A family of chemical compounds formed by the addition of chlorine to a biphenyl system (a dual-ring structure comprising two benzene rings linked by a single carbon-carbon bond). They are categorized as a TOMP (Toxic Organic Micro Pollutant). PCBs are released into the atmosphere through the incomplete combustion of fuel or the processing of old electrical transformers or capacitors and are particularly long-lived, with atmospheric destruction of PCBs usually caused by reaction with OH radicals. The ultraviolet wavelengths that destroy PCBs do not make it to the troposphere because they are blocked by the ozone layer, but photolysis can occur in the upper atmosphere. Bioprocessing by organisms in soil and water is a much more common, though very slow destruction route for PCBs. PCBs have been found in some organism on all the earth's continents.