What Causes a Fart: Gas Sources and Why It Smells

Flatulence comes from two main sources: swallowed air and gases produced by bacteria fermenting undigested food in your large intestine. The smell, which makes up a tiny fraction of the total volume, is almost entirely the work of sulfur-containing compounds generated by specific gut microbes. Most of what actually leaves your body during a fart is odorless hydrogen, nitrogen, and carbon dioxide, but the trace sulfur gases pack an outsized punch to anyone nearby.

The Two Roads Gas Takes Into Your Gut

Every time you eat, drink, chew gum, or even talk, you swallow small amounts of air. That air is mostly nitrogen and oxygen. Some of it gets belched back up, but whatever makes it past your stomach and into the small intestine has to travel the full length of the digestive tract and eventually exit as flatulence. This swallowed air accounts for a meaningful portion of the gas you pass, especially the odorless kind.

The second source is far more interesting. When food you eat reaches your large intestine without being fully broken down and absorbed, the trillions of bacteria living there go to work on it. These microbes ferment carbohydrates, fiber, and other residues that your own digestive enzymes couldn’t handle. The process generates gases as byproducts, primarily hydrogen and carbon dioxide, and in some people, methane. Hydrogen is the single largest gaseous product of this microbial activity. Concentrations of hydrogen in intestinal gas can range from undetectable to over 40 percent, depending on the individual and what they’ve eaten.¹ The gut is home to trillions of microbes, including thousands of bacterial species and hydrogen-consuming archaea, all of which shape the final gas mixture that ends up as flatulence.²

What a Fart Is Actually Made Of

If you could capture and analyze a typical day’s worth of flatulence from a healthy person, you’d find it dominated by just a handful of gases, none of which smell. In a study of healthy volunteers, the median daily hydrogen output was about 361 milliliters, carbon dioxide about 68 milliliters, and the remaining bulk (a median of roughly 213 milliliters) was mostly nitrogen, presumably from swallowed air.³ Only three of the volunteers in that study produced measurable methane, and one of them generated far more than the other two.

The variation between people is striking. Daily hydrogen output ranged from as low as 42 milliliters to over 1,000 milliliters across participants, a more than twenty-fold spread.³ What this means in practical terms is that two people eating the same meal can produce vastly different amounts of gas, and neither one is abnormal. The composition of your gut bacteria, how quickly food moves through your system, and how much air you swallow all contribute to where you fall on that spectrum.

Why Farts Smell

Hydrogen, nitrogen, carbon dioxide, and methane are all odorless. If your flatulence were made of nothing else, nobody would ever notice. The smell comes from sulfur-containing gases that constitute a vanishingly small fraction of the total volume but have an extremely low detection threshold, meaning your nose picks them up at concentrations measured in parts per million.

Research that directly analyzed the odor of human flatus identified three primary sulfur culprits. Hydrogen sulfide was the dominant one, present at a mean concentration of about 1.06 micromoles per liter. Methanethiol came in second at roughly 0.21 micromoles per liter, and dimethyl sulfide third at about 0.08 micromoles per liter. Flatus odor correlated strongly with hydrogen sulfide concentration specifically.⁴ In plain terms, the “rotten egg” smell of a particularly offensive fart is mostly hydrogen sulfide. The other two sulfur compounds add layers to the odor but contribute less to the overall stench.

Hydrogen sulfide is the end product of sulfate-reducing bacteria in your colon.⁵ These bacteria thrive on sulfate and sulfur-containing amino acids from the food you eat. When you consume foods rich in sulfur, such as eggs, cruciferous vegetables like broccoli and cabbage, garlic, onions, and red meat, you’re feeding the bacteria that produce these smelly gases. The connection between diet and odor is direct and, for most people, predictable enough to manage.

How Often Is Normal

Studies looking at flatulence frequency in healthy people on their usual diets consistently land in the same ballpark: about ten times per day on average. One study tracking subjects over a full week found they passed gas around ten times daily, with an upper limit of normal (defined statistically) at about twenty times per day.⁶ A separate study using a placebo-controlled design found essentially the same average of about ten episodes per day.⁷

If you’re in the range of five to twenty times daily, you’re within the normal distribution for a healthy person. People often worry that they’re passing gas too frequently, but the research suggests that anything up to about twenty episodes a day doesn’t indicate a problem on its own. What matters more clinically is whether excess gas is accompanied by pain, bloating, or changes in bowel habits, which may point to something beyond normal variation.

