A fart is roughly 99% odorless gas. The five main components are nitrogen, hydrogen, carbon dioxide, methane, and oxygen, all of which have no smell whatsoever. The characteristic odor comes from trace sulfur-containing compounds that make up less than 1% of the total volume. Your body produces between 500 and 2,000 milliliters of intestinal gas every day, and the average person passes it about 15 times in 24 hours, though anywhere from a handful to 40 times falls within the normal range.
The Five Main Gases
Nitrogen is typically the most abundant gas in flatulence, ranging from 11% to 92% of the total mix. Most of it comes from air you swallow while eating, drinking, or talking. Hydrogen can range from 0% to 86%, carbon dioxide from 3% to 54%, and methane from 0% to 54%. Oxygen rounds it out at 0% to 11%. Those wide ranges aren’t typos. The chemical profile of a fart varies enormously from person to person and even from hour to hour, depending on what you’ve eaten, what bacteria live in your gut, and how much air you’ve swallowed.
The variation in hydrogen and carbon dioxide is largely driven by bacterial fermentation in the colon. When gut bacteria break down fiber and complex carbohydrates that survived digestion in the small intestine, they release these gases as byproducts. A meal high in beans, whole grains, or cruciferous vegetables gives your gut bacteria more material to ferment, which means more gas.
Where the Gas Actually Comes From
Intestinal gas has two main sources. The first is swallowed air, called aerophagia. Every time you eat, drink, chew gum, or smoke, you swallow small amounts of air. This air is mostly nitrogen and oxygen. Some of it gets absorbed or burped back up, but a portion travels all the way through the digestive tract and exits as flatulence. This is why eating quickly or drinking carbonated beverages can make you gassier.
The second and more interesting source is your gut microbiome. Trillions of bacteria in your large intestine ferment the carbohydrates your body couldn’t digest higher up in the tract. This fermentation produces hydrogen, carbon dioxide, and in some people, methane. The balance of bacterial species in your colon determines which gases dominate and how much total gas you produce. Two people eating the exact same meal can have very different results depending on the composition of their gut bacteria.
Why Only Some People Produce Methane
Not everyone’s farts contain methane. Estimates suggest that only 30% to 62% of humans harbor the specific microorganisms needed to produce it. These aren’t actually bacteria at all. They’re archaea, a separate domain of life, and the dominant species in the human gut is called Methanobrevibacter smithii. These organisms consume hydrogen produced by other gut microbes and convert it into methane.
If you’re a methane producer, it doesn’t necessarily mean you’re gassier. In fact, methane production can reduce the total volume of gas because the archaea are consuming hydrogen that would otherwise need to exit on its own. Whether or not you carry these organisms appears to be influenced by genetics, early-life exposures, and the overall ecology of your gut.
What Makes It Smell
Since the five major gases are all odorless, the smell comes entirely from sulfur-containing compounds present in tiny concentrations. Hydrogen sulfide is the primary culprit, producing the classic rotten-egg smell. Methanethiol adds a decaying-vegetable quality, and dimethyl sulfide contributes a sweeter, cabbage-like note. Together, these trace gases make up less than 1% of total flatulence volume, yet they’re detectable by the human nose at extraordinarily low concentrations, parts per billion in some cases.
The amount of sulfur gas you produce depends heavily on diet. Foods rich in sulfur-containing amino acids, like eggs, meat, cheese, and cruciferous vegetables such as broccoli, cauliflower, and cabbage, give gut bacteria more sulfur to work with. Garlic and onions are also significant contributors. When bacteria in the colon break down these sulfur compounds, the smelly byproducts get mixed into the rest of the gas. This is why a high-protein or cruciferous-heavy meal tends to produce more pungent flatulence even if the total volume of gas isn’t particularly high.
Why Farts Are Flammable
Both hydrogen and methane are combustible gases, which means flatulence can technically catch fire. This is not an urban legend. The concentration of these gases varies widely between individuals, so not every fart is equally flammable, and people who produce neither significant hydrogen nor methane may find the trick doesn’t work at all. The flammability is a direct consequence of bacterial fermentation: the same process that makes you gassy also produces fuel. For obvious safety reasons, this is worth knowing as a fact of chemistry rather than testing in practice.
How Diet Changes the Mix
What you eat is the single biggest factor you can control when it comes to both the volume and composition of your flatulence. Foods high in fermentable carbohydrates, sometimes grouped under the acronym FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols), provide the most raw material for gut bacteria. Beans, lentils, onions, wheat, and certain fruits are common examples. These foods pass through the small intestine partially or fully undigested and arrive in the colon ready for bacterial fermentation.
Fiber-rich diets generally increase gas production because fiber is the primary fuel for colonic bacteria. This is a normal, healthy process. The gas is a side effect of your microbiome doing exactly what it’s supposed to do: breaking down complex plant material, producing short-chain fatty acids that nourish the cells lining your colon, and maintaining a healthy gut environment. Reducing gas production to zero would mean starving your gut bacteria, which isn’t desirable.
Carbonated drinks add gas more directly by introducing carbon dioxide into the digestive tract. Sugar alcohols found in sugar-free gum and diet foods (like sorbitol and xylitol) are poorly absorbed in the small intestine and ferment readily in the colon, often producing noticeable bloating and flatulence even in small amounts.
What’s Normal and What Isn’t
Passing gas 15 times a day is average, but the range of normal extends from just a few times to around 40. Total daily gas volume of 500 to 2,000 milliliters is typical. If you’ve recently increased your fiber intake, a temporary spike in flatulence is expected and usually settles within a few weeks as your gut bacteria adjust.
A sudden, persistent change in gas volume, odor, or frequency that doesn’t line up with a dietary change can sometimes signal a shift in gut health. Conditions that impair carbohydrate absorption, like lactose intolerance or celiac disease, allow more undigested material to reach the colon, increasing fermentation and gas. Excessive flatulence paired with bloating, pain, diarrhea, or constipation may point to an underlying digestive issue worth investigating.