Style is subjective, so buy frames that you enjoy wearing. However, in terms of general eye protection, look for sunglasses that block 99 or 100 percent of UV radiation. They may be labeled "UV absorption up to 400 nm," which means the same thing. Shades labeled "cosmetic" block only about 70 percent of UV rays. Make sure the lenses are dark enough to keep your eyes comfortable, but not so dark that they reduce your vision. As a general rule of thumb, when considering sunglasses, try them on and take a look in the mirror; if you can see your eyes, the lenses aren't dark enough to screen out visible light.

If you spend a lot of time outdoors in intense glare from sunlight bouncing off snow or water, you should wear sunglasses that block blue light. Medium to dark lenses with a grey, or a slightly brown or green tint, will filter out most blue light.

Most sunglasses have plastic lenses. These lenses are tougher than glass and less likely to shatter. If you buy plastic lenses, look for a pair with a scratch-resistant coating. Check the lenses for distortion by putting the sunglasses on and looking at a rectangular pattern, such as floor tiles. If the lines stay straight when you move your head up and down, and side-to-side, then the amount of distortion is acceptable.

Sunglasses are made with different kinds of lenses to meet different needs:

• Regular lenses reduce the brightness of everything evenly.
• Polarizing lenses are designed to cut glare due to reflection. This means they are good for driving and outdoor activities in the snow or on water.
• Photo-chromic lenses change with the intensity of UV light by turning darker when outdoors and lighter when indoors. If you wear these for driving, choose sunglasses that are fairly dark.
• Mirror lenses reflect all or part of the light instead of absorbing it. They offer no performance advantage as they scratch easily. You should choose a pair with a scratch-resistant coating.

Sunglasses UV Protection Levels

Price does not dictate how much UV protection a pair of sunglasses will provide. Look for a label that lists the type and amount of protection. Manufacturers follow voluntary industry standards when labeling these products. Sunglasses that comply with industry standards are grouped in three categories:

• Cosmetic sunglasses have lightly tinted lenses for use in sunlight that is not harsh. They block from 0 to 60 percent of visible light and UVA rays, and between 87.5 and 95 percent of UVB rays. These glasses are not usually recommended for daylight driving.
• General purpose sunglasses block from 60 to 92 percent of visible light and UVA rays, and between 95 and 99 percent of UVB rays. These sunglasses are good for driving, and are recommended whenever sunlight is harsh enough to make you squint.
• Special purpose sunglasses block up to 97 percent of visible light and up to 98.5 percent of UVA rays. They also block at least 99 percent of UVB rays, and are suitable for prolonged sun exposure. These sunglasses are not recommended for driving.

Note of caution: The U.S. government does not monitor labeling claims made by sunglass manufacturers, and the chemical coating that absorbs the radiation is invisible, so you can't see whether it's there. To be sure your shades offer adequate protection you should have them tested with a UV meter. Most opticians and some sunglass stores will test them for free.