Dear Science,

Summer is here! (Well, sorta here in Seattle.) With my particularly pasty body, I know what happens when I skip the sunscreen. But here's my question: How does sunscreen work, particularly the new stuff that really does go on clear? The old-school sunscreen, which caked on a white paste, makes sense. What I don't understand is how something that ends up clear blocks sunlight.

Sun-Loving Ghost

Let's talk radiation. If we were back in the 17th century, we'd be having a nasty debate: Is light a particle (team Newton!) or a wave (team Huygens!)? In a delightfully fair resolution, it's both a wave and a particle. Let's start with thinking of light as a particle first. Sunburns (and ultimately wrinkles and cancer) are caused by molecules in your body being damaged by these light particles (called photons). Think of these photons as bullets: The more energy the bullet contains, the more damage it can do. Now, let's add in the wave way of thinking. As the wavelengths of light get shorter, each particle contains more energy. Ultraviolet (UV) light starts at wavelengths just below what eyes can see and extends all the way down to X-rays. At the transition between visible light and UV light, this energy level is finally high enough to cause changes in the chemical structure of key molecules in the body—typically kicking around electrons. The goal of a sunscreen or sunblock is to prevent these high-energy particles from reaching your body.

What can a sunscreen do with sunlight? The energy can be reflected back. Here comes the white paste! The active ingredient in these sunblocks is typically a metal oxide, like titanium dioxide or zinc oxide. The idea here isn't too far from wrapping yourself in aluminum foil—the sunlight never makes it to your skin. The other trick is to chemically absorb the UV light. Chemical sunscreens are molecules crafted to love UV light, absorbing high-energy photons in order to change their shape. Since the molecule doesn't reflect light, it can be clear. Likewise, so long as these sunscreen molecules are crafted to absorb only wavelengths shorter than visible light, you can't see it after it's on.

Here's the problem with chemical sunscreens, part of a simmering controversy in the scientific and medical fields: What UV light most often does to electrons is split happy pairs into angry lone electrons. These lone electron-containing molecules are called free radicals—and can wreak all kinds of havoc in the body. Some chemical sunscreens form free radicals as a part of absorbing sunlight. Free radicals don't cause sunburn but can cause horrible DNA damage that can lead to melanoma. If you want to play it safe, stick with a nonchemical sunblock (based on titanium dioxide or zinc oxide). Or wear a hat.

Absorbingly Yours,

Science

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