Why Do We Sweat? How Your Body’s Built-In Air Conditioning Works

On a summer day or halfway through a run, beads of sweat appear on your forehead, your shirt gets damp and you might wish you could switch it off. But sweating is one of the body’s most important survival tools. Without it, you would overheat dangerously in minutes of hard work.

A built-in cooling system

Your body works best at around 37 °C. Every movement produces heat, and on hot days heat also flows in from the surroundings. To get rid of it, the body has two main tricks: widening blood vessels in the skin to release heat, and sweating.

A region of the brain called the hypothalamus acts as a thermostat. When it senses rising temperature, it sends signals through the nerves to the sweat glands. They release a watery, slightly salty fluid onto the skin. When this fluid evaporates, it absorbs a lot of heat from the skin, cooling the blood just beneath it. This is the same physics that makes you feel cold when you step out of a swimming pool.

Two kinds of sweat glands

• Eccrine glands cover almost the whole body, with the highest numbers on the palms, soles and forehead. Humans have about 2 to 4 million of them. They produce the watery sweat that cools us.
• Apocrine glands are found mainly in the armpits and groin. They become active at puberty and release a thicker, oilier sweat, especially when we are stressed.

Why sweat smells

Fresh sweat is almost odourless. The smell comes from bacteria living on the skin, which break down substances in apocrine sweat into smelly compounds. That is why armpits smell more than, say, your forearm, and why washing and deodorant help. Antiperspirants go a step further and temporarily plug the sweat ducts.

How much can we sweat?

Sweat rates vary hugely. A person resting in mild weather loses only a little, but during hard exercise in the heat, people can sweat more than 2 litres an hour. Sweat also carries sodium and small amounts of other minerals. During long events, such as marathons, losing a lot of fluid and salt without replacing it can lead to dehydration, while drinking far too much plain water can also be dangerous.

Humans have an unusually powerful sweating system. Many scientists think it helped our ancestors hunt and travel over long distances under the hot African sun, outlasting animals that had to stop and pant.

Why humidity is so uncomfortable

Sweat only cools when it evaporates. In dry air, it evaporates quickly, so you may barely notice you are sweating. In humid air, the air is already full of water vapour, so sweat evaporates slowly and drips off instead. You keep sweating but cool down less, which is why a humid 30 °C can feel worse than a dry 35 °C and why heat waves with high humidity are more dangerous.

Nervous sweating

Sweaty palms before an exam or a job interview are a different kind of sweating. Emotional stress activates the body’s “fight or flight” system, which triggers sweat on the palms, soles and armpits. Scientists think sweaty palms and feet may once have helped our ancestors grip better when running or climbing.

In short

• We sweat mainly to cool down; the brain’s hypothalamus acts as the thermostat.
• Sweat cools only when it evaporates, carrying heat away from the skin.
• Humans have about 2–4 million eccrine sweat glands.
• Sweat itself is nearly odourless; bacteria cause body odour.
• In the heat, people can lose more than 2 litres of sweat an hour, along with salt.
• Humid air slows evaporation, so sweating cools us less.