Is it true we’re supposed to wear sunscreen indoors even if we’re not going to be out in the sun?
Yes, you should.
I know that feels counterintuitive, but let’s start by pondering this question: Why do people in the United States tend to get more skin cancers on their left side, while people in Australia appear to get more sun damage on the right?
The answer is UVA exposure while driving.
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As a brief reminder, visible light is the only light the human eye can see, as opposed to the shorter ultraviolet (UV) waves, which we can’t see. Standard window glass blocks most UVB – the rays that cause sunburn – but does little to stop visible light or UVA, which are known to accelerate skin aging.
The clearest evidence for this comes from a 2011 study published in the Journal of the American Academy of Dermatology, in which researchers analyzed more than 82,000 melanoma cases and roughly 2,400 Merkel cell carcinoma cases in the United States. Both cancers were slightly more common on the left side. In Australia, where drivers sit on the right side, older studies have found sun-damaged precancerous patches more commonly on the right.
Experimental data on the glass used in luxury cars in 2003 found that a left-sided driver next to a closed car window receives five to six times more UV radiation on their left arm than the right.
“Unless you’re in a box – in a box with no windows – you should still be wearing sunscreen on a daily basis,” said Temitayo Ogunleye, a professor of clinical dermatology at the University of Pennsylvania Perelman School of Medicine. “You’re still getting a decent amount of cumulative UVA exposure.”
So, yes, I do wear sunscreen every day despite working indoors. I commute at least 45 minutes to my office each way. I also face a nice large window behind my computer screen when I’m there. So even though I don’t actually step foot outside most of the day (tragically), my skin is getting UVA rays for much of my indoor time.
How light exposure affects your skin
UVA rays can reach beneath our skin’s surface, which is why they play a big role in premature skin aging and wrinkle formation in addition to cancer.
It turns out visible light may also impact our skin.
“Visible light matters even more for people with melasma, hyperpigmentation, or darker skin tones,” said Charlene Lam, an associate professor of dermatology at Penn State Health. “It triggers pigment-producing cells more aggressively in darker skin types and can worsen dark spots.”
An oft-cited 2010 study confirmed these effects: Researchers exposed people with various skin tones to both visible light and UVA. The scientists found that among people with darker skin tones, visible light actually produced darker and more persistent pigmentation than UVA did. Among volunteers with lighter skin, the same visible light exposure produced no observable pigmentation at all.
4 tips for picking an effective sunscreen
Look for iron oxide, especially if you’re concerned about hyperpigmentation or dark spots
When you’re shopping for sunscreen, you’ll come across two main types: chemical, such as those containing ingredients like oxybenzone or avobenzone, or mineral, such as those containing ingredients like zinc oxide or titanium dioxide.
Ordinary chemical sunscreen is largely ineffective against visible light. That’s why Lam recommends a mineral sunscreen with iron oxide – the ingredient that a 2015 randomized trial found helps block visible light and reduces recurrence of dark spots. (The study was supported by Laboratoire Bioderma, a skin care company.)
Iron oxide is often used as a tint for mineral sunscreens. As someone with a darker skin tone myself, I’m only too familiar with the whitish-gray cast that many mineral sunscreens leave on my skin. So, at least for me, the addition of iron oxide as a tint is a win-win: It can help these sunscreens appear less obvious while also bolstering visible light protection.
SPF 30 is better than you may think
It feels like there should be a major upgrade when you buy something labeled SPF 50 or even 100. But actually, the difference isn’t particularly striking.
“SPF 30 blocks about 97 percent of UVB. SPF 50 blocks about 98 percent,” Lam said. “That 1 percent jump is why I tell patients not to obsess over 30 versus 50.”
If that’s news to you – you’re not alone. I didn’t learn this until medical school.
Choose a broad-spectrum option
“SPF is the relative number of Joules of UV light needed to cause mild sunburn in sun protected skin versus sun unprotected skin,” said Jenna Lester, associate professor of dermatology and director of the Skin of Color Program at the University of California at San Francisco.
This means that SPF just tells you about UVB, which causes sunburn – and it tells you nothing about UVA.
So presuming you also want UVA protection (as I do), you need to pick a sunscreen that specifically says, “broad spectrum.” This means the sunscreen protects against both UVB and UVA.
Go for the bigger bottle
SPF numbers are calculated assuming a specific, generous amount of product – about 2 grams per square centimeter of skin in lab testing. “If you’re not using the equivalent amount, you’re not getting the SPF sun protection that’s on the label,” Lester said.
For your face, that roughly equates to a thick line of sunscreen along your pointer and middle finger. For your body, that means about the amount in a shot glass.
This is why the experts I spoke with agree that the most important sunscreen is whichever one you’ll actually wear routinely – and not be precious with.
As a surgeon who helps reconstruct skin affected by cancer, Lam finds the tougher spots to operate on are a few key forgotten areas: “Lips, the tops of ears and the scalp – especially if you’re thinning up top,” she said.
What I want my patients to know
For now, the main light to focus on in terms of skin damage is really from the sun – not, for example, the blue light emitted from our computers and smartphones. Skin damage from these devices is “theoretically possible but still understudied,” Lester said.
Overall, my recommendation is to wear sunscreen every day, even if you’re going to be indoors. But you especially should remember to wear it if you’re going outside.“The amount of exposure that you’re getting outdoors is still orders of magnitude higher than what you’re getting indoors,” Lester said.
