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A polychlorinated biphenyl (PCB; CAS number ) is a synthetic organic chemical compound of chlorine attached to biphenyl, which is a molecule composed of two benzene rings. There are 209 configurations of organochlorides with 1 to 10 chlorine atoms. The chemical formula for a PCB is C12H10−xClx. Of the different PCB arrangements and orientations 130 are used commercially. Polychlorinated biphenyls were widely used as dielectric and coolant fluids in electrical apparatus, cutting fluids for machining operations, carbonless copy paper and in heat transfer fluids. Due to PCBs' environmental toxicity and classification as a persistent organic pollutant, PCB production was banned by the United States Congress in 1979 and by the Stockholm Convention on Persistent Organic Pollutants in 2001. According to the U.S. Environmental Protection Agency (EPA), PCBs cause cancer in animals and are probable human carcinogens.〔(【引用サイトリンク】title=Health Effects of PCBs )〕 Many rivers and buildings including schools and other sites are contaminated with PCBs, and there have been contaminations of food supplies with the toxins. Some PCBs share a structural similarity and toxic mode of action with dioxin. Other toxic effects such as endocrine disruption (notably blocking of thyroid system functioning) and neurotoxicity are known. The maximum allowable contaminant level in drinking water in the United States is set at zero, but due to water treatment technologies, a level of 0.5 parts per billion is the de facto level. ==Physical and chemical properties== PCBs were produced and marketed as mixtures of many congeners. PCB mixtures are characterized by chlorine content. Less-chlorinated PCBs are odorless, tasteless, clear to pale-yellow, viscous liquids, while highly chlorinated mixtures are more viscous and deeper yellow. They are formed by electrophilic chlorination of biphenyl with chlorine gas. PCBs have low water solubilities — 0.0027-0.42 ng/L for Aroclors,〔 and low vapor pressures at room temperature, but they have high solubilities in most organic solvents, oils, and fats. They have dielectric constants of 2.5~2.7, very high thermal conductivity,〔 high flash points (from 170 to 380 °C)〔 and are chemically fairly inert, being extremely resistant to oxidation, reduction, addition, elimination, and electrophilic substitution. The density varies from 1.182 to 1.566 kg/L.〔 Other physical and chemical properties vary widely across the class. As the degree of chlorination increases, melting point and lipophilicity increase, and vapour pressure and water solubility decrease.〔 PCBs readily penetrate skin, PVC (polyvinyl chloride), and latex (natural rubber). PCB-resistant materials include Viton, polyethylene, polyvinyl acetate (PVA), polytetrafluoroethylene (PTFE), butyl rubber, nitrile rubber, and Neoprene.〔 The resistance of PCBs to oxidation and reduction in the natural environment makes them very stable compounds, not decomposing readily. They have a long half life (8 to 15 years) and are insoluble in water, which contributes to their stability. Their destruction by chemical, thermal, and biochemical processes is extremely difficult, and presents the risk of generating extremely toxic dibenzodioxins and dibenzofurans through partial oxidation. Intentional degradation as a treatment of unwanted PCBs generally requires high heat or catalysis (see Methods of destruction below). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Polychlorinated biphenyl」の詳細全文を読む スポンサード リンク
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