4 things you didn't know about sunscreen

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Starting next year, sunscreen—and the way its marketed—will change. This is good news. The changes correct some rather glaring examples of consumer misinformation. And it's also important news … at least, from the perspective of this redhead.

New Food and Drug Administration regulations mean that, by the summer of 2012, there will be no such thing as "waterproof" sunscreen. That's because, frankly, there already wasn't such a thing. A sunscreen might be more water resistant than a competitor. But you can't assume that one application of the "waterproof" stuff will stay with you through hours of pool time. Next year, sunscreen bottles will be honest about that fact, and they'll tell you how long you can expect water resistance to last.

The other big change: What the sunscreen protects you from. Under the new regulations, only broad-spectrum sunscreens—the kind that protect you from both the UVA and UVB wavelengths of solar radiation—with SPF values of 15 and higher, can claim to prevent skin cancer. Anything else must tell you that it's just for preventing sunburn.

These changes bring marketing into line with evidence, which always a good thing where public health is concerned. But, to me, they also bring up an important issue that remains unresolved. More and more Americans are using sunscreen. But that's not the same thing as more Americans understanding sunscreen. Information is important, and packaging can only be expected to do so much. With that in mind, I present this short guide to the seldom-discussed details behind an everyday product.

1) There's not absolute proof that sunscreen prevents cancer. You should still use it, anyway.

Here's the funny thing about cancer: It doesn't happen instantly. Instead, you get exposed to a carcinogen and, maybe (depending on how much carcinogen you were exposed to, how long you were exposed to it for, and other factors like genetics) you might develop a cancer 30 or 40 years later. By then, it's very difficult to determine cause and effect, both for the carcinogens themselves AND for things that prevent cancers from happening. This is a fact that's easy to forget. But it rears its head every time we try to talk about cancer—whether it's skin cancer caused by the sun, fears about brain cancers caused by cell phones, or the cancers that follow a nuclear power plant meltdown.

Here's the funny thing about sunscreen: Americans haven't been using it for very long. "The connection between sun exposure and skin cancer wasn't even talked much about at all just 30 years ago," says Christopher Arpey, M.D., professor of dermatology at the Mayo Clinic. "To say, 'you did this when you were 20 and now here you are 40 years later, and this is why you've got skin cancer or don't have it,' that's hard to do. It takes a really long time to build that evidence."

We do know that using sunscreen prevents sunburns and premature aging of the skin, but those are different things from cancer. The risks and the evidence available also depend on the type of cancer. Reynold Tan, Ph.D., is an interdisciplinary scientist with the FDA's Office of Nonprescription Products/Division of Nonprescription Regulation. He says there's stronger evidence supporting sun exposure as a cause of, and sunscreen use as a preventative of squamous cell carcinoma—a specific type of skin cancer that's been strongly linked to repeated, chronic skin damage from solar radiation. On the other hand, you have something like melanoma, which is known to have a strong genetic component, and there the outlook is still a lot more hazy.

But, despite the uncertainty here, sunscreen is still important. While we don't have all the evidence yet, the weight of evidence that does exist supports sunscreen use as a way to prevent skin cancer. "We know that regular sunscreen users develop fewer precancers over time," Dr. Arpey says. Even if sun exposure isn't the only cause of skin cancer, there's a good chance that it's an important cause. There's a lot we don't know, but we can still act on the information we do have.

2) SPF isn't a percentage, and it is kind of relative.

To understand what Sun Protection Factor really means, you have to know what happens in the studies that determine which sunscreen gets awarded what SPF value. These experiments are done with people, volunteers who have sunscreen applied to some parts of their bodies, but not to others. Then, they're exposed to UV light, and researchers measure how much longer it took the protected parts to burn compared to the unprotected parts.

SPF 30 just means that it took a dose of UV 30x larger to induce a sunburn on the protected skin, compared to unprotected. But that's not the same thing as saying that, if you put on some SPF 30, you can stay out in the sun 30x longer.

"It's a dose," Tan says. "And dose is intensity and time. So, depending on the intensity of the sun on that particular day, it's not necessarily 30x longer. Also the 30x higher dose is measured in a laboratory. When you're outside, you're sweating and you're rubbing it off on your clothes."

One downside to the new regulations is that they'll make this even more confusing. That's because the experiment I just told you about, and the SPF rating that comes out of it, really only applies to UVB wavelengths—the stuff that's most associated with causing sunburns. It says nothing about protection against UVA, which is most associated with causing cancer.

Beginning next year, companies will have a big incentive to get their products certified as "broad spectrum," protective against UVA and UVB. To do that, the FDA runs a different test, putting sunscreen on a slide and examining it with a spectrophotometer. By observing which wavelengths of light get absorbed, and which don't, the researchers can see how well the sunscreen does at protecting a person from UVA.

The SPF ratings on "Broad Spectrum" certified sunscreens will take this into account, Tan says. But those ratings will be relative only to other sunscreens. "Broad Spectrum SPF 30, compared to Broad Spectrum SPF 15, provides higher UVA and UVB protection," he says. "But you can't really say it's double the UVA protection. The only way to objectively measure that would be to look at absorption curves themselves and see how well a product works against each individual UVA and UVB wavelengths, but that's not very helpful for consumers."

3) You don't use enough sunscreen

At least, not enough to assume that the SPF rating on the bottle applies to you. Remember the experiment that determines a sunscreen's SPF rating? In those studies, sunscreen is slathered on thick—2 milligrams per centimeter squared, according to Reynold Tan. In real life, he says, people use something like 1/2 to 1/4 as much.

This isn't necessarily a problem, Dr. Arpey says. He's more worried about compliance—whether people are using sunscreen at all—than whether they're following manufacturer's instructions to the letter. If you're more likely to use sunscreen when you put on less, then put on less. But just remember that, when you do, your sunblock isn't operating at the advertised strength.

4) Sunscreen absorbs solar radiation.

Well, anyway, some sunscreens do. There are really two types. You can get sunscreen that works by simply being a physical barrier, reflecting the light away from you. That's how products with zinc oxide or titanium dioxide as the active ingredients work.

Everything else involves large molecules that mimic natural melanin by absorbing energy from the light, rather than blocking it. And that brings up another question: What do the molecules of active ingredient in those types of sunblock do with all that energy? The answer involves atoms and electrons.

You know that electrons go around the nucleus of an atom. The regions of space those electrons move through are called "orbitals." It's helpful—although not wholly accurate—to imagine them as the Sun, encircled by the orbits of planets. Unlike planets, though, electrons can move from one orbital to another, when they absorb the right amount of energy. Imagine Earth bouncing up suddenly from its orbit to that of Mars. That's essentially what happens with the electrons in the atoms of your sunscreen's active ingredient are hit with solar radiation.

"Electrons dispel the energy by moving to a higher orbital," Tan says. "Then they come back to stable lower orbitals." This process happens quickly, he says, over and over and over during the time you're wearing sunscreen. And it gives the radiation energy from the sun something to do besides penetrate your skin.

Image: Beer, cigarettes and sun block: Roskilde Festival 2009 essentials., a Creative Commons Attribution (2.0) image from wouterkiel's photostream