Risk Assessment of Polychlorinated Biphenyls at Hazardous Waste Sites

We recently reported a hazardous waste site in which polychlorinated biphenyls (PCBs) were at such high concentrations in soil that significant levels were found in indoor residential air. This paper uses the waste site to illustrate the complexities in exposure assessment and toxicity assessment of PCBs. Aroclors are commercial mixtures of different arrays of the 209 individual PCB congeners. What is actually found at a given site will be a complex mixture of individual congeners, the exact composition depending on the makeup of the original material released at the site and the subsequent environmental fate and transport of released material. Recently, individual congener analysis has begun supplementing the reporting of Aroclors. Unfortunately, long term toxicity studies, and hence toxicity criteria, are only available for Aroclors rather than individual congeners. The PCB mixtures at a waste site may be quite different than the mixture on which toxicity criteria are based. U.S. EPA currently provides upper bound cancer slope factors of 2.0 for High Risk and Persistence PCBs, 0.4 for Low Risk and Persistence PCBs, and 0.07 for Lowest Risk and Persistence PCBs, based on bioassays of Aroclors 1260, 1254, 1242, and 1016. Although the soil contaminant at our hazardous waste site was identified as Aroclor 1260, 98% of the PCBs detected in indoor air were monochlorinated biphenyls. A further complication in toxicity assessment is that noncancer toxicity can result from relatively low dose levels. For example, all doses of Aroclor 1016 predicting a cancer risk above 5E-6 are also associated with an unacceptable noncancer hazard. Another consideration for toxicity assessment is the dioxin-like activity of co-planer PCBs. Exposure assessment of PCBs is complicated by the bioaccumulative properties. This necessitates consideration of the potential for indirect exposure to PCBs from the waste site through food pathways, including fish, poultry and livestock, and breast milk. Another exposure assessment consideration is the near ubiquitous ambient low level presence of PCBs in environmental media including air, soil, and food.