Toxic equivalent

Toxic equivalency factor (TEF) expresses the toxicity of dioxins, furans and PCBs in terms of the most toxic form of dioxin, 2,3,7,8-TCDD. The toxicity of the individual congeners may vary by orders of magnitude.

With the TEFs, the toxicity of a mixture of dioxins and dioxin-like compounds can be expressed in a single number - the toxic equivalency (TEQ). It is a single figure resulting from the product of the concentration and individual TEF values of each congener.

The TEF/TEQ concept has been developed to facilitate risk assessment and regulatory control. While the initial and current set of TEFs only apply to dioxins and dioxin-like chemicals (DLCs), the concept can theoretically be applied to any group of chemicals satisfying the extensive similarity criteria used with dioxins, primarily that the main mechanism of action is shared across the group. Thus far, only the DLCs have had such a high degree of evidence of toxicological similarity.

There have been several systems over the years in operation, such as the International Toxic Equivalents for dioxins and furans only, represented as I-TEQ_DF, as well as several country-specific TEFs. The present World Health Organisation scheme, represented as WHO-TEQ_DFP, which includes PCBs is now universally accepted.

Humans and wildlife are rarely exposed to solitary contaminants, but rather to complex mixtures of potentially harmful compounds. Dioxins and DLCs are no exception. This is important to consider when assessing toxicity because the effects of chemicals in a mixture are often different from when acting alone. These differences can take place on the chemical level, where the properties of the compounds themselves change due to the interaction, creating a new dose at the target tissue and a quantitatively different effect. They may also act together (simple similar action) or independently on the organism at the receptor during uptake, when transported throughout the body, or during metabolism, to produce a joint effect. Joint effects are described as being additive (using dose, response/risk, or measured effect), synergistic, or antagonistic. A dose-additive response occurs when the mixture effect is determined by the sum of the component chemical doses, each weighted by its relative toxic potency. A risk-additive response occurs when the mixture response is the sum of component risks, based on the probability law of independent events. An effect-additive mixture response occurs when the combined effect of exposure a chemical mixture is equal to the sums of the separate component chemical effects, e.g., incremental changes in relative liver weight. Synergism occurs when the combined effect of chemicals together is greater than the additivity prediction based on their separate effects. Antagonism describes where the combined effect is less than the additive prediction. Clearly it is important to identify which kind of additivity is being used. These effects reflect the underlying modes of action and mechanisms of toxicity of the chemicals.

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