One of the strongest risk factors for skin cancer is direct exposure to the sun. Sunlight emits ultraviolet (UV) radiation, which can damage the DNA of the cells in our bodies. UV exposure causes mutations in DNA, and these accumulated mutations can lead to skin cancer. Melanin is a pigment present in the skin that gives skin its color. Melanin also helps protect skin against UV radiation and sun damage. In addition to protective effects of the skin, most of the damaging ultraviolet radiation from the sun is filtered out by the Earth’s atmosphere. However, at higher altitudes, there is less atmospheric filtering of UV radiation. Therefore, ultraviolet radiation exposure is increased at high altitudes, which explains the higher risk of skin cancer in regions with high elevation.
High Altitudes and Skin Cancer Risk
Leadville, Colorado is the highest United States city, with an elevation of 10,152 feet (3,094 m). The city is not only high in altitude, but also has high rates of skin cancer. To find out why, we need to take a closer look…or rather, a “farther look,” starting as far away as the sun.
Ninety-two million miles away from Earth, the sun emits radiation, which is basically energy. The earth’s atmosphere helps shield us from this radiation. But being at a higher altitude means there is less atmosphere between your skin and the sun, leaving it more vulnerable to radiation damage and ultimately skin cancer. To understand more about this process, we will explain in more detail principles of radiation, the skin, and skin cancer.
Energy—The Electromagnetic Spectrum
To understand the radiation from the sun, we will begin with an overview of all possible forms of radiation. These different forms are classified into a system called the electromagnetic spectrum (Figure 1) . Light and radiation behave very differently than the objects we interact with in everyday life. For example, you can not hold light in your hand in the same way that you can hold an apple. This is because light can behave like a wave. But believe it or not, light can also behave like a particle (like a very, very small apple!) and these particles of light are called photons. This means that electromagnetic radiation can behave like a particle or it can behave like a wave. Albert Einstein helped develop the theory of particle-wave duality .
Electromagnetic radiation can be described in terms of two main features: wavelength and frequency. Wavelength is the distance between successive crests of a wave. Frequency refers to the number of waves that pass a fixed point in unit time. In the electromagnetic spectrum, different forms of radiation are distinguished by their wavelengths, which range from extremely long (radio wavelengths are as long as skyscrapers) to very short (gamma ray wavelengths are the same length as a single atomic nucleus). In Figure 1, you will notice that the frequency is inversely related to wavelength. This means that as wavelength increases frequency decreases, and vice versa. Higher frequencies (and therefore, shorter wavelengths) correspond to higher energy. In general, higher energy radiation is more damaging to the human body .
Ultraviolet (UV) Radiation
Let us get back to sunlight. The sun emits mostly visible, ultraviolet, and infrared light . We will focus on ultraviolet light because it is primarily responsible for the sun’s effects on human health. There are three main types of ultraviolet light. UV-A has the longest wavelength and lowest energy, UV-B has a medium wavelength and energy, and UV-C has the shortest wavelength and highest energy.
UV-A composes 95% of the ultraviolet radiation that reaches the Earth, but it only has a small contribution to tanning and skin cancer development . UV-B leads to tanning and is the major culprit of sunburn and skin cancer . UV-C could cause the most detrimental health effects, but it is entirely filtered by the Earth’s atmosphere and does not reach the Earth’s surface.
What does it mean that the atmosphere “filters” the UV-C light? The ozone layer is a shield-like region around the Earth that absorbs most of the sun’s ultraviolet radiation. This prevents the high-energy, dangerous UV-C waves from reaching the Earth. This protective process is called atmospheric shielding. As an analogy, glass windows in your house absorb almost all UV-B light. This explains why you do not get sunburned when you hang out in a sunny spot in your living room. You can not see the filtration of light through the glass window but it is happening, just as the ozone layer is filtering out UV-C light! While the ozone layer is responsible for much of the atmospheric shielding, the entire atmosphere (from the Earth’s surface to the stratosphere) provides shielding effects .
