The Difference Between Smelly And Odorless Farts Is Easily Explained By Science

Smelly farts, especially those that linger, can be so offensive that they clear a room, but that doesn't scare off flatologists, scientists who study flatulence. Through research, they've been able to identify several facts. For instance, there's a scientific reason you can't help farting more on an airplane, and silent farts smelling worse than loud farts is actually a smelly fart myth based in science. One landmark study published in Gut even discovered the reason some farts are more odorous than others, and it all comes down to the gas content.

For the study, researchers at the Minneapolis Veterans Affairs Medical Center gathered fart samples from six women and 10 men between the ages of 18 and 47. The participants had no gastrointestinal issues, and all except three followed a diet of about 8 ounces (200 grams) of pinto beans the night and morning before the gas collection. Two hours beforehand, they were also given a dose of lactulose, a medication that increases bowel movements. Each fart — three to nine samples from each participant — was collected in a separate impermeable bag using a rectal tube.

Analyzing the gas, the researchers identified three sulfur-containing gases: hydrogen sulfide, methanethiol, and dimethyl sulfide. There was about five times as much hydrogen sulfide as methanethiol, leading the scientists to identify it as the main culprit behind why some farts are so malodorous. As a bonus, they found that the female participants' farts contained significantly more hydrogen sulfide and a higher odor intensity than the male participants' farts.

Although sulfur-containing gases are responsible for the malodorous nature of farts, they account for less than 1% of all the fart gases. The most prominent gas is nitrogen, which isn't surprising because nitrogen is important for all living things. Others include hydrogen, carbon dioxide, methane, and oxygen — all of which are also odorless. Primarily, the gases in a fart are a natural byproduct of large intestinal bacteria breaking down food. As a result, the amount of sulfur-containing gases produced are directly related to the types of foods that you've eaten.

Hydrogen sulfide is responsible for making farts smell like rotten eggs, which makes sense because it's the same compound that emanates from rotten eggs. Meanwhile, methanethiol has a more cabbage- or garlic-like odor, and dimethyl sulfide has a milder cooked-vegetable scent. A wide variety of foods can produce smelly farts.

According to a study published in Heliyon, some fruits — like durian, kiwi, strawberries, and watermelon — and dairy products — such as milk, mature cheeses, and yogurt — are rich in sulfur compounds. Eggs, fish, poultry, and red meat can also produce significant amounts of hydrogen sulfide, while garlic and onions produce methanethiol and dimethyl sulfide. Additionally, volatile sulfur compounds are found in cruciferous vegetables as well as legumes, nuts, and seeds.