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Here's What All Those SPF Ratings Really Mean

Christina Marfice

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Christina is a reporter based in Boise, Idaho. She's a veteran vegetarian, a political junkie and a huge grammar snob. On the weekends, she can usually be found binging on Netflix, playing the piano or petting her cats, Daisy and Dandelion.

Let's decode your sunscreen bottle

With skin cancer being the most common form of cancer in the U.S., it's important we do everything we can to keep our skin safe and protected from harmful UV rays. And dermatologists agree: The single best way to protect your skin is to wear sunscreen, and not just on vacation or during the summer. Every. Single. Day.

More: Myths About Sunscreen You Should Stop Believing ASAP

The only problem with that is that sunscreen can be a little confusing. There are so many different types, and so many SPF levels — what do they all even mean? SheKnows spoke to Dr. William Wooden, director of operative services at Indiana University Health, to get the skinny on everything on your sunscreen bottle so you can get into the sun with as much protection as possible.

So here it is: your handy guide to sunscreen SPF. Read on to learn all you need to keep your skin fresh and healthy all summer long — and for years to come.

What even is SPF, anyway?

SPF stands for sun-protection factor. It's basically a rating system invented by Franz Greiter in 1962, to measure a sunscreen's capacity for blocking UV rays from reaching your skin. According to Wooden, "the number indicates how long you can be exposed to sunlight without burning. It measures the length of time your skin will be protected from burning compared to the length of time it would take your skin to burn without sunscreen."

More: Do You Really Need Different Sunscreens for Your Body and Face?

How does SPF work?

Like Wooden said, the SPF number on your sunscreen bottle is basically a multiplier that tells you how long it will take your skin to burn. "For example, if you are very light skinned and would be red after 30 minutes in the sun, an SPF 10 sunscreen would allow you to be exposed for up to 10 times longer, so 300 minutes or 5 hours, before you burn," he said. "It’s important to remember to put on enough, and keep in mind that activity, water and sweat can remove the sunscreen."

So the higher the SPF, the better, right?

Well, sort of. Most people don't put on enough sunscreen in the first place to get the full effects, which automatically reduces the SPF protection their skin is getting. Being in water or sweating in warm temperatures can also cause sunscreen to wear off more quickly. And according to Wooden, once you hit SPF 30, you're blocking most of the sun's rays, so anything higher is only marginally better.

"SPF 15 to 30 will provide you with good protection, blocking 93 to 97 percent of harmful UVB rays," he said. "Going beyond SPF 30 will block an additional 1 to 2 percent of rays."

Then what kind of sunscreen is best?

The American Academy of Dermatology recommends sunscreen with an SPF no lower than 15. It says SPF 15 to 30 is usually best, but to reapply sunscreen every 1 to 2 hours. Wooden agrees.

"I recommend using sunscreen with SPF 15 to 30," he said. "Since SPF 30 will block 97 percent of UVB rays, it provides good protection. The key is to use enough and reapply. It’s also important to remember additional skin-protection methods, like shade and sun-protective clothing."

More: 6 Peeling Skin Remedies We All Need After a Bad Sunburn

This post was sponsored by Olay.

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