Development of the Risk Index for Individual Hazardous Substances

3 Development of the Risk Index for Individual Hazardous Substances 

For the purpose of developing a comprehensive and risk-based hazardous waste classification system, a simple method of calculating the risk posed by mixtures of radionuclides and hazardous chemicals in waste is needed. The method should properly account for the presence of subtances in waste that cause deterministic or stochastic responses. As a first step in accomplishing this, it is necessary to specify a method for calculating the risk posed by individual substances with associated deterministic or stochastic responses. 

NCRP assumes that the risk from disposal of any hazardous substance in waste can be described by means of a dimensionless risk index. The risk index for the ith hazardous substance is defined as the calculated risk from disposal of that substance, based on an assumed exposure scenario, relative to a specified allowable risk for the assumed type of disposal system. Based on this definition, the risk index is written as  

The risk index in Equation 6.2 is expressed in terms of risk (i.e., the probability that an adverse response will occur during an individual s lifetime). This definition is consistent with the fundamental objective of developing a risk-based hazardous waste classification system. However, the use of health risk per se in calculating the risk index presents some difficulties because risk is not proportional to dose for substances that cause deterministic effects. For this type of substance, the risk is presumed to be zero at any dose below a nominal threshold. Since the allowable dose 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 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, it is generally useful to express the risk index as the ratio of a calculated dose [e.g., sieverts, mg (kg d)-1] to an allowable dose that corresponds to an allowable risk  

NCRP believes that use of a risk index expressed in terms of dose is acceptable and desirable as long as (1) the units of the numerator and denominator are consistent at a conceptual level, (2) the assumptions embodied in the proportionality constants between dose and response for substances that cause stochastic responses are clearly stated, and (3) the allowable doses are adjusted when the proportionality constants between dose and response for substances that cause stochastic responses or the thresholds for substances that cause deterministic responses change significantly. 

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