Polychlorinated biphenyls (PCBs) are organochlorine compounds with the formula C₁₂H_10−xCl_x; 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. They are highly toxic and carcinogenic chemical compounds, formerly used in industrial and consumer electronic products, whose production was banned internationally by the Stockholm Convention on Persistent Organic Pollutants in 2001.

Polychlorinated biphenyl

Chemical structure of PCBs. The possible positions of chlorine atoms on the benzene rings are denoted by numbers assigned to the carbon atoms.
Identifiers
CAS Number	1336-36-3
ECHA InfoCard	100.014.226
UN number	UN 2315
CompTox Dashboard (EPA)	DTXSID5024267
Properties
Chemical formula	C12H10−xClx
Molar mass	Variable
Appearance	Light yellow or colorless, thick, oily liquids
Hazards
NFPA 704 (fire diamond)	1 2 0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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.

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. 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 US 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.