Encyclopedia of Environmental Science and Engineering, Volume I and II

874 PCBs AND ASSOCIATED AROMATICS

A poisoning very similar to the Japanese Yusho incident

occurred in Taiwan in 1978–1979 and is known as the “Yu-

Cheng” incident. In this case also, a leaking heat exchanger

caused the contamination of rice oil during the manufac-

turing process with a mixture of Kanechlor 450 (48% Cl)

and Kanechlor 500 (54% Cl). 1,843 cases of poisonings

were reported during the period 1978–1979 (Masuda and

Yoshimura;^5 Kunita et al. ;^6 Chen et al.^7 ). The PCB concen-

tration in the oil was less in this incident than in Yusho but

the higher dose rates to the victims resulted in approximately

the same total dose.

Hsu et al.^8 have estimated that the minimum effective

dose of PCBs in the Yu-Cheng incident was 0.3 to 0.5 g. The

ratio of PCDFs to PCBs in the Yu-Cheng rice oil was found

by Chen et al.^7 and Kunita et al.^6 to be approximately 1 : 300

and therefore the minimum effective total dose for PCDFs is

of the order of 1 mg.

The assessment of potential toxicity is exacerbated by

the fact that not all PCB, PCDF, PCDD, and so on, isomers

are equally toxic. It is therefore not sufficient to analyze for

the presence of a class of compounds but also necessary to

quantitate the biologically active isomers within a class.

There are 209 PCB isomers contained within the 10

homologous series of PCBs, see Table 10. The series result

from the number of chlorine atoms which substitute hydro-

gen on the biphenyl rings. A commercial arocior mixture

may contain 60 or more different isomers.

PCBs are absorbed in the fatty tissues of humans. The

ultimate fate of the PCB isomers depends upon their phar-

macokinetics so that the analysis of isomers remaining in a

tissue may show little resemblance to the isomers originally

ingested. Less chlorinated PCBs tend to be metabolized more

readily than their higher chlorinated congeners and therefore

arociors such as aroclor 1260 or 1254 tend to bioconcentrate

more than aroclor 1242.

There have been several studies which have concluded

that past exposure to PCBs is associated with excess mortal-

ity or morbidity from cancer in human populations (Bertazzi

FIGURE 16 PCB metabolism in birds and mammals.

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