Because of the possible threat to public health posed by photochemical air pollution, a need exists for experimental studies of short-term respiratory effects of air pollutant exposure in humans. Such studies require rigorous control and comprehensive documentation of the experimental air environment and exposure conditions to ensure that results are both reliable and relevant to public health questions. In addition to biochemical, behavioral, and clinical evaluations, comprehensive pulmonary testing is required to assure that effects at different levels of the respiratory tract are detected. An experimental design based on these principles is described. Studies using this design have shown a wide range of sensitivity to the pollutant ozone and important adverse health effects in sensitive individuals under exposure conditions similar to those experienced during ambient pollution episodes.