As I recall from my CO training several years ago, and explained in Rick's reference, CO is lighter by 1 molecule. So while there is technically a difference, it is a very small one and air movement that is in the home will overcome it.

What is advised is keeping the detectors out of corners and if on walls, not up against the ceiling. I am aware of no requirement that restricts low placement. One consideration that is seldom mentioned - dead end hallways. At night, when bedroom doors are likely to be closed, air movement can be substantially reduced and delay CO's reaching a detector at the far end of the hall.

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Eric Barker
Moraine Woods Consulting, Inc.
Barrington, IL