Ozone is a secondary pollutant with many distinctive characteristics in respect to its sources and modes of formation within regions of the troposphere and in the stratosphere. The scales of intermediate and longer range transport influencing the atmospheric distribution of O3 will be discussed. As a result of these various processes, atmospheric O3 concentrations can vary substantially during each day, from day to day, and with season of the year as well as with geographical location. In contrast to ozone, sulfur dioxide is a primary pollutant. Both the sources of SO2 and its mechanisms of removal can be much different than for ozone. Sulfur dioxide exposures tend to be of concern on a local rather than on a regional scale. Because of the wide variations in distribution in O3 and in SO2 with location, local meteorology with the movements of weather systems, and season, duplication of ambient air exposures in experimental regimes is always difficult.