Why Does Hair Turn Gray? What Researchers Know and Do Not Know
Gray hair is one of the most visible signs of aging, yet scientists are still working out exactly why it happens.
What gray hair is
A 2018 review in the journal Annals of Dermatology says hair turns white when the follicle loses its pigment, melanin, completely. Hair called gray is usually a mix of white hairs and still-pigmented hairs. A single hair can also fade gradually from black through gray to white.
How common it is
The review cites the "50-50-50 rule": about half of people have about 50 percent gray hair by age 50. Graying often starts at the temples, then spreads to the front, top and sides of the head, and reaches the back last. It can run in families, and the review says it may be inherited in an autosomal dominant manner. Graying before the age of 30 is generally called premature graying, although the criteria differ.
What seems to go wrong
The traditional view is that the pigment-making cells in the hair bulb, called melanocytes, run out of their ability to make pigment. The review says damage from reactive oxygen species, which are harmful by-products of cell chemistry, may play a part. It lists inflammation, ultraviolet light, cigarette smoking, psychological stress and some chemicals as possible triggers of this kind of stress.
A newer idea focuses on melanocyte stem cells, which live in a part of the follicle called the bulge and supply new pigment cells. Research the review describes, mainly in mice, suggests graying may come from these stem cells failing to maintain themselves rather than from a problem in the bulb cells. The authors propose that active hair growth may add stress that eventually depletes these stem cells.
Other links
The review says graying is associated with autoimmune conditions such as vitiligo and with some rare premature aging syndromes. It may also reflect nutritional deficiencies, especially of vitamins, and the use of drugs such as chloroquine.
Can it reverse?
The review notes that graying is sometimes reversed even in older people. Some white hairs begin to make pigment again after exposure to radiation or certain signalling molecules. One explanation is that pigment cells that are not yet active remain in the follicle and restart when stimulated.
What is still unknown
The authors state plainly that the exact cause of hair graying in humans is not known, and that much of the evidence comes from mice and is only partial.
Sources
- Three Streams for the Mechanism of Hair Graying, Jo and others, Annals of Dermatology, 2018: https://pmc.ncbi.nlm.nih.gov/articles/PMC6029974/
Facts checked: October 14, 2026.