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Sometimes your farts stink, and sometimes they don't—and it's not because of methane.

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"Don't fart naked near food." That was a tongue-in-cheek conclusion from a couple of Australian scientists in 2001 who conducted an experiment on farts. A nurse had asked them if farting in a sterile environment, like the surgery room she worked in, could cause infections. It's kind of a weird question, but not maybe entirely ridiculous. So these researchers looked into whether farts were potentially harmful, or just stink clouds. And their experiment was just one of many that analyzed what farts are made of. 

Flatus, as medical journals call it, is partially produced by swallowing air as you go about your life. But farts are mostly a by-product made by the trillions of microbes that live in your gut, digesting carbohydrates and spewing out gasses. Humans fart up to 20 times per day or more, and release mostly odorless gasses like nitrogen, oxygen, carbon dioxide, and hydrogen. But there's a tiny percentage of the stuff that smells. Really bad. It may surprise you that methane isn't the culprit. It's flammable, and a greenhouse gas, but it doesn't smell. Not to mention, not all of us have gut bacteria that produce methane in the first place. Fart smells are actually due to a handful of compounds tht contain sulfur: hydrogen sulfide, methanethiol, and dimethyl sulfide. 

Hydrogen sulfide is a colorless gas that smells like rotten eggs. It's flammable, and in large quantities, is also highly poiisonous. Like some other toxins, hydrogen sulfide messes with enzymes that let the mitochondria in your cells make energy. Fortunately for us, farts contain tiny amounts of it. So we smell it, but we're not in any real danger. Methanethiol is another sulfurous chemical, and it smells like rotting vegetables. It's produced by bacteria that are also involved in things like terrible breath and making some cheeses.