Propane is naturally odorless, but non-toxic chemicals are added that give it a sulfur- or rotten egg–like smell that helps to alert anyone nearby to a leak. The smell may escape from stoves or appliances when the gas is first turned on if it is not immediately lit. If the smell is present with no valves turned on or any identifiable source, there may be a leak, and you should exit the home until a professional can find the source.