Why Ethyl Mercaptan is Added to Propane

Since its discovery in 1910, propane has become an important source of fuel in the United States.  Because it becomes a liquid when pressurized, propane is more portable and easily stored than many other fuels. When released from pressure it becomes a gas that can be burned to provide heat.   Propane provides warmth for many US homes, particularly in rural areas where natural gas is not always available.  It is also used to power outdoor grills and camping stoves, to dry crops on farms, and even as fuel for vehicles.   

Since propane gas cannot be seen and does not naturally have any smell or taste, it can be difficult to detect a propane leak.  While small amounts of propane in the air are not considered hazardous, breathing large amounts of propane is dangerous.  The propane begins to take up enough space in the lungs that there is not sufficient room for oxygen.   Symptoms of propane inhalation can include dizziness, headache, confusion, nausea, vomiting and lightheadedness.  Another significant danger of undetected leaking propane is the risk of fire and explosions.  Because propane gas is highly flammable, leaking propane could be easily ignited from a small spark or even from static electricity.  

To make propane gas noticeable to humans, a small amount of ethyl mercaptan, also called ethanethiol, is added to propane.  This propane is then referred to as “odorized”.   The addition of an odorizing agent to propane makes it possible for humans to detect a propane leak in most cases.  However, some of the ethyl mercaptan does dissipate during storage, and this may lead to a dangerous situation if gas leaks cannot be detected by smell.

There are several reasons that ethyl mercaptan is the most common odorant added to propane. First of all, it has a very strong, unpleasant odor.  Secondly, the ethyl mercaptan does not affect the propane, nor does it damage propane tanks.   Finally, when the propane gas is used, the ethyl mercaptan burns with the propane.

The smell of ethyl mercaptan is sometimes compared to skunk spray or rotten eggs.  This is not surprising, as ethyl mercaptan molecules contain a hydrogen sulfide group, and it is also hydrogen sulfide which gives rotten eggs their distinctive odor.  In the propane industry, ethyl mercaptan is often referred to as “stench”.  It is generally added when the propane is loaded from the refinery into a tanker truck to be taken to distribution facilities.   It may be added later if the propane is expected to be stored underground for several months.  During underground storage the ethyl mercaptan can entirely dissipate, and more would have to be purchased and added before the propane could be sold to distributors.  

If a person smells an unpleasant odor in an area where propane may be present, he should immediately extinguish any open flames and ensure that all people evacuate the area.  As soon as he reaches an area far from the propane leak, he should notify the local fire department and the propane supplier that a leak is suspected.   No one should use any telephones or light switches in an area where propane gas might be present.   If it is safe to do so and someone present is trained in propane safety, the main gas supply valve on the propane tank should be turned off.