What you should know before buying sunglasses

These include "eyecare provider offices, or brick-and-mortar and online department stores and sunglass specialty shops -- as they offer sunglasses that meet the necessary standards for proper UV protection," Bazan wrote in an email. He adds that shoppers "should be wary when purchasing sunglasses from online auction sites, street vendors and flea markets, as sunglasses available at these locations may not meet the necessary standards for proper UV protection."

That's all good, but does UV protection wear off over time?

It doesn't, says Dr. Jeff Pettey, assistant professor of ophthalmology at the Moran Center University of Utah and a spokesman for the American Academy of Ophthalmology.

"The UV protection is embedded," Pettey said, explaining both the technology and the process have changed over time, advancing far enough that "routinely, even on the cheapest pair of glasses," the protection is built in for life.

"In the testing we've done, we've never had a pair of sunglasses that didn't meet that UV protection," Pettey said.

He acknowledges that in the early 1990s, tests on children's sunglasses showed that not all lived up to their UV protection claims, but more recently, "we just have not seen that."

Still, he suggests buying from a reputable retailer just to be safe.

"There's no guarantee, because you can't say for certain where your glasses are coming from," Pettey said, adding that there's a test you yourself can perform at any local optical shop that has a UV light meter.

"You can take your glasses in and have them tested," said Pettey. This is a handy test for when you doubt your sunglasses have the UV protection claimed by a retail tag or if they're simply old and you want to make sure.

As far as a "hard requirement," UV protection is it, he said. Tint doesn't matter, polarization doesn't matter, and although bigger is always better, "UV protection is the essential piece."

The inessentials, though, may also play a role in eye health.

Thau says there are two parts to sunglass protection: One is non-visible radiation, and the second has to do with visible light -- how much brightness they block.

"When you're in the bright sun, like the beach, you do want something 75% or darker to block you from visible light," Thau said. Too much exposure to visible light "does bleach your receptors, and some studies have indicated it can impair your night vision and your color vision perception."

Yet blocking visible light has a downside if your sunglasses are not up to UV snuff, suggests Dr. James H. Diaz, an environmental medicine specialist and anesthesiologist.

"The darker the sunglass lenses, the more the pupils will dilate and allow more UV light to enter the eye," Diaz wrote in an email. This is true of blue light, which ranges in length from 400 to 440 nanometers.

"The longer the retinas are exposed to unfiltered blue light, the greater the risk of macular degeneration," Diaz said. However, the National Eye Institute does not list protection against blue light as necessary when purchasing sunglasses. In fact, research has shown blue light exposure is good for us, as it helps regulate our circadian rhythms and so affects both mood and cognition.

"Orange and yellow lenses provide the best protection from blue light, and blue and purple lenses provide the least protection," Diaz said.

Thau noted color is not crucial in protecting eye health. "Most popular colors are gray, green and brown. They are the least distorting for color perception, with gray being the most neutral," she said.

People who have color vision deficiencies generally find that they see much better with brown lenses, while "green seems to give more contrast," said Thau.

Whether you opt to filter out blue light or not, a good pair of UV-blocking sunglasses can protect both your short-term and long-term health.

"Skin around the eyelid is the thinnest in the body, so it is susceptible to skin cancers," Thau said. This thin skin is most likely to develop basal cell and squamous cell cancers, so the recommendation is to wear the largest pair of sunglasses possible to protect the eyelids and surrounding skin.

Meanwhile, Pettey warns that cancers of the eye itself, including squamous cell carcinomas and malignant melanomas, also can result from sun exposure.

"The same damage that occurs to our skin occurs to the eye," he said: specifically "eye burn," a form of short-term damage similar to a sunburn.

Thau says sun exposure can also cause photokeratitis, an inflammation of the cornea, with temporary symptoms of blurry vision, light sensitivity and a burning or gritty sensation. Too much sunlight may also lead to a thickening and/or yellowing of the conjunctiva, the membrane covering the eye. Though unsightly and annoying -- your eyes will feel too dry when this happens -- this doesn't cause blindness, says Thau.

Other conditions caused by too much unprotected time in the sun may have longer-term consequences, according to Pettey. Pterygium, for instance, is a growth of fleshy tissue that can cover part of the cornea and hurt your vision. This is sometimes called "surfer's eye."

"Inside of the eye, as far as function, increased UV light leads to increased progression of cataracts and also likely increased progression of macular degeneration, both of which are conditions that cause loss of sight," Pettey said.

Thau says the latter is the more serious of the two complications.

"Cataracts can be removed surgically, but macular degeneration is yours for life," she said. "It literally causes damage to the photo receptors. It's like damaging film in a camera, and you cannot replace the film."

One other long-term danger of looking directly at the sun is solar retinopathy. Just like your mother told you, don't ever look directly at a solar eclipse, such as the one coming August 21.

"If you were to look at that level of radiation, even for a few seconds, without the brightness of the light telling you to look away because it would be painful, you can actually cause burns on the retina in the back," Thau said. These burns cause permanent damage to your sight, and regular sunglasses are not enough protection for a solar eclipse.

Though the sun is the main cause of UV radiation damage, artificial sources such as tanning beds, lasers and welding machines also produce UV radiation that might damage vision.

One other consideration at least for some is their location on the globe, Diaz said.

"We have more sunny days in the South, especially in Florida, and the West, especially in California, than in other areas of the US, and therefore, we see more sun-related injuries," said Diaz, who has researched this topic. Naturally, this is also true for the sunniest spots around the globe.

"Another problem in the coastal South and all coastal areas is the reflected magnification of UV radiation off of surface waters," Diaz said. UV-blocking sunglasses protect against these reflections, but there is a risk of the same complications that result from direct sunlight.

Beyond the standard UV recommendations, does polarization matter?

"I like to fish, and polarized lenses will reduce reflection and glare off surface waters and allow one to see at a greater depth," Diaz said.

Thau says this is not mandatory, "it's just an optional add-on benefit you can have."

Though most of us are concerned with eye health, the Vision Council reports that only 31% of Americans always wear sunglasses when outside.

They're not merely a fashion statement, insists Thau, but if that gets you to wear them, go with it. She herself owns five pairs and has been known to put them on even when sitting in a bright room.

To be safest, Thau recommends that an annual comprehensive eye examination with a credentialed doctor to learn more about eye health and which sunglasses might be most beneficial in any given circumstance.

"It's also important for children to start with some protection early, because it's cumulative damage over time," Thau said. "My family does not walk out the door without their sunglasses on, except at night."