centration ¯Cj for the interval tj. A person's total exposure E to an airborne pollutant is the summation over all the microenvironments M in which the person is in contact with the pollutant:
The latter equation includes the totality of all locations and activities that the person can occupy and engage in.
To obtain the total exposure of a population Epop of N persons, it is necessary to sum the individual exposures Ei of all the persons in the population from i = 1 to N:
Generally, the amount of time spent in each microenvironment is averaged over the exposed population,
so that the average population exposure is given by
Thus, it is necessary to estimate the atmospheric concentration of the pollutant to which people are exposed to obtain Cj and their activity patterns to obtain tj.
It is often impossible or impractical to measure the exposures of individuals or populations directly, and instead mathematical models are used to estimate exposures. Microenvironmental concentrations are estimated with concentration models, which are based on the physics and chemistry of the environment. The time spend by an individual in a microenvironment with a pollutant is another important input to an exposure model. Population-exposure models combine data representing the time-activity patterns of an entire population with pollutant concentrations.
Gaussian-plume models are used by the Environmental Protection Agency (EPA) to estimate the concentration of a pollutant at locations some distance from an emission source. The models have this name because they represent the