Mercaptan, also known as methanethiol, is added to natural gas to make it smell. Natural gas is colorless and odorless, so mercaptan is added as an odorant to make it easier to detect gas leaks. Mercaptan is an organic gas composed of carbon, hydrogen, and sulfur, and is found naturally in living organisms, including the human body where it is a waste product of metabolism. Mercaptan is detectable by the human nose at concentrations as small as only 10 parts per billion, making it an effective odorant. Natural gas distributors began adding mercaptans to natural gas after a deadly school explosion in 1937 at the New London School in New London, Texas.