894 PCBs AND ASSOCIATED AROMATICS
TABLE 24
The most active compounds are those with Cl substitution at the 2,3,7,
and 8 positions
Relative potencyPCDF TCDF TCDDDibenzofuran <10^4 <10^6
2,8-diCDF <10^4 <10^6
2,4,6-triCDF 3 10 ^53 10 ^7
2,3,8-triCDF 4.5 10 ^3 4.8 10 ^5
1,4,6,8-tetraCDF <10^4 <10^6
2,3,6,8-tetraCDF 2.0 10 ^4 2.1 10 ^6
2,4,6,8-tetraCDF <10^4 <10^8
2,3,7,8-tetraCDF 1 9 10 ^2
1,3,4,7,8-pentaCDF 4.8 10 ^1 5.2 103
1,2,4,7,8-pentaCDF 3.0 10 ^3 3.2 10 ^5
1,2,3,4,6,8,9-heptaCDF 3.8 10 ^3 4.1 10 ^5
Source: Reference 66: Bradlaw, J. and Casterline, J., Jr.; Induction of enzyme
activity in cell culture: PA rapid screen for detection of planar polychlorinated
organic compounds; J. Assoc. Off. Anal. Chem. 62 , 904 (1979).1016 is pyrolysed because of the PCB isomer distribution
present in the undegraded product.
Erickson et al.^23 have determined PCDF formation from
PCBs under fire conditions when PCB contaminated oils are
burned under a variety of conditions. The results indicated
that the optimum conditions for PCDF formation from PCBs
occurred near 675°C for 0.8 s or longer, with 8% excess
oxygen. The residence time of the PCBs in the hot zone did
not significantly affect the PCDF formation in the range 0.3
to 1.5 s. A linear relationship was found for PCDFs formed
versus the amount of PCBs present. A 4% conversion effi-
ciency of PCBs was observed.
The New York State Department of Health has developed
the concept of “2,3,7,8-TCDD equivalents” as a measure of
the effective toxicity of polychlorinated aromatic chemicals
present in a mixture. The parameter is used as a determinant
for the assessment of exposure and risk. The algorithm used
to derive the exposure equivalents takes into consideration
the relative toxicity of individual compounds. In short, if the
heat exchanger fluids involved in the Yusho and Yu-Cheng
incidents had been Aroclor 1016 rather than more highly
chlorinated PCBS, it is very unlikely that the events would
not have been noticed.
PCB concentrations in the blood of two groups of patients
were statistically correlated,^31 in the period 1973–1980 and 27
patients in 1981–1984. The AHH activity in the 1973–1980
group showed a positive correlation with blood PCB concen-
tration but the correlation was less statistically significant for
the 1981–1984 group. Both group AHH activities showed
a correlation to their former PCB in blood concentrations
but much less correlation to the later PCB concentrations.
Consequently, AHH activity of Yusho patients at the onset of
symptoms was probably higher than in its later stages sincePCBs and PCDFs had been eliminated from the body. Yusho
patients with higher AHH activity showed more serious skin
lesions than patients with lower AHH activity.
Furst et al.^70 have analysed the distribution of PCDF and
PCDD isomers in human milk samples from West Germany
and found that all of the samples contained a “typical” pat-
tern of PCDFs and PCDDs. The first study of this type was
reported by Rappe and his co-workers^71 on the identifica-
tion of specific isomers in human milk samples derived from
Sweden and West Germany. In addition, the levels of the
congeners identified in all of the samples were of nearly the
same proportion in each.
However, there was relatively little correlation between
either the PCDFs or PCDDs with the concentration of PCBs.
This suggests that the sources of contamination may not be
the same. Interestingly, the correlation coefficients for iso-
mers which differed by one chlorine are strikingly higher
than those for compounds which differ by more than one
chlorine. The PCDFs show a similar consistency. No expla-
nation was offered for the observation but it would be inter-
esting to extend the study to draw possible correlations
between the isomer distributions found in foods and those
which are excreted in breast milk.
The selective accumulation of tetra- to octa-chlorinated
dibenzo-p-dioxins and dibenzofurans in foods and human
tissues in northern Japan was reported by Y. Takizawa and
H. Muto in 1987.^72 They found that the distribution of iso-
mers was similar in food samples and in human tissue sam-
ples. Foods with a high fat content such as fish/shellfish and
meats contained high concentrations of PCDFs and PCDDs
relative to non-fat foods such as vegetables. The following
table illustrates the levels of contamination in daily foods in
northern Japan (Table 26).C016_003_r03.indd 894C016_003_r03.indd 894 11/18/2005 1:12:37 PM11/18/2005 1:12:37 PM