What Eccrine Sweat Is Made Of
Eccrine glands produce the watery sweat that appears over much of the body, especially during heat or exercise. It is mostly water, but not pure water: it also contains dissolved salts and small amounts of other substances, including waste products. The mix is usually nearly scentless when it reaches the skin. Strong body odor often develops later, when skin bacteria break down compounds in sweat and other skin secretions.
How Evaporation Cools You
Sweat cools the body only when it evaporates. To change from liquid into vapor, water molecules need energy; they take that energy as heat from the skin. As more sweat evaporates, the skin loses more heat, helping keep the body’s internal temperature from climbing too high.
That is why sweat dripping off without evaporating does little cooling. In humid air, the atmosphere already holds substantial water vapor, so sweat evaporates more slowly. Air movement and light, breathable clothing can help carry vapor away from the skin; heavy, damp clothing can trap it.
The cooling effect comes from water leaving the skin as vapor, not from sweat being cold. In dry, breezy conditions, a small amount may evaporate quickly and cool efficiently; in muggy conditions, sweat can bead or run off while the body still feels hot.
