This article was first published in The Skeptical Inquirer.
Fresh off the beach in Tenerife—laying on my stomach in bed, unable to turn over due to the red sunburn covering my back, and unable to sleep from the pain—I got to wondering about sunscreen and such. What exactly does SPF mean? Will a tan really protect me from further burns? Is aloe the best for a burn? Would people (including myself) take sunburns more seriously if we stressed that they are literal radiation burns?
Returned from paradise, with a peeling back and deeply contrasting tan lines, I’ve researched and busted some of my own internally held myths.
Why Is the Sun Dangerous Anyway?
The parts of sunlight that are dangerous to us are those on the higher energy side of the spectrum—ultraviolet light (UV). UV is divided into three categories: UVA, UVB, and UVC. UVC is the most energetic, and therefore the most damaging to our skin, but it is thankfully blocked almost entirely by the ozone layer.
UVB is, in many respects, the most famous of the three, as sun protection factor (SPF) is measured by a product’s ability to block just UVB wavelengths. UVB is also responsible for suntans and sunburns. UVA on the other hand contributes little to our short-term visible signs of sun exposure but is responsible for wrinkling and aging of the skin, as well as cancerous melanoma.
A sunburn is, as I said, literally a radiation burn. It is caused by ultraviolet radiation directly damaging the DNA of our skin cells. One of the main ways it does that is by causing thymine—one of the repeating molecules that make up our DNA—to form links with neighboring thymines. Our bodies recognize the radiation damage and trigger some protective mechanisms such as molecular-level DNA repair, killing off the too badly damaged cells (which is why we peel after a bad sunburn) and increased melanin production.
What Does SPF Actually Mean?
SPF was introduced in 1974. To understand how exposed a given SPF leaves you, remember that it is a sun protection factor. An SPF of 30 lets through 1/30 of UVB radiation, meaning that 3.33 percent of the radiation gets through to the skin, while 97percent is blocked.
Image from https://guavahealth.com/article/decoding-spf-what-you-need-to-know-about-sunscreen-protection
Unfortunately, SPF only tells us how protected we are against UVB radiation. UVA wavelengths can also cause photoaging, so it’s important to get a sunscreen that protects against both types of radiation. In the United States, FDA rules state that sunscreens marketed as “broad spectrum” must contain UVA protection proportional to their UVB protection. In the European Union, sunscreens carrying a “UVA” seal have at least one-third of their SPF as UVA protection.
Fun fact: SPF is literally measured on volunteers for new products. Often their bums!
Will the Tan I Get After This Protect Me at Least?
There are two different ways that the Sun makes our skin darker. First, and immediately, UVA radiation oxidizes existing melanin and causes it to darken and melanocytes, the cells that make melanin, to release any they are storing. This skin darkening happens quickly but doesn’t last more than a day or two.
The second process of tanning is melanogenesis. Over the first few days after UVB exposure, your body actually produces more melanin. This darkening of the skin lasts longer and does offer modest sun protection—an SPF of about 2–5 depending on the study.
Although tans do offer a small amount of UVB protection, their UVA protection is negligible. Even then, don’t overestimate the protection of a tan. An SPF of 5 means 20 percent of UVB radiation is reaching your skin, more than enough to cause a burn.
How Does Sunscreen Work?
Sunscreens basically, to quote Tim Wogan, “intercept UV photons before they hit cells and do anything nasty.” Sunscreens are typically divided into organic versus inorganic (or mineral) types. But they all contain components that absorb UV photons and dissipate them. In mineral sunscreens, those compounds are typically titanium oxide or zinc oxide, whereas there are many FDA-approved “UV filter” compounds for organic formulations.
Why Do I Have to Reapply It?
After a metal oxide or an organic molecule in sunscreen absorbs a photon’s energy, that energy is dissipated, and a negligible amount of heat is generated. Sometimes, however, absorbing that energy can lead to a molecule breaking up, rendering the application of more sunscreen necessary.
Also, swimming, sweating, and friction will remove it from the skin.
How Often Do I Need to Reapply?
