In France, most of the ventilation systems in dwellings now consist of exhaust vents linked up with a fan, and air inlets. A survey conducted by the CSTB shows that actual ventilation rates are frequently different from prescribed values and that a lot of problems encountered are related to occupant behaviour, - The duration of exhaust flowrate peak value was measured ; it was shown that this duration was dependent on the kind of command and its location in the room. - draughts through air inlets were a major concern. - a lots of air vents did not operate correctly because of fouling. Reasons were that the inhabitants had not always a high consciousness of the necessity of cleaning, and that, moreover, a lot of air vents were not easily dismountable. Among conclusions of the survey, are the following : - air vents should be easily dismountable for cleaning and recommendations for it should be given to the inhabitants. - air inlets , exhaust vents and fan command should be correctly located.