Putting on sunscreen is like applying an invisible supernatural force field. Summer may be the season when people’s thoughts turn to sunscreen, but rarely do we think about exactly how it works. It may seem like magic, but it’s science that makes sunscreen work.
The UV Alphabet
Skin is harmed by two different kinds of ultraviolet light—UV-A rays and UV-B rays. UV-A rays have the longest wavelength, and they penetrate deepest into our skin, causing premature aging and increasing our cancer risk. UV-B rays have a shorter wavelength, and although our atmosphere absorbs many of these rays, the ones that make it to earth cause sunburn. The sun also generates UV-C rays, but the atmosphere completely blocks these and they never make it to earth. The American Academy of Dermatology recommends this clever way to remember the UVs—UV-A rays cause aging, while UV-B rays cause burns.
Sun protection products work by either absorbing or reflecting away the harmful UV rays. There are two kinds of sun protection—chemical barriers and physical barriers. Chemical barriers soak into the skin like lotions, and the molecules in them absorb UV rays and release them as heat before they can do any damage. On the other hand, physical barriers sit on the surface of the skin, like the chalky, white zinc oxide used on the noses of lifeguards. It’s like covering your body in a thin mirror. Even though many people use the terms sunscreen and sunblock interchangeably, they are actually two different products. A sunblock must have an SPF of twelve or higher and contain a physical barrier, while sunscreens can be any product with an SPF of two or higher. Even a plain white tee-shirt has an SPF of about three. Since they are easier and more comfortable to use, most products we use today are sunscreens.
UVA vs. UVB: Do We Have to Choose?
The strength of sunscreen is measured by its SPF—sun protection factor. SPF only refers to protection against UV-B rays and sunburn; it makes no guarantee that a product will protect from the aging effects of UV-A rays. An SPF of 15 indicates that a person can be in the sun for fifteen times longer without burning than they could be if they weren’t wearing sunscreen. Although the formulation of SPF is standardized across different brands, the number has a different interpretation for everyone, since people naturally have different amounts of melanin and pigment in their skin. For someone with fair skin who burns after ten minutes, SPF 4 would only give them forty minutes of protection. Someone with darker skin, who can spend an hour in the sun without burning, would be afforded 240 minutes, or four hours of protection from the same product.
Sun protection doesn’t increase in proportion to a product’s SPF. That is, an SPF of 30 isn’t twice as strong as an SPF of 15. Sunscreen with SPF 15, the minimum number recommended by dermatologists, filters out about 93 percent of UV rays. SPF 30, on the other hand, filters out 97 percent. Doubling the SPF only provides about 4 percent more protection. Even an SPF of 50 only protects against 99 percent of the sun. Looking for the absolute highest possible SPF won’t hurt, but it won’t filter out 100 percent of the sun’s rays, and it doesn’t give anyone the license to stay out in the sun for hours. No matter what the SPF of a specific sunscreen is, most products wear off after two hours, making anything above SPF 30 a moot point. Even if the math says that an SPF 90 would protect for fifteen hours, there’s no way the product would last that long. Actual SPF depends on many factors, including how liberally the product was applied and whether the wearer has been sweating or swimming. Even products labeled as water-resistant or waterproof will wear off and need to be reapplied at least every two hours, probably even more often.
The Full Spectrum
SPF has no bearing on whether a sunscreen protects against exposure to UV-A exposure. For UV-A protection, look for sunscreens that explicitly offer broad-spectrum protection, because those products contain ingredients to fend off both types of sunlight. Many chemical sunscreens now contain microparticles of zinc oxide or titanium dioxide (the active ingredients in physical sunblocks) to act as microscopic mirrors and reflect away the sun, along with the ingredients that absorb the rays. Some new sunscreens are featuring ingredients that are longer-lasting and better-absorbed than older ingredients. When shopping for a sunscreen, look for ingredients like Parsol 1789 (avobenzone), oxybenzone, or Mexoryl SX, which all offer protection against both types of UV rays. If you don’t use sunscreen very often (although everyone should wear it every day), remember to throw away sunscreens that are more than a couple of years old. The FDA requires sunscreens to stay potent for about three years, but after the expiration date, the active ingredients begin to degrade and become ineffective.
I’m a sunscreen evangelist these days because there’s no way to get sun safely. Like most people, I hated the smelly, tropical-scented glop I had to wear to the beach when I was a kid, but I’ve now embraced the idea of fewer wrinkles, fewer burns, and looking years younger than I am. I love the idea of sunscreen as my personal invisible force field, and every time I reapply, I happily imagine those millions of tiny mirrors reflecting the sun back up into the sky—where it belongs.