There’s something lurking at the bottom of your makeup bag, and it’s not pretty.

According to Mayo Clinic Health System Dermatology physician assistant Mary Duh, old and expired cosmetics harbor dangerous amounts of bacteria. This not only directly affects the individual wearing the makeup, but it also can affect anyone they come in contact with.

“Makeup can be infected with bacteria after only one use. The bacterium builds up over time and can cause harm to a person’s skin, eyes, lips and overall health,” said Duh. “When makeup gets old, it starts to break down, and this can cause issues from irritation and inflammation to rashes, blisters, eye infections and pink eye.”

In rare cases, women have even been temporarily or permanently blinded by an eye cosmetic, according to the Food and Drug Administration.

Duh offers these rules for healthy cosmetic use:

- Look through your old makeup for chemical changes, including: a rancid odor, color change, changes in texture or consistency, and makeup becoming crusty/clumpy.

- Protect your skin from the sun. Most foundations have some level of SPF. Expired makeup is less effective for sun protection.

- Never share cosmetics. Cross contamination occurs when two or more people use the same application tools and makeup. The main danger with sharing makeup is passing an infection or virus.

- Wash your hands and cosmetic tools before applying eye cosmetics. The bacteria on your hands increases the chance for an eye infection.

- Don’t store cosmetics above 85 degrees. Cosmetics held for long periods in hot cars, for example, are more at risk of having the preservatives weaken.

- Date makeup when you buy it. Write the month and year on the package with a permanent marker.