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EPA-Expo-Box (A Toolbox for Exposure Assessors)

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Indirect Estimation
(Scenario Evaluation)

Overview

Indirect Estimation

EPA’s Guidelines for Exposure Assessment (U.S. EPA, 1992) defines scenario evaluation as "an approach to quantifying exposure by measurement or estimation of both the amount of a substance contacted, and the frequency/duration of contact, and subsequently linking these together to estimate exposure or dose." As such, scenario evaluation is an “indirect estimation” method that relies on an exposure scenario—or a set of facts, assumptions, and inferences about how exposure takes place—to estimate exposures or doses (in contrast to Point-of-Contact measurement approaches, which more directly measure exposures or doses, or Exposure Reconstruction, which estimates exposure using information on an effect or outcome).

Indirect estimation of exposure or dose ultimately requires quantitative values to use as inputs to exposure/dose equations, such as the example shown in the adjacent figure. The inputs to these equations are obtained through the development of exposure scenarios that involve data or assumptions on the sources and releases of a stressor of interest, fate and transport mechanisms, and concentrations of contaminants at the point of exposure. Information about receptor populations and exposure factors (e.g., activities and time frame over which exposure occurs) are also needed. Calculation tools like the general equation shown in the adjacent figure and more complex integrative models can be used to quantify exposures or doses for the populations of interest. Additionally, Point-of-Contact measurement approaches can be used to validate results of scenario evaluation assessments.

Indirect Estimation of Potential Dose: Example

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