Observed variability in response to toxic agents can be attributed to either dynamic or kinetic factors. Dynamic differences in response are due to the dose; host factors such as age, genetic background, and disease state; and environmental factors such as the form of the agent, atmospheric conditions, and the presence of other agents. The relative importance of a given route will depend upon the chemical form of the agent, and the individual and environmental conditions present during the exposure. Many believe, incorrectly, that some agents are toxic and others are harmless. A dose-response relationship can be constructed for each type of effect an agent is capable of producing. The duration of exposure may be the most important determinant of the dose, and therefore, of the agent’s toxicity. The partition coefficient is a measure of an agent’s distribution between an aqueous and lipid phase of a solution.