A fart is mostly odorless gas. Roughly two-thirds of the volume is nitrogen, with the rest split among carbon dioxide, hydrogen, methane, and small amounts of oxygen. The smell comes from trace sulfur compounds that make up less than one percent of the total mixture but pack an outsized punch. The science of flatulence turns out to be surprisingly well-studied, touching on gut microbiology, diet, diagnostic technology, and even space travel.
The Five Main Gases
Intestinal gas is dominated by just five components. Nitrogen is the most abundant, accounting for about 65% of the total volume on average. Carbon dioxide contributes roughly 10%, methane around 14% in people who produce it, hydrogen about 3%, and oxygen around 2%.1ScienceDirect (Biomedical Technology). Ignored roles of gases in digestive diseases These proportions shift depending on what you ate, who lives in your gut, and where along the digestive tract the gas is measured. But the broad picture is consistent: none of these five gases have any smell at all. You could fill a balloon with them and nobody would notice a thing.
Nitrogen and oxygen mostly come from swallowed air. Every time you eat, drink, or even swallow saliva, you gulp small amounts of atmospheric gas. Some of it gets belched back up; the rest travels down through the intestines. Carbon dioxide, hydrogen, and methane, on the other hand, are produced inside you, mostly by bacteria fermenting undigested food in the large intestine. That fermentation is the reason high-fiber meals generate more gas than, say, a piece of plain white bread.
Where the Smell Actually Comes From
The foul odor of flatus is almost entirely the work of sulfur-containing trace gases. A landmark study collecting and analyzing flatus from healthy volunteers found that hydrogen sulfide was the dominant smelly component, present at an average concentration of about 1.06 micromoles per liter. It was followed by methanethiol and dimethyl sulfide, at much lower concentrations. The correlation between hydrogen sulfide levels and perceived malodor was strong and statistically significant.2PubMed Central. Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour In plain terms, the more hydrogen sulfide in a given sample, the worse the judges said it smelled.
Hydrogen sulfide is the same compound responsible for the smell of rotten eggs. Your gut bacteria produce it when they break down sulfur-containing amino acids, which are abundant in foods like eggs, meat, cruciferous vegetables (broccoli, cabbage, cauliflower), garlic, and onions. So the link between eating certain foods and producing particularly pungent gas is not folklore; it traces directly to the sulfur content of what you eat and how your particular microbial community processes it.
Skatole and Indole Are Not the Culprits You Think
A persistent belief holds that skatole and indole, two compounds produced from the amino acid tryptophan during bacterial fermentation, are the primary source of fecal and flatulence odor. In reality, when researchers isolated these compounds from feces and tested their smell in purified form, they found that skatole and indole produced a mothball-like odor rather than the characteristic stench people associate with them. The major contributors to the actual smell were methyl sulfide compounds, not skatole and indole as had long been assumed.3Gastroenterology. Gas-chromatographic and mass-spectrometric analysis of the odor of human feces
Skatole and indole are present in feces and flatus, and in high concentrations they are certainly unpleasant. Research on pig manure, for example, has found that p-cresol, indole, and skatole together account for most of the odor impact from volatile organic compounds in livestock settings.4PLoS ONE. Effect of Storage Period on the Changes of Odorous Compound Concentrations and Bacterial Ecology for Identifying the Cause of Odor Production from Pig Slurry But in human flatulence specifically, the sulfur gases are the stars. The misconception likely persists because skatole and indole are so closely associated with feces in popular understanding that their role gets inflated.
How Much Gas Is Normal
People pass gas more often than they tend to admit. In a study tracking healthy subjects over a week on their usual diets, the average was about 10 episodes per day, with an upper limit of normal around 20.5PubMed. Factors influencing frequency of flatus emission by healthy subjects A separate study that actually measured total daily gas volume using a rectal catheter (volunteers ate their normal diet plus 200 grams of baked beans) found volumes ranging from 476 to 1,491 milliliters over 24 hours, with a median of 705 milliliters. Women and men in that study expelled equivalent amounts.6PubMed Central. Investigation of normal flatus production in healthy volunteers
A study of 120 people keeping three-day diaries found somewhat different patterns by sex. Men averaged about 13 episodes per day and women about 7, with individual ranges spanning from 1 to 53 in men and 1 to 32 in women. Flatus frequency correlated significantly with dietary fiber intake. The study also found that men reported more aromatic flatus than women, which correlated with beer consumption.7PubMed. Flatus emission patterns and fibre intake Whether the gender gap in frequency is biological or reflects differences in diet and reporting is still debated, but the consistent finding across studies is that anywhere from a handful to a couple dozen episodes per day falls within the normal range.
