Insofar as risk is a measure of the probability that an adverse outcome would occur, health risk perception necessarily starts with the adverse health effect that we perceive or understand. Often times, adverse health effects may be described by one or more types of toxic reactions, and their definitions are not always uniformly accepted or followed.

In spite of this problem, according to most textbooks of toxicology (Hall et al., 1997; Hayes, 1994; Klaassen, 1996; Lu, 1996; Williams et al., 2000), allergic reaction is defined as an immunologically mediated response resulting from previous sensitization by a toxic agent or by one structurally similar to it. Idiosyncratic reaction is an abnormal response that is genetically determined and may take the form of extreme sensitivity to low doses of a toxic agent or extreme insensitivity to high doses. Reversible and irreversible reactions are, respectively, those in which the tissue has the capacity for regeneration from injury and those in which the tissue does not have this capacity. Immediate and delayed reactions are, respectively, those that occur or develop rapidly and those that do not. Local and systemic reactions are, respectively, those responding at the site of contact with the agent and those responding beyond the site of contact following adsorption and distribution of the agent.

For purposes of toxicity testing, adverse health effects may also be described as skin or eye irritations, dermal sensitization, acute, subchronic, chronic, developmental, reproductive, genetic, mutagenic, carcinogenic, and more.