Abstract: This invention relates to systems for clearing smoke from buildings when a fire occurs therein. Smoke detectors are provided in communal units in the building and means are provided for maintaining the pressure in accommodation units of the building above that in the communal units under normal conditions, so that there is a flow of air from the accommodation units to the communal units through the cracks in the doorways separating the accommodation units from the communal units. This flow of air ensures that, if a fire occurs in any one of the accommodation units, smoke will be carried into one of the communal units where it will be detected by one of the smoke detectors. When smoke is detected, conditions are reversed, so that the pressure in the communal units is maintained above that in the accommodation units. As a result, there will be a flow of air from the communal units to the accommodation units, once again through the cracks in the doorways.