The main goal of this study was to evaluate the magnitude of outdoor exposure to fine particulate matter (PM 10 ) potentially experienced by the population of metropolitan Mexico City. With the use of a geographic information system (GIS), spatially resolved PM 10 distributions were generated and linked to the local population. The PM 10 concentration exceeded the 24-hr air quality standard of 150 µg/m 3 on 16% of the days, and the annual air quality standard of 50 µg/m 3 was exceeded by almost twice its value in some places. The basic methodology described in this paper integrates spatial demographic and air quality databases, allowing the evaluation of various air pollution reduction scenarios. Achieving the annual air quality standard would represent a reduction in the annual arithmetic average concentration of 14 µg/m 3 for the typical inhabitant. Human exposure to particulate matter (PM) has been associated with mortality and morbidity in Mexico City; reducing the concentration levels of this pollutant would represent a reduction in mortality and morbidity and the associated cost of such impacts. This methodology is critical to assessing the potential ben-