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carcinogenic potency may decline in homologues more chlorinated than TCDD. It was therefore
assumed that PCDDs and PCDFs that are not chlorinated on the 2,3,7,8 positions or do not have at
least one ring position open are noncarcinogenic. Additionally, it was also considered that the
2,3,7,8-isomer PentaCDD has a carcinogenic potency equivalent to TCDD, and that 2,3,7,8-isomer
HeptaCDD is equivalent in carcinogenic potency to 2,3,7,8-isomer HexaCDD. The potencies for the
homologous PCDDs were also used for the PCDFs. Using this approach, the potency of a given
concentration of PCDDs would be 2% of the potency of TCDD. The potency of a mixture of PCDFs
would be 3% of the potency of TCDD.
Since 1986, another toxicity equivalency factor (TEF) scheme was developed during an international
symposium (NATO/CCMS, 1988a,b), and it was adopted by US EPA (US EPA, 1989) and the
Department of Toxic Substances Control (DTSC) (DTSC, 1992). The international scheme, referred
to as ITEFs, is based on experimental cancer and noncancer data for many 2,3,7,8-PCDDs and
2,3,7,8-PCDFs and on the assumption that the mechanism of all PCDD/PCDF-related biologic effects
are based on initial binding to a specific protein, the Ah receptor. Because the ITEF scheme
incorporates more experimental data from cancer and noncancer studies for more PCDDs/PCDFs than
does the CTEF scheme, the replacement of the CTEFs by the ITEFs is appropriate for use in risk
assessment. This approach also increases uniformity among Cal/EPA guidelines. The TEFs contained
in the dioxin TAC (CDHS, 1986) document and the ITEFs are listed in Table 8. A detailed discussion
of the ITEFs and their use can be found in Appendix A of this document. The cancer unit risks and
potency factors for chlorinated dibenzo-p-dioxins and dibenzofurans listed in this chemical summary and
in the Hot Spots Unit Risk and Cancer Potency Values table have been generated by applying the
appropriate ITEFs to the cancer unit risk and potency factor for 2,3,7,8-TCDD calculated in the dioxin
TAC document.
Table 8: Toxicity equivalency factors for chlorinated dibenzo-p-dioxins and
dibenzofurans (relative to 2,3,7,8-TCDD)
Compound
(Dioxins)
Congeners CDHS -TEFs ITEFs
Mono-, Di-, Tri-CDDs 0 0
TetraCDD 2,3,7,8- (1.0) (1.0)
others 0
PentaCDD 2,3,7,8- 1.0 0.5
others 0 0
HexaCDD 2,3,7,8- 0.03 0.1
others 0 0
HeptaCDD 2,3,7,8- 0.03 0.01
others 0 0
OctaCDD 0 0.001
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