A couple of weeks ago, we heard about a new makeup line and asked for samples to try (yes, it’s a rough life). We were told they had only one sample set—and it had been touched (code word: contaminated!) by other people. So there was no way I was trying that product—and not just because I’m a germ-phobe (which, admittedly, I am).

Allure has reported more than once on the sanitary pitfalls of used makeup. And now Jefferson Medical College in Philadelphia is confirming the dangers: In a study of tester makeup in department stores, specialty stores, and drugstores, they found that 100 percent of it was “tainted”…with E. coli, staph, and strep bacteria. Elizabeth Brooks, the biological sciences professor who conducted the study, left us with this little gem on The Huffington Post: “Whenever you see E. coli, you should think ‘E. coli equals feces.’ That means someone went to the bathroom, didn’t wash their hands, and then stuck their fingers in that moisturizer.” Lovely.

But besides grossing you out, contaminated makeup testers can actually make you sick, pass along oral herpes and pink eye, for example. So we wondered if it’s ever safe to use cosmetics that another person has touched. And it turns out it depends on where you intend to apply them, says cosmetic chemist Ron Robinson, founder of beautystat.com. Not surprisingly, you’re at the highest risk for infection if you use shared products on your lips or eyes. (Everywhere else, Robinson characterizes your risks as pretty low.) And while wiping off or sharpening products can minimize contamination, they aren’t exactly going to clean them either. “With sharpeners, you’d have to make sure that the sharpener itself doesn’t get contaminated, as that can also help to spread germs to a clean pencil,” Robinson says. And wiping a product off with a tissue only helps if you really work at it, removing a good chunk of the product in one strong swipe. (A knife works, too.) But better yet, ask for samples in individual packets—they’re the cleanest options around.