The Harvard Air Pollution Health Study has been a ten year prospective study of respiratory symptoms and pulmonary function of children and adults living in six U.S. communities. Indices of acute and chronic effects of air pollution exposures have been studied. Evidence is presented for adverse effects of ambient and indoor air pollution on children. Relationships between ambient TSP concentrations and hospital emergency room admissions, temporary decreases in pulmonary functions and prevalence of community bronchitis all indicate a slight adverse effect. Refinements of these relationships will occur when fine fraction and acid sulfate aerosol concentrations are incorporated into the health analysis. Exposures to cigarette smoke at home are associated with increased reported respiratory symptoms in children. There is a negative relationship between maternal smoking and age and sex adjusted height for children. Results from indoor and personal exposure studies have lead to the design of an acute symptoms and indoor air pollution study. Between 1985 and 1988 1800 children will be tracked for a year while respirable particles, nitrogen dioxide, water vapor and air exchange will be measured in their homes. Using continuous sulfate/sulfuric acid monitors built at Harvard, we are characterizing the magnitude, duration and frequency of acid aerosol events in each of our study cities. This information will be utilized in the analysis of the respiratory symptom data. The Harvard Air Pollution Health Study is providing information on the relationship among health variables and air pollutant exposures. In addition, this study will add to our understanding of lung growth and aging and the risk factors associated with chronic respiratory disabilities.