Why does the sun lighten hair but darken skin?

Sunny beach hairWe have all experienced the effects of sunlight on our skin and hair, whether sunbathing at the beach or using tanning beds at the salon. Oddly, the sun lightens our hair, but darkens our skin. This strange quirk of nature comes down to a matter of life and death (and simple chemistry):

Responsible for these colour-changing effects is the natural pigment ‘melanin’, from the Greek ‘melas’, meaning ‘black’ or ‘dark’. This pigment is found in both skin and hair. Specialised cells known as ‘melanocytes’ produce two types of melanin: eumelanin (a black/brown pigment) and pheomelanin (a red pigment). Different amounts of one or the other give rise to different coloured skin and hair.

Ultraviolet (UV) radiation from the sun’s rays is able to chemically change melanin in a process known as ‘oxidation’. Sunlight falling on the skin causes melanocytes to respond by producing more eumelanin, darkening the skin and helping to protect us from this potentially harmful UV radiation. However, whereas skin cells are alive, hair ‘cells’ are dead. Consequently, melanin that is damaged by UV radiation cannot be replaced in hair, and so the pigment colour is lost. This is called ‘bleaching’.

Melanins are efficient light absorbers and protect the deeper layers of skin from being damaged by UV radiation, thus reducing the risk of skin cancer. It sounds unbelievable, but the skin’s ability to respond to high levels of potentially harmful light can be explained by the fact that our skin has ‘eyes’. Researchers have found photoreceptors in the skin similar to the ones in the eye, which trigger melanin production within seconds after exposure to UV light.

However, this melanin line of defence is not perfect, so don’t throw your sunblock and summer hat away. Traditional sun protection is still number one company for a trip to the beach.

By Eileen Brandenburger

Wendy Zukerman. (November 2011). Skin ‘sees’ the light to protect against sunshine. New Scientist [web site]. http://www.newscientist.com/article/dn21127-skin-sees-the-light-to-protect-against-sunshine.html#.VNSNirCsWQM. (Accessed 6 February 2015).

Michaela Brenner et al. Photochem Photobiol. 84(3), 539–549. 2008.

Nadine L. Wicks et al. Current Biology, 21, 1906–1911, 2011.

Photo Credit: Thiago CPR via Compfight cc

Article by Eileen Brandenburger

February 7, 2015

Eileen is a passionate chemist currently pursuing a PhD at the Manchester Institute of Biotechnology. Holding an MSc in chemistry, she left her home near Frankfurt to Germanise UK science. In between pipetting and attempting to be a proper biochemist, she likes to swap the lab coat for a tutu and perform on stage.

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