Synthetic fibers are not porous and therefore don’t actually soak in sweat, which is a mixture of water and other chemicals from your body that can cause odors to develop. To be sure, that’s part of the appeal of these clothes—they keep you dry as you work out because they wick moisture away. But there’s a downside: “The moisture evaporates, leaving behind the odor-causing chemicals on the surface,” says Rico de Paz, the Consumer Reports chemist who tests laundry detergents. “The synthetic fibers act like a strong magnet for these odor-causing chemicals, making them difficult to wash out, and they build up over time.”