POLYCHLORINATED-BIPHENYLS (PCBS) - ENVIRONMENTAL-IMPACT, BIOCHEMICAL AND TOXIC RESPONSES, AND IMPLICATIONS FOR RISK ASSESSMENT

Commercial polychlorinated biphenyls (PCBs) and environmental extracts contain complex mixtures of congeners that can be unequivocally ident ified and quantitated. Some PCB mixtures elicit a spectrum of biochemi cal and toxic responses in humans and laboratory animals and many of t hese effects resemble those caused by 2,3,7,8-tetrachlorodibenzo-p-dio xin (TCDD) and related halogenated aromatic hydrocarbons, which act th rough the aryl hydrocarbon (Ah)-receptor signal transduction pathway. Structure-activity relationships developed for PCB congeners and metab olites have demonstrated that several structural classes of compounds exhibit diverse biochemical and toxic responses. Structure-toxicity st udies suggest that the coplanar PCBs, namely, 3,3',4,4'-tetrachlorobip henyl (tetraCB), 3,3',4,4',5-pentaCB, 3,3',4,4',5,5'-hexaCB, and their monoortho analogs are Ah-receptor agonists and contribute significant ly to the toxicity of the PCB mixtures. Previous studies with TCDD and structurally related compounds have utilized a toxic equivalency fact or (TEF) approach for the hazard and risk assessment of porychlorinate d dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) cong eners in which the TCDD or toxic