Almost every bottle of sunscreen at the drugstore says it’ll reduce your risk of skin cancer, but that’s not why sunscreen was invented. In fact, sunscreen is much, much older than our understanding of skin cancer. People were processing stuff from nature to make sunscreen millennia ago. For example, ancient Greeks and Egyptians smeared all kinds of things—like oil, myrrh, and rice bran—all over themselves to try and prevent tanning.
But the roots of modern sunscreens can be traced to a single product: Ambre Solaire, created by Eugène Schueller in 1935. Back then, the link between the sun and skin cancer was not well understood. In fact, Ambre Solaire was invented nine years before anyone realized that DNA carried our genetic information, 18 years before we knew the structure of DNA, and more than 40 years before we knew that cancer could be caused by DNA mutations. That’s because Ambre Solaire was invented to try and prevent sunburn, not skin cancer. In 2012, the FDA’s sunscreen labeling rules officially went into effect, specifically allowing manufacturers to claim that a sunscreen “decreases the risk of skin cancer.” To figure out why the FDA allows manufacturers to make this claim, let’s look at two of the most common active ingredients in sunscreens sold in the United States: zinc oxide and oxybenzone (also known as benzophenone‑3).
You might have read that zinc oxide is a type of “physical” sunscreen, and oxybenzone is a type of “chemical” sunscreen, and that the former reflects photons like a shield and the latter absorbs them like Whitney Houston’s bodyguard absorbs bullets in her Oscar-nominated hit The Bodyguard.
That’s wronger than an Oreo in orange juice. What they actually do is a lot weirder. Let’s look at oxybenzone:
To give you a sense of size, there would be roughly 700,000,000,000,000,000,000 molecules of oxybenzone in a typical quarter-size splooge of sunscreen, and if you apply the recommended dose to your skin, you’d be spreading about 8,400,000,000,000,000,000,000 molecules of oxybenzone over every square inch of your exposed body.
When an ultraviolet photon from the sun hits a molecule of oxybenzone on your skin, it sets off a somewhat complicated chain of events. First, the photon crashes into an oxybenzone molecule, putting it in an excited state, which just means it has more energy than it did before. The molecule looks the same:
We just add a little * to show that excited state. But what happened to the photon? It’s gone. Vanished. Poof. Oxybenzone absorbed it, preventing it from hitting your DNA and potentially damaging it. So far, this actually does sound kind of similar to what a bodyguard would do: take a bullet for someone else. But wait. There’s more.
Because oxybenzone is in an excited state, you now have an excited-state molecule on your skin, which might be just as damaging as having a high-energy photon hit your skin. But oxybenzone can get rid of that extra energy through the power of DANCE!
First, some of the electron density in the carbon-oxygen double bond moves upward toward the hydrogen, which dissipates a bit of the energy:
Then, one of the bonds linking the two rings together rotates, twisting the right-hand ring out of the screen, like a propeller making a quarter-turn.
That leads to the ring on the right smacking a nearby molecule (say, water).
And that makes the water molecule vibrate a little more than before. So, tl;dr: Energy carried by the ultraviolet photon was dissipated by oxybenzone’s wriggling and transferred to the water molecule.
Then the bond that turbo-prop’d into the water molecule rotates back:
And we’re almost back to where we started:
Notice that oxybenzone has managed to dance itself back to the way it was at the very beginning, before it was hit by a photon. So this series of sloppy dance moves that generate heat is actually a cycle: An ultraviolet photon goes in; molecular motion comes out. Molecular motion is measured by something you know well: temperature. So oxybenzone essentially converts light energy to heat energy.1
Zinc oxide and titanium dioxide (the so‑called physical sunscreens) also cyclically absorb photons and convert them into heat energy, though the exact mechanism is different. Health blogs, news articles, and even dermatologists say they “reflect” or “scatter” UV light. In fact, some sources suggest they reflect or scatter only as little as 5 percent of UV light and absorb the rest. My suspicion is that the confusion arose because some formulations of zinc/ titanium sunscreens look like white cream cheese spread out on your skin. People just assumed that, since the sunscreens were scattering visible light—making you look like a bagel waiting for its salmon—they must also be scattering UV light. But whether something reflects visible light can be unrelated to whether it reflects UV light.
Back to oxybenzone. Its convert‑UV‑photon‑to‑heat cycle happens fast: It takes roughly ten-trillionths of a second for a molecule of oxybenzone to get back to the way it was.2 This means that one molecule of oxybenzone can absorb about 90,000,000,000 UV photons per second. If you apply the FDA-recommended amount of SPF 30 sunscreen, what you’re doing is enhancing your skin’s ability to harmlessly dissipate the energy from well over 700,000,000,000,000,000,000,000,000,000,000 ultraviolet photons crashing into you per second.
