Int. J. Environ. Res. Public Health 2012, 9 2013 determine whether Congress has spoken directly to the issue. Since many environmental statutes direct EPA to take actions necessary to “protect the public health” or to address a condition that may pose a “substantial” or “significant” thread to public health, a court would likely find that Congress has not directly spoken to whether EPA can consider cumulative risks in making such decisions. If a court does find a statute ambiguous under Chevron Step One, it must defer to any reasonable agency interpretation under Chevron Step Two. Chevron suggests that if an agency makes 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. Given the plausible argument that EPA has authority to use cumulative risk assessment to support decision-making under a statute, there remains the question of whether EPA’s use of cumulative risk assessment in a particular circumstance can withstand arbitrary or capricious review under the APA. Whether a particular cumulative risk analysis is appropriate for consideration in an Agency policy or regulatory decision is a fact-driven inquiry requiring judicial examination on a case-by-case basis. Therefore, there is no way to make a broad statement of general applicability regarding how to construct a cumulative risk assessment analysis that would be upheld in a court of law; in different factual circumstances, the sufficiency of evidence of the risks and exposure effects shown by the analysis may be different. A reviewing court’s inquiry into the reasonableness of a cumulative risk assessment likely would examine whether the data and assumptions used in the assessment were rational, based on the available information, and whether the Agency’s conclusions were reasonable based on the analysis. If data and assumptions are sufficient for a court to decide “whether the agency’s conclusions are rationally supported,” the court must affirm ([79], p. 1052 n.7). In many current circumstances, making reasonable quantitative estimates of impacts using cumulative risk assessments would require access to data that currently are nonexistent or insufficient. Recently, potential users in the scientific community have suggested the necessity and value of having the following cumulative effects data and tools: larger emissions inventories, air quality monitoring networks, modeling software, and pollution inventories expanded to unregulated operations [101–104]. The writers believe that EPA and the scientific community should focus on developing the necessary data and tools to provide a sound cumulative risk assessment framework. The Agency then should develop guidelines for a cumulative risk assessment methodology, including guidance for evaluating qualitative cumulative effects. To support EPA use of cumulative effects methodologies in environmental decision-making, future research could include analyses of specific EPA decisions under particular statutes. EPA acknowledges the situation-specific nature of legal challenges in its recent EJ Legal Tools document [36]. EJ Legal Tools analyzes EPA’s statutes and their relevant regulatory standards to protect public health or welfare and the environment for how they might provide opportunities to ensure that Federal policies have no disproportionately high and adverse human health or environmental effects on minority or low-income communities ([36], p. 2). The Agency explains that without the context of specific applications, the EJ Legal Tools document does not attempt to fully characterize the legal risks of interpreting EPA’s “more vague” legal authorities ([36], p. 2). Although we discuss EPA’s general authority in this article, future research could include a closer examination of how specific legal