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Polychlorinated biphenyls (PCBs) are a class of organic compounds composed of two interconnected phenyl rings, with a number (one to 10) of substituted chlorine atoms attached to these rings. There are a variety of variations, forms or congeners of PCBs, all of which have slightly different chemical properties.
PCBs are a member of a class of pollutants referred to as organochlorines and are conservative contaminants, that is, they persist in the environment for a long time, and they also bioaccumulate: PCBs that are not excreted build up in the tissues of contaminated organisms. As PCBs are lipophilic (fat soluble), they primarily tend to be stored in adipose (fat) tissues of organisms.
PCBs have been produced since the 1930s and have been used in insulating fluid for industrial transformers and capacitors; hydraulic fluids; surface coatings for copy paper; in synthetic resins, rubbers, paints, and waxes; and in lubricating oils, asphalts, and flame retardants. Due to environmental health concerns about this class of chemicals, many countries began banning the production of PCBs in the late 1970s and early 1980s. Until these bans, world production of PCBs was approximately one million tons-most of which has ultimately ended up entering the marine environment.
The structure of PCBs means that they can sometimes mimic hormones, which in turn leads to disruption of biological systems and health effects. As a result, reproductive failures have been recorded in several mammal species fed PCB-laced diets. Other PCB-induced reproductive defects include: altered menstrual cycles, embryo reabsorption, abortions, stillbirth, impaired infant survival or low birth weights, and impaired infant growth. PCBs can also affect the immune system, impacting lymphocyte production and increasing susceptibility to viral, bacterial, and protozoan infections. Other documented effects of PCBs include liver toxicity, thyroid and skin damage, cancer promotion, behavioral changes and impaired neurological development, and “reduced intelligence.”
Two well-known outbreaks of disease related to PCB poisoning have occurred. In 1968 rice paddies in Japan contaminated with PCBs led to an outbreak of the disease yusho, affecting 1,700 people. Symptoms, which included darkening of skin and acnelike skin eruptions, were recorded with PCB doses of 0.07 milligrams per kilogram of victim body weight. Another PCB-induced disease outbreak occurred in 1979 in Yu-Cheng, Taiwan, affecting 2,000 people. A more recent poisoning event occurred in January 1999, in Belgium, when 500 tons of animal feed contaminated with 50 kilograms of PCBs (and one gram of dioxins) were distributed to Belgian, Dutch, German, and French farms. Very quickly decreases in egg production were noted in some poultry farms, and ultimately over two million chickens were slaughtered due to contamination fears. When the incident was publicized in May 1999, it led to a major political and economic crisis. It has been estimated that due to PCB contamination alone, up to 6,776 extra cancer deaths could occur in the Belgian population.
As noted above, PCBs are lipid soluble, and one notable lipid-rich substance in humans is breast milk. In several areas of the world relatively high levels of PCBs have been reported in human breast milk and concerns have been expressed about the potential effects. One particular concern is that the effects of vitamin K metabolism may result in hemorrhagic disease in newborn infants. However, although some studies have shown correlations between breast milk PCB concentration and thyroid hormonal levels and impaired neurological development in infants, other studies contradict some of these results. Nonetheless the situation is one that requires monitoring and consideration.
Studies showing high levels of human PCB contamination have often noted a high seafood diet. Since the majority of PCBs have ultimately ended up in the marine environment, fish and other marine species with fat-containing tissues are at risk of PCB contamination. In particular, PCB contamination is a problem for marine mammal species as they are often top predators with long life spans, and may bioaccumulate and biomagify higher levels of PCB contaminants in their tissues. The thick blubber layers of many marine mammal species may also act as a PCB store, and their fat rich milk may pass on high levels of contaminants to their offspring. Marine mammal species that consume other marine mammals may be particularly at risk from PCB contamination, and several studies have linked reproductive and immune system abnormalities, high neonatal mortality rates, and even mass mortality events in several species of whales, dolphins, and seals to high levels of PCBs and other organochlorine contaminants. In turn, human communities in Alaska, Canada, Greenland, or other areas who consume high concentrations of whale or seal tissues in their diet may also be at greater risk of elevated dietary intakes of PCB as a result of marine mammal contamination.
- Brouwer et al., “Report of the WHO Working Group on the Assessment of Health Risks for Human Infants from Exposure to PCDDS, PCDFS and PCBS,” Chemosphere (v.37, 1998);
- E. Reynolds and S.A. Rommel, eds., Biology of Marine Mammals (Smithsonian Institution Press, 1999);
- Safe, “Polychlorinated Biphenyls (PCBs) and Polybrominated Biphenyls (PBBs): Biochemistry, Toxicology and Mechanisms of Action,” CRC Critical Reviews in Toxicology (v.13, 1984);
- van Larebecke et al., “The Belgian PCB and Dioxin Incident of January-June 1999: Exposure Data and Potential Impact on Health,” Environmental Health Perspectives (v.109, 2001).