Farts are almost entirely gas, not poop particles. About 99% of a fart is made up of odorless gases like nitrogen, oxygen, hydrogen, carbon dioxide, and methane. The smell comes from tiny amounts of sulfur-containing compounds produced by bacteria in your gut. These are gas-phase molecules, not solid or liquid bits of fecal matter floating through the air.
That said, the full answer is a little more nuanced than a simple “no.” Under very specific conditions, flatulence can carry trace amounts of bacteria. Here’s what actually happens when you pass gas, what that smell really is, and whether any of it poses a health risk.
What a Fart Is Made Of
The gases in flatulence come from two sources: swallowed air and bacterial fermentation in your colon. Nitrogen makes up the largest share at roughly 65%, mostly from air you swallow while eating. Carbon dioxide accounts for about 10%, methane around 14%, and hydrogen about 3%. Oxygen fills in the remaining small percentage.
None of these gases smell like anything. The distinctive odor comes from sulfur compounds, primarily hydrogen sulfide (the classic rotten-egg smell) and methanethiol (a more pungent, barnyard-like odor). These are volatile molecules produced when gut bacteria break down food, especially sulfur-rich foods like eggs, broccoli, and meat. They’re present in very small concentrations, but your nose is extremely sensitive to them. Importantly, these are true gases dissolved at the molecular level. They are not microscopic flecks of stool.
How Your Body Keeps Gas and Stool Separate
Your body has a surprisingly sophisticated system for telling the difference between gas and feces. The internal anal sphincter, which handles 80% to 85% of the muscle tone keeping your rectum closed, works with a rich network of nerve endings in your rectal lining. Together, they can “sample” what’s pressing against the rectal wall and determine whether it’s solid, liquid, or gas.
When gas builds up, a reflex causes the internal sphincter to relax just enough for those nerve endings to assess the contents. If it’s gas, your body can selectively release it while keeping everything else in place. This is why healthy adults can pass gas without losing bowel control. The system isn’t perfect (anyone who’s had a stomach bug knows that), but under normal circumstances it reliably separates gas from stool.
The Petri Dish Experiment
The most widely cited experiment on this topic was conducted by Australian researcher Karl Kruszelnicki and published in the BMJ. A colleague was asked to pass gas onto two petri dishes from about 5 centimeters away: once while fully clothed and once with pants down.
The results were telling. The petri dish exposed to a bare-bottomed fart grew two types of bacteria overnight, one commonly found in the gut and one found on skin. The researchers concluded that the gut bacteria came from the fart itself, while the skin bacteria were likely blasted off the surrounding skin by the force of the gas. The petri dish exposed to a clothed fart grew nothing.
Two important details often get lost when this experiment is cited. First, neither type of bacteria that grew was harmful. The researchers compared them to the “friendly” bacteria found in yogurt. Second, the clothing completely blocked any bacterial transfer, meaning that in any real-world scenario where you’re wearing underwear or pants, the fabric acts as a filter.
How Clothing Filters What Comes Through
Fabric is a surprisingly effective barrier against tiny particles. Research on common clothing materials found that even a single layer of cotton can filter a meaningful percentage of particles, and the efficiency climbs significantly with tighter weaves and higher thread counts. When two different fabric types are layered together (cotton and silk, for example), filtration rates exceeded 80% for very small particles and over 90% for larger ones.
Most people wearing underwear plus pants have at least two layers of fabric between their body and the outside air. The sulfur gases responsible for the smell pass through easily because they’re individual molecules, far smaller than any bacteria. But bacteria and any microscopic droplets are physically large enough to get trapped in fabric fibers. This is why the Kruszelnicki experiment found bacterial growth only from the unclothed fart. Your clothing is doing real work as a filter, even if it can’t stop the smell.
Why the Smell Doesn’t Mean Particles
It’s natural to assume that if you can smell something, physical particles of that thing must be reaching your nose. But smell doesn’t work that way. Odor is carried by volatile compounds, molecules light enough to float freely in air as a gas. Hydrogen sulfide, the primary culprit in fart odor, has a molecular weight so low that it behaves nothing like a solid particle. It moves through the air the same way the scent of perfume does.
Your nose can detect hydrogen sulfide at extraordinarily low concentrations, parts per billion. So the fact that a fart smells strong doesn’t mean it’s carrying a heavy load of material. It means a vanishingly small amount of a very potent gas molecule has reached your nostrils. No fecal matter needs to be present for that to happen.
Is There Any Actual Health Risk?
In practical terms, no. During the early days of the COVID-19 pandemic, researchers explored whether flatulence could theoretically transmit the virus, since SARS-CoV-2 was found in stool samples. The conclusion: no published evidence supported fart-based transmission. China’s Centers for Disease Control explicitly noted that pants act as a sufficient barrier.
The bacteria that gut microbiome studies associate with gas production, species like Bacteroides and Bilophila, are normal residents of your colon. Even in the unlikely event that a few of them hitched a ride on a fart, they’re organisms your body already hosts in enormous quantities. They’re not the kind of pathogens that cause infectious disease through casual exposure.
The genuine fecal-oral transmission risks in daily life come from inadequate handwashing after using the toilet, contaminated food and water, and aerosols generated by toilet flushing (which can launch droplets containing bacteria into the air with real force). Flatulence, especially through clothing, is not a meaningful contributor to that list.