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. 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. 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. Structures of the twelve dioxin-like PCBs Coplanar or non- ortho The coplanar group members have a fairly rigid structure, with their two phenyl rings in the same plane. 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. Toxic effects vary depending on the specific PCB. About 130 different individual PCBs are found in commercial PCB products. PCBs are typically used as mixtures of compounds and are given the single identifying CAS number. There are 209 different chemical compounds in which one to ten chlorine atoms can replace hydrogen atoms. In PCBs, some of the hydrogen atoms in biphenyl are replaced by chlorine atoms. PCBs are derived from biphenyl, which has the formula C 12H 10, sometimes written (C 6H 5) 2. PCB-resistant materials include Viton, polyethylene, polyvinyl acetate (PVA), polytetrafluoroethylene (PTFE), butyl rubber, nitrile rubber, and Neoprene. PCBs readily penetrate skin, PVC (polyvinyl chloride), and latex (natural rubber). Intentional degradation as a treatment of unwanted PCBs generally requires high heat or catalysis (see Methods of destruction below). They can generate extremely toxic dibenzodioxins and dibenzofurans through partial oxidation. PCB mixtures are resistant to acids, bases, oxidation, hydrolysis, and temperature change. PCBs do not easily break down or degrade, which made them attractive for industries. As the degree of chlorination increases, melting point and lipophilicity increase, and vapour pressure and water solubility decrease. Other physical and chemical properties vary widely across the class. The density varies from 1.182 to 1.566 g/cm 3. They have dielectric constants of 2.5–2.7, very high thermal conductivity, and high flash points (from 170 to 380 ☌). They have low vapor pressures at room temperature. 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. The compounds are pale-yellow viscous liquids. Physical and chemical properties Physical properties
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. Environmental Protection Agency (EPA), PCBs cause cancer in animals and are probable human carcinogens.
The International Agency for Research on Cancer (IARC), rendered PCBs as definite carcinogens in humans. With the discovery of PCBs' environmental toxicity, and classification as persistent organic pollutants, their production was banned by United States federal law in 1978, and by the Stockholm Convention on Persistent Organic Pollutants in 2001. īecause 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. Polychlorinated biphenyls were once widely deployed as dielectric and coolant fluids in electrical apparatus, carbonless copy paper and in heat transfer fluids. PCB warning label on a power transformer known to contain PCBsĪ polychlorinated biphenyl ( PCB) is an organic chlorine compound with the formula C 12 H 10− x Cl x.
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