A stench from the French

A cloud of sulfurous gas emanating from a factory in France made the news in January. It spread northwards on the wind, covering vast swathes of southern England, and southwards to the French capital, Paris. The gas came from the specialist lubricants and paints firm Lubrizol, in Rouen.

'Le pong' - as some newspaper editors dubbed it - caused considerable disruption and discomfort. A French football match was postponed and lots of people complained about the smell.

However, the biggest disruption was caused by the specific nature of the gas and an unfortunate coincidence. Lubrizol said that the gas was 'mercaptan'. Chemically speaking, mercaptans are a class of compounds containing an S-H group. They are the sulfur analogues of alcohols, also known as thiols. These compounds, along with related thioethers like dimethylsulfide, are also characterised by their extremely noxious odours (reminiscent of rotting eggs, overcooked cabbage, sweat and a host of other foul aromas) at anything above the lowest of concentrations.