Polychlorinated biphenyls (PCBs) are a group of synthetic chlorinated organic compounds that are of public health concern because of their persistence in the environment and their potential cancer-causing and endocrine-disruptive effects. There are 209 individual PCB congeners of varying toxicity, but a smaller number accounts for most of the commercially distributed mixtures. PCBs usually have been sold as clear oily mixtures whose lubricating, insulating, and coolant properties have led to their being widely distributed for many industrial and commercial uses. They are relatively inert, making them particularly valuable for such uses as fireproofing, but their lack of reactivity is also responsible for environmental persistence and for bioaccumulation in the food chain.
Heavy exposure to PCBs due to contamination of cooking oil occurred in two episodes in Japan and Taiwan in which skin manifestations, including chloracne, were prominent effects. Also observed were abnormal hepatic function, neurophysiological alterations, and developmental effects in offspring.
Exposure to high levels of PCBs at the workplace can cause skin and upper respiratory tract irritation. Environmental exposure to PCBs primarily occur through ingestion of contaminated seafood, but exposure can also occur through children ingesting contaminated soil and through the skin in landfill sites. PCBs are stored in fat and are present in breast milk. Inhalation of PCBs can also occur, particularly when there are indoor sources. PCBs can be carried long distances in air, which accounts for their distribution and accumulation in food chains in otherwise pristine arctic areas.
The different PCB congeners have different rates of persistence, bioaccumulation, and toxicity. In general, as the extent of chlorination increases, the rate of metabolism and detoxification decreases. The position of the chlorine atoms on the phenyl rings also affects metabolic rate and toxicity. Contamination of commercial PCB mixtures, particularly with chlorinated dibenzofurans, may also contribute to toxicity.
Acute toxicity due to PCBs is not of concern. However, PCBs are definitely carcinogenic to laboratory animals and are considered to be carcinogenic to humans. Concern about the potential for endocrine-disruptive effects, including developmental abnormalities, is similar to the concern for other persistent chlorinated compounds. Polybrominated biphenyls (PBBs) are in many ways similar to PCBs although far lesser amounts have been produced. An episode of PBB-contaminated cattle feed in Michigan led to human consumption of contaminated meat and dairy products, resulting in evidence of immunological dysfunction.
PCBs have been among the persistent organic pollutants (POPs) that have been considered to be of particular concern in many national and international deliberations. The United States banned the manufacture of PCBs in 1977, but PCB mixtures still remain in old electrical equipment and other items manufactured before 1977. There is also substantial PCB contamination of landfills and rivers. The Hudson River has been heavily contaminated by dumping from an electrical-device manufacturing facility. There has been much controversy concerning whether dredging the Hudson River to remove PCBs may do more harm than good by causing pockets of PCBs to be stirred up and to enter the ecosystem food chain.
The United States Food and Drug Administration has established allowable tolerances for PCBs in a variety of foods, particularly dairy products and seafoods. With the possible exception of the arctic, in recent years there has been a decline in PCB levels in human fat and in the general environment.
BERNARD D. GOLDSTEIN
(SEE ALSO: Toxicology)
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