So, to summarize: Our species has engineered a creamy white splooge that you spread over your body to convert the potentially DNA-damaging energy from hundreds of million septillions of ultraviolet photons per second into mostly harmless heat.
On one level, modern sunscreen isn’t so far from smearing yourself with clays, minerals, or a mixture of sand and oil like the ancient Egyptians or Greeks did. But on another level, modern sunscreens are some mind-bending magico-chemical spellwork.
Our species should be patting ourselves on the back right now.
But does our little magic trick actually work?
That’s not just a philosophical question. It’s a practical one. Let’s say you’re at the drugstore buying a bottle of sunscreen because your dermatologist threatened to go on a hunger strike if you didn’t. Which one do you pick? Nobody would blame you for spending hours in the sunscreen aisle, completely confused. Bemused. Befuddled. Overwhelmed.
It’s not you. Sunscreens bear the most incomprehensible labels you’re likely to be confronted with. A representative example:
(This sunscreen is fictitious, and any resemblance to a real sunscreen is entirely coincidental.)
It doesn’t seem like it, but the label actually contains many of the clues we’ll need to figure out the practical (and philosophical) questions of whether a sunscreen works.
Let’s start with SPF. Both Merriam-Webster.com and the Oxford English Dictionary define “SPF” as “sun protection factor.” Both of these storied repositories of our hallowed English language have it wronger than peanut butter on pepperoni. “SPF” should really stand for “sunburn protection factor.” (Remember, Ambre Solaire was invented so that pasty white Europeans could get a suntan without risking a sunburn.)
SPF is kind of tough to wrap your head around. The first thing to know is that it’s not spit out by an algorithm; it’s a quantity that some unfortunate person in a nondescript medical office building somewhere actually measures. The procedure, which is mandated by federal law, goes roughly like this:
Find a white person (not off-white or cream; they have to be printer-paper-white).3
Cut out a stencil with two rows of rectangular boxes and put it over their lower back.
Smear a very specific amount (2.0 milligrams per square centimeter) of sunscreen through the bottom row onto their back and wait for it to dry.
Using a lamp that is designed to put out only ultraviolet light, give this white person increasing doses of ultraviolet light (as you go left to right on the stencil).
Wait a day, and then see how much ultraviolet light was needed to just barely give them a sunburn on the top row (no sunscreen) vs. the bottom row (with sunscreen).
Then calculate the SPF like this:
- Repeat with a bunch more white people and take the average of the SPFs you found.
So if you’re in the drugstore and holding two bottles of sunscreen in your hands, one SPF 25 and one SPF 50, you know that both sunscreens have been tested in a lab somewhere, by humans and on humans, and that the SPF 50 lets in roughly half the amount of sunburn-causing ultraviolet energy as the SPF 25 sunscreen. This is true of every legitimate sunscreen product in every major market in the world. So sunscreen really does work, in the sense that it unequivocally reduces your risk of sunburn.
When it comes to actually interpreting what the SPF means, we sometimes have trouble. Have you ever heard something like: If it takes 20 minutes for your unprotected skin to start turning red, using an SPF 15 sunscreen theoretically prevents reddening 15 times longer—about five hours. This is sorta technically true, but unfortunately, it leads to people doing math like this:
Suppose you think it takes you 20 minutes to burn without sunscreen. If you slather on SPF 100, you might think you can gallivant in the sun for 33 hours and not get burned. That’s some hot nonsense. Here’s why: First, you have no idea what the “time it normally takes me to burn” is. Second, that number is not fixed. It changes dramatically based on the time of day, time of year, where you are on Earth, what’s underneath you (sand? snow?), and what’s above you (clear sky? clouds?). And third, you almost never get the full protection of the SPF listed on the label. Why? Many reasons, the simplest of which is: Few of us, studies show, apply as much sunscreen as they use in the official test, 2 milligrams per square centimeter of skin.
That’s a lot of sunscreen. I tried putting that much on one summer and I felt like I had walked through an I Can’t Believe It’s Not Butter! carwash. For this reason, most people seem to apply half this amount or less. And this leads to another misconception: that people put on “too little” sunscreen. This is . . . meaningless.
Nobody tells you how much butter to put on yourself; you just go for however much feels right. Same with sunscreen. Just be aware that “what feels right” is probably about half what the FDA mandates. That’s actually one reason the bottle says to reapply: because it knows you didn’t put on “enough” the first time around.
Another very popular—and also wrong—interpretation of the SPF goes something like this: Once you get above SPF [insert random number between 10 and 30 here], the number doesn’t really make much difference. This myth is in the New York Times and Consumer Reports, on Gizmodo and the Encyclopædia Britannica’s website, and in peer-reviewed scientific articles written by practicing dermatologists. And everybody’s reasoning is very similar. It’s largely based on a table showing what percent of sunburn-causing UV light is absorbed by sunscreens of different SPFs:
Well-meaning people look at the table above and they write sentences like these:
An SPF of 15 blocks about 93 percent of the UVB radiation, while an SPF of 30 blocks out 97 percent of the UV radiation. This is only a 4 percent difference ...
