876 PCBs AND ASSOCIATED AROMATICS
It is not yet possible to estimate the sensitivity of humans
to the toxic effects of PCDDs. It has been suggested that
humans are resistant to the toxic effects of PCDDs. The sup-
position is not presently verifiable because of the lack of char-
acterization of human exposure. There is no doubt, however,
that the 2,3,7,8-tetrachlorodibenzo-dioxin (2,3,7,8-TCDD)
isomer is a potent toxin which has caused impaired reproduc-
tion and/or birth defects in every species tested and has been
described as the most toxic compound known to man.
In addition to PCDFs and PCDDs mentioned above there
are several classes of polychlorinated aromatic hydrocar-
bons which may be contained as impurities or degradation
products of PCBs. These classes include polychlorinated
terphenyls (PCTs), polychlorinated quaterphenyls (PCQs),
polychlorinated quaterphenyl ethers (PCQEs), polychlori-
nated naphthalenes (PCNs), polychlorinated biphenylenes(PCPs), polychlorinated pyrenes (PCPYs) and polychlori-
nated chrysenes (PCCYs).
Polychlorinated terphenyls have been found (Allen
et al.^13 ) to have qualitatively similar enzyme inducing effects
to those of PCBs but appear to be less potent (Goldstein^14 ).
Mixtures of PCBs and PCTs have been marketed but were
not widely used.
Polychlorinated quaterphenyls were found as contami-
nants of Yusho and Yu-Cheng oils but for a long time were
an unrecognized interference in the quantitation of PCB
contamination in these incidents. Studies by Hori et al.^15
and Kunita et al.^6 showed that the toxic effects of PCQs are
similar to PCBs.
Polychlorinated quaterphenyl ethers (PCQEs) have
been identified in Yusho and Yu-Cheng oils. This class of
compounds does not seem to have been tested for toxicityFIGURE 18 Compounds of concern.C016_003_r03.indd 876C016_003_r03.indd 876 11/18/2005 1:12:30 PM11/18/2005 1:12:30 PM