Int. J. Environ. Res. Public Health 2012, 9 2005 the Supreme Court explained that the power of an administrative agency to administer a Congressionally created program “necessarily requires a formulation of policy and the making of rules to fill any gap left, implicitly or explicitly, by Congress” ([49], p. 843, citing [52], p. 231). In fact, the “agency’s interpretation need not be the only permissible reading of the statute, nor the interpretation that the court might have originally given the statute” ([53], p. 581, citing [49], p. 843). Chevron suggests that if an agency interprets its enabling statute to permit making a regulatory decision rationally based on some factor or analysis that the applicable Federal statute did not specifically prohibit the agency from considering, a court would not overturn the decision simply because of reliance on the unstated factor or analysis ([50], pp. 1667–1668, 1676–1678; [54,55]). In particular, courts have made clear that agencies may consider substitute risks of proposed regulations (i.e., “risk-risk” or “health-health” tradeoffs that arise when regulation of one health problem gives rise to another health problem), even where the statute the agency is implementing does not direct the agency to consider tradeoffs ([50], pp. 1672–1674, citing [56–58]). Courts have also often found it permissible for agencies to consider cost factors in promulgating decisions where the statute does not mention cost or feasibility ([50], pp. 1676–1678, citing [59–61]). Although EPA use of cumulative risk assessment is a different issue, the point here is that courts have repeatedly found agency authority to implement a statute by considering criteria not mentioned in that statute. Under the rubric of Chevron, then, a court might uphold EPA’s assertion of authority to make a decision based on the results of a cumulative risk assessment even where a statute is vague, ambiguous, or silent on the subject. Section 2.2 addresses arguments that a stakeholder challenging such an EPA interpretation of its statutory authority would likely raise. 2.2. What Might a Challenger Assert as a Counter-Argument to EPA’s Claim of Authority? A stakeholder challenging EPA’s interpretation of the Agency’s statutory authority to consider the results of a cumulative risk assessment might argue that such a statutory interpretation is impermissible because the overall legislative structure for protecting human health and environmental resources prohibits considering multiple stressors outside of the pollutants addressed by a specific statute. In other words, Congress has addressed environmental pollutants and problems on a piecemeal basis, or pollutant-by-pollutant. Therefore, the argument would continue, when establishing standards for protecting the nation’s waters, for example, Congress did not intend a statute that addresses water pollutants to provide EPA with authority to consider the combined health impacts of air pollutants and water pollutants. A challenger making this argument would likely cite FDA vs. Brown (quoted above), where the Supreme Court refused to defer to a Federal agency’s interpretation of its statutory authority, finding that the result of such agency interpretation would be inconsistent with other Congressional and agency action [51]. Applying the Supreme Court’s reasoning in FDA vs. Brown, one could argue that because Congress addressed different pollutants and the protection of different media under different statutory schemes, it did not intend for EPA to consider the cumulative effects of all pollutants when determining how stringently to limit one type of pollutant. On the other hand, in Massachusetts vs. EPA, the Supreme Court found that EPA could find carbon dioxide to be an air pollutant under the Clean Air Act based on that pollutant’s contribution to global