Are Gasoline Vapors Lighter Than Air?

Gasoline quickly evaporates when exposed to the atmosphere; the vapors are not lighter than air. This characteristic is not uncommon, according to the Canadian Centre for Occupational Health and Safety, which notes, “The vapors from nearly all flammable and combustible liquids are heavier than air.”

Because gasoline vapors are heavier than air, they sink through the normal atmosphere. Flammable, explosive amounts of vapors can therefore collect around floors or in basement structures, pits, sewers, sumps and trenches while people nearby remain unaware. For this reason, storing gasoline inside buildings is discouraged. The Wisconsin Department of Health Services advises, “An explosion is possible if the vapors are lit by a spark or flame, such as the pilot light in a water heater, stove or furnace.” Once ignited, the vapor trail can flash back to the leaking storage vessel. This can result in an explosion.

Many jurisdictions mandate a maximum amount of gasoline a homeowner can store -- often the amount is 25 gallons -- and carry in a vehicle, and the type of container that must be used. The experts at Marathon Petroleum say, “Only store gasoline in containers with approved labels as required by federal or state authorities.” They further note that glass containers must never be used.

Even if the vapors do not ignite or explode, they can have harmful effects. Experiments on rats have suggested that the vapors may be carcinogenic, and certain of gasoline’s components have been demonstrated to be cytotoxic, meaning they cause damage to cells.