In apartment buildings it is assumed that a fire is most likely to occur in an apartment. The principle is for the occupant of the apartment which is on fire to escape and the other residents to stay put (they may not even know there is a fire). As the occupant of the ‘fire’ flat escapes, it is probable that some smoke will get into the common corridor or lobby. It is important to ventilate this smoke to prevent it getting in to the common stair and it is the protection of this common stair that is the primary objective. This ventilation also affords some protection to the common corridors/lobbies but this is a secondary benefit – not a primary requirement.