What Does SPF Mean?

When it comes to the SPF number on our sunscreen bottles, most people shrug and grab the highest one, figuring bigger is better.

SPF 30? 50? 75? Bring it on—it will work, right?

There’s actually much more to it than that, and it’s all rather scientific. Simply put, SPF, or Sun Protection Factor, is a measure of how well a sunscreen will protect your skin from UVB rays.

Two kinds of UV radiation—UVA and UVB—can damage the skin. UVB rays are usually the culprit behind those pink, peeling sunburns, but UVA rays can be just as problematic. We have UVA rays to thank for wrinkles and leathery, saggy skin.

Wearing sunscreen with an SPF rating gives you a certain amount more time of safe sun exposure. It’s fairly simple to calculate how long you can stay in the sun safely, as long as you know the SPF number and how long it takes you to develop a sunburn normally.

To calculate the safety of your sunscreen, multiply the number of minutes you can sit outside without burning by the SPF number. For example, if you would normally burn after 10 minutes in the sun, applying sunscreen with an SPF rating of 15 would allow you to stay in the sun safely for about 150 minutes. Of course, this is a rough estimate, and it really depends on your skin type, the amount of sunscreen you use, and the intensity of the sunlight you’re in.

Interestingly, the SPF scale is not linear. Contrary to what you might think, upgrading from SPF 15 to SPF 30 doesn’t actually double your protection. SPF 15 blocks 93 percent of UVB rays, while SPF 30 blocks 97 percent, and SPF 50 blocks 98 percent of UVB rays.

For best protection, most experts recommend using a minimum SPF of 15, and applying the proper amount—about 9 teaspoons for full body coverage—and reapplying every 2 hours. It turns out that most of us under-apply sunscreen, dramatically reducing the SPF protection.

So what about those damaging UVA rays? Just because a sunscreen has a high SPF doesn’t mean you’re being protected from them. SPF is only a measure of how well a sunscreen protects you from sunburn, which you get only from UVB rays. UVA rays penetrate deeper into the skin than UVB, and will cause you to get a tan instead of burn. UVA radiation also creates free radicals, which lead to skin damage, wrinkles, and even skin cancer.

UVA radiation makes up 95 percent of all UV light reaching the earth’s surface. It passes through clouds and glass, and is more or less the same strength throughout the day and the year. Many conventional sunscreens, even those intended for children, offer little or no protection from UVA rays. In order to protect the skin from UVA rays,  your sunscreen needs to contain something more powerful, like zinc oxide or titanium dioxide.

Photo credit: Paul Delmont