Indirect Exposure Assessment at the U.S. EPA
In the early 1980s, exposures and subsequent health impact assessments from contaminants emitted into the air from stationary sources focused on the inhalation pathway. This 'direct' pathway of exposure was thought to be the most critical pathway, as it is for many contaminants. However, by the latter 1980s, the focus at the Environmental Protection Agency (EPA) shifted to contaminants that would persist in the environment and could bioaccumulate up the food chain. Consumption of impacted food products, and other 'indirect' pathways of exposure, such as soil-related exposures, were shown to result in exposures that exceeded inhalation exposures by two to up to four orders of magnitude. A historical background of indirect exposure assessment at EPA is provided in this paper, followed by an overview of modeling methodologies commonly used in indirect exposure assessments. These methodologies are demonstrated on a contaminant of primary focus for indirect exposure impact, dioxins. Two examples are provided. In the first, an air-to-beef model validation exercise is described. In the second, an indirect exposure assessment on a municipal solid waste incinerator in Ohio conducted by the EPA is summarized. This incinerator emitted very large amounts of dioxin, more than any single source known to EPA. Models were used to predict the movement of dioxins from the stack to a nearby hypothetical farm, where individuals in the farm family were exposed to dioxins from consumption of home-produced beef and milk. The predicted lifetime cancer risk of 2.8W10-4 based on these food pathways was used by EPA's Region 5 to initiate regulatory activity on this incinerator.