Sweat is a watery fluid produced by glands in your skin, primarily to cool your body down. It’s mostly water with small amounts of salt, potassium, and other minerals dissolved in it. Your body can produce anywhere from about half a liter of sweat per day while resting in a cool room to 10 liters per day during heavy exercise in the heat. Sweating is one of the most important ways humans regulate body temperature, but it also plays roles in immune defense and even communication.
How Your Body Decides to Sweat
The process starts in a small region of the brain that acts as your internal thermostat. Temperature sensors throughout your skin and inside your body constantly send readings to this control center. When your core temperature starts rising, whether from exercise, a hot environment, or fever, the brain sends signals through the nervous system to activate sweat glands across your skin.
Those nerve signals travel along a specific branch of the nervous system (the same one that controls your heart rate and digestion). The glands respond by pushing fluid out onto the skin’s surface. As that thin layer of moisture evaporates, it pulls heat away from the skin, cooling the blood flowing beneath it. This evaporative cooling is remarkably effective, which is why humans can sustain physical activity in heat that would overwhelm most other mammals.
Two Types of Sweat Glands
Your body has two distinct kinds of sweat glands, and they serve different purposes.
Eccrine glands are the workhorses of temperature control. They’re spread across nearly your entire body, with the highest concentrations on your palms and the soles of your feet. These glands open directly onto the skin’s surface and produce the thin, watery sweat you’re most familiar with. They’re active from birth and respond to both heat and emotional stress.
Apocrine glands are concentrated in specific areas: your armpits, groin, ear canals, and around the nipples. They open onto hair follicles rather than directly onto the skin, and they don’t kick in until puberty. Their secretion is thicker and milkier than eccrine sweat. In many other mammals, apocrine glands are the primary cooling system, but in humans their exact function remains unclear. The leading theory is that they contribute to body odor and possibly chemical signaling between people.
What Sweat Is Made Of
Fresh sweat starts as a fluid that closely resembles blood plasma, minus the proteins and blood cells. As it travels through the gland’s duct toward the skin surface, the duct lining reabsorbs much of the sodium and potassium, leaving behind a fluid that’s overwhelmingly water.
The minerals that remain are mostly sodium, chloride (together, essentially salt), potassium, calcium, and trace amounts of magnesium. Sodium and chloride are the dominant electrolytes, and their concentrations actually increase the longer you exercise. In studies measuring sweat composition during workouts, sodium levels nearly doubled between the 15-minute and 65-minute marks, while potassium levels dropped by about a third over the same period. This is why prolonged exercise in heat can deplete your salt stores faster than you might expect.
Emotional Sweat vs. Heat Sweat
Not all sweating is about temperature. Stress, anxiety, fear, pain, and sexual arousal all trigger sweating through a different pathway. You’ve probably noticed that nervous sweat tends to show up in specific spots: your palms, the soles of your feet, your armpits, and your forehead. That’s because emotional triggers activate both eccrine and apocrine glands, while heat-driven sweating is almost entirely an eccrine response.
The difference matters beyond just location. Emotional sweating can start almost instantly, before your body temperature changes at all. It’s driven by adrenaline and related stress hormones rather than by temperature signals from the brain’s thermostat. This is why your palms get clammy before a job interview even in an air-conditioned room.
Why Sweat Smells
Fresh sweat is essentially odorless. Body odor develops when bacteria living on your skin break down compounds in sweat, particularly the thicker secretions from apocrine glands in your armpits. Specific species of Corynebacterium that naturally inhabit armpit skin are among the primary culprits. These bacteria use specialized enzymes to break apart molecules in apocrine sweat, releasing volatile fatty acids that produce the characteristic smell.
This is why body odor is concentrated in the armpits and groin rather than across your whole body. It’s also why freshly washed skin doesn’t smell even when you start sweating, since it takes time for bacterial populations to rebuild and begin processing the sweat.
Sweat as Immune Defense
Beyond cooling, sweat plays a surprisingly active role in protecting your skin from infection. Eccrine glands secrete a natural antibiotic peptide that remains effective across the wide range of salt concentrations and pH levels found in human sweat. This substance helps regulate the microbial community on your skin and can limit infection by harmful bacteria in the first hours after they land on the surface. It’s one layer of a broader immune system built into the skin itself.
How Much You Can Sweat
Sweat output varies enormously depending on fitness, heat exposure, and how acclimatized you are. At rest in comfortable temperatures, your skin loses about 500 milliliters (roughly two cups) of moisture per day, much of it as “insensible” evaporation you never notice. During intense exercise in the heat, sweat rates can reach 3 to 4 liters per hour. A healthy but unacclimatized person tops out at about 1.5 liters per hour, while highly trained, heat-adapted individuals can sustain 2 to 3 liters per hour.
That adaptation is real and measurable. People who regularly train in hot conditions develop the ability to start sweating sooner, produce more sweat, and lose fewer electrolytes per liter of sweat. The body essentially becomes more efficient at its own cooling system over the course of one to two weeks of repeated heat exposure.
When Sweating Becomes a Medical Issue
Some people sweat far beyond what temperature or activity would explain. This condition, called hyperhidrosis, affects roughly 3% of the U.S. population. It’s diagnosed when excessive sweating persists for six months or more, occurs symmetrically on both sides of the body (typically palms, soles, underarms, or face), and interferes with daily life. A hallmark of the primary form is that sweating decreases or stops during sleep, suggesting it’s tied to the waking nervous system rather than an underlying disease.
On the opposite end, some people sweat too little. This can result from nerve damage, certain medications, skin conditions, or genetic factors. Insufficient sweating is potentially dangerous because it removes the body’s primary cooling mechanism, raising the risk of heat-related illness during exercise or high temperatures.