The various types of toxic reactions listed in the previous slide are problematic in terms of their actual use. For example, acute toxicity involves an adverse effect that by definition is manifested within a relatively short time interval following dosing or exposure. Yet the upper end of this interval is not well-defined. Also, a side effect of a drug may be outweighed by its therapeutic effect(s). Or some people may even argue that the intangible long-term undesired health effects of a pesticide are no match to the more tangible immediate economic gains. Academically, much of the implication of mutagenesis for carcinogenesis remains unknown. Still another example is that irritation scores given in a skin sensitization test are often based on some degree of subjective evaluation.

The problems or uncertainties with defining and interpreting these adverse health effects, along with the risk perception discussed in Slide 6, indeed have a great impact, directly or indirectly, on the way in which health risk assessments are performed or toxicity studies are pursued. These different health risk perceptions help explain in part why U.S. EPA (Lecture 2, Slide 19), U.S. FDA (Lecture 2, Slide 17), and other U.S. federal health agencies have applied different terminology to the safe human dose for certain adverse effects.