Unless you're the type to sport your Oakley, Gucci, or Ray-Ban sunglasses indoors, you know that shades are meant to protect your eyes from the sun’s harmful ultraviolet (UV) rays and reduce glare. (Looking cool is simply a bonus.) But if you’re grabbing a pair you’ve had for years, you might be wondering if their effectiveness is reduced over time. Do sunglasses' UV protection ever “expire”? 

The answer is probably not. Here’s why.

Most sunglasses have a clear layer of UV coating that blocks the sun’s rays. The coating can be over the lenses, or, in the case of more expensive sunglasses, inside them. Either way, there's no evidence continued sun exposure wears down the material.

"In general, UV protection does not degrade over time," Mark Blecher, M.D., attending physician at Wills Eye Hospital in Philadelphia, Pennsylvania, tells Mental Floss. "The coatings on the surface can scratch off, but it depends on the quality."

Wearers may want to check the effectiveness of glasses either because they fear they've been scratched or because they've bought a cheaper pair and are unsure about the presence or quality of the UV coating. Your local optical shop can perform a test, or you can do one easily at home. Take a UV flashlight and shine it through the lenses so the beam hits a newer $5 to $100 bill. If you can see the watermark on the bill, the lenses aren’t blocking the UV light and it’s time to discard the glasses. If the watermark isn’t visible, you’re good to go.

Whatever you do, don't opt for sunglasses that only have tint to reduce glare and no UV coating. According to Blecher, that's worse than wearing no glasses. "If you're wearing sunglasses just to avoid glare, your pupils will dilate and you'll get more UV exposure than if you weren't wearing any glasses at all," he says.

Without sunglasses, UV light can damage the cornea and conjunctiva, causing pterygia, or scar tissue. Over time, prolonged exposure can also accelerate cataract formation. And they're not strictly summer gear: If you're going to be out in the sun, you'll want to shield your eyes.

If you need a new pair, make sure they have a 400 UV label, which means the lenses block 99 percent of UV rays, and that they cover both UVA and UVB. (The tint of the lenses doesn’t matter when it comes to UV blocking.) Polarized lenses don’t have anything to do with UV rays, but may reduce glare. And try to opt for bigger lenses, as they help reduce UV exposure around the eyes. Tom Cruise and his Ray-Bans had the right idea all along.