Int. J. Environ. Res. Public Health 2012, 9 1.3. What this Article is About 2002 This article explains what EPA must demonstrate to survive a legal challenge to a decision based on the results of a cumulative risk assessment. In short, the Agency must show that it has statutory authority to use such a methodology, and present evidence to support an argument that the Agency has acted rationally in the exercise of that authority. This article examines how a court would determine whether there is a defensible case that EPA has the statutory authority to use cumulative risk assessment and its products in environmental decision-making, and what evidence might be sufficient to show the rational exercise of that authority. Research suggests that the legal viability of using cumulative risk assessment to project cumulative effects depends upon the specific statutory authority under which EPA is acting, and the scientific soundness of the analysis at issue. In the context of judicial review in the United States, this inquiry is highly fact-driven and hinges on the reasonableness of the conclusions drawn from the technical analyses. As noted above, the analysis in this article focuses on how a court would review a challenge to EPA’s consideration of cumulative effects by basing an environmental decision on the results of a cumulative risk assessment. The reader should note that the analysis presented here is equally applicable to any construct of cumulative effects EPA might use. A reviewing court would still examine the specific statutory language at issue, the data used by EPA, and the Agency’s application or use of such data in a specific factual circumstance. Although this article presents what we believe are legally permissible interpretations of EPA authority, we acknowledge that using cumulative risk assessment as we suggest could involve legal and policy interpretations that veer from longstanding interpretations of the Agency’s program legislation and regulations. We acknowledge further that a broad use of cumulative risk assessment as the basis for EPA decision-making will require considering a mix of scientific, political, financial, human resource, and other factors. We hope that the discussion in this article can serve the continuing dialogue on this topic and advance the real-world analysis of using cumulative risk assessment in EPA policy and regulatory activity. 1.4. The Context for Judicial Review of the Issues in the United States Under the Administrative Procedure Act (APA), persons aggrieved or adversely affected by agency actions, including agency promulgation of rules, have the right to seek judicial review of those actions ([44]; [45], p. 7). Most statutes establishing regulatory programs also provide for court review of agency rules ([45], p. 7). It is in this context of challenging EPA action in a U.S. court that a stakeholder might argue the Agency lacked statutory authority to consider some criteria or analytical method that EPA used in making its decision (e.g., a cumulative risk assessment). Some environmental statutes explicitly state that human health or environmental risks must be considered, while other statutes give EPA less specific directives to protect the public health [46]. Specific legislative references to risk supply clear authority for EPA to use risk assessments in administrative decision-making. In other statutes, however, EPA has decided to key regulatory choice to level of risk without an explicit statutory mandate to do so. For example, Congress may broadly direct EPA to do what is necessary “to protect the public health;” or to address a condition that is “unsafe,” that may pose “substantial” or “significant” threat to health or safety, or that rises above a