A fart is your body’s way of releasing gas that builds up in your digestive tract. That gas comes from two main sources: air you swallow and gas produced by bacteria in your large intestine as they break down food. The average healthy adult passes gas between 14 and 23 times a day, often without even noticing.
Where the Gas Comes From
Every time you eat, drink, or swallow saliva, small amounts of air travel down into your stomach and intestines. This swallowed air is mostly nitrogen and oxygen, and it accounts for a significant portion of the gas in your gut. Eating quickly, chewing gum, drinking through a straw, or talking while eating all increase the amount of air you take in.
The second, more interesting source is your gut bacteria. Your large intestine is home to trillions of microbes, and they earn their keep by breaking down food your small intestine couldn’t fully absorb. This process, called fermentation, is the same basic chemistry that turns grapes into wine or cabbage into sauerkraut. The byproducts are gases: carbon dioxide, hydrogen, and in some people, methane.
Certain foods are especially productive because they contain short-chain carbohydrates (often called FODMAPs) that your small intestine simply can’t break down. These sugar chains pass through to the large intestine intact, where bacteria feast on them and convert them into gas. Beans, onions, garlic, wheat, and many fruits fall into this category. Your small intestine even draws in extra water to help push these undigested carbohydrates along, which is why gas-producing foods can also cause bloating.
What’s Actually in a Fart
Five odorless gases make up about 99% of every fart: nitrogen, oxygen, carbon dioxide, hydrogen, and methane. The proportions vary wildly from person to person. Nitrogen can range from 11% to 92% of a single fart, hydrogen from 0% to 86%, and carbon dioxide from 3% to 54%. This variation depends on how much air you’ve swallowed, what you’ve eaten, and which bacteria live in your gut. Not everyone even produces methane. Only about a third of people harbor the specific microbes responsible for it.
That remaining 1% is what makes things unpleasant. The signature rotten-egg smell comes primarily from hydrogen sulfide, a colorless, pungent gas produced when gut bacteria break down sulfur-containing amino acids found in protein-rich foods. Bacteria in genera like Desulfovibrio also generate hydrogen sulfide by processing sulfate, a compound naturally present in many foods and drinking water. This is why high-protein meals, eggs, cruciferous vegetables like broccoli and cauliflower, and sulfur-rich foods like garlic tend to produce the most pungent results.
How Your Body Decides to Release It
Gas doesn’t just leak out randomly. Your body has a surprisingly sophisticated system for managing what leaves and when. Two rings of muscle, the internal and external anal sphincters, guard the exit. The internal sphincter works automatically, while the external one is under your conscious control.
As gas accumulates in the large intestine, it’s pushed toward the rectum by the normal wave-like contractions that move everything through your gut. When enough gas stretches the rectal wall, it triggers something called the rectoanal inhibitory reflex. This reflex briefly relaxes the internal sphincter, allowing a tiny sample of rectal contents to contact specialized sensory nerve endings in the upper anal canal. This “sampling” process is how your body distinguishes between gas, liquid, and solid. It’s the reason you can (usually) confidently release gas without worrying about anything else coming with it.
If the sensors confirm it’s just gas and the social situation permits, you relax the external sphincter and let it pass. If the timing is wrong, you can consciously squeeze the external sphincter to hold it in. The gas doesn’t disappear, though. It either gets reabsorbed into the bloodstream and eventually exhaled through your lungs, or it waits for a less conspicuous moment.
What Makes the Sound
The sound of a fart works on the same principle as a brass instrument. As gas passes through the narrow opening of the anal canal, it causes the surrounding skin to vibrate, much like a trumpet player’s lips buzz against the mouthpiece. The pitch and volume depend on several factors: how fast the gas is moving, how much gas there is, and how tightly the sphincter muscles are contracted.
A large volume of gas released through a relaxed opening tends to produce a lower, longer sound. A small burst squeezed through a tighter opening creates a higher pitch. Larger people generally produce lower-frequency sounds because their anal tissue is thicker and vibrates at a lower resonant frequency. Body position matters too. Sitting compresses the buttocks against each other, changing the vibration dynamics compared to standing.
Why Some Days Are Worse Than Others
Your gas output on any given day is shaped almost entirely by what you’ve eaten in the previous 6 to 24 hours, since that’s how long food takes to reach the large intestine where bacterial fermentation happens. A meal heavy in beans, whole grains, dairy (if you’re lactose intolerant), or cruciferous vegetables will produce noticeably more gas than a simple meal of rice and chicken.
Carbonated drinks add gas directly to your stomach, though most of that carbon dioxide gets absorbed or burped out before it reaches the other end. Artificial sweeteners like sorbitol and xylitol, common in sugar-free gum and candy, are notorious gas producers because they’re poorly absorbed in the small intestine and heavily fermented by colon bacteria.
Stress and anxiety can also increase gas symptoms, not necessarily by producing more gas, but by speeding up gut motility and making you more aware of normal sensations. Antibiotics can temporarily change your gut bacteria population, altering both the volume and smell of gas until your microbiome recovers. Even the simple act of increasing fiber intake, while healthy in the long run, produces a temporary spike in gas as your gut bacteria adjust to the new food supply. This usually settles down within a few weeks as your microbiome adapts.