TABLE 4-3 TEF Values from WHO (1998)


TEF valuea







3,3’,4,4’-tetrachlorobiphenyl (PCB 77)


aTEF = Toxicity Equivalency Factor, a numerical index that is used to compare the toxicity of different congeners and substances.

SOURCE: Van den Berg et al., 1998.

exposure as well as the role of specific PCB congeners or classes of congeners in health outcomes (Schantz et al., 2003; Ulbrick and Stahlmann, 2004).

Toxicity and Recommended Intake Limits for Dioxins, DLCs, and PCBs

Toxicity and Estimates of Risk The biological activity of dioxins, DLCs, and PCBs varies due to differences in toxicity and half-life of the various congeners. Variations in toxicity among congeners are related to a number of factors, including binding interaction at the cellular level with the arylhydrocarbon receptor (AhR) and variability in pharmacokinetics in vivo. Not all factors apply to all congeners; for example, many PCBs that do not have dioxin-like characteristics do not bind to the AhR. Van den Berg et al. (1998) describes factors used to determine the TEF values for dioxins, DLCs, and PCBs that include (but are not universal to all congeners):

  • Structural relationships between congeners;

  • Binding to the AhR;

  • Toxic responses mediated through AhR activation; and

  • Persistence and bioaccumulation.

The TEF value expresses the activity or toxicity of a specific congener relative to the toxicity of reference congeners, 2,3,7,8-TCDD; it is assigned a TEF of 1 and the toxicity of other congeners is expressed relative to TCDD (Van den Berg et al., 1998; IOM, 2003; SACN, 2004). Examples of some TEF values established by WHO are shown in Table 4-3. Toxicity can be additive in a mixture of congeners and so the Toxicity Equivalency (TEQ) of a mixture of DLCs is calculated by multiplying the concentration of each congener by its TEF, and summing across all DLCs in the mixture.

The Toxic Equivalency system is difficult to use, but it does permit extrapolation from 2,3,7,8-TCDD, a congener for which much is known.

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