Our Dangerous Sun

By Paul Skelly

Imagine living on a world so inhospitable, standing outside could kill you. The very light source you would use to see would also destroy your body and slowly kill you. You don’t have to imagine; that planet is called Earth. Luckily, our bodies have pigments that help protect our cells from ultraviolet damage. For additional protection, we can use sunscreen.

The sun, the star at the center of our solar system, gives off huge amounts of electromagnetic radiation. Some of this we see as visible light, but another, larger portion, of this spectrum is ultraviolet light.

Electromagnetic radiation – light – has properties that allow it to act both as a wave and as a photon. The waves with their varying wavelengths are what give us our wide spectrum of electromagnetic radiation types. Light also acts as a photon, meaning it hits objects with a certain amount of energy. This energy can be damaging to the cells of your body.

A photon is the energy carrying property of light. The Planck-Einstein relation (E=hf) tells us that the energy (E) of a photon is proportional to its frequency (f), where h is Planck’s constant. Ultraviolet light has a relatively high frequency and gives off a lot of energy that can penetrate deep into the skin and damage our cells.

Sunscreen is often advertised as a way to protect against skin cancer and UV light, but how does it work?  Sunscreen works by preventing harmful UV light from reaching our cells and damaging them. There are two ways to block UV light from our cells: physical blockers which reflect the light, and chemical blockers which absorb the energy.

Physical blockers work by reflecting or scattering light. UV light travels as a wave, and therefore can be reflected by certain materials. Just like different color paints get their color from reflecting light of that wavelength, different materials in sunscreen, such as zinc oxide or titanium dioxide can reflect UV light. Reflecting the light will prevent it from penetrating in and damaging the skin.

Zinc oxide and titanium dioxide work together to offer full spectrum protection from UV light. UV light is 100-400 nm, and using a machine called a UV/visible light spectrometer, the absorbance of light of different materials can be measured. Zinc oxide and titanium dioxide together have an absorbance pattern that covers a broad range of the UV spectrum, offering high protection. In addition, the refractive index tells us that light hitting the particles are slowed, reducing their energy. Refractive index is determined by n=c/v, where n is the refractive index, c is the speed of a light, a constant number, and v is the speed of light when it penetrates an object. For titanium dioxide, the refractive index is 4, compared to air’s refractive index of 1. This higher number means light is slower when travelling through titanium dioxide.

UV light as a photon carries energy with it, and this energy is what causes the damage to our cells. However, chemical blockers, made of organic compounds, can hijack this energy and use it to destroy itself instead of destroying our cells. The chemical blockers have bonds in them that will react with the energy in UV light to break the bond and release heat, destroying the blockers in the process. This reduces the energy reaching our cells, reducing the damage from sun exposure. These blockers are destroyed in the process of blocking UV light, which is why it is important to reapply sunscreen every two hours.

These are only the artificial products. Our body has its own sunscreen: melanin. Melanin works by absorbing UV light, and is believed to absorb over 99.9% of UV light that hits it. People with darker skin don’t just have more melanin, but have different types of melanin that is larger and believed to be better at absorbing UV light. Like physical blockers, melanin also works to scatter UV light and reduce its penetration. The melanin from people with darker skin colors has been found to have a lower amount of UV light penetrating into the skin.

Sunscreen is advertised with its SPF number, which means sun protectant factor. A higher SPF means that it blocks a higher percentage of UV light, but only to a certain degree as there is a limit to how much UV light can be blocked.

UV light is especially dangerous not just because of sunburns, but because it can damage DNA. That same energy that chemical blockers absorb to break their own bonds can also disrupt the bonds of our DNA. UV light carries enough energy that when it hits DNA, it can mutate the DNA. Any mutation to DNA is cause for concern, and if the body doesn’t detect the mutation and fix. it or destroy the cell, cancer can form.

Skin cancer has been a growing problem for the past 15 years. In the United States, sunscreen hasn’t been used enough and more and more people are being diagnosed with skin cancer. UV light exposure is the number one cause of skin cancer, and those with lighter skin are disproportionately affected.

The world is a dangerous place. Just walking outside can be harmful for over half the US population. The very sun that gives us life is also damaging our DNA and penetrating deep into the skin. Thankfully this energy can be absorbed or reflected by artificial and chemical sun blockers. UV light can’t be avoided, but proper use of sunscreen can reduce its damage.