Composite Risk Index for All Hazardous Substances

Hazardous waste generally can contain mixtures of substances that cause stochastic or deterministic responses. The composite risk index for any mixture of hazardous substances in a given waste can be represented as the sum of risk indexes for multiple substances that cause stochastic or deterministic responses given in Equations 6.4 and 6.5  

the index j denotes whether the waste is being investigated for classification as exempt or low-hazard. It is included because 

Given the form of the deterministic risk index in Equation 6.5, which results in zero or integer values, the composite risk index for all hazardous substances in Equation 6.6 also can be expressed as the maximum of the separate risk indexes for multiple substances causing stochastic or deterministic responses  

If the risk index for all substances that cause deterministic responses in the waste (RId) in Equation 6.5 is zero (i.e., the doses of all substances that cause deterministic responses are less than the allowable values), classification is determined solely by the risk index for all substances that cause stochastic responses (RP) in Equation 6.4 the latter must be nonzero based on the assumption of a linear, nonthreshold dose-response relationship. On the other hand, if the risk index for all substances that cause deterministic responses is unity or greater, the calculated risk exceeds the allowable risk for the waste class of concern without the need to consider the risk posed by substances that cause stochastic effects. The only advantage of the form of the composite risk index in Equation 6.6 is that it indicates more explicitly that the total risk posed by a given waste is the sum of the risks posed by the two types of hazardous constituents, however approximate that representation may be. 

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Use of the composite risk index in classifying waste. Given the risk indexes for mixtures of substances causing stochastic or deterministic effects calculated using Equations 1.5 and 1.6, respectively, the composite risk index for all hazardous substances is calculated using Equation 1.4. This procedure assumes that induction of stochastic effects is independent of exposures to substances causing deterministic effects, and vice versa. 

For the purpose of illustrating how the composite risk index in Equation 6.6 would be used to classify a hypothetical waste, it is helpful to simplify Equations 6.4 and 6.5. This is done by assuming that the summation over all responses (index r) has been calculated, that only one waste classification boundary represented by the index j is being considered (i.e., the boundary between exempt and low-hazard waste, based on a negligible risk, or the boundary between low-hazard and high-hazard waste, based on an acceptable risk), and that the modifying factor (F) is unity. Further, the calculated dose in the numerator of the risk index is denoted by D and the allowable dose in the denominator is denoted by L. Then, the composite risk index for all hazardous substances in the waste, expressed in the form of Equation 6.6, can be written as ... 

The boundaries between different waste classes would be quantified in terms of limits on concentrations of hazardous substances using a quantity called the risk index, which is defined in Equation 6.1. The risk index essentially is the ratio of a calculated risk that arises from waste disposal to an allowable risk (a negligible or acceptable risk) appropriate to the waste class (disposal system) of concern. The risk index is developed taking into account the two types of hazardous substances of concern substances that cause stochastic responses and have a linear, nonthreshold dose-response relationship, and substances that cause deterministic responses and have a threshold dose-response relationship. The risk index for any substance can be expressed directly in terms of risk, but it is more convenient to use dose instead, especially in the case of substances that cause determinstic responses for which risk is a nonlinear function of dose and the risk at any dose below a nominal threshold is presumed to be zero. The risk index for mixtures of substances that cause stochastic or deterministic responses are given in Equations 6.4 and 6.5, respectively, and the simple rule for combining the two to obtain a composite risk index for all hazardous substances in waste is given in Equation 6.6 or 6.7 and illustrated in Equation 6.8. The risk (dose) that arises from waste disposal in the numerator of the risk index is calculated based on assumed scenarios for exposure of hypothetical... 

Using the separate risk indexes for chemicals that cause deterministic effects, chemicals that cause stochastic effects, and radionuclides obtained in Sections 7.1.7.4 to 7.1.7.6, the composite risk index for all hazardous substances in the waste is given by ... 

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