A series of field studies involving repetitive functional measurements in relatively small populations of healthy children and adults engaged in normal outdoor activities has shown that significant decrements in respiratory function are associated with exposures to ozone (O3) at concentrations below the national ambient air quality standard. The ability to detect such effects can be attributed, at least in part, to the study design criteria used, which emphasized maximization of signal-to-noise ratios. Locations were selected to ensure relatively high exposures to relatively uniformly distributed secondary pollutants, with minimal exposure to local sources of primary pollutants. Populations were selected that would be engaged in active recreation out of doors. Populations of healthy persons were used to minimize variability in baseline function. It was found that the magnitude of the O3-associated decrements in respiratory function was dependent on the variability in sensitivity to O3 among the population, the minute ventilation during outdoor activity, and the duration of the outdoor exposure.