Farts that don’t smell are the normal state of affairs, not a sign that something is missing. Nearly all of what you pass is gas with no odor at all.1Wikipedia. Flatulence So the real question is why gas ever smells, and the answer is a trace ingredient whose amount shifts from day to day.
What Your Gas Is Actually Made Of
Over 99% of fart volume is odorless gas: oxygen, nitrogen, carbon dioxide, hydrogen and methane.2Wikipedia. Flatulence Hydrogen, carbon dioxide and methane are made in the gut, and together they form about three-quarters of the gas in a typical person.3Wikipedia. Flatulence Nitrogen is different. It comes mainly from air you swallow, not from digestion.4Wikipedia. Flatulence Someone whose gas is mostly swallowed air passes gas with almost nothing to smell.
Swallowed air comes from everyday habits. Eating fast, gulping drinks and talking while you eat all pull in extra air, and chewing gum and fizzy drinks add more. That air passes through mostly as nitrogen and oxygen, neither of which has a smell. Some of it comes back up as a burp, so only part travels on to the intestines. On a day with lots of swallowed air, the gut-made gases, the ones that can carry sulfur traces, make up a smaller share of what you pass.
The average person passes gas around 20 times a day, about a liter in total.5Ask Dr. Universe. Do all animals pass gas? Do cats fart?6Ask Dr. Universe. Do all animals pass gas? Do cats fart? Volume and smell are separate things. A large, loud fart can have no odor, while a small one can smell strong if it carries a little sulfur.
Methane is made only by certain microbes. Some people’s guts carry plenty of them and others carry few or none, so the same meal can leave one person with a lot of methane in their gas and another with almost none. Methane itself has no smell.
The Tiny Ingredient That Causes the Smell
Sulfur compounds are only about 1% of gas, yet they cause the smell.7UnityPoint Health. Passing Gas: Different Smells and What They Mean That’s because some sulfur gases smell strong even in tiny amounts.8GastroDoxs. Sulfur-Smelling Gas: Common Causes Hydrogen sulfide gives the rotten-egg smell and is the most common one.9UnityPoint Health. Passing Gas: Different Smells and What They Mean Methanethiol smells like rotting vegetables or garlic.10UnityPoint Health. Passing Gas: Different Smells and What They Mean Dimethyl sulfide smells like cabbage, with a sweeter note.11UnityPoint Health. Passing Gas: Different Smells and What They Mean
Human noses pick up sulfur gases at extremely low concentrations, far below what any instrument would call a meaningful share of the gas. This sensitivity likely helps us avoid spoiled food, which is why these smells register as so unpleasant. A small shift in sulfur production, from a single meal for example, can move a fart from unnoticeable to obvious.
Hydrogen sulfide is the most plentiful sulfur gas, about five times more than methanethiol.12PubMed Central. Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour The stronger the hydrogen sulfide level, the worse people rate the smell.13PubMed Central. Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour
Most smelly farts blend all three, so the smell is rarely one pure note.
How Your Gut Bacteria Decide the Smell
Gut bacteria break down food and release hydrogen as a byproduct.14H2 HUBB. Fiber, Gut Bacteria & Molecular Hydrogen: How Your Body Makes Hâ‚‚ Two groups then compete for that hydrogen: methane-making microbes, and bacteria that turn sulfur into hydrogen sulfide.15PubMed Central. Methanogens outcompete sulphate reducing bacteria for H2 in the human colon In lab mixtures, the methane makers grabbed hydrogen faster and cut hydrogen sulfide production about sixfold.16PubMed Central. Methanogens outcompete sulphate reducing bacteria for H2 in the human colon Another lab study, though, found sulfide levels similar whether methane makers were present or not.17Gut / BMJ Publishing Group. Competition for hydrogen by human faecal bacteria: evidence for the predominance of methane producing bacteria So having methane makers may help explain low-smell gas, but it isn’t settled.
Your own bacteria mix matters too. Each person carries their own set of gut microbes, built up over years rather than fixed at birth. Long-term eating patterns, past courses of antibiotics and other medicines, and life history all shape which microbes thrive.
You may also not be the most reliable judge of your own gas. People get used to their own smells over time, so yours may seem milder to you than it would to someone else. The simplest check is to ask someone you trust whether they ever notice it, rather than concluding it has none.
