I have seen industrial ventilation systems in many factories that are designed to remove “heavier-than-air” vapours from the ground. Unfortnately the factory owner has often wasted many thousands of dollars (pounds or whatever), as in most cases, vapour fail to sink to the ground.
Above – an expensive ventilation system at a paint factory deigned to remove heavier-than-air vapours from the floor. The measured solvent concentration was uniform from the floor to at least 10 m.
The myth is perpetuated in things like safety videos like this clip, extracted from a YouTube video on asbestos. Click here to see the clip
The reasons for the myth can be appreciated in two ways:
1. Qualitatively:
Imagine a large bag of 1000 marbles. If one is marble substituted with a ball bearing, then would you really notice the difference? The one ball bearing in 999 marbles is the analogous to 1000 ppm (parts per million) of a heavier-than-air vapour in 990,000 parts of air. The exposure limit for most flammable vapours is around 100 ppm, well below the explosive limit (say 3% or 30,000 ppm). Click on the image below for a larger version.
Data source: 2004 ACGIH TLV® list and Lees (1996).
The concentration is on a logarithmic scale in ppm and most exposure limits are less than 1% of the lower Explosive/Flammable limit. A nominal 15,000 ppm has been plotted for asphyxiants (A) corresponding displacement of oxygen from 21% to 19.5%.
So if your have a safety problem, you have a severe health problem, if the vapour cloud is uniform.
2. Quantitatively: (trust me, I have a calculator!)
Imagine a million air molecules with unit density, with 1000 molecules of solvent (10 times the occupational exposure limit for many solvents) with twice the vapour density compared to air. The density of the vapour- air mix is
Now consider the ideal gas equation, PV = nRT, with the usual symbolic meanings to Pressure, Volume, moles (n), the gas constant R, and absolute Temperature (T) in kelvins (0°C is 273.15°K). The air pressure at the floor and ceiling is effectively the same, ignoring the weight of the air column in the room and the volume of air is constant, so the “nRT” right side of the equation at floor and ceiling is the same, or n1T1 = n2T2. For air temperatures of 20°C at the floor and 21°C at the ceiling gives
T1 = 20°C = 273 + 20 = 293°K at floor
T2 = 21°C = 273+ 21 = 294°K at ceiling
giving
A mere 1°C temperature difference from floor to ceiling is going to produce much greater (34x) convective forces than the density of a vapour at 1000 ppm. This means that in most workplaces, slight air currents will readily mix the air and a blanket of “heavier-than-air” vapour will not form near the floor.
Comment
In many workplaces, the temperature difference between floor and ceiling is at least a few degrees.
It is possible to get a layer of vapour near the ground, but the concentrations would have to be high and the conditions isothermal (no temperature difference). You may find this in the bilge of a boat, particularly when the owners have been away and gas or fuel has leaked; or in a trench.
A slightly expanded version of this can be downloaded from dbOHS.com in the Downloads section or click here
David Bromwich, April 2008
Thanks for writing this.