I think (looking at martins figures) that the problem is that co is lighter than oxygen, but not lighter than the nitrogen in the air, since the nitrogen makes up the bulk of the air (78%), and since it and the co are the same weight, it doesnt rise comparative to most of the atmosphere, but builds up at the higher elevations (or more to the point the oxygen sinks down to be just above the carbon dioxide)
without air circulation (ventilation) therefore the co builds up in the nitrogen and displaces the oxygen we need. on convection alone if the co levels are increasing, then it wont flow out of the ceiling vents as its mixed in with the nitrogen, but is slowly displacing the heavier oxygen. so stirring the air about will help, but the co will get you as its mixing with the nitrogen faster than it can flow though the ceiling vents. that and if the incoming air contains the co, then its not refreshing the oxygen.