U-shaped dose-response curves: implications for risk characterization of essential elements and other chemicals.

The existing risk assessment and management model is a general framework that can incorporate the assessment of numerous types of chemicals, such as essential elements. The general nature of the framework, however, often precludes precise statements of risk and only gives assurances of accuracy by way of a judicious use of conservative assumptions. A mode-of-action framework may provide approaches that are both more precise and accurate, and that may be used for characterizing both risk and benefit of essential elements, but such a framework needs to address the differing severity of adverse effects. Current dose-response data gaps often hinder the evaluation of the risk and benefit of several essential elements; however, new modeling methods, such as categorical regression, need to be further explored. Finally, the essentiality of certain elements provides evidence that two thresholds likely exist in an individual for adverse effect, one at low doses for undernutrition, and another at high doses for toxicity, for the element of concern. This evidence may aid in the estimation of such thresholds in populations.