Fresh underarm sweat has almost no smell. The odor shows up when bacteria on the skin break it down into smaller, smellier molecules (source). Researchers showed this as early as 1953: sweat kept free of bacteria stayed odorless for two weeks, and the same sweat with skin bacteria smelled strong within six hours (study, summary).

The main culprit molecule has been described as smelling of "sweat and onion" (source). A 2020 study traced the enzyme that makes it to underarm staph bacteria, and found those bacteria picked up that ability tens of millions of years ago (source).

So what is sweat for? Mostly cooling. It's largely water and salt, and a major review found no direct evidence that sweating is a good way to remove toxins (source).

Your clothes play a part too. In a small study at Ghent University, polyester shirts worn for a spin class smelled worse than cotton ones, and one group of bacteria turned up almost only on the synthetics (source). A 2025 laundry study found that a cool 30 °C wash removed known odor compounds from polyester, while the living bacteria on the fabric actually increased, and that drying clothes in humid air made the smell worse (source).

Why can't you smell yourself? Your nose gets used to a smell you're around all the time, and its sensitivity comes back once you're away from it (source).

One more thing people often mix up: antiperspirants use aluminum salts to reduce sweat (source). Deodorants work on odor, not on sweat.

Several of these are small studies, and we say so when we cite them.

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