In a sealed room of approximately 60 m3, sidestream cigarette smoke was maintained at a constant level of approximately 20 ppm during a 3-hr experimental period. Air concentrations of carbon monoxide, nitrogen dioxide, hydrogen cyanide, acrolein, and other aldehydes were measured in situations with and without persons present. It was demonstrated that these smoke constituents were encountered in lower concentrations with than without persons present. It was further demonstrated that measurement of carboxyhemoglobin concentrations was not a good general indicator for exposure to various tobacco smoke constituents because of the vast differences in cigarette equivalent time values between the different smoke constituents. Subjective discomfort was estimated by means of questionnaires distributed every 30 min during the experimental period and was found to be almost identical irrespective of exposure to whole sidestream smoke or to only its gas phase. Exposure of volunteers to acrolein caused considerably less discomfort than exposure to sidestream smoke or to the gas phase alone.