PCBs AND ASSOCIATED AROMATICS 871
FIGURE 13 Likely substitution patterns.in the rat yields four monohydroxy-, four dihydroxy-, and
two trihydroxy-metabolites. The mechanism is shown in
the figure below as epoxidation of the biphenyl nucleus
followed by epoxide ring opening accompanied by a 1,
2-chlorine shift. Subsequent cyclisation of the hydroxylated
derivatives produced either by metabolism or as a result of
photo degradation in the environment or as a reaction by-
product of a PCB disposal method, can lead to the formation
of chlorinated dibenzofurans. The transformation of only
0.002% of a major constituent of an Aroclor mixture to the
corresponding chlorinated dibenzofurans would produce con-
centrations in the mixture corresponding to the values reported
by Vos et al. [Vos, 1970] as toxicologically significant.
Table 12 shows the concentrations of toxic PCB congeners
which have been found in commercial PCB mixtures.The toxic isomers of interest are shown in Figure 17.
The manufacturing process for PCBs caused the chlo-
rination of the hydrocarbons present to yield a number of
polysubstituted products. In some cases, the initial reaction
mixture was contaminated with trace quantities of dibenzo-
furans and naphthalenes. The polychlorinated dibenzofuran
(PCDFs) and polychlorinated naphthalene (PCNps) products
are compounds of concern even in trace quantities. Based on
a vast amount of published information in scientific journals
the causative agent of health effects attributed to PCBs is
most likely due to the presence of PCDFs-see the section
on “The Perception of PCB Health Effects; the Yusho and
Yu-Cheng Poisonings.”
Table 13 indicates the concentrations and types of
polychlorinated dibenzofurans in commercial products.C016_003_r03.indd 871C016_003_r03.indd 871 11/18/2005 1:12:28 PM11/18/2005 1:12:28 PM