This is wronger than meat loaf at a clambake. To see why, let me try to sell you a couple “bullet-resistant vests.” Vest A stops 93 percent of bullets. Vest B stops 97 percent of bullets. It seems like there’s only a 4 percent difference between the two vests, but consider this: If someone shot one hundred bullets at you and you were wearing vest B, you’d be hit by three bullets. In vest A, you’d be hit by seven—more than double vest A. Ditto with photons: the number of photons blocked by the sunscreen is totally irrelevant. The number that matters is how many get through.
With that in mind, let’s add a column to the table above:
There. Now we have a much better sense of how two different SPFs relate to one another: you can see that SPF 100 absorbs twice as many sunburn-causing photons as SPF 50, and SPF 30 absorbs twice as many as SPF 15 (assuming you put the same amount of sunscreen on, of course).
So should you go for the highest SPF available? In the late 2000s, sunscreen manufacturers certainly thought so: they were constantly trying to outdo each other by making ever-higher-SPF sunscreens. I tend to go for the highest SPF I can find, but this is definitely not a one-size-fits-all approach. There are legitimate reasons you might not want to use ultra-high SPF sunscreens. Using a lower SPF sunscreen might be a good way to psychologically trick yourself into reapplying.
Wait. What?
The logic goes like this: if you apply SPF ELEVENTY BILLION sunscreen, you might think, Oh, this is plenty to keep me 100 percent protected for the entire day, so I can just apply once and that’s it. Unfortunately, that’s not true. Any sunscreen—no matter what SPF—will eventually get washed away by all your beachy activities, or toweled off, or diluted by your sweat. So if you’re going to spend all day in the sun,4 you need to reapply. If, however, you’re using only SPF 30, you might not feel so protected, and may consistently reapply it throughout the day.
By the way, you may have noticed that sunscreen labels tell you to “apply liberally 15 minutes before sun exposure.”
Why?
Because sunscreen is not moisturizer. You don’t want to rub it under the top layer of your skin; you want it to form a protective barrier on top of your skin. So, contrary to basically everything you’ve been taught your entire life, the right way to put on sunscreen is to spread it very lightly over the surface of your skin, then let it dry. As it dries, it binds to the top layer of your skin. That’s what the 15-minute waiting period is for. If you put on sunscreen and then immediately put on your clothes, you might unintentionally wipe it off before it has a chance to bind to the top layer of your skin.
So. Does sunscreen work?
It absolutely does reduce your risk of sunburn. That’s crystal clear, because every commercial sunscreen is smeared on a person and the SPF is calculated by actually observing how much more ultraviolet light it takes to give that person a sunburn while they’re wearing sunscreen.
But the picture is a little muddier when it comes to skin cancer.
There are two basic types of skin cancer: melanoma and non-melanoma. A vast majority of skin cancers are non-melanoma, which can be further divided into either squamous cell carcinoma (SCC) or basal cell carcinoma (BCC). If you had to get cancer but could choose the type, you’d likely pick BCC: it tends to be very slow growing and rarely metastasizes. On the other hand, melanoma is often much more serious. It accounts for a minority of skin cancer cases but causes most of the deaths.
We absolutely know that the sun causes skin cancer. The question is whether using sunscreen protects against it. Intuitively, it seems like it would: we know it absorbs sunburn-causing ultraviolet photons. But as cancer researcher John DiGiovanna says, “Sunscreen isn’t a suit of armor. It can be overcome by too much sun.” Unless you’re submerged in a pool of sunscreen, some solar photons will absolutely get through to your skin; that’s one reason the FDA doesn’t allow manufacturers to use the word “sunblock.” But there’s also this:
Photons have different energies
different-energy photons can do different things to your skin
and different sunscreens may absorb photons of differing energy differently.
That’s a mouthful. Let’s break it down.
In Copenhagen in 1932, at the Second International Congress of Light—which sounds like some sort of Illuminati gathering—a bunch of physicists got drunk and created arbitrary divisions within ultraviolet light. You’ve almost certainly seen these arbitrary divisions before: they’re called UVA and UVB, and maybe your dermatologist explained them to you roughly like this:
UVB CAUSES SUNBURN (AND SOME CANCERS).
UVA CAUSES WRINKLING (AND SOME CANCERS).
