Methanethiol

Methanethiol (also known as methyl mercaptan) is a colorless gas with a smell like rotten cabbage. It is a natural substance found in the blood, brain, and other tissues of people and animals. It is released from animal feces. It occurs naturally in certain foods, such as some nuts and cheese. It is also one of the main chemicals responsible for bad breath and the smell in flatulence. The chemical formula for methanethiol is CH3SH; it is classified as a thiol.

Occurrence
Methanethiol is released from decaying organic matter in marshes and is present in the natural gas of certain regions, in coal tar, and in some crude oils.

In surface seawater, methanethiol is the primary breakdown product of the algal metabolite dimethylsulfoniopropionate (DMSP). Marine bacteria appear to obtain most of their protein sulfur by the breakdown of DMSP and incorporation of methanethiol, despite the fact that methanethiol is present in seawater at much lower concentrations than sulfate (~0.3 nM vs. 28 mM). Bacteria in oxic and anoxic environments can also convert methanethiol to dimethyl sulfide (DMS), although most DMS in surface seawater is produced by a separate pathway. Both DMS and methanethiol can be used by certain microbes as substrates for methanogenesis in some anoxic sediments.

Methanethiol is a weak acid, with a pKa of ~10.4. This acidic property makes it reactive with dissolved metals in aqueous solutions. The environmental chemistry of these interactions in seawater or fresh water environments such as lakes has yet to be fully investigated.

The United States material safety data sheet (MSDS) lists methanethiol as a colorless, flammable gas with an extremely strong and repulsive smell. At very high concentrations it is highly toxic and affects the central nervous system. Its penetrating odor provides warning at dangerous concentrations. An odor threshold of 0.002 ppm has been reported. The United States OSHA Permissible Exposure Limit is listed as 10 ppm.

Uses
Methanethiol is manufactured for use in the plastics industry, as a precursor in the manufacture of pesticides, and as a jet fuel additive. It is also released as a decay product of wood in pulp mills. Due to the extremely low odor threshold of thiols in general, they may be added to otherwise odorless gasses, enabling people to detect leaks by smell.

St. Petersburg incident
On December 26, 2005, dozens of people at a St. Petersburg, Russia Maksidom home supplies store became ill when gas suspected to be methanethiol was released. The store had received letters threatening to disrupt business during the winter holiday season. Three other stores belonging to the same chain found boxes with glass containers and timers that also might have been rigged to release the gas.