By now we all know that the sun can kill:About 1 million new skin cancers are diagnosed each year in the United States,and about half of all new cancers are skin cancers, according to the AmericanAcademy of Dermatology. And thanks to increased awareness about skin cancer, wealso know the summer mantra: Wear sunscreen, wear sunscreen, wear sunscreen.What we may not know, however, is how to choose and use sunscreen properly, sowe can maximize our protection each time we apply it and step out into the sun.Here, rules to follow for a safe summer:

Rule #1: Go with an SPF of 15 or Higher

Sun protection factor (SPF) has to do with the amount oftime a product protects the skin from the reddening caused by ultraviolet rays,compared to how long it would take without the product. If you would normallyburn in 20 minutes, a sunscreen with an SPF of 15 will protect you for 15 timeslonger, or about five hours.

"Fifteen is probably sufficient for most people most ofthe time," says Dr. Neil S. Goldberg, a dermatologist who practices in theNew York communities of Bronxville and White Plains. "But if somebody hashad skin cancer... or burns very easily, they should probably use an SPF of 25or higher." Goldberg says that anything less than 15 is probably worthless,whether you are sun-sensitive or not.

Past an SPF of 15, the incremental sun-protective benefitdiminishes. An SPF of 15 blocks out 93 percent of harmful rays; an SPF of 25about 96 percent; and an SPF of 30 about 97 percent.

Even if you're not a particularly sun-sensitive person, itmay be worthwhile to choose an SPF higher than 15. The level of protectionindicated on a product is only reached if the correct amount of sunscreen isused (1 ounce per use is considered optimal). However, most people apply toolittle, and while it's best to try to use the recommended amount, if you use ahigher number SPF, you will get greater protection using less product.

Rule #2: Get Broad-Spectrum Protection

While SPF is a universal measure of protection against UVBrays, known to cause sunburns and many forms of skin cancer, there is currentlyno standard for UVA rays, which are less potent but more prevalent. UVA raysare thought to play a role in the wrinkling and aging of skin, and maycontribute to skin cancer. Some sunscreens now offer what is called"broad-spectrum" protection, or protection against both UVA and UVBrays.

Avobenzone (Parsol 1789) chemically absorbs UVA rays. Somequestion exists as to whether it becomes less protective when exposed tosunlight. Results of studies to determine this should be available in the nextyear or two. For now, Dr. Henry W. Lim, chairman of the department ofdermatology at the Henry Ford Health System in Detroit, calls avobenzone"the best UVA protection on the market in the U.S."