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When you go to buy sunscreen, you might look for one thing: SPF.
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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.
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Except… that's not totally true.
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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.
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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.
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First, as a disclaimer, it's worth noting that countries approach sunscreen regulations differently.
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This episode is a good general overview, but keep in mind that things might be different where you live.
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Okay, that being said, onward.
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Essentially, SPF measures how much radiation is blocked from reaching your skin.
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So if it's SPF 30, it will protect you against thirty times more exposure than if you didn't wear sunscreen.
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That translates to roughly 97% protection, and it explains why anything higher than SPF 30 isn't that much more effective.
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SPF 50, for example, blocks fifty times the radiation, or 98% of it.
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Which isn't that different.
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Still, regardless of what that number says, SPF only tells you how protected you'll be from a specific type of radiation.
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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.
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It doesn't have to show protection against UVA radiation, which causes wrinkles, premature skin aging, and again, skin cancer.
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So even if you're using SPF one thousand, that doesn't mean you're protected against UVA.
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If you want to avoid both kinds of rays, you'll need to look for broad-spectrum sunscreen.
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Another thing worth knowing about SPF is that the way it's tested isn't bulletproof.
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To figure out how much UVB something blocks, you'd think scientists would use some sort of skin-like compound in a Petri dish.
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Like, just shine some light on this stuff, and see how much radiation gets through.
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But while that is how they test broad-spectrum sunscreen, it isn't how they test SPF.
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To get an SPF rating, researchers use volunteers.
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First, they shine a UV lamp on one spot on participants' bare backs to get a baseline for their tendency to burn.
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Then, a lab tech applies a gob of sunscreen to a different spot and allows it to dry.
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Finally, the tech turns the UV light back on, and sees how much longer the participants last before turning red again.
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If they burned in four minutes unprotected, but 60 minutes with sunscreen, that earns an SPF fifteen rating.
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And if this process seems sketchy and full of issues…
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Yes.
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Yes, it does.
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For one, people turn red at different amounts of exposure.
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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.
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And then there's the amount of sunscreen used in these tests.
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While there's tons of variability between testers, protocol calls for no less than two milligrams of sunscreen per square centimeter of skin.
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And that's significantly more than people tend to use in real life.
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In one study, it was more than twice the amount.
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Ultimately, these tests just aren't representative of what happens in the real world.
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They don't take into account things like skin color, or how much sunscreen people actually use.
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So when you're buying this stuff, it's probably worth keeping those things in mind.
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For example, maybe this means it's worth putting on sunscreen more often than you currently do.
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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.
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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.
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This will let scientists get way more data points and will improve accuracy.
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At the end of the day, SPF can be a helpful way to understand what you're buying.
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To make an informed decision, you just need to know what it says and where those numbers are coming from.
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And it's also worth knowing that not all hope is lost, either.
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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.
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So, it's always worth lathering up!
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If you've ever wished you just didn't have to wear sunscreen — well, I've got some sad news for you.
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Almost all amphibians, reptiles, birds, and fish can make their own sunscreen.
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Mammals are kind of the odd ones out.
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If you want to learn more, you can watch our episode about the missing sunscreen gene after this.