home edit2 whole TSD November 2002 PDF format - OEHHA

scheme. TEFs for two major noncancer toxicity endpoints, teratogenicity and immunotoxicity, are
presented in Table 3. An important feature of this table is the relatively high toxicity of the 2,3,4,7,8-
pentaCDF, based on teratogenicity (TEF = 0.1) and immunotoxicity (TEF = 0.8).
A comparison of the CTEF and ITEF schemes (Table 2) shows the CDHS to ITEF ratios are variable.
For example, the CTEF/ITEF for 1,2,3,7,8-pentaCDF is 20, whereas the same ratio for the 2,3,7,8-
hexaCDD and hexaCDF congeners is 0.3. In each case, the CTEF was based on an assumed
equivalence of carcinogenicity, whereas the ITEF was based on experimental data from cancer and
noncancer studies. For the octa-congeners of PCDD and PCDF, the ITEF value is 0.001 whereas no
TEF was developed for the CTEF scheme. Although the ITEF = 0.001 is small, its use in the ITEF
scheme recognizes a heretofore unknown toxicity property of the octa congeners.
The ultimate use of a TEF scheme is to estimate the TEQ level in a sample. Therefore, a comparison of
TEQs in various environmental samples, as estimated by the CTEF and ITEF schemes, is of interest. In
Table 4, the TEQs, present in seven contaminated samples, are compared by the CTEF and ITEF
methodologies. The results suggest that where differences occur, they are less than 10-fold and appear
to be due largely to differences in the TEFs for the PCDFs, in particular the tetra- and penta-congeners.
When studying the results in Table 4, the reader should keep in mind the ITEF scheme was developed
from a much larger data base than the CTEFs. The CTEF scheme also assumes the 2,3,7,8-penta
congeners are equivalent to 2,3,7,8-TCDD, the 2,3,7,8-hepta-congeners are equivalent to the 2,3,7,8-
hexa-congeners, and the PCDFs are equivalent to their PCDD homologues. In the ITEF schemes, such
assumptions are not made but are based on available information.
Public Health Impact
Currently, two schemes, the CTEFs and the ITEFs, are in use in the State of California for risk
assessment of PCDD/PCDF contaminated samples. The CTEF scheme was incorporated into the Hot
Spots Risk Assessment Guidelines (CAPCOA, 1993), while the ITEF scheme was adopted by the
DTSC (DTSC, 1992). The ITEF scheme incorporates more data on congener specificity and multiple
end-points. The use of multiple endpoints is important for PCDD- and PCDF-induced toxicities,
because the evidence is clear that exposure to this class of compounds leads to many responses that
may be related by a similar mechanism of action, i.e. initial binding to the Ah receptor. Hence, the ITEF
scheme provides a more complete picture of the relative toxicity of the various congeners than does the
CTEF scheme. Since the TEFs are applied to carcinogenic and noncancer evaluations in risk
assessment, the ITEF scheme is more appropriate than the CTEF scheme.
The results of the two schemes are relatively consistent. Although the CTEF scheme sometimes results
in TEQs up to 4-times greater than the ITEF scheme, the differences in the TEQs are generally within
the uncertainties of the estimated carcinogenicity or noncancer toxicity of 2,3,7,8-TCDD.
Consequently, the impact using the ITEFs in place of the CTEFs to estimate PCDD/PCDF exposure
related risk, will be minimal.
570