Polychlorinated Biphenyls and Polybrominated Biphenyls - Organic Pollutants: An Ecotoxicological Perspective - page 150

6.4 Summary
PCB mixtures were once used for a variety of purposes, and came to cause wide-
spread environmental pollution. Over 100 different congeners are present in com-
mercial products such as Aroclor 1248 and Aroclor 1254. PCBs are lipophilic, stable,
and of low vapor pressure. Many of the more highly chlorinated PCBs are refractory,
showing very strong biomagnification with movement along food chains.
The toxicology of PCBs is complex and not fully understood. Coplanar PCBs
interact with the Ah-receptor, with consequent induction of cytochrome P4501A1/2
and Ah-receptor-mediated toxicity. Induction of P4501A1 provides the basis of
valuable biomarker assays, including bioassays such as CALUX. Certain PCBs, for
example, 3,3′,4,4′-TCB, are converted to monohydroxymetabolites, which act as thy-
roxine antagonists. PCBs can also cause immunotoxicity (e.g., in seals).
PCBs have been implicated in the decline of certain populations of fish-eating
birds, for example, in the Great Lakes of North America. Although their use is now
banned in most countries and very little is released into the environment as a conse-
quence of human activity, considerable quantities remain in sinks (e.g., contaminated
sediments and landfill sites), from which they are slowly redistributed to other com-
partments of the environment. There continues to be evidence that PCB residues are
still having environmental effects, for example, on birds and fish.
PBB mixtures have been used as fire retardants. Many of their constituent conge-
ners are highly persistent, and there was a major environmental accident in the United
States in which farm animals and humans became heavily contaminated by them.
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