From the relationship between the degree of air pollution due to tobacco and its acute effects on healthy persons it is possible to estimate the still tolerable level of air pollution for healthy adults. The results and conclusions of the studies published by now on acute effects of passive smoking on patients with bronchial asthma are partially contradictory and do not allow a reliable estimation of the upper limit of tobacco smoke exposure for this particularly sensitive group. Because of methodological problems in studies with symptomatic asthmatics, further studies should be conducted which investigate patients with hayfever and asthma out of the pollen season, clinically healthy subjects with nonspecific bronchial hyperreactivity upon pharmacological challenge, and subjects who claim sensitivity to tobacco smoke.