Int. J. Environ. Res. Public Health 2012, 9 2009 In the United States, the APA provides the basic framework within which Federal agencies must operate in promulgating rules, issuing policy statements, and adjudicating rights [44]. Under this statute, a court may assess the validity of regulatory agency decision-making, and “hold unlawful and set aside agency action, findings, and conclusions found to be arbitrary, capricious, an abuse of discretion or otherwise not in accordance with law” [76]. In applying this “arbitrary and capricious” standard, a court must conduct a “searching and careful” review of the agency’s record ([67], p. 43). The court must vacate the action if “the agency has relied on factors which Congress has not intended it to consider, entirely failed to consider an important aspect of the problem, offered an explanation for its decision that runs counter to the evidence before the agency, or is so implausible that it could not be ascribed to a difference in view or the product of agency expertise” ([67], p. 43) (Figure 2). Figure 2. Under the APA, the court will conduct a “searching and careful” review and ask whether the agency made a decision that was not “arbitrary and capricious”. (Note that the steps are not necessarily sequential.) Relied on factors Congress did not intend it to consider Failed to consider an important aspect of the problem Offered an explanation that runs counter to the evidence before the agency Was so implausible it could not be ascribed to a difference in view or stemming from the agency’s expertise Agency decision likely to be upheld if a rational basis for decision is presented NO NO NO NO YES YES YES YES Challenge likely to be upheld Although the APA requires courts to perform a “searching and careful” inquiry into the facts underlying the agency’s decisions, courts will “presume the validity of agency action as long as ‘a rational basis for it is presented’” ([56], p. 362, citing [77], p. 1145) (Figure 2). U.S. courts generally give an “extreme degree of deference to an agency that is evaluating scientific data within its technical expertise,” reviewing the agency’s action to “ensure that [the agency] has examined the relevant data