Why Do I Smell Like a Fart? The Scientific Reasons

The sulfurous, rotten-egg smell we associate with flatulence comes from a tiny family of sulfur-containing gases, and those same gases can escape your body through routes that have nothing to do with passing gas. Hydrogen sulfide is the main culprit behind fart odor, but it also shows up in breath, sweat, and even the volatile compounds your skin bacteria produce. So if you smell like a fart but cannot trace it to the obvious source, the explanation usually lives somewhere along a chain that runs from your gut bacteria to your liver, your mouth, your skin, or your wardrobe.

What Actually Makes That Smell

More than 99 percent of the gas your intestines produce is made up of hydrogen, carbon dioxide, and methane, all of which are odorless. The stink comes from less than one percent of the total volume, a sliver dominated by sulfur-containing compounds. A classic study that had volunteers collect flatus in gas-tight bags and then ran the samples through gas chromatography found that hydrogen sulfide was the dominant odor compound, present at roughly five times the concentration of methanethiol (which smells like rotting cabbage) and more than ten times the concentration of dimethyl sulfide. Malodor correlated strongly with hydrogen sulfide concentration specifically, not with the total volume of gas.

1PubMed Central. Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour

These sulfur gases are produced when anaerobic bacteria in your colon break down sulfur-containing amino acids from protein and reduce sulfate from food and water. One particularly prolific group, bacteria in the genus Desulfovibrio, generates hydrogen sulfide by converting sulfate, a compound found naturally in many foods and in drinking water.

2PubMed Central. What Stinks? The Role of Hydrogen Sulfide in the Gut

This matters because hydrogen sulfide is not confined to your intestines. It is a small, volatile molecule that crosses biological membranes easily. It can diffuse into your bloodstream, travel to your lungs, and leave through your breath. It can seep through your skin. And the same bacterial chemistry that produces it in your gut can occur in your mouth, on your skin, and in pockets of your tonsils. That is why the “fart smell” can seem to follow you even when you are not actually passing gas.

Your Diet and Gut Bacteria

The simplest explanation for smelling sulfurous is that your gut is producing more hydrogen sulfide than usual. High-protein diets are a common trigger because sulfur-containing amino acids like cysteine and methionine, abundant in eggs, meat, dairy, and cruciferous vegetables like broccoli and cauliflower, are the raw material gut bacteria ferment into hydrogen sulfide. Beer, wine, and dried fruits preserved with sulfites also deliver sulfate directly to your colon, feeding the sulfate-reducing bacteria that churn out the gas.

2PubMed Central. What Stinks? The Role of Hydrogen Sulfide in the Gut

Meanwhile, unabsorbed carbohydrates that reach the colon get fermented by other bacteria, producing the bulk gases (hydrogen, carbon dioxide, methane) that increase the total volume of flatulence and can carry the trace sulfur gases out with them more frequently. Beans and legumes are famous for this, but so are onions, garlic, and high-fiber foods when you increase your intake quickly. The more gas your gut produces overall, the more often the sulfurous trace compounds get expelled, and the more likely you are to notice the smell on your clothes, your skin, or lingering around you.

3ScienceDirect. Intestinal gas production by the gut microbiota: A review

When the Problem Is Bacterial Overgrowth or Malabsorption

Sometimes the issue is not just what you eat but how your gut handles it. Small intestinal bacterial overgrowth, or SIBO, occurs when bacteria that normally live in the colon migrate upward into the small intestine, where they start fermenting food before it can be properly absorbed. Research on people with diarrhea-predominant irritable bowel syndrome found that hydrogen sulfide levels in exhaled breath were distinctly altered in those who tested positive for SIBO, driven by the activity of sulfate-reducing bacteria colonizing the wrong part of the digestive tract. Breath hydrogen sulfide could even track the progression and resolution of the overgrowth.

4Journal of Breath Research. Hydrogen sulphide in exhaled breath: a potential biomarker for small intestinal bacterial overgrowth in IBS

Malabsorption is a related issue. When conditions affecting the small intestine, pancreas, liver, or biliary tract prevent nutrients from being properly digested or absorbed, undigested material reaches the colon in larger quantities, giving bacteria more substrate to ferment. The result is often bloating, increased gas production, and a more persistent sulfurous odor that can permeate breath and body odor alike. Lactose intolerance is one well-known example, but many other conditions, from celiac disease to pancreatic insufficiency, can cause malabsorption with the same downstream effects on gas production.

5PubMed Central. Small and Large Intestine (I): Malabsorption of Nutrients

Your Mouth Can Smell Like Your Gut

Hydrogen sulfide and methanethiol are not exclusive to the colon. The same types of anaerobic bacteria that produce them in your intestines also thrive in your mouth, particularly on the back of your tongue. Research has shown that the proteolytic, anaerobic bacteria residing on the tongue play an essential role in the development of halitosis by breaking down proteins and dead cells into volatile sulfur compounds.

