Question: is carbon monoxide (co) lighter than air

Answer: Yes, I made the suggestion about not needing them on the ceiling into a stern warning. If you install them on the ceiling (unless they're specifically designed for it - most aren't) it could be deadly. = incorrect Carbon monoxide is not heavier than air - in fact it is slightly lighter as CO has the RMM of 28 whereas air is slightly more than this. It is therefore almost neutral density. However in a normal room, thermal convection current dominate transport of gases in a room. The ceiling is a relatively good place to have a CO detector as long as air flow from convectional currents in not affected and any boundary layer is overcome.