The Methane Divide

Not everyone produces methane. Only a subset of the population, estimated at somewhere between a third and a half of adults in Western countries, harbors the specific archaea (a type of microorganism distinct from bacteria) responsible for methane production in the gut. The most well-studied of these is Methanobrevibacter smithii, which consumes hydrogen produced by other gut microbes and converts it into methane.²

This creates an interesting metabolic trade-off. In people who harbor methane-producing archaea, some of the hydrogen that would otherwise end up in their flatulence gets consumed and converted to methane instead. Methane itself is also odorless, so this doesn’t change the smell. But it does change the overall volume and composition of the gas. The gas composition data from healthy volunteers illustrate this clearly: only three out of the study group produced any measurable methane at all, and their outputs ranged from a trivial 3 milliliters per day to a substantial 120 milliliters.³

Whether you’re a methane producer or not appears to be relatively stable over time and is influenced by factors including genetics, early microbial colonization, and possibly long-term dietary patterns. Hydrogen that isn’t consumed by methanogens can also be used by sulfate-reducing bacteria, the same ones that produce hydrogen sulfide. So the balance between these microbial communities in your gut doesn’t just determine what your flatus is made of, it also influences how it smells.

Why Certain Foods Are Worse Than Others

The foods most associated with gas production are the ones that deliver undigested carbohydrates to the colon. Beans are the classic example because they contain oligosaccharides, complex sugars that humans lack the enzyme to break down. These pass through the small intestine intact and arrive in the colon as a feast for fermentative bacteria. The result is a surge of hydrogen and carbon dioxide production.

But beans are hardly the only offender. Other major contributors include:

Cruciferous vegetables: Broccoli, cauliflower, Brussels sprouts, and cabbage contain both fermentable fiber and sulfur compounds, making them a double threat for both volume and smell.
Dairy products: For people who don’t produce enough lactase, the lactose in milk and cheese reaches the colon undigested and gets fermented.
Whole grains and high-fiber foods: Fiber is healthy for many reasons, but the portion that resists digestion feeds colonic bacteria.
Alliums: Garlic, onions, and leeks are rich in fructans and sulfur-containing compounds, contributing to both gas volume and odor.
Sugar alcohols: Sorbitol, xylitol, and mannitol, common in sugar-free gums and candies, are poorly absorbed and readily fermented.

The smell side of the equation has its own dietary triggers. Foods high in sulfur-containing amino acids, particularly cysteine and methionine, give sulfate-reducing bacteria the raw material to produce hydrogen sulfide. Red meat, eggs, and certain dairy products are especially rich in these amino acids. Research has also found that various probiotic bacterial strains differ substantially in how much volatile sulfur compounds they produce, with some strains generating much higher amounts than others even within the same species.⁸ Your personal mix of gut bacteria, shaped over years by your diet and environment, determines how efficiently sulfur in your food gets converted to smelly gases.

When Gas Becomes a Medical Concern

For most people, flatulence is a normal and healthy byproduct of digestion. But some conditions amplify gas production, change how gas moves through the gut, or make normal gas volumes feel unbearable.

Small intestinal bacterial overgrowth, commonly called SIBO, occurs when bacteria that normally belong in the colon colonize the small intestine instead. These misplaced bacteria start fermenting food earlier in the digestive process than they should, producing gas in a part of the gut that isn’t designed to handle it. Prevalence estimates for SIBO have ranged widely, from about 2.5 to 22 percent depending on the population studied and the diagnostic method used, and the condition becomes more common with age and in people with other gastrointestinal disorders.⁹

Bloating, which often accompanies excess flatulence, doesn’t always mean you’re producing more gas than usual. Research into gas transit through the intestines has found that some people with bloating have impaired reflex control of gas propulsion, meaning their guts don’t move gas along efficiently even when the total volume is normal.¹⁰ The problem in these cases isn’t overproduction but poor clearance. Gas pools in parts of the intestine, causing distension and discomfort, even though the total amount generated may be perfectly normal. Studies of intestinal gas dynamics have highlighted these subtle motility dysfunctions as an underappreciated factor in gas-related symptoms.¹¹

Other conditions that commonly increase gas or gas-related discomfort include irritable bowel syndrome, celiac disease, lactose intolerance, and fructose malabsorption. In each case, the underlying mechanism is similar: something that should have been digested and absorbed upstream arrives in the colon intact, where bacteria ferment it. The distinguishing feature of these conditions is that the excess fermentation happens because of a specific digestive failure, not just because you ate a lot of beans.

Practical Ways to Reduce Gas and Odor

If your concern is volume, the most straightforward approach is reducing the amount of fermentable substrate that reaches your colon. Cutting back on the foods listed above, or introducing them gradually to let your microbiome adapt, can help. But there are also targeted interventions worth knowing about.

