Your skin cracks and bleeds, your lips shrivel to prunes and your nose feels stuffy and inflamed. Not only is dry winter air uncomfortable, but experts say those skin changes weaken your body’s defenses against sickness-causing germs. It’s no wonder Americans buy roughly 10 million humidifiers each year. But depending on how frequently you clean your machine—and the type of water you fill it with—you may be inhaling harmful microorganisms. “While moisture can be a positive thing, it also poses problems,” says Janice Nolen, an indoor air specialist and assistant vice president for national policy at the American Lung Association.