II 



Risk Comparisons 



Summary of Assumptions 



concentration for selected consumption rates and species (e.g., Figure 

 8) aid in rapid interpretation of tissue contamination data. 



Interpretation of carcinogenic risk assessment results may be based on 

 comparison of estimated health risks for the study area with: 



• Estimated health risks for consumption of fishery species from 

 a reference area 



• Estimated health risks for consumption of alternative foods 

 (e.g., charcoal-broiled steak, marketplace foods). 



An example of comparison with reference-area risk estimates is shown 

 in Figure 7 above. Comparative risks for alternative foods can be 

 summarized in a table or histogram. Wilson and Crouch (1987) point 

 out the importance of comparing the results of risk assessments with 

 similar assessments of common activities to provide perspective for 

 interpretation of the results by risk managers and the general public. 

 Risk comparisons should be based on consistent exposure analysis and 

 risk extrapolation models. Analogous exposure scenarios should be 

 used for each risk estimate being compared (i.e., either worst case, 

 plausible-upper limit, average, or lower limit). A single model should 

 be applied consistently to calculate exposure and risk. A linear ex- 

 trapolation model, such as Equations 2 and 6 above, is justified in 

 general if the excess risk attributed to the contaminant of concern is 

 regarded as a marginal risk, added to a background of relatively high 

 cancer incidence from all other causes not being modeled (Crump et 

 al. 1976; Omenn 1985). 



When interpreting the results of risk assessments, risk managers may 

 define an acceptable level of risk to provide a criterion forjudging the 

 significance of potential health effects. The term "acceptable risk" is 

 used to denote the maximum risk considered tolerable by an individual 

 or a regulatory agency. An acceptable risk level has not been strictly 

 determined by EPA. Although acceptable risk levels must be defined 

 on a case-specific basis, some perspective can be gained by examining 

 previous risk management decisions. For example, past regulatory 

 decisions by U.S. federal agencies have allowed environmental risks as 

 high as 10' to 10'" when the exposed population was relatively small 

 (Travis et al. 1987). For exposures of the entire U.S. population, the 

 acceptable risk level has usually been defined as 10 . 



Assumptions underlying the risk assessment model and estimates of 

 model variables should be summarized in a concise format (see Table 

 7 for summary of some assumptions and numerical estimates used in 

 the approach presented in this manual). Specific assumptions adopted 

 on a case-by-case basis should be summarized in a similar fashion. 



70 



