polychlorinated biphenyl (PCB) One of a range of compounds, first synthesized in 1881 and manufactured in 1929 and used mainly as liquid insulators in heavy-duty electrical transformers. They were detected in the environment in 1966 and associated with reduced reproduction in marine birds and mammals; they are also believed to compromise the immune system. Restrictions on their use were imposed in North America and Europe in the early 1970s, but they continue to cause contamination, because of their persistence and as a consequence of careless disposal of old equipment containing them.

polychlorinated biphenyl (PCB) Any of several stable mixtures – liquid, resinous or crystalline – of organic compounds. They are fire-resistant and are used as lubricants and heat-transfer fluids. The use of PCBs has been restricted since 1973 because they are toxic and their resistance to decomposition in streams and soils poses a threat to wildlife.