Alpha-galactosidase, sold under the brand name Beano, is an enzyme that breaks down the oligosaccharides in beans and other legumes before they reach the colon. In a double-blind crossover study, subjects who took alpha-galactosidase with a test meal had significantly fewer flatulence events over the following hours compared to those who took a placebo.¹² A separate study confirmed that the enzyme reduced both breath hydrogen excretion (a proxy for colonic gas production) and the severity of flatulence when taken at a sufficient dose during a bean-heavy meal.¹³ It’s worth noting that alpha-galactosidase works specifically on the sugars found in legumes and certain vegetables. It won’t help with gas from lactose, fiber, or other sources.

If your concern is smell rather than volume, the target shifts to sulfur. Bismuth subsalicylate, the active ingredient in Pepto-Bismol, binds hydrogen sulfide in the colon. In lab and human testing, bismuth subsalicylate produced a greater than 95 percent reduction in fecal hydrogen sulfide release.¹⁴ That’s a striking effect on the primary odor compound, though bismuth isn’t something you’d want to take daily long-term due to potential side effects including darkened stools and, with chronic use, bismuth toxicity.

Dietary adjustments targeting sulfur specifically can also help. Reducing intake of high-sulfur foods like eggs, red meat, and cruciferous vegetables tends to reduce odor even if it doesn’t change gas volume much. Some people find that charcoal-lined underwear pads or seat cushions absorb sulfur gases effectively enough to matter in social situations, though the evidence on these products is more anecdotal than clinical.

Why Your Hydrogen Goes Where It Goes

Not all the hydrogen your gut bacteria produce ends up in your flatulence. There are actually three exit routes: it can leave as flatus, diffuse into the bloodstream and get exhaled through the lungs (which is how breath hydrogen tests work for diagnosing conditions like lactose intolerance), or get consumed by other microbes in the gut.¹ The microbes that consume hydrogen include the methane-producing archaea already discussed, sulfate-reducing bacteria that convert it to hydrogen sulfide, and acetogens that use it to make acetate.

This means the ecology of your gut directly determines the balance between gas volume and gas odor. Someone whose gut is dominated by methanogens may produce less total flatus volume but the gas contains methane. Someone whose gut harbors more sulfate-reducing bacteria may produce similar or lower volumes but with a stronger smell, because hydrogen is being diverted into hydrogen sulfide rather than escaping as odorless gas. And someone with fewer hydrogen-consuming microbes may pass larger volumes of gas that is relatively odorless. These microbial ecosystems are remarkably stable within individuals but vary enormously between people, which is part of why flatulence is such a personal and unpredictable experience.

Swallowed Air and the Habits That Increase It

While bacterial fermentation gets most of the attention, swallowed air is worth understanding separately because it’s the one gas source you can directly control through behavior. Every swallow sends a small bolus of air into the stomach. Most people swallow air at a steady, low rate, but certain habits accelerate the process:

Eating or drinking quickly: Gulping food means gulping air.
Chewing gum: Repeated swallowing without food pushes excess air down.
Carbonated drinks: The dissolved carbon dioxide adds gas directly, though much of it is released as a belch before reaching the intestine.
Smoking or vaping: The inhale-swallow cycle introduces air.
Loose-fitting dentures: Extra saliva production and altered chewing increase swallowing frequency.

Nitrogen from swallowed air is not consumed by gut bacteria and can’t be absorbed into the blood, so it has no exit route except flatulence. This is why some people who don’t eat particularly gassy foods still pass a lot of gas. If the volume seems high but there’s little odor, swallowed air is a likely contributor. Slowing down while eating, avoiding gum, and cutting back on carbonated beverages are the simplest ways to reduce this component.

The Fiber Paradox

One of the more frustrating aspects of gut health advice is that the foods that are best for your long-term digestive health are often the same ones that produce the most gas in the short term. High-fiber diets feed beneficial bacteria, promote regular bowel movements, and are associated with lower risks of colorectal cancer and cardiovascular disease. They also deliver large amounts of fermentable material to the colon.

The good news is that this effect tends to diminish over time. When you gradually increase fiber intake rather than making a sudden switch, your gut microbiome shifts to accommodate the new substrates. Populations of hydrogen-consuming bacteria expand, fermentation becomes more efficient, and the initial surge in gas production settles down. People who eat consistently high-fiber diets often produce less noticeable gas than people who eat fiber only occasionally, because their microbial community is adapted to it. If you’re adding more fiber to your diet, the standard advice to increase intake slowly over a couple of weeks is backed by real physiology: you’re giving your microbes time to rebalance.

The smell component doesn’t follow the same trajectory, though. A high-fiber diet that also includes sulfur-rich foods can continue to produce odorous gas even after your microbiome has adapted to the fiber load. Volume and odor respond to different dietary inputs and are mediated by different microbial pathways, which is why someone can eat a plant-heavy diet with modest gas volume but still produce impressively smelly flatulence if their meals include a lot of garlic, onions, and cruciferous vegetables.