Molecule of the Month: What lurks within that nasty niff: Methyl mercaptan causes halitosis and smelly socks, but it also has industrial uses, says John Emsley

GUIDELINES for acceptable levels of smelly molecules in the air we breathe are to be published next month. At the top of European Odour Standards list will be a molecule we often breathe out: methyl mercaptan (chemical formula CH3 SH), the main cause of bad breath, which we can easily detect when someone speaks to us but not when we produce it ourselves.

In Japan, you can test your breath with a device called the Oral Checker, 10,000 of which have been sold at pounds 60 each. This halitosis detector, the size of a powder compact and patented in 1988 by Katunori Nakamura, works on the principle that a metal oxide, such as that of tin, changes its electrical resistance when it absorbs a gas.

Methyl mercaptan cannot help but have a high profile. Sometimes it is produced naturally from bacteria in the environment: the shore of the Firth of Forth near Edinburgh often exudes it, much to the distress of the residents of the select suburb of Cramond.

When the Pan Britannica Industries plant at Waltham Abbey in Essex, which makes the insecticide dimethoate, accidentally released methyl mercaptan last summer, some local people were so sickened that they sought hospital treatment; others rang British Gas, which is not surprising because similar sulphur compounds are used to odourise natural gas.

Bad breath is caused by several molecules, but the chief culprits are derivatives of sulphur: hydrogen sulphide, dimethyl sulphide and especially methyl mercaptan, which is by far the worst smelling; our noses can detect as little as one part per billion in the air.

Hydrogen sulphide, H2 S, the traditional stink of the chemistry lab, is much less smelly, as is dimethyl sulphide, which forms part of the aroma of fresh coffee.

Professor Graham Embery, of the Dental School of the University of Wales College of Medicine in Cardiff, who is investigating the sulphur-containing molecules in the mouth, says methyl mercaptan arises from the activity of bacteria that are always present.

If the smell of methyl mercaptan is very strong, it indicates gum disease, where deep pockets of the bacteria have released the gas from the amino acids of broken-down protein. Cysteine and methionine, which contain sulphur atoms, are the amino acids responsible; methionine is essential for all living things, and animal protein contains up to 4 per cent of it.

Mr Embery and Professor Gunnar Rolla, of the Dental School of the University of Norway in Oslo, have written a book, Clinical and Biological Aspects of Dentifrices, which devotes a whole chapter to halitosis. Mr Embery advises those who suspect they are breathing out methyl mercaptan to use a toothpaste containing anti- plaque agents such as zinc or tin salts. These metals interfere with the enzymes in the bacteria that produce methyl mercaptan. Mouthwashes containing these salts are being tested.

Traditionally, mouthwashes do little more than clean the mouth and disguise the offending smell.

They consist of water and alcohol, with benzoic acid and natural flavours such as thymol and menthol. A good rinse with an ordinary mouthwash will remove about half the oral bacteria, but more can be removed with one that contains an emulsion of oil, say Drs Mel Rosenberg and Ervin Weiss, of Tel Aviv University. They claim that 90 per cent of offending bacteria stick to the oil drops. Another way to clean the mouth is to increase saliva flow by chewing gum.

Our feet can also harbour microbes that give off methyl mercaptan, especially if we provide an environment of unwashed socks and unventilated shoes. Staphylococci and aerobic coryneform bacteria flourish in the increasingly alkaline conditions within unchanged shoes and socks.

A chemical answer is to insert charcoal-filled insoles into our shoes, which absorb the methyl mercaptan between the layers of carbon. Since the amount of methyl mercaptan is tiny, the insoles will work for weeks.

Methyl mercaptan is the simplest member of a series of compounds in which there can be chains of up to 20 carbon atoms attached to sulphur. We encounter mercaptans with three and four carbons when we smell a gas leak. One with 18 carbons is used in silver polishes.

Methyl mercaptan is used in pesticides, especially weedkillers for cereal crops such as wheat, maize and rice, and in the manufacture of methionine, an amino acid that may be deficient in our diet. Some animal feeds are fortified with methionine, thus increasing the amount they offer us in meat and milk.

Although methyl mercaptan is used industrially, a big drawback is its low boiling point of 6C. Luckily it can easily be turned into chemically similar dimethyl disulphide (DMDS), a yellow liquid that boils at 110C. This is only slightly less smelly, but it is much safer to transport around, and is the material imported into Britain.

DMDS is also employed in the making of pesticides, but its chief use is to regenerate the catalysts that become coated with tar when they are used to refine petrol.

Elf Atochem, of Thatcham in Berkshire, the world's largest producer of sulphur-based chemicals, imports DMDS from Rotterdam and Lacq, in south- west France, whose natural gas wells produce large amounts of H2 S.

This is combined with methanol to form methyl mercaptan, which is then converted to DMDS and other mercaptans. Mike Fraser, of Elf Atochem, says the company blends DMDS with other mercaptans to odourise natural gas. 'The potency of these simple compounds is illustrated by the small amounts needed,' he says. 'A wineglass of these liquids is all that is required each day to odourise the gas for a city the size of Southampton.'

The author is science writer in residence at the department of chemistry, Imperial College, London.