Assessing RiskFeedback

In the Risk Characterization, the toxicity assessment (which identifies dose levels associated with adverse effects) is integrated with the exposure assessment (which identifies the potential dose for each type of receptor) to quantify potential health risks. The risk characterization reveals which COPCs are posing the most significant risks and which exposure pathways are important. The risk characterization process is depicted in the Figure below (cf, Risk Assessment Guidance for Superfund, Human Health Evaluation Manual, Part A, Interim Final).

For each exposure pathway and COPC, a cancer risk and/or noncancer chemical hazard is calculated. In the case of radionuclides, an annual radiation dose may calculated. The risks for each COPC may then be summed across exposure pathways where the pathways pertain to the same individual and exposure period. Risks may also be summed across COPCs for carcinogenic effects, and also for noncancer chemical hazard and for radiation dose. In practice, it is customary to sum cancer risks across multiple chemicals, but the summation of chemical hazard across COPCs should be qualified by knowledge of the major effects, target organs, and mechanism of action of the individual chemicals. EPA guidance for assessing human health risk from exposure to mixtures of chemicals is available via EPA's Office of Solid Waste and Emergency Response. EPA's 1995 Guidance for Risk Characterization provides a broad perspective on the role of risk characterization within a risk management framework.

An important aspect of Risk Characterization is an Uncertainty Analysis that describes the level of confidence in the results and any known biases. Uncertainty pertaining to Data Collection and Evaluation encompasses sample collection activities, laboratory sample preparation and analysis, and data preparation and analysis. The use of the DQO process described above is intended to ensure that uncertainty related to data quality and representativeness has a minimal impact on the confidence in the risk assessment results. There are a number of potentially important uncertainties in the Exposure Assessment. Some of these may include,

There also uncertainties in the accuracy of the EPA criteria used to quantify chemical toxicity and in the assumptions employed when evaluating health effects across multiple exposure pathways and chemicals. Additionally, EPA toxicity criteria are generally based on models that incorporate protective biases to ensure that health risks are not underestimated for potentially sensitive receptors. The appropriate scope and complexity of an uncertainty analysis is discussed in the context of a tiered approach to risk assessment in Volume III (Process for Conducting Probabilistic Risk Assessment) of EPA's Risk Assessment Guidance for Superfund.