used in setting standards or guidelines for individual chemicals. Although both draw heavily on toxicity assessments—for example, as appear on or are used by EPA’s Integrated Risk Information System (IRIS) web site—the application of the toxicity assessments typically differs considerably between the two. Site-specific risk assessments are often concerned with simultaneous evaluation of multiple chemicals, multiple pathways of exposure, multiple routes of exposure, and multiple receptors. Standard-setting or guideline-setting generally evaluates at one time single chemicals, single routes of exposure, and single receptors, although there are exceptions, such as disinfection byproducts in drinking water.
The committee’s task of evaluating the potential for a cumulative risk assessment of phthalates has to take into account that such cumulative assessments are commonly performed already, and any recommendations of the committee should be compared with the current EPA approach as described in RAGS and related guidance. Accordingly, the following sections discuss what is typically required in the exposure-assessment, toxicity-assessment, and risk-characterization parts of a risk assessment. The approaches are evaluated for what they imply about cumulative assessment of phthalates in various EPA programs, such as those involving Superfund, air toxics, and drinking water.
The exposure-assessment component of a risk assessment of hazardous chemicals according to RAGS (EPA 1989a) requires evaluation of exposure of all the relevant, although not personally identified, people (“receptors”) to all the relevant chemicals through all the relevant pathways by all the relevant routes of exposure for all relevant periods. The products of exposure assessment are estimates of exposure of defined receptors to each chemical disaggregated by periods and exposure pathways. This section provides an idealized general description, not a critical review, of the current practice of exposure assessment.
The relevant receptors to evaluate are typically intended to be persons who experience the “reasonable maximum exposure” (RME) and persons who experience “central-tendency” (CT) exposures. The RME is the highest exposure that is expected to occur (EPA 1989a, 1992), and EPA (2001) advises that risk managers using probabilistic risk assessment should select the RME from the upper end of the range of risk estimates, “generally between the 90th and 99.9th percentiles” (EPA 2001). Later discussion focuses on persons who experience the RME because their exposure usually forms the basis of EPA decision-making (CT estimates may be needed for some pathways of the RME, as described below).