Risk assessment generic exposure dose

In general, calculation of the risk or dose from waste disposal in the numerator of the risk index in Equation 6.2 or 6.3 involves the risk assessment process discussed in Section 3.1.5.1. As summarized in Section 6.1.3, NCRP recommends that generic scenarios for exposure of hypothetical inadvertent intruders at waste disposal sites should be used in calculating risk or dose for purposes of waste classification. Implementation of models describing exposure scenarios for inadvertent intruders at waste disposal sites and their associated exposure pathways generally results in estimates of risk or dose per unit concentration of hazardous substances in waste. These results then are combined with the assumptions about allowable risk discussed in the previous section to obtain limits on concentrations of hazardous substances in exempt or low-hazard waste. 

For each generic exposure scenario to be used in classifying waste, and taking into account all relevant exposure pathways in each scenario, calculate the dose per unit concentration of each hazardous substance in the waste. These doses generally would be the highest values calculated over an assumed time frame for the risk assessment (see Section 6.4.5.3), taking into account the time-dependence of the concentrations of hazardous substances in the waste. For example, the quantity calculated for radionuclides would be the annual effective dose (sievert) per unit activity concentration (Bq nr3), and the quantity calculated for hazardous chemicals would be the dose (intake, mg kg 1 d-1) per unit concentration (kg m 3). 

EPA risk assessment practice, (and the Risk Assistant software) reflects this distinction between generic and site-specific information in the prediction of the risks associated with an agent at a specific site. The results of authoritative Hazard Identifications and Dose-Response Assessments are not expected to be re-evaluated by the risk assessors evaluating specific sites these assessors are expected to concentrate on site-specific Exposure Evaluation and Risk Characterization. 

Because the significance of exposure has only been considered over the past few years, there is not as wide a selection of exposure models available as that for fate models. The latter have been applied for several decades to the calculation of ambient exposure levels compared with some standard values. Papers illustrative of human exposure assessments in this symposium include one on airborne pollutant exposure assessments by Anderson (2), a generic approach to estimating exposure in risk studies by Fiksel (5), and a derivation of pollutant limit values in soil or water based on acceptable doses to humans by Rosenblatt, Small and Kainz (6). 

Although the Hazard Identification and Dose-Response Assessment of an agent are generic, the risks that the agent poses are specific to a particular location and set of circumstances in which exposure to the agent occurs. In order to assess site-specific risks of an agent, the assessor must perform a specific Exposure Evaluation for the site, and combine it with information on the Hazards posed by the agent to yield a Risk Characterization for the site. 

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