Similar to the Reassessment, Part III, the chapter here is organized into sections on sources, environmental fate, environmental media and food, background exposures, and potentially highly exposed populations and sensitive populations. This chapter has three major sections: an overview and commentary on all aspects of the dioxin exposure assessment with an effort to point out strengths, limitations, and omissions; the committee’s findings; and specific recommendations.

OVERVIEW AND COMMENTARY ON EPA’S EXPOSURE CHARACTERIZATION

In this section, the committee provides summary and commentary on key issues related to exposure characterization for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, also referred to as dioxin), other dioxins, and dioxin-like compounds (DLCs). This information includes sources, environmental fate, environmental media and food concentrations, background exposures, and potentially highly exposed populations and particularly sensitive developmental stages.

For sources and environmental fate, EPA had a clearly articulated stepwise approach that the committee primarily accepted with some commentary. The other steps in the exposure assessment are not as easy to track, summarize, and critique. To comment on these steps, the committee used a format that went beyond the simple narrative.

Sources

Summary of the EPA Approach

The type, geographic distribution, and time history of the sources and associated emission magnitudes of TCDD, other dioxins, and DLCs are essential inputs for risk characterization. In Part III of the Reassessment, EPA discusses sources and emissions estimates for 1987 and 1995. More recently, EPA issued a report that includes the year 2000 update on sources and emissions estimates (EPA 2005b). These reports consider emissions of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) compounds and dioxin-like polychlorinated biphenyl (PCB) compounds. PCDDs and PCDFs have never been intentionally produced outside research laboratories. They are released to the environment as unintended by-products from various combustion, industrial, and biological processes. PCBs have been produced commercially in large quantities in the United States and other industrialized countries but are no longer commercially produced in the United States and Europe.

Sources of TCDD, other dioxins, and DLCs considered in the Reassess-



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