Editing Polychlorinated biphenyl (section)

==Physical and chemical properties==
===Physical properties===
The compounds are pale-yellow [[viscosity|viscous]] [[liquid]]s. They are hydrophobic, with low [[water (molecule)|water]] [[solubility|solubilities]]: 0.0027–0.42&nbsp;[[nanogram|ng]]/L for Aroclors [[brand]],<ref name="unep"/>{{page needed|date=October 2015}} but they have high solubilities in most organic [[solvent]]s, oils, and [[fat]]s. They have low [[vapor pressure]]s at room temperature. They have [[dielectric constant]]s of 2.5–2.7,<ref>{{cite web|title=PCB Transformers and Capacitors from management to Reclassification to Disposal |url=http://www.chem.unep.ch/pops/pdf/PCBtranscap.pdf |archive-url=https://web.archive.org/web/20030621170714/http://www.chem.unep.ch/pops/pdf/PCBtranscap.pdf |url-status=dead |archive-date=2003-06-21 |website=chem.unep.ch |publisher=United Nations Environmental Program |access-date=2014-12-30 |pages=55, 63}}</ref> very high [[thermal conductivity]],<ref name="unep" />{{page needed|date=October 2015}} and high [[flash point]]s (from 170 to 380&nbsp;°C).<ref name="unep" />{{page needed|date=October 2015}}

The density varies from 1.182 to 1.566&nbsp;g/cm<sup>3</sup>.<ref name="unep"/>{{page needed|date=October 2015}} 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.<ref name="unep"/>{{page needed|date=October 2015}}

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.<ref>{{Cite book |url=https://books.google.com/books?id=mOvJCEEzlAgC |title=Halogenated Biphenyls, Terphenyls, Naphthalenes, Dibenzodioxins and Related Products | veditors = Kimbrough RD, Jensen AA |date=2012 |publisher=Elsevier |isbn=9780444598929|page=24}}</ref> They can generate extremely toxic [[Polychlorinated dibenzodioxins|dibenzodioxins]] and [[dibenzofuran]]s through partial oxidation. Intentional degradation as a treatment of unwanted PCBs generally requires high heat or [[catalysis]] (see [[#Methods of destruction|Methods of destruction]] below).

PCBs readily penetrate [[absorption (skin)|skin]], [[Polyvinyl chloride|PVC]] (polyvinyl chloride), and [[latex]] (natural rubber).<ref name="anzecc">{{cite book |title=Identifying PCB-Containing Capacitors |publisher=Australian and New Zealand Environment and Conservation Council |pages=4–5 |year=1997 |isbn=978-0-642-54507-7 |url=http://www.environment.gov.au/settlements/publications/chemicals/scheduled-waste/pubs/pcbid.pdf |access-date=2007-07-07 |url-status=dead |archive-url=https://web.archive.org/web/20121003010739/http://www.environment.gov.au/settlements/publications/chemicals/scheduled-waste/pubs/pcbid.pdf |archive-date=2012-10-03}}</ref> PCB-resistant materials include [[Viton]], [[polyethylene]], [[polyvinyl acetate]] (PVA), [[polytetrafluoroethylene]] (PTFE), [[butyl rubber]], [[nitrile rubber]], and [[Neoprene]].<ref name="anzecc"/>

===Structure and toxicity===
PCBs are derived from [[biphenyl]], which has the formula C<sub>12</sub>H<sub>10</sub>, sometimes written (C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>. 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 registry number|CAS number]] {{CAS|1336-36-3}}. About 130 different individual PCBs are found in commercial PCB products.<ref name="unep">{{cite book | author=UNEP Chemicals | title=Guidelines for the Identification of PCBs and Materials Containing PCBs | issue=1 | publisher=United Nations Environment Programme | year=1999 | pages=40 | url=http://www.chem.unep.ch/pops/pdf/PCBident/pcbid1.pdf | access-date=2007-11-07 | url-status=dead | archive-url=https://web.archive.org/web/20080414094255/http://www.chem.unep.ch/pops/pdf/PCBident/pcbid1.pdf | archive-date=2008-04-14}}</ref>{{rp|2}}

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 [[Congener (chemistry)|congener]]s.

[[File:dl-PCB beschriftet.svg|thumb|upright=2|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. It renders their structure similar to [[polychlorinated dibenzo-p-dioxins|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.<ref>{{cite journal | vauthors = Safe S | title = Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action | journal = Critical Reviews in Toxicology | volume = 13 | issue = 4 | pages = 319–395 | year = 1984 | pmid = 6091997 | doi = 10.3109/10408448409023762 }}</ref><ref>{{cite journal | vauthors = Safe S, Bandiera S, Sawyer T, Robertson L, Safe L, Parkinson A, Thomas PE, Ryan DE, Reik LM, Levin W | display-authors = 3 | title = PCBs: structure-function relationships and mechanism of action | journal = Environmental Health Perspectives | volume = 60 | pages = 47–56 | date = May 1985 | pmid = 2992927 | pmc = 1568577 | doi = 10.1289/ehp.856047 | jstor = 3429944 | bibcode = 1985EnvHP..60...47S }}</ref>

;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.<ref>{{cite journal | vauthors = Winneke G, Bucholski A, Heinzow B, Krämer U, Schmidt E, Walkowiak J, Wiener JA, Steingrüber HJ | display-authors = 3 | title = Developmental neurotoxicity of polychlorinated biphenyls (PCBS): cognitive and psychomotor functions in 7-month old children | journal = Toxicology Letters | volume = 102–103 | pages = 423–428 | date = December 1998 | pmid = 10022290 | doi = 10.1016/S0378-4274(98)00334-8 }}</ref><ref>{{cite book |title=A Risk Management Strategy for PCB-Contaminated Sediments |url=https://www.nap.edu/catalog/10041/a-risk-management-strategy-for-pcb-contaminated-sediments |author=National Research Council (United States) |publisher = National Academies Press |location=Washington, D.C. |date=2001 |doi=10.17226/10041 |isbn=978-0-309-07321-9}}</ref>

Di-''ortho''-substituted, non-coplanar PCBs interfere with intracellular [[signal transduction]] dependent on [[Calcium in biology|calcium]] which may lead to [[neurotoxicity]].<ref>{{cite journal | vauthors = Simon T, Britt JK, James RC | title = Development of a neurotoxic equivalence scheme of relative potency for assessing the risk of PCB mixtures | journal = Regulatory Toxicology and Pharmacology | volume = 48 | issue = 2 | pages = 148–170 | date = July 2007 | pmid = 17475378 | doi = 10.1016/j.yrtph.2007.03.005 }}</ref> ''ortho''-PCBs can disrupt [[thyroid hormone]] transport by binding to [[transthyretin]].<ref>{{cite journal | vauthors = Chauhan KR, Kodavanti PR, McKinney JD | title = Assessing the role of ortho-substitution on polychlorinated biphenyl binding to transthyretin, a thyroxine transport protein | journal = Toxicology and Applied Pharmacology | volume = 162 | issue = 1 | pages = 10–21 | date = January 2000 | pmid = 10631123 | doi = 10.1006/taap.1999.8826 | bibcode = 2000ToxAP.162...10C | url = https://zenodo.org/record/1229980 }}</ref>
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