Approaches to Estimating Dose-Response Relationships for Radionuclides and Hazardous Chemicals

2 Approaches to Estimating Dose-Response Relationships for Radionuclides and Hazardous Chemicals 

An important technical issue that requires resolution in developing a comprehensive and risk-based waste classification system concerns the approaches that should be used to estimate health risks from a given exposure to radionuclides and hazardous chemicals. NCRP believes that reasonably consistent approaches should be used for all hazardous substances. Otherwise, some hazardous substances 

The use of MLEs of probability coefficients for radionuclides but UCLs for chemicals that induce stochastic responses is the most important issue that would need to be resolved to achieve a consistent approach to estimating risks for the purpose of waste classification. For some chemicals, the difference between MLE and UCL can be a factor of 100 or more. The difference between using fatalities or incidence as the measure of response is unlikely to be important. Use of the linearized, multistage model to extrapolate the dose-response relationship for chemicals that induce stochastic effects, as recommended by NCRP, should be reasonably consistent with estimates of the dose-response relationship for radionuclides, and this model has been used widely in estimating probability coefficients in chemical risk assessments. The difference in the number of organs or tissues that are taken into account, although it cannot be reconciled at the present time, should be unimportant. 

2 Approaches to Estimating Dose-Response Relationships for Substances That Cause Deterministic Responses. Most of the factors that must be considered in developing reasonably consistent approaches to estimating risk for radionuclides and chemicals that induce stochastic responses discussed in the previous section do not apply to substances that induce deterministic responses. For purposes of health protection, incidence generally is the appropriate measure of response for substances that cause deterministic responses. Furthermore, an accounting of deterministic responses 

In classifying waste, deterministic responses generally should be of concern only for hazardous chemicals (see Section 3.2.2.1). Therefore, the only important issue for risk assessment is the most appropriate approach to estimating thresholds for induction of responses in humans. The primary concern here is that consistent approaches should be used for all substances that induce deterministic effects. NCRP s recommendation that nominal thresholds in humans should be estimated using the benchmark dose method and a safety factor of 10 or 100, depending on whether the data were obtained in a study in humans or animals (see Section 6.1.2.1), is intended to provide consistency in estimating thresholds for all substances that cause deterministic effects. 

---