 They are generally drawn from the scientific literature or governmental statistics.For example, they may report informations such as amount of different food eaten by specific populations, divided by location or age, breathing rates, time spent for different modes of commuting, showering or vacuuming, as well as information on types of residences.Such information can be combined with contaminant concentrations from "ad-hoc" studies or monitoring network to produce estimates of the exposure in the population of interest.