1. If Most of a Fart Has No Smell, What Is the Smell?
Doctors call it flatus (the gas) and flatulence (passing it). Most of its gas is the same kind of thing found in air or produced by digestion: nitrogen, oxygen, hydrogen, carbon dioxide and methane, and none of these has a smell. The smelly part is a very small fraction. I could not find an exact official percentage, so "nearly all" is the safest way to put it.
A small amount can still be noticed because some gases are both intensely smelly and easy for the human nose to detect. Think of one person shouting in a quiet line of a hundred. This is only a picture, not an explanation of how noses work. The U.S. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) says the odor of flatus "may be due to sulfur" [1]. Which sulfur gas matters most?
2. Which Gas Did Scientists Find Behind the Smell?
In 1998 a team at the Minneapolis Veterans Affairs Medical Center in the United States, led by Fernando Suarez and Michael Levitt, tried to find out. They asked 16 healthy adults to eat pinto beans and a sugar called lactulose, which increases the amount of gas the gut makes. The researchers collected each passage of gas through a thin tube placed in the rectum, measured sulfur gases, and had two judges rate how bad each sample smelled [2].
Three sulfur gases turned up, and one clearly led:
- Hydrogen sulfide: 1.06 micromoles per liter (the smell of rotten eggs)
- Methanethiol (also called methyl mercaptan): 0.21
- Dimethyl sulfide: 0.08
Put side by side, hydrogen sulfide was about 5 times as concentrated as methanethiol and about 13 times as concentrated as dimethyl sulfide. Adding the three averages gives 1.35, and hydrogen sulfide is about 78% of the combined average concentrations of these three measured gases. This last percentage is my own rough arithmetic from the reported averages, not a number the authors give [2].
The key result: the more hydrogen sulfide a sample held, the worse the judges rated it, and the link was statistically very strong (p ≤ 0.001). The authors concluded that sulfur-containing gases are "the major, but not the only" smelly parts of flatus. One caution: the volunteers had eaten beans and lactulose to make more gas, so these numbers are not what an ordinary meal would give [2].
So hydrogen sulfide is the main suspect. Where does it come from?
3. How Do Gut Bacteria Make Hydrogen Sulfide?
The hydrogen sulfide in your large intestine is made by bacteria, not by your own cells. Your large intestine is home to a huge community of bacteria. Food that your stomach and small intestine cannot fully digest, such as some sugars, starches and fiber, reaches the large intestine, and bacteria there break it down and produce gas [1]. The rest of the gas can come from swallowed air.
A 2020 scientific review of sulfur chemistry in gut bacteria describes two routes to hydrogen sulfide. In one, sulfate-reducing bacteria such as Desulfovibrio use sulfate (a sulfur-containing salt found in food and water) in a process that produces hydrogen sulfide. In the other, bacteria including E. coli and Clostridium break down cysteine, an amino acid that contains sulfur and is part of proteins [3].
That review is about what bacteria do, not about how much smell a fart has. It lists sulfate in foods such as bread, sausages and cabbage-family vegetables, but it does not measure how those foods change the odor of gas. So the claim that "eating more meat makes smellier gas" is plausible but is not shown by the sources I could read, and I will not state it as fact. If hydrogen sulfide is the main culprit, could it simply be removed?
4. Does Charcoal Remove the Smell, and What Is Left Unexplained?
The same study tested two ways to fight the odor. Zinc acetate, a chemical that binds hydrogen sulfide and methanethiol, lowered the sulfur gases but did not totally remove the smell. Activated charcoal, a very porous form of carbon that traps gas molecules on its surface, removed "virtually all" of it. In a separate test with eight people, a cushion lined with charcoal soaked up more than 90% of sulfur gases that were released near the body [2]. That was a laboratory device, so it does not show how well products sold in stores work.
This points to a puzzle. If sulfur gases are not the only smelly part, why did charcoal remove almost all of the odor, while zinc acetate, which only targets sulfur, did not? One possibility, and it is only my guess, not something the paper states, is that charcoal also traps other smelly molecules. Nobody can say from this paper alone.
What is known: hydrogen sulfide and other sulfur gases carry most of the smell. What is not known from these sources: what the rest is, and how big its share is.
5. How Can You Explore This Yourself?
With an adult, look at the outside of a deodorizing product at home, such as a shoe or fridge odor absorber. Check whether the label mentions "activated charcoal" and what it says the product is for. Do not open the package or touch what is inside. Reading the label is enough. A label does not prove the product works as well as the charcoal in the experiment.
If you want more, the abstract of the 1998 Gut paper is free online. A good question to take further: how little hydrogen sulfide can a human nose detect? The numbers I found vary a lot between sources, so I left them out. If gas ever causes pain, or your digestion changes suddenly, tell an adult and ask a doctor. NIDDK says that passing gas up to about 25 times a day is considered normal [1].