This is not exactly true, but a perfectly fine simplification for our purposes. Early sunscreens absorbed UVB photons very well and absorbed UVA photons . . . not so well. You might call these sunscreens “narrow spectrum.” And narrow spectrum works great for shielding against sunburn-causing UVB photons, but to protect against a fuller range of the sun’s photonic assault you also need to absorb UVA photons. Hence the "broad spectrum" on the label.
The FDA allows any sunscreen that has an SPF of 15 or higher that also passes its broad-spectrum test to say that it “decreases the risk of skin cancer . . . caused by the sun.” What’s the evidence for that claim?
Um . . .
Well . . .
It’s kind of embarrassing to admit this, but so far there appears to have been only one randomized controlled trial that tested whether sunscreen could reduce the risk of skin cancer, and that trial was mostly focused on non-melanoma skin cancers. It found that sunscreen didn’t change the number of people who got squamous or basal cell carcinomas, but it did reduce the number of squamous cell carcinoma tumors diagnosed per person. This is not exactly the type of ironclad evidence you’d hope for, though I will point out two factors in this trial’s defense. First, it was conducted in the 1990s, which means it used pretty old sunscreen technology. If we redid the trial with modern sunscreens, we would expect a more dramatic result. Second, the control group in the trial was not prevented from using sunscreen; that would have been unethical. They were allowed to use sunscreen, but they used less than the full‑on‑sunscreen group. If people in the control group had been prevented from using sunscreen, we would also expect a more dramatic result.5
What about melanoma? Again, the evidence here is . . . less than ideal. The only randomized controlled trial on melanoma in adults was actually a continuation of the trial we just talked about. Both this trial and a couple of cohort studies suggest that sunscreen does have a protective effect.
Data on melanoma rates reveals a bit of a paradox: even though lots of white people throughout the world use sunscreen, melanoma rates have not gone down or even stayed flat. In fact, over the past thirty years, they’ve nearly tripled. If sunscreen protects against skin cancer, why are melanoma rates rising?
One explanation could be that people enjoy tanning and burning the living crap out of themselves more than they used to, so even though they use sunscreen, they also expose themselves to way more sun than they used to. Under this hypothesis, melanoma rates would be even higher if people didn’t use sunscreen.
But there’s another hypothesis. It was advanced by a Belgian epidemiologist named Philippe Autier, and although it’s supported by two (small) randomized controlled trials he’s helped conduct, it remains controversial. Autier believes that sunscreen use among white people who like to sunbathe actually increases total UV exposure, which could lead to melanoma. His thinking goes like this: White people like to intentionally expose themselves to the sun to get a tan, but they don’t like to burn. So they buy ultra-high-SPF sunscreen, which effectively absorbs most of the photons that cause sunburn. But because they’re not getting sunburned, these white people stay out in the sun much longer than their bodies would otherwise let them.
Basically, Autier believes that sunscreen lets you circumvent your biochemical “GTFO of the sun!” alert, thus allowing you to overdose on sun exposure. In a 2011 paper, he went so far as to say that the recommendation to reapply sunscreen—which is required by law in the United States—“probably represents a form of abuse.”
That’s pretty wild.
But where does it leave us?
As sunscreen expert Brian Diffey told me, it leaves us with “a dilemma.” On the one hand, the evidence that sunscreen protects against skin cancer is not as robust as evidence would be for, say, a new cancer drug. But on the other hand, we know that photons from the sun cause skin cancer, and we also know that our bodies do not respond well to too much sun. So what’s my take-home message here? Basically this: it’s better to avoid getting hit with ultraviolet photons. I don’t tan myself for fun, whether in the sun or in a tanning bed, and when I’m outside, I try to stay in the shade. Does this mean I avoid the sun like a vampire? Absolutely not. We all need a baseline level of ultraviolet light to make vitamin D (assuming you’re not getting it from your diet). Without enough of this essential nutrient, you can end up with rickets or osteomalacia, and low-but-not-dire levels might increase your risk of osteoporosis. Plus, sun exposure feels damn good, especially these times when we’re cooped up at home all day (...if we’re lucky). So don't be afraid to take that walk around the block. Uncover your arms! (Do throw on a mask and keep your distance, though!)
What if you have to be in the sun for a loooooong time? Should you wear sunscreen?
Here, I’d say . . . yeah, sure. Sunscreen will reduce the number of ultraviolet photons interacting with the molecules in your skin, and that might reduce your risk of skin cancer. So I say go for it. But I also think it’s a good idea to wear a hat. And clothes.
All illustrations courtesy of George Zaidan.
Sources for all of this information can be found here.
Adapted from: Ingredients: The Strange Chemistry of What We Put In Us and On Us by George Zaidan Copyright © 2020 by George Zaidan Published by arrangement with Dutton an imprint of Penguin Publishing Group/Random House/The Knopf Doubleday Group/The Crown Publishing Group, a division of Penguin Random House LLC
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