Sweating used to be taboo. (Remember when women used to assert that they didn’t sweat—they glowed?) But peruse any fashion or beauty site today, and you’ll see that sweat is in style—at least as long as you do it at the gym. From infrared saunas to hot yoga, towel-soaking activities are touted not only as relaxation tools, but also as ways to stay healthy by flushing toxins out of our bodies.

Too bad you can’t sweat away toxins any more than you can sweat actual bullets. We sweat primarily to cool ourselves, not to excrete waste or toxic substances. That’s what our kidneys and liver are for.

Of course, there’s often a grain of truth at the heart of a myth, and sweat detox is no exception. Though sweat is made up mostly of water, it can contain trace amounts of hundreds of substances, including some that are toxic.

“You always have to ask how much,” says chemist Joe Schwarcz. Plenty of substances are found at very low levels in sweat. But just because something is present doesn’t mean there’s enough of it to create a health risk.

Schwarcz directs McGill University’s Office for Science and Society, which debunks science myths; the group is inundated with questions about medical scams and quackery, including many that promise to “detoxify” the body. “Whenever you see this concept of ‘detox’ in the popular media, you’re usually looking at silliness,” Schwarcz says.

Is sweat even toxic?

Most “detox” products and diet plans are pretty vague about what toxins exactly we need to rid ourselves of. Pesticides? Metals? Whatever makes up processed cheese? Whatever they are, toxins sound nasty and we want them out. And because we can’t see them, it’s pretty easy to convince people that fasting or drinking green stuff or sweating a lot will do it.

But when you consider how toxic substances actually accumulate inside us and the body’s means of getting rid of them, you’ll realize that most detox plans make about as much sense for your health as a tapeworm diet.

(You can’t detox your uterus either—another popular myth.)

To begin with, the type of sweat we produce when we’re hot or exercising comes from eccrine glands. You have about three million of them all over your body. Because we make this sweat to cool ourselves, it makes sense that it’s more than 99 percent water. Dissolved in that water are small amounts of minerals like sodium and calcium, plus small amounts of various proteins, lactic acid, and urea.

Urea—which is produced in the liver by the breakdown of proteins in food—is a waste product, so it’s true that sweating flushes a teensy bit from the body. But the process plays only a minor role in our bodies’ waste removal systems; for the most part, your kidneys handle the heavy lifting, and the vast majority of urea leaving the body is in urine. Only if your kidneys are failing does sweat become an important way for your body to get rid of that particular waste product.

As for man-made pollutants, the levels found in sweat are so low that they’re essentially meaningless, says Pascal Imbeault, who led a 2018 study calculating these levels. As an exercise physiologist at the University of Ottawa in Canada, Imbeault studies fat—specifically, what happens to the pollutants stored in body fat. Known as persistent organic pollutants, these include pesticides, flame retardants, and polychlorinated biphenyls, or PCBs, which have been banned but are still found in the environment.

These are the kinds of chemicals that many people think of as “toxins” in our food and environment. But as Imbeault notes, even that term isn’t quite right: Toxins are harmful substances made by living organisms like plants, animals, or bacteria; manmade substances that are toxic are called toxicants.

Whatever you call them, one reason you won’t find high levels of persistent organic pollutants in sweat comes down to basic chemistry: Sweat is made mostly of water, and these fat-loving substances don’t dissolve well in it. (That’s why we say people who don’t get along are “like oil and water”; they don’t mix.)

(Is tap water safe to drink? Here’s what you need to know.)

Imbeault and his colleagues found that a typical person doing 45 minutes of high-intensity exercise could sweat a total of two liters in a day—normal background perspiration included—and all that sweat would contain less than one-tenth of a nanogram of these pollutants.

To put that in perspective: “The amount in sweat is .02 percent of what you ingest every day on a typical diet,” Imbeault says. If you really pushed it on your exercise regime, you might release up to .04 percent of your average daily intake of pollutants. In other words, there’s no way you could sweat enough to get rid of even one percent of the tiny amount you’ll eat in your food that day.

Keep in mind that the levels of pesticides and other pollutants in most people’s bodies are also extremely low to begin with. It’s a testament to analytical chemistry that we can detect a compound down to parts per trillion, Schwarcz says. But that doesn’t mean it’s harming you at that concentration, or that incrementally decreasing it will have any health effect.

So what can you do about these pollutants?

But back to that grain of truth: Small amounts of heavy metals like lead and bisphenol A (BPA) contained in plastics do make their way into sweat, because these pollutants dissolve more readily in water than the fat-loving ones do. But again, the amount removed by sweating is relatively low—and because far more BPA leaves the body through urine than through sweat, you’re more likely to rid yourself of that chemical on the toilet than in the sauna.

(What BPAs do to your body—and how to limit your exposure.)

Not that you need to start gulping down gallons of water, either. Instead, the best way to reduce your BPA exposure is to avoid eating and drinking out of containers made with it, according to the very practical researchers at the National Institute of Environmental Health Sciences.

Likewise, if you’re concerned about pesticides and other pollutants in your food, you’re better off avoiding them in the first place than trying to sweat them out later. And to help filter out whatever you do take in, you can keep your kidneys healthy—by avoiding smoking, high blood pressure, and heavy use of nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen. Dehydration also stresses the kidneys, so ironically, sweating heavily without drinking enough water could harm your body’s ability to cleanse itself.

As much as we’d all love a quick fix, a boring old healthy lifestyle is still the best we can do. All the same, none of this has stopped the growing sweat-detox industry, which now also touts infrared saunas that use light instead of electric heaters or steam to create heat.

Sauna use has been correlated with better cardiovascular health—possibly because when we’re hot, our heart beats faster, as in moderate exercise. But no credible science demonstrates that saunas, infrared or otherwise, can cleanse us of toxins.

Sweat therapy can also be dangerous if taken too far. For one thing, most people should not stay in a sauna for more than 10 minutes at a time, according to the American College of Sports Medicine.

(Here’s what extreme heat does to the body.)

And like most things, just because a little bit is good doesn’t mean more is better. In 2011, a self-help guru in Arizona was convicted on three counts of negligent homicide when three people died after a two-hour sweat-lodge ceremony. The same year, a 35-year-old woman in Quebec died after a detoxification spa treatment plastered her with mud, then wrapped her in plastic and put a cardboard box over her head. She lay under blankets for nine hours, sweating. Hours after the treatment, she was dead from extreme overheating.

“It’s the old story of wanting to provide a simple solution to a complex problem,” Schwarcz says. “Hope is so precious, but some people use hope for selling crazy stuff to people who are vulnerable.”

Indeed, a huge wellness industry makes it almost impossible to sort through all the claims for diets, pills, and exercise regimes that promise to clean you out, firm you up, or make your skin glow. Who doesn’t want to find that “one weird trick” for losing weight, or not losing your hair, or whatever the clickbait is promising this time?

It’s too bad there’s no one weird trick for blasting away environmental pollutants. But you can still feel good about going to the gym and working up a sweat—for the exercise. The guy using the treadmill after you might not love your sweat. But your heart will thank you.