Editing Polychlorinated biphenyl (section)

===Biosphere===
In the [[biosphere]], PCBs can be degraded by the [[sun]], [[bacteria]] or [[eukaryote]]s, but the speed of the reaction depends on both the number and the disposition of chlorine atoms in the molecule: less substituted, ''meta''- or ''para''-substituted PCBs undergo biodegradation faster than more substituted congeners.{{Citation needed|date=May 2013}}

In bacteria, PCBs may be dechlorinated through [[reductive dechlorination]], or oxidized by [[dioxygenase]] enzyme.{{Citation needed|date=May 2013}} In eukaryotes, PCBs may be oxidized by the [[cytochrome P450]] enzyme.<ref>{{cite journal | vauthors = Tiedje JM, Quensen JF, Chee-Sanford J, Schimel JP, Boyd SA |display-authors=3| title = Microbial reductive dechlorination of PCBs | journal = Biodegradation | volume = 4 | issue = 4 | pages = 231–240 | date = 1993–1994 | pmid = 7764920 | doi = 10.1007/BF00695971 | s2cid = 2596703 }}</ref>
[[File:Biomagnification.svg|thumb|[[Biomagnification]] is the increasing concentration of a substance, such as a toxic chemical, in the tissues of tolerant organisms at successively higher levels in a food chain.]]
Like many lipophilic toxins, PCBs undergo [[biomagnification]] and [[bioaccumulation]] primarily due to the fact that they are easily retained within organisms.<ref name=Schmidt>{{cite journal | vauthors = Schmidt C, Krauth T, Wagner S | title = Export of Plastic Debris by Rivers into the Sea | journal = Environmental Science & Technology | volume = 51 | issue = 21 | pages = 12246–12253 | date = November 2017 | pmid = 29019247 | doi = 10.1021/acs.est.7b02368 | bibcode = 2017EnST...5112246S | url = http://oceanrep.geomar.de/43169/4/es7b02368_si_001.pdf }}</ref><ref>{{cite journal | vauthors = Ferreira I, Venâncio C, Lopes I, Oliveira M | title = Nanoplastics and marine organisms: What has been studied? | journal = Environmental Toxicology and Pharmacology | volume = 67 | pages = 1–7 | date = April 2019 | pmid = 30685594 | doi = 10.1016/j.etap.2019.01.006 | bibcode = 2019EnvTP..67....1F | hdl = 10773/37415 | s2cid = 59306677 | hdl-access = free }}</ref>

Plastic pollution, specifically [[microplastics]], are a major contributor of PCBs into the biosphere and especially into marine environments. PCBs concentrate in marine environments because freshwater systems, like rivers, act as a bridge for plastic pollution to be transported from terrestrial environments into marine environments.<ref>{{Cite journal| vauthors = Ma P, Wei Wang M, Liu H, Feng Chen Y, Xia J |display-authors=3|date=2019-01-01|title=Research on ecotoxicology of microplastics on freshwater aquatic organisms|journal=Environmental Pollutants and Bioavailability|volume=31|issue=1|pages=131–137|doi=10.1080/26395940.2019.1580151 |doi-access=free|bibcode=2019EnvPB..31..131M }}</ref> It has been estimated that 88–95% of marine plastic is exported into the ocean by just 10 major rivers.<ref name=Schmidt/>

An organism can accumulate PCBs by consuming other organisms that have previously ingested PCBs from terrestrial, freshwater, or marine environments. The concentration of PCBs within an organism will increase over their lifetime; this process is called bioaccumulation. PCB concentrations within an organism also change depending upon which [[trophic level]] they occupy. When an organism occupies a high trophic level, like [[orcas]] or [[human]]s, they will accumulate more PCBs than an organism that occupies a low trophic level, like [[phytoplankton]]. If enough organisms with a trophic level are killed due to the accumulation of toxins, like PCB, a [[trophic cascade]] can occur.

PCBs can cause harm to human health or even death when eaten.<ref>{{cite news | vauthors = Faber H |title=Hunters who eat ducks warned on PCB hazard |url=https://www.nytimes.com/1981/10/08/nyregion/hunters-who-eat-ducks-warned-on-pcb-hazard.html |newspaper=New York Times |date=October 8, 1981}}</ref> PCBs can be transported by birds from aquatic sources onto land via feces and carcasses.<ref>{{cite journal | vauthors = Rapp Learn J |title=Seabirds Are Dumping Pollution-Laden Poop Back on Land |website=Smithsonian.com |date=November 30, 2015 |url=http://www.smithsonianmag.com/science-nature/seabirds-are-dumping-pollution-laden-poop-back-land-180957288/?no-ist|access-date=30 November 2015}}</ref>