Recent Developments and Future Directions

The threshold of toxicological concern (TTC) concept has been developed to provide criteria for risk assessment decision-making in the absence of detailed information on chemicals. The approach involves estimating a tolerable human exposure value for all chemicals below which there is a very low probability of an appreciable risk to human health (Kroes et al. 2004, 2005), based on their chemical structures, compared to an extensive toxicity database. As utilized by U.S. FDA in their Threshold of Regulation procedure, structural alerts for high-potency carcinogenicity are included, to increase the assurance of safety. 

The TTC concept was adopted by the Joint FAOAVHO Expert Committee on Food Additives (JECFA) to evaluate flavoring agents in food, and it is also now used by the European Food Safety Authority. In the TTC decision-tree approach of Kroes et al. (2004), proteins, heavy metals, and dioxins were excluded because the database used to derive TTC values did not include proteins and heavy metals, and the extreme species-dependence of the dioxins and related compounds made it less useful for this category (compared to the existing toxicity equivalence factor method). This approach could be much more widely used to categorize trace chemicals in the environment as well as help prioritize the thousands of untested chemicals for further evaluation. 

While scientific advances are being made, political influences are also important. This can be seen in some cases such as the court-forced incorporation of the nonlinear method for EPA s assessment of chloroform instead of the linear extrapolation method. This brings to mind the importance of evaluation of carcinogenesis mechanisms by scientists rather then being settled in the courts. International decisions and deliberations are also affecting regulation of carcinogens (or at least the debate about them) in the United States. An example is the precautionary principle, which is often applied in the context of the impact of human actions on human health and the environment, in which the consequence of actions 

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