Assessment of toxicity is now one of the three important subprocesses in health risk assessment (RA). This subprocess requires the conduct of hazard identification and dose-response assessment. Dose-response assessment is an important step in RA in that the toxicities of concern should be related quantitatively to doses that induced them in order to appreciate the doses estimated in the exposure assessment. The task of performing dose-response assessment rests on two basic principles. The first is to identify the toxicities involved. The second is to extrapolate the existing dose-response relationships to low doses at which no observation on the effects is feasible or yet available.

Hazard identification is not an arbitrary activity or task in RA. Its scope is generally as broad as the spectrum of adverse effects outlined in Slide 7. In the United States, this task has a focus more on carcinogenic, developmental, reproductive, and neurotoxic effects, partly because of legislative actions and partly because these types of adverse effects are what the public perceives as the most dreadful. Although acute lethality, eye irritation, skin sensitization, and some others are also dreadful, they are considered by many people to be controllable and observable events (see Slide 6).

Because zero or de minimus risk is the ultimate goal of any RA, a great deal of efforts has been made to identify health hazards with valid biologic models and high quality toxicity data. To this end, data from epidemiologic studies, animal studies, and PB-PK type biologic modeling are often weighted, and are used to complement or supplement one another.