When someone exercises, their body sweats to help them maintain a steady temperature. When this sweat mixes with bacteria on the skin and then dries, it can smell. People usually refer to this body odor as BO.
Why do I smell so bad after running?
Ammonia is toxic, so the body reverts to the more primitive removal system through sweat. When the sweat concentration of ammonia rises, you will likely notice the smell after runs. The natural movement of air across the body probably keeps the odor down while you are outdoors and moving during your runs.