The complex nature of polychlorinated dibenzo-p-dioxins, dibenzofurans and biphenyls, which are usually generated together in occupational or environmental exposure complicates the risk evaluation for humans. This is a concept introduced to facilitate risk assessment and regulatory control of exposure to these mixtures. 2,3,7,8-TCDD has been assigned a toxic equivalency factor (TEF) of 1.0. TEF values for individual congeners of dioxins, furans, and biphenyls in combination with their concentration can be used to calculate the total TCDD toxic equivalents concentration (TEQs) contributed by all dioxin-like congeners in the mixture using appropriate equations. Compounds are included in the scheme and assigned a TEF if they show structural relationships to PCDD or PCDF, bind to the aryl hydrocarbon receptor, elicit aryl hydrocarbon receptor mediated biochemical and toxic responses, and persist and accumulate in the food chain.