What chemical makes farts smell?

Three sulphur-containing gases account for most of the odour: hydrogen sulphide, methanethiol and dimethyl sulphide. A 1998 study in Gut correlated the concentration of these compounds with odour ratings from human judges, and hydrogen sulphide showed the strongest relationship. Together they make up a tiny fraction of the total volume — the other 99% is odourless.

What is a fart actually made of?

Mostly nitrogen, hydrogen, carbon dioxide, methane and oxygen. Nitrogen and oxygen arrive as swallowed air. Hydrogen, carbon dioxide and methane are produced by gut bacteria fermenting carbohydrates. None of these five gases has a smell.

Why do eggs and meat make farts smell worse?

Because odour comes from sulphur, and foods rich in sulphur-containing amino acids give gut bacteria more raw material. Eggs, meat, garlic, onions, cabbage, broccoli and cauliflower are all high in sulphur compounds. Beans produce a lot of gas by volume but not necessarily a strong smell — volume and odour are separate problems.

Sometimes. Hydrogen and methane are both flammable, and both appear in intestinal gas. But roughly a third of adults produce methane at all, and the proportions vary hugely between people and meals. This is a genuinely dangerous thing to test and burns are a documented outcome.

Why do farts smell?

Roughly 99% of it is completely odourless. The entire reputation of the fart rests on a trace fraction of sulphur-containing gases — parts per million, doing enormous reputational damage.

Key facts

Odourless share: ~99% by volume
Main odour compound: Hydrogen sulphide (H₂S)
Also involved: Methanethiol, dimethyl sulphide
Detection threshold: H₂S is detectable at parts per billion
Source of sulphur: Sulphur-rich foods, fermented by gut bacteria

What is in there

By volume, intestinal gas is five things: nitrogen, hydrogen, carbon dioxide, methane and oxygen. Nitrogen and oxygen were swallowed — they are simply air that took the long route. Hydrogen, carbon dioxide and methane are manufactured in the colon by bacteria fermenting carbohydrates that made it past the small intestine undigested.

Not one of those five gases has a smell. If flatus were only those five, it would be an entirely silent problem, socially speaking.

The 1% that does the work

The definitive work here is a 1998 study published in Gut by Suarez, Springfield and Levitt. They collected samples, measured the concentration of individual trace gases, and had human judges rate the odour of each — an experimental design that deserves more respect than it usually gets.

Three compounds correlated with the ratings:

Hydrogen sulphide (H₂S) — the strongest relationship with perceived odour. Smells of rotten eggs. The human nose detects it at concentrations measured in parts per billion.
Methanethiol — rotting cabbage. Also present in bad breath, and in the compound added to natural gas so leaks can be smelled.
Dimethyl sulphide — sweeter, closer to cooked vegetables or the sea.

The same study found that activated charcoal cushions substantially reduced the sulphur gases reaching the outside world, which is either useful information or the funniest sentence in gastroenterology, depending on your day.

Where the sulphur comes from

Sulphur enters through food. Sulphur-containing amino acids — cysteine and methionine — are abundant in eggs, meat, fish and dairy. Cruciferous vegetables such as broccoli, cabbage, cauliflower and Brussels sprouts carry sulphur in a different form, as glucosinolates. Garlic and onions bring their own.

Sulphate-reducing bacteria in the colon convert this material into hydrogen sulphide. More sulphur in, more hydrogen sulphide out. It is a fairly direct relationship, and it explains why a big steak-and-eggs weekend produces something different from a lentil-heavy one.

Volume and smell are separate problems

This is the part people get wrong. Beans are famous for gas, and the reputation is earned — they carry raffinose and other oligosaccharides that humans cannot digest and gut bacteria love. But that fermentation produces hydrogen and carbon dioxide: a lot of volume, not much smell.

Conversely a modest amount of gas from a sulphur-heavy meal can clear a room. If you want quiet, work on the fermentable carbohydrates. If you want inoffensive, work on the sulphur. They are different levers. The food guide separates them properly.

Why some smell worse than others

Time in the colon. Slower transit means longer fermentation and more sulphur compounds. This is the mechanism behind the folk belief that held-in gas comes back worse.
Your bacterial mix. How many sulphate-reducing bacteria you host varies enormously between individuals.
Recent meals. The window is hours, not days — what you ate this afternoon is what you are working with tonight.
Concentration. A small volume carrying the same amount of sulphur is more concentrated, and therefore more noticeable.

Frequently asked questions

Can you make them smell less?

Reducing sulphur-rich foods is the direct lever. Beyond that, the evidence is thin and mostly commercial. Activated charcoal showed an effect in the 1998 study when used as a seat cushion; charcoal taken orally has much weaker support.

Why does my own smell less bad to me?

Partly olfactory adaptation — continuous exposure to an odour reduces the response within minutes. Partly familiarity: your own gut flora produce a consistent profile that your brain has stopped flagging as novel.

Is hydrogen sulphide dangerous?

At industrial concentrations, seriously so. At the concentrations produced by a human digestive system in an ordinary room, no. The gap between the two is many orders of magnitude.

Do dogs and other animals produce the same gases?

Broadly yes — the mechanism is bacterial fermentation in a mammalian gut, and the same sulphur chemistry applies. Diet drives the differences, which is why a dog fed table scraps becomes notorious.

This is general information, not medical advice. FartRank is a game. If your symptoms are painful, sudden, persistent, or come with weight loss, bleeding or a change in bowel habits, talk to a doctor — not to a leaderboard.

Sources

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Published 26 August 2026 · by Furiosa Studio