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Body + BrainBody & Brain

Cool down with the slick science of sweat

Under extreme conditions, a human can produce more than three gallons of sweat in a single day.

ByKatherine J. WuNOVA NextNOVA Next
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Shall I compare thee to a summer’s day? Thou art making me perspire. Image Credit: Tomwang112, iStock

In case you haven’t gotten the memo, summer is definitely in full swing. That means peak season for ice cream, sunblock, and a regular deluge of pit-drenching, clothes-soaking, decorum-destroying sweat.

But before you scoff at this oft-maligned bodily fluid, consider this: Our signature brand of sweat is part of what makes us humans unique—and there’s more to the salty stuff than you might think. (And don’t forget that perspiration is, apparently, 99 percent of genius.) So before you slather on that antiperspirant, chill out with some of the slick science behind nature’s OG thermostat.

I also promise to use the word “moist” no more than three times throughout the remainder of this article.

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Sweat’s not gross. You’re gross.

Let’s be real: Sweat isn’t glamorous. As Angela Lamb, a dermatologist at the Icahn School of Medicine at Mount Sinai, puts it, “sweat is kind of the urine of the skin.”

That doesn’t make perspiration useless. The primary function of this salty stuff, which is secreted by the 2 to 4 million sweat glands that stud our skin, is to keep the body from overheating. As sweat evaporates, it absorbs heat from the body, and what was once warm and moist (strike one) becomes relatively dry and cool.

All that vaporizing magic (er, science) happens thanks to the sweat produced by eccrine glands, which speckle most of the body, but are especially abundant on our palms and foot soles. Plenty of triggers can send eccrine glands into overdrive, including heat waves, intense exercise, and strong emotions like fear or anxiety. Spurred into action, the glands pump out a dilute mixture of water and a wee bit of salt.

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In its purest form, sweat is actually odorless. But when the stuff from sweat glands—especially apocrine sweat glands—is processed by the bacteria living on your skin, things can get rank. Image Credit: Giulio Fornasar, iStock

While eccrine glands tend to hog the spotlight, there’s another culprit behind the body’s slick sheen: apocrine glands. These are mostly restricted to the armpits and the genital region, and secrete an oily, protein-rich version of sweat in times of stress or sexual excitement. (Fun fact: Mammary glands and the glands that produce earwax—more on that later—are modified apocrine glands.)

No matter where it hails from, though, sweat in its purest form is actually odorless, says Sarina Elmariah, a dermatologist at Massachusetts General Hospital. Until, that is, it hits the surface of skin. There, bacteria nosh on the unscented secretions—especially the rich, fatty apocrine formulation—and churn out that infamously gnarly stank as a byproduct.

Why you won’t find a dog wiping its brow

Humans aren’t the only animals that squeeze juice out of their bodies to keep cool—and there are actually a lot of mammals with sweat glands. But most of our furry brethren rely instead on the evaporative action of a surface that churns out liquid all the time: the moist (strike two) lining of the airway. That’s what dogs are doing when you see them panting, and it’s thought that the early ancestors of humans might have done it, too.

It’s not totally clear why humans swapped out panting for perspiring, or even if one tactic is necessarily superior to the other, says Jason Kamilar, an anthropologist studying primate evolution at the University of Massachusetts Amherst. But the transition probably occurred when our ancestors migrated out of sheltered, humid rainforests into habitats with more sun exposure and drier air. Then, as early humans shed much of their body hair, rapid cooling via sweat likely became a boon during long daytime hunts, says Yana Kamberov, a geneticist studying the evolution of sweat at the University of Pennsylvania.

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You might pee like a racehorse, but you don't exactly sweat like one. Most of the sweat glands on the equine body are apocrine. Image Credit: vesilvio, iStock

Other animals caught on to the perks of perspiration, too. It’s not uncommon to glimpse a horse drenched after a hard gallop (but that’s actually apocrine sweat glistening on their hides). Some mammals with soft paws, like cats and dogs, have eccrine glands in their foot pads, but that sweat is mostly for surface tension, which provides traction, Kamberov says. The feathertail glider (Acrobates pygmaeus), a type of flying squirrel, actually pumps so much sweat out of its feet that it can scurry up vertical panes of glass.

(You may have seen that hippopotamuses’ “sweat” is red. It’s true that they produce a rust-hued substance called blood sweat, but technically, it’s neither blood nor sweat. Its function does overlap with perspiration proper, though, and also works as a sunscreen.)

Don’t even try sweating the small stuff.

Maybe you’re already a cool cat (with some super-steady foot pads) and you don’t need sweat to chill out. That’s fine—it’s good for other things, too: Sprinkled in with the salts in sweat is a whole suite of proteins with antimicrobial, or microbe-fighting, properties that may play a role in bolstering immunity.

