Why do my armpits smell like onions?

Armpits don't always smell so fresh — after all, there are a ton of sweat glands residing there smooshed in between your torso and arm that are specifically designed to excrete a certain type of sweat (via WiseGeek). While you're probably familiar with regular old BO, you might wonder why your armpits sometimes smell like… onions?

For starters, let's take a little closer look at the type of sweat glands you have hanging around in your pits. The human body has a few types of sweat glands, two of which are called the eccrine and apocrine glands. The eccrine glands are located basically all over your body and secrete a fine sheen of sweat, and the apocrine glands are nestled in your armpit, genital area, and around your nipples.

The apocrine glands are quite different from their peers found in the rest of the body — they secrete thicker, stickier sweat than those eccrine glands do. Also, bacteria on the surface of the skin is always hard at work breaking down this thick, sticky sweat, which is the whole reason your armpits reek. 

Armpit onion smell comes from bacteria eating your sweat

Body odor can vary from person to person, though, and some people tend to have more onion-y sweat than other people. It turns out there's a scientific reason why "onion" pops up as a descriptor. According to NPR, when your armpit bacteria gobble up your sweat, they turn the odorless human sweat molecules into other products called thioalcohols. 

While pure human sweat is completely odorless, these specific compounds produced by the bacteria are definitely not. They smell really distinct — like rotten eggs, onions, or meat (hungry yet?). The reason for the similarity to onions, though, is because actual onion scent is also caused by thioalcohols (via Journal of Food Science). You will also, interestingly, find thioalcohols in skunk spray (via Gizmodo). 

So, while you may be glad to know that smelling like an onion is totally normal, you still might not be super thrilled to do so. Slap on some deodorant and maybe the onion smell will stay away.