Oct 5, 2016 3 min read

DEAR DOCTOR K: During my last eye exam, my eye doctor advised me to buy a pair of “high quality” sunglasses. But she didn’t tell me what she meant by high quality. Can you help?

DEAR READER: When you buy sunglasses, it’s natural to look for a style that looks good on you and is comfortable. But don’t fail to consider the most important detail: the amount of harmful ultraviolet (UV) radiation the lenses screen out. Without proper UV protection, sunglasses can work against you by enabling you to see comfortably in light that is harming your eyes.

UV radiation can penetrate the clouds, even on overcast winter days. That’s why the American Academy of Ophthalmology recommends that everyone wear sunglasses whenever they are outdoors.

Since 1998, the FDA has regulated nonprescription sunglasses as medical devices. It requires lenses to be impact-resistant, nontoxic and nonflammable. Beyond that, though, options vary, and some choices are better than others. You’ll want to consider the following:

THE LABEL

Look for 99 percent or 100 percent UV protection, or UV400, which means the lenses absorb wavelengths up to 400 nanometers, thus blocking all harmful UV rays.

THE SIZE

The larger the lenses, the better protection they offer. Wraparound lenses are the best because they prevent UV rays from entering at the side.

THE SHADE

Although it may seem counterintuitive, darker isn’t automatically better. The darkness of the lens affects only the ability to filter out visible light. The protection from UV light is conferred by coatings applied to the lens.

Yet shading is important in protecting you from glare — a different problem from UV light. For many people, very bright light causes an unpleasant sensation. It makes you squint, and it’s harder to see clearly.

Lenses come in different shades, for different situations. For example, dark lenses are best for a sunny day on the water, while lighter tints may be better choices for overcast days. These days, you can get “photochromic” lenses that change their shade depending on the amount of light. Indoors, they are clear. Outside in bright sun, they become a dark shade.

OPTICAL QUALITY

When you’re trying on sunglasses, focus on a vertical edge or line and move your head back and forth. If the line wiggles, the lens may have an optical defect.

THE FIT

You want the frames to fit comfortably, with the lenses directly in front of your eyes.

THE COST

There is no relationship between the price tag on a pair of sunglasses and the protection it offers. As long as the labels specify 99 percent or 100 percent UV protection, or UV400, and have no optical defects, an inexpensive pair from a dollar store will do the job as well as pricier sunglasses from a designer boutique. If you want glasses that change their shade depending on the amount of light, such prescription lenses will cost you more.

Most people who buy expensive sunglasses are spending their money on how they think the glasses look, not on the “high quality” of the glasses.

Dr. Komaroff is a physician and professor at Harvard Medical School.