In controlled clinical studies, volunteers are deliberately exposed to specific air pollutants under conditions simulating ambient exposures, and health-related responses are documented. Studies of the health risks of air pollution need to be scientifically rigorous and clearly relevant to "real-world" pollution exposures. Their results should be confirmed by independent replication if they are to be used as a basis for air quality regulations. Well-designed controlled clinical studies readily meet these criteria, and complement the other methods of scientific risk assessment--animal toxicology and epidemiology. Clinical studies, toxicology, and epidemiology all have provided important information about air pollution health effects. A better understanding of the interrelationships of findings from these different fields is needed.