POLYCHLORINATED BIPHENYL

(PCB) any of a class of highly stable organic compounds that are prepared by the reaction of chlorine with biphenyl. A commercial mixture of such chlorinated isomers of biphenyl provides a colourless, viscous liquid that is relatively insoluble in water, does not degrade under high temperatures, and is a good dielectric. Because of these qualities, PCBs are particularly useful as lubricants, heat-transfer fluids, and fire-resistant dielectric fluids in transformers and capacitors. They also are good plasticizers and have found application in paints, paper coatings, and certain packaging materials. PCBs came into widespread industrial use during the 1930s and '40s, but since the mid-1970s the production and application of these chemicals have been restricted because they have been found to be injurious to living organisms. PCBs were never intended to be released into the environment, but they found their way into the air, water, and soil via industrial and municipal waste disposal and leaks from mechanical equipment. The high resistance of PCBs to decomposition ensures that they remain in soils and bodies of water for many years, enabling them to accumulate and enter the food chain. PCBs are particularly toxic to fishes and invertebrates and are fatal to these animals even in small concentrations. PCBs cause liver dysfunction, dermatitis, and dizziness in humans exposed to them. The chemicals are also suspected of being carcinogenic. PCB concentration levels in the environment have dropped since the manufacture and use of the compounds were curtailed in several countries.