The smell comes from sulfur. Specifically, trace sulfur-containing gases make up only about 50 parts per million of any given fart, but your nose is extraordinarily sensitive to them. The vast majority of intestinal gas, roughly 99%, is completely odorless. It’s that tiny fraction of sulfur compounds that makes the difference between a silent pass and clearing a room.
What Actually Creates the Smell
About three-quarters of every fart is carbon dioxide, hydrogen, and methane, all produced by bacteria in your gut. Another quarter is just swallowed air (oxygen and nitrogen). None of these have any smell at all. The stink comes entirely from volatile sulfur compounds produced when gut bacteria break down sulfur-containing foods.
Three compounds do most of the damage. Hydrogen sulfide is the dominant one, present at concentrations about five times higher than the other two. It smells like rotten eggs. Methanethiol, the second most concentrated, smells like decomposing vegetables or rotten cabbage. Dimethyl sulfide rounds out the trio with a sweeter, garlic-like odor. When judges in a study published in the journal Gut were asked to rank these gases individually, hydrogen sulfide was consistently rated the most offensive.
Your nose can detect these compounds at incredibly low concentrations. That’s why even a tiny shift in sulfur gas production can turn an unremarkable fart into something memorable.
Your Gut Bacteria Are Doing the Work
The bacteria living in your large intestine are the factories behind smelly gas. Some species, including members of the Fusobacterium and Escherichia groups, produce hydrogen sulfide by breaking down sulfur-containing amino acids like cysteine and taurine. These amino acids show up whenever you eat protein-rich foods.
A separate group of bacteria takes a different route entirely. Anaerobic bacteria in the Desulfovibrio genus produce hydrogen sulfide by processing sulfate, a compound found naturally in many foods and even drinking water. So even if you cut back on protein, sulfate from other dietary sources can still feed these bacteria and keep the smell going.
The balance of your gut microbiome matters enormously here. Two people can eat the exact same meal and produce very different-smelling gas, simply because their bacterial populations differ. If your gut harbors more sulfate-reducing bacteria than average, you’ll tend to produce more hydrogen sulfide from the same foods.
Foods That Make It Worse
Sulfur enters your gut through several categories of food. The biggest contributors fall into three groups:
- High-protein foods: Turkey, beef, eggs, fish, and chicken all contain methionine and cysteine, two sulfur-rich amino acids. Red meat is particularly notorious because its high protein content means more raw material for bacteria to ferment. Nuts, seeds, grains, and legumes are plant-based sources of the same amino acids.
- Cruciferous vegetables: Broccoli, cauliflower, cabbage, kale, arugula, and radishes are all packed with sulfur compounds. These are some of the healthiest foods you can eat, but they reliably increase sulfur gas production.
- Allium vegetables: Garlic, onions, leeks, scallions, and shallots belong to this group. They contain sulfur compounds that survive digestion and reach the colon, where bacteria convert them into odorous gases.
Sulfates in drinking water and certain supplements (glucosamine sulfate, chondroitin sulfate, MSM) can also contribute. If you’ve recently started a joint supplement and noticed a change in your gas, that connection is real.
Why Some Days Are Worse Than Others
Transit time plays a major role. The longer food sits in your colon, the more time bacteria have to ferment it and produce sulfur gases. If you’re constipated or just having a slow digestive day, bacteria get extra hours to work on whatever you’ve eaten. The result is more concentrated, more pungent gas.
This is also why a high-protein, low-fiber meal can produce especially foul-smelling results. Fiber speeds transit through the colon, while protein without fiber tends to move slowly, giving bacteria a longer fermentation window. A steak dinner without much in the way of vegetables is a textbook setup for bad-smelling gas the next day.
The average person passes gas about 15 times a day, though anywhere from 3 to 40 times falls within the normal range. Frequency alone doesn’t indicate a problem. What matters more is whether the smell, volume, or pattern has changed significantly from your personal baseline.
When Bad Smells Signal a Bigger Problem
Persistently foul-smelling gas, especially paired with other digestive symptoms, can point to malabsorption. This happens when your small intestine fails to properly break down or absorb nutrients, sending undigested food to the colon where bacteria ferment it aggressively.
Fat malabsorption is one common pattern. Fats that aren’t absorbed in the small intestine pass into the colon and produce greasy, runny, particularly smelly stools alongside increased gas. Carbohydrate malabsorption works similarly: unabsorbed carbohydrates get fermented by colonic bacteria into gases and short-chain fatty acids, producing both bloating and odor.
Several conditions can cause this. Celiac disease damages the lining of the small intestine, reducing its ability to absorb nutrients. Pancreatic insufficiency means your body doesn’t produce enough digestive enzymes to break food down properly before it reaches the colon. Parasitic infections like giardiasis can also disrupt normal absorption. If your gas has become dramatically worse and you’re also experiencing diarrhea, greasy stools, unexplained weight loss, or persistent bloating, those symptoms together warrant investigation.
Hydrogen sulfide-producing bacterial overgrowth in the small intestine is another possibility that researchers are actively studying, though standardized testing for it isn’t widely available yet.
How to Reduce the Smell
The most direct approach is dietary. Cutting back on the sulfur-heavy foods listed above, even temporarily, can help you identify which ones are your biggest triggers. You don’t have to eliminate them permanently. Simply knowing that a meal heavy in eggs, garlic, and broccoli will produce results 12 to 24 hours later lets you plan accordingly.
Increasing fiber intake (gradually, to avoid making gas worse in the short term) helps speed transit time through the colon, giving bacteria less opportunity to produce sulfur gases. Staying hydrated supports the same goal.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol, has a specific and well-documented effect on gas odor. It works by chemically binding to hydrogen sulfide in the colon, converting it into bismuth sulfide, which is insoluble and odorless. In a study published in Gastroenterology, subjects who took it for several days showed markedly decreased hydrogen sulfide release from fecal samples. It doesn’t reduce gas volume, but it directly neutralizes the compound most responsible for the smell.
Probiotics may help shift the bacterial balance in your gut over time, though results vary widely between individuals and strains. Reducing carbonated drinks and eating more slowly (to swallow less air) can decrease overall gas volume, even if the sulfur content of each episode stays the same.