From
http://www.homesafe.com/coalert/detect.htm
When considering where to place a carbon monoxide detector, keep in mind that although carbon monoxide is roughly the same weight as air (carbon monoxide's specific gravity is 0.9657, as stated by the EPA; the National Resource Council lists the specific gravity of air as one), it may be contained in warm air coming from combustion appliances such as home heating equipment. If this is the case, carbon monoxide will rise with the warmer air.
For this reason, the makers of First Alert®, a leading brand in carbon monoxide detector technology, suggests mounting the detector on the ceiling. This also puts the detector out of the way of potential interference, such as pets or curious children.
Installation locations vary by manufacturer. Manufacturers’ recommendations differ to a certain degree based on research conducted with each one’s specific detector. Therefore, make sure to read the provided installation manual for each detector before installing.