Vapor Density

Vapor Density

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.

• Gases that are heavier than air— those with a vapor density greater than one—will tend to lie low.
• Gases that are lighter than air— those with a vapor density less than one—will tend to rise and accumulate in traps along the top of the space.
• Gases that weigh about the same as air—those with a vapor density of close to one—tend to disperse within the atmosphere.

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.