Ventilation requirements have been derived primarily from those for odor control and secondarily from criteria for concentrations of air contaminants. The study reported here looked at criteria for both odor and physical acceptability and paid particular attention to the differences between smoking and nonsmoking occupancy in an environmental chamber. The results implied that, under nonsmoking conditions and with moderate humidity, between 5 and 10 cfm (2.5 and 5 L/s-1) of fresh air per occupant should satisfy 75% of visitors, but that, under smoking conditions, many times as much fresh air is needed for both odor acceptability and compliance with customary criteria values for smoke.