You’re sitting at home and enjoying your favorite show, with no intention of going outside. Do you still need to wear sunscreen?

You may think that if you roll out of bed, get in your car and drive to work, where you sit at your desk all day, that you don’t need sunscreen. Or, maybe you’re indulging in a lazy Sunday, binge-watching your favorite show, and you have no intention of brushing your hair, much less putting on sunscreen.

But, is it really the case that you don’t need sunscreen indoors? Or is that a common misconception?

Setting aside the obvious health benefits of getting moving, getting outside and getting exercise, you may, indeed, be harming your skin and upping your rates of skin cancer, if you don’t wear sunscreen — even indoors. That’s because, while glass effectively blocks most ultraviolet (UV) rays, it does not block all of them in equal measure. To better understand this, it’s also important to know the difference between two types of UV rays: UVA and UVB.

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UVA rays are generally linked to the aging of skin cells and tend to be the cause of wrinkles, sunspots and other signs of sun damage. UVB rays, on the other hand, are stronger and can directly damage the DNA in skin cells. UVB rays are also the principal cause of sunburns and are linked to most skin cancers.

According to the American Cancer Society (ACS), the glass typically used in car, home and office windows is designed to block most UVB rays, but it does not offer protection from all UVA rays. So even if you’re indoors, if you’re close to a window you still run the risk of exposure to UVA rays and possible skin damage.

“Unfortunately, there is a lot of misinformation out there about the use of sunscreen, leading individuals to avoid its use,” says Maria Teresa Ochoa, MD, a dermatologist at USC Dermatology of Keck Medicine of USC and professor of clinical dermatology at the Keck School of Medicine of USC. “Sunscreen is one of the most important tools in preventing skin cancer,” Ochoa explains.

So, if your desk or prime TV-watching spot is next to a window, it’s best to err on the side of caution and take the necessary steps to protect your skin. And, it’s not just indoors that you have to be vigilant: It’s in your car, too. Studies have shown that, in the United States, sun damage is significantly worse on the left side of the face and left arm than it is on the right side.

While most windshields are treated to protect against UVA and UVB rays, more often than not, the side and rear windows are not. That means, if you’re driving an hour each way to work, you’re getting two hours a day of exposure (and that’s not counting time at your desk or time actually spent outdoors), if you’re not wearing a broad spectrum UVA and UVB sunscreen.

To be on the safe side, apply sunscreen on a daily basis, whether you plan to be outdoors or not. Your skin will thank you for it down the line.

by Anne Fritz

If you are local to Southern California and are in search of a primary care physician, call (800) USC-CARE (800-872-2273) or visit www.keckmedicine.org/request-an-appointment/ to schedule an appointment.