Nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide are important ambient air pollutants. High-intensity, confined space exposure to NO2 has caused catastrophic injury to humans, including death. Ambient NO2 exposure may increase the risk of respiratory tract infections through the pollutant's interaction with the immune system. Sulfur dioxide (SO2) contributes to respiratory symptoms in both healthy patients and those with underlying pulmonary disease. Controlled human exposure studies have demonstrated that experimental SO2 exposure causes changes in airway physiology, including increased airways resistance. Both acute and chronic exposure to carbon monoxide are associated with increased risk for adverse cardiopulmonary events, including death. However, studies have not demonstrated a clear dose-dependent health risk response to increasing amounts of these pollutants except at high concentrations. In addition, a number of studies examining the effects of ambient level exposure to NO2, SO2, and CO have failed to find associations with adverse health outcomes.