How sweat turns into body odor
Sweat itself is mostly water and salt, and the fluid freshly released from sweat glands has little smell. The familiar odor develops later, when skin microbes use some of the substances in sweat as food and release smelly by-products.
The armpits are especially prone to this process. They are warm, often covered, and home to bacteria that can break down compounds in sweat. Those compounds are not all the same: sweat from the glands concentrated in the armpits contains more odor-forming material than the watery sweat that mainly cools the skin elsewhere. As microbes process it, their waste products create the characteristic scent.
Why a workout can make the effect noticeable
During exercise, the body makes more sweat to carry heat away from the skin. Moisture and warmth also give microbes a favorable environment, while a shirt can hold sweat close to the body. The longer sweat and skin remain in contact, the more opportunity microbes have to transform odor-forming compounds.
That is why the smell is not a direct measure of how much a person sweated, or of how clean they are. It reflects a mix of sweat chemistry, skin microbes, time, and conditions around the skin. Washing removes sweat and some microbes; antiperspirants reduce the amount of sweat available, while deodorants mainly mask or limit odor. The smell appears when skin microbes process sweat—not at the moment the body produces it.
