A recent study by University of Arizona microbiologist Charles Gerba shows that it’s none other than my favorite spot: The break room. About 75 percent of break room faucet handles contained a high level of microorganisms—such as mold, yeast, bacteria—and just under 48 percent of office microwave handles suffered the same grimy fate. (Only about a quarter of keyboards, by comparison, are as germ-ridden.) So if you’re trying to stay healthy this back-to-school season (good luck!), treat your hands to a few squirts of hand sanitizer after you wash out your coffee mug or zap that popcorn.

And don’t forget to check out 14 more surfaces where germs lurk.