Step 2.3 Evaluating Exposures for Possible Health Effects

Studies in which specific amounts of a chemical are observed to cause indetifiable health effects in animals or humans.

Studies that compare an exposed population to a similar population that is not exposed in order to determine the possibility of an association between the exposure and any increased rate of a specific illness in the exposed population.

Sensitive populations are those people in a community who are unusually susceptible to exposure to a chemical. Sensitive populations may include children, older adults, people with various types of health problems, and those who are exposed to a chemical for a longer period of time than the general population (for example, people who are exposed to chemicals in their workplace).

The route of exposure is the way in which the chemical enters the body. Studies reviewed should be carefully selected so that the route of exposure for the study population is similar to that of the people in the community being evaluated.

Bioavailability is the percentage of the total amount of a chemical that is absorbed into the bloodstream. Only the absorbed amount of the chemical is available to cause harm. For example, people who are gardening might get soil that contains a pesticide on their hands. When they are eating, they might accidentally eat some of the soil containing the pesticide. Only a portion of the pesticide will be absorbed into the bloodstream because some amount of the chemical will remain attached to the soil and will pass through their bodies without being absorbed.

Chemicals often exist in different forms. Slight differences in the form can affect how a chemical moves in the environment, its bioavailability, and how it can affect a person's health. For example, one form of chromium, chromium III, is an essential nutrient, needed in the human diet. Another form of chromium, chromium VI, has been associated with cancer when it is inhaled.

Toxicokinetics describe how a chemical, once contacted, enters the body (absorption), moves around the body (distribution), changes within the body (metabolism), and is removed from the body (excretion). Chemicals, once metabolized, might become more or less toxic.