What, Exactly, Is SPF?

You may think you know, but you probably have no idea

Illustrated by Sarah Lutkenhaus.

Okay, so: We know you know what sunscreen is. We know you know you should be wearing it. But do you know how it actually works? Like, how it helps protect your skin from the sun and, as a result, the damage it can inflict? Do you know there are different kinds of rays that you should be worried about? Do you know the difference between sunscreen labeled 15 versus 30? Honestly, do you even SPF, bro?

A lot of questions, but here are some answers.

Sunscreen works like this…
Most products are made up of minerals like zinc oxide or titanium dioxide, which act as a physical block against the sun. Others might contain organic chemicals like avobenzone or oxybenzone, which absorb radiation through chemical bonds.

SPF is…
It stands for sun protection factor, which measures how well sunscreen protects the skin against UVB light.

What SPF isn’t…
A catchall for protection against all rays. Most sunscreens protect against UVB rays, which cause sunburn and most skin cancers, but do nothing to protect against UVA rays, which are responsible for premature aging and have also been found to increase skin cancer risk, Dr. Joshua Zeichner tells us. “When choosing a sunscreen, look for the words ‘broad spectrum’ to make sure you’re getting proper UVA protection as well,” he advises. Might we also point you to this article about Asian sunscreens, most of which typically protect from both rays.

What do the different numbers mean…
Let’s use SPF 30 as an example. If you typically start burning without any protection after 15 minutes in the sun, then an SPF 30 sunscreen will protect you for 30 times that duration. So, more or less seven hours. Another way to look at it is this: SPF 15 protects you against 93 percent of UVB rays, SPF 30 against 97 percent, SPF 50 against 98 percent, and 100 guards you from 99 percent. Essentially, the higher the number, the better the protection…

But, there’s a catch…
“While theoretically, there is little difference between any sunscreen above an SPF 30, in the real world, we are not applying as much as we should and we are not reapplying,” Dr. Zeichner says. So, starting out with a higher SPF, he says, “is like an insurance policy to give you the highest quality of protection.”

SPF 100 is…
Real! Not just some marketing ploy from beauty brands to get you to spend more money! There’s an actual study proving its effectiveness! It’s also something you should look into. Yes, we know what Dr. Zeichner just said, but he also said that most people aren’t adamant about the whole reapplication thing, so it’s better to be safe and protected than sorry and sunburnt.

All of this is null and void if…
You don’t use the product correctly! Dr. Zeichner recommends using a quarter-sized dollop of sunscreen for your face and one ounce (the size of a shot glass) for the rest of your sun-exposed body. And, very importantly: In order to get adequate protection, you should be reapplying every two hours, as sunscreen loses its effectiveness over time. Also, “no sunscreen is completely waterproof,” he says. “Some sunscreens are labeled as water resistant, but you should still reapply immediately after swimming or after heavy sweating.”

Doctor's orders, friends.