Polychlorinated biphenyls (PCBs) are toxic and carcinogenic chemical chlorine compounds that were used until the 1980s primarily in transformers, electrical capacitors, in hydraulic systems as hydraulic fluids, and as plasticisers in paints, sealants, insulants and plastics. PCBs are now among the twelve organic toxins known as the "dirty dozen", which were banned worldwide by the Stockholm Convention of 22 May 2001.  

source https://www.chemie.de/lexikon/Polychlorierte_Biphenyle.html