When it comes to sunscreen, you'd think a higher SPF would protect against more UV rays than a lower one. It does, but not by much. SPF 15 sunscreen blocks 93% of UVB radiation. SPF 30 blocks around 97%, and SPF 50 blocks 98%.
"As you get higher and higher, it's not really a practical difference," American Academy of Dermatology president Dr. David M. Pariser told the New York Times. Here's why: companies calculate SPF by comparing the time it takes a person to burn unprotected with the time it takes for them to burn wearing sunscreen. Therefore, if you burn after 20 minutes with no sunscreen, you should theoretically be able to last for 15 times longer -- a whopping five hours -- with SPF 15. But sunscreen itself doesn't usually last that long. Sweat, friction, and simple quirks of product formulation can make it wear off, which is why dermatologists recommend reapplying every two hours. That means it doesn't really matter whether you get the SPF 30 or the SPF 100, since the formula will probably wear off before the difference in protection becomes important.
What is important is applying sunscreen properly: for a full-body application, you should use an ounce of sunscreen, or roughly the volume of a shot glass. And because SPF is only a measure of UVB protection, you should look for a full-spectrum formula that protects againsts both aging-related UVA and sunburn-related UVB rays.