INTRODUCTION The estimation of population exposure to air pollutants contributes greatly to the effective development of adequate ambient air quality or emission standards and pollutant emission control plans. Therefore@ it is crucial to develop technical tools that can provide quantitative information on population exposure to air pollutants. Such information can be useful in bringing greater realism and certainty to air pollutant risk assessment@ and encouraging a more explicit and objective accounting of the relationships between pollutant emissions and resultant air-quality-related human health effects in air quality planning. To these ends@ Systems Applications has performed work under contract to the American Petroleum Institute since 1982 to review the state of the art in population exposure modeling and to further develop and improve an existing population exposure model (Hayes@ Roth@ and Seigneur@ 1983; Hayes@ Seigneur@ and Lundberg@ 1984@ 1985; Hayes and Lundberg@ 1985). The population exposure model that was selected as the basis of the work is the National Ambient Air Quality Standard (NAAQS) Exposure Model (NEM) that was developed for the U.S. Environmental Protection Agency. This study addressed in particular@ the application of HEM to the estimation of exposure to urban ozone patterns.