For this use, air has a molecular weight of 28.97 atomic mass units, and all other gas and vapour molecular weights are divided by this number to derive their vapour density.
This can be seen by dividing the molecular weight of Acetone, 58.1[4] by that of air, 28.97, which equals 2.
With this definition, the vapour density would indicate whether a gas is denser (greater than one) or less dense (less than one) than air.
The density has implications for container storage and personnel safety—if a container can release a dense gas, its vapour could sink and, if flammable, collect until it is at a concentration sufficient for ignition.
Even if not flammable, it could collect in the lower floor or level of a confined space and displace air, possibly presenting an asphyxiation hazard to individuals entering the lower part of that space.