You’re not imagining that your synthetic gym shorts emit an unholy reek after a hard workout. The stench is real, and there’s a reason for it, a new study in Applied and Environmental Microbiology shows. Scientists asked volunteers to sniff both cotton and polyester T-shirts that healthy people had worn in an intense, hourlong spin class, but not until the sweaty shirts had “incubated” for 28 hours. (Letting the shirts stew gave bacteria time to break the fatty acids found in sweat down to the smaller, airborne molecules that make us stink.) After determining that the polyester shirts did, indeed, smell worse than the cotton ones, the researchers took a microscopic look at the fabric to determine why. They found that the bacteria micrococci—the main culprit in turning sweat rank—grows much better on polyester than on cotton, likely because moisture is not absorbed, but rather stored, between synthetic fibers. The team is now trying to determine precisely why polyester is such a refuge for micrococci. In the meantime, they recommend that gymgoers worried about their offensive BO switch to cotton.