The usual way of extracting the waste air from a building is to convey the air above the roof. However, it would be easier to install exhaust and intake vents on the outer wall of the building. This paper presents the results of full-scale experiments thal were carried out on a three-storey building using tracer gas and video recording. To assess the measured concentration on the building wall, the odour threshold values of exhaust air during cooking or smoking were used. The results were in agreement with the theory underlying the use of roof exhausts near the exhaust opening but at longer distances (5-10 m) the measured concentrations were higher than predicted The highest two-minute mean intake air concentration was 0.5% of the exhaust air concentration. This relative concentration is smaller than the odour threshold value of 0.6%. The averaging time over which concentrations were measured is important. In this study two-minute averages were used. Instantaneous values were almost an order of magnitude higher. On the basis of the tests performed so far (exhaust and intake are located on the same straight wall, exhaust jet and wind have no obstruction and wind velocity is rather low), the wall exhaust system seems to be acceptable in residential buildings.