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What is UV light and what does it do to our skin?


We’ve all heard that UV light is bad for our skin, but how bad is it really? In this post we'll find out what UV light is, how it works, and what it does to your skin at a cellular level. Learn all you need to know about UV light to protect your skin and stay safe.

What is UV light?

rays of sunlight shining through a cloud

UV light is short for "ultraviolet light." It's part of the invisible light spectrum, meaning we can't see it. However UV light is all around us, even on rainy or cloudy days. It's the sun's UV rays that are responsible for tanning our skin. However long-term exposure can cause signs of premature aging, and even cancer.

UV light is just one part of the electromagnetic spectrum. The whole spectrum is the range of all possible frequencies, lengths and energies. It includes everything from radio waves that can be miles long, through to gamma rays, which are less than one thousandth of a millimeter in length!

How does the electromagnetic spectrum work?

the electromagnetic spectrum

Light is a form of energy that moves in waves. The length of the wave determines what kind of light it is. Our eyes can pick up some wavelengths, and they are what we call the visible spectrum. That's the rainbow!

The visible spectrum is in the middle of the electromagnetic spectrum. Either side are wavelengths that are too long, or too short, for us to see.

At the bottom of the spectrum are electromagnetic waves with long lengths. These include radio waves, microwaves, and infrared light. At the top of the spectrum, the waves are very short. Here you'll find ultraviolet light, X-rays, and gamma radiation.

What types of electromagnetic rays are harmful to our cells?

x-ray of a person's wrist

You can probably tell that the shorter the wavelength, the more dangerous the rays become. All the wavelengths across the entire electromagnetic spectrum are types of radiation. That includes visible light! However only the shortest wavelengths have enough energy to do damage to our cells. The more compressed (shorter) a wavelength is, the more energy it has.

This energy is powerful enough to strip electrons from atoms. It's called ionizing radiation. Gamma rays, X-rays, and some UV rays are classed as ionizing radiation. They can do massive damage to our cells. All other types of rays on the electromagnetic spectrum are classed as non-ionizing radiation.

Microwaves are the only really harmful waves at the lower end of the spectrum. They move at the right frequency to affect some molecules, such as water. Microwaves make those molecules vibrate, which creates heat. That's how we can cook using microwaves.

Microwaves that occur naturally, or are generated by electronics such as cell phones, are generally too weak to do us any harm. Microwaves are also easy to shield against, which is why the microwave oven in your kitchen doesn't hurt you.

What is the most harmful type of UV light?

woman sunbathing on a cloudless day

All UV light is harmful to our cells, but only some types of UV rays are classed as ionizing radiation. That's because UV light can be as short as 100 nanometers, or as long as 399 nanometers. That's a big difference! The shortest UV rays have four times as much energy as the longest. Because of this, scientists split UV light into three different types.

  • UVA — has the longest wavelength, 315-399nm
  • UVB — is in the middle, 280-314nm
  • UVC — has the shortest wavelength, 100-279nm

We know that shorter wavelengths are the most dangerous, so that means UVC is the most harmful type. However only the shortest UVC wavelengths (under 125 nm) have the energy to be classed as ionizing radiation.

The good news for us is no UVC light ever reaches us. It all gets absorbed by the ozone layer. Sometimes we make UVC light, because its ionizing properties make it a really powerful antibacterial cleaner. But you don't have to worry about UVC exposure when you step outside.

Instead what matters most to us are UVA and UVB rays. Of these, UVB has the most energy and is most harmful. UVB light can't ionize your cells, but it still does a lot of damage.

How does UV light damage DNA?

graphic render of broken dna strand

UVA and UVB light can both harm our cells. They do this by damaging our DNA. DNA is made of four chemical molecules. These are:

  • Adenine
  • Cytosine
  • Guanine
  • Thymine

The way these molecules bond creates the coded information about how to make every atom and cell of our bodies. For example a chain of three adenine molecules (AAA) is the code for the amino acid phenylalanine, which your body uses to make proteins. Molecules of cytosine, thymine, and adenine (CTA) are the code for a different amino acid, aspartic acid. How amazing is that!

