Mercaptan is also known as methanethiol and is a harmless but pungent-smelling gas which has been described as having the stench of rotting cabbages or smelly socks.

It is often added to natural gas, which is colourless and odourless, to make it easier to detect.

The gas is an organic substance, made of carbon, hydrogen and sulphur, and is found naturally in living organisms, including the human body where it is a waste product of normal metabolism. It is one of the chemicals responsible for the foul smell of bad breath and flatulence.

People who have eaten asparagus can experience the distinctive smell of mercaptan in their urine within 30 minutes of consuming the vegetable, which contains substances that are quickly broken down to methanethiol. However, not everyone is able to smell mercaptan in their urine as a genetic mutation in some people means they are immune to the odour.