There's not really a good reason to buy super-high SPF sunscreen

Sunscreen can be a lifesaver. And it feels like it should be so easy - just grab something with a nice big number on it and you're set to go, right?

Wrong. It turns out, there's more to sunscreen than SPF.

Let's start with what we're actually trying to block. There are two main types of solar radiation that hit your skin when you're sunbathing: UVA and UVB. UVB is what creates tans and burns, but there's a lot more UVA and it can reach further into your skin and contribute to skin cancer.

But SPF doesn't really acknowledge UVA radiation at all, and it doesn't take into account any of the negative effects of sun besides burns. There's no similar scale for blocking UVA rays, so before you even worry about that SPF number, be sure you're using a sunscreen that's labeled "broad spectrum." That means the sunscreen provides a similar protection against UVA rays as well.

So what does SPF mean?

No sunscreen can stop all dangerous radiation from reaching the cells in your body. SPF 15 translates to blocking about 93% of UVB radiation, SPF 30 blocks nearly 97%, SPF 50 about 98%, and SPF 100 about 99%. So the difference between SPF 100 and SPF 30 isn't nearly as stark as the difference between SPF 30 sunscreen and bare skin.

While a few percentage points of extra coverage isn't a bad thing, practically, there's not really a good argument for buying super-high SPF sunscreen.

The Food and Drug Administration, which regulates how sunscreens are sold, is considering simplifying the entire upper half of the SPF scale into simply "SPF 50+" because it hasn't seen any data that show that higher SPF numbers are more beneficial than SPF 50.

Basically, once you get up to SPF 50, it's more important to prioritize "broad spectrum" over a sky-high SPF.

But the single most thing to do with sunscreen is to remember to actually slather it on - generously. The average person applies between a quarter and half as much sunscreen as testers use to calculate SPF. So you're probably getting significantly less protection than you think.

Experts recommend about an ounce of sunscreen for an adult. Having trouble picturing that? To cover your an average body, you'll need about two tablespoons: the size of a golf ball, or enough to fill a shot glass (that's about a teaspoon per limb).