From:
http://en.wikipedia.org/wiki/Thiol
"Many thiols have strong odors resembling that of garlic. The odors of thiols are often strong and repulsive, in particular for those of low molecular weight.
The spray of skunks consists mainly of low-molecular-weight thiols and derivatives.[9][10][11][12][13]
These compounds are detectable by the human nose at concentrations of only 10 parts per billion.[14]
Human sweat contains (R)/(S)-3-methyl-3-sulfanylhexan-1-ol (MSH), detectable at 2 parts per billion and having a fruity, onion-like odor. Women emit significantly more MSH than men.[15]
(Methylthio)methanethiol (MeSCH2SH; MTMT) is a strong-smelling volatile thiol, also detectable at parts per billion levels, found in male mouse urine. Lawrence C. Katz and coworkers showed that MTMT functioned as a semiochemical, activating certain mouse olfactory sensory neurons, attracting female mice.[16] Copper has been shown to be required by a specific mouse olfactory receptor, MOR244-3, which is highly responsive to MTMT as well as to various other thiols and related compounds.[17]
Thiols are also responsible for a class of wine faults caused by an unintended reaction between sulfur and yeast and the "skunky" odor of beer that has been exposed to ultraviolet light.
Not all thiols have unpleasant odors. For example, furan-2-ylmethanethiol contributes to the aroma of roasted coffee, whereas grapefruit mercaptan, a monoterpenoid thiol, is responsible for the characteristic scent of grapefruit. The effect of the latter compound is present only at low concentrations. The pure mercaptan has an unpleasant odor.
Natural gas distributors were required to add thiols, originally ethanethiol, to natural gas (which is naturally odorless) after the deadly New London School explosion in New London, Texas, in 1937. Many gas distributors were odorizing gas prior to this event. Most gas odorants utilized currently contain mixtures of mercaptans and sulfides, with t-butyl mercaptan as the main odor constituent. In situations where thiols are used in commercial industry, such as liquid petroleum gas tankers and bulk handling systems, an oxidizing catalyst is used to destroy the odor. A copper-based oxidation catalyst neutralizes the volatile thiols and transforms them into inert products."
