According to the National Fire Incident Reporting System, there are about 2,500 propane fires and explosions annually in the US, killing about 20 people and injuring about 330 others. In many of these cases, survivors report that they never smelled gas. The propane industry has known for at least 40 years that propane can lose its odor under a variety of circumstances. Propane, or LP gas, has no odor in its natural state. A strong-smelling substance called ethyl mercaptan is added to propane to warn consumers of a leak.
Experts say that the odor of propane can fade as it is stored in tanks.
Ethyl mercaptan reacts with rust and metal surfaces inside steel tanks, causing the odorant to leech out of the gas. Research done in the laboratories of the Arthur D. Little Co. for the CPSC found that the odor can significantly fade within five to seven days after a propane tank is filled. When the odor of propane fades, the culprit tanks are usually either brand-new or old tanks that have not been in continuous use. Experts believe that after exposure to mercaptan for months or years, a steel tank will become seasoned and no longer promote the loss of odor. If an empty propane tank is allowed to remain open to the air, a new layer of rust can form on the inside.