Hair turns gray as people age because the follicles at the base of the hair shaft cease to produce melanin. Each follicle contains a finite number of pigment cells. These pigment cells produce melanin, a chemical that gives the hair shaft — the visible strand — its color, whether black, brown, blond, red or any shade in between. The darkness or lightness of the hair depends on how much melanin each strand contains. Melanin is the same pigment that makes skin more tan with continued exposure to the sun.
The Graying Process
With age, the pigment cells in a hair follicle gradually die off. As they do, that strand of hair will no longer grow in with as much color and will show up as more silver, gray or white as it continues to grow. Eventually, if all the pigment cells die, the hair grows in completely gray. The graying of a full head of hair typically occurs over a long period of time. From the time a person notices the first gray hair strands, it can take 10 years or more to complete the process.
Genetics and Timing
The age at which a person gets gray hair is determined by his or her genes. A general estimate can be made by observing the ages at which his or her parents or grandparents went gray. Some people go gray at young ages — sometimes in their teens — and others might not go gray before age 50. Many people show their first gray hairs during their late 30s or early 40s.
Other Causes
Genetics is the most common cause of gray hair, but other things can contribute to graying, including a poor nutrition and a lack of B vitamins, particularly pantothenic acid. Anemia, which is the lack of iron in the blood; thyroid problems; and even smoking also can contribute to gray hair. Smokers are believed to gray at an earlier rate because smoking depletes oxygen in the body's tissues. Some treatments for diseases, such as cancer or acquired immune deficiency syndrome (AIDS), can cause hair to turn gray.
Myths
Some theories about gray hair have been determined to be myths. For example, it has been said that a sudden shock or fright can cause hair to turn gray overnight, but that theory has been discredited. Although parents often like to claim that the stress of having children has caused them to get gray hair, this theory also has been discounted.