What’s that smell?
Propane has no color, taste or smell in its natural state. To warn users of a leak, propane is odorized with ethyl mercaptan (C2H6S), also known as ethanethiol.
Mercaptans are a class of organic chemicals that contain sulfur. These compounds have a low molecular weight and are highly volatile. Mercaptans have a strongly disagreeable rotten odor, and are listed in the Guinness Book of World Records as one of the smelliest substances found on this planet.
Mercaptans naturally occur in a variety of sources, including crude petroleum. Skunk spray contains butyl mercaptan (butanethiol). Slicing through onion or garlic releases allyl mercaptan. Methyl mercaptan (methanethiol) is produced by decaying animals and plants. Mercaptans are also in flatulence.
The odorant used most often in natural gas is tertiary butyl mercaptan (TBM). For a number of technical reasons, odor fade does not appear to be as prevalent with natural gas.
Back to Propane and Odor Fade
More information:
Wikipedia: Ethanethiol
TOXNET Harardous Substances Data Bank: ethyl mercaptan