Therefore, the higher the elevation above sea level, the less atmospheric shielding, and the more UV exposure.
Understanding the Skin
To understand how UV exposure causes skin cancer, we must understand the skin as well. The skin is composed of three main layers . The epidermis is the outermost, waterproof layer. The dermis lies underneath the epidermis and contains connective tissue, hair follicles, and sweat glands. Finally, the subcutaneous tissue, also called the hypodermis, contains adipose tissue (fat) and more connective tissue (Figure 2).
The epidermis contains melanin, a pigment that determines skin complexion. Melanin is efficient at absorbing UV radiation . In fact, melanin protects the body from 99.9% of UV-B light—so we can say melanin is photoprotective . Usually, UV light stimulates the production of melanin . This explains why sunbathing may lead to tanning in most individuals. However, some individuals never tan and only sunburn. The greater the number of sunburns a person gets, the higher the risk of skin cancer. This is because sunburns often indicate a high level of UV exposure, capable of causing DNA damage .
The Link Between UV Light and Skin Cancer
When melanin absorbs UV light, the energy in the light is not absorbed by the DNA. If UV light energy is absorbed by DNA, this can cause mutations. DNA mutations can be dangerous changes to the DNA that can lead to the development of skin cancers (Figure 2). These mutations can occur in genes that promote cell growth (oncogenes, get turned on) or in genes that prevent cell division (tumor suppressors, get turned off). Skin cancers usually occur as one of three types: squamous cell carcinoma, basal cell carcinoma, and melanoma. UV-induced DNA damage can cause any of these cancers.
It is important to recognize that sunlight is not all bad. There are several health benefits of sunlight, including vitamin D production, positive effects on mood, healing properties for some diseases, and prevention of some cancers . However, despite these benefits, excessive sun exposure is strongly linked to the development of skin cancers. In high-altitude locations, like Leadville, Colorado at 10,152 feet, people have higher risks for skin cancer. At this altitude, there is decreased atmospheric shielding from UV radiation, which leads to 50% more UV exposure than at sea level. So, the next time you go skiing in the mountains, make sure to wear your sunscreen!
Radiation: ↑ The release of energy either as particles or waves.
Atmosphere: ↑ The air and gases surrounding the earth or other planet.
Electromagnetic Spectrum: ↑ The range of wavelengths and frequencies over which radiation can exist.
Wavelength: ↑ Wavelength is the distance between successive crests of a wave.
Frequency: ↑ Frequency refers to the number of waves that pass a fixed point in unit time.
Atmospheric Shielding: ↑ Absorption of cosmic radiation by Earth’s atmosphere.
Melanin: ↑ A dark brown pigment located in the outer layer of skin, which is responsible for skin coloration and tanning; it also protects against UV light.
Photoprotective: ↑ Able to prevent damage from sunlight.
Mutations: ↑ Changes in the DNA sequence.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
 ↑ WikimediaCommonsContributors. 2020. EM Spectrum Properties: Wikimedia Commons, the free Media Repository. WikimediaCommonsContributors. Available online at: https://commons.wikimedia.org/w/index.php?title=File:EM_Spectrum_Properties_edit.svg&oldid=446224839 (accessed August 31, 2021).
 ↑ Sliney, D. H. 2016. What is light? The visible spectrum and beyond. Eye. 30:222–9. doi: 10.1038/eye.2015.252
 ↑ D'Orazio, J., Jarrett, S., Amaro-Ortiz, A., and Scott, T. 2013. UV radiation and the skin. Int. J. Mol. Sci. 14:12222–48. doi: 10.3390/ijms140612222
 ↑ Mead, M. N. 2008. Benefits of sunlight: a bright spot for human health. Environ. Health Perspect. 116:A160–7. doi: 10.1289/ehp.116-a160
 ↑ Nasti, T. H., and Timares, L. 2015. MC1R, eumelanin and pheomelanin: their role in determining the susceptibility to skin cancer. Photochem Photobiol. 91:188–200. doi: 10.1111/php.12335