Risk Index for Waste Classification

For the purpose of developing the waste classification system described in Section 1.4.1, a simple method of evaluating risks to the public posed by radionuclides and hazardous chemicals in waste is needed. The term risk generally refers to the probability of harm, combined with the potential severity of that harm. In the context of hazardous waste disposal, risk is the probability of a response in an individual or the frequency of a response in a population taking into 

NCRP recommends that risks to hypothetical individuals at waste disposal sites should be evaluated in classifying waste, as described in the following section, and that the risk to an individual that arises from disposal of any hazardous substance be expressed in the form of a dimensionless risk index (RI). The risk index for the ith hazardous substance (Rid is defined in terms of the risk that arises from disposal of that substance relative to a specified allowable risk for an assumed type of disposal system (e.g., municipal/industrial landfill for disposal of exempt waste) as  

Risk is not always a useful measure of health impact in evaluating the risk index, because risk is not proportional to dose when a hazardous substance is assumed to have a threshold dose-response relationship. For this type of substance, the risk is presumed to be zero at any dose below a nominal threshold. Since the allowable dose of such substances should always be less than the threshold in order to prevent the occurrence of adverse responses, expressing the risk index in terms of risk would result in an indeterminate value when the dose is below the threshold and, more importantly, a lack of distinction between doses near the nominal thresholds and lower doses of much less concern. For any hazardous substance, including carcinogens for which risk is assumed to be proportional to dose without threshold, a generally useful form of risk indexes (Rid is in terms of dose  

Given the definition of risk indexes (Rid in Equation 1.1 or 1.2 and assuming that risks from exposure to the different hazardous substances in waste are additive, waste classes are defined by the requirement on each waste class and associated disposal system that  

Adding risk indexes (RR) for noncarcinogenic substances and combining risk indexes (RR) for carcinogenic and noncarcinogenic substances requires care, however, due to the assumed forms of the dose-response relationships. The evaluation of Equation 1.3 for mixtures of hazardous substances is described in Section I.5.5.4. 

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A risk index for waste classification to be used in conjunction with the framework is then developed, the combination of these constituting the recommended risk-based waste classification system and... 

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