Science Questions

Why does hair lighten in the Sun?

Sun, 28th Sep 2008

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Rachel, Manchester asked:

If melanin causes the colour of our skin and our hair why does our skin go dark and our hair go light in the sun?


Ben - This is basically because hair is dead. Once it comes out of you, once itís grown, all the melanin it the sunwill ever have has already been put into it. It gets broken down by the sun but in your skin the melanin gets replaced when melanocytes put new melanin into your skin.


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Hi Rachel

the reason is that hair is a filament of protein that's produced by a hair follicle and extruded out through the skin. Within the hair follicle populations of cells called melanocytes add forms of melanin called phaeomelanin and eumelanin to the hair, giving it its colour.

But once the hair leaves the follicle and grows out onto your scalp it is beyond the reach of the melanocytes and therefore the melanin it carries cannot be replaced; sunlight falling on the hair can therefore photolyse (break apart) the melanin molecules, bleaching the hair.

Skin is different. A suntan is the result of damage to the basal layers of the skin by ultraviolet rays. This UV stimulus triggers melanocytes to increase their activity, adding more melanin to the overlying skin. As the cells are present, more sun means more UV and this means more melanocyte activity meaning more melanin. Unlike the hair the permanent presence of the melanocytes means they can alter the composition of the overlying skin.

Chris chris, Sat, 27th Sep 2008

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