TEF concept in environmental health assessment

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Scope

How should the TEF concept be accounted for when making an environmental health assessment?

Rationale

The TEF methodology provides a mechanism to estimate potential health or ecological effects of exposure to a complex mixture of dioxin-like compounds. However, the TEF method must be used with an understanding of its limitations. This methodology estimates the dioxin-like effects of a mixture by assuming dose-additivity and describes the mixture in terms of an equivalent mass of 2,3,7,8-TCDD. Although the mixture may have the toxicological potential of 2,3,7,8-TCDD it should not be assumed for exposure purposes to have the same environmental fate as 2,3,7,8-TCDD. The environmental fate of the mixture is still the product of the environmental fate of each of its constituent congeners. Different congeners have different physical properties such as vapor pressure, practical vapor partition, water octanol coefficient, photolysis rate, binding affinity to organic mater, water solubility, etc. Consequently, both the absolute concentration of a mixture in an environmental medium and the relative concentration of congeners making up an emission will change as the release moves through the environment. For some situations, treating emission as equivalent to exposure, which assumes that modeling fate and exposure can be reasonably accomplished by treating a mixture as if it were all 2,3,7,8,-TCDD, is a useful but uncertain assumption. However, for many risk assessments the differences in fate and transport of different congeners must be taken into consideration and TEQ must be calculated at the point of exposure if more accurate assessments are to be achieved. Similarly, many dioxin releases are associated with the release of non-dioxin-like compounds such as pesticides, metals, and non-dioxin-like PHAHs, and their risk potential may also need to be assessed in addition to dioxin-related risk.[1]

There are instances where exposures to PCBs are the major problem. The TEF methodology provides risk assessors with a useful tool to estimate potential dioxin-related health risks associated with these exposures. Typically, the congener makeup of environmental exposures to PCBs does not resemble the congener profile of any of the commercial mixtures produced. Because the environmental mixtures do not resemble the commercial mixtures, it is not clear that using total PCB concentrations and comparing them to any of the commercial mixtures provides an accurate assessment of the potential risks. However, the use of the TEF methodology allows for the estimation of the risk associated with the dioxin-like effects of the mixture and may provide a more accurate assessment of the risk in conjunction with the use of total PCBs. The Agency has recently published an application of this approach to the evaluation of PCB carcinogenicity (U.S. EPA, 1996; Cogliano, 1998)

Result

The AhR mediates the biochemical and toxicological actions of dioxin-like compounds and provides the scientific basis for the TEF/TEQ methodology. In its 20-year history, this approach has evolved, and decision criteria supporting the scientific judgment and expert opinion used in assigning TEFs have become more transparent. Numerous countries and several international organizations have evaluated and adopted this approach to evaluating complex mixtures of dioxin and related compounds. It has become the accepted, interim methodology, although the need for research to explore alternative approaches is widely endorsed. Although this method has been described as a "conservative, order of magnitude estimate" of the TCDD dose, experimental studies examining both environmental mixtures and laboratory-defined mixtures indicate that the method provides a greater degree of accuracy when all effects are considered and may not be as conservative as sometimes described. Clearly, basing risk on TCDD alone or assuming all chemicals are as potent as TCDD is inappropriate on the basis of available data. Although uncertainties in the TEF methodology have been identified, one must examine the utility of this method in the broader context of the need to evaluate the public health impact of complex mixtures of persistent bioaccumulative chemicals. The TEF methodology decreases the overall uncertainties in the risk assessment process (U.S. EPA, 1999); however, this decrease cannot be quantified. One of the limitations of the TEF methodology in risk assessment is that the risk from non-dioxin-like compounds is not evaluated. This applies to both industrial/synthetic as well as natural ligands which are not considered to be dioxin-like, in addition to non-AhR ligands which may be interacting with dioxin-like chemicals in modulating their impacts on biological systems. Future research should focus on the development of methods that will allow risks to be predicted when multiple mechanisms are present from a variety of contaminants.

Since TEFs were first proposed in the 1980's, there have been several expert panels charged with evaluating and assigning TEF values to dioxin-like congeners. The development of the TEF methodology can be seen as an iterative process in which as more data was collected and our knowledge base on the mode of action and biological effects of these chemicals accumulated, the later panels provided more accurate assessments of the chemicals included in the TEF methodology. For example, the initial TEF proposals assigned values to all tetra-, penta-, hexa-, hepta- and octa-chlorinated dioxin and dibenzofuran congeners. Later evaluations assigned TEF values only to the 2,3,7,8-chlorine substituted congeners. The most recent expert panal to reevaluate and assign TEF values to dioxin-like congeners was the WHO panel convened in 1997 (Van den berg, 1998). This group of experts assigned TEF values to dioxin-like PCBs and revised TEF values for several of the chlorinated dioxins and dibenzofurans. The WHO98 TEF values are based on the most recent data available and it is recommended that these values supercede previous TEF values.

Thus, in summary, the WHO98 TEF values, which include dioxins, furans and dioxin-like PCBs, are the recommended TEF values. These are the TEF values recommended for use in human health risk analysis.

See also

Dependencies

References

  1. U.S.EPA (2003): Toxic Equivalency Factors (TEF) for Dioxin and Related Compounds. In: Exposure and Human Health Reassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds. Part II: Health Assessment for 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and Related Compounds. Chapter 9. NAS Review Draft NCEA-I-0836. December 2003. www.epa.gov/ncea.
    DISCLAIMER This document is a draft. It has not been formally released by the U.S. Environmental Protection Agency and should not at this stage be construed to represent Agency policy. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.