Polychlorinated biphenyls (PCBs) are organochlorine compounds with the formula C12H10−xClx. They were once widely used in a range of industrial and consumer electronic products including the manufacture of carbonless copy paper, as heat transfer fluids, and as dielectric and coolant fluids for electrical equipment. Some of them are highly toxic and have been classified as carcinogens. This has led to production being banned internationally by the Stockholm Convention on Persistent Organic Pollutants in 2001. The manufacture of PCBs has declined drastically since the 1960s when 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. However, because of their longevity and widespread use as a coolant in electric transformers, PCBs still persist in built environments. The International Agency for Research on Cancer (IARC) has classified 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. 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 similar 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 the ban. 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.
Physical and chemical properties
Physical properties The compounds are pale-yellow viscous liquids. They are hydrophobic, with low water solubilities: 0.0027–0.42 ng/L for Aroclors brand, but they have high solubilities in most organic solvents, oils, and fats. They have low vapor pressures at room temperature. They have dielectric constants of 2.5–2.7, very high thermal conductivity, and high flash points (from 170 to 380 °C). The density is 1.44 g/cm3 at 30 °C. Other physical and chemical properties vary widely across the class. As the degree of chlorination increases, melting point and lipophilicity increase, and vapor pressure and water solubility decrease. PCBs do not easily break down or degrade, which made them attractive for industries. PCB mixtures are resistant to acids, bases, oxidation, hydrolysis, and temperature change. They can generate 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). 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.
Structure and toxicity PCBs are derived from biphenyl, which has the formula C12H10, sometimes written (C6H5)2. In PCBs, some of the hydrogen atoms in biphenyl are replaced by chlorine atoms. There are 209 different chemical compounds in which one to ten chlorine atoms can replace hydrogen atoms. PCBs are typically used as mixtures of compounds and are given the single identifying CAS number 1336-36-3 . About 130 different individual PCBs are found in commercial PCB products. Toxic effects vary depending on the specific PCB. In terms of their structure and toxicity, PCBs fall into two distinct categories, referred to as coplanar or non-ortho-substituted arene substitution patterns and noncoplanar or ortho-substituted congeners.
Coplanar or non-ortho The coplanar group members have a fairly rigid structure, with their two phenyl rings in the same plane. It renders their structure similar to polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans, and allows them to act like PCDDs, as an agonist of the aryl hydrocarbon receptor (AhR) in organisms. They are considered as contributors to overall dioxin toxicity, and the term dioxins and dioxin-like compounds is often used interchangeably when the environmental and toxic impact of these compounds is considered. Noncoplanar Noncoplanar PCBs, with chlorine atoms at the ortho positions can cause neurotoxic and immunotoxic effects, but only at concentrations much higher than those normally associated with dioxins. However, as they are typically found at much higher levels in biological and environmental samples they also pose health concerns, particularly to developing animals (including humans). As they do not activate the AhR, they are not considered part of the dioxin group. Because of their lower overt toxicity, they have typically been of lesser concern to regulatory bodies. Di-ortho-substituted, non-coplanar PCBs interfere with intracellular signal transduction dependent on calcium which may lead to neurotoxicity. ortho-PCBs can disrupt thyroid hormone transport by binding to transthyretin.
Mixtures and trade names Commercial PCB mixtures were marketed under the following names:
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