A Guide to SPF and How It Works

If you’re at the beach, it’s likely in your bag. You spray it, rub it in and re-apply it diligently during the summer months. Sunscreen is crucial to protecting your skin from cancer and early aging, but what exactly is the SPF that’s in sunscreen and how does it work? Learning more about your sunscreen—like how and when to use it—can help you make the best decisions to keep you and your family safe from the sun.

What is SPF?

You’re likely familiar with the term SPF—which stands for Sun Protection Factor—but what does it mean and how does it work? SPF is an indicator of a sunscreen’s ability to prevent UVB (ultraviolet B) rays from damaging your skin. For example, according to the Skin Cancer Foundation, if it takes 20 minutes for your unprotected skin to start turning red, a sunscreen with an SPF of 15 prevents reddening 15 times longer, or about five hours.

How does sunscreen work?

How your sunscreen works can be explained in percentages. The higher the SPF, the greater percentage of UV rays it blocks. For example, an SPF of 15 filters out about 93 percent of all incoming UVB rays. SPF 30 keeps out 97 percent and SPF 50 keeps out 98 percent. Sunscreens with higher SPF ratings block slightly more UVB rays, but it’s good to keep in mind that no sunscreen offers 100% protection from the sun. SPF 15 or SPF 30 are generally recommended for proper protection. If your skin is very light and susceptible to burning, you may consider applying a higher SPF.

What’s more important than the SPF rating you choose—experts say—is making sure to use broad-spectrum, water-resistant sunscreen and to apply it liberally half an hour before going outside. Reapply your sunscreen at least every two hours or after any activity that may have affected its protection—like swimming or sweating.

Who should use sunscreen?

Anyone over the age of six months should use a sunscreen. Even if you’re not planning a day at the beach or even any outdoor activities, it’s still recommended to apply some SPF (at least 15-30) each day to make sure you’re protected. Make sure to keep children younger than six months from sun exposure by using clothing, shade and staying indoors during peak hours.

Remember that sunscreen is only one part of protecting your skin from the sun. Apply sunscreen in addition to practicing other sun safety precautions—like seeking shade, wearing sunglasses and a hat, and keeping your arms and legs covered.  

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