The sulfur compounds that make flatulence smell bad are also produced by bacteria living in your mouth, on your skin, and deep in your gut, and they can reach your nose through routes that have nothing to do with passing gas. Hydrogen sulfide, the signature “rotten egg” gas in flatus, is the same molecule generated by tongue bacteria, tonsil bacteria, and even sweat-dwelling microbes. Your body can also absorb sulfur gases from the intestine into the bloodstream and exhale them through the lungs. So when you catch a whiff of something unmistakably fart-like and you know you haven’t let one go, the smell is likely real and coming from somewhere else on or in your body.
The Same Sulfur Compounds, Many Different Sources
Flatulence gets its distinctive odor primarily from hydrogen sulfide, with smaller contributions from methanethiol and dimethyl sulfide. A study measuring the chemical makeup of flatus found that hydrogen sulfide was the dominant sulfur compound, and its concentration correlated directly with how bad the smell was judged to be.1PubMed Central. Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour These three gases are not unique to the intestines. Any environment where bacteria break down sulfur-containing amino acids can generate them. Your mouth, your armpits, even your tonsils host bacterial communities that produce these exact molecules. The reason a fart-like smell can seem to follow you around is that your body has multiple sulfur-gas factories running at once.
Your Mouth and Throat
One of the most common non-flatulence sources of a sulfurous smell is your own breath. Anaerobic bacteria on the tongue, particularly in the coating at the back, break down sulfur-containing amino acids like cysteine and methionine into volatile sulfur compounds. Researchers have confirmed that hydrogen sulfide, methanethiol, and dimethyl sulfide levels in the mouth are all significantly higher in people with halitosis compared to those without it.2Acta Odontologica Scandinavica. Characterization of oral bacteria in the tongue coating of patients with halitosis using 16S rRNA analysis A review of dietary sulfur and oral health found that this process is driven by the tongue’s biofilm, a layer of anaerobic microbes that thrive in low-oxygen pockets of the mouth.3PubMed Central. Dietary Sulfur Compounds and Halitosis: Bridging Food Science, Microbial Metabolism, and Oral Health Because the exact same gases are involved, breath from a dirty tongue can smell indistinguishable from a fart.
Tonsils are another underappreciated contributor. Chronic tonsillitis, or even tonsil stones (those small, white, calcified lumps that form in the crevices of the tonsils), can harbor sulfur-producing bacteria. A study measuring volatile sulfur compounds in patients with chronic tonsillitis found elevated levels of hydrogen sulfide and methanethiol, which dropped significantly after the tonsils were removed.4PubMed Central. Assessment of Volatile Sulfur Compounds in Adult and Pediatric Chronic Tonsillitis Patients Receiving Tonsillectomy If you have persistent bad breath that smells sulfurous and you can sometimes dislodge small, foul-smelling lumps from the back of your throat, tonsil stones are a strong candidate.
Skin Bacteria and Sweat
Your skin, especially in warm, enclosed areas like the groin, armpits, and between the buttocks, is colonized by bacteria that can produce sulfurous and other foul-smelling compounds. A review of microbiota and body malodor noted that unpleasant smells from the body are typically caused by odorants produced by resident bacterial communities on the skin, in the mouth, and elsewhere.5PubMed Central. Microbiota and Malodor-Etiology and Management Apocrine sweat glands, concentrated in the armpits and groin, release a thick secretion that is initially odorless. Bacteria such as Corynebacterium and Staphylococcus species then break down the fats and proteins in that sweat into volatile compounds, some of which overlap chemically with the sulfur gases in flatus.
This is why the smell can seem to come from your lower body even when you haven’t passed gas. Sweat trapped between skin folds or under tight clothing near your backside creates an environment where bacteria thrive. The resulting odor wafts upward and can easily be mistaken for flatulence. The distinction matters because the solution is different: this kind of smell responds to better hygiene in skin folds, breathable fabrics, and sometimes topical antibacterials, not dietary changes aimed at your gut.
Gut Gases That Bypass the Back Door
Here is a pathway most people never consider: hydrogen sulfide produced by bacteria in your intestines can cross the gut wall, enter your bloodstream, circulate through your body, and get exhaled through your lungs. Animal studies suggest that a substantial portion of circulating hydrogen sulfide in the body actually originates from the metabolic activity of gut bacteria. In other words, some of the gas your gut microbes produce never becomes a fart; it enters your bloodstream instead and leaves your body through your breath. This means your exhaled air can carry a sulfurous, fart-like odor that has nothing to do with the digestive tract’s normal gas exit route.
