A fart is the release of gas that has built up inside your digestive tract. It exits through the rectum, sometimes audibly, sometimes silently, and it happens to every person on Earth at least 14 times a day. The total volume a healthy person expels in 24 hours ranges from about 500 to 1,500 milliliters, with a typical day landing around 700 ml.
Where the Gas Comes From
Intestinal gas has two main sources: air you swallow and gas your gut bacteria produce. Every time you eat, drink, or even just swallow saliva, small amounts of air travel down into your stomach. Most of that swallowed air is nitrogen and oxygen, and it accounts for a large share of the gas in your upper digestive tract. Some of it gets burped back up, but whatever passes into the intestines eventually works its way out the other end.
The second, more interesting source is fermentation. When food reaches your colon without being fully digested, bacteria break it down and release gases as a byproduct. Unabsorbed carbohydrates, fiber, and certain sugars are the primary fuel for this process. The more undigested material that arrives in your colon, the more gas your bacteria produce.
What’s Actually in a Fart
Roughly 99% of a fart is completely odorless. A large meta-analysis of studies on intestinal gas composition found the average breakdown looks like this:
- Nitrogen: about 65%
- Methane: about 14%
- Carbon dioxide: about 10%
- Hydrogen: about 3%
- Oxygen: about 2%
None of those gases smell like anything. The odor comes from trace sulfur compounds that make up less than 1% of the total volume. Hydrogen sulfide is the biggest contributor, responsible for that classic rotten-egg quality. Two other sulfur compounds, methanethiol and dimethyl sulfide, add their own unpleasant notes. Even in tiny concentrations, these substances are potent enough for your nose to detect immediately.
Not Everyone Produces Methane
One quirk of human biology: methane production varies dramatically from person to person. The microbe responsible is a type of archaea called Methanobrevibacter smithii, and it’s found in more than half of adults but is completely absent in others. Studies using breath tests show that the percentage of people who produce detectable methane ranges from 34% to 87% depending on geographic and ethnic population. In people who lack these methane-producing organisms, their gas composition simply shifts toward higher proportions of hydrogen and carbon dioxide instead.
Why Some Foods Make It Worse
Foods that produce the most gas are the ones your small intestine can’t fully absorb, leaving more material for colonic bacteria to ferment. Beans are the famous example, but the list is much longer. Certain sugars are especially efficient at generating gas. Gram for gram, sugar alcohols (like sorbitol, found in sugar-free gum and some fruits) and inulin (a fiber in onions, garlic, and wheat) produce about 2.5 times more fermentation than the same weight of lactose or fructose.
Dairy is a major trigger for the roughly 68% of the world’s population that has reduced ability to digest lactose. When lactose passes undigested into the colon, bacteria ferment it rapidly. Cruciferous vegetables like broccoli, cabbage, and Brussels sprouts contain sulfur-containing compounds, which is why they tend to produce particularly smelly gas rather than just a higher volume of it.
Carbonated drinks add gas more directly. The carbon dioxide dissolved in soda or sparkling water enters your stomach and either comes back up as a burp or continues through your intestines. Eating quickly, chewing gum, and drinking through a straw all increase the amount of air you swallow, adding to the overall gas load.
What Makes a Fart Loud or Silent
Sound is produced by vibration of the anal sphincter as gas passes through. The volume and pitch depend on several factors: the speed of the gas flow, the tightness of the sphincter muscles, and the volume of gas being released at once. A large bolus of gas exiting quickly through a tighter opening produces a louder sound. Smaller amounts passing slowly tend to be silent. Body position matters too, since it changes the angle and pressure dynamics at the exit point.
When Gas Signals Something Else
Flatulence by itself, even frequent flatulence, is normal. But when it comes alongside other persistent symptoms, it can point to a digestive problem worth investigating. The combination of chronic diarrhea, unintentional weight loss, and greasy or unusually foul-smelling stools that float is a classic pattern for malabsorption syndromes, conditions where your intestines aren’t properly absorbing nutrients from food. Celiac disease, lactose intolerance, and pancreatic insufficiency can all present this way.
A sudden, lasting change in your gas patterns, especially paired with bloating, abdominal pain, or changes in stool consistency, is more meaningful than simply passing gas often. If you’re otherwise healthy, eating well, and just gassy, your gut is almost certainly doing exactly what it’s supposed to do.