We’ve all been there: You’re running late to work, realize you’re taking a class later that day, and quickly shove the closest pair of workout clothes — clean or not — into your bag. When you’re sweating through those lunges and burpees that night, you remember that these were the same leggings you wore to power through last night’s HIIT session. Ew, right?Do they smell? Are other people noticing? Come to think of it, how often should you wash these things? And does it matter if you're wearing them to a cardio class rather than a more relaxed hour of yoga? Jeans can handle a few wears before needing some suds, but you're not sweating furiously in your jeans like you do in your leggings. Do your body (and your fellow workout partners) a favor and read up on how often you should really wash your leggings.Not to scare you, but there’s a lot the eye can’t see when it comes to what’s living in your workout clothes. Yes, um, living. As you wear your clothes, especially tight-fitting pieces like leggings, bacteria and yeast rub off on your body. Many of these bacteria stick to your clothes and can multiply the next day, even after your clothes have dried. If you slip back into these pieces for a second time, you're introducing the new bacteria and yeast to your skin, increasing the chance of infection or at least irritation. Plus, bacteria and infection aside, odor fatigue does exist. While your leggings may have passed your initial sniff test, it’s possible that you’ve just become used to your body’s odor. After you wear the garment a second time and sweat even just a little, the stench will return and likely be worse.There are some pieces you can get away with wearing a few times before washing, but you have to check the label. If your garment is made from cotton or a performance fabric, like water-resistant nylon, you can probably wait to wash it until after a few wears. However, keep in mind that odor-causing bacteria grows more on some fabrics than others, according to research from Ghent University in Belgium. The team studied bacteria growth on seven different textiles, including cotton and polyester. While cotton grew very few stink-causing germs, the sweat microbes got trapped in-between the synthetic fibers of polyester and created a place for the bacteria to flourish. So, if you want to spend less on loads of laundry and decrease the risk of nasty bacteria growth, stick to leggings made of cotton or nylon.The choice between washing or wearing again comes down to how much of a sweaty beast you are in the gym. If you’re pushing yourself to the limit and letting off some major steam — literally — toss those leggings in the hamper after one wear. But if you’re participating in a restorative yoga or Pilates session, you may be able to get by sporting those puppies a second or third time. (However, if you’re going, ahem, commando, treat your leggings like your undergarments and wash them after every wear.)Finally, no matter how many extra minutes it will take you to get to work, consider grabbing the clean pair of leggings from your dresser rather than the crumpled ones on your bedroom floor. Your skin, and your classmates, will thank you.