Propane runs grills, RVs, forklifts, and the furnaces of millions of homes that aren't on a natural gas line. It is so common that people stop thinking of it as dangerous. Firefighters never stop thinking of it that way.
Propane is extremely flammable. It is stored as a liquid under pressure, but the moment it escapes it becomes a gas that ignites in air at concentrations between roughly 2% and 10%. That is a wide flammable range, and it takes very little: a light switch, a pilot light, a static spark from your clothes. Its autoignition temperature is around 850°F, but you almost never need that, because ignition sources are everywhere.
The Property That Makes Propane Dangerous
Propane is heavier than air. Natural gas rises and disperses. Propane sinks and pools. A leak in a garage, basement, boat cabin, or RV fills the low spots first and can sit there invisibly until something ignites it. That is why propane appliances are not allowed in basements in many codes, and why a propane smell low to the ground is an emergency, not a curiosity.
Pure propane is odorless. The rotten egg smell is ethyl mercaptan, added on purpose so you can detect a leak. If you smell it, trust it.
How Propane Fires and Explosions Happen
- Grill fires. A leaking hose or connection, or a grill lit with the lid closed after the gas has been on for a few seconds. The lid closed flash is the classic backyard injury.
- Tank fires. A tank valve leaking near an ignition source produces a jet flame. On its own a jet flame is manageable. The danger is heat on the tank itself.
- BLEVE. If a propane tank is heated by an external fire, the liquid inside boils, pressure climbs past what the relief valve can vent, and the tank ruptures violently. Firefighters call this a boiling liquid expanding vapor explosion. It is why we set up far away from a tank that is being heated by fire and why you should too. We explain it in detail in what is a BLEVE.
- Indoor heaters. Propane heaters not rated for indoor use produce carbon monoxide and, if they leak, fill an enclosed space with gas. See our article on using a propane heater indoors.
If You Smell Propane
- Don't touch light switches, appliances, or phones inside the space. Any of them can spark.
- Get everyone out and leave the door open behind you.
- If you can safely reach it, shut the tank valve.
- Call 911 or your propane supplier from outside, well away from the building.
- Don't go back in until it has been checked.
Handling Propane Tanks Safely
- Store tanks upright, outdoors, in the shade, and never in a garage, basement, or vehicle.
- Check grill connections with soapy water each season. Bubbles mean a leak.
- Open the grill lid before you turn on the gas, and light it promptly.
- Don't overfill. Tanks are filled to 80% on purpose to leave room for expansion.
- Replace tanks past their recertification date and any tank with rust, dents, or a damaged valve.
- Transport tanks upright and secured, with the windows cracked, and take them out of the car as soon as you get home.
Propane is safe when it stays where it is supposed to be. Every propane call we run started with a leak that someone smelled and ignored, or a tank that was stored somewhere it never should have been. For the natural gas version of this question, read is natural gas flammable.
