What It Is Actually Made Of

We receive a certain kind of letter. It says, in effect: you are a humour publication, you will not tell me the actual facts, and I would quite like the actual facts.

Here are the actual facts. They are, as usual, better than the jokes.

The bulk of it is nothing

By volume, a typical fart is somewhere between 20 and 90 per cent nitrogen, which you swallowed. Not metaphorically — literally swallowed, in small quantities, with food, with drink, while talking, while chewing gum, while being anxious. Air goes in the top, most of it comes back out the top as a burp, and a fraction takes the long route.

The rest of the bulk is hydrogen, carbon dioxide, and — in about a third of people — methane, all produced by bacteria in the large intestine feeding on the carbohydrates your small intestine failed to absorb.

All four of these gases are completely odourless.

Add them up and you have accounted for something like 99 per cent of the volume of the thing. Ninety-nine per cent of a fart, chemically speaking, is a non-event: colourless, harmless, and — a fact worth sitting with — indistinguishable from a deep breath in a swamp.

The one per cent

The remaining fraction, well under one per cent by volume, is mostly sulphur compounds. Hydrogen sulphide, methanethiol, dimethyl sulphide, and a small supporting cast.

These are the entire story.

The human nose detects hydrogen sulphide at concentrations of a few parts per billion. To put that in some kind of scale: that is a sensitivity roughly comparable to noticing a single grain of sugar dissolved in an olympic swimming pool, except that instead of noticing it you find it personally offensive.

This asymmetry is the whole design. The gas that makes up ninety per cent of the volume, you cannot detect at any concentration whatsoever. The gas that makes up a rounding error, you can detect at a few billionths, and you will detect it from across a room, and you will detect it while asleep.

You are not equipped with a fart detector. You are equipped with a sulphur detector of extraordinary precision, and the reason is that sulphur compounds are also the signature of decay, of spoiled protein, and of a great many things that would kill you. Your revulsion is not a social convention. It is several hundred million years of your ancestors declining to eat the wrong thing.

Every embarrassment you have ever suffered on this subject was administered by a safety mechanism that has no idea it is being rude.

Why some are worse

Sulphur in, sulphur out. The compounds have to come from somewhere, and where they come from is sulphur-containing amino acids — cysteine and methionine — which are abundant in eggs, meat, dairy, and the brassicas: broccoli, cabbage, cauliflower, sprouts.

The much-maligned bean is a slightly different mechanism. Beans are rich in oligosaccharides that humans lack the enzyme to break down, so they arrive in the large intestine intact and are enthusiastically received. The result is volume, not necessarily odour — beans make the sound, eggs make the problem. These are two different complaints and they are constantly confused, including by people who have been dealing with both for decades.

On the sound

The sound has nothing to do with the chemistry, which is why our sequencer is a musical instrument and not a laboratory.

Pitch is a function of aperture and pressure, exactly as in a brass instrument or a balloon: tighter aperture, higher note. The tissue involved is soft and irregular, so the tone is rich in harmonics and buzzy rather than pure — closer to a reed than to a flute. Duration is a function of volume and flow rate. Everything you would predict from first principles turns out to be true, which is unusual and rather satisfying.

It also means the sound carries no information about the odour at all. None. The loud ones are not the bad ones; if anything the correlation runs slightly the other way, since a large volume of odourless nitrogen makes noise and dilutes the sulphur, while a small quiet volume does neither.

Everyone knows this. It is the empirical basis of the entire Silent School and it has been common knowledge, without ever once being taught, in every human culture we are aware of.

The number

Between 500 millilitres and 1.5 litres a day. Somewhere between 10 and 25 events. Every person you have ever met, every day of their lives, including the ones you would least expect and especially the ones who claim otherwise.

We include this last figure not for the joke but because it is the single most reassuring fact in this article, and because a surprising number of people appear to believe they are unusual in this respect.

You are not unusual. You are one of eight billion people producing about a litre of mostly-nitrogen a day, and about one per cent of it is being read out to the room by a sulphur detector that was calibrated, long before there were rooms, for something else entirely.