Sunlight that has passed through the earth's atmosphere contains two types of ultraviolet rays — A and B. Ultraviolet B (UVB) rays are the ones that burn or tan your skin. But ultraviolet A (UVA) rays are sneakier. While they don't make you burn, they do cause wrinkles. Exposure to ultraviolet light is believed to be the number one cause of all types of skin cancer. What can you do about it? Read on.

Sizing Up SPF

The sun-protection-factor (SPF) rating system, which was introduced in 1978, is mainly a measure of the time it takes your skin to burn — and that depends on how fair or dark your skin is. If your unprotected skin turns pink after, say, 10 minutes of sun exposure, then it would take 15 times as long, or 150 minutes, when you're wearing an SPF 15. The SPF number also indicates what percentage of UV rays the product blocks. An SPF 2 blocks 50 percent of rays, an SPF 15 blocks 93 percent, and an SPF 30 blocks 97 percent. Any SPF higher than 30 gives you only marginally more protection, which is why doctors often say that SPF 30 is as high as anyone needs to go. (Some sunblock makers charge a little more for higher SPFs.)

Bear in mind: No sunscreen, no matter how high the number, blocks 100 percent of UV rays. Sunscreen should not be considered a free pass to spend all day in direct sunlight.

Using SPF

Use the following checklist to see if a sunscreen measures up.

Broad-Spectrum Protection: A sunscreen with this term on its label will filter out most of the UVA and UVB rays.

Physical Blocks: These ingredients, such as titanium dioxide and zinc oxide, reflect the widest range of UVA and UVB radiation. When the particles are big enough, physical blocks work like a mirror, reflecting UV radiation back into the environment. And these days you don't need to sport the white lifeguard nose to wear a physical block; zinc and titanium particles are now micronized, which means they're so fine they're invisible, but still strong enough to form a curtainlike barrier.

Chemical Blocks: Synthetic screens, including Parsol 1789, benzophones, cinnamates, and salicylates, absorb UV light instead of reflecting it. Dermatologists agree that a sunscreen should ideally contain both physical and chemical ingredients. If you have sensitive skin, however, look for sunscreens with only physical blocks to avoid skin reactions.

Water-Resistant or Sweat-Resistant: A sunscreen with one of these terms on its label will maintain its SPF for up to 40 minutes in the water, and it will stand up to humidity and perspiration. If you spend a lot of time swimming or exercising, however, look for a "waterproof" or "sweatproof" product, which will maintain its SPF for up to 80 minutes in water and withstand heavier sweating.

Antioxidants: New additions to many sunscreens, vitamins C and E have been shown to ward off wrinkles and possibly skin cancer. These antioxidants neutralize harmful, cancer-causing free radicals in the skin before damage can occur. "Vitamins C and E offer extra protection against UV rays," explains Sheldon R. Pinnell, a professor emeritus of dermatology at Duke University Medical Center, in Durham, North Carolina.