Unfortunately two of these molecules, cytosine and thymine, are at risk from UV damage. That's because they absorb UV wavelengths, so they soak up that energy like a sponge whenever we go out in the sun. When there is only one cytosine or thymine molecule, the energy isn't strong enough to do much hard. But when two of the same molecules are together, the combined energy separates their bond.

Not only does this start to break down the code in parts of our DNA, it also creates reactions with the surrounding molecules. Sometimes the separated molecules will break up another bond to pair with them instead. This causes damage to our DNA.

While that all sounds super scary, it happens all the time. The rogue bonds are called cytosine or thymine dimers, and around 50-100 of them form in each skin cell for every single second you spend unprotected in sunlight. That's a lot of damage, but then DNA contains a lot of information. If you unraveled the DNA in just one of your cells, it would be over six feet long.

Once these dimers have formed, that part of your DNA is broken. This causes problems for your cell when it's time to replicate, because it can't read the broken DNA to recreate it properly.

Of the two, thymine dimers are the least problematic. Our bodies usually guess how to handle these breaks in DNA and the dimer gets repaired during the replication process. However cytosine dimers are harder for your body to handle. They often get paired incorrectly during replication, and instead of fixing the problem, it causes a mutation. If this mutation happened on an important gene, such as a tumor suppressing gene, then it can lead to cancer.

What are the signs of UV light damage on the skin?

smiling older woman with sun damaged skin

Both UVA and UVB light are capable of damaging our skin, but most of the damage at a DNA level is caused by UVB light. UVA light doesn't have enough energy to get deep inside your cells and cause reactions between DNA molecules.

The damage done by UVA light is mostly on the surface. Wrinkles, dryness, skin discoloration, and other signs of premature aging are associated with UVA light exposure. However that doesn't mean UVA is completely safe. Some scientists believe UVA light does play a role in skin cancer formation.

UVB light damage happens deeper inside your cells. UVB exposure is responsible for most sunburn, liver spots, and skin cancers. It can also cause uneven lesions on the skin such as actinic keratosis (precancerous dry skin patches) or solar elastosis, which is caused by excess elastin and results in deep wrinkles.

UV light doesn't just harm your skin cells, either. It can cause eye damage such as inflamed or burned corneas, cataracts, or growths in the white of the eye called pterygium. And because UV light is causing all this damage, it also weakens your immune system as your body tries to recover.

Can UV damage be reversed?

woman on a beach applies sunscreen

Our bodies are awesome at healing themselves, but some damage simply can't be fixed. Sunburned cells die off — that's why we peel after we burn — and get replaced by healthy new cells. If the sunburn was very deep, it may take longer for it to fully heal. Remember the cellular turnover cycle slows down as we age, so those damaged cells will hang around for longer the older we get.

DNA damage is also hard to heal. While it might get repaired during the cell's next replication, it might not. And all those mutations add up over time, resulting in cumulative damage and an ever-increasing risk of serious side effects such as cancer.

When we talk about damage at a DNA level, or even a cellular level, we're talking about numbers so massive that they're impossible to picture. The average person has about 35 billion skin cells. If each one of them suffers 50-100 cytosine or thymine per second every time you go outdoors, that's more DNA damage than you can imagine.

What's truly amazing is that our bodies eventually heal almost all of it. But some damage will always fall through the cracks, and over a lifetime it accumulates to a point where it becomes visible on our faces.

Right now, there is no reliable way of undoing all the damage that UV light does. There is no magic cream or lotion that can fix broken DNA. All we can do is support our bodies as they heal themselves. Using skincare products that increase the cellular turnover cycle will help replace damaged cells faster. And taking supplements that give our bodies everything they need to build healthy new cells will support the replication process. But none of that can undo the damage completely.

The best, most effective way to protect yourself against the damage caused by UV light is to prevent it from happening in the first place. That means wearing sunscreen or sunblock every time you go outside.

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