Background:A unique, active, timely, low-cost surveillance system for the metropolitan area of Mexico City was established in the mid-1990s.Methods:The system obtained upper and lower respiratory tract symptoms and eye symptoms from daily interviews for a systematic sample of the general nonhospitalized population living within a 2-km radius of air pollution monitors during 1996-1997.Results:Ozone increments (10 ppb) were associated with upper respiratory symptoms (odds ratio [OR] = 1.003; 95% confidence interval [CI] = 1.002-1.004) and ocular symptoms indicators (OR = 1.005; CI = 1.004-1.007), and with a higher risk of lower respiratory symptoms indicator among nonsmokers (OR = 1.003; CI = 1.002-1.005). Increases in relative humidity reduced the risk of increments of sulfur dioxide on the 3 acute health indicators. Association of PM10with health indicators varied among the 5 regions. During emergency episodes, symptoms increased sharply when ozone reached 281 ppb, a finding that resulted in a change in the ozone criteria for emergency declaration from 294 to 281 ppb.Conclusions:This system has been low cost, timely, and useful for local decision making.