Sun protection factor (SPF) is the key component of an effective sunscreen. SPF refers to the amount of protection the sunscreen will give you, or how much longer you can spend outdoors before you get sunburned compared to if you weren't wearing sunscreen.
Here's how SPF works:
Say you normally can stay outside in the sun for 20 minutes without burning. If you slather on an SPF 15 sunscreen, you can stay outside for 15 times longer -- about five hours -- before burning. The higher the SPF, the greater the protection the sunscreen provides. An SPF 15 sunscreen blocks 93 percent of UVB rays, an SPF 30 blocks 97 percent, and an SPF 50 blocks 98 percent. The SPF of a sunscreen only relates to UVB rays -- there currently is no rating system for UVA protection, but it's in the works.
It would seem logical that an SPF 30 sunscreen would be twice as effective as an SPF 15 sunscreen, and an SPF 100 would be six times more effective, but that's not really how it works. Although a higher SPF sunscreen will protect you slightly better, the numbers game in sunscreens is mostly marketing (in fact, the Food and Drug Administration has proposed capping the SPF at 50+). Dermatologists say you're fine using an SPF 30 or SPF 45, but just make sure to reapply the sunscreen every two hours or so [source: WebMD]. If your skin is starting to take on a reddish tinge, reapply sooner.