"Dielectric fluids in transformers and capacitors often contain polychlorinated biphenyls (PCBs) or chlorobenzenes. These substances may generate polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzo-p-dioxins (PCDDs) under certain conditions of combustion/pyrolysis. When electrical equipment containing these fluids is involved in an accidental fire, the resulting smoke, soot, and residues may be contaminated with PCDDs, PCDFs, and other chlorinated hydrocarbons. The full report contains a review of several laboratory studies investigating the sources of PCDDs and PCDFs as well as the conditions under which they are formed. In addition, some data from sites of actual fire incidents are available and are discussed. Chlorobenzenes and PCBs do not form PCDDs and PCDFs when heated in the absence of oxygen. During fires, the dielectric fluid of transformers or capacitors may be leaked or vented from ruptured casings. With exposure to oxygen, PCBs can produce PCDFs and chlorobenzenes can produce PCDDs. The particular isomers of PCDDs and PCDFs formed are related to the number of chlorine substituents in the reacting material."