What Causes Skin Tanning?

Summertime, Credit; canva.com

What causes the skin to tan?

After a fun day at the beach, you come back home burning red. You should’ve listened to your mother when she told you for the hundredth time: don’t forget to wear sunscreen.

The largest organ of the body

Have you ever really thought about your skin? Our entire body is covered by it and its derivatives (hair, nails, and sweat glands ), making the skin the largest organ in the body. The skin, or as scientists call it the integumentary system , creates a barrier between the external and internal worlds. It serves important functions like protection (injury, microbes, and environment), thermoregulation  (regulation of body temperature), energy storage (increase in weight), and sensation (touch and temperature).

The layers of the skin

The skin is not just an elastic sheet, but a complicated structure made up of three basic layers.


1. Epidermis: the outermost layer with keratinocytes  associated with protection and skin color.


2. Dermis: middle layer with hair follicles  and sweat glands associated with sensation and regulation of body temperature.


3. Hypodermis: deepest layer made up of fat cells associated with insulation against cold weather, energy storage, and cushioning.

The components of the skin
The components of the skin, Credit: WIkimedia/BruceBlaus

Skin deep, literally. But despite the multiple functions and intricate details of each layer, our main focus in this article will be on the epidermis to better understand how it protects our skin from spending too much time under the sun.

Melanin to the rescue - Skin tones: how much melanin are you producing?

Human skin tone is remarkable for its beautiful diversity. Our skin color is determined by our genetics, as in what genes  we inherit from each of our parents. But what sort of instructions do these genes provide for the skin cells?


The color of our skin is produced by specialized cells called melanocytes located in the lowest layer of the epidermis. These are the cells that the genes regulate to produce the skin pigment known as melanin. The amount of melanin we produce determines the color of our skin.

Melanocyte, Credit: Wikimedia/BruceBlaus

It’s not just about the color, though. Melanin plays an important role in protecting your skin against the sun’s harmful ultraviolet (UV) rays, which can burn or kill skin cells and even damage the DNA enough to cause skin cancer.

Early human migration and vitamin D

Species adapt to their environment to maximize survival. Our skin tone underwent such a process, but this was the result of years of evolution and natural selection.


According to the evidence we have, modern humans (homo sapiens) originated in Africa. Groups of humans started to gradually move throughout the rest of the continents, away from the equator with the blazing sun. Although the sun’s UV light can be damaging to our skin cells, that same light plays an important role in producing vitamin D required for healthy bones and strong immunity. In regions like Africa where the sun is very strong, high melanin and dark skin were vital to protect our ancestors from UV but allow enough of it to go through the skin and stimulate vitamin D production. However, as our ancestors migrated north, sunlight became weaker and was unable to dodge the protective melanin to induce our skin to make vitamins. This put our ancestors at risk and natural selection began to select individuals with lighter skin; only humans with lower levels of melanin were able to produce vitamin D. This variation in skin tone was actually an evolutionary response that took years to manifest!

Out of Africa: early expansions of our ancestors
Out of Africa: early expansions of our ancestors, Credit: WIkimedia/NordNordWest

Why does your skin tan?

In addition to our genes, some of our traits are affected by the outside world to some extent. This allows our body to react to the outside world to improve survival. Tanning is one such example.

UV light and skin tanning
UV light and skin tanning, Credit: Wikimedia/scientificanimations.com

The UV light in the sunlight triggers the melanocytes to start producing more melanin. Melanin is then stored and sent out in delivery packages called melanosomes and eventually distributed to the other skin cells. This process leads to a gradual increase in the color of our skin to create a natural sunscreen that will prevent UV light from reaching the deeper layers of the skin and causing major damage.

Melanin in action
Melanin in action, Credit: Wikimedia/OpenStax College

This process is reversible, and with time and the absence of continuous sunlight our skin will go back to the original color set out by our genes.

How does sunscreen work?

Even though our skin produces this natural sunscreen, melanin is not enough to fully prevent sunburn, skin damage, and, potentially, cancer resulting from UV radiation. You can increase your protection against UV light by applying sunscreen and staying hydrated, while you still enjoy your time under the sun.

Broad spectrum sunscreens rely on physical and chemical processes to ward off two of the most harmful types of UV radiation: UVA and UVB. The sunscreen formula includes naturally occurring minerals that sit on your skin to create a mirror-like barrier that reflects UV light away. At the same time, other chemicals in sunscreen absorb the UV rays and undergo a chemical reaction that neutralizes the radiation. But where does “SPF” fit into this? SPF stands for “sun protection factor” and is often misunderstood as a measure of how strong the sunscreen is. This factor actually refers to how long it will take for UV light to redden your skin while wearing that sunscreen. For example, it will take 30 minutes longer for sunburn to happen while wearing sunscreen with SPF30 as compared to unprotected skin.

UVA vs UVB, Credit: canva.com

UVA can reach the dermis, causing chronic skin damage including premature aging and wrinkles. UVB can only reach the epidermis, causing skin burning and eventually cancer.

The facts are simple. As nice as a sunny day at the beach is, you can avoid the pain of a sunburn by simply applying your sunscreen a couple of minutes before running off to a fun day with your friends!


Integumentary system: the body’s outer layer, made up of skin, nails, hair, and the glands and nerves on the skin.


Thermoregulation: A mechanism by which mammals maintain inner body temperature.


Keratinocytes: The adult skin cells that are mostly found on the outermost layer of the skin.


Hair follicles: Bulbs that anchor each hair into the skin.


Sweat glands: Small structures of the skin that produce sweat.


Gene: The basic unit of inheritance that carries the information that determines our traits.

Flesch Kincaid Grade Level: 9.8

Flesch Kinkaid Reading Ease: 56

Montagna, W. (2012). The structure and function of skin. Elsevier.


Cichorek, M., Wachulska, M., Stasiewicz, A., & Tymińska, A. (2013). Skin melanocytes: biology and development. Advances in Dermatology and Allergology/Postępy Dermatologii i Alergologii30(1), 30–41.


Brenner, M., & Hearing, V. J. (2008). The protective role of melanin against UV damage in human skin. Photochemistry and Photobiology84(3), 539–549.


Abdulla, F. R., Feldman, S. R., Williford, P. M., Krowchuk, D., & Kaur, M. (2005). Tanning and skin cancer. Pediatric Dermatology22(6), 501–512.


  • Joseph Sahakian
    : Author
    Joseph Sahakian has a Master's in Biology and a minor in Psychology from Notre Dame University. Joseph is a strong advocate of scientific communication and he tries his best to break the jargon barrier to make scientific progress accessible to all. In his free time, Joseph enjoys reading and creative writing.

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