The Red Book emphasized both the need for generically applicable defaults and the need for flexibility in their application. Thus, the Red Book and Science and Judgment pointed out that scientific data could shed light, in the case of specific substances, on one or more of the information gaps in a risk assessment for which a generally applicable default had been applied. The substance-specific data might reveal that a given default might be inapplicable because it is inconsistent with the data. The substance-specific data might not show that the default had been ill chosen in the general sense but could show its inapplicability in the specific circumstance. Thus, there arose the notion of substance-specific departures from defaults based on substance-specific data. Much discourse and debate have attended the question of how many data, and of what type, are necessary to justify such departures, and the committee addresses the matter in this chapter. EPA recently altered its view on the question of “departures from defaults,” and this chapter begins by examining this view in relation to its central theme.


The committee recognizes that defaults are among the most controversial aspects of risk assessments. Because the committee considers that defaults will always be a necessary part of the risk-assessment process, the committee examined EPA’s current policy on defaults, beginning with an eye toward understanding its applications, its strengths and weaknesses, and how the current system of defaults might be improved.

EPA began articulating a shift toward its current policy on defaults in the Risk Characterization Handbook (EPA 2000a) when it stated,

For some common and important data gaps, Agency or program-specific risk assessment guidance provides default assumptions or values. Risk assessors should carefully consider all available data before deciding to rely on default assumptions. If defaults are used, the risk assessment should reference the Agency guidance that explains the default assumptions or values (p. 41).

EPA’s staff paper titled Risk Assessment Principles and Practices (EPA 2004a) reflected a further shift in the agency’s practices on defaults:

EPA’s current practice is to examine all relevant and available data first when performing a risk assessment. When the chemical- and/or site-specific data are unavailable (that is, when there are data gaps) or insufficient to estimate parameters or resolve paradigms, EPA uses a default assumption in order to continue with the risk assessment. Under this practice EPA invokes defaults only after the data are determined to be not usable at that point in the assessment—this is a different approach from choosing defaults first and then using data to depart from them (p. 51).

EPA’s revised cancer guidelines (EPA 2005a) emphasize that the policy is consistent with EPA’s mission and make clear that the general policy applies to cancer risk assessments:

As an increasing understanding of carcinogenesis is becoming available, these cancer guidelines adopt a view of default options that is consistent with EPA’s mission to protect human health while adhering to the tenets of sound science. Rather than viewing default options as the starting point from which departures may be justified by new scientific information, these cancer guidelines view a critical analysis of all of the available information that is relevant to assessing the carcinogenic risk as the starting point from which a default option may be invoked if needed to address uncertainty or the absence of critical information (p. 1-7).

Those statements may reflect the agency’s current perspective on the primacy of scientific data and analysis in its risk assessments; the agency commits to examining all relevant

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