Fart myths and facts

Gas production is a by-product of bacterial fermentation in the colon, which does not take days off. Healthy adults pass gas somewhere between 5 and 25 times daily. People who insist they never do are counting, not measuring.

Production continues overnight and the anal sphincter relaxes during sleep. Most people pass gas several times without waking. This is the single biggest reason self-reported counts come in below measured ones.

Hydrogen and methane are both flammable and both occur in intestinal gas. But only about a third of adults produce methane at all, and the mix varies enormously by person and by meal. Also: burns from attempting this are documented in medical literature, which tells you how often people try it. Do not.

Uncomfortable, yes. Dangerous, no. Retained gas is partly reabsorbed through the intestinal wall and eventually exhaled through the lungs; the rest leaves later. It does not accumulate indefinitely. Habitual holding can contribute to bloating and cramping, which is an argument for finding a corridor rather than for alarm.

Unusually, the folklore has a mechanism. Longer residence in the colon means more time for bacteria to generate sulphur compounds, and sulphur compounds are the entire source of the odour.

Methane is a minority component and absent entirely in roughly two thirds of adults. By volume, intestinal gas is mostly nitrogen, hydrogen and carbon dioxide. Nitrogen is swallowed air, which means a substantial part of a fart was recently the room.

Hydrogen sulphide is genuinely toxic at industrial concentrations. The amounts a human digestive system produces in an ordinary room are lower by many orders of magnitude. The smell is a social problem, not a toxicological one.

Worst for volume, not for smell. Beans ferment into hydrogen and carbon dioxide — plenty of gas, little odour. Eggs, meat and cruciferous vegetables produce far less volume and far more smell. The two problems have different causes and different fixes.

The gas-producing oligosaccharides are water-soluble, so soaking and then discarding the water removes a portion before cooking. Rinsing tinned beans achieves a smaller version of the same thing. This is one of the few pieces of kitchen folklore with clean chemistry behind it.

Studies of volunteers on controlled diets have not found a consistent difference in frequency. Diet, microbiome and swallowed air explain far more of the variation between two individuals than sex does. The perceived gap is mostly a difference in social reporting.

Gas often rises with a high-fibre diet, and high-fibre diets have good evidence behind them. But gas volume is not itself a health measure, and plenty of things that increase it — lactose intolerance, sugar alcohols — are not signs of anything good.

Not close. Ruminants ferment on an entirely different scale and produce large amounts of methane, mostly through belching rather than the other end. Human flatulence is climatically irrelevant, which is a genuinely useful thing to know at a dinner party.