The classic definition of vapor density is “the mass of a certain volume of a particular gas or vapor divided by the mass of the same volume of hydrogen” at the same temperature and pressure. However, for safety considerations, vapor density is more commonly defined with respect to air rather than hydrogen, with air having an arbitrary value of one (Air = 1). Generally, without accounting for thermal updrafts, relative humidity, and other influencing factors, if a gas or vapor has a vapor density less than one it will rise in air and conversely if a gas or vapor has a vapor density greater than one it will sink in air. Vapor density therefore has important safety implications in the rising up or sinking down of a toxic or flammable gas in ambient air.  
 
For example, some healthcare providers have pivoted from the use of canvas tents as COVID-19 testing sites to more durable multi-bay metal sheds to better withstand the winter elements. With these structures having less natural air flow due to a tighter skin, a greater potential for carbon monoxide (CO) gas buildup exists from the exhaust of idling vehicle engines. Where carbon monoxide, a toxic gas, is insidious in having no odor, color, or taste, the need for detection in the way of sensor installation inside these sheds is critical. But at what height should a carbon monoxide detector be mounted? The answer lies in knowing carbon monoxide’s vapor density. The vapor density for carbon monoxide is 0.97, as referenced in Section 9: Physical and chemical properties of an applicable product safety data sheet. So, comparing CO’s vapor density of 0.97 to that of air (1), reveals that carbon monoxide is slightly lighter than air. With this in mind, the Environmental Protection Agency recommends CO detectors be placed on a wall about 5 feet above the floor.

In contrast, hydrogen sulfide (H2S), a colorless, flammable, and hazardous gas (largely due to its asphyxiant property) has a vapor density of 1.19 and is therefore heavier than air. The gas tends to travel along the ground and collects in low-lying areas such as underground vaults and enclosed utility spaces. Commonly referred to as sewer or swamp gas, hydrogen sulfide has a rotten egg odor due to its sulfur component. With continuous low-level exposure or at high concentration, olfactory fatigue occurs making it undetectable to the sense of smell even though it’s still present. Acute adverse effects can happen at high concentrations leading to unconsciousness, coma, and death. The high flammability of H2S gas, as it seeps along the  ground, can migrate to an ignition source and cause flashback or explosion.  Check out this OSHA Fact Sheet on hydrogen sulfide gas for more information on its hazardous properties, health effects, and protective measures.

As illustrated in these two examples, knowing the vapor density of a gas can be extremely helpful in taking appropriate action when there’s a sudden release or presence of a particular gas in the work environment. And while section 9 of a safety data sheet might be more to the liking of Bill Nye the Science Guy and safety geeks, it certainly warrants review for a thorough understanding of a chemical product’s properties. Hmm, maybe there should be a Mylar helium balloon that says, Vapor Density Can Be Uplifting.  I wonder how many people would be down with that?