Indicator approach, 2,3,7,8-TCDD

The indicator approach to estimation of spatial distributions of TCDD concentrations consists in estimating first the conditional probability distribution of any unknown concentration. Estimates of the unknown concentration are then derived, together with their confidence intervals. This approach has several outstanding features ... 

Toxic equivalency factors (TEFs) are estimated relative to 2,3,7,8-TCDD, which is assigned a value of 1. They are measures of the toxicity of individual compounds relative to that of 2,3,7,8-TCDD. A variety of toxic indices, measured in vivo or in vitro, have been used to estimate TEFs, including reproductive effects (e.g., embryo toxicity in birds), immunotoxicity, and effects on organ weights. The degree of induction of P450 lAl is another measure from which estimations of TEF values have been made. The usual approach is to compare a dose-response curve for a test compound with that of the reference compound, 2,3,7,8-TCDD, and thereby establish the concentrations (or doses) that are required to elicit a standard response. The ratio of concentration of 2,3,7,8-TCDD to concentration of test chemical when both compounds produce the same degree of response is the TEF. Once determined, a TEF can be used to convert a concentration of a dioxin-like chemical found in an environmental sample to a toxic equivalent (TEQ). 

The wasting syndrome is a characteristic effect of exposure to 2,3,7,8-TCDD in animals and, in its most severe form, is usually associated with lethality, particularly in rodents. The fact that the wasting syndrome has not been demonstrated in humans does not necessarily indicate that humans are insensitive to this effect of dioxins, but may indicate that human exposure has not approached acutely high enough levels. 

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