Polychlorinated hbiphenyls (PCBs) and polychlorinated terphenyls (PCTs) are complex organic chemicals used widely for their insulating properties. They are liquids of various densities containing chlorine, hydrogen and carbon and resemble mineral oils in appearance and consistency. They are not naturally occurring and are classified as synthetic oils. PCBs and PCTs have been manufactured since 1930, production reaching a peak in 1970. As the evidence linking PCBs with harm to wildlife and disease in man accumulated, the production and use of PCBs were restricted or eliminated in several industrialized countries; other Countries are considering similar steps. However, this is only a partial solution because these compounds continue to persist in the environment and to accumulate in human body tissue.