As discussed in the last slide, health risk assessment (RA) can be defined either as a public health discipline, or straightly as a quantitative process wherein the risk is assessed and later characterized. Yet throughout this lecture, RA pretty much takes on the latter connotation in an effort to lead to a fuller treatment of health risk characterization (RC).

In its simplest term, RA as well as RC is a quantitative process comparing an estimated exposure level to a level pre-established as safe or insignificant. The risk involved is assumed to be proportionally greater if the
exposure at issue gets increased farther (within a reasonable region) above the safe level. In most cases, this dose-response relationship can be readily supported by toxicity studies in which test subjects are dosed at 3 or more levels. These toxicity studies are hence conducted not simply for hazard identification (HI), a key component of the RA paradigm initially developed by the National Research Council (NRC, 1983). About a decade later, NRC (1994) revised its initial RA paradigm to combine HI and dose-response assessment into a single component called toxicity assessment, realizing that the data and tasks required for dose-response assessment and for HI are almost inseparable.

In Lecture 3, references were given for several regulatory guidelines on the proper conducting of acute and long-term animal toxicity studies. The art and science of identifying and assessing hazards from exposure to environmental contaminants are not readily reflected in those guidelines or in earlier lectures. For completeness, they are thus discussed in the next two slides.