6PubMed. Assessing the contribution of anaerobic microflora of the tongue to oral malodor

Tonsil stones are another underappreciated source. These small, calcified lumps form in the crevices of the tonsils and harbor a diverse community of anaerobic bacteria from genera like Fusobacterium, Porphyromonas, and Prevotella, all of which produce volatile sulfur compounds. When you cough one up and crush it, the smell is unmistakable. But even intact, tonsil stones can release enough sulfurous gas to make your breath smell like flatulence to anyone standing nearby.

7PubMed. Composition of the bacterial flora in tonsilloliths

This means a person whose “fart smell” seems to come from their face rather than their backside likely has an oral source. Tongue scraping, improved dental hygiene, and addressing tonsil stones can make a dramatic difference. If the smell persists despite good oral care, a dentist or ENT specialist can help investigate further.

Sulfur in Your Sweat

Body odor is not just one smell. The familiar sweaty, oniony aroma of armpits has its own sulfur chemistry that overlaps with the compounds in flatulence. Fresh sweat is nearly odorless, but bacteria on your skin transform odorless precursor molecules in sweat into volatile compounds you can smell. Research isolating bacteria from armpit sweat identified several species of Corynebacterium and Staphylococcus that generated strong sweat odors from collected sweat material. One strain of Staphylococcus haemolyticus stood out for producing a distinctly sulfury character, rather than the more typical musty or acidic sweat smell.

8PubMed. 3-Methyl-3-sulfanylhexan-1-ol as a major descriptor for the human axilla-sweat odour profile

The specific mix of bacteria on your skin varies from person to person and even between your left and right armpits. If your particular microbial community happens to be heavy on sulfur-compound producers, your sweat can genuinely take on an eggy, flatulence-like quality rather than the more socially familiar “BO” smell. Stress sweat, which comes from apocrine glands concentrated in the armpits and groin, tends to contain more protein-rich secretions that bacteria love to break down, so the sulfury smell may be worse during anxious moments than during ordinary exercise.

Why Your Clothes Might Be the Real Problem

You shower, you apply deodorant, and an hour later you catch a whiff of something foul rising from your shirt. The fabric itself may be to blame. Different textiles interact with body odor compounds in very different ways. A study measuring how fibers adsorb and release volatile compounds found three distinct patterns: cotton showed low adsorption and low release of odor molecules, polyester showed high adsorption and continuous release, and wool showed high adsorption but low overall release.

9Textile Research Journal. Textile binding and release of body odor compounds measured by proton transfer reaction – mass spectrometry

In practical terms, polyester acts like a sponge that keeps wringing itself out. It soaks up volatile sulfur compounds from your sweat and then steadily emits them back into the air, creating a persistent smell that can follow you around even after the sweat itself has evaporated. Cotton lets less odor accumulate in the first place. Wool traps odor compounds but holds onto them more tightly, releasing less into the surrounding air. This is consistent with the common observation that synthetic workout clothes tend to smell worse than natural-fiber alternatives, even after washing.

10Textile Research Journal. Odor in textiles: A review of evaluation methods, fabric characteristics, and odor control technologies

If your “fart smell” seems worse in certain outfits, pay attention to fabric composition. Switching to cotton or merino wool base layers, especially for undershirts and underwear, can cut down on the sulfur-compound reservoir sitting against your skin all day.

Liver and Kidney Problems That Change How You Smell

Your liver and kidneys are your body’s main chemical-processing plants for waste products. When either one falters, sulfur-containing and nitrogen-containing compounds that would normally be broken down or excreted can accumulate in the blood and escape through the lungs and skin, producing distinctive odors.

Liver disease creates a condition known as fetor hepaticus, a musty, sulfurous breath odor. Analysis of breath samples from liver patients found elevated levels of dimethyl sulfide, acetone, and certain ketones. Dimethyl sulfide, a compound also present in flatus, was identified as the primary driver of the characteristic smell.

11PubMed. GC-MS analysis of breath odor compounds in liver patients

Kidney failure produces its own set of odor changes. When the kidneys cannot clear urea effectively, ammonia builds up in the blood, crosses the blood-lung barrier, and shows up in exhaled breath, giving it a sharp, fishy quality that some describe as fart-like or worse.

12PubMed. Recent analytical approaches to detect exhaled breath ammonia with special reference to renal patients

Research on volatile organic compounds in uremia confirmed that compounds including ammonia and hydroxyacetone accumulated as kidney function declined and were cleared by dialysis, reinforcing that the smell is a direct consequence of waste buildup.

13PLOS ONE. Volatile Organic Compounds in Uremia

Additionally, kidney disease can worsen or unmask trimethylaminuria-like symptoms. Trimethylamine, a compound with an intense fishy or fecal odor, is normally produced by gut bacteria from dietary choline and carnitine and then converted by the liver into an odorless form excreted in urine. In kidney failure, this clearance pathway breaks down at both ends: uremia impairs the liver’s oxidation of trimethylamine, and the kidneys cannot excrete the odorless product efficiently. The result is trimethylamine seeping out through sweat, breath, and urine.

