PCBs or polychlorinated biphenyls are a class of 209 synthetic chlorinated aromatic compounds composed of two fused benzene rings and 1-10 chlorine atoms. First produced in the 1930ís, due to their stability and fire resistance, they found widespread use as electrical and capacitor insulating fluids in the form of commercial PCB mixtures known in the U.S. as Aroclors and by other names worldwide. Other uses included carbonless copy paper, adhesives, hydraulic fluids, fluorescent light ballasts and gas pipeline lubricants. Later, it was found that they were resistant to environmental degradation, bioaccumulative, toxic to both humans and animals and probably carcinogenic. For these reasons, production and sale of new PCB mixtures, except for research and testing purposes were legally prohibited starting in the late 1970ís.