Just How Sweaty Can Humans Get?

This summer, I, like so many other Americans, have forgotten what it means to be dry. The heat has grown so punishing, and the humidity so intense, that every movement sends my body into revolt. When I stand, I sweat. When I sit, I sweat. When I slice into a particularly dense head of cabbage, I sweat.

The way things are going, infinite moistness may be something many of us will have to get used to. This past July was the world’s hottest month in recorded history; off the coast of Florida, ocean temperatures hit triple digits, while in Arizona, the asphalt caused third-degree burns. As human-driven climate change continues to remodel the globe, heat waves are hitting harder, longer, and more frequently. The consequences of this crisis will, on a macroscopic scale, upend where and how humans can survive. It will also, in an everyday sense, make our lives very, very sweaty.

For most Americans, that’s probably unwelcome news. Our culture doesn’t exactly love sweat. Heavy perspirers are shunned on subways; BO is a hallmark of pubescent shame. History is splattered with examples of people trying to cloak sweat in perfumes, wash it away by bathing, or soak it up with wads of cotton or rubber crammed into their shirts, dresses, and hats. People without medical reason to do so have opted to paralyze their sweat-triggering nerves with Botox. Even Bruce Lee had the sweat glands in his armpits surgically removed, reportedly to avoid on-screen stains, several months before his death, in 1973.

But our scorn of sweat is entirely undeserved. Perspiration is vital to life. It cools our bodies and hydrates our skin; it manages our microbiome and emits chemical cues. Sweat is also a fundamental part of what makes people people. Without it, we wouldn’t be able to run long distances in high heat; we wouldn’t be able to power our big brains and bodies; we wouldn’t have colonized so much of the Earth. We may even have sweat to thank (or blame) for our skin’s nakedness, says Yana Kamberov, a sweat researcher at the University of Pennsylvania. Her team’s recent data, not yet published, suggest that as human skin evolved to produce more and more sweat glands, fur-making hair follicles disappeared to make room. Sweat is one of the “key milestones” in human evolution, argues Andrew Best, a biological anthropologist at the Massachusetts College of Liberal Arts—on par with big brains, walking upright, and the expression of culture through language and art.

Humans aren’t the only animals that sweat. Many mammals—among them, dogs, cats, and rats—perspire through the footpads on their paws; chimpanzees, macaques, and other primates are covered in sweat glands. Even horses and camels slick their skin in the heat. But only our bodies are studded with this many millions of teeny, tubular sweat glands—about 10 times the number found on other primates’ skin—that funnel water from our blood to pores that can squeeze out upwards of three, four, even five liters of sweat an hour when we need them to.

Our dampness isn’t cost free. Sweat is siphoned from the liquid components of blood—lose too much, and the risks of heat stroke and death shoot way up. Our lack of fur also makes us more vulnerable to bites and burns. That humans sweat anyway, then, Best told me, is a testament to perspiration’s cooling punch—it’s so much more efficient than merely panting or hiding from the heat. “If your objective is to be able to sustain a high metabolic rate in warm conditions, sweating is absolutely the best,” he said.

Read: You’re probably drinking enough water

And yet, in modern times, many of us just can’t seem to accept the realities of sweat. Americans are, for whatever reason, particularly preoccupied with quashing perspiration; in many other countries, “body odor is just normal,” says Angela Lamb, a dermatologist at Mount Sinai’s Icahn School of Medicine. But the bemoaning of BO has cultural roots that long predate the United States. “I’ve read discussions well back into antiquity where there are discussions about people whose armpits stink,” says Cari Casteel, a historian at the University of Buffalo. By the start of the 20th century, Americans had been primed by the recent popularization of germ theory to fear dirtiness—the perfect moment for marketers to “put the fear in women, and then men, that sweat was going to kibosh your plans for romance or a job,” says Sarah Everts, the author of The Joy of Sweat. These days, deodorants command an $8 billion market in the United States.

The Joy of Sweat: The Strange Science of Perspiration

By Sarah Everts

Our aversion to sweat doesn’t make much evolutionary sense. Unlike other excretions that elicit near-universal disgust, sweat doesn’t routinely transmit disease or pose other harm. But it does evoke physical labor and emotional stress—neither of which polite society is typically keen to see. And for some, maybe it signifies “losing control of your body in a particular way,” says Tina Lasisi, a biological anthropologist at the University of Michigan. Unlike urine or tears, sweat is the product of a body function that we can’t train ourselves to suppress or delay.

We also hate sweat because we think it smells bad. But it doesn’t, really. Nearly all of the sweat glands on human bodies are of the so-called eccrine variety, and produce slightly salty water with virtually no scent. A few spots, such as the armpits and groin, are freckled with apocrine glands that produce a waxy, fatty substance laced with pheromones—but even that has no inherent odor. The bacteria on our skin eat it, and their waste generates a stench, leaving sweat as the scapegoat. Our species’ approach to perspiration may even make us “less stinky than we could be,” Best told me. The expansion of eccrine glands across the body might not have only made our skin barer; it’s also thought to have evicted a whole legion of BO-producing apocrine glands.

As global temperatures climb, for many people—especially in parts of the world that lack access to air-conditioning—sweat will be an inevitability. “I suspect everyone is going to be quite drippy,” Kamberov told me. Exactly how slick each of us will be, though, is anyone’s guess. Experts have evidence that men sweat more than women, and that perspiration potential declines with age. But by and large, they can’t say with certainty why some people are inherently sweatier than others, and how much of it is inborn. Decades ago, a Japanese researcher hypothesized that perspiration potential might be calibrated in the first two or three years of life: Kids born into tropical climates, his analyses suggested, might activate more of their sweat glands than children in temperate regions. But Best’s recent attempts to replicate those findings have so far come up empty.

Read: The man who did not sweat

Perspiration does seem to be malleable within a lifetime. A couple of weeks into a new, intense exercise regimen, for instance, people will start to sweat more and earlier. Over longer periods of time, the body can also learn to tolerate high temperatures, and sweat less copiously but more efficiently. We sense these changes subtly as the seasons shift, says Laure Rittié, a physiologist at Glaxo-Smith Kline, who has studied sweat. It’s part of the reason a 75-degree day might feel toastier—and perhaps sweatier—in the spring than in the fall.

But we can’t simply sweat our way out of our climatic bind. There’s a ceiling to the temperatures we can tolerate; the body can leach only so much liquid out at once. Sweat’s cooling power also tends to falter in humid conditions, when liquid can’t evaporate as easily off of skin. Nor can researchers predict whether future generations might evolve to perspire much more than we do now. We no longer live under the intense conditions that pressured our ancestors to sprout more sweat glands—changes that also took place over many millions of years. It’s even possible that we’re fast approaching the maximal moistness a primate body can produce. “We don’t have a great idea about the outer limits of that plasticity,” Jason Kamilar, a biological anthropologist at the University of Massachusetts at Amherst, told me.

For now, people who are already on the sweatier side may find themselves better equipped to deal with a warming world, Rittié told me. At long last: Blessed are the moist, for they shall inherit the Earth.