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Sunscreen Dos and Don'ts for Summer

More than two million cases of skin cancer are diagnosed every year -- and most skin cancers are caused by too much sun exposure, according to the American Cancer Society.

Special Section: Dr. Jennifer Ashton
Video Series: Dr. Ashton's Health and Wellness

So, how high of an SPF should you use?

CBS News Medical Correspondent Dr. Jennifer Ashton explained "SPF" stands for "sun protection factor" and it stands for the amount of time you can stay in the sun without burning. For example, an SPF of 30 means you'll get 300 minutes in the sun without burning.

Ashton pointed out on a 90 SPF bottle of sunscreen, "It does not mean 90 percent of the sun's rays are blocked."

And what's the difference between UVA and UVB rays?

UVA, Ashton said, refers to "aging rays," while UVB refers to "burning rays."

She said, "Both can contribute to skin cancer, but the burning rays, typically affects the surface of the skin, makes it more red. The UVA rays, which can penetrate window glass are what really ages us. It gives us wrinkles. Both you want to avoid in excess."

Ashton suggested these sunscreens recommended by Consumer Reports:
Target's Up & Up Sport Continuous SPF 30
Walgreens Sport Continuous SPF 50
Banana Boat Sport Performance Continuous SPF 30
Aveeno Continuous Protection SPF 50

Ashton added, "You don't need to spend a lot of money, but when you're talking about sprays, you have to make sure you cover all the surfaces, you put it on 30 minutes before, and you (cover) things like ears, neck and (all other exposed areas)."

And don't put on your sunscreen when you're going out into the sun. Ashton reminded viewers you must put it on 30 minutes before you go outside because it needs time to be absorbed into your skin.

Sunscreen also has an expiration date. Ashton said after three years it's time to throw out your supply.

But you should be going through your sunscreen rather quickly in a season, anyway, if you're using the correct amount. Ashton recommended using about the amount of a shot glass for each application, which more than double the amount, she said people usually use.

"You really want to apply it liberally," she said. "Most people don't put on a lot."

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