Diet Is the Biggest Lever
What you eat determines both how much gas you produce and how bad it smells. Dietary fiber is the clearest driver of volume. Fiber passes through the small intestine largely undigested and arrives in the colon, where bacteria feast on it and produce hydrogen, carbon dioxide, and methane as byproducts. A meta-analysis of intestinal gas composition confirmed a positive correlation between fiber intake and the volume of these fermented gases.8PubMed. Meta-Analysis of the Composition of Human Intestinal Gases This is why beans, lentils, whole grains, and many vegetables are reliably gas-producing: they deliver large amounts of fermentable substrate to the colon.
FODMAPs, a group of short-chain carbohydrates found in foods like wheat, onions, garlic, certain fruits, and dairy products, are another major contributor. These molecules are poorly absorbed in the small intestine and rapidly fermented by colonic bacteria. When researchers put people on a short-term FODMAP-restricted diet, breath hydrogen, breath methane, and gastrointestinal symptoms including flatulence all dropped significantly.9PubMed Central. Impact of Short Duration FODMAP Restriction on Breath Gases and Gastrointestinal Symptoms This is not a recommendation to cut these foods permanently, since many of them feed beneficial bacteria. But for people dealing with uncomfortable levels of gas, a temporary reduction in FODMAPs is one of the better-studied approaches.
Smell, as mentioned above, tracks with sulfur intake. The simplest way to reduce offensive odor is to moderate consumption of high-sulfur foods like eggs, red meat, and cruciferous vegetables. But there is considerable individual variation: the same meal can produce barely noticeable gas in one person and room-clearing emissions in another, depending on the specific bacterial species populating their colon.
The Methane Question
Not everyone produces methane. Only about 20% of healthy people in Western populations exhale meaningfully elevated levels of this gas, a trait determined largely by which microbes have colonized their gut.10PubMed Central. Reduced B12 uptake and increased gastrointestinal formate are associated with archaeome-mediated breath methane emission in humans Methane in the gut is produced not by bacteria but by archaea, a separate domain of microbial life. The dominant player is a single strain of the species Methanobrevibacter smithii. Research has shown that the difference between high and low methane emitters is not a fundamentally different microbial community but rather the massive predominance of this one particular strain: high emitters had roughly a thousand-fold increase in M. smithii compared to low emitters.10PubMed Central. Reduced B12 uptake and increased gastrointestinal formate are associated with archaeome-mediated breath methane emission in humans
This means the gas composition reported by studies using averages can be misleading. If you happen to be a non-methane producer, that 14% methane figure does not apply to you; your gas is mostly nitrogen, carbon dioxide, and hydrogen instead. The factors that determine whether M. smithii takes up residence in your colon include genetics, diet, and the availability of certain metabolites. One preprint found that the methane-driving strain matched M. smithii strain KB11 with 100% sequence identity, and that dietary habits and host genetics both played a role in determining whether someone was a high emitter.11bioRxiv. Methane emission of humans is explained by dietary habits, host genetics, local formate availability and a uniform archaeome
Methane production is clinically relevant because it has been linked to slower intestinal transit time. People who produce large amounts of methane tend to report more constipation, and elevated methane has been associated with conditions like irritable bowel syndrome. The gas itself may slow down gut motility, creating a feedback loop where slower transit gives archaea more time to produce methane, which further slows transit.
Can You Actually Reduce the Smell?
Activated charcoal is one of the most popular over-the-counter remedies marketed for flatulence. The theory is that charcoal, a potent adsorbent, should trap sulfur gases before they escape. The evidence does not support this. In a controlled trial, ingesting activated charcoal produced no significant reduction in the release of any sulfur-containing gas from fecal samples, and it did not meaningfully change total fecal gas output or abdominal symptoms.12PubMed. Failure of activated charcoal to reduce the release of gases produced by the colonic flora
Bismuth subsalicylate (the active ingredient in Pepto-Bismol) is a different story. When tested, it produced a dose-dependent reduction in hydrogen sulfide release from fecal samples, with treatment resulting in greater than 95% reduction in fecal hydrogen sulfide.13PubMed. Bismuth subsalicylate markedly decreases hydrogen sulfide release in the human colon Bismuth binds sulfide in the gut, converting it to bismuth sulfide, which is insoluble and odorless. The catch is that you need to take bismuth regularly for the effect to persist, and long-term daily use is not recommended because bismuth can accumulate in the body. As a short-term fix before a social event, it has genuine evidence behind it. As a permanent solution, it is not practical.
