Editing Polychlorinated biphenyl (section)

===Air===
A small volume of PCBs has been detected throughout the Earth's atmosphere. The atmosphere serves as the primary route for global transport of PCBs, particularly for those congeners with one to four chlorine atoms.<ref>{{cite book |doi=10.1016/B0-12-369400-0/00785-7 |chapter=Polychlorinated Biphenyls (PCBS) |title=Encyclopedia of Toxicology |date=2005 |last1=Kulkarni |first1=Swarupa G. |last2=Mehendale |first2=Harihara M. |pages=509–511 |isbn=978-0-12-369400-3 }}</ref>

In the atmosphere, PCBs may be degraded by [[hydroxyl radical]]s, or directly by [[photolysis]] of carbon–chlorine bonds (even if this is a less important process).<ref>{{cite journal |vauthors=Xin ML, Yang JW, Li Y |title=The mechanism for enhanced oxidation degradation of dioxin-like PCBs (PCB-77) in the atmosphere by the solvation effect |journal=Chemistry Central Journal |volume=11 |issue=1 |pages=61 |date=July 2017 |pmid=29086899 |pmc=5503851 |doi=10.1186/s13065-017-0291-3 |doi-access=free }}</ref>

Atmospheric concentrations of PCBs tend to be lowest in rural areas, where they are typically in the [[picogram]] per cubic meter range, higher in suburban and urban areas, and highest in city centres, where they can reach 1&nbsp;[[nanogram|ng]]/m<sup>3</sup> or more.<ref>{{cite journal |last1=Diamond |first1=Miriam L. |last2=Melymuk |first2=Lisa |last3=Csiszar |first3=Susan A. |last4=Robson |first4=Matthew |title=Estimation of PCB Stocks, Emissions, and Urban Fate: Will our Policies Reduce Concentrations and Exposure? |journal=Environmental Science & Technology |date=15 April 2010 |volume=44 |issue=8 |pages=2777–2783 |doi=10.1021/es9012036 |pmid=20170162 |doi-access=free }}</ref> In [[Milwaukee]], an atmospheric concentration of 1.9&nbsp;ng/m<sup>3</sup> has been measured, and this source alone was estimated to account for 120&nbsp;kg/year of PCBs entering [[Lake Michigan]].<ref>{{cite journal |vauthors=Wethington DM, Hornbuckle KC |title=Milwaukee, WI, as a source of atmospheric PCBs to Lake Michigan |journal=Environmental Science & Technology |volume=39 |issue=1 |pages=57–63 |date=January 2005 |pmid=15667075 |doi=10.1021/es048902d |bibcode=2005EnST...39...57W}}</ref> In 2008, concentrations as high as 35&nbsp;[[nanogram|ng]]/m<sup>3</sup>, 10&nbsp;times higher than the EPA guideline limit of 3.4&nbsp;ng/m<sup>3</sup>, have been documented inside some houses in the U.S.<ref name="rudel2008"/>

Volatilization of PCBs in soil was thought to be the primary source of PCBs in the atmosphere, but research suggests ventilation of PCB-contaminated indoor air from buildings is the primary source of PCB contamination in the atmosphere.<ref>{{cite journal |vauthors=Jamshidi A, Hunter S, Hazrati S, Harrad S |title=Concentrations and chiral signatures of polychlorinated biphenyls in outdoor and indoor air and soil in a major U.K. conurbation |journal=Environmental Science & Technology |volume=41 |issue=7 |pages=2153–2158 |date=April 2007 |pmid=17438756 |doi=10.1021/es062218c |bibcode=2007EnST...41.2153J}}</ref>