Dioxin in Air

Dioxins include a group of chemicals that share certain similar chemical structures and biological characteristics. 2,3,7,8,tetrachlorodibenzo-p-dioxin (TCDD) is the most well-studied and the most toxic of these compounds.

Scientists believe that dioxins cause toxic effects in similar ways. Because of this and because exposure is typically to variable mixtures of dioxins, EPA and others use Toxicity Equivalency Factors (TEFs) that compare the potential toxicity of each of the individual dioxins to the relative toxicity of TCDD. With such factors, the toxicity of a mixture can be expressed in terms of its Toxicity Equivalents (TEQs), which is the amount of TCDD it would take to equal the combined toxic effect of all the dioxin-like compounds found in that mixture. In this approach, the concentration of each dioxin is multiplied by its respective TEF. If a result is reported as non-detected the common conservative practice for determining its concentration is to set it to ½ the detection level. The products of the concentrations and their respective TEFs are then summed in order to obtain a single TCDD TEQ value for the complex mixtures of dioxins found in the sample.

Since the 1970's, different sets of TEFs have been developed and used for evaluating mixtures of dioxin compounds. Because a uniform set of TEFs does not exist and because dioxin is currently under reassessment by EPA, two sets of TEFs are discussed here. They are the International set and the World Health Organization (WHO) set.

The International approach was initially adopted by EPA in 1989 (U.S. Environmental Protection Agency, 1989). This procedure assigns TEFs for a total of 17 compounds. The current written EPA policy adopts this approach. Another notable approach for TEFs was established by WHO (Van den Berg et al, 1998). This approach also includes a total of 17 compounds. The WHO approach for developing TEF values differs from the International approach for 3 compounds, 2 of which would not alone significantly change any TCDD TEQ value. However, for one compound (a penta form of the dioxin) the TEF for the WHO 1998 approach is twice as high (1 vs. 0.5) as that of the International approach. EPA is currently in the process of formally evaluating the WHO approach as a part of the Dioxin Reassessment.

The results of the dioxin monitoring data are presented using the International approach.

References

U.S. Environmental Protection Agency (1989). Interim procedures for estimating risks associated with exposures to mixtures of chlorinated dibenzo-p-dioxins and dibenzofurans (CDDs and CDFs) and 1989 update. Washington, DC: Risk Assessment Forum. EPA/625/3-89/016)

Van den Berg et al. 1998. Toxic Equivalency Factors (TEFs) for PCBs, PCDDs, PCDFs for Humans and Wildlife. Environmental Health Perspectives 106 (12)