When gut bacteria produce excess hydrogen sulfide, the consequences can extend beyond just a smell. A case registry of patients with hydrogen sulfide-related small intestinal bacterial overgrowth found that the most common symptoms were bloating, constipation, and abdominal pain.6medRxiv. Hydrogen sulfide small intestinal bacterial overgrowth case registry A low-sulfur diet and bismuth were the treatments most likely to help, with about three-quarters of patients on each treatment responding positively.6medRxiv. Hydrogen sulfide small intestinal bacterial overgrowth case registry If you consistently smell sulfurous and also have digestive symptoms, this kind of bacterial overgrowth is worth discussing with a doctor.
Foods That Shift the Balance
What you eat directly affects how much sulfur your body’s bacteria have to work with. High-sulfur foods like eggs, cruciferous vegetables (broccoli, cauliflower, cabbage), garlic, onions, and red meat all supply the amino acids and other sulfur-containing compounds that bacteria convert into volatile sulfur gases. This happens both in the gut, where intestinal bacteria ferment undigested food residues, and in the mouth, where oral bacteria act on sulfur compounds that linger after a meal.
Research has confirmed that dietary factors influence volatile sulfur compound formation not only through direct substrate availability in the mouth but also through broader metabolic effects that occur outside the oral cavity.3PubMed Central. Dietary Sulfur Compounds and Halitosis: Bridging Food Science, Microbial Metabolism, and Oral Health Eating a sulfur-heavy meal can produce odor from multiple body sites at once: your breath smells because oral bacteria have more substrate, your skin smells because your sweat carries sulfur metabolites, and your gut produces more hydrogen sulfide that enters circulation and eventually gets exhaled. This multi-front effect explains why, after certain meals, you can smell fart-like without actually producing more gas through the usual route.
Your Clothes Might Be Amplifying It
Fabric choice has a surprisingly large impact on body odor. Synthetic fabrics like polyester are worse than natural ones like cotton at trapping and promoting bacterial growth. A study that collected T-shirts from people after an intense cycling session and incubated them found that polyester shirts smelled significantly worse and more intensely than cotton shirts. The polyester harbored higher levels of Micrococcus bacteria, which were largely absent from the cotton.7PubMed Central. Microbial odor profile of polyester and cotton clothes after a fitness session The structure of synthetic fibers creates a surface that bacteria adhere to more easily, and the material retains oils and sweat that feed those bacteria even after washing.
This means your underwear and pants could be acting as an odor reservoir. If you wear synthetic-blend underwear or exercise pants that you’ve washed repeatedly but that still seem to hold a smell, the fabric itself may be the problem. Switching to cotton or merino wool for garments close to the body, especially around the groin and buttocks, can make a noticeable difference. Washing synthetic workout clothes with a sports-specific detergent or adding white vinegar to the rinse cycle can help break down the bacterial biofilm that builds up in the fibers over time.
Medications and Supplements That Cause Sulfurous Odors
Certain drugs and supplements can make your breath or body smell like sulfur through their own metabolism. A systematic review of drug-related halitosis identified several culprits. Dimethyl sulfoxide (DMSO), used topically for some musculoskeletal conditions, caused halitosis in all patients who applied it because the body metabolizes it into dimethyl sulfide, which gets exhaled. Fish-oil supplements can produce a fish-scented breath that some people describe as sulfurous. And certain experimental cancer drugs have been documented to produce a garlic-like odor on patients’ breath due to the exhalation of sulfur-containing metabolites.8PubMed Central. Drug-related Halitosis: A Systematic Review
Supplements containing N-acetylcysteine (NAC), methylsulfonylmethane (MSM), or high-dose garlic extract are frequent offenders as well. These all supply sulfur to your metabolic pathways, and the excess exits through your breath and sweat. If you started a new supplement or medication around the time the smell appeared, that is worth investigating before you assume the problem is dietary or hygiene-related.
When Your Nose Is Lying to You
Sometimes the fart smell is not coming from your body at all. It is being generated by your brain. Phantosmia, the perception of a smell that has no external source, is more common than most people realize. A population-based study of older adults found that phantom smells were a recognized phenomenon, with smoky or burnt odors being the most frequently reported type.9PubMed Central. Phantom Smells: Prevalence and Correlates in a Population-Based Sample of Older Adults Sulfurous and fecal-type phantom smells are also reported, though less commonly studied.
Parosmia, a related condition where real smells are distorted into unpleasant ones, became widely recognized during the COVID-19 pandemic. After a viral infection damages olfactory neurons, the regenerating nerve cells can misfire, causing normal smells to register as foul. Research into the triggers of parosmia found that certain highly volatile molecular compounds with low detection thresholds tend to be disproportionately perceived during recovery, creating a distorted smell profile in which the unpleasant components of an odor dominate while the pleasant ones are lost.10PubMed Central. Post-viral olfactory loss and parosmia This means your own body’s normal smells, or even the smell of food and coffee, could register as something rotten or sulfurous during recovery from a respiratory infection.
