Dr. Loomis and colleagues at the University of North Carolina and Dr. Víctor Borja-Aburto of the Instituto Nacional de Salud Pública in Cuernavaca, Mexico, collected mortality, air quality, and weather data from records and monitoring stations in Mexico City from 1990 through 1992. Using statistical techniques, the investigators evaluated the association between mortality and ambient levels of ozone, sulfur dioxide, and total suspended particles, both individually and in a model that included all three pollutants.