Do I really need to wear sunglasses every time I go out?

Q: I wear prescription eyeglasses, so I never use sunglasses when I’m outside, as it’s annoying to carry two pairs. Is this bad for my eyes?

Shielding your eyes from sun exposure is, as far as experts are concerned, not up for debate.

“Sunglasses are sunscreen for your eyes,” said Laura Di Meglio, an assistant professor of ophthalmology at the Wilmer Eye Institute at Johns Hopkins Medicine. “They help protect your eyelids, the skin around your eyes and the surface of the eyes.”

But do people who wear eyeglasses need a pair of sunnies, too? It depends, said Sheri Rowen, an ophthalmologist and cataract surgeon. People assume they need tinted lenses, she explained, but some of your bases are covered if you wear prescription frames.

We asked doctors what you should know about keeping your eyes safe.

What does the sun do to your eyes?

Sunlight contains three types of ultraviolet radiation – UVA, UVB and UVC – but only the first two reach earth’s atmosphere. UVB radiation is believed to be more dangerous for the eyes than UVA. However, both are present whether it is sunny or not, and both can increase your risk for conditions that affect your eyesight.

For example, cataracts, a protein build-up that clouds the lens of the eye, can occur naturally as we age. But UV radiation can cause cataracts to form earlier, said Bryce St Clair, an assistant professor of ophthalmology at the Wilmer Eye Institute. The sun’s UV rays can also worsen macular degeneration, a progressive eye disease that can cause vision loss.

Leaving your eyes unprotected can also cause two types of growths, pterygium and pinguecula.

Pinguecula is typically more minor, appearing as a small yellow spot or a raised bump on the whites of your eyes. It can cause discomfort, but does not usually affect your vision.

Pterygium, however, can be more serious. “A pterygium is something that actually grows into your cornea,” Di Meglio said. “If those get bad enough, they can affect your vision”, and in some cases, they may require surgery, she added.

The area around your eyes is vulnerable, too. Five per cent to 10 per cent of skin cancers in the US develop on the eyelid. “Almost every cell type is found within and around the eye,” said Dagny Zhu, a cornea, cataract and refractive eye surgeon based in California. “Any of those can mutate when exposed to too much UV.”

Finally, there are aesthetic concerns. Even if you are protected from UV rays, which can accelerate signs of ageing, bright light can make you squint, which contributes to wrinkles.

“Squinting is your body’s natural way of saying, ‘I’m experiencing too much light’,” St Clair said. “Wearing sunglasses helps relax your eye muscles, which prevents things like crow’s feet.”

So do you need two pairs of glasses?

Your clear prescription frames can have as much protection as a dark lens, St Clair said, and most reputable retailers offer them as a standard part of a clear lens.

Modern eyeglass lenses are often made from polycarbonate, which has UV protection built in. And non-polycarbonate lenses often come with a special coating that provides UV protection.

If you want to be certain that your glasses have sun protection, you can bring them to a local optical shop to have them checked, said Emily Schehlein, a Michigan-based ophthalmologist and spokesperson for the American Academy of Ophthalmology.

But if carrying a second pair annoys you, do not worry too much – unless sunlight irritates your eyes, Rowen said. Brightness alone does not increase your risk of cancer or other eye growths, but excessive amounts can cause uncomfortable side effects, like eye fatigue, dry eye, headaches and visual discomfort, she added. Tinted frames reduce that visible light, so they can help.

But tinted or not, glasses cover only a fraction of your face, so Rowen recommends adding a hat and sunscreen for more UV protection, especially on days when you will be spending hours outdoors.

And whether you are shopping for sunglasses or eyeglasses, look for a label that says “UV400” or “100 per cent UVA/UVB protection”. These terms are essentially synonymous; they mean the lenses block 99 per cent to 100 per cent of ultraviolet light.

Sunglasses are regulated by the Food and Drug Administration, so it is illegal to label a pair UV400 if it does not actually have that protection.

But all the experts warned against using those US$3 (S$3.90) sunglasses you might snag on the boardwalk. Extremely dark lenses that do not have UV protection can actually damage your eyes.

“All they do is make your pupils open up and absorb more light,” Rowen said. “If you have no UV protection and your pupils are big, you’re going to get straight-on UV exposure,” she added.

Ultimately, whether you bring two pairs of glasses is a matter of personal comfort, as long as you are avoiding situations where direct sunlight has an unobstructed path to your retina. “We need light, we need sun, we need to be outside,” Rowen said. “We don’t need UV hitting our eyes.” NYTIMES