Determining the "MARGIN of Incremental Exposure": An Approach to Assessing Non-Cancer Health Effects of Dioxins
U.S. Environmental Protection Agency (EPA) guidance issued in April 1994 for performing screening level risk analyses of emissions from facilities that burn hazardous waste does not address the evaluation of non-cancer health effects from dioxin emissions. Historically, EPA has evaluated the carcinogenic risks from exposures to dioxins, but has not considered their potential non-cancer health effects because reference dose (RfD) values do not exist for any congener of polychlorinated dibenzo(p)dioxin or dibenzofuran (PCDD/PCDF). EPA's Draft Dioxin Reassessment concluded, "...calculation of an RfD based on human and animal data and including standard uncertainty factors to account for species differences and sensitive subpopulations would likely result in reference intake levels on the order of 10 to 100 times below the current estimates of daily intake in the general population". Rather than compare a site-specific incremental dose of dioxin to an RfD, which is traditionally done in non-cancer exposure and risk assessments, the Draft Reassessment recommended that the potential for non-cancer effects for dioxin and related compounds should be evaluated using a "margin of exposure" analysis, considering both background and any additional, site-specific incremental exposure. It is EPA policy not to use the Draft Reassessment as a basis for making policy decisions until it has been made final. However, based upon public concerns about potential non-cancer health effects of dioxins emitted during combustion of hazardous wastes, the EPA Office of Solid Waste and Emergency Response (OSWER) independently decided, in 1995, to assess the Margin of Incremental Exposure (MOIE) to dioxins on a provisional and site-specific basis, pending the development of Agency-wide policy on the issue. The MOIE is defined as the ratio of a hypothetical individual's exposure to dioxins from a given source - in this case, a facility that burns waste - to the average background exposure of dioxins for the general U.S. population. This paper evaluates the methodologies used to calculate MOIEs in the risk assessments of five different waste combustion facilities, indicating similarities and differences, and areas in which further MOIE methodology development would be useful.