Polychlorinated biphenyls (PCBs) are highly carcinogenic chemical compounds, formerly used in industrial and consumer products, whose production was banned in the United States by the Toxic Substances Control Act in 1976 and internationally by the Stockholm Convention on Persistent Organic Pollutants in 2001. PCBs are organochlorine compounds with the formula C12H10−xClx; they were once widely used in the manufacture of carbonless copy paper, as heat transfer fluids, and as dielectric and coolant fluids for electrical equipment. Because of their longevity, PCBs are still widely in use, even though their manufacture has declined drastically since the 1960s, when a host of problems were identified. With the discovery of PCBs' environmental toxicity, and classification as persistent organic pollutants, their production was banned for most uses by United States federal law on January 1, 1978, under Title 15 U.S.Code 2605(e) Polychlorinated biphenyls, in subsection (2)(A) established that after January 1, 1978, "no person may manufacture, process or distribute in commerce or use any PCB in any manner other than in a totally enclosed manner (...)" EPA may also regulate PCB disposal. The International Agency for Research on Cancer (IARC) rendered PCBs as definite carcinogens in humans. According to the U.S. Environmental Protection Agency (EPA), PCBs cause cancer in animals and are probable human carcinogens. Many rivers and buildings, including schools, parks, and other sites, are contaminated with PCBs and there has been contamination of food supplies with the substances. Moreover, because of their use as a coolant in electric transformers, PCBs still persist in built environments. Some PCBs share a structural similarity and toxic mode of action with dioxins. Other toxic effects such as endocrine disruption (notably blocking of thyroid system functioning) and neurotoxicity are known. The bromine analogues of PCBs are polybrominated biphenyls (PBBs), which have analogous applications and environmental concerns. An estimated 1.2 million tons have been produced globally. Though the EPA enforced the federal ban as of 1978, PCBs continued to create health problems in later years through their continued presence in soil and sediment, and from products which were made before 1979. In 1988, Japanese scientists Tanabe et al. estimated 370,000 tons were in the environment globally, and 780,000 tons were present in products, landfills, and dumps or kept in storage. Source: Wikipedia (en)