co can exist at high or low level, it can even linger in a corner of a cabin at a dangerous level while the rest of the space is ok, because of this detecters should be placed at head height, but remember whilst your sleeping where,s your head(hopefully not in the gutter!!) , all to do with the weight s of the different gases ,which i cant think of the exact answer at the mo!!
hear,s what i was trying to explain http://carbonmonoxidedetectorbeeping.com/correct-understanding-about-carbon-monoxide-weight-to-air/