The present chapter addresses the general assessment methods and covers identification of the studies considered, their evaluation, and the weight-of-evidence assessment. These issues are also addressed within the context of the specific health outcomes evaluated (see Chapters 4-5).
IRIS has the overall purpose of evaluating human health effects that may arise from exposure to environmental contaminants (EPA 2010a). An IRIS assessment addresses noncancer and cancer effects as appropriate and provides descriptive and quantitative information:
“Noncancer effects: Oral reference doses and inhalation reference concentrations (RfDs and RfCs, respectively) for effects known or assumed to be produced through a nonlinear (possibly threshold) mode of action. In most instances, RfDs and RfCs are developed for the noncarcinogenic effects of substances” (EPA 2010a).
“Cancer effects: Descriptors that characterize the weight of evidence for human carcinogenicity, oral slope factors, and oral and inhalation unit risks for carcinogenic effects. Where a nonlinear mode of action is established, RfD and RfC values may be used” (EPA 2010a).
A sequence of activities is involved in conducting IRIS assessments and in calculating RfCs and unit risk estimates. Figure 2-1 is a generic schema that describes the steps used to generate the draft IRIS assessment and the actions needed at each step. The figure is the committee's representation of that process, as gleaned from the assessment. Although the draft IRIS assessment does not explicitly acknowledge these steps, they are implicit in the approach and are ordered as shown.