Farts smell because of a tiny fraction of their total gas content: sulfur compounds produced by bacteria in your large intestine. About 99% of flatulence is made up of five odorless gases (nitrogen, oxygen, carbon dioxide, hydrogen, and methane). The remaining 1% or less contains trace sulfur-based gases that your nose can detect at astonishingly low concentrations, some as little as 0.5 parts per billion.
What’s Actually in a Fart
The bulk of intestinal gas is completely scentless. Nitrogen alone can make up anywhere from 11% to 92% of a single episode of flatulence, while carbon dioxide ranges from 3% to 54%, hydrogen from 0% to 86%, and methane from 0% to 54%. These ratios vary wildly from person to person and even meal to meal. Oxygen makes up a small share too, mostly from swallowed air.
None of those gases have any smell. The entire odor experience comes from sulfur-containing gases present in trace amounts. The three main culprits are hydrogen sulfide, methanethiol, and dimethyl sulfide. Hydrogen sulfide is the dominant one, present at roughly five times the concentration of methanethiol. When judges in a study published in the journal Gut were asked to describe the individual smells, they characterized hydrogen sulfide as “rotten eggs,” methanethiol as “decomposing vegetables,” and dimethyl sulfide as “sweet.” The first two do most of the damage. Dimethyl sulfide, despite its presence, probably contributes little to how bad things smell.
Why Your Nose Picks Up So Little
The human nose is remarkably sensitive to hydrogen sulfide. According to the Agency for Toxic Substances and Disease Registry, people can detect it at concentrations as low as 0.5 parts per billion. That’s an almost absurdly small amount. For perspective, if you filled a billion cups with clean air, you’d only need to add hydrogen sulfide to half of one cup before someone could smell it. This extreme sensitivity is why even a tiny amount of sulfur gas in an otherwise odorless cloud of nitrogen and carbon dioxide produces such a noticeable stink.
The Bacteria Behind the Smell
Your colon is home to trillions of microbes, and several types are directly responsible for producing sulfur gases. The most studied group is called sulfate-reducing bacteria, which use sulfate as fuel for their metabolism and release hydrogen sulfide as a byproduct. The dominant species in this group belong to the genus Desulfovibrio, accounting for roughly 64% to 81% of sulfate-reducing bacteria in the colon.
But sulfate reduction isn’t the only route to smelly gas. Many common gut bacteria, including strains of E. coli, Clostridium, and Streptococcus, can break down cysteine (a sulfur-containing amino acid from food) and release hydrogen sulfide in the process. Another intestinal resident, Bilophila wadsworthia, takes a different approach entirely: it breaks down taurine (a compound found in bile) and reduces the resulting sulfite into hydrogen sulfide. So multiple bacterial pathways converge on the same stinky end product.
Foods That Make It Worse
Since gut bacteria produce sulfur gases from sulfur-containing compounds in food, what you eat has a direct effect on how your gas smells. The biggest dietary contributors fall into a few categories:
- High-protein animal foods: Turkey, beef, eggs, fish, and chicken are rich in methionine and cysteine, two sulfur-containing amino acids that gut bacteria readily convert to hydrogen sulfide.
- Cruciferous vegetables: Broccoli, cauliflower, cabbage, kale, arugula, and radishes contain sulfur compounds that feed sulfate-reducing bacteria.
- Allium vegetables: Garlic, onions, leeks, scallions, and shallots are naturally high in sulfur.
- Legumes and grains: Lentils, chickpeas, oats, and walnuts provide cysteine, giving bacteria more raw material to work with.
Eggs are a particularly common offender because they’re dense in both methionine and cysteine. A high-protein meal with eggs and red meat, followed by a side of broccoli or garlic, is more or less a recipe for potent gas later that day.
How Much Gas Is Normal
The average person passes gas about 15 times a day, but the normal range is enormous, anywhere from 3 to 40 times daily. Frequency alone doesn’t signal a problem. Most of those episodes are odorless or barely noticeable, since the majority of gas is just swallowed air and fermentation byproducts with no sulfur content. The ones you notice are the ones where sulfur-producing bacteria had plenty of fuel to work with.
When Smelly Gas Signals Something Else
Persistently foul-smelling gas, especially when paired with other symptoms, can point to digestive issues worth paying attention to. Malabsorption conditions, where the intestines fail to properly absorb fats, sugars, or other nutrients, commonly produce excessive bloating, flatulence, and unusually bad-smelling stool and gas. When fats aren’t absorbed properly, stool becomes greasy, light-colored, and particularly foul.
Celiac disease, Crohn’s disease, and parasitic infections can all damage the intestinal lining enough to cause malabsorption. Bacterial overgrowth in the small intestine, where bacteria that normally live in the colon migrate upward and multiply, also interferes with digestion and increases gas production. If your gas has recently become much more frequent or smelly, and you’re also dealing with diarrhea, weight loss, or abdominal pain, those symptoms together are worth investigating.
Reducing the Smell
The most straightforward approach is reducing sulfur-rich foods in your diet and seeing if things improve. Cutting back on eggs, red meat, and cruciferous vegetables for a week or two can give you a clear signal about which foods are driving the problem.
There’s also a pharmacological option. Bismuth subsalicylate (the active ingredient in Pepto-Bismol) has been shown to markedly decrease hydrogen sulfide in the colon. The mechanism is straightforward: bismuth reacts with sulfide to form bismuth sulfide, an extremely insoluble compound that effectively traps the gas before it can become airborne. Research published in Gastroenterology confirmed that bismuth itself, not the salicylate portion of the compound, is the active ingredient responsible for this effect.
Increasing fiber intake gradually can also shift the composition of your gut bacteria over time, favoring species that produce less sulfur. Rapid changes in fiber, though, tend to increase gas volume in the short term, even if the smell eventually improves.