Human farts are mostly odorless gases: nitrogen, carbon dioxide, methane, hydrogen, and small amounts of oxygen. These five gases make up over 99% of every fart. The smell comes from trace sulfur compounds that account for less than 1% of the mixture but pack an outsized punch to your nose.
The Five Major Gases
Nitrogen is the dominant component, averaging about 65% of total flatus volume. It enters your digestive tract primarily through swallowed air. Every time you eat, drink, or swallow saliva, you’re taking in small gulps of the same air around you, which is roughly 78% nitrogen. Most of this gets absorbed or burped back up, but some travels all the way through to the exit.
Methane comes in second at around 14%, followed by carbon dioxide at about 10%. Hydrogen averages around 3%, and oxygen rounds things out at about 2%. Unlike nitrogen and oxygen, which largely come from swallowed air, hydrogen, carbon dioxide, and methane are produced inside your gut by microbial fermentation. When bacteria in your large intestine break down undigested carbohydrates and fiber, these gases are the byproducts.
These proportions vary considerably from person to person and meal to meal. Nitrogen alone can swing from about 44% to 86% depending on how much air you swallow and what your gut bacteria are doing with the food you send them.
Why Farts Smell
None of those five major gases have any scent. The smell comes entirely from volatile sulfur compounds, present in tiny concentrations but detectable by the human nose at extraordinarily low levels. The main offenders are hydrogen sulfide (the classic rotten-egg smell), methyl mercaptan, and dimethyl sulfide.
Your gut bacteria produce these sulfur compounds through two main pathways. The first is breaking down sulfur-containing amino acids found in dietary protein, particularly cysteine, methionine, and taurine. The second is reducing inorganic sulfur from food additives like sulfites and sulfates, as well as from bile acids your own body produces. In Western diets, protein appears to be the bigger contributor. Studies show that fecal sulfide concentrations rise proportionally with meat intake, and highly processed, low-fiber diets paired with animal protein produce more hydrogen sulfide than plant-heavy diets.
This is why certain meals produce noticeably worse-smelling gas. Foods rich in sulfur compounds (eggs, cruciferous vegetables like broccoli and cabbage, garlic, onions, red meat) give your gut bacteria more raw material to work with. Beans and lentils, by contrast, tend to increase gas volume without necessarily making it smell worse, because they’re rich in fermentable carbohydrates rather than sulfur.
How Much Gas Is Normal
A study of healthy volunteers found total daily flatus volume ranged from 476 to 1,491 milliliters, with a median of about 705 ml. That’s roughly the volume of a wine bottle, spread across anywhere from 10 to 25 episodes per day. Passing gas up to about 20 times daily falls within the normal range, though most people significantly undercount because many episodes happen during sleep or go unnoticed.
Volume and frequency spike after meals high in fiber, resistant starch, or fermentable sugars (sometimes grouped under the acronym FODMAPs). Wheat, onions, beans, and dairy (in lactose-intolerant individuals) are common triggers. This is normal bacterial fermentation doing exactly what it’s supposed to do.
Not Everyone Produces the Same Gas
One of the more interesting differences between people is methane production. Gut methane comes from archaea, ancient microorganisms distinct from bacteria, that thrive in oxygen-free environments like the large intestine. The dominant species is Methanobrevibacter smithii, found in over 90% of the archaeal community in human feces. These organisms consume hydrogen produced by other gut microbes and convert it into methane.
Researchers historically divided people into “methane producers” and “non-producers,” but more sensitive instruments have shown that everyone produces at least some methane. The more accurate distinction is between high and low producers. Whether you fall into one category or the other depends on your age, sex, ethnic background, and the specific microbial community you’ve cultivated in your gut over a lifetime. Geography and ethnicity appear to play a significant role, with studies finding different rates of high methane production across populations even when they live in the same country.
Can You Actually Light a Fart on Fire?
Yes, and the chemistry checks out. Both hydrogen and methane are flammable gases. Hydrogen’s lower flammability limit in air is 4%, meaning a mixture of at least 4% hydrogen will ignite with a spark. Methane’s threshold is similar, around 4.7%. Since the average fart contains roughly 3% hydrogen and 14% methane, many farts carry enough methane alone to be combustible when they mix with air at the right ratio. The blue flame sometimes reported is consistent with methane combustion. People with higher hydrogen concentrations in their gas have an even more flammable mix.
This isn’t just a party trick concern. Surgeons have to account for intestinal gas during certain procedures involving electrical instruments in the colon, where a spark in an enclosed space filled with hydrogen and methane could cause real problems.
When Gas Changes Signal Something Else
A noticeable shift in the smell, volume, or frequency of your gas can reflect changes happening in the gut. Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis alter the composition of volatile organic compounds in both stool and flatulence. The underlying inflammation, malabsorption, and shifts in gut bacteria associated with these conditions change what’s being fermented and how, leading to distinctly different odor profiles.
Food intolerances are a more common cause of changes. Lactose intolerance, fructose malabsorption, and celiac disease all result in undigested material reaching the colon, where bacteria ferment it into excess gas. If your gas volume or odor has changed significantly and persistently, it often reflects a change in what’s being absorbed (or not absorbed) higher up in the digestive tract, rather than a problem with the gas itself.