The weight of a flammable vapor compared to air (Air=1). Vapors with a high density are more dangerous and require better ventilation because they tend to flow along the floor and collect in low spots.
Vapor density is defined as the relative weight of a gas or vapor compared to air, which has an arbitrary value of one. If a gas has a vapor density of less than one it will generally rise in air. If the vapor density is greater than one the gas will generally sink in air.
This concept is important for responders because it will indicate where the gas or vapors can generally be expected to be located at hazmat releases. Accordingly, responders can better select their staging and equipment set-up areas as well as the hazard isolation zones for the incident. Additionally, knowledge of where the gas may be found can better define the monitoring instrument strategy at each incident.