Here’s why gym clothes smell so rank—and how to freshen them up

Whether or not you’ve resolved to get into shape this January, Muscle Month is here to teach you a thing or two about stretching, contracting, lifting, tearing, gaining, and so much more.

There’s an episode of 30 Rock where Tina Fey’s character, Liz Lemon, lugs around her dirty, rancid gym bag on the subway, creating a protective stink bubble, so people don’t bother her. As a New Yorker who strongly values her personal space—paradoxical, I know—I’ve occasionally considered adopting this method.

But time after time, I remember just how foul that gym-clothes stink can get if you let it fester. And sometimes, especially with clothes made from synthetic fibers, unpleasant odors linger even after multiple washes. All of this begs the question: Why do gym clothes stink so badly? And what can we do to to freshen them up?

Why do my clothes stink?

Well, to understand why your clothes stink, you must first understand why you stink.

OK, fine. Why do I stink?

It all begins with sweat, of which humans have two kinds: eccrine and apocrine. Eccrine sweat is plentiful, watery, slightly salty, and it doesn’t really smell bad. You secrete it all over your body, unlike apocrine sweat, which only comes from glands in your pits and your groin. Apocrine sweat is the nasty stuff—a thick, oily fluid made up of fatty compounds.

But apocrine sweat still doesn’t stink on its own. Your skin’s microbiome—the cast of bacterial, fungal, and viral characters living on you right now—love to eat those fatty, oily compounds in apocrine sweat. And the molecular leftovers of that feeding frenzy are what stink. That’s how B.O. is made.

Your skin’s microbiome eats stuff to create other smells, too. Lucy Dunne, an apparel engineer at the University of Minnesota, says bacteria also eat dead skin cells. “They are specialized in what they consume,” she says. “That’s why your feet smell differently than your armpits. It’s also why your feet smell like cheese—it’s the same bacteria eating your dead skin that turns milk into cheese.”

Those B.O. molecules get on my clothes and make them smell bad, right?

Right! Those “odorous compounds,” as they call them in the biz, cling to the fibers in your clothes. And all of that watery eccrine sweat carries the compounds around your body and onto your clothes, exacerbating the stink.

Things get interesting, though, when it comes to the way odorous compounds cling to different kinds of fabrics. Let’s consider the two big players in athletic apparel: cotton and polyester. According to Rachel McQueen, a textile scientist at the University of Alberta who focuses on odor, cotton is hydrophilic, or water-loving. That means it soaks up a lot of water and sweat while you work out, which is why your cotton tee can feel kind of heavy if you’re perspiring. All that warmth and wetness makes a lovely habitat for B.O.-causing bacteria, which proceed to happily stink up you and your clothes. Cotton also runs into smelly problems if you leave it wet in your hamper or gym bag for a while. It retains water so well that bacteria and fungi like mold and mildew thrive, concocting a musty body odor hellstorm. (This was Liz Lemon’s secret.)

But as stinky and sweat-soaked as your cotton tank top can get, polyester can get do much worse. McQueen says polyester is hydrophobic, or water-hating. That means it’s really great at soaking up sweat and then quickly getting rid of it through evaporation. This is what companies mean when they say a garment can “wick sweat,” and it’s what makes dry-fit workout tops so magical.

The problem is, polyester is oleophilic, a.k.a. oil-loving. So while it wicks away plenty of watery eccrine sweat to keep you feeling dry, any of the oily apocrine sweat compounds and already-digested odorous compounds that pass through the clothing cling to polyester fibers for dear life. There, they take on a new, especially foul kind of scent.

“Body odor itself is different on polyester,” says McQueen. “People know it when they smell it. It’s not your body odor, it’s your body odor on polyester. It’s a repulsive smell. And the reason for it is likely because of the selective way polyester will retain certain types of odorants.” The kinds of odorous compounds that love to hang on to polyester combine to become especially pungent.

Skin microbiome scientist Chris Callewaert—whose colleagues affectionately call “Dr. Armpit”—has put this to the test in his lab. He and his research team at Ghent University in Belgium had people take an hour-long spin class wearing cotton, polyester, or mixed blend apparel. Then, by testing which species of bacteria clung to the fibers of which garment, they found that “synthetic fibers harbor more malodor-associated bacteria,” Callewaert says. “Cotton had more of the non-smelly bacteria. The difference in microbiome [on different fabrics] helps explain the vast difference in odors between synthetic and cotton fibers.”

Why is it so hard to get rid of that gross workout smell?

A lot of the difficulty of cleaning sweaty clothes has to do with just how tightly the smelly compounds grab on to polyester, but a lot also has to do with the way we do laundry. Washing loads in cold water, a rising trend that is thought to save energy and better preserve clothes’ quality, isn’t great for getting B.O. molecules to let go of their new fiber friends. High-efficiency washers, too, have problems here, because they use less water—that makes it more difficult to pry stink compounds off of polyester or kill odor-causing bacteria.

Plus, in another Dr. Armpit study, Callewaert found that washing clothes at 30 degrees Celsius (about 86 degrees Fahrenheit) with non-specialized detergent and on a regular wash cycle didn’t consistently kill or even remove the problem bacteria. It just mixed them up.

That’s why you might smell absolutely rank in the first few minutes of your warm-up when wearing these “freshly-washed” synthetic garments. Your non-smelly sweat moistens the outfit and is quickly evaporated by design—but when that happens, parts of some lingering odorous compounds also evaporate into the air, making you smell like you just did two hours of Bikram yoga when all you did was jog a lap and do a few lunges. As you continue your workout, new odorous compounds stick to your clothes, and the cycle continues.

So what can I do to fix the lingering synthetic stink?

The easiest solution might be to just go buy a specially-formulated laundry detergent. For gym stink, Wirecutter suggests Tide Plus Febreeze Sport. Wirecutter’s test found that the detergent got the scent of bacon grease out completely. And according to Mary Johnson, a principal scientist at Tide, it’s specially formulated to eat up and wash away the odors that love polyester so much, even in cold water.

“Forty-three percent of consumers in North America wash all or most of their laundry loads in cold water,” Johnson says, adding that the company’s sport detergents have special polymers (long chains of molecules) that grab onto dirt and odor-causing compounds, preventing them from sticking back on clothes in the wash. The sport detergents also have an enzyme that gobbles up some stink-compounds, even in cold water. Stink, Johnson says, “is a big issue. People are troubled by odors. They don’t want their kid to be the stinky one on the team, and in yoga class, people don’t want others to notice their smell. Without stink, people are willing to be more social and hang out with friends after yoga class, not go home because they’re stinky and embarrassed.”

There are also a lot of household tricks that might remedy your perpetual stink. Callewaert suggests drying clothes out in the sun, weather permitting. “Ultraviolet light kills bacteria and breaks down chemical compounds,” he says. Soaking garments in vinegar or even straight vodka might work too, since those liquids kill bacteria and “capture” odorous molecules like the polymer in Tide does. (Just make sure to wash them after the soak, so you don’t smell like you bathed in vinegar or vodka.) Bleach is also an effective option for reducing stink, but it obviously lifts color, too, so use it at your own risk.

As we learn more about the microbes at the root of human gym stench, what fabrics they prefer, and what their weaknesses are, scientists and companies will be able to formulate more effective detergents, and even workout wear that doesn’t stink. (The latter solution seems promising—some companies even have apparel with microbe-killing silver threads woven in.)

Until then, we wait, doing our best to use science to fight gym stench and avoid becoming a nose-wrinkling Liz Lemon on public transit.