A Comparative Exposure Assessment of EPA Indirect Guidance on Dioxin Using Point Estimates And Monte Carlo Techniques. K. L. Hentz, K.A. Williams, and N. J. Karch, Karch & Associates, Inc., 1 701 K Street NW, Washington, DC 20006
The EPA is currently struggling with risk assessment guidelines for combustor emissions, particularly for dioxins/furans. There are two guidance documents available for the assessment of indirect exposure to dioxin. These two documents, Estimating Exposure to Dioxin-like Compounds and Methodology for Assessing Health Risks Associated with Indirect Exposure to Combustor Emissions, differ significantly in their recommended exposure parameters. If the recommended point estimates are used to predict risks from exposure to dioxin, the resulting risks can differ by as much as 30-fold. This exposure assessment compares the results of the single point risk estimates for the two guidance documents and the results of a Monte Carlo analysis. Although one of the guidance documents only addresses dioxin/furan exposure, this exposure assessment also evaluates the potential risks from indirect exposure to metal combustor emissions. The recommended point estimates in the guidance documents are very conservative, and the combination of several conservative parameters results in overall risks that are extremely unlikely to exist. Because Monte Carlo techniques utilize probability density functions instead of point estimates, more realistic predictions of risk are derived. In EPA's most recent exposure assessment guidelines, it is recommended that several risk descriptors be used to demonstrate the range of potential risks. Monte Carlo is one possible technique which not only predicts a range of potential risks, but also resolves some of the discrepancies between the two indirect exposure guidance documents.