The toxicity of active PODD/PCDFs is mediated through the aryl hydrocarbon receptor; a toxic equiva- lency factor (TEF) is used, assuming that the effects are additive and act via a common mechanism to cause toxicity. Since these compounds are considered to act by a common mechanism and because they occur as mix tures in the environment, they are commonly assessed and regulated as a class. However, detailed toxicolog ical information is available only for TCDD The TEF system was initiated for dioxins and furans in 1998 by North Atlantic Treaty organization's Committee on the Challenges of Modern Society (NATO/CCMS) scheme, dopted internationally and termed as International- TEFs (I-TEFs). The toxicity of PCDD/PCDF's is ex- pressed as toxic equivalents (TEQs) where the most toxic congener, TCDD, is rated as 1 and the less toxic congeners as fractions of this. Many of the other PCDDs and PCDFs are less potent than TCDD but vary con siderably in their respective concentrations. Each con- gener can be assigned a potency value relative to TCDD (TEF). When a TEF is multiplied by the congener concentration level, a TEQ is obtained. The toxicity of a mixture is stated as TEQ (TCDD equivalents) and is assumed to be equal to the sum of the concentration of individual congeners multiplied by their potencies (TEFi)