Propane vapor is heavier than air by what factor, contributing to hazards when refueling in enclosed spaces?

• A. One and a half times heavier
• B. Equal to air
• C. Three times heavier
• D. Half as heavy

Answer: (A)

Propane vapor density relative to air determines where it will collect when released. Propane is heavier than air, about one and a half times as dense. Because of that, in enclosed spaces it tends to settle toward the floor and pool in low areas, near drains or corners, rather than mix evenly with the air. That pooling creates a flammable vapor-air mixture close to ground level, which can ignite if a spark or flame is present, posing a serious fire or explosion hazard during refueling. The statement that propane vapor is one and a half times heavier than air best describes why these hazards occur. If it were equal to air, it wouldn’t preferentially accumulate low; if it were three times heavier, the pooling would be even more pronounced but the known typical factor is about 1.5; if it were half as heavy, it would rise away from the floor, which isn’t the case for propane.