The change in the perception of polluted air was studied during the first 15 min of exposure in climate chambers. Subjects, 16 female and 16 male, ages 18-30 y, served as air quality judges during 42 exposures. They were exposed to different concentrations of human bioeffluents (500-4000-mu-L/L CO2), tobacco smoke (0.25-2.5-mu-L/L CO), and emissions from building materials (-). The subjects voted every 2 min on scales for odor intensity and acceptability of the air quality, while they were exposed to constant levels of the air pollution. Perception of bioeffluents reached a low level independent of concentration after a few minutes. Adaptation to tobacco smoke caused acceptability to increase, but votes still depended on concentration. Ventilation for comfort may be reduced considerably if a few minutes of discomfort are acceptable or if the occupants are exposed to a gradually increased pollution level during the first 10 min or more of their stay in a space.