Mixtures of polychlorinated biphenyls (PCBs) were manufactured for commercial purposes from 1929 onwards. They were chiefly used as coolants and insulation fluids in transformers and capacitors, and as lubricants and fire retardants. Most PCBs do not easily react to other chemicals and are heat-resistant. There are some 180 commonly known PCBs. According to the World Health Organization (WHO), production of PCBs between 1939-1976 was over a million tonnes. Half of this lies in landfills. The remainder pollutes water bodies in which industrial effluent has been released.

PCBs are not very water soluble, but mix easily with oil or fat. When organisms like algae and phytoplankton are exposed to contaminated water, they accumulate PCBs. Humans can be poisoned by eating fish feeding on these organisms. However, electrical equipment maintenance workers are directly exposed to PCBs. Symptoms of PCB poisoning include skin and respiratory problems, headaches, fatigue, dizziness, depression, impotence and sleep and memory disturbances. PCB poisoning is also linked to cancer.

Some 1,200 people died in the Yusho tragedy in Japan in 1968 after consuming rice bran oil contaminated with polychlorinated compounds. In Taiwan, 2,061 died in a similar case in 1978. Children born to women exposed to PCBs in these tragedies had abnormal growth. Recent studies have established that the deaths were due to poisoning by polychlorinated dibenzofurans (PCDFs), which were in the mixture that contaminated the oil.