Why Hair Turns Gray!

From FASEB Federation of American Societies for Experimental Biology: H2O2-mediated oxidative stress affects human hair color by blunting methionine sulfoxide repair.

Gray hair, according to new findings, is caused by a massive build up of hydrogen peroxide due to wear and tear on hair follicles. The peroxide winds up blocking the normal synthesis of melanin, your hair's natural pigment.
All hair cells make a tiny bit of hydrogen peroxide, but as you age, the amount increases. Essentially, you bleach our hair pigment from within, and your hair turns gray and then white.
Researchers made this discovery by examining cell cultures of human hair follicles. They found that the build up of hydrogen peroxide was caused by a reduction of an enzyme that breaks up hydrogen peroxide into water and oxygen.
They also discovered that hair follicles could not repair the damage caused by the hydrogen peroxide because of low levels of the enzymes MSR A and B, which normally serve this function. The high levels of hydrogen peroxide and low levels of these enzymes also disrupt the formation of tyrosinase, another enzyme that leads to the production of melanin in hair follicles.