As with natural gas and other fuel gases sold on the retail market, an odorant is added to propane to effect a recognizable scent. If propane gas is allowed to leak undetected and accumulate until the flammable range of 2.15% to 9.6% in air is reached, a source of ignition can result in catastrophic consequences.Although there are different formulations for odorants used in propane, ethyl mercaptan (EtSH) has been proven by scientific research to be the most effective and widely used odorant for propane in the United States. Ethyl mercaptan acts as a warning agent by providing an odor similar to rotten eggs that has been shown to be detectable by most people at a few parts per million (ppm) in air. For a proper perspective, the lower limit of propane gas flammability (2.15%) equates to 21 500 ppm in air. Any odorant that is used in propane shall provide a detectable odor when propane is at a concentration of at least one-fifth the lower limit of flammability in air or, in other words, at a level of 4300 ppm. Title 49 of the U.S. Code of Federal Regulations (49 CFR) Part 173.315(b)(1) Note 2 states that adding 1 lb (0.45 kg) of ethyl mercaptan to 10 000 gal (37 854 L) of liquid propane has been recognized to be an effective odorant that meets this requirement. Some individuals are unable to detect the odor of ethyl mercaptan either temporarily or permanently, and it is possible that propane could be delivered with insufficient odorant or no odorant.Under certain conditions ethyl mercaptan can react with materials that it contacts resulting in a diminished ability to act as a warning agent. This phenomenon is known as odor fade. It is the intent of this technical bulletin to provide information on odor fade to both tank and cylinder fabricators and to those that handle and fill liquefied petroleum gas (LPG) containers.