polychlorinated biphenyl

Any of a group of chlorinated isomers of biphenyl (C₆H₅)₂. They are dangerous industrial chemicals, valuable for their fire-resistant qualities. They constitute an environmental hazard because of their persistent toxicity. Since 1973 their use had been limited by international agreement. In December 2000, 122 nations agreed a treaty to ban the toxic chemicals known as persistent organic polluters (POPs), which include PCBs, although they are unlikely to be totally eliminated until about 2025.