When you sweat, your body runs its built-in cooling system. A region of your brain detects rising body temperature and sends chemical signals to millions of sweat glands in your skin, which push a mostly-water fluid to the surface. As that fluid evaporates, it pulls heat away from your body. But cooling is only part of the story. Sweating also affects your electrolyte balance, your skin’s defenses, and even how you smell.
How Your Brain Triggers Sweating
The process starts in the hypothalamus, a small structure deep in your brain that acts as your internal thermostat. When your core temperature rises, whether from exercise, hot weather, or a fever, the hypothalamus recognizes the shift and fires off chemical messengers to your sweat glands. This happens automatically. You don’t decide to sweat any more than you decide to digest food.
You have two types of sweat glands. Eccrine glands are spread across nearly your entire body, with the highest concentration on your palms, the soles of your feet, and your forehead. They become active in infancy and produce a thin, watery sweat. Apocrine glands are clustered in your armpits and groin. They activate at puberty and produce a thicker, oilier fluid that empties into hair follicles rather than directly onto the skin surface. Both types respond to heat, but apocrine glands are also strongly triggered by stress and emotional arousal.
What Sweat Is Made Of
Eccrine sweat is about 99% water. The remaining 1% is mostly sodium and chloride, the same two components in table salt. Smaller amounts of potassium, calcium, magnesium, zinc, copper, and iron are also present, along with traces of proteins, amino acids, and vitamins. Very small quantities of heavy metals and environmental chemicals like BPA can show up as well, though the amounts are negligible compared to what your kidneys filter out daily.
Apocrine sweat has a different makeup. It contains lipids and proteins that eccrine sweat lacks, which is why it feels oilier. This composition is also what makes it a feast for skin bacteria, and the reason your armpits smell while your forearms typically don’t.
How Evaporation Actually Cools You
Sweat sitting on your skin doesn’t cool you. Evaporation does. When liquid water turns to vapor, it absorbs a significant amount of energy from the surface it’s leaving. At normal skin temperature, evaporating one gram of sweat removes about 580 calories of heat energy from your body. (These are physics calories, much smaller than food calories, but the total adds up fast when millions of glands are active.)
This is why humid days feel so miserable. When the air is already saturated with moisture, your sweat can’t evaporate efficiently. It just drips off, taking very little heat with it. Dry heat, by contrast, lets sweat evaporate quickly, which is why 95°F in the desert can feel more manageable than 85°F in a tropical climate. A healthy average-sized person produces roughly 500 milliliters of sweat per hour during exercise, a little over two cups. Elite athletes and people working in extreme heat can lose considerably more.
What Happens to Your Electrolytes
Every drop of sweat carries salt with it. Sodium losses typically range from 25 to 75 milliequivalents per liter, which varies widely from person to person based on genetics, fitness level, and how acclimatized you are to the heat. If you’re sweating for a short workout, this loss is trivial. During prolonged exercise lasting several hours, the math changes.
The risk most people worry about is dehydration, and that’s a legitimate concern. But drinking too much plain water without replacing salt creates a different, potentially more dangerous problem called hyponatremia, where sodium levels in your blood become dangerously diluted. Since sweat always contains less salt than your blood, losing sweat without drinking anything would actually concentrate your blood sodium. It’s the combination of heavy sweating and aggressive water intake without electrolytes that drives sodium levels down. This is why sports drinks exist and why endurance athletes pay close attention to both fluid and salt replacement.
Where Body Odor Really Comes From
Fresh sweat is nearly odorless. The smell people associate with sweating is produced by bacteria on your skin that feed on the proteins and lipids in apocrine sweat. Different bacterial species produce different odors. Certain Corynebacterium species break down sweat compounds into fatty acids that smell goat-like or cumin-like. Staphylococcus hominis, common in the armpit, produces a sulfur-containing compound responsible for that rotten-onion quality some people notice. And if your feet smell, that’s likely a different bacterium breaking down the amino acid leucine in your foot sweat into a cheesy-smelling compound called isovaleric acid.
This is why body odor is so individual. Everyone hosts a slightly different community of skin bacteria, which means the same amount of sweat can produce vastly different smells from person to person. It’s also why antiperspirants (which block sweat) and deodorants (which target bacteria or mask odor) take two completely different approaches to the same problem.
Sweat Protects Your Skin
Beyond cooling, sweat plays an active role in skin defense. Your eccrine glands continuously secrete an antimicrobial peptide called dermcidin, which kills bacteria on the skin’s surface without needing any trigger from injury or inflammation. In lab studies, dermcidin destroyed 100% of Staphylococcus aureus and 97% of E. coli at tested concentrations. It also showed significant activity against the bacterium responsible for inflammatory acne.
People with inflammatory acne have been found to produce lower concentrations of dermcidin in their sweat, which may allow acne-causing bacteria to proliferate more easily in hair follicles. This doesn’t mean sweating cures acne, but it does mean that your sweat is doing quiet antimicrobial work you probably never think about. Sweat also contributes to your skin’s slightly acidic surface layer, sometimes called the acid mantle, which serves as a chemical barrier against harmful microbes.
Does Sweating Detoxify Your Body?
The short answer is: barely. Your sweat does contain trace amounts of heavy metals, urea, and environmental chemicals, but the quantities are tiny compared to what your liver and kidneys process every hour. Your kidneys filter roughly 180 liters of fluid per day. Your sweat output, even on a hot day, is a fraction of that.
There is some historical and clinical evidence that sweating can modestly reduce blood urea levels. Dialysis patients in warmer climates have shown slightly lower blood urea in summer months compared to winter, with an average difference of about 16 mg/dL, suggesting that perspiration contributes a small assist. But this is supplemental at best. For anyone with normally functioning kidneys, sweat’s contribution to detoxification is negligible. The marketing behind “sweat out your toxins” programs dramatically overstates what the science supports.
When Sweating Goes Wrong
Some people sweat excessively under conditions that wouldn’t trigger sweating in most others: sitting at a desk, shaking someone’s hand, or standing in a cool room. This is hyperhidrosis, and it can be focal (affecting just the palms, feet, or underarms) or generalized across the body. Focal hyperhidrosis usually runs in families and often starts in childhood or adolescence. It’s not dangerous, but it can significantly affect quality of life, from slippery handshakes to soaked clothing.
On the opposite end, hypohidrosis is the inability to sweat enough. Some people are born with genetic mutations that impair or completely eliminate sweating. Others develop reduced sweating from nerve damage, certain medications, or skin conditions that block sweat glands. Hypohidrosis is the more medically concerning of the two because without adequate sweating, your body loses its primary cooling mechanism. People who can’t sweat properly are at high risk for heat exhaustion and heatstroke, particularly during exercise or in hot environments. The main warning sign is noticing that you stay dry in situations where everyone around you is visibly sweating.