Everybody burps and farts around 2.5 litres of gas per day, which comes from the air we breathe, the drinks we quaff and the bacteria in our digestive system. Our bodies get rid of this excess gas via the mouth and anus. Unlike cows, methane is not a major constituent of our farts. So if someone sets a fart alight, it’s usually hydrogen gas that’s burning.
A small amount of swallowed air makes it into the intestines, but most of the gas there is produced by the bacteria that help to digest our food.
The gas is mainly hydrogen and carbon dioxide. The smell comes from sulphur compounds that are only present in trace amounts.
Fart or poo?
The nerve endings in your anus allow you to distinguish between a build-up of gas and a solid stool, so you can pass wind safely.