But there are limits to perspiration’s power. The body can’t really “sweat out toxins,” for instance. Heavy metals and chemicals like bisphenol A (BPA), which is found in plastics, can technically hitch a ride out of the body via the sweat glands, but they do so at concentrations too low to make much of a difference, Lamb says. Most of the body’s detoxifying power lies in the liver, gut, kidneys—and if you ingest a toxin, you’d be wise to choose a hospital over a steam room.

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A feathertail glider (Acrobates pygmaeus) in Queensland, Australia. Sweat glands on these flying squirrels' feet help them scale vertical structures. Image Credit: Dash Huang, flickr

Sweating also can’t replace a workout, and simply secreting stuff from these glands won’t be enough to “trick” a body into thinking it’s exercising. In spite of what some sauna operators might want you to think, perspiration doesn’t really confer any health benefits on its own, Lamb says.

Sweaters: They’re not one-size-fits-all.

Some people sweat more, others sweat less, and we only kind of understand why.

There’s probably at least some genetic component, Lamb says, because sweatiness often runs in families. In extreme cases, certain mutations can even lead to medical conditions in which people chronically sweat too much or too little.

But the genetics of sweat isn’t all doom and gloom. A delightful series of studies has uncovered a curious connection between earwax type and sweat odor. As it turns out, the same genetic mutation that causes people to make wet, sticky, brown earwax also bequeaths unto them hyperactive apocrine sweat glands—the more pungent perspirers (again, thanks to those pesky skin microbes). This mutation is particularly prevalent in populations with European ancestry, while those of East Asian descent are more likely to have dry, crumbly, grayish earwax and more apathetic apocrine glands.

Of course, your slick factor is also a product of certain lifestyle factors. Body size, for instance, appears to play a role, and weight gain may increase your propensity to perspire. The reason comes down to math: Higher mass decreases the body’s surface area to volume ratio, making it harder for internal heat to escape.

And as anyone who’s overindulged in spicy food can attest, sweat can reflect diet. Capsaicin—the kicky substance in chili peppers—tends to cue profuse perspiration. The things you nosh on can also change the composition of your sweat, resulting in distinct odors: Consider the savory funk of meat sweats, or the acrid tang that tends to follow a meal heavy in alliums like garlic or onions. Weirdest of all may be the herb fenugreek, which can imbue bodily secretions with the cloying aroma of maple syrup.

But when it comes to characterizing the sweat spectrum on a global scale, there are far more questions than answers. Kamilar and his student Andrew Best are currently attempting to catalogue some of this diversity by testing a decades-old theory that proposes that childhood conditions can dictate what percentage of an individual’s sweat glands stay active into adulthood—or retire young. One part of this idea is that people who spend their first two years of life in hot, dry conditions with limited access to artificial cooling might grow up sweatier than their peers whose parents kept the AC on high.

One person’s skin urine is another person’s… beverage?

So, no, we can’t use sweat as a replacement for sunscreen, or purge ourselves of toxins through our armpits. All the same, it seems the future of sweat is bright and shiny.

This brackish bodily fluid can already be used as a diagnostic for cystic fibrosis, a condition that affects how the body secretes mucus, sweat, and other substances. But researchers like Jason Heikenfeld, an electrical engineer at the University of Cincinnati, think perspiration can be used for much more.

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A combination of genetic and lifestyle factors affect how much you sweat. Image Credit: tommaso79, iStock

It turns out eccrine sweat is a surprisingly accurate and noninvasive diagnostic proxy for blood, Heikenfeld says. The space just beneath the surface of our skin is chock full of vessels, and what’s secreted onto the surface of our skin tends to tug many of the chemicals present in blood along with it—including hormones and, potentially, certain types of drugs.

A system that analyzes the composition of sweat could prove useful for monitoring dehydration in athletes, or tracking levels of stress hormones in those suffering from chronic anxiety, Heikenfeld says. If sweat provides a comprehensive enough picture of a person’s health status, it may even someday play a role in helping doctors tailor treatments to individual patients.

Heikenfeld’s group isn’t the only one developing sweat sensors. Still other perspiration pioneers are looking into powering small electronic devices with sweat, while others have managed to laboriously purify it into drinkable water (though the latter effort was mostly intended to raise awareness about the 780 million people worldwide who lack access to clean drinking water).

One of the biggest barriers to all these applications, of course, is availability. Sweat production can be copious, but most people don’t secrete on command. Heikenfeld’s work is addressing this as well, and he and his team now have a way to stimulate small patches of skin to produce just enough sweat for a sensitive chemical analysis.

Stay moist (!), folks. You might just be cooler for it.

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