Isn't carbon monoxide heavier than air? If so shouldn't CO detectors be put at floor level?

molecular weight of CO ~ 28 (12+16), while the main ingredients of air are O2 = 32 (2×16) and N2 = 28; mixed together, that’s heavier than CO. Alone from that, I’d say CO is lighter. Of course, other factors come into play for the density, but this is probably the main thing for that. Well, otherwise have a look at http://en.wikipedia.org/wiki/Carbon_monoxide, where the box on the right says “lighter than air”.
So CO rises. Therefore, again, keep your head down in case of smoke ;-)!