The correct answer is: the presence of hydrogen peroxide in the scalp. Some see it as a sign of wisdom, others the beginning of the end. Greying hair is a condition that lies in wait for all humans once they reach adulthood (but usually after the age of thirty). Even though it’s a problem that has interested scientists for ages, it was only in 2009 that a British study* was able to describe the mechanism behind the phenomenon. Here’s an interesting fact: the process involves a substance well known to those who voluntarily dye their hair: oxygenated water, or hydrogen peroxide (H2O2). Hydrogen peroxide is naturally present in the hair bulb. In the hair follicle, stem cells that produce hair use melanocytes, specialized cells that inject melanin, or pigment, into the new cells. This is what gives hair and skin their color. The melanocytes naturally contain enzymes called catalases that can remove an oxygen atom from the peroxide, thus breaking it down into harmless water and oxygen. It is thought that diseases like vitiligo are the result of serious malfunctions in the anti-oxidant processes taking place in the melanocytes. Each hair lasts about three and a half years. After that, it falls out and is replaced by a new one, grown from the same follicle. After about ten complete cycles, or 35 years, the process of hair pigmentation starts to degrade. The activity of the melanocytes doesn’t degrade during a growth cycle, however; the hair will have the same color at the beginning of the cycle as it does at the end. Why does the onset of greying vary from person to person, though? Melanocytes are highly susceptible to oxidative stress; UV radiation or even high stress levels are lethal to them. There are also genetic differences in melanocyte production. But whatever the cause, as soon as melanocytes begin to lose the antioxidant battle, the hydrogen peroxide in the blub can accumulate and do its thing, turning the hair grey (poor in melanin) or white (completely lacking in melanin). There is some good news, however, that emerges from our understanding of the phenomenon. The longer they accumulate oxidative stress, the higher risk melanocytes have of mutating into cancerous cells. Their elimination, which is evidenced by increasingly grey hair, is thus a reflection of a healthy natural defense mechanism.. * Senile hair graying: H2O2-mediated oxidative stress affects human hair color by blunting methionine sulfoxide repair, J. M. Wood et al, the Journal of the Federation of American Societies for Experimental Biology, 2009. An extensive review of the research on greying has been published in 2013 in the International Journal of Cosmetic Sciences.