Both CO and nitrogen are molecular weight 28; oxygen is 32. The weighted average for air is 29. CO will be distributed fairly well throughout the room by molecular diffusion alone. If the molecules stratified by molecular weight difference, we'd have oxygen down low and nitrogen high, and you'd be asphyxiated just by standing up. Thermal currents due to air rising at interior walls and sinking by cold exterior walls helps the mixing.
However, as was said above, smoke and CO are produced by incomplete combustion, and sources usually involve fire or some other heat source, and both these pollutants tend to be transported upward by thermal convection initially. Early detection is important, so a high mount usually is best, although one could argue that optimum placement really depends on the configuration of a particular house or room and where any smoke or CO most likely would be introduced.
|