Once every two hours is enough unless you go swimming or sweat so profusely that your sunscreen has been washed away!
Is Any of it Really Waterproof?
To quote the FDA website: “No sunscreen is waterproof.”
Eventually, all sunscreens will come off. But some are water resistant for a certain period of time, typically forty or eighty minutes.
Am I Applying Enough?
When SPF is tested and measured, it is based on an application of 2 mg of sunscreen for every 1 cm2 of exposed skin. According to the FDA guidelines, a 5’4” adult who weighs 150 lbs and is only wearing bathing suit bottoms, would use about 30 mL or 1 ounce of sunscreen. One study found that, in practice, participants applied sunscreen to a density ranging from 0.39 to 1.0 mg per cm2.
So, no. I am definitely not applying enough sunscreen.
Oh, and I should be rubbing it in slowly. A 1991 study found that if one is too vigorous, some sunscreen is removed in the process.
Is All Sunscreen Bad for Coral?
It’s true that a couple ingredients found in older formulas of organic sunscreens—oxybenzone and octinoxate—are harmful to coral reefs, causing bleaching and disruptions to their homeostasis that can have far-reaching ecological implications. But any sunscreen without those two ingredients is not harmful to coral, as far as we know. Oxybenzone-free varieties aren’t hard to find, but one safe option would be a titanium oxide mineral sunscreen.
Can Sunscreen Lead to a Vitamin D Deficiency?
I already split my living time between sunny London, England, and Canada, so my vitamin D levels are notoriously at risk due to low sun exposure. Does wearing sunscreen mean that I miss out on the UVB radiation my skin needs to make vitamin D?
Organizations such as Health Canada and the American Academy of Dermatology do not recommend getting vitamin D from sun exposure, but rather through diet and supplements. They believe that the risks of sun exposure outweigh the benefits.
As it turns out, we don’t need that much exposure to the sun to get enough vitamin D. One study found that for white adults in Manchester, UK, thirteen minutes of midday summer sunlight exposure three times per week with 35 percent of the body exposed gave sufficient vitamin D levels. Given that I walk dogs for 3–5 hours most weeks, I think, combined with all the fortified oat milk in my coffee and the supplements I take during the winter, I’ll be ok.
Does Sunscreen Actually Expire?
It’s not like I’m trying to use expired sunscreen, but we all know certain expiration dates matter more than others. Unfortunately for my cheap self, sunscreen is one case where expiration dates do matter, at least for organic varieties.
As time passes, the molecules responsible for catching those UV rays and saving your skin can break down, particularly if stored improperly. The older an organic sunscreen is, the more likely that it is no longer effective. Mineral sunscreens, however, are largely stable for years.
Speaking of proper storage, do not do what I did and fill transparent upcycled keychain Purell containers with sunscreen to easily carry around with you. Sunscreen containers are opaque for a reason! Store them away from heat and light so that their UV-catching power is saved for your skin.
Is There Any Evidence for Using Aloe on a Sunburn?
As I grit my teeth and my loving fiancé rubbed aloe gel over my lobster-red back, I wondered if this was even the right thing to do. Why aloe for a burn?
As it turns out, the medical care for mild to moderate sunburn is basically just supportive. That is to say, there’s nothing to be done besides things to make us more comfortable. Anti-inflammatory medicines can be given for pain, a lot of water to combat dehydration, and cooling compresses or showers to physically remove the heat.
Aloe is cooling, so it can help with pain and heat and may have some local anti-inflammatory properties that decrease local redness. It’s hydrating for the skin, which is good when the peeling begins, and putting it on lets us feel like we’re doing something.
But nothing we do actually decreases the healing time of a sunburn. It just makes us feel a bit better while we heal.
Social media trends, misinformation, changes in public health messaging, and a strong appeal to nature have all tried to challenge the prevailing medical conclusion that sunscreen is safe and UV exposure is not. But I have a feeling that if those considering believing in anti-sunscreen pseudoscience skip the sunscreen and get a sunburn as bad as the one I have now, they might reconsider. I know my use has gotten quite fastidious since that day.
