As part of the on-going Harvard Study on the Health Effects of Sulfur Dioxide and Respirable Particulates, the authors have developed monitoring equipment for acidic particles that can be used in multiple field settings. Preliminary data suggest that these strong acid aerosol measurements may correlate with respiratory symptoms more closely than similar measurements of particulate matter less than 15 micro m in size. These results have led to the beginning of a U.S.-Canadian cooperative study to assess the chronic effects of acid aerosols on the health of North American children. Communities are being selected on the basis of anticipated levels of H2SO4 in ambient air along with predicted levels of ozone and nitrates. Each community will undergo a 1-year period of every other day, 24-hr monitoring with newly developed monitoring equipment that will allow for quantification of H ion concentrations, as well as for specific measures of ozone and acid fractions. At the end of the 1-year period, while measurements are still being made, approximately 600 children aged 7 to 11 in each of up to 24 communities will be assessed with standardized questionnaires completed by parents, and pulmonary function will be measured in the children while in school. By estimating chronic exposure from the year-long measurement of acid aerosols and consideration of specific criteria for selecting communities to study; the authors hope to minimize potential confounding to allow us to assess the chronic impact of strong acid in the atmosphere on the respiratory health of these children.