This symbol can keep you safe from expired cosmetics, plus the makeup technique doctors say to avoid

LOS ANGELES (KABC) -- Makeup trends come and go, but serious eye infections can stick around if you're not careful.

Doctors say expired cosmetics are a huge concern. Many consumers probably don't know of a little known symbol located right on the box that can help you out. Eyecare experts discuss the potential dangers in your makeup bag.

At 16, Kaylie Hiraga of Cypress follows all the latest trends on social media including how to apply make up.

While she knows each item has an expiration date, she didn't know about a tiny cream jar symbol on every box that gives you the shelf life.

"That tells you how long that makeup is good for after you open it," explained Talin Amadian, a doctor of optometry with Woodley Optometry. She said expired makeup is a top reason why many of her patients come in with nasty eye infections.

"If your mascara goes from 3 to 6 months, I would err on the side of caution. Don't keep it for more than 3 months," Amadian said.

She said every time you use mascara or lip gloss and put the applicator back, you're participating in a science experiment.

"That just kind of creates this petri dish for bacterial overgrowth," she said.

But it's not just poor hygiene creating problems. Amadian said even the preservatives in makeup that guard against bacteria can be harmful.

"A very common one is something called BAK, benzalkonium chloride," Amadian said.

BAK is common in mascara and eye makeup removers, and it can be toxic to corneal tissue. Another ingredient is tar.

"Derivatives of coal tar, believe it or not, are included in a lot of black eyeliners, mascaras, etc. Carbon black is a really big one." she said.

Look for these ingredients on any products that go near your eyes, especially eyeliner pencils.

"Same with any liquid eyeliners," Amadian said.

And no matter what social media influencers tell you, experts say avoid a technique called tight lining. That's when you use your eyeliner on the inner part of your bottom lid. All eye care practitioners agree that's a bad idea.

"Anything that you put there is going to seep into your gland, and that can lead to long-term repercussions," she said.

Amadian added that it's really important to wash your hands before you apply makeup and to wash your makeup brushes daily. Hiraga said getting rid of makeup past its prime is the first thing she'll tell her friends.

"Look at the expiration dates because it could be bad for you," Hiraga said.

It'll help make room for the next big trendy find.