Nondioxin-like PCBs

PCBs can be divided into "dioxin-like PCBs" and "nondioxin-like PCBs." Congeners substituted in only the meta and para positions were approximate isostereomers of 2,3,7,8-tetrachlorodibenzo-p-dioxine (TCDD). Toxicological studies confirmed that these nonortho substituted "coplanar" PCBs bind to the Ah Receptor and induce a variety of in vitro and in vivo dioxin-like effects [74,132] and are classified as dioxine-like PCBs. All other PCBs thus fall into the nondioxin-like classification. Maximum dioxine-like activity is obtained for nonortho-PCBs when there is two or more meta, and both para positions are occupied [94,108]. Table 20.4 reports the toxicity of nonortho-PCB congeners expressed in terms of toxicity equivalent factor (TEF) [109,110,134]. TEF is calculated by comparing the toxicity of each congener with that of TCDD (TEF = 1). 

New information on levels of dioxin-like compounds in human adipose tissue and blood has been published [166]. These measurements show that concentrations of dioxin-like PCBs can be more than an order of magnitude higher than concentrations of TCDD. Comparison with other published information suggests much higher levels of nondioxin-like congeners of PCBs and the possibility that concentrations of both types of congeners will depend heavily on previous human activities such as fish consumption. These data are consistent with the previous statement that dioxin-like PCBs may account for approximately one-third of the total TEQ in the general population. 

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