Farts smell because of a rounding error. Over 99% of a fart by volume is odorless gas. The entire reputation rests on the remaining trace, which is mostly volatile sulfur compounds, and your nose is extraordinarily good at those. Less than 1% of the product is responsible for 100% of the complaints.
The short answer
Gut bacteria ferment what you could not digest and make gas. Almost all of it has no smell. The part that does is a trace of sulfur compounds: hydrogen sulfide (rotten eggs), methyl mercaptan (decomposing vegetables) and dimethyl sulfide. Eat more sulfur rich protein and the trace gets louder.
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Stenchi Gas Index
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The Stenchi Gas Index is a joke scale and the point values are ours. The foods come from Wikipedia's Flatulence article. Real digestion varies by person, gut bacteria and a lot more than dinner.
What is in a fart
Per Wikipedia, intestinal gas is swallowed air, gas that is already in food and drink, or the product of gut fermentation. Normal subjects make most of it, 74%, from hydrogen, carbon dioxide and methane. Here is the full manifest, split by what the nose can do about it.
No smell at all
Over 99% of the volume| Compound | Smell | Where it comes from |
|---|---|---|
| Nitrogen | None | Not made in the gut. It is swallowed air. |
| Oxygen | None | Also swallowed air, along with the nitrogen. |
| Carbon dioxide | None | Made in the gut by bacteria fermenting leftovers. |
| Hydrogen | None. Flammable. | Made in the gut by fermentation. |
| Methane | None. Flammable. | Made by gut microbes, but not in everyone: in one study only five of nine adults had methane producing archaea. |
The part with opinions
Under 1% of the volume| Compound | Smell | Where it comes from |
|---|---|---|
| Hydrogen sulfide | Rotten eggs | Possibly the most abundant sulfur compound in flatus. Tracked best with bad odor in one study. |
| Methyl mercaptan (methanethiol) | Decomposing vegetables | The biggest contributor in other reports, from people not on a special diet. Also in the durian. |
| Dimethyl sulfide | Unpleasantly sweet, like wild radish | Third in that study's odor ranking. |
| Dimethyl disulfide and trisulfide | Garlic and Limburger cheese | Present in flatus. The same two are in the corpse flower. |
| Indole and skatole | Mothballs | Blamed for decades. Wikipedia says they probably do not contribute greatly. |
The three that matter are all present above their smell perception thresholds. Wikipedia describes hydrogen sulfide, methyl mercaptan and dimethyl sulfide as rotten eggs, decomposing vegetables and unpleasantly sweet, wild radish. For scale, our Smell Index entry for hydrogen sulfide says half of a test panel can detect it at 0.47 parts per billion. A trace the body makes anyway, on a detector that good, produces the situation you are here about.
Why yours are worse than somebody else's
Volume and smell are separate dials. Volume follows fermentable carbohydrate, the beans, onions, dairy and starches in the toy above. Smell follows sulfur. Wikipedia says diets high in protein, especially sulfur containing amino acids, have been demonstrated to significantly increase the odor, and it lists the Brassica vegetables as commonly reputed to add pungency.
The rest is your gut. The mix of microbes in the colon differs a lot between people, and it is normally very resistant to change. Some people are simply set up to make more gas. That is not a failing, it is a lease you did not read.
When it is not a joke
Wikipedia says that when gas is excessive or foul it can be a sign of a health disorder, including irritable bowel syndrome, celiac disease, non-celiac gluten sensitivity and lactose intolerance. It can also come from some medicines, including ibuprofen, laxatives, antifungals and statins, and from infections such as giardiasis.
So: a new change that sticks around, or gas that comes with other symptoms, is a question for a doctor or a pharmacist. It is not a question for a satirical deodorant company, and our shampoo cannot treat any of it.
How many is normal
- 8 to 20 a day. That is Wikipedia's normal range for episodes. It is a wide range, so nobody is the average.
- 476 to 1,491 mL a day. The normal total volume, which depends a lot on diet. Each episode is 5 to 375 mL.
- The first one of the day is bigger. The volume of the first flatulence after waking is significantly larger than the rest. Suggested reasons include gas building up during sleep and a burst of gut activity in the first hours of the day.
- Your own are easier to live with. People are unfazed by, and may enjoy, the smell of their own, and not of anyone else's. The speculation is familiarity. It has not been studied much, which tells you what scientists will and will not do for a grant.
The long record, with sources
Five real entries, oldest first. Anything in quotation marks is as given on the Wikipedia pages linked below.
“People who produce at will without any stench such rhythmical sounds from their fundament that they appear to be making music even from that quarter.”
Wikipedia has this as early as the historical record goes, and the important phrase is “without any stench.” That was the compliment. The standard of the field has not changed in about fifteen centuries.
Blew out a candle from several yards away, for an audience that included the Prince of Wales, King Leopold II of the Belgians and Sigmund Freud.
