It therefore makes intuitive sense that a heavy gas will accumulate in low lying areas and lighter gases will collect in high areas. We often see published advice from gas detection vendors for example that sensors for ammonia (mol. wt. = 17 g/mol) which is lighter than air (av. mol. wt ~ 29 g/mol) should be placed up near the ceiling and monitors for heavier gases such as carbon dioxide (mol. wt. 44 g/mol) should be placed near floor level. Even though it makes intuitive sense, does gas stratification occur in practice?
Stratification is the extent to which the heavier gases tend to settle to the bottom and the lighter gases rise to the top of an initially uniform air mixture, in the absence of bulk air movement. In well ventilated areas, gas stratification is irrelevant. The air movement from the ventilation will mix up the room air sufficiently that the gas and vapor concentrations will be uniform with height above the ground. Therefore, in well ventilated work environments, such as a hospital sterile process department with a high air turnover (typically at least 10 air exchanges per hour), we recommend placing gas monitors for toxic gases about five feet of the ground so that they correspond to the breathing zone of individuals regardless of the identity or molecular mass of the gas or vapor being detected.
Stratification is widely believed by many to occur in locations where there is little air movement; however a 2009 paper by Badino, which discusses stratification of air in caves, provides a very good mathematical analysis which goes a long way to answering the stratification question. His analysis shows that stratification does occur, but it requires a column of static air several kilometers high to have a major impact; and so stratification will not be relevant to most occupational safety gas monitoring applications.
Caves full of deadly carbon dioxide do exist, as do other confined spaces such as sewers, storage vessels etc.; but Badino argues that these arise not because of stratification but because these gases and vapors form in the caves and diffuse out very slowly causing a local high concentration. He also points out that many of these situations are also dangerous because of the low oxygen concentration, which he argues is due to the oxygen being consumed in the reaction with organic matter rather than stratification. These confined spaces present a significant danger to anyone entering them, regardless of whether the mechanism is stratification, diffusion or another cause. Therefore, whenever entering a confined space, especially one with little air movement, it is important to follow the normal confined space entry procedures and regulations.
Theilacker and M. J. White conducted a study of gas diffusion and stratification after a helium leak at Fermi Lab. Since helium is such a light atom (mol. wt. = 4 g/mol) they had expected it to displace oxygen from ceiling, but they saw no difference in the readings of the oxygen monitors as a function of height above the floor. The conclusion of their studies with both helium and sulfur hexafluoride, a large heavy molecule (mol. wt. 146 g/mol) was that "modest gas velocities will fully mix the spilled gases with air. The gases remained fully mixed over long distances in tunnels, or for long times in enclosed spaces." In other words stratification is not a issue with gases under normal working conditions, even if your normal work environment is 25 feet underground in a four mile long particle accelerator tunnel.
While these two papers are not the end of the story, the take home message is the same as we have been saying for many years. In most work environments with good ventilation, stratification of gases is not going to occur to a significant extent. Thus if measuring the concentration of a lighter than air gas such as ammonia or a heavier than air gas such as ethylene oxide, for workplace safety applications, in both cases the monitors should be placed at head height or about 5' off the ground. However, confined spaces, especially those with little air movement, present real dangers, even if the cause is not gas stratification, and so normal confined space entry procedures and regulations should be followed.