As every good vampire hunter knows, the sun's rays are lethal to the undead, unless they wear a daylight ring made by witches so that they can stroll around during the day. Depending on your budget, they're available on eBay (seriously) for anywhere between $1 and $110.

But for mere mortals, protecting your eyes from the sun means not only getting good UV-blocking sunglasses, but replacing them every two years if you wear them two to three hours daily.

According to a recent report in BioMedical Engineering Online, that's what sun-savvy Brazilians do.

Here's what they know, and you might not:

Sunglasses age with time and exposure to sunlight. That means the UV protection originally provided by your shades can break down. Then your eyes are exposed to more potentially cornea- and retina-damaging solar radiation than is healthy.

Ten percent of cataracts are directly attributable to UVB exposure, and UVA light may contribute to macular degeneration.

Your shades should block as much UVA and -B light as possible. The top-notch Brazilian standard: Sunglasses should block out UV light that's got an intensity of 380 to 400 nanometers (400 being the strongest UV ray).

True, if you wanted to measure how your sunglasses are holding up, you'd have to use a spectrometer, but replacing them every couple of years and making sure your new ones block out 99 to 100 percent of UV rays and screen out 75 to 90 percent of visible light should protect your eyes.