According to the text, Hazardous Waste Management, "toxicology deals with the adverse effects caused by exposure of living organisms to chemical substances." The toxicity of a substance is difficult to determine. In general, the toxicity of a chemical is determined through animal studies which are extrapolated to humans. The extrapolated values are calculated for the most sensitive humans. For example, children are a main focus when determining risk because their immune systems are much weaker than the average healthy middle-aged person. Children are more succeptable to have an adverse effect through exposure to a hazardous chemical. The method of exposure to a contaminant also determines the toxicity level. Chemicals can enter the body through three different routes: dermal contact, ingestion, and inhalation. The entry route adversely determines the effect of a chemical on an individual.