This evidence may be in the form of case reports, such as physicians' reports of an unusual number of cases of a specific illness. Because human information is very limited for most toxic air pollutants, scientists often conduct studies on laboratory animals, such as rats. Animal studies are performed under controlled laboratory conditions. Scientists can study a variety of health effects by exposing animals to pollutants at varied concentrations and for varied time periods. When relying on animal studies only, scientists need to be satisfied that health effects in humans are likely to be the same as those in the animals tested.
Scientists try to use animal species with body functions that are similar to humans.
An exposure assessment estimates how much of a pollutant people inhale during a specific time period, as well as how many people are exposed. There are many sources of toxic air pollutants. For example, a factory smokestack or thousands of automobiles crossing a busy intersection each day could be the source of a pollutant of concern. So the first step in an exposure assessment is to decide which sources are giving off the pollutant of concern. Engineers use either monitors or computer models to estimate the amount of pollutant released from the source and the amount of pollutant at different distances from the source.