sure to air pollution adversely affects the growth of lung function during rapid lung development and if it has clinically significant adverse effects on final lung function attained during adolescence. Study Population. Fourth-grade children (average age: 10 years) were enrolled and followed prospectively for 8 years. Methods. Fourth-grade children (n 1759) were recruited from elementary schools in 12 communities of southern California. Spirometric data were attained annually for 8 years. Forced vital capacity (FVC), forced expiratory volume in the first second (FEV1), and midexpiratory flow rate were evaluated. The attrition rate of subjects was 10% of subjects per year. Air pollution–monitoring stations were set up in the 12 study communities to measure exposures to ozone, acid vapor, nitrogen dioxide, particulate matter, elemental carbon, and organic carbon. Linear regression was used to examine the relationship of air pollution to the FEV1 and other spirometric measures. Results. Over the 8-year period, deficits in the growth of FEV1 were associated with exposure to nitrogen dioxide (P .005), acid vapor (P .004), particulate matter with an aerodynamic diameter of 2.5 m (PM2.5) (P .04), and elemental carbon (P .007) even after adjustment for several potential confounders and effect modifiers. Associations were also observed for other spirometric measures. Exposure to pollutants was associated with clinically and statistically significant deficits in the FEV1 attained at the age of 18 years. For example, the estimated proportion of 18-year-old subjects with a low FEV1 (defined as a ratio of observed to expected FEV1 of 80%) was 4.9 times as great at the highest level of exposure to PM2.5 as at the lowest level of exposure (7.9% vs 1.6%; P .002). Exposure to ozone was not proven to be a contributor to lungfunction deficit. Conclusions. The results of this study indicate that current levels of air pollution in the communities evaluated have chronic, adverse effects on lung development in children from the age of 10 to 18 years. This long-term exposure leads to clinically significant deficits in attained FEV1 as children reach adulthood. Reviewers’ Comments. This prospective study illustrates that chronic exposure to particular air pollutants negatively impacts the progression of lung function. From a public health perspective, it is important to consider and ameliorate environmental exposures that can have an adverse effect on final attainment of lung function.