Good morning, Gents:

As a scientist, let me wade in and address a few things. The whole notion of whether CO (or CO2) is heavier than air or lighter than air is simply not important for a lot of reasons.

For a start, heavier-than-air gases don’t settle to the lower portion of a structure and lighter-than-air gases don’t accumulate at the upper areas of a building. That's a myth.

Posit: Imagine for a moment that one could “see” helium (a lighter-than-air gas), and imagine that it was blue. And imagine for a moment that one could “see” toluene vapor (an heavier-than-air gas), and imagine that it was yellow. Now, take an air-tight see-through chamber made of glass, of say, ten cubic meters, that contains “air.” Carefully displace the upper portion of the chamber with helium so that the upper one foot layer is all blue. Now carefully displace the lower portion of the chamber with toluene, so that the lower one foot layer is yellow.

Now imagine that the chamber has absolutely NO ventilation, and absolutely NO air movement, and in fact, the chamber is isothermic so there are no convection currents. Now, wait an hour, and guess what? The contents of the chamber is green – the blue layer will be gone and the yellow layer will be gone!

What happened? As the two different gases were being introduced into the chamber, the mass density of the material was different relative to the displaced material (the air), and so even visually, one could see a distinction, and the displaced material was occupied preferentially by another material. However, what you could not see was that the blue helium as also immediately mixing with the air, and so was the yellow toluene. The molecules don't "know" they are heavier or lighter than their surrounding neighbors; they are in constant motion, and are busy zinging along a straight path until they hit another molecule which changes their direction. The molecules are NEVER stationary, but are in constant motion.

“Air,” helium, and toluene vapor (just like oxygen, carbon monoxide, carbon dioxide, etc) are “ideal gases” and they will completely and perfectly occupy any given volume at EXACTLY the same concentrations. So, after a few minutes, no matter where in the chamber we measure the concentration of gas, it will be the same as in all other areas of the chamber.

So although the helium temporarily occupied the upper portion of the chamber, and the toluene temporarily occupied the lower portion, within a very short period of time, through the process of molecular diffusion, the two gases mixed perfectly.

Now, if that can happen quickly in an air-tight chamber with no convection currents, imagine how much more quickly perfect mixing will occur in an open structure with air movement.

The proper placement of a CO meter in a property has more to do with occupancy, proximity to a combustion source, topography of the structure, thermal by-passes, and so forth. Each of these parameters vastly overwhelms any consideration of molecular weight.

Which brings me to a pet peeve of mine: I go nuts when the commentators during an NFL game talk about the hang time of flight of a ball during game in the fog or rain, wherein the presumption is made that the ball is hindered in flight by the high humidity, since the commentators foolishly believe that the air is “thicker” due to the high humidity. In fact, moist air has a lower density than dry air (that is, water vapor is a lighter-than-air-gas).

Cheers!
Caoimhín P. Connell
Forensic Industrial Hygienist
Forensic Industrial Hygiene

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG