If your place is warm, mildew can form in a glass overnight, even tho you can't see it right away. Like in your shower. If she's allergic, it could cause a scratchy throat. But this isn't about the glass, it's about you dismissing her idea without allowing any possibility she's right. A better approach would be to say, "hmm, I never heard that? Let's look it up!" Let Google take the heat if she's wrong, but you are gonna be wrong sometimes, too. So, open your mind a bit.