In my position as Tech/Customer Support here at TC Life Safety, I get this question a lot. And since we are dealing with Life Safety, I don't want to just give you my opinion without backing it up with the science of the issue.
The density of a gas is proportional to the weight of a single molecule of that gas. That would mean that gases with similar atomic weights would mix more evenly whereas a lighter gas would simply float over a heavier one.
So with that in mind let's look at the atomic weights of gases that make up our air supply.
Atomic weights:
- Hydrogen(H)=1
- Helium(He)=4
- Carbon(C)=12
- Nitrogen(N)=14
- Oxygen(O)=16
Add them up for the molecular weights of pure gases:
- Hydrogen: H2 = (1+1) = 2, very light
- Helium: He = (4) = 4, very light
- Nitrogen: N2 = (14+14 ) = 28, about neutral
- Oxygen: O2 = (16+16) = 32, slightly heavy
- Carbon Dioxide: CO2= (12+16+16)= 44, heavy
- Carbon Monoxide: CO = (12+16) = 28, about neutral
- Methane: CH4= (12+4*1) = 18, light (majority part of natural gas)
- Water (Steam): H2O= (2*1+16) = 18, light
- Ethane: C2H6=(2*12+6*1)= 30, about neutral (minority part of natural gas)
- Propane: C3H8=(8 + 3*12)= 42, heavy
- Butane: C4H10=(10+4*12)= 58, heavy
- Pentane: C5H12=(12+5*12)= 70, lightest part of gasoline
For mixed gases just take a proportionate average:
- Air is 80% N2 + 20% O2
- Air = 0.8(28) + 0.2(32) = 29 (exactly neutral, by definition)
So pure carbon monoxide has an atomic weight of 28 or about 3% lighter than air. But usually it is made in modest concentrations, mixed in with the normal combustion products: CO2, H2O, which are always mixed with the 80% Nitrogen that never participates in burning. Then that mixes with room air, making an even smaller concentration.
The key here is that it mixes, it can be found evenly near the ceiling, the floor and even at "mouth height" which is where the original detectors were supposed to be mounted (as if CO was only hovering around where people breathed).
Relatively speaking they are the same density. However, keep in mind that CO comes from poor ignition, so it is usually warmer than the surrounding air and will tend to rise then mix as it cools to the room temperature. Thus it is prudent to detect near a source of CO for the earliest warning. Also, if you are investing in a combination CO and Smoke Detection device, it must be mounted on the ceiling or as high on a wall as possible in order for the smoke detection component to function properly.
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