There is also olfactory reference syndrome, a psychiatric condition in which a person becomes convinced they emit an offensive body odor when they do not. This goes beyond ordinary self-consciousness about smell. People with this condition may shower repeatedly, avoid social situations, and seek medical evaluations despite no objective odor being detectable.11PubMed. The Nose Knows… or Does it? Olfactory Reference Syndrome in Patients Presenting for Assessment of Unusual Body Odor If trusted people around you consistently tell you they cannot detect the odor, and no medical cause has been found, this possibility is worth exploring with a mental health professional.
Rare Metabolic Conditions
A small number of people have a genetic condition that genuinely changes their body odor in dramatic ways. Trimethylaminuria, sometimes called “fish odor syndrome,” is caused by mutations in the gene for a liver enzyme called FMO3. Normally this enzyme converts trimethylamine, a fishy-smelling compound produced during digestion, into an odorless form that gets excreted in urine. When the enzyme does not work properly, trimethylamine builds up and is released through sweat, breath, and urine, producing a strong odor that can range from fishy to fecal to sulfurous depending on the person and the day.12PubMed. A spectrum of molecular variation in a cohort of Italian families with trimethylaminuria: identification of three novel mutations of the FM03 gene
The condition is inherited in a recessive pattern, meaning you need two copies of the faulty gene to be fully affected. However, research has identified multiple genetic variants that reduce FMO3 activity to varying degrees, meaning some people are carriers or have partial enzyme impairment without full-blown trimethylaminuria.13PubMed. FMO3 allelic variants in Sicilian and Sardinian populations: trimethylaminuria and absence of fish-like body odor These individuals might notice their body odor worsens after eating foods high in choline or carnitine, like eggs, liver, and certain fish, without ever reaching the diagnostic threshold for trimethylaminuria. If your odor problem is persistent, resists all hygiene measures, and worsens after protein-rich meals, a urine test for trimethylamine can either rule this out or point toward dietary management strategies.
A Practical Checklist for Tracking Down the Source
Because so many different sources can produce the same sulfurous smell, narrowing down the cause requires some detective work. A few questions can help you sort through the possibilities:
- Is it your breath? Cup your hands over your mouth and nose and exhale. If the smell is strong, oral sources like tongue coating, tonsil stones, or gum disease are likely. A dentist or ENT specialist can evaluate further.
- Is it your skin? Smell your armpits and groin area separately. If the smell concentrates in skin folds, bacteria on the skin surface are the likely culprit, and the fix involves targeted washing and fabric changes.
- Is it your clothes? Sniff the garment separately from your body after wearing it. Synthetic fabrics can develop a persistent bacterial odor that survives regular washing.
- Did it start with a new food, supplement, or medication? Sulfur-rich foods, fish oil, DMSO, and certain other compounds can all produce a body-wide sulfurous odor that resolves when you stop taking them.
- Can others smell it? If nobody else can detect the odor despite your certainty, phantosmia, parosmia, or olfactory reference syndrome deserve consideration.
- Do you have digestive symptoms too? Bloating, constipation, and abdominal pain alongside the smell could suggest bacterial overgrowth in the small intestine, which warrants a breath test from a gastroenterologist.
Most cases turn out to involve mundane causes: a tongue that needs better scraping, a sulfur-rich diet, or synthetic underwear that has become a bacterial breeding ground. Occasionally, the answer is more surprising, like a post-viral change in how your brain processes smell. The variety of possible sources is exactly why this is a frustrating symptom to pin down on your own, but it also means there is almost always something you can do about it once you identify which factory is producing the gas.
Why the Smell Seems Worse to You Than to Others
Human noses are extraordinarily sensitive to sulfur compounds. Hydrogen sulfide can be detected at concentrations as low as a few parts per billion, which is why even tiny amounts of these gases register as an intense stench. Your own nose sits closer to most of the sources discussed above than anyone else’s does. Your breath passes directly through your nasal cavity. Your skin odor rises from your body in a thermal plume that your nose intercepts before it disperses. This proximity effect means you are often smelling yourself at concentrations that would be undetectable to someone standing a normal conversational distance away.
There is also an attentional component. Once you notice an unpleasant smell and start wondering whether others can detect it, you become hypervigilant. You sniff more often, you interpret ambiguous sensations as confirmations, and your anxiety about the smell can make you sweat more, which in turn feeds the skin bacteria that produce the odor. Breaking this cycle sometimes requires nothing more than an honest assessment from someone you trust, or a formal evaluation from a clinician who can measure volatile sulfur compounds in your breath or run a metabolic panel to rule out rarer causes.