Sweating starts in your brain. When your body temperature rises, a region deep in the brain called the hypothalamus detects the change and sends signals through your nervous system to millions of tiny glands embedded in your skin. Those glands pull water and salts from surrounding tissue, push the mixture to your skin’s surface, and as it evaporates, it carries heat away from your body. It’s one of the most efficient cooling systems in the animal kingdom.
How Your Brain Triggers Sweating
Temperature sensors in your skin detect warmth and relay that information through your spinal cord to a processing station in the brainstem. From there, signals travel to the hypothalamus, which acts as your body’s thermostat. When the hypothalamus decides you’re getting too warm, it fires off commands through the sympathetic nervous system, the same branch of your nervous system that handles fight-or-flight responses.
Here’s where sweat glands are unusual. Most sympathetic nerves use adrenaline as their chemical messenger, but the nerves connected to your sweat glands release acetylcholine instead. This chemical binds to receptors on the gland cells and triggers them to start producing sweat. Tiny muscle-like cells wrapped around each gland then contract to squeeze the fluid up through a narrow duct and onto your skin.
Two Types of Sweat Glands
Your body has two distinct kinds of sweat glands, and they serve very different purposes.
Eccrine glands are your primary cooling system. You have roughly 2 to 4 million of them spread across nearly your entire body, with the highest concentrations on your palms and soles (250 to 550 glands per square centimeter). Your face, trunk, and limbs have about two to five times fewer glands per square centimeter than your palms. Eccrine glands produce a thin, watery fluid that’s mostly water and salt, and they’re active from the day you’re born.
Apocrine glands are a different story. They’re larger, located mainly in your armpits, groin, scalp, and around the nipples, and they empty into hair follicles rather than directly onto the skin surface. Their secretion is thicker, containing lipids, proteins, sugars, and ammonia. Though present at birth, apocrine glands don’t start producing anything until puberty. In many animals, apocrine glands are the main thermoregulators, but in humans their function appears to be limited to scent production.
What Sweat Is Made Of
When your eccrine glands first produce sweat deep in the skin, it’s chemically similar to blood plasma, with sodium concentrations around 135 to 145 millimoles per liter. But as this fluid travels up through the gland’s duct, the cells lining the duct reabsorb most of the sodium and chloride back into your body. By the time sweat reaches the surface, its sodium concentration has dropped dramatically, typically falling between 10 and 70 millimoles per liter. The result is a fluid that’s mostly water with a relatively small amount of salt, plus traces of potassium, urea, and ammonia.
This reabsorption process is why sweat tastes salty but isn’t nearly as salty as blood. It’s also why sodium losses vary so much between people. Some individuals reabsorb salt more efficiently than others, which is part of the reason some people finish a hard workout with white salt stains on their clothing while others don’t.
How Evaporation Actually Cools You
The cooling doesn’t come from sweat sitting on your skin. It comes from sweat evaporating. When liquid water converts to vapor, it absorbs a significant amount of energy from whatever surface it’s on. For sweat evaporating directly from skin, each gram that evaporates removes about 2,430 joules of heat from your body. That’s roughly 580 calories of thermal energy per gram, which is remarkably efficient.
This is why humid environments feel so oppressive. When the air is already saturated with moisture, your sweat can’t evaporate efficiently. It drips off instead of vaporizing, and you lose the cooling benefit even though your glands are working overtime. Dry heat, by contrast, allows sweat to evaporate quickly, keeping your skin temperature lower even when the air is scorching.
Emotional Sweating Works Differently
Not all sweating is about temperature. The clammy palms you get before a job interview or during a scary movie are a separate process called emotional sweating. This type of sweating concentrates on the palms, soles, and sometimes the forehead and armpits, and it’s driven by different brain circuits than thermal sweating.
The amygdala, a brain structure involved in processing fear and emotion, plays a central role. Research on a patient with damage to both amygdalae showed that emotional sweating on the palms could not be triggered by any of the usual methods: deep breathing, mental math, exercise, or touch. Thermal sweating still worked normally. This tells us the brain keeps two separate sweating systems, one for cooling and one for emotional responses, even though they use many of the same glands.
Where Body Odor Comes From
Fresh sweat is essentially odorless, whether it comes from eccrine or apocrine glands. The smell develops when bacteria living on your skin break down the proteins and lipids in apocrine secretions. Species like Corynebacterium are particularly active in the armpit, where they convert compounds in apocrine sweat into a collection of pungent molecules.
The main culprits include a branched acid called E3M2H, which produces a strong, sharp smell, and a related compound with a rancid, cheesy odor that’s released when Corynebacterium bacteria cleave it from a larger molecule in sweat. A third compound contributes an onion-like smell. This bacterial process is why deodorants target bacteria rather than sweat itself, and why the armpits (loaded with apocrine glands and warm, moist conditions ideal for bacteria) are the epicenter of body odor rather than, say, your forehead, which sweats plenty but has only eccrine glands.
Eccrine sweat can contribute to odor too, mainly through ammonia. Some of this ammonia is transported directly from the bloodstream, while some is produced within the sweat glands themselves. The “gym clothes” smell that develops over time is largely ammonia and other eccrine byproducts breaking down in fabric.
How Much You Can Sweat
At rest in a comfortable environment, you barely notice your sweat output. During intense exercise in heat, however, your body can produce staggering volumes. Well-trained athletes commonly sweat 1 to 2 liters per hour, and in extreme conditions, rates above 2 liters per hour have been documented. Over the course of a full day in hot conditions, total sweat losses can reach 10 liters or more.
This volume creates a real hydration challenge. The simplest way to gauge your personal sweat rate is to weigh yourself before and after exercise. Each kilogram (about 2.2 pounds) lost represents roughly one liter of sweat that needs replacing. If you’re consistently losing weight during activity, you need to drink more. If you’re gaining weight, you’re overdrinking, which carries its own risks.
Sodium losses vary widely between individuals, with whole-body sweat sodium concentrations ranging from about 10 to 70 millimoles per liter. Someone sweating at 2 liters per hour at the high end of sodium concentration is losing substantially more salt than someone sweating the same volume at the low end. This individual variation is why blanket hydration advice doesn’t work equally well for everyone.
What Changes Your Sweat Response
Several factors influence how much and how quickly you sweat. Fitness level matters: regular aerobic training causes your sweat glands to activate at a lower core temperature and produce more sweat per gland, essentially making your cooling system more responsive. This is one reason well-conditioned athletes start sweating earlier during exercise than untrained people.
Heat acclimatization amplifies this effect further. Spending 10 to 14 days exercising in hot conditions increases your total sweat output while simultaneously improving your glands’ ability to reabsorb sodium, so you lose less salt per liter of sweat. Your body also expands its blood plasma volume, giving it more fluid to work with.
Age and sex play roles too. Apocrine glands activate at puberty, which is why body odor becomes an issue in adolescence. Older adults tend to have reduced sweat output per gland, making them more vulnerable to overheating. Men generally produce higher sweat volumes than women during the same exercise intensity, though much of this difference disappears when you account for body size and fitness.