If a vapor has a density greater than air, how does it typically behave in a room?

• A. It rises to the ceiling.
• B. It mixes with air evenly immediately.
• C. It tends to stay near the floor, since it is heavier than air.
• D. It will not disperse.

Answer: (C)

Vapors with density greater than air tend to pool at the bottom because gravity pulls them downward and they don’t rise into the room’s air mass as easily as lighter vapors do. In a typical room, such a vapor will accumulate near the floor, seeping into corners, along walls, and under doors. Diffusion and mixing with the surrounding air happen slowly, so you don’t get an even, immediate spread throughout the space. Heat, drafts, and ventilation can stir and lift some of it, but the default behavior is low-level pooling. This is why responders emphasize monitoring and ventilating at floor level and considering pathways the vapor might travel along the floor.