Determining whether a vapor will rise or fall is determined by which property?

• A. Vapor dispersion
• B. Vapor density
• C. Vaporization
• D. Specific gravity

Answer: (B)

Vapor density is the property that tells you whether a vapor will rise or settle. It compares the vapor’s density to the density of air. If the vapor density is less than 1, the gas is lighter than air and will tend to rise toward the ceiling. If the vapor density is greater than 1, the gas is heavier than air and will tend to pool near the floor. This understanding helps predict where the hazard will accumulate and guides ventilation and entry strategies.

Vaporization is simply a phase change from liquid to gas and doesn’t address vertical movement. Vapor dispersion describes how the vapor spreads, which is influenced by density but isn’t the rule governing rise or fall. Specific gravity compares density to water, not to air, so it isn’t the right metric for predicting whether a vapor will rise or fall.