Risk assessment--the mother of all uncertainties. Disciplinary perspectives on uncertainty in risk assessment.

Uncertainty in the detection and evaluation of chemical hazards to health leads to challenges when conducting risk assessments. Some of the uncertainty has to do with data, some with incomplete understanding of processes, and some with the most fundamental ways of viewing the questions. True variability--across space, in time, or among individuals--complicates the search for understanding many important aspects of risk. A few statistical and toxicologic tools are available to assess uncertainty. Three methods of classifying uncertainty are briefly discussed. In addition, our disciplinary background may influence how we view and discuss variability and uncertainty. We rarely know as much as we think we do (and not just in risk assessment). Great uncertainty is likely to remain an important part of risk assessment for some decades to come.