The higher the SPF, the stronger the sunscreen—that’s how most of us think SPF works, right? And while that’s not entirely wrong, it’s also not the whole story. To keep us all in the know (and a little more protected as we cram in these final weeks of summer sun), we sat down with dermatologist Anthony Rossi to figure out how SPF really works.
SPF—which stands for “sun protection factor”—is a measure of how well a sunscreen will protect your skin from UVB rays, which cause sunburns, wrinkles and skin cancer. What exactly SPF is measuring is a little more complicated.
“It’s a measure of how much UV energy it takes to produce a sunburn,” Rossi explains. “Applying a product with 10 SPF will allow your skin to undergo 10 times the UV intensity that would otherwise cause a sunburn.”
Now, that may sound like you can time your reapplication based on the SPF number—20 SPF lasts twice as long as 10 SPF, surely?—but this is absolutely not the case. Rossi says you should never think of SPF as directly correlated to the amount of time your sunscreen will continue doing its duty.
“The sun’s intensity changes throughout the day and varies by geographic location, and lighter skin burns more easily than darker skin,” Rossi says. “So I encourage people to reapply sunscreen every two hours, whatever the SPF may be.”
Additionally, Rossi explains that the SPF scale isn’t linear, so higher SPFs (especially those above 30) don’t offer up much more sun protection than their supposedly weaker counterparts:
- SPF 15 blocks 92.5 percent of UVB rays
- SPF 20 blocks 95 percent of UVB rays
- SPF 30 blocks 97 percent of UVB rays
- SPF 50 blocks 98 percent of UVB rays
- SPF 75 blocks between 98 and 99 percent of UVB rays
So next time you’re at the store and the SPF 75 sunscreen is twice the price of SPF 30, save yourself a penny—as far as the sun is concerned, both screens provide your skin with pretty much the same amount of protection.