Polychlorinated dibenzofurans, thermal

Of additional concern is the thermal conversion of PCBs to polychlorinated dibenzofurans, especially when PCB—filled electrical transformers are involved (16). Further, PCB isomers with no chlorine atoms substituted in the ortho. ortho -biphenvl positions are particularly potent as inducers of aryl hydrocarbon hydroxylase activity. The potency of certain dibenzofurans and non-ortho, ortho chlorine substituted PCBs in inducing activity of these enzymes is similar to that of the highly toxic 2,3,7,8-tetrachloro-dibenzo- -dioxin (15. 17., 18). 

Erickson MD, Swanson SE, Flora JD, et al. 1989. Polychlorinated dibenzofurans and other thermal combustion products from dielectric fluids containing polychlorinated biphenyls. Environ Sci Technol 23 462-470. 

Polychlorinated dibenzofurans (PCDFs) are formed as inadvertent by-products in the production and use of polychlorinated biphenyls (PCBs), formerly used as an insulator in electrical transformers and, in combination with polychlorinated dibenzo-p-dioxins (PCDDs), in the production of chlorophenols and have been detected as contaminants in these products. PCDFs and PCDDs also may be produced in thermal processes such as incineration and metal processing and in the bleaching of paper pulp with free chlorine. PCDFs are also found in residual waste from the production of vinyl chloride and the chlor-alkali process for chlorine production. The relative amounts of PCDF and PCDD congeners produced... 

Thermal destruction tests on PCBs indicate that essentially complete destruction occurs in well designed incinerator systems. The potential for formation of polychlorinated dibenzofurans (PCDFs) and dibenzo-p-dioxins (PCDDs) during thermal destruction of PCBs can be examined by thermochemical equilibrium calculations. The calculations predict that, under oxidizing conditions, formation of PCDFs and PCDDs is not thermodynamically favored. However, under pyrolytic conditions (absence or near absence of oxygen), as may arise in an inadequately designed or operated incinerator, thermochemical equilibrium calculations indicate that trace amounts of possible precursors to PCDFs and PCDDs can form [192]. 

Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) [1,2] are environmental pollutants particularly relevant for their toxicity. Their structure formulas are shown in Figure 21.1. Each of the carbon atoms at the 1,23/4-, and 6,73,9-positions can bond either with chlorine or with hydrogen atoms, so yielding 210 possible congeners (75 PCDDs and 135 PCDFs), different in the number and/or position of chlorine atoms. The PCDDs and PCDFs are chemically and thermally stable and very persistent in the... 

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