OR OR AND AND Int. J. Environ. Res. Public Health 2012, 9 2008 Table 1. Factors a court might consider when determining whether an agency has authority to apply a new interpretation of its authorizing legislation. Can an agency change its interpretation of authorizing legislation? Unlikely if… Likely if… • Old interpretation has received court approval; • The agency provides a rationale for the change; • Old interpretation is consistent with other • New evidence supports a different interpretation Congressional or agency action; to satisfy the statutory mandate; • New interpretation is arbitrary, capricious, or • The agency provides adequate notice of and an abuse of discretion. opportunity to comment on methodology change. It would seem, then, that under the Chevron test, a court might uphold an EPA interpretation that a statutory mandate to “protect the public health” may authorize the use of a cumulative risk assessment methodology, even if this determination overturns a longstanding interpretation of the underlying legislation. Such a result seems most likely where the Agency reasonably interpreted the statute as permitting consideration of cumulative effects derived from a cumulative risk assessment methodology, and where EPA has strong scientific evidence to support the assertion that cumulative effects are a significant concern in the circumstance at issue ([11], p. 378). However, assuming that a court would uphold EPA’s interpretation or reinterpretation of a vague or ambiguous statute to permit the use of cumulative risk assessment, there remains a further legal hurdle. A challenger still could assert that even assuming the Agency had authority to use the methodology and results of the analysis, there were flaws in the conduct of the analysis itself or in the use of the results. In such a case, EPA must show that there was a “rational basis” for its decision–that its actions were not “arbitrary and capricious.” A U.S. court would analyze these issues under the rubric of the APA [44]. 3. If EPA Overcame a Challenge to Its Authority, What Evidence must It Offer that a Decision Based on Data Derived From a Cumulative Risk Assessment was Rational? As noted above, EPA often supports or justifies its decision-making by estimating risks associated with various pollutants or stressors. This activity usually results in an administrative rule setting a particular health, ecological, or technology-based standard or policy. Assuming a court has found that EPA had authority to consider cumulative risk in an ambiguous statutory directive, how might a court assess whether there is a rational basis for using a cumulative risk assessment methodology and the results deriving from it? This inquiry is particularly important given that the product of any risk assessment is a qualitative or quantitative statement regarding the probability of, and degree to which exposed populations or systems will be harmed ([38], p. 26; [11], p. 354). In other words, how does EPA survive the “arbitrary and capricious” test when it employs a methodology that necessarily involve a series of assumptions, which are estimated to best reflect the Agency’s understanding of real- world conditions, and inevitably contain some amount of uncertainty ([13], p. 19)?