Why does hair get gray?

Sooner or later almost everyone’s hair goes gray, but the cause has never been clear. Last spring a team of Japanese researchers said they think they have found the trigger: radiation-induced stress.
Within every hair follicle is a population of melanocyte stem cells. Over time these cells split into two populations. One produces pigment for the hair before dying off, while the other becomes a new melanocyte stem cell. In a stress-free world, these cells would replenish themselves indefinitely and we would keep our youthful hair color until our dying day (baldness notwithstanding). But stress free this world is not—nor is the lab of dermatologist Emi Nishimura at Kanazawa University. There, she and her colleagues bombarded brown- and black-haired mice with DNA-damaging radiation. The consequences, as described in a paper published in Cell this June: The melanocytes that originally went on to rejuvenate instead only matured and died. The brown- and black-haired mice soon went gray.
Researchers posit that the melanocyte die-off may be a way for the body to shed potentially cancerous, radiation-stressed cells. It is too early to blame your spouse for your silver strands, though—emotional stress has not yet been shown to harm stem cells.