Int. J. Environ. Res. Public Health 2012, 9 2011 to protect the public health, and the decision that of an expert administrator, we will not demand rigorous step-by-step proof of cause and effect” ([87], pp. 1064–1065, citing [12], p. 28). According to the court, because the assumptions EPA applied in its risk assessment methodology bore a rational relationship to the real world, given the incomplete information faced by the Agency, the court was obligated to uphold EPA’s technical judgment ([87], p. 1070). In Mossville Environmental Action Now vs. EPA (2005), the D.C. Circuit remanded an EPA determination to use vinyl chloride as a surrogate for other hazardous air pollutants. The court reasoned that it could not assess the rationality of the Agency’s analysis, because EPA failed to memorialize evidence of the correlation the Agency claimed existed between vinyl chloride and the other pollutants in the record ([88], p. 1243). Thus, a court will find an EPA decision arbitrary and capricious if EPA fails to show a rational relationship between its conclusions or assumptions and the evidence before the Agency as contained in the record. In summary, a review of the case law indicates that when a stakeholder challenges the quality of the data or technical process relied on by EPA or suggests that other data is more persuasive, courts are likely to defer to EPA’s expertise and uphold the final agency action. In contrast, challengers tend to succeed when the record under review shows data gaps or missing steps in EPA’s logic that preclude meaningful review of EPA’s decision-making process ([74], pp. 318–319). Given the inherent scientific judgment in the selection of data and assumptions at various steps of a risk assessment, a court will attempt to ensure that EPA performed the most rigorous analysis possible given the available data ([87], pp. 1069–1070). If the data and assumptions upon which a cumulative risk assessment is based are such that a reasonable policy-maker could not interpret the results of the risk assessment with reasonable confidence that those results bear a rational relation to the real world (i.e., if the uncertainty of the risk assessment findings is too high), a court would likely find it inappropriate to consider such health effects in agency decision-making ([79] quoting [89], p. 1373). This is because a court will overturn an agency decision where the agency fails to provide sufficient information to demonstrate a rational connection between the factors that agency examined and the conclusions it reached. For example, in Northwest Coalition for Alternatives to Pesticides vs. EPA, the 9th Circuit Court of Appeals rejected EPA’s choice of a safety factor as arbitrary where the court was unable to determine whether there was reliable data supporting the Agency’s choice of that factor. The court found that EPA failed to explain the connection between the toxicological data and the safety factor selected ([79], p. 1052). Therefore, if EPA does not explain how its reliance on the results of a cumulative risk assessment relates to its statutory directive, or if EPA does not explain how its decision is supported by the results, a court would likely overturn that decision. It follows that when a challenger argues that an EPA decision is flawed due to a faulty risk assessment, to avoid a ruling that it has been arbitrary and capricious, the Agency must show a rational relationship between the assumptions used in the risk assessment and what is known from real-world data [87,90]; and that it considered all important aspects of the problem ([67], p. 43). A plausible way for EPA to show that its use of results from a cumulative risk assessment was not arbitrary and capricious is to present evidence that the cumulative risk assessment was based on reasonable methodologies and assumptions, given the available data, and that EPA’s use of the risk assessment results were reasonable based on the analysis.