A fart is a pocket of gas that builds up in your digestive tract and gets pushed out through your anus by pressure from your abdominal muscles and the relaxation of two ring-shaped muscles called sphincters. The average person does this about 8 to 14 times a day, though up to 25 times is still considered normal. The process involves swallowed air, bacterial fermentation, and a surprisingly elegant system of muscles that lets your body decide when and how to release that gas.
Where the Gas Comes From
Intestinal gas has two main sources: swallowed air and bacterial fermentation. Every time you eat, drink, or swallow saliva, small amounts of air travel down into your stomach and intestines. Most of this air is nitrogen and oxygen, the same gases in the atmosphere around you. Some of it gets burped back up, but the rest continues through your digestive tract.
The second, more interesting source is the trillions of bacteria living in your large intestine. When food, particularly carbohydrates, reaches your colon without being fully absorbed, bacteria break it down through fermentation. This process generates hydrogen, carbon dioxide, and in about one-third of people, methane. The specific mix depends on your gut bacteria, which is as unique as a fingerprint. Two people eating the exact same meal can produce very different gas profiles.
Five gases make up more than 99 percent of what comes out: nitrogen, oxygen, carbon dioxide, hydrogen, and methane. Nitrogen dominates the mix, largely from swallowed air. None of these five gases have any smell at all, which is why most farts are actually odorless.
Why Some Farts Smell Terrible
The smell comes from sulfur compounds that make up roughly 1 percent of your flatulence. That tiny fraction punches well above its weight because the human nose is extraordinarily sensitive to sulfur. Three compounds do most of the damage:
- Hydrogen sulfide produces the classic rotten egg smell and is the most common culprit.
- Methanethiol smells like rotting vegetables or garlic.
- Dimethyl sulfide adds a cabbage-like sweetness to the overall odor.
What you eat directly controls how much sulfur your gut bacteria have to work with. High-sulfur foods include eggs, meat, poultry, garlic, onions, broccoli, cabbage, Brussels sprouts, and cauliflower. A steak dinner with a side of broccoli is essentially handing your gut bacteria a sulfur buffet. Meals built around rice, bread, or fruit tend to produce gas with far less odor.
How Your Body Holds and Releases Gas
Your anus has two concentric rings of muscle that work together like a gate system. The internal sphincter is involuntary. It contracts in a rhythmic wave pattern, cycling 20 to 40 times per minute, and is responsible for keeping your anus closed at rest without any conscious effort on your part. The external sphincter is the one you control voluntarily, using a combination of slow-twitch fibers (for sustained holding) and fast-twitch fibers (for those urgent clench moments).
When gas accumulates in your rectum and creates pressure, it triggers a reflex that briefly relaxes the internal sphincter. This serves a clever purpose: it allows a tiny sample of rectal contents to reach the sensitive nerve endings in your upper anal canal, so your body can distinguish between gas, liquid, and solid. This “sampling reflex” is how you know whether it’s safe to let one go or whether you need to find a bathroom.
If you decide to release the gas, you relax your external sphincter and pelvic floor muscles. Your abdominal muscles tense up and funnel pressure downward toward the pelvis. The gas escapes through the now-open canal. If you decide to hold it, you squeeze the external sphincter, and the gas retreats back into the rectum to wait for a better moment. It doesn’t disappear. Some of it may get reabsorbed into the bloodstream and eventually exhaled through your lungs, but most of it simply waits.
What Creates the Sound
The sound of a fart works on the same principle as a brass instrument. When gas passes through the anal opening, it causes the skin around the anus to vibrate, much like lips buzzing against a trumpet mouthpiece. The pitch and volume depend on several physical factors: how fast the gas is moving, how tight the sphincter is at the moment of release, and the size of the opening.
A tighter sphincter with a small opening and fast-moving gas produces a higher-pitched, louder sound. A relaxed sphincter with a wider opening lets gas pass more quietly. This is also why body size matters. Larger animals tend to produce lower-frequency sounds because the vibrating tissue is thicker and larger, giving it a naturally lower resonant frequency. Silent farts typically happen when the sphincter is relaxed enough that gas escapes slowly without causing vibration.
Foods That Produce More Gas
Certain carbohydrates are poorly absorbed in the small intestine, which means they arrive in the colon intact and ready for bacteria to ferment. Beans are the famous example, but the list is long: lentils, whole grains, dairy (if you’re lactose intolerant), onions, apples, pears, and most cruciferous vegetables like broccoli and cabbage. Carbonated drinks add gas directly by introducing carbon dioxide into your stomach.
Fructose, the sugar found in fruit and many sweetened drinks, causes extra gas in people who have difficulty absorbing it. Lactose in dairy does the same for the roughly 65 percent of adults worldwide who lose the ability to fully digest it after childhood. In both cases, the undigested sugar reaches the colon and becomes fuel for gas-producing bacteria.
Fiber-rich foods tend to increase gas production as well, which creates an awkward tradeoff: the foods that are best for your gut health are often the same ones that make you gassier. Your gut bacteria do adapt over time, though. Gradually increasing fiber intake over a few weeks typically causes less gas than a sudden dietary change.
When Gas Signals Something Else
Passing gas up to 25 times a day falls within the normal range. Some people sit closer to a handful of times daily, while others hit 40, and the average lands around 15. The number alone isn’t a reliable indicator of a problem.
What matters more is a significant change from your baseline, especially when it comes with other symptoms. Persistent bloating, abdominal pain, diarrhea, or unintentional weight loss alongside increased gas can point to conditions like lactose intolerance, fructose malabsorption, or small intestinal bacterial overgrowth, a condition where bacteria that normally live in the large intestine colonize the small intestine and start fermenting food too early in the digestive process. That premature fermentation creates extra gas and can interfere with nutrient absorption.
For most people, though, gas is simply the byproduct of a healthy digestive system doing its job. A gut with no gas production would be a gut with no bacterial activity, and that would be far worse for your health than a few awkward moments a day.