Dear Seth,
Since I’m a fluffy cat, my fur protects me from the sun. But my hairless cat and human friends use sunscreen when they go outside.
I asked my friend Michael Smerdon how it works. He’s a biochemist at Washington State University.
He told me that the sun puts out different kinds of light. Some light is visible to humans. Some is invisible—like UV light.
UV light can hurt your eyes and skin. It can even damage your DNA and cause cancer.
Here’s how that happens. DNA is also called your genetic code. It determines the traits that make you, you—like if you have brown eyes or weird-looking toes. It even influences what your personality is like.
DNA looks like a ladder that’s twisted or a spiral staircase. That shape is a double helix. The sides of the ladder are made of sugars. The “rungs” of the ladder are pairs of chemical bases—known as A, T, G and C—that hold onto each other.
Your cells copy your DNA when your body makes new cells or builds things your body needs. When that happens, a protein in your cells “unzips” the rungs of the DNA ladder. Then a replicating protein runs down one side of your DNA like a train on a track. It reads the code and builds a new piece of DNA that matches up with it.
But sometimes DNA absorbs UV light and gets damaged. Usually, the damage is where two T bases have gotten glued together—called a lesion or dimer. Most of the time, DNA repair proteins get to the damage before the replicating protein does. They snip out and replace the problem spot.
But sometimes that doesn’t happen, and the replicating protein gets stuck on the track.
Then your cell has two options. It can die. Or it can paste any base into that spot to get unstuck and keep going down the track. That can change the genetic code at that spot. That’s called a mutation. It’s different from the original code.
Sometimes mutations are no big deal. Other times they’re the start of cancer.
Fortunately for us, the Earth protects us from lots of the UV light that comes our way. It’s surrounded by a protective layer of oxygen—called the ozone layer—that blocks out some of the UV light.
“If it wasn’t filtered out by the ozone layer, you and I and everybody else on Earth would all be dying of skin cancer or some other direct effect from that UV light—literally within months,” Smerdon said.
But some UV light still gets through. That’s why you need sunscreen. It blocks or absorbs the UV light that gets through the ozone layer.
There are two ways sunscreen protects you.
A physical or mineral sunscreen acts like a barrier. It stops UV light trying to get into your skin and reflects it, so the UV light bounces off.
A chemical sunscreen works by sucking in the UV light. It changes that light into heat that puffs off into the air, usually with small bits of water.
Some sunscreen contains both kinds of ingredients, so it blocks and absorbs. With that sunscreen smeared on your skin, UV light can’t get through to hurt your skin or DNA.
But don’t forget that you must reapply sunscreen every few hours to keep your skin safe. That’s how you prevent damage that could make you sun-scream.
Sincerely,
Dr. Universe