According to Jeffrey Benabio, MD, a dermatologist at Kaiser Permanente in San Diego hair goes gray when color-producing cells stop producing pigment.
Naturally occurring hydrogen peroxide can also build up in the hair, bleaching the color.
According to Dr. Benabio, Caucasian people typically start going gray in their mid-30s, Asians in their late 30s, and African-Americans in their mid-40s.
Half of all people have a significant amount of gray hair by the time they turn 50.
A Caucasion person is considered to be prematurely gray if his or her hair turns gray by age 20; gray before 30 is early for African-Americans.
Dr. Benabio says that going gray, by itself, does not mean you have a medical problem, except in rare cases.
Contrary to popular belief, stress has not been shown to cause gray hair. Scientists don’t know exactly why some people go gray early, but genes play a large role.
Also, a vitamin B-12 deficiency or problems with your pituitary or thyroid gland can cause premature graying that’s reversible if the problem is corrected, Benabio says.
Some research has suggested a connection between premature graying and lower bone density later in life. But in 2007, a study of about 1,200 California men and women showed no such link.
"Your level of bone density is related to activity level, your weight, your height, your ethnicity. It’s not related to your hair or the things controlling the color of your hair," says researcher Deborah J. Morton, PhD, of the University of California, San Diego.
So why does our hair turn gray or white?
Dr. Desmond Tobin, professor of cell biology from the University of Bradford in England, suggests that the hair follicle has a “melanogentic clock” which slows down or stops melanocyte activity, thus decreasing the pigment our hair receives. This occurs just before the hair is preparing to fall out or shed, so the roots always look pale.
Moreover, Dr. Tobin suggests that hair turns gray because of age and genetics, in that genes regulate the exhaustion of the pigmentary potential of each individual hair follicle.
This occurs at different rates in different hair follicles. For some people it occurs rapidly, while in others it occurs slowly over several decades.
In a February 2005 Science article (Nishimura, et al.) Harvard scientists proposed that a failure of melanocyte stem cells (MSC) to maintain the production of melanocytes could cause the graying of hair. This failure of MSC maintenance may result in the breakdown of signals that produce hair color.
There are other factors that can change the pigmentation of hair, making it lighter or darker. Scientists have divided them by intrinsic (internal) and extrinsic (external) factors:
In 2009, scientists in Europe described how hair follicles produce small amounts of hydrogen peroxide.
This chemical builds on the hair shafts, which can lead to a gradual loss of hair color. (Wood, J.M et al. Senile hair graying: H2O2 mediated oxidative stress affect human hair color by blunting methionine sulfoxide repair. FASEB Journal, v. 23, July 2009: 2065-2075).