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
Send your science questions to
dearscience@thestranger.com.
Would it be socialism to outlaw chemical sunscreens that cause cancer?
I understand that as a US Citizen it’s my job to protect the profits of private corporations, but I’d rather not get cancer. Do I have to get cancer to be a real American?
It’s not government’s job to regulate industry, right, because that’s big government, and so I can understand why chemical sunscreens have the right to ask for my money in exchange for cancer.
So you shouldn’t tell people that things give them cancer. You have a responsibility to the makers of cancer causing sun screen, and if you worked for the government, and did that, then that would be socialism. Right?
Oh Joy! I was just diagnosed with Melanoma, likely caused by frequent childhood sunburns back in the early ’60s. I just filled my house with cans of clear sunscreen at every door. It is probably like closing the barn door after the horse has left, but after the Dr. hacks a huge chunk of your skin out, you really don’t like the sun too much. I wonder if they make a male berka….
The reason many newer sunscreens go on “clear” is a result of nanotechnology — when the titanium dioxide particles are small enough, they become clear, rather than the white that we are used to. (Virtually any material that is “white” uses titanium dioxide for color, and even many that are not white use it as a base color.)
Reasons for me to use the “white paste” sunscreens: I’m sensitive to chemical sunscreens, and they make me feel like I already have a sunburn. And no matter how non-greasy or oil free they promise they are, wearing them still makes me feel like I’ve been dipped in a vat of vegetable oil.
Of course, finding a non-greasy zinc or titanium sunblock isn’t easy either. I have to order them on the internet instead of just going to the local drugstore. It’s easier to just stay out of the sun altogether when I can. On the one hand, I look about 10 years younger than I actually am, which is nice, on the other I’m too embarrassed to wear shorts in public because my legs just about give off their own light.
Uhhhm, no. Your last paragraph is incorrect. Free radicals coming from the components in the sunscreen (generated by sunlight) are not going to transport through your dermal cell walls. Now, free radicals created inside the cell (because you’re not using sunscreen) could cause DNA damage…resulting in melanoma.
Another disturbingly unreferenced article. Another simmering controversy in medicine that this physician has never heard of. And a great mystery: how do those free radicals, subatomic particles, make their way unperturbed through layers and layers of dead skin cells full of keratin and whatnot, down to living cells and beyond them to the cells that regenerate to replace the upwardly migrating skin cells, you know, the ones that accumulate DNA damage over the years and eventually cause precancerous and cancerous lesions (not just melanomas)?
Here is the reference: Hanson, Gratton, and Bardeen; Free Radical Biol. and Med., 2006. Sunscreen enhancement of UV-induced reactive oxygen species in the skin. (or Google “sunscreen and free radicals” for reviews of the paper because it has private content)
The study was done on a skin cell line. The issue is that NOBODY KNOWS if the free radicals can penetrate cell layers in actual human skin or not because NOBODY HAS LOOKED YET. That’s why it’s a controversy. Also, there is evidence to suggest that more things that we put on our skin absorbs into the underlying tissues and capillaries than we ever thought before (see EWG website for a start) because certain synthetic chemicals that are only in skin-applied cosmetics/lotions (i.e. not in food) appear in people’s blood and urine. Remember that just because you learned some stuff about the epidermis from one class in med school 20-something years ago — from a teacher whose knowledge was itself 20 years out of date — does not mean that you are up-to-date with modern scientific knowledge. Also “I can’t think how it would happen” is not an acceptable substitution for “I’ve looked and it doesn’t happen,” thanks.
More research needed, please, on which substances can penetrate skin and under which conditions. And then it will trickle down to physicians over the next 40 years.