A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why does the sun darken skin but lighten hair?

August 26, 1994

Dear Cecil:

Why does the sun darken skin but lighten hair?

Cecil replies:

My representative little Ed, well-intended but feeble as always, was able to get through the first part of this answer on the air but was obliged to have the second part explained to him by another caller. Explained to him incorrectly, as it turned out, but when all you know is that you don't know you can't complain when you get shown up by someone who doesn't even know that.

Sun darkens skin because it triggers the production of melanin, a brownish-black pigment that helps filter out harmful ultraviolet rays. It lightens hair because the UV light triggers the breakdown of these selfsame melanin molecules into simpler and evidently less colorful compounds. The exact mechanism by which this is accomplished is not as clear as it might be. "The ionic pathway probably begins by nucleophilic attack of the peroxide anion on the o-quinone grouping," says one medical text, clearly written by the kind of guy you wouldn't want to have season tickets next to at the ballgame. The melanin in both skin and hair is meant to protect the other tissue, but in skin it's renewed (and thus the skin gets darker) whereas in hair it's not, since hair is no longer living. An answer little Ed will spit out quicker next time it's asked.

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