In the past, no scientifically valid approach for determining the exposures of an urban population to a pollutant was available. Recently, EPA successfully field tested two separate but complementary approaches for determining population exposures to air pollution: (a) direct measurement of exposure profiles of a representative random sample of the population with personal exposure monitors, and (b) an indirect approach which combines human activity pattern data with microenvironmental concentration data to estimate exposure profiles. In the winter of 1982-83, the two approaches were field tested. The effort provides a rich human exposuee data base and a methodology that can be applied to other pollutants and other urban areas.