As people eagerly head out into the sun to soak up the last weeks of summer, inevitably some will end up picking up a cheap pair of sunglasses — because you left your good ones at home, or in the back of an airplane seat or in a cab, or just because those ones the street vendor is hawking are so cute. Yet, according to ophthalmologists, spending the $5 for trendy glasses that don’t block out ultraviolet rays may ultimately cost you more than going without.

Dr. Wayne Bizer, a Fort Lauderdale based eye doctor and spokesman for the American Academy of Ophthalmology says that sunglasses without protection from the two most harmful types of UV light — UVA and UVB — can actually do more harm to your eyes than if you were forced to squint.

As everyone knows, when it’s really bright out you squint and your pupils constrict to tiny dots — as small as half a millimeter in diameter — to limit the amount of light getting in. When you put on sunglasses, the effect is the same as being in darkened room, the pupil dilates to let in more light.

Glasses that don’t block UV rays may offer some relief from visible light and reduce your need to squint, but the additional exposure to UVA and UVB can be harmful. “Let’s suppose you put on a very dark pair of sunglasses,” Bizer says, “Your pupil opens up, it dramatically changes in size from half a millimeter to 5 or 6 millimeters. It’s an enormous increase, and now you’re allowing much more [harmful light] into your eyes than if you didn’t wear the cheap sunglasses at all.”

“Sunglasses without UVA and UVB protection simply filter out the ambient light, the glare,” Bizer says. “They don’t do anything to protect you.” And that additional exposure to UVA and UVB rays may increase the risk for cataracts, macular degeneration and even development of ocular melanoma — a very rare type of cancer.

So how can you make sure that you’re buying a good pair of sunglasses? You don’t necessarily have to spend a ton of money. Of course $100 polarized and polycarbonate lenses will come with the full range of protection, but even cheap drug store glasses are often guaranteed to block some UVA and UVB — a feature that manufacturers can add to plastic lenses simply by dipping them in a special solution, Bizer says, and that can even be added to clear lenses.

He recommends buying from a reliable source, and checking for stickers or tags that guarantee to block UVA and UVB. “You should be able to buy a decent pair of [relatively cheap] sunglasses at a reputable place,” he said. That, Bizer says, probably means skipping the street vendors, who are often flogging wares that don’t comply with FDA standards. —By Tiffany Sharples