Why does hair turn gray?

Dear Cecil: One of us is getting gray hair. He is only 22 and vain. Why is this happening? Does this mean he won’t go bald? Do dark-haired people turn gray earlier or is it only more noticeable? There’s some speculation that it means he’s run out of pigment? Could this be true? A friend, Tempe, Arizona

Cecil replies:

The phenomenon of graying is not very well understood. One of the few things that medical science knows for sure, though, is that there’s no correlation between gray hair and balding–which gives your friend no guarantee one way or the other.

Hair color is determined by a pigment known as melanin that’s distributed through the middle of the hair shaft. The range of color, from blond to brown to black, is determined by the number, size, and, needless to say, color of the pigment granules. Darker hairs show higher trace amounts of copper, which indicates that the metal-based melanin molecules are more highly developed in dark-haired people. That’s about it for empirical observation; the rest is mainly speculation.

For some reason the cells that manufacture melanin, known not very imaginatively as “melanocytes,” can slow down or stop completely. When this happens, the hair, naturally, begins to lose its color, turning yellow and then gray. Air bubbles, which may mysteriously work their way into the hair shaft, also contribute to graying by blocking the passage of the melanin. Graying seems to be genetically determined, but again, the connection is not all that clear. The process is the same for light- and dark-haired people, but the pigment loss, of course, is more obvious in darker hairs.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.