Shining Light On Great Sunscreen Debate

It used to be that the SPF ratings of sunscreens topped out at 30.

No more.

Federal officials suggest capping them at 50, but now, "SPF creep" has hit triple digits, with Neutrogena's SPF 100+ sun block, leading some dermatologists to complain that this is merely a numbers game that confuses consumers.

On The Early Show Friday, CBS News correspondent Medical Correspondent Dr. Jennifer Ashton explained how much SPF is enough and how to properly use sunscreen to truly protect yourself.

A sunscreen's SPF, or sun protection factor, measures how much the product shields the sun's shorter-wave ultraviolet B rays, known as UVB radiation, which can cause sunburn

Twenty years ago, 15 was the highest one. Then, 30 became the standard. Now, the most popular is 30. But we also have 50, 75 and that 100 SPF that was just launched.

Using 100 instead of 50 mean doesn't mean you're getting double the protection, Ashton pointed out. And a rating of 100 doesn't mean it blocks out 100 percent of the sun's rays. "All it does," she explained, "is tell you how long you can be outside without burning. So 100 would be 100 times longer than if you didn't have protection on."

There isn't such a thing as 100 percent protection. SPF 100 means 99 percent blocking of UVB rays. The only way to do that, she said, is "stayng inside!"

The difference in UVB protection between an SPF 100 and SPF 50 is marginal. Far from offering double the blockage, SPF 100 blocks 99 percent of UVB rays, while SPF 50 blocks 98 percent. SPF 30, that old-timer, holds its own, deflecting 96.7 percent.

So, what a higher SPF really does is enable you to take more time before reapplying the sunscreen. The higher the SPF, the more time before you must reapply. A person who turns red after 20 minutes of unprotected sun exposure is theoretically protected 15 times longer if he or she adequately applies SPF 15. Because a lot of sunscreens rub off or don't stay put, dermatologists advise reapplication every two hours or after swimming or sweating. The key here is not the SPF, but how you're using sunscreen. Are you applying it right?

The amount is very important. To get the SPF on a bottle, you must apply the amount of a shot glass. If you use half as much, you're getting half the SPF.

When you put it on is also important: Twenty minutes before sun exposure is ideal.

And how often you reapply is really crucial for avoiding sun damage and staying protected.