Farts smell bad because of sulfur-containing gases produced by bacteria in your gut. These gases make up less than 1% of flatulence, but they pack an outsized punch. The other 99% of what you pass is completely odorless.
Most of a Fart Has No Smell at All
Five odorless gases account for 99% of flatulence: nitrogen, oxygen, carbon dioxide, hydrogen, and methane. The proportions vary wildly from person to person. Nitrogen alone can range from 11% to 92% of total gas volume, and hydrogen anywhere from 0% to 86%. Your intestines produce between 500 and 2,000 milliliters of gas every day, and the average person passes gas about 15 times daily (though anywhere from a handful to 40 times falls within the normal range).
None of that gas smells like anything. The stink comes entirely from trace compounds that exist in tiny concentrations.
The Sulfur Compounds Behind the Smell
The main offender is hydrogen sulfide, the same compound responsible for the smell of rotten eggs. Your nose can detect it at concentrations as low as half a part per billion, which is why even a minuscule amount produces a noticeable odor. That extreme sensitivity is an evolutionary trait. Hydrogen sulfide is toxic at high concentrations, so humans evolved to notice it immediately.
A second compound, methanethiol, forms when gut bacteria break down methionine, an amino acid found in meat, fish, and dairy. Methanethiol has a smell often compared to rotting cabbage or sulfurous decay. Together, hydrogen sulfide and methanethiol are responsible for the characteristic “rotten” quality of smelly gas.
Your Gut Bacteria Do the Heavy Lifting
The smell doesn’t come from food itself. It comes from what your gut bacteria do with that food. Sulfate-reducing bacteria, predominantly from the genus Desulfovibrio, consume hydrogen in your colon and produce hydrogen sulfide as a byproduct. These bacteria compete with other microbes, like methane-producing organisms, for the same fuel. The balance between these bacterial populations partly determines whether your gas leans more toward odorless methane or pungent hydrogen sulfide.
When you eat foods rich in sulfur-containing proteins, you’re essentially giving these bacteria more raw material to work with. More sulfur in, more hydrogen sulfide out.
Foods That Make It Worse
Sulfur enters your gut through two main routes: sulfur-containing amino acids in protein and sulfur-based compounds in certain vegetables.
- Cruciferous vegetables like broccoli, cauliflower, and Brussels sprouts contain glucosinolates, sulfur-rich compounds that become especially gassy during digestion.
- Allium vegetables like garlic, onions, and leeks are naturally high in sulfur compounds.
- High-protein animal foods like eggs, red meat, fish, and dairy provide sulfur-containing amino acids that gut bacteria ferment into hydrogen sulfide and methanethiol.
- Beer and wine contain sulfites that add to the sulfur load in your gut.
- Dried fruits and coconut milk are less obvious sources that also contribute.
The relationship is fairly direct: the more sulfur-containing food you eat, the more hydrogen sulfide your gut bacteria produce, and the worse your gas smells. A meal heavy in eggs and broccoli, for example, gives bacteria both amino acid-based and vegetable-based sulfur to work with.
Why Some Days Are Worse Than Others
Diet is the biggest variable, but it’s not the only one. How quickly food moves through your digestive tract also matters. When transit slows down, as it does during constipation, bacteria have more time to ferment food in your colon. Longer fermentation means more thorough breakdown of proteins, which increases the production of sulfur gases and other smelly byproducts. This is one reason constipation often comes with particularly foul-smelling gas.
Your gut microbiome itself shifts over time based on your long-term diet, antibiotic use, and other factors. Someone whose bacterial population skews heavily toward sulfate-reducing species will consistently produce smellier gas than someone with a different microbial balance, even eating the same meal.
When Smelly Gas Signals Something Else
Foul-smelling gas on its own is almost always just a byproduct of normal digestion. But a sudden, persistent change in how your gas smells can sometimes reflect a digestive problem. Small intestinal bacterial overgrowth, where bacteria multiply abnormally in the small intestine, produces excess gas and can cause diarrhea and weight loss alongside unusually smelly flatulence. Celiac disease, gastroparesis, and chronic constipation can all shift gas patterns as well.
The combination of severely smelly gas with other symptoms is what matters most: unintentional weight loss, blood in your stool, persistent diarrhea or constipation, vomiting, or heartburn alongside increased gas are signals worth getting checked out. Smelly gas by itself, without those accompanying symptoms, is just your bacteria doing their job.