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Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are likely formed by thermal synthesis of a variety of primary precursors. Highest levels of these compounds are expected, however, when the starting material requires only one or two reaction steps for their formation, as is the case with chlorophenols, chlorobenzenes and polychlorinated biphenyls (PCBs). Laboratory pyrolyses have indeed shown that PCBs give significant yields of PCDFs, and chlorobenzenes give both PCDFs and PCDDs. In addition, a variety of other chloroaromatic compounds are formed. From these experiments and from accidents involving PCB fires, it is known that PCDFs are the most important toxic compounds associated with PCBs. Most commercial PCBs contain PCDFs in the low ppm range. PCDF concentration does not increase during normal operation in electrical equipment. Accidents (fires and explosions) involving PCBs can give PCDF levels in soot of up to 1000 ppm and higher. Effective thermal destruction of PCB is possible in modern incineration units, provided high temperatures, excess air and sufficient residence times are used. Exact figures for minimum temperature and residence time cannot be given, since feedstock and incinerator construction greatly influence destruction efficiency. Effluents from EPA-licensed incinerators used for PCB destruction contain only very low levels of PCDDs and PCDFs.

Formation of polychlorinated dibenzofurans and dioxins during combustion, electrical equipment fires and PCB incineration.