14PubMed Central. Trimethylaminuria (fish malodour syndrome) in chronic renal failure

These are not subtle effects. People with advanced liver or kidney disease often report that others notice their smell before they do. If you have an unexplained persistent body odor that is new and accompanied by fatigue, nausea, changes in urine color, or swelling, it is worth mentioning to a doctor. The odor may be a clinical clue.

Trimethylaminuria as a Standalone Condition

You do not need liver or kidney disease to have a metabolic odor problem. Trimethylaminuria, sometimes called fish-odor syndrome, is a genetic condition in which the liver enzyme responsible for converting trimethylamine into its odorless form does not work properly. People with this condition excrete excessive amounts of free trimethylamine through sweat, urine, and breath.

15JAMA Dermatology. Trimethylaminuria (Fish-Odor Syndrome): A Case Report

The odor is often described as fishy, but it can also be perceived as fecal or garbage-like, and people with the condition frequently describe being told they smell like flatulence. Trimethylamine is produced from dietary precursors including choline, lecithin, and carnitine, which are abundant in eggs, organ meats, legumes, and certain fish. Reducing intake of these foods can lower the smell, though it rarely eliminates it entirely. The condition is underdiagnosed partly because the smell fluctuates with diet and hormonal cycles, and partly because many people are too embarrassed to bring it up with a doctor. A urine test measuring the ratio of trimethylamine to its oxidized form can confirm the diagnosis.

Phantom Smells and Neurological Causes

Sometimes you smell something foul and nobody around you can detect it. This can be a phantosmia, or phantom smell, a neurological phenomenon in which your brain generates an odor perception without any external source. Phantom smells are more common than most people realize, particularly among older adults, and the perceived odors are almost always unpleasant: burning, chemical, or rotten.

Research on olfactory hallucinations associated with primary headache disorders found that the phantom smells typically lasted anywhere from five minutes to an hour, appeared shortly before or at the same time as headache pain, and were highly specific and unpleasant, most commonly described as a burning smell.

16PubMed. Olfactory hallucinations in primary headache disorders: case series and literature review

Phantosmia can also accompany seizure disorders, sinus infections, Parkinson’s disease, and head injuries. If you are the only person who can smell the offending odor, and especially if it comes and goes with headaches or other neurological symptoms, the problem may be in your olfactory processing rather than on your body. A neurologist or ENT doctor can help sort this out.

Practical Steps for Tracking Down the Source

Because so many different body systems can produce sulfurous smells, narrowing down the cause matters more than reaching for a general solution. A few questions can help you triangulate:

  • Where is it strongest? If the smell is worst near your face, think oral sources: tongue coating, tonsil stones, gum disease. If it concentrates in your armpits or groin, skin bacteria are the likely culprit. If it seems to come from everywhere, a systemic metabolic cause or your clothing might be involved.
  • When does it appear? A smell that spikes after meals points toward dietary sulfur and gut fermentation. One that worsens with stress or exercise suggests sweat-related causes. A smell that is constant and unaffected by showering or changing clothes raises the possibility of a metabolic or organ-related issue.
  • Can others smell it? If not, consider phantosmia. Ask someone you trust to give you an honest answer.
  • Does it respond to dietary changes? Cutting back on high-sulfur foods (eggs, cruciferous vegetables, garlic, onions, red meat) for a week or two is a low-cost experiment. If the smell diminishes, your gut bacteria were likely the main drivers.

For gut-related odor, probiotics and dietary adjustments are the first-line approach, though the evidence for specific probiotic strains reducing sulfur gas production is still limited. For oral sources, tongue scraping, flossing, and a dental checkup are straightforward. For skin odor, switching to natural-fiber clothing and using antibacterial body wash on odor-prone areas can make a real difference. For any persistent, unexplained change in body odor that does not respond to hygiene or dietary adjustments, a visit to a primary care doctor is warranted. They can check liver and kidney function with basic blood work and order a urine trimethylamine test if the pattern fits.

The Role of Smell Sensitivity and Social Anxiety

One underappreciated piece of this puzzle is that your own nose may be playing tricks on you. Olfactory sensitivity varies enormously between individuals, and people who are anxious about body odor tend to become hypervigilant, detecting faint traces of normal body chemistry that others would never notice. There is a recognized condition called olfactory reference syndrome in which a person becomes preoccupied with the belief that they emit an offensive odor, despite no one else being able to detect it. This is distinct from phantosmia in that the person may actually be smelling real but trace-level compounds that fall well within the range of normal human body chemistry.

If you have asked multiple trusted people whether you smell and they consistently say no, and if the worry is affecting your daily life, it is worth discussing with a mental health professional rather than continuing to hunt for a physical cause. The distress is real even when the odor is not socially detectable, and effective treatments exist. On the other hand, if other people can confirm the smell, the investigation shifts firmly back to the biological sources covered above. Either way, the path forward starts with an honest conversation with someone whose nose you trust.