Think slathering on the highest-number sunscreen at the beach or pool will spare you skin cancer and premature wrinkles?

Probably not, if you're in the sun a lot.

That's because you don't need a sunburn to suffer the effects that can cause various types of skin cancer.

Sunscreens generally do a good job filtering out the ultraviolet rays that cause sunburn — UVB rays. But with sunburn protection, many people get a false sense of security that keeps them under the harsh sun much longer.

That adds to the risk of eventual skin cancer — both deadly melanoma and the more common and less-threatening basal and squamous cell cancers.

And most sunscreens don't defend nearly as well against the UVA rays that penetrate deep into the skin and are more likely to cause skin cancer and wrinkles. That's true even for some products labeled "broad-spectrum UVA/UVB protection."

Experts say the best protection against UVA is a sunscreen that includes zinc oxide, titanium dioxide or avobenzone. Consumers should also look for those that are water-resistant and have an SPF of 30 or better, indicating strong protection against UVB rays, and apply liberally and often.

More important, limit time in the sun, particularly from 10 a.m. to 4 p.m., and cover up, including wearing a hat and sunglasses.

Often, product labels are confusing or bear misleading claims. For example, the SPF, or sun protection factor, refers only to defense against the less harmful UVB rays.

"I don't think people understand they're only getting protection from part of the spectrum," said Dr. Sandra Read, a spokeswoman for the American Academy of Dermatology.

"You're accumulating this damage, and you don't know it."

Many sunscreens say little about when to reapply — doctors say at least every two hours and after swimming or sweating. Nor do they say much about how much to use, roughly two tablespoons for an adult.

"Most people who use an SPF 15 get the protection equivalent to an SPF 5 because they put it on" too thinly, said Dr. Martin A. Weinstock, chairman of the American Cancer Society's skin cancer advisory group and a Brown University professor.

Although a higher SPF number means more protection, the difference is small: SPF 15 blocks about 93 percent of UVB rays and SPF 50, often more expensive, blocks about 98 percent.

Most sunscreens work by reacting chemically with the skin, so they don't start absorbing damaging rays right away and must be applied a half-hour before going outside, something many labels fail to note.

And such claims as "waterproof" and "sunblock" are unsupported, according to the Food and Drug Administration, which years ago proposed replacing them with the more-accurate terms "water resistant" and "sunscreen."

Southern Arizonans probably face a higher risk of developing skin cancer than people in any other part of the country, experts at the Arizona Cancer Center say.

They say probably because Arizona does not have a skin-cancer registry to track cases and compare state numbers with the rest of the country and world.

What is known is that Arizona has one of the highest skin-cancer rates in the world, said Dr. David Alberts, a cancer prevention expert and director of the Arizona Cancer Center.

Cancer Center researchers reported two years ago that the rates of occurrence of all types of skin cancer are in the range of four to seven times higher in Southern Arizona than in Minnesota or New Hampshire.

In the early 1990s, Cancer Center researchers determined that skin-cancer rates for Southern Arizona and Australia are nearly identical, and higher than elsewhere in the world.