Once those levels of bacteria get too high, dirty makeup brushes can lead to breakouts, irritation, and even infection if used over damaged skin (a popped zit, a scratch, etc.). To avoid that, Cook-Bolden recommends washing them at least once a week, and even more so if you can. "[Try] pairing it with another weekly activity so it becomes a habit," she says. And, it's also important to be mindful of where and how you store them. "If someone took a damp brush and threw it in a drawer, it’s the perfect place for microbials [to grow] because it’s dark and warm," says Ni'Kita Wilson, cosmetic chemist and CEO of Skinects