Other assumptions, such as general approaches or assumptions under- 

 lying models that are commonly used to estimate risk, can be sum- 

 marized in the text of a risk assessment document. Some additional 

 assumptions involved in applying the risk assessment approach 

 described in this manual include the following: 



• Adverse effects in experimental animals are indicative of ad- 

 verse effects in humans (e.g., Hfetime incidence of cancer in 

 humans is the same as that in animals receiving an equivalent 

 dose in units of mg per surface area) 



• Dose-response models can be extrapolated beyond the range 

 of experimental observations to yield plausible-upper-bound 

 estimates of risk at low doses 



• A threshold dose does not exist for carcinogenesis 



• A threshold dose (e.g., NOAEL) exists for noncarcinogenic 

 effects 



• The most sensitive animal species is appropriate to represent 

 the response of humans 



• Cumulative incidence of cancer increases in proportion to the 

 third power of age (this assumption is used to estimate lifetime 



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