Differences in Indirect Risk Methodologies: Dioxin Emissions from Combustion Facilities

Abstract

Guidance for performing indirect risk evaluations is in a state of flux; yet, EPA and the public are insisting that estimates of indirect risks associated with combustion of industrial wastes must be derived. Two different guidance documents exist for estimating these risks: one that focuses on dioxin exposures and one that has a more general application. This paper evaluates the differences in indirect risk estimates derived from methodologies presented in these two documents and discusses those assumptions that may contribute to these differences. The influence of an individual exposure pathway on indirect risk estimates varies depending on the methodology applied. Indirect risk estimates differ by 30-fold for the same exposure pathway when comparing these methodologies. Assumptions about vapor uptake, food consumption rates, and the fraction of local food consumption appear to drive final estimates. A major discrepancy between these two documents is differences in assumptions about soil dermal contact for children. Many EPA default assumptions and values are extremely conservative and often unrealistic; therefore, use of site-specific data will be essential when estimating indirect risks associated with combustor emissions. Also, application of a toxic equivalency approach may have some benefit; risk estimates are reduced 16- to 18-fold using this approach.