The Science of Sweating It Out

Many cultures have ritualistic sweating ceremonies—if not currently, then at some point in their histories. Marbled hammams dot the Middle East; Native Americans have sweat lodges; Koreans frequent jjimjilbangs; Russians drink vodka in banyas; and the Finns have exported saunas across the Western world. For many people, there’s something both calming and cathartic about perspiring in vast quantities.

Mesmerizing towel work is the crux of the Aufguss ceremony, because it distributes thick gusts of hot wind around the sauna. Like the opposite of a winter windchill, which makes you feel colder, the steamy gale makes the sauna feel hotter. A good Aufguss master can work up enough wind over the course of the ten-minute ceremony so that your hair blows in the breeze, even as sweat floods down your skin.

The outpouring is not only sweat: Scientists have figured out that in a steamy sauna, between 30% and 55% of the liquid that rolls down your body is actually condensed water. Skin temperature in a sauna rises to a few degrees above normal (to about 109°F), but the rest of the space is typically about 175°F to 195°F, and the steam is above 210°F. Because your body is one of the coolest objects in the room, evaporated water condenses on it like kettle steam on a cold winter window.

At the end of an Aufguss ceremony, it’s not uncommon to float out on a wave of euphoria that is the product of both brain biochemistry and basic physiology. When your skin temperature spikes in a sauna, so does your pulse. After 10 to 15 minutes inside, your heart can be beating about 120 to 150 beats per second. For many people, this is the equivalent of mild exercise. Sauna sessions boost blood levels of epinephrine, growth hormone and endorphins—the latter of which are, incidentally, also hormones often held responsible for a runner’s high. With a sauna, you get the happiness without the mileage.

In trying to capitalize on humanity’s love of a good sweat, spa entrepreneurs often promise patrons a cornucopia of health benefits, many of which border on pseudoscience, if not rubbish. Going to the sauna is not a smart chemical detox strategy; in fact, sweating is not a detox strategy at all, and calling it one reveals a fundamental misunderstanding about how the human body works.

Sweat is sourced from the liquid parts of blood, minus the big stuff like red blood cells, platelets and immune cells. Any chemicals floating around in the circulatory system can emerge in sweat—the good stuff like glucose and hormones and the unwelcome stuff like heavy metals and urea. But if you actually detoxed by sweating, you would have to expel all the liquid in your blood to get rid of the nasty stuff. This would leave you completely dehydrated and likely dead. Instead, your kidneys filter your blood of problematic chemicals and dispatch them out in urine.

Sweat is responsible solely for cooling us down; anything that emerges with it is incidental, just along for the ride from the blood plasma to our skin’s surface. Which is why humans need to replenish our precious bodily fluids after an epic sweat, an issue investigated by Michael Zech of Dresden’s University of Technology.

Sitting in a sauna, gazing down at the perspiration pouring off his body, Dr. Zech wondered how long it would take for a sip of water to transit from his lips to his pores. Before his next sweat, he slipped a chemical tracer into his favorite sauna rehydration beverage. He drank a little over a pint of it, stripped down and stepped into the sauna. At regular intervals, he captured his sweat droplets in small glass vials.

At his lab, Dr. Zech checked the samples and found that it took less than 15 minutes for the tracer to transit through his stomach, be absorbed by the intestine, get filtered through the liver and kidney, enter his bloodstream, lap through his circulatory system to reach the veins in his skin, diffuse through his dermis toward the sweat glands and then escape out of the millions of pores on his skin. His question answered, Dr. Zech went back to sweating for fun instead of science.

Serious scientists have not neglected the sauna, but many questionable health claims about it come from dubious research done decades ago. For example, there’s an oft-repeated claim that going to the sauna boosts your immune system and helps prevent colds in winter. The evidence for this comes from a handful of studies from the 1970s and 1980s that even a proponent called “mostly retrospective and poorly controlled.”

Going to the sauna has been shown, however, to be excellent for your heart. This conclusion is based on a large study of Finnish men that has gone on since the mid-1980s. The men who went to the sauna regularly had a lower incidence of sudden cardiac death, fatal coronary heart disease, fatal cardiovascular disease and mortality altogether—which means, in short, that going to the sauna regularly could help extend one’s life.

Of course, for Finnish men, “going to the sauna regularly” means more than four times a week. Given the widespread cultural habit of regular sweating among Finns, the scientists used the men who went to the sauna once a week as a base reference and compared their health to the men who went more often.

Still, the finding is striking. Even though you are, in principle, relaxing in the sauna, your circulatory system is not. It’s on full-blast, trying to take hot blood from your interior and bring it to veins near your skin’s surface, where sweat evaporation cools the blood swooshing by. All that blood pumping through the circulatory system gives your heart a workout and provides knock-on biochemical effects that likely activate plaque-clearing and other benefits to your circulatory system.

But don’t cancel your gym membership yet. Sitting in a sauna doesn’t burn nearly as many calories as a workout, and you don’t build or strengthen muscle. Still, for people who can’t exercise, going to the sauna might be a good first step toward heart health. Catching an Aufguss ceremony can lend it a touch of artistry besides.

—Ms. Everts teaches journalism at Carleton University in Ottawa, Canada.This essay is adapted from her new book, “The Joy of Sweat: The Strange Science of Perspiration,” published by W.W. Norton.