Butanethiol, also known as butyl mercaptan, is a volatile, clear to yellowish liquid with a fetid (extremely foul-smelling) odor, commonly described as “skunk” odor. In fact, butanethiol is structurally similar to several major constituents of a skunk’s defensive spray. The scent of butanethiol is so strong that the human nose can easily detect it in the air at concentrations as low as 10 parts per billion.