Application of the Modifying Factor in Risk Index

The modifying factor in the risk index in Equation 6.2 or 6.3 represents any factors deemed important in classifying waste other than those explicitly accounted for in the calculated risk or dose from waste disposal in the numerator or the allowable risk or dose for the waste class of concern in the denominator. The modifying factor generally can be substance- or waste-specific. 

NCRP notes that the modifying factor in the risk index should be applied independently of the requirement to achieve a negligible risk or dose for exempt waste or an acceptable (barely tolerable) risk or dose for nonexempt waste, in order to provide regulatory flexibility in classifying particular wastes. NCRP believes that such flexibility is highly desirable to promote cost-effective management and disposal of waste, provided it is applied in a transparent manner. 

4 Development of the Composite Risk Index for Multiple Substances 

The risk index defined in Equation 6.1 (see Section 6.2.1) is intended to provide a measure of the potential risk that arises from disposal of any waste that contains hazardous substances. In Section 6.3, the general definition of the risk index is elaborated and recommendations on suitable approaches to calculating the risk index for individual hazardous substances are presented. For purposes of developing a comprehensive and risk-based waste classification system, a simple method of calculating the risk from disposal of mixtures of hazardous substances is needed. The method must take into account that the allowable concentrations of particular hazardous substances in waste of a given class generally will be lower when multiple substances are present than when only a single substance is present. Such a method is presented and discussed in this Section. 

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