Many people don't think seriously about sunscreen -- or the possible consequences of not wearing any -- until they find a spot that shouldn't be there or see someone go through a bout of cancer treatment.

Once you decide that you want to protect your skin, you still have to decide what kind of protection to don to avoid ultraviolet rays, both UVA and UVB.

"What many people do not know is that the SPF of a sunscreen is only a measure of its UVB protection. Proper sunscreen must take into consideration the UVA coverage as well," said Dr. Peter Kopelson, a dermatologist and owner of the Kopelson Clinic in Beverly Hills, Calif.

He said consumers should look for sunscreen that contains one of the following UVA blockers: avobenzone (also called Parsol 1789), mexoryl, titanium dioxide or zinc oxide.

"I like titanium and zinc oxide because they filter UVA and UVB and act instantly. The downside is that they can be opaque, and people don't like their appearance," said Dr. Kenneth Beer, a dermatologist in West Palm Beach, Fla., and author of "Palm Beach Perfect Skin."

Avobenzone can also irritate skin and eyes, so check ingredient lists carefully if you have sensitive skin.

Beer said that most people who spend only a little time outdoors can use SPF 30.

But those with outdoor jobs or who spend a lot of time in the sun should go to SPF 50 and be sure to get any exposed areas. That can include the back, arms, legs and neck.

He said the most important thing is to experiment and find something that you will be comfortable using everyday -- something that isn't too greasy and doesn't cause a rash or have any other problems.

Whatever product you choose, be sure to apply it 20 to 30 minutes prior to being in the sun. That gives chemicals that react with the skin time to get going.

If you're swimming or sweating a lot, you may need to re-apply frequently. Otherwise, every two hours should suffice.