considering Environmental Justice During the Development of an Action, issued by the EPA Administrator (head of the Agency) [15]. Addressed to EPA program managers, the Interim Guidance states that managers should consider cumulative effects in Agency action that may affect EJ populations [15]. However, because EPA has often faced legal challenges alleging that the Agency is over-reaching, EPA program offices may hesitate to apply a broad interpretation of the Agency’s public health statutory authority. When presented with a challenge to its use of cumulative risk assessment in determining the probability and extent of exposure to pollutants, how might EPA persuade a court that broad or unspecific statutory language gives the Agency this authority? What might a challenger assert as a counter-argument to EPA’s claim of authority? How might a court evaluate whether a reinterpretation of authority to permit the use of a cumulative risk methodology was valid? If the Agency overcame a challenge to its authority, what evidence must it offer that a decision based on data derived from a cumulative risk assessment was rational? Sections 2.1 through 2.3 address the first three questions. Section 3 addresses the remaining question. 2. How a Court would Examine EPA’s Assertion of Statutory Authority to Use Cumulative Risk Assessment Methodologies in Decision-Making [48] 2.1. How Might EPA Persuade a Court that Broad or Unspecific Statutory Language Gives the Agency Authority to Use Cumulative Risk Assessment in Decision-Making? Because environmental statutes may present broad mandates to protect public health or the environment, which EPA must interpret in its implementation, the question arises how reviewing courts might examine an EPA assertion that it has authority under such a mandate to use cumulative risk assessment and to act on data derived from that analysis. In the United States, when courts review an agency’s interpretation of a statute it implements, they analyze the issue under the framework laid out in the 1984 Supreme Court decision, Chevron U.S.A., Inc. vs. Natural Resources Defense Council (Chevron) [49]. In Chevron, the Supreme Court created a two-part framework for reviewing agency interpretations in such circumstances (Figure 1). First, a reviewing court must examine statutory language to decide whether Congress has directly spoken to the precise question at issue. If the statute is clear, the court “must give effect to the unambiguously expressed intent of Congress”, and strike down any conflicting agency interpretation ([49], pp. 842–843). However, if the relevant statutory terms do not unambiguously resolve the issue, courts must defer to the agency’s interpretation as long as it is a “reasonable” one. In specific cases of agencies acting where their authorizing statute is vague or ambiguous, a court’s analysis of reasonableness can involve