Most farts smell like rotten eggs, rotten cabbage, or a faintly garlicky sulfur. That distinctive stink comes from a tiny fraction of the gas you actually pass. The bulk of a fart is odorless, made up of nitrogen, oxygen, carbon dioxide, hydrogen, and sometimes methane. The smell comes almost entirely from sulfur-containing compounds that make up, on average, just 50 parts per million of each fart.
The Three Compounds Behind the Smell
Three sulfur gases do most of the heavy lifting when it comes to odor. Hydrogen sulfide is the classic rotten-egg smell. Your nose can detect it at concentrations as low as 0.5 parts per billion, which is why even a trace amount makes its presence known. Methanethiol adds a rotting-cabbage quality. Dimethyl sulfide contributes a garlic-like note. Together, these three compounds account for the sulfurous smell profile of flatulence, even though they represent an almost negligibly small portion of the total gas volume.
There are also two other odor compounds, indole and skatole, that carry a more fecal quality. Even at extremely low concentrations relative to the odorless gases, all of these trace compounds dominate what you actually smell. Think of it like a drop of food coloring in a glass of water: the amount is tiny, but it changes everything.
Why Some Farts Smell Worse Than Others
The intensity of the smell depends largely on how much sulfur your gut bacteria have to work with. Bacteria in your large intestine break down sulfur-containing amino acids from the food you eat, producing hydrogen sulfide as a byproduct. The more sulfur-rich material reaching your colon, the more pungent the result.
A controlled feeding study in humans found that fecal sulfide concentrations rose dramatically with meat intake, ranging from 0.22 mmol/kg on a meat-free diet to 3.38 mmol/kg when participants ate 600 grams of meat per day. That’s roughly a 15-fold increase. Dietary protein from meat is one of the most significant drivers of sulfide production in the large intestine, because meat is rich in sulfur-containing amino acids like cysteine and methionine.
A separate group of bacteria, anaerobic sulfate-reducers, also produce hydrogen sulfide by converting sulfate, a compound naturally present in many foods and drinking water. So the smell isn’t just about protein. Foods high in sulfate or other sulfur compounds, like cruciferous vegetables (broccoli, cauliflower, cabbage), eggs, garlic, and onions, can all intensify the odor.
What Different Smells Can Tell You
Not all fart smells are the same, and the character of the odor loosely reflects what’s being fermented in your gut.
- Rotten eggs: Heavy hydrogen sulfide production, typically from high-protein meals or sulfur-rich foods.
- Sour or acidic: Often linked to carbohydrate fermentation. When sugars and starches aren’t fully absorbed in the small intestine, bacteria in the colon ferment them into gases and short-chain fatty acids.
- Fecal or sewage-like: Indole and skatole, which are produced when bacteria break down the amino acid tryptophan.
Odorless farts are common too. If the gas is mostly swallowed air or the byproduct of fermenting low-sulfur foods, there may be no noticeable smell at all. Many people pass gas 10 to 20 times a day, and a good portion of those are silent and scentless.
When the Smell Changes Noticeably
A sudden shift in how your gas smells usually tracks with a change in diet. Adding more red meat, eggs, protein shakes, or cruciferous vegetables to your routine will predictably increase sulfur gas production. Cutting back on those foods tends to reduce it. Interestingly, even food additives play a role: one study found that altering the amount of a common food dye (Red 40) in the diet changed sulfide levels in the gut, because hydrogen sulfide reacts with certain compounds found in dyes and medications.
Persistently foul-smelling gas that comes with other symptoms can sometimes point to a digestive issue. Fat malabsorption causes undigested fats to reach the colon, producing particularly smelly gas alongside greasy, runny stools. Carbohydrate malabsorption leads to excessive fermentation, causing bloating, gas pain, and fatty stools. Conditions like small intestinal bacterial overgrowth (SIBO) can damage the lining of the small intestine and contribute to both malabsorption and increased gas production. On its own, smelly gas is normal. Paired with persistent bloating, changes in stool, or unexplained weight loss, it’s worth investigating.
Why Your Nose Is So Sensitive to It
Human smell receptors are extraordinarily tuned to sulfur compounds, likely because many of them signal spoiled food or environmental hazards. Hydrogen sulfide is detectable at 0.5 parts per billion, making it one of the lowest-threshold odors the human nose can pick up. For context, that’s roughly equivalent to detecting a single drop of liquid in a large swimming pool. This extreme sensitivity is why a fart can fill a room even though the actual amount of smelly gas released is vanishingly small. At higher concentrations, hydrogen sulfide actually smells sweet rather than sulfurous, but the levels present in flatulence stay well within the rotten-egg range.