Why Does Sunlight Lighten Hair But Darken Skin?
Short answer: Skin protects itself, hair doesn't.
The sun’s ultraviolet rays damage skin and hair. So both rely on a pigmented polymer called melanin for protection. Melanin both absorbs and scatters UV rays, keeping them away from your cells’ fragile DNA. But melanin degrades over time and loses its color from prolonged exposure.
In hair, the result is a bleached or yellowed effect. But because hair cells are dead—comprised only of lipids, water, pigments, and structural proteins—these light strands remain in this damaged state until new hair with fresh melanin grows to replace them.
Skin cells, on the other hand, are alive and can react and adapt to UV rays. When sun hits the skin, the body cranks out a hormone that binds to melanin-making cells, causing them to produce more melanin for additional protection. This melanin populates the lower epidermis and becomes darker as it disperses to the upper layers. Over time, this process leads to a suntan, which serves to protect you better.
Prolonged exposure to UV light can eventually damage skin’s cellular DNA, though, and those damaged cells put you at a higher risk for skin cancer. Tanning and repeated sunburns only multiply those risks. So feel free to sun your hair until it’s golden blond—but slather on that sunscreen.
This article was originally published in the May/June 2016 issue of Popular Science.