Understanding what causes hair to turn white--or gray or silver--involves taking a close look at the structure of the hair itself. According to an October 2007 article in Scientific American, there are two components to hair involving special cells called keratinocytes, which are responsible for producing the physical properties of the hair, and melanocytes, which deliver melanin that give hair color. Brown or black melanin (euelanin) and yellow or red melanin (phoemelanin) blend together in varying proportions to create the vast array of human hair color.
Stem cells at the base of the hair follicle replenish the number of keratinocytes and melanocytes. Once melanocyte stem cells are depleted over a period of time, no more melanin is produced. When hair loses some or most of its melanin, it appears gray or silver in hue. But a total absence of melanin results in hair that's completely transparent. Such hair appears white in color to the human eye.
What Causes White Hair
When it comes to when the first stands of white hair first begin to appear, the primary culprit is genetic makeup, notes Dr. Leo Cooney of Yale University School of Medicine in a March 2009 article in the New York Times. Gray and white hair--a physical sign that a person is getting older--also can occur prematurely, and this too is based on hereditary factors. But there are other causes of white hair, such as a condition called vitiligo, certain types of anemia and thyroid disorders. Stress is often mentioned as a cause of white hair. Professor Ralf Paus at the University Hospital Schleswig-Holstein in Lübeck, Germany, told Scientific American that it's conceivable that stress hormones could interfere with melanin production.
White Hair Myths
A common myth is that sudden shock or fear can cause hair to turn white suddenly. Historical figures Thomas More and Marie Antoinette purportedly turned gray the night before they were to be executed. But this is most likely anecdotal. Cecil Adams of "The Straight Dope" states that there is a rare medical condition called diffuse alopecia areata that causes melanin-rich hair to fall out in a short period of time while leaving unpigmented hair intact. A person with this condition could conceivably appear to have gone gray practically overnight.