As CO (carbon monoxide) detectors become more popular, the placement of these detectors comes into question. CO has the approximately the same density as normal air and as such it makes very difficult to find the optimum position to mount these detectors. As a number of these detectors are mains powered via a standard plug socket it is a common myth that the correct height is just above floor height. This is not necessarily the correct position it is just convenient as this is where most mains 220 outlets are fitted. In fact the best height is where you are trying to measure the effects of CO, for example if it is to protect someone sleeping in a room then the best height is about the same level as the bed.
Placing detectors on the ceiling will work but if the CO source is low in the room CO will have to fill the room before the detector goes into alarm.