Your farts stink because of sulfur. Specifically, tiny amounts of sulfur-containing gases produced by bacteria in your large intestine give flatulence its signature smell. These gases make up only about 1% of what you actually release, but they’re potent enough to clear a room. The other 99% of a fart is completely odorless.
What’s Actually in a Fart
Most of a fart is boring. Between 20 and 90% is nitrogen (swallowed air), up to 50% can be hydrogen, and the rest is a mix of carbon dioxide, oxygen, and sometimes methane. None of these gases have any smell at all. You could release enormous volumes of them and nobody would notice, aside from the sound.
The smell comes from three sulfur compounds, all present in trace amounts. Hydrogen sulfide is the dominant one, responsible for that classic rotten-egg quality. It shows the strongest correlation with how bad a fart smells. The second contributor, methanethiol, smells like decomposing vegetables. The third, dimethyl sulfide, has a slightly sweet odor. A study published in the journal Gut found hydrogen sulfide concentrations in flatus were roughly five times higher than methanethiol and thirteen times higher than dimethyl sulfide. When the overall concentration of all three goes up, the stink intensifies proportionally.
There are also compounds called indole and skatole, which are produced when protein breaks down in your colon. These have a distinct fecal smell and add another layer to the odor profile beyond the sulfur gases.
Your Gut Bacteria Do the Dirty Work
You don’t produce these smelly gases yourself. Bacteria in your large intestine do. When food reaches your colon, trillions of microbes ferment whatever your small intestine didn’t fully absorb. Carbohydrates that slip through get broken down into gases and short-chain fatty acids. This process generates the bulk of your gas volume but not much of the smell.
The real stink factories are sulfate-reducing bacteria. These microbes thrive on sulfur-containing compounds in your diet, and they release hydrogen sulfide as a byproduct. The more sulfur-rich food that reaches your colon, the more raw material these bacteria have to work with, and the worse things smell on the way out.
Protein creates a separate problem. Your body is efficient at absorbing protein in the small intestine, but when you eat more than your body can use, the excess travels to the colon. There, bacteria break it down through a process called putrefaction, which produces indole, skatole, and additional sulfur gases. This is why a high-protein meal often leads to particularly foul-smelling gas.
Foods That Make It Worse
Sulfur is everywhere in the diet, which is why completely odorless farts are rare. The biggest dietary sources include:
- Cruciferous vegetables: broccoli, Brussels sprouts, red cabbage, radishes, and turnips
- Alliums: onions, leeks, and garlic
- Meat and poultry: especially beef, ham, and organ meats
- Eggs and aged cheeses: whole eggs, cheddar, Parmesan, and Gorgonzola
- Legumes: soybeans, black beans, kidney beans, and split peas
- Certain drinks: beer, wine, and cider
Asparagus is a well-known offender. So are condiments like mustard and horseradish. Even dried fruits like apricots and figs contain meaningful amounts of sulfur. When you eat a meal loaded with several of these foods, you’re essentially giving your sulfate-reducing bacteria a feast. The result shows up (and smells) hours later.
Beans deserve a special mention because they cause both volume and odor. They contain complex carbohydrates your small intestine can’t break down, so your colon bacteria ferment them aggressively. This produces large quantities of gas. Combine that with the sulfur content in many legumes, and you get farts that are both frequent and foul.
How Often Is Normal
The old estimate was about 14 times a day, give or take six. But newer research from the University of Maryland, using wearable gas-sensing devices, found the actual average is closer to 32 times per day. Some participants hit 59, and unpublished data from the same lab recorded well over 100 in a single day. Most of these passages are small, odorless, and go completely unnoticed.
The smelly ones stand out precisely because they’re unusual. A fart only has a noticeable odor when sulfur compound concentrations spike above the threshold your nose can detect. Since hydrogen sulfide is detectable at extremely low concentrations, even a modest increase in sulfur fermentation can produce a smell that seems disproportionate to the amount of gas released.
Why Some Farts Smell Worse Than Others
Day-to-day variation in fart odor comes down to what you ate, how long it sat in your colon, and which bacteria are most active at the time. A high-protein, high-sulfur meal like steak with broccoli and a glass of red wine is a recipe for potent gas 6 to 24 hours later. A meal of plain rice and chicken breast produces far less sulfur for bacteria to work with.
Transit time matters too. When stool moves slowly through the colon, as happens with constipation, bacteria have more time to ferment and produce sulfur gases. The longer food residue sits, the more thoroughly it gets broken down and the more concentrated those gases become. This is why constipation often comes with particularly smelly flatulence.
Your personal microbiome also plays a role. Everyone’s gut bacteria population is slightly different, and some people naturally carry higher concentrations of sulfate-reducing bacteria. This means two people can eat the exact same meal and produce very different levels of odor. Gut bacteria populations shift over time based on your long-term dietary patterns, so a sudden increase in sulfur-rich foods can temporarily amplify the smell until your microbiome adjusts.
When Smelly Gas Signals a Problem
Foul-smelling gas by itself is almost always dietary. But persistently terrible-smelling gas combined with other symptoms can point to a digestive issue. Malabsorption, where your small intestine fails to properly absorb nutrients, sends undigested food to the colon in larger quantities than normal. The result is excess fermentation, more gas, and worse smells. Malabsorption also produces fatty stools that are greasy, runny, and particularly foul.
Celiac disease, parasitic infections like giardia, and conditions affecting pancreatic function can all cause malabsorption. The pattern to watch for isn’t just smelly gas on its own, but smelly gas alongside bloating, diarrhea, weight loss, or stools that look oily or float persistently.
Reducing the Smell
The most straightforward approach is reducing sulfur-heavy foods, especially in combination. You don’t need to eliminate them entirely. Cutting back on the biggest offenders for a few days typically produces a noticeable difference.
Bismuth subsalicylate, the active ingredient in some common over-the-counter stomach remedies, is one of the few compounds shown to directly reduce hydrogen sulfide. The bismuth binds to hydrogen sulfide in the colon, converting it into an insoluble compound that doesn’t release odor. In clinical testing, taking it four times daily for several days reduced hydrogen sulfide release by over 95%. The bismuth itself is the active component, not the salicylate portion of the compound.
Eating less protein than your body needs to absorb in the small intestine also helps. When protein intake matches your body’s capacity, very little reaches the colon for putrefaction. The excess is where the trouble starts. Spreading protein intake across meals rather than loading it into one sitting gives your small intestine a better chance of absorbing it before bacteria get involved.