Human health is probably the most important reason for concern over atmospheric ozone pollution. Controlled studies of humans cannot answer some questions concerning possible long-term harmful effects, but they can provide the most definitive means of documenting short-term, adverse effects which are likely to occur in ambient exposures. Lung function tests, biochemial analyses of red blood cells, and clinical evaluations have revealed harmful short-term effects in some volunteers exposed to ozone concentrations of 0.4 to 0.5 ppm under conditions simulating ambient exposures. One laboratory has reported adverse changes after exposures to 0.1 ppm, but these findings have not been independently confirmed as yet. Factors which may modify toxic responses to ozone include preexisting disease, biological adaptation, and coexisting pollutants which may enhance toxicity.