What chemical does fart produce?: The scientific breakdown of flatulence

September 29, 2025 •

4 min read

The human body produces an average of 14 to 23 farts per day, a natural byproduct of our digestive process. The answer to what chemical does fart produce is not one simple substance, but a complex mix of gases created by the fermentation of undigested food in the large intestine.

Quick Summary

A fart is a complex mixture of gases, primarily nitrogen, hydrogen, carbon dioxide, and methane, with trace amounts of sulfur compounds that cause the characteristic odor. The composition varies significantly from person to person based on diet and gut bacteria. This natural process is a sign of a healthy digestive system at work.

Key Points

Gas composition: Farts are a mixture of odorless gases like nitrogen, hydrogen, carbon dioxide, and methane, with tiny amounts of sulfur compounds responsible for the smell.
Diet plays a role: The specific chemicals and intensity of odor are heavily influenced by your diet, especially sulfur-rich foods and certain carbohydrates that feed gut bacteria.
Healthy gut, more gas: A healthy, diverse gut microbiome that effectively ferments fiber and other undigested foods can produce more gas, which is a sign of good digestive health.
Odor explained: The distinct smell comes from volatile sulfur compounds such as hydrogen sulfide, methanethiol, and dimethyl sulfide, produced by gut bacteria.
Swallowed air contribution: A portion of the gas is simply swallowed air, which is primarily nitrogen and oxygen and has no odor.
Potential red flags: While normal, consistently excessive or odorous gas, especially with other symptoms like pain, can indicate food intolerances or other digestive issues.

The chemistry behind the toot

Intestinal gas, medically known as flatus, is a normal and healthy part of human digestion. While often considered a humorous or embarrassing topic, the chemical composition of a fart is a fascinating window into the workings of our gut microbiome and the food we consume. The main bulk of a fart is typically made up of a few common, and mostly odorless, gases. The more memorable aspect—the smell—comes from a tiny fraction of sulfur-containing compounds.

The primary, odorless components

The vast majority of the gas expelled during flatulence is completely odorless. This portion is a combination of swallowed air and gases produced by gut bacteria during digestion.

Nitrogen and oxygen

These two gases, which make up most of the air we breathe, can be swallowed during eating or drinking, or while talking. While a lot of swallowed air is released through burping, some travels down the digestive tract and is eventually released as part of a fart.

Carbon dioxide

Produced by both bacterial and human metabolism, carbon dioxide is another major component. It is a product of fermentation by gut bacteria and is also generated during metabolic processes within the body itself.

Hydrogen and methane

These two flammable gases are produced exclusively by the anaerobic bacteria and archaea that live within the large intestine. The exact proportion of each is highly dependent on an individual's diet and unique gut flora. Not everyone produces methane, and the prevalence can vary, possibly correlating with factors like diet or even constipation.

The stinky stuff: Sulfur compounds

Though they make up less than 1% of the total volume, volatile sulfur compounds are responsible for the distinctive and sometimes potent odor associated with farts. These are the byproducts of gut bacteria breaking down sulfur-containing amino acids found in certain foods. The more sulfur in the diet, the more pungent the resulting gas.

Hydrogen sulfide ($H_2S$)

This gas is notorious for its rotten-egg smell. It is a very common component of smelly flatulence and is produced when bacteria ferment sulfur-rich foods.

Methanethiol ($CH_3SH$)

Similar in smell to rotting vegetables or garlic, methanethiol is another sulfur compound that contributes to the overall odor. It's produced when bacteria break down the amino acid methionine.

Dimethyl sulfide (($CH_3)_2S$)

Often described as having a sweet, cabbage-like odor, dimethyl sulfide adds another layer of aroma to flatulence.

Comparison of key fart components


Gas	Primary Source	Odor	Flammability	Proportion (Approx.)
Nitrogen	Swallowed air	Odorless	No	20-90%
Hydrogen	Gut bacteria	Odorless	Yes	0-50%
Carbon Dioxide	Gut bacteria/metabolism	Odorless	No	10-30%
Methane	Archaea in gut	Odorless	Yes	0-10%
Hydrogen Sulfide	Gut bacteria (sulfur-rich foods)	Rotten eggs	Trace	<1%
Methanethiol	Gut bacteria (sulfur-rich foods)	Rotting vegetables	Trace	<1%

The role of diet and gut flora

What you eat has a profound impact on the chemical composition of your farts. High-fiber foods, especially certain carbohydrates and vegetables, provide more material for gut bacteria to ferment, leading to higher volumes of gas.

Here is a list of foods known to contribute to gas:

Cruciferous vegetables: Broccoli, cauliflower, cabbage, and Brussels sprouts contain high levels of sulfur compounds.
Legumes: Beans and lentils are high in oligosaccharides, complex sugars that are difficult to digest and ferment in the large intestine.
Dairy products: Lactose intolerance means the body can't break down the sugar in milk, leading to fermentation by gut bacteria.
High-fructose foods: Pears, prunes, and other fruits contain fructose, a sugar that can be poorly absorbed by some people.

In addition to diet, the specific types and quantity of bacteria and archaea in your gut play a huge role. Everyone's microbiome is unique, which is why people have different amounts and smells of flatulence. A thriving, diverse gut microbiome is actually a sign of good digestive health, even if it means producing more gas.

Is excessive gas a sign of a problem?

While farting is normal, excessive or unusually foul-smelling gas can sometimes indicate an underlying issue. This may include food intolerances like lactose intolerance, irritable bowel syndrome (IBS), or even certain infections. Keeping a food diary can help identify trigger foods. However, in most cases, changes in flatulence are simply a reflection of dietary changes and are not a cause for concern. If accompanied by other symptoms like abdominal pain, bloating, or changes in bowel habits, it's a good idea to consult a doctor. For more information on digestion and gut health, visit the National Institute of Diabetes and Digestive and Kidney Diseases at NIDDK.NIH.gov.

Conclusion: Understanding a normal bodily function

In summary, the chemical composition of a fart is mostly odorless gases from swallowed air and bacterial activity, but the small portion of sulfur compounds is what creates the smell. This natural process is a key part of our digestive system. Rather than being a source of embarrassment, understanding the complex chemistry of flatulence offers a deeper appreciation for the intricate processes happening within our bodies.

Frequently Asked Questions

There isn't one primary chemical, but a mixture of gases. The largest components are often nitrogen (from swallowed air), hydrogen, and carbon dioxide, which are all odorless.

The unpleasant odor comes from trace amounts of volatile sulfur compounds, such as hydrogen sulfide, methanethiol, and dimethyl sulfide, which are produced by bacteria breaking down sulfur-rich foods.

Yes, absolutely. Foods high in sulfur, fiber, or certain carbohydrates and sugars (like fructose and lactose) can significantly change the volume and smell of your gas.

Yes, it is very normal. Many farts are odorless because they are composed mostly of nitrogen, carbon dioxide, and other non-smelly gases. The smell only occurs when specific sulfur compounds are present.

While it is a normal bodily function, a sudden, persistent increase in flatulence or a change in odor, especially when accompanied by pain or bloating, could be a sign of a food intolerance or a digestive condition. Consult a doctor if you are concerned.

The frequency and volume of flatulence vary widely among individuals due to differences in diet, gut microbiome composition, and how much air is swallowed during eating and drinking.

Foods that commonly produce smelly gas include cruciferous vegetables (broccoli, cabbage), legumes (beans, lentils), and high-protein foods that contain sulfur-rich amino acids.

Yes, methane is a flammable gas, and since it is a component of flatulence, farts can be flammable, particularly if they have a higher concentration of methane and hydrogen.