What term describes whether a vapor tends to rise or settle in air?

• A. Vapor pressure
• B. Vapor density
• C. Vapor velocity
• D. Vapor concentration

Answer: (B)

In hazmat scenarios, how a vapor behaves in air is described by its vapor density. This value compares the vapor’s density to that of air and tells you whether it will rise or settle. If the vapor density is less than that of air, it tends to rise; if it’s greater than air, it tends to settle in low areas. This is why vapor density is the term that specifically describes buoyant behavior of vapors.

Vapor pressure, vapor velocity, and vapor concentration describe other properties: how readily a substance becomes vapor, the speed of the vapor, and how much vapor is present in a given volume, respectively. Understanding vapor density helps you predict where the vapor will accumulate—near the ceiling if it’s lighter than air, or near the ground if it’s heavier than air—and plan safety measures accordingly.