Foods That Make Gas Smell More or Less
Beans, vegetables, fruits, grains and, for some people, dairy cause gas.18Journal of Functional Foods. Intestinal gas production by the gut microbiota: A review Their indigestible carbohydrates are fermented by bacteria in the colon.19Journal of Functional Foods. Intestinal gas production by the gut microbiota: A review High-fiber diets raise gas volume, hydrogen and carbon dioxide.20PubMed. Excretion of breath and flatus gases by humans consuming high-fiber diets So a plant-heavy diet can bring lots of gas that smells little.
If your gas became milder after weeks of eating more plants and less meat or eggs, the diet change is a likely explanation, since less sulfur-carrying protein reaches your gut bacteria. Animal proteins are rich in sulfur-carrying building blocks that bacteria can turn into hydrogen sulfide.21Specialists in Gastroenterology. Low Sulfur Diet A heavy steak or egg meal can bring the rotten-egg smell back for a day or so.
Beans have a reputation for smelly gas, but their main effect is adding volume, because their carbohydrates feed gas-making bacteria. How strong that gas smells depends more on how much sulfur the whole meal supplies. Beans served with meat or eggs combine the volume from the fiber with the sulfur from the protein, and that is when the worst combination appears.
Sugar-free sweeteners like maltitol can cause gas at around 40 g in one sitting.22PubMed Central. Gastrointestinal Disturbances Associated with the Consumption of Sugar Alcohols with Special Consideration of Xylitol: Scientific Review and Instructions for Dentists and Other Health-Care Professionals Slower digestion gives bacteria more time to make stronger odors,23UnityPoint Health. Passing Gas: Different Smells and What They Mean so fiber and water, which speed things along, may reduce odor.24UnityPoint Health. Passing Gas: Different Smells and What They Mean
To see your own pattern, jot down meals and how your gas seemed for a few days, then look for the meals that came before the smelliest releases. One day of notes proves little, so look for the same link appearing more than once.
Why Some Farts Smell and Others Don’t
Much gut gas is absorbed into the blood or eaten by other microbes, and only a modest share leaves as farts.25Ovid. Intestinal gas homeostasis: disposal pathways What reaches the exit is a changing leftover. In one study, women’s gas had more hydrogen sulfide while men passed more volume.26New York Post. Women’s farts smell worse than men’s — and there’s science to back it up Changes in air pressure, as at high altitude, increase gas without making it smell more.27Wikipedia. High-altitude flatus expulsion – Wikipedia
Timing plays a part. Gas soon after a meal is often swallowed air pushing through and tends to be odorless, while gas from food that has reached the colon and been fermented is more likely to carry sulfur and smell. These are general tendencies, not rules, because meal size, content and gut speed all shift the timing.
The same variation explains why your gas differs from a partner’s. Different people have different bacteria, diets and digestion speeds, and all of these affect how much sulfur gas forms.
Can You Make Gas Smell Less
In tests, activated charcoal removed nearly all fart odor.28Gut / BMJ Publishing Group. Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour Zinc acetate cut sulfur gas but didn’t remove all the odor.29Gut / BMJ Publishing Group. Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour Cutting sulfur-rich foods for a while reduces rotten-egg gas.30Specialists in Gastroenterology. Low Sulfur Diet So if you want milder gas, try smaller portions of meat and eggs, with more fluids and fiber.
Charcoal pads and charcoal-lined underwear work by trapping odor-carrying gas as it leaves your body. They act at the exit, not in the gut, so they do nothing to what your bacteria produce or how much gas you make. They suit specific situations like a meeting or a long trip, rather than being a way to change your digestion.
If you add fiber, raise it gradually over a couple of weeks so your gut bacteria can adjust. Expect more gas volume at first, which usually settles as your gut adapts. Drink water alongside it, because water keeps things moving and lets fiber do its job.
When Gas Smell Is Worth a Doctor’s Visit
No smell on its own is not a recognized warning sign. Consistently foul gas with bloating or cramps is different, and can signal poor digestion or a bacteria imbalance.31Chicago Colonoscopy Screening. When to Worry About Flatulence: A Guide to Understanding Your Digestive Signals Bacterial overgrowth in the small intestine can raise rotten-egg gas.32Specialists in Gastroenterology. Low Sulfur Diet See a doctor if a strong smell persists along with pain, diarrhea or weight loss.
A doctor will usually ask how long the change has lasted, since a recent shift means more than a lifelong pattern. They will ask about new medicines, supplements or diet changes that might explain it, and about other symptoms such as bloating, greasy stools or weight loss. Depending on the answers, they may test for food intolerance or small-intestine bacterial overgrowth.
A sudden loss of smell overall is a different matter. If nothing smells, including food, the issue is your nose. Colds and infections can dull smell, and that is separate from what your gas contains.