Farts smell because of a tiny fraction of their total gas. About 99% of what you pass is completely odorless, made up of nitrogen, oxygen, carbon dioxide, hydrogen, and methane. The remaining 1% consists of volatile sulfur compounds, and that sliver is entirely responsible for the smell. Your nose is extraordinarily sensitive to these compounds, which is why even a trace amount can fill a room.
The Sulfur Compounds Behind the Smell
The primary culprit is hydrogen sulfide, the same gas that gives rotten eggs their distinctive odor. Your nose can detect it at concentrations as low as 0.0005 parts per million, which means you’re essentially equipped with a built-in hydrogen sulfide alarm. Two other sulfur gases, methanethiol and dimethyl sulfide, contribute additional layers of stench. Together, these three compounds create the spectrum of flatulence odors, from mildly unpleasant to room-clearing.
These gases form when bacteria in your large intestine break down sulfur-containing amino acids (the building blocks of protein) and sulfate compounds from food and drinking water. The process is a normal byproduct of digestion. Every human produces these gases. The variation in how bad your farts smell comes down to what you eat, which bacteria dominate your gut, and how quickly food moves through your system.
Your Gut Bacteria Are the Gas Factory
Your colon hosts trillions of bacteria, and specific groups specialize in producing sulfur gases. The most notable are sulfate-reducing bacteria, predominantly from a genus called Desulfovibrio, which is the most common sulfate-reducing bacterium found in the guts of healthy American adults. These microbes consume hydrogen and sulfate compounds, then release hydrogen sulfide as a waste product. Other bacteria, including species of Fusobacterium and E. coli, generate hydrogen sulfide by breaking down sulfur-containing amino acids like cysteine.
The balance of your gut bacteria matters. People with more sulfate-reducing bacteria tend to produce smellier gas. Meanwhile, people whose guts are dominated by methane-producing microbes (methanogens) may pass less odorous gas, though methane brings its own quirk: it slows intestinal transit, meaning food sits in the colon longer, which can increase bloating and potentially give sulfur-producing bacteria more time to work.
Foods That Make It Worse
High-sulfur foods are the most direct path to foul-smelling gas. Cruciferous vegetables like broccoli, cauliflower, cabbage, and Brussels sprouts are rich in sulfur compounds. Eggs, red meat, dairy, garlic, and onions also supply the sulfur-containing amino acids that gut bacteria ferment into hydrogen sulfide. Even drinking water contains sulfate in varying amounts, feeding those sulfate-reducing bacteria.
Protein-heavy diets tend to produce smellier (though not necessarily more voluminous) gas, because the breakdown products of protein are what generate sulfur compounds. Carbohydrate-rich foods, on the other hand, tend to increase the volume of gas without necessarily making it smell worse. Beans, lentils, wheat, and foods high in certain short-chain carbohydrates called FODMAPs (found in onions, garlic, apples, dairy, and artificial sweeteners like sorbitol) are fermented rapidly by gut bacteria, producing large amounts of odorless hydrogen and carbon dioxide. The result: more frequent gas, but not always more pungent gas.
The smelliest farts often come from meals that combine both high-protein and high-fiber foods, giving bacteria plenty of sulfur to convert and plenty of fermentable material to keep the process going.
How Often Is Normal
The long-standing estimate was about 14 times per day, give or take six. But a University of Maryland study using more precise tracking found the real average is closer to 32 times per day. Some participants passed gas 59 times in a single day, and unpublished data from the same research group recorded well over 100. Most of these episodes are odorless or nearly so, which is why you don’t notice the majority of them. The ones you do notice are the ones where sulfur compounds happen to be concentrated.
Why Some Farts Smell Worse Than Others
Several factors determine whether a particular episode is mild or devastating. The most significant is what you ate 6 to 24 hours earlier, since that’s roughly how long it takes food to reach the colon where fermentation happens. A steak-and-broccoli dinner provides both the protein and the sulfur-rich vegetables that maximize hydrogen sulfide output.
Transit time also plays a role. When food moves slowly through the colon (due to constipation, dehydration, or naturally slower motility), bacteria have more time to ferment and produce gas. Methane itself contributes to this cycle: research shows it slows the contraction speed of intestinal muscles, which delays transit further and can increase gas buildup. Conversely, when transit is fast (during a bout of diarrhea, for instance), there’s less fermentation time and often less odor.
Holding gas in doesn’t make it smell worse per se, but it does allow more gas to accumulate, so the eventual release carries a higher concentration of sulfur compounds in a single burst.
When Smelly Gas Signals a Problem
Occasional foul-smelling gas is completely normal. Persistently extreme odor, especially combined with other symptoms, can point to a digestive issue worth investigating. Malabsorption syndromes, where your small intestine fails to properly absorb nutrients, cause undigested food to reach the colon in larger quantities than usual. Bacteria feast on this surplus, producing excess gas, bloating, and characteristically foul-smelling stools that appear pale, greasy, and bulky.
Lactose intolerance is one of the most common causes of unusually gassy, smelly digestion. If you notice bloating, cramping, and watery diarrhea 30 to 90 minutes after consuming dairy, you may lack enough of the enzyme that breaks down lactose. Small intestinal bacterial overgrowth (SIBO), where bacteria colonize the small intestine in abnormal numbers, can also produce excessive and particularly odorous gas along with bloating, diarrhea, and nutrient deficiencies.
The distinguishing feature between normal smelly gas and a medical issue is the pattern. If foul-smelling gas is a daily problem regardless of what you eat, or if it comes with persistent diarrhea, unexplained weight loss, or fatty stools, that’s worth bringing up with a doctor. If it only happens after you demolish a plate of eggs and cabbage, your gut is just doing its job.