Atawhai Te Hau, of Villa Maria College, asks :-
Why does sunlight make your hair go lighter?
Stan Simpson, a chemist and past Director of the Wool Research Organisation, Lincoln, responded.
All animals, including humans, have only two forms of coloured cells in their hair. Melanin is the chemical name for the material which fills either a yellow or brown-black type of granule. There is no precise size to the melanin molecules, as they appear to form polymers from complex molecules (called quinones) in the hair fibre as it grows.
Melanins absorb selective colours of sunlight. The reflected light remaining after absorption is what we see as fair, brown, grey or black hair, the latter involving complete absorption of sunlight. Almost all of the packets of light energy merely heat the hair, but occasionally a quantum of light energy will break a weak chemical bond and hence the molecule. Lots of types of these photochemical reactions are well known. The effect is to gradually destroy the light-absorbing chemical structure of melanin, and as a result our hair is progressively bleached.
Sunlight also slowly destroys synthetic dye colours used in textiles and in industry a number grading system is used to describe the resistance of the dye to fading. Melanins fall in the middle of the range, although the effects of sunlight bleaching are noticed more readily with lightly coloured hair.