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Asked by Ben to Chris, Josh, Rebecca, Rob, Susan on 16 Jun 2015.
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Rebecca Dewey answered on 16 Jun 2015:
We all have melanin in our skin, but some people have more than others. In general, people with darker skins have more than people with lighter skins, and people with black skins have the most of all. When the melanin in our skin is exposed to the UVA and UVB in sunlight, it turns dark, and causes a tan. Over time, this can break down again, and so you lose the tan after a while.
Melanin is opaque to UV rays and so protects people with tanned or darker skin (to a small extent) or people with black skin (to a greater extent) from the damaging effects of the sun. However, it also blocks out the light that helps our skin to make vitamin D.
Originally humans had a lot of melanin in their skin (more like the amount that people with black skin have now) but as prehistoric humans moved around the planet and those who moved to places with lower sunlight levels (such as Scandinavia etc.) did better with paler skin because they got more sunlight and were able to produce vitamin D!
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Susan Cartwright answered on 16 Jun 2015:
Why do we tan? – Because too much ultraviolet light exposure causes skin cancer. Increasing the pigmentation of the upper layers of the skin reduces the amount of UV light that gets to the sensitive layer, and thus reduces this risk (not enough, which is why you should wear sunscreen if you’re out in the sun!).
How do we tan? – Your skin responds to UV radiation by producing a chemical called melanin, which is an effective absorber of UV radiation. It is also quite an effective absorber of visible light, i.e. it is dark-coloured, so as your skin produces more melanin it appears darker. This process is less efficient in blondes and redheads, which is why they have light-coloured hair (melanin is also responsible for dark hair colour), and also why they tend to burn in the sun rather than tan, and are at greater risk of skin cancer.
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