It’s gym day, but you haven’t done your laundry yet. So you dig out a gym shirt from your hamper, give it a sniff and a quick once-over, and it’s good to go for round two, right?

Not exactly. Re-wearing sweaty old clothes can be a gamble, unless you’re okay with breakouts and wafting an odor strong enough to knock the guy off the treadmill next to you.

Problem is, the article of clothing might look fine—and even smell fine too, to an extent—but that doesn’t mean it’s going to stay that way.

Here’s why: The sweat our body produces is actually odorless, says Kay Obendorf, Ph.D, professor emeritus of fiber science at Cornell University. It’s the oily sebum and the bacteria that your sweat picks up from your skin that can cause the stink.

The more bacteria in your sweat, the greater the odor. And your workout provides the perfect warm, moist breeding ground for the bugs, since your body temperature increases as you sweat. That means a good dose of stinky sweat to seep into your clothing. 

And that’s especially true if you wear sports fabrics like spandex and polyester, which have been shown to collect the most odor-causing, micrococcus bacteria as compared to cotton, according to a study by Ghent University in Belgium.

If you don’t wash your clothing right away, the bacteria can continue to multiply even after your workout—meaning your shirt gets progressively stinkier, explains Obendorf. So the shirt that may have passed the sniff test before you left the house could be a little riper when you finally take it out of your gym bag later. Plus, if you sweat again in an already-stinky shirt, well, you’re just going to make it even worse.

What’s more, excess dirt and dried sweat on your clothing can react with the oxygen in the air, leaving you with yellowish stains that are difficult stains to remove.

And the same factors that make your clothing stink can be bad for your skin, too.

“People put back on their workout clothing that they sweat in, and as a result, the fabric becomes a little rougher, so they get worse chafing during physical activity, says Arielle Nagler, M.D., assistant professor of dermatology at NYU Langone Health.” That can lead to itchy skin rashes and skin irritation.

You might notice more breakouts, too. That’s because re-wearing the clothes provides the perfect environment for acne, Dr. Nagler says. Chalk it up to the same bacterial breeding ground that makes you stink.