When you fart, your body expels a small pocket of gas that has built up in your large intestine. The gas passes through two ring-shaped muscles at the end of your digestive tract, vibrates the surrounding skin, and exits your body. Most people do this up to 25 times a day, and the whole process involves a surprisingly complex chain of events.
Where the Gas Comes From
Gas accumulates in your digestive system from 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. That accounts for most of the nitrogen and oxygen in your gas.
The second source is your gut bacteria. When food reaches your large intestine, trillions of microbes go to work breaking down whatever your body didn’t absorb higher up. Certain carbohydrates are especially difficult for your small intestine to digest, so they arrive in the colon mostly intact. Gut bacteria ferment these leftovers and produce hydrogen, carbon dioxide, and sometimes methane as byproducts. The more undigested material that reaches your colon, the more gas your bacteria produce.
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 episode, hydrogen from 0% to 86%, and carbon dioxide from 3% to 54%. Some people produce virtually no methane at all, while in others it accounts for more than half the volume. This variation depends on your diet, your unique gut bacteria, and how much air you’ve swallowed.
The remaining 1% is what you actually smell. Trace amounts of sulfur-containing compounds, particularly hydrogen sulfide (the chemical behind the scent of rotten eggs), give farts their distinctive odor. Because your nose is extremely sensitive to hydrogen sulfide, even tiny concentrations are detectable. Foods rich in sulfur, like eggs, cruciferous vegetables, and meat, tend to produce smellier gas.
How Your Body Releases It
Your rectum has two concentric muscles that control what leaves your body. The internal anal sphincter operates automatically, relaxing slightly when gas or stool arrives so your body can “sample” what’s there. Your brain receives a signal distinguishing between gas and solid matter. If the timing and setting are right, you voluntarily relax the external anal sphincter and your pelvic floor muscles, and a slight increase in abdominal pressure pushes the gas out.
If you’re in a meeting or on a first date, your external sphincter stays clenched. You consciously override the signal, and the gas stays put until you choose to let it go.
Why Farts Make Noise
The sound comes from vibration. As pressurized gas passes through the tight opening of the anal sphincter, it causes the surrounding skin to vibrate, much like how buzzing your lips into a trumpet mouthpiece creates sound. The pitch and volume depend on several factors: how fast the gas is moving, how tightly the sphincter is contracted, and the physical size of the opening. A large volume of gas escaping quickly through a tense sphincter produces a louder, higher-pitched sound. A slow, relaxed release is quieter or silent. Larger bodies tend to produce lower-pitched sounds because the vibrating tissue is thicker.
What Happens If You Hold It In
Holding in a fart doesn’t cause any real harm, but it does take a detour. Gas that isn’t released through the usual exit gets reabsorbed into your bloodstream through the intestinal wall. From there, it travels to your lungs and is quietly exhaled with your breath. In the meantime, the trapped gas can stretch your intestinal walls, causing bloating, discomfort, or nausea. The pressure eventually resolves one way or another, whether you release the gas later, absorb it, or breathe it out without ever knowing.
Foods That Increase Gas Production
Certain carbohydrates are poorly absorbed in the small intestine and arrive in the colon ready to be fermented. Nutritional science groups these under the term FODMAPs, a category of short-chain sugars found in a wide range of everyday foods. The major culprits include:
- Fructans and GOS: found in wheat, rye, onions, garlic, and legumes like beans and lentils
- Lactose: found in milk, soft cheeses, and yogurt
- Excess fructose: found in honey, apples, and high-fructose corn syrup
- Sugar alcohols (sorbitol and mannitol): found in some fruits, vegetables, and sugar-free gum or candy
When gut bacteria ferment these sugars, gas is the direct byproduct. This is why a bowl of bean chili or a large glass of milk can leave you noticeably gassier a few hours later. People vary in how well they absorb each type of carbohydrate, which is why the same meal can affect two people very differently.
When Excess Gas Signals Something Else
Farting 25 times a day falls within the normal range, and even exceeding that number isn’t automatically a concern. But a significant, sustained increase in gas, especially paired with other symptoms, can point to an underlying issue. Celiac disease, for instance, causes the small intestine to react to gluten and produces symptoms like diarrhea, weight loss, bloating, and increased gas. Irritable bowel syndrome, particularly the diarrhea-predominant type, is another common cause of chronic gassiness.
Symptoms worth paying attention to include unexplained weight loss, fever, blood in your stool, unusually severe pain, or a sudden change in your digestive patterns that doesn’t resolve. Worsening bloating and gas can also signal pelvic floor dysfunction, where the muscles responsible for coordinating bowel movements stop working together properly. For most people, though, gas is simply evidence that your gut bacteria are doing their job.