Why Do Some Farts Stink and Some Farts Don't?

Chloe Effron
Chloe Effron / Chloe Effron

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Everyone farts, every day. But not all farts are the same. Some farts make no sound but are really stinky. Others are loud but don't smell. Some you can smell clear across the room, but others you can let out without anyone noticing. (Secret: I’m farting right now!)

The smell of your booty bomb depends on what you’ve been eating. Broccoli, cabbage, onions, eggs, and meat all contain a lot of sulfur, a chemical that helps give rotten eggs their stink. When your body digests, or breaks down, those foods, the teeny-tiny creatures in your gut called bacteria feast upon the proteins in the food that contain sulfur. This process creates smelly gases like methanethiol (METH-ain-THIGH-all). When those gases leave your body, they end up as pungent farts.

All smells are chemicals in the air that your nose can pick up. Farts are made up of chemicals like oxygen, nitrogen, hydrogen, methane, carbon, and the super villain of stinkiness: sulfur. (Sulfur is the reason that skunk spray smells so gross!) The more sulfur in your toots, the more likely they are to clear the room. You also swallow air as you eat or talk. Some of that comes out of your rear end too.

Let's talk about bacteria again. When you eat carbohydrates (CARB-oh-HIGH-drates) like potatoes, bread, and vegetables, your stomach doesn’t fully digest them. They pass into the small intestine and then the large intestine, which are really long tubes leading to your anus (where poop and farts come out). Bacteria break the carbohydrates into smaller pieces. That releases gases like hydrogen and carbon dioxide. Those gases don’t smell. But other bacteria in the gut take those odorless chemicals and make them into compounds (mixes of chemicals) that do smell. Hydrogen sulfide (HIGH-dro-jen SULL-fide) is the smelliest of the gases that might come out of your butt.

For more fart science, check out AsapSCIENCE’s illustrated video explanation of why your farts don’t smell as bad as other people’s do.