The study that identified hydrogen sulfide as the main culprit also tested a charcoal-lined cushion designed to filter flatus as it escaped. That device did reduce odor perceived by judges, suggesting that external filtration works even if internal charcoal does not.2PubMed Central. Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour Charcoal-lined underwear and seat cushions have since become a small commercial niche, and the basic principle is sound even if the products vary in quality.
When Gas Signals Something Else
Passing gas is normal. But a sudden, persistent increase in flatulence, especially when accompanied by bloating, abdominal pain, or changes in stool, can point to underlying conditions. One of the more common culprits is small intestinal bacterial overgrowth (SIBO), where bacteria that normally reside in the colon proliferate in the small intestine, fermenting food earlier in the digestive tract than they should. SIBO was found to be relatively common among patients referred for gastrointestinal symptoms, with one study finding it in about 60% of symptomatic patients.14PubMed Central. The role of small intestinal bacterial overgrowth and false positive diagnosis of lactose intolerance in southwest Hungary—A retrospective observational study
SIBO also complicates the diagnosis of food intolerances. Breath tests for lactose intolerance, for instance, measure hydrogen and methane spikes after consuming lactose. But if SIBO is present, the bacterial overgrowth in the small intestine can produce those gas spikes regardless of whether the person actually has trouble digesting lactose. One study found that among patients who tested positive for SIBO, 81% also tested positive for lactose intolerance, compared to 58% of SIBO-negative patients. A similar pattern held for sorbitol intolerance, where the rates were 54% in SIBO-positive patients versus 14% in SIBO-negative ones.15Annals of Clinical & Laboratory Science. Small Intestinal Bacterial Overgrowth May Increase the Likelihood of Lactose and Sorbitol but not Fructose Intolerance False Positive Diagnosis In other words, some people told they are lactose intolerant may actually have SIBO masquerading as an intolerance.
Hydrogen Sulfide as a Signaling Molecule
Hydrogen sulfide is not just a waste product. At low concentrations, it functions as a signaling molecule in the body, with roles in nerve signaling, blood vessel dilation, and immune regulation. In the gut specifically, it has both pro-inflammatory and anti-inflammatory effects, which makes its relationship to intestinal disease complicated. Too little hydrogen sulfide can impair protective mechanisms, while too much can damage the intestinal lining. Maintaining a balanced level appears to be important for intestinal health, and researchers have been investigating whether the gas plays a role in inflammatory bowel disease, though its influence is not yet well determined.16PubMed Central. Role of Hydrogen Sulfide in Inflammatory Bowel Disease
This dual nature is part of why the gut microbiome’s sulfur metabolism matters beyond just the smell of your gas. The same bacterial pathways that produce the hydrogen sulfide in your flatus are involved in maintaining or disrupting intestinal barrier function. It is an active area of research, and one where the simple framing of “sulfur gases are bad” misses the biological picture entirely.
Swallowable Sensors and the Future of Gas Diagnostics
One of the more surprising recent developments in flatulence science involves ingestible electronic pills. Researchers have developed a wearable platform that uses magnetic-field-based 3D localization to track an ingestible capsule through the gut with millimeter-scale resolution, achieving accuracy under 2.2 millimeters in some configurations. The pill carries optoelectronic sensors that can detect oxygen and ammonia concentrations in real time as it moves through the digestive tract.17Cell Reports Physical Science. 3D gas mapping in the gut with AI-enabled ingestible and wearable electronics
The clinical promise here is significant. Currently, diagnosing conditions like SIBO, carbohydrate malabsorption, and motility disorders relies on indirect methods like breath tests, which measure gas that has already been absorbed into the bloodstream and exhaled through the lungs. A sensor that directly maps gas concentrations along the length of the digestive tract could provide a much more precise picture of where fermentation is happening and how fast gas is moving. The technology is still in early stages, but it represents a shift from treating intestinal gas as a nuisance to treating it as diagnostic information.
Flatulence in Sealed Environments
When you share a sealed, recirculated-air environment with other people for months, methane becomes an engineering problem. In spacecraft and space station life-support systems, atmospheric trace contaminants must be carefully managed. Methane is among the most difficult to remove directly, and NASA and other space agencies have spent considerable effort evaluating whether dedicated methane removal is necessary or whether the gas can simply be tolerated at the low concentrations produced by a small crew.18TTU DSpace. Is Direct Methane Removal in Human Space Flight Required? Methane itself is not toxic at the concentrations produced by human flatulence, but it is flammable when it accumulates, and in a closed-loop life-support system, even low-level buildup over weeks requires either active removal or careful monitoring. The International Space Station uses catalytic oxidation to handle trace gases, but as missions get longer and crews get larger, the engineering margins get tighter. It is one of the more unusual downstream consequences of the same colonic fermentation that produces the gas in the first place.