A lot of workout clothing advertised as "antibacterial" or "anti-odor" contains microscopic particles, or nanoparticles, of silver. ... Sounds great, right? Except that silver nanoparticles aren't bound to that antibacterial clothing, and the teeny particles slough off every time you wash your sweaty clothes. And, according to a new study in the journal Environmental Science & Technology, they get absorbed by your skin when you sweat, exposing you to untested, and potentially unhealthy, levels of this metal.