Wikipedia says it was a common misconception that he passed intestinal gas on stage. He was allegedly able to move air into his rectum and control its release, which is why it was a theater act and not a hazard.
“For inventing Under-Ease, airtight underwear with a replaceable charcoal filter that removes bad-smelling gases before they escape.”
The prize rationale, as listed on Wikipedia. He received the patent in 1998. Stenchi's whole catalog points the other way, and nobody has given us a prize. On the science: Wikipedia says the evidence for swallowed activated charcoal is mixed.
A proposed agricultural emissions research levy “promptly became known as a ‘fart tax’ or ‘flatulence tax.’”
It targeted agricultural emissions. Per Wikipedia, 90 to 95% of the methane livestock release comes from exhaling and burping, so the nickname was funnier than it was accurate.
A farting app earned nearly $10,000 in one day.
A better day than any Stenchi product has had, since every Stenchi product is sold out and none of them can be bought.
You and a durian have a molecule in common
Methyl mercaptan, the methanethiol on the table above, is also one of the two thiols the durian releases. Thiols are also what skunk spray is made of, and a related sulfur compound, ethanethiol, is added to gas on purpose so you can smell a leak. Your digestive system is, in a small way, a sulfur plant. Nothing in our catalog has been this honest.
Questions people ask in the elevator
Why do farts smell?
Because of a trace. Over 99% of the volume of a fart is odorless gas: oxygen, nitrogen, carbon dioxide, hydrogen and methane. The remaining under 1% includes volatile sulfur compounds, mainly hydrogen sulfide, methyl mercaptan and dimethyl sulfide, which are present above the thresholds at which a nose can perceive them.
Why do my farts smell so bad?
Mostly what you ate and what lives in your gut. Wikipedia says diets high in protein, especially sulfur containing amino acids, significantly increase the odor, and the cruciferous vegetables (broccoli, cabbage, cauliflower, Brussels sprouts) are commonly reputed to increase it too. Gut bacteria differ a lot between people. Wikipedia also notes that when gas is excessive or foul it can be a sign of a health disorder such as irritable bowel syndrome, celiac disease, non-celiac gluten sensitivity or lactose intolerance, and some medicines (ibuprofen, laxatives, antifungals, statins) are linked to it. This is a humor page. If the change is new and it lasts, ask a doctor or a pharmacist.
What is in a fart?
Mostly gases with no smell. Nitrogen and oxygen come from swallowed air. Hydrogen, carbon dioxide and, in some people, methane are made by bacteria fermenting what you could not digest. Hydrogen, carbon dioxide and methane together make up about 74% of the volume in normal subjects. The smell comes from the trace compounds, mainly sulfur ones.
What gas makes farts smell like rotten eggs?
Hydrogen sulfide. Wikipedia says that in one study its concentration correlated convincingly with how bad the odor was rated, followed by methyl mercaptan and dimethyl sulfide. Other researchers found methyl mercaptan the biggest contributor in people not on a special diet. Wikipedia describes the three as smelling like rotten eggs, decomposing vegetables and something unpleasantly sweet, like wild radish.
How many times a day is it normal to fart?
Wikipedia gives the normal range as 8 to 20 episodes a day, with a normal total volume of 476 to 1,491 mL per 24 hours and 5 to 375 mL per episode. It varies hugely from person to person, and diet is the main factor.
Why do I not mind the smell of my own farts?
Nobody knows for certain. Wikipedia says people find other people's flatus unpleasant and are unfazed by, and may even enjoy, their own, and that one speculative explanation is familiarity: survival in nature may depend on detecting and reacting to foreign smells. There has been little research on it.
Are silent farts worse?
There is no good evidence either way that we could find. Per Wikipedia, the sound depends on the volume of gas, the tension of the sphincter and the speed of the gas, while the odor comes from trace sulfur compounds. Those are different variables, so silence and smell are not the same thing, and the saying is folklore, not chemistry.
Can a fart catch fire?
Hydrogen and methane are flammable, and Wikipedia says flatus can be ignited if it contains adequate amounts of them. That is a fact about gas, not a suggestion. Fire and the human body are a bad combination, so do not test it.
Do cow farts cause global warming?
Less than the joke suggests. Wikipedia says the idea that livestock methane is in their flatus is an erroneous belief. Livestock account for around 20% of global methane emissions, and 90 to 95% of that is released by exhaling or burping, not farting.
More stinks like this
Farts are not on the Stenchi Smell Index, because ranking one would mean ranking our readers. These are the sulfur flavored entries.
Sources
- Wikipedia: Flatulence
- Wikipedia: Le Pétomane
- Wikipedia: List of Ig Nobel Prize winners (2001)
- Wikipedia: Hydrogen sulfide
The facts, numbers and quotations come from Wikipedia's Flatulence article and the pages above, which cite the studies behind them. The jokes, the Dinner Forecast and its Gas Index, and the Stenchi products are ours. This is a humor page, not medical advice. Do not diagnose anything from it. Stenchi products are fictional and cannot be bought.
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