The vapor density of a gas is the weight relative to which substance?

• A. Water
• B. Air
• C. Carbon
• D. Nitrogen

Answer: (B)

Vapor density is a comparison of how heavy a gas is relative to the surrounding air, at the same temperature and pressure. This reference lets you predict how the gas behaves in a space: gases lighter than air tend to rise and mix upward, while gases heavier than air stay near the floor and can pool in low areas. In firefighting practice, air is the standard reference for vapor density, so the weight is described relative to air. That’s why the correct answer is air. For context, a gas with a vapor density less than air’s will rise, whereas a gas with a vapor density greater than air’s will linger low to the ground.