Int. J. Environ. Res. Public Health 2012, 9 2007 North Carolina vs. EPA, the Agency was interpreting its statute in such a way that conflicted with another provision of the statute ([63], pp. 911–912). In contrast, the analysis in this article is suggesting that EPA could interpret broad authority to protect the public health to include a consideration of background effects from multiple stressors that might affect how the pollutant at issue impacts human health or the environment. This analysis does not suggest that EPA could interpret its statutory directive to conflict with another provision of the statute. In fact, as discussed in greater detail below, EPA must assess all criteria that Congress has directed the Agency to consider in making its determinations under a particular statute ([67], p. 43). If EPA concludes that basing a decision on results from a cumulative risk assessment is the best way to carry out a statutory mandate to protect the public health, and does so in a way that is not in conflict with any other provisions of the statute, a court likely would defer to the Agency’s reasonable interpretation of its statutory authority ([49], p. 843). For these reasons, the authors conclude that North Carolina vs. EPA may be consistent with the D.C. Circuit’s application of Chevron’s principle of deference to reasonable EPA interpretations of the Agency’s statutory authority. Assuming that EPA can overcome the arguments discussed in this section, there is another principle of statutory construction to consider: whether a U.S. Federal agency may change a longstanding interpretation of its authorizing legislation. Section 2.3 addresses how a court would view an EPA interpretation of its statute that differed from how the Agency had interpreted the same statute in the past. 2.3. How might a Court Evaluate whether a Reinterpretation of Authority to Permit the Use of a Cumulative Risk Methodology was Valid? Some EPA offices may hesitate to use cumulative risk assessment in program decision-making where the Agency previously has not considered cumulative risks in implementing that program. Courts may question the validity of an agency’s reinterpretation of its substantive legislation [68]— especially if the earlier interpretation received court approval [69,70], or appears to be more consistent with other Congressional and agency action ([51], pp. 138–139). However, the Supreme Court has recognized that agency interpretations of their substantive authorities are not immutable, and thus may be changed when appropriate ([49], pp. 863–864). In the Chevron case, the Supreme Court stated, “An initial agency interpretation is not instantly carved in stone. On the contrary, the agency, to engage in informed rulemaking, must consider varying interpretations and the wisdom of its policy on a continuing basis” ([49], pp. 863–864). Therefore, courts’ deference to reasonable EPA interpretations under Chevron Step Two (Figure 1) extends not only to EPA’s initially selected interpretation, but also to subsequent decisions to change its preferred interpretation ([71], p. 742; [72], p. 521; [73], pp. 22–27; [74] p. 317; [75]). In such cases, the court would apply the presumption that when Congress left an ambiguity in a statute meant for implementation by an agency, Congress understood that the implementing agency—not the reviewing court—would have discretion to resolve the ambiguity ([71] citing [49]). Based on a review of the case law, Table 1 presents some of the factors a court might consider when determining whether an agency has authority to changes its interpretation of a statute that agency implements.