Toxic equivalency factor approach for risk assessment of combustion by-products

Hazard and risk assessment of individual toxic chemicals utilizes data obtained from chronic toxicity and carcinogenicity studies in laboratory animals for regulating emission, cleanup or intake levels for individual chemicals. This approach can be utilized for problems associated with a single chemical emitted from a point source; however, in most situations, toxic chemicals are formed and emitted into the environment as complex mixtures of different structural classes of toxic/carcinogenic chemicals. Methodologies for risk assessment of complex mixtures have been developed for polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) which are found as industrial and combustion by-products. This methodology is based on the common mechanism of action for these chemicals and utilizes the most toxic PCDD/PCDF congener, 2,3,7,8-tetrachIorodibenzo-p-dioxin (TCDD), as a reference compound. The relative potencies or toxic equivalency factors (TEFs) are assigned to the other relevant PCDD/PCDF congeners and the TEF = EC₅₀ or ED₅₀(TCDD)/EC₅o or ED₅₀(test congener). Thus, for any complex mixture of PCDDs and PCDFs, the TCDD-like toxicity or toxic equivalents (TEQ) for the mixture can be calculated using the equation: TEQ = X [TEE x PCDD.L X X [TEE X PCDFJ_nwhere TEE is the TEF for the individual PCDD or PCDF congener and n is the number of compounds in each mixture. The TEF approach is currently used by regulatory agencies for risk management of PCDDs and PCDFs as well as being considered for polychlorinated biphenyls (PCBs). This paper discusses the problems associated with the TEF approach for risk assessment of halogenated aromatic and polycyclic aromatic hydrocarbons.