Glossary / Physical properties
Vapor density
Vapor density is the weight of a gas or vapor compared with an equal volume of air, with air given a value of 1. A figure above 1 means the vapor is heavier than air and tends to sink, while a figure below 1 means it tends to rise.
Explanation
Vapor density predicts where a released vapor will go. Heavier-than-air vapors settle into floor drains, pits, trenches, basements, tanks and other low areas, and they can travel along the ground a long way from where they were released. Lighter-than-air gases such as hydrogen rise and disperse upward, though they can pool under ceilings and in roof spaces.
This matters for both fire and health. A flammable vapor that sinks may find an ignition source far from the spill, and a toxic or asphyxiating gas that collects in a low spot can create an oxygen-poor atmosphere that is easy to walk into. It is a key reason Confined space entries require air testing at several levels.
Do not confuse it with Specific gravity or Relative density, which compare liquids or solids with water. Vapor density uses air as the reference. Real behavior also depends on temperature and ventilation: a cold dense gas can hug the floor, and a warm one may rise at first before cooling and sinking.
Air movement can override the effect indoors. Fans, drafts and ventilation patterns will mix gases, so vapor density should guide sampling and ventilation placement, not replace monitoring.
What to do and watch for
- Test the air at low and high levels when entering pits, tanks and trenches.
- Place exhaust pickup points near the floor for heavy vapors.
- Remember heavy flammable vapors can reach ignition sources far from the spill.
Where it appears on an SDS
- Section 9: Physical and chemical properties. Gives vapor density relative to air (air = 1), or states that no data is available.
SDS content and wording vary by supplier. This page is general reference and does not replace the current SDS for your product.