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Go to Brilliant.org/SciShow to learn how you can take your STEM skills to the next level! [♪ INTRO] There’s nothing better than sitting by a window on a sunny, crisp Fall day and watching the world go by. But next time you sit in front of a window, it might be worth putting on some sunscreen to prevent skin damage.

Sunlight is made out of different wavelengths of light. Some aren’t visible to humans, like ultraviolet, also known as UV light. UV light is sorted into three types or classes based on how much energy it has.

And that amount of energy determines what light is able to pass through the atmosphere, or glass for that matter. For example, UV-C has the most energy, but it’s blocked by the ozone layer in our stratosphere. This is because UV-C has just the right amount of energy to break up those ozone molecules.

So, UV-C uses up all the energy it's got by breaking those bonds and it can’t go any farther through the atmosphere. UV-B has mid-range energy. Some of it can still break up some of the oxygen and ozone, but while most of the UV-B is interacting with those molecules, some of it goes through the atmosphere and makes it to the ground, reaching you.

And UV-B has the right energy to interact with your skin, which results in UV damage to your skin cell’s DNA, causing sunburns. UV-A has the lowest energy of the three. But don’t let that lull you into a false sense of security.

Its long wavelength means that it passes through the oxygen, the water, and everything else in the atmosphere pretty much undisturbed, so most of the UV exposure you will get is UV-A. This radiation, unlike UV-B, penetrates deeper into your skin because it hasn’t found anything to interact with yet. That is, until it hits the inner layers of your skin.

It interacts with your dermis, wreaking havoc. To be clear, both UV-A and UV-B are bad for you. They work in slightly different ways, but they’re both good at breaking important cellular bonds and increasing your risk of skin cancers and premature aging.

But if you’re behind glass, you can block some of those wavelengths. Most glass is actually really good at blocking UV-B light because it’s made up of impurities that are efficient at absorbing that wavelength. This is also why the glass gets hot after sitting in the Sun; all that UV-B gets absorbed by the impurities, which dissipates as heat.

And since UV-B is the main cause of sunburns, you’re unlikely to get burnt behind a window, unless you spend a very long time there. But there’s another caveat. Unfortunately for your skin, UV-A is a different story.

Some of it can pass through regular glass, because its energy level just isn’t right for any bond-breakage or interaction at all. So it can still reach your skin to cause skin damage. However, if the glass is covered with a solar film, UV-A can be blocked because it’s able to interact with the tint and not cross.

But that’s not the case for most windows like the ones in airplanes or some cars. So, dermatologists, other doctors, and health agencies have started recommending that you wear sunscreen even if you’re indoors. And given what we’ve just learned about windows, it seems like good advice to follow.

If you want to learn more about the world around us and how it impacts our bodies’ you should check today’s sponsor, Brilliant! They’re a website and app built off the principle of active problem solving: where you can learn by doing. And they’ve just made their courses even more interactive.

Like the course “Waves and Light” where you’ll explore waves in all their glory because everything we hear and see is based on how our bodies interpret waves. So if you’d like to learn more about the waves of our everyday life or other STEM topics, you can check them out at Brilliant.org/SciShow, where you can also get 20% off an annual premium subscription to Brilliant. So thanks for your support! [♪ OUTRO]