How does Hair Turn Gray?
"Gray hair" is actually a misnomer. "Gray hair" is hair with a reduced level of melanocytes, so the hair has a color, the same color as was previous, but the color appears paler.
A person's melanocytes produce different amounts of melanin during a lifetime, and different kinds of melanin as well ... and in different places across the body's mosaic of hair! Children may have pale hair which darkens at adolescence: an increase of melanins. When people mature, their melanocytes may cease producing melanins, and their hair turns gray, then white.
A hair may gradually "turn gray", with the melanocytes gradually producing fewer melanins during the life of the hair, or a hair may fall out and be replaced by a considerably less pigmented hair.
At any given time, 85% - 90% of a person's hairs are growing. The other hairs are resting, getting ready to fall out. A person's cranial hair grows from 3 years to 7 or more years, then stops growing for several months, then falls out. This happens over and over during a person's life. At some point, a new hair grows in and the bulbar melanocytes produce less or no melanin in the new hair. That's a new gray hair. Even though ageing people may blame each gray hair on a specific worry, and recount these trials to the exasperation of their children, gray hairs do not arise from suffering (or even wisdom).
The onset of gray hair is primarily fixed by heredity. On average, 50% of white people are 50% gray by age 50. Their average age for the first gray hair is age 34 +/- 10 years. Average onset for Asian people is age 39 +/- 9 years. Average onset of graying for Blacks is 44 years +/- 10 years. The first few strands are random, then form a pattern. Temples gray, and the top of the head gray first. Then graying proceeds from the top of the head downwards.
Sometimes children grow single gray hairs at age 8. Premature gray for Caucasians is defined as gray onset before age 20 in Caucasians, before age 25 in Asians, and before age 30 in blacks. Some diseases cause prematurely gray hair, such as pernicious anemia, hyper/hypo-thyroidism, osteopenia, progeria and pangeria, and prematurely gray hair can be caused by ..... cigarette smoking!
D.J. Tobin, R. Paus
Graying: gerontobiology of the hair follicle pigmentary unit
Experimental Gerontology 36 (2001) 29-54
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