It stands for sun protection factor, and based on that name, it seems like it should tell you how well you'll be protected from the sun.

Except… that's not totally true.

Because while SPF might sound like a perfect, rational scientific unit, there are a lot of flaws in the way it's named and the way it's tested.

So if you want to be as protected from the sun as possible, you're going to have to know a lot more than that one number.

First, as a disclaimer, it's worth noting that countries approach sunscreen regulations differently.

This episode is a good general overview, but keep in mind that things might be different where you live.

Okay, that being said, onward.

Essentially, SPF measures how much radiation is blocked from reaching your skin.

So if it's SPF 30, it will protect you against thirty times more exposure than if you didn't wear sunscreen.

That translates to roughly 97% protection, and it explains why anything higher than SPF 30 isn't that much more effective.

SPF 50, for example, blocks fifty times the radiation, or 98% of it.

Which isn't that different.

Still, regardless of what that number says, SPF only tells you how protected you'll be from a specific type of radiation.

The rating you see on sunscreen bottles is only required to show protection against UVB rays, the type of radiation that causes sunburns and leads to skin cancer.

It doesn't have to show protection against UVA radiation, which causes wrinkles, premature skin aging, and again, skin cancer.

So even if you're using SPF one thousand, that doesn't mean you're protected against UVA.

If you want to avoid both kinds of rays, you'll need to look for broad-spectrum sunscreen.

Another thing worth knowing about SPF is that the way it's tested isn't bulletproof.

To figure out how much UVB something blocks, you'd think scientists would use some sort of skin-like compound in a Petri dish.

Like, just shine some light on this stuff, and see how much radiation gets through.

But while that is how they test broad-spectrum sunscreen, it isn't how they test SPF.

To get an SPF rating, researchers use volunteers.

First, they shine a UV lamp on one spot on participants' bare backs to get a baseline for their tendency to burn.

Then, a lab tech applies a gob of sunscreen to a different spot and allows it to dry.

Finally, the tech turns the UV light back on, and sees how much longer the participants last before turning red again.

If they burned in four minutes unprotected, but 60 minutes with sunscreen, that earns an SPF fifteen rating.

And if this process seems sketchy and full of issues…

Yes.

Yes, it does.

For one, people turn red at different amounts of exposure.

So if you used a bunch of pale people in one study and a bunch of people with darker skin in another, you'd get totally different results.

And then there's the amount of sunscreen used in these tests.

While there's tons of variability between testers, protocol calls for no less than two milligrams of sunscreen per square centimeter of skin.

And that's significantly more than people tend to use in real life.

In one study, it was more than twice the amount.

Ultimately, these tests just aren't representative of what happens in the real world.

They don't take into account things like skin color, or how much sunscreen people actually use.

So when you're buying this stuff, it's probably worth keeping those things in mind.

For example, maybe this means it's worth putting on sunscreen more often than you currently do.

If nothing else, though, the good news is that researchers are at least aware that there's a problem here, and they've been working on protocols to hopefully get more consistent results.

Their ideas include using robots that spread sunscreen evenly in all conditions and shifting towards standardized in vitro testing using chemicals in a dish — no human skin involved.

This will let scientists get way more data points and will improve accuracy.

At the end of the day, SPF can be a helpful way to understand what you're buying.

To make an informed decision, you just need to know what it says and where those numbers are coming from.

And it's also worth knowing that not all hope is lost, either.

Even if this is a bit of a flawed system, multiple studies have shown that consistent use of sunscreen, even at low SPF, reduces your risk of skin cancer.

So, it's always worth lathering up!

If you've ever wished you just didn't have to wear sunscreen — well, I've got some sad news for you.

Almost all amphibians, reptiles, birds, and fish can make their own sunscreen.

Mammals are kind of the odd ones out.

If you want to learn more, you can watch our episode about the missing sunscreen gene after this.