Characterization of risks posed by combustor emissions

Risk characterization is the final step of the risk assessment process as p racticed in the U.S. EPA. In risk characterization, the major scientific ev idence and "bottom-line" results from the other components of the risk asse ssment process, hazard identification, dose-response assessment, and exposu re assessment, are evaluated and integrated into an overall conclusion abou t the risks posed by a given situation. Risk characterization is also an it erative process; the results of a specific step may require re-evaluation o r additional information to: finalize the risk assessment process. Risks po sed by atmospheric emissions are an example of an involuntary human health risk which typically receives a great deal of public attention. Characteriz ation of the risks posed hv atmospheric emissions typically requires the us e of mathematical models to evaluate: I) the environmental fate of emitted pollutants, 2) exposures to these pollutants, and 3) human dose-response. I ntegration of these models results in quantitative risk estimates. The conf idence in a quantitative risk estimate is examined by evaluating uncertaint y and variability within individual risk assessment components. Variability arises from the true heterogeneity in characteristics within a population or an event; on the other hand, uncertainty represents lack of knowledge ab out the true value used in a risk estimate. U.S EPA's 1997 Mercury Study wi ll illustrate some aspects of the risk characterization process as well as the uncertainty and variability encountered in the risk assessment process.