Parents often cite having teenagers as the cause of gray hair.
This is a good hypothesis, but scientists continue to investigate why
hair turns gray. In time, everyone’s hair turns gray. Your
chance of going gray increases 10-20% every decade after
Initially, hair is white. It gets its natural color from a type
of pigment called melanin. The formation of melanin begins before
birth. The natural color of our hair depends
type and amount of melanin in the middle layer of the hair shaft
Hair has only two types of pigments: dark (eumelanin)
and light (phaeomelanin). They blend together to make up
the wide range of hair colors.
Melanin is made up of specialized pigment cells called
melanocytes. They position themselves at the openings on the
skin’s surface through which hair
grows (follicles). Each hair grows from a single follicle.
The process of hair growth has three phases:
- Anagen: This is the active growth stage of the hair fiber and
can last from 2- 7 years. At any given moment 80-85% of our hair
is in the anagen phase.
- Catagen: Sometimes referred to as the transitional phase, which
is when hair growth begins to “shut down” and stop
activity. It generally lasts 10- 20 days.
- Telogen: This occurs when hair growth is completely at rest
and the hair fiber falls out. At any given time, 10-15 % of our
hair is in the telogen phase, which generally lasts 100 days
for scalp hair. After the telogen phase, the hair growth process
starts over again to the anagen phase.
As the hair is
being formed, melanocytes inject pigment (melanin) into cells
containing keratin. Keratin is the protein
that makes up our
hair, skin, and nails. Throughout the years, melanocyctes continue
to inject pigment into the hair’s keratin, giving it a
With age comes
a reduction of melanin. The hair turns gray and eventually
So why does our hair turn gray or white?
Dr. Desmond Tobin, professor of cell biology from the University
of Bradford in England, suggests that the hair follicle has a “melanogentic
clock” which slows down or stops melanocyte activity, thus
decreasing the pigment our hair receives. This occurs just before
the hair is preparing to fall out or shed, so the roots
always look pale.
Moreover, Dr. Tobin suggests that hair turns gray because of age
and genetics, in that genes regulate the exhaustion of the pigmentary
potential of each individual hair follicle. This occurs at different
rates in different hair follicles. For some people it occurs
rapidly, while in others it occurs slowly over several decades.
In a February 2005 Science article (Nishimura, et al.) Harvard
scientists proposed that a failure of melanocyte stem cells (MSC)
to maintain the production of melanocytes could cause the graying
of hair. This failure of MSC maintenance may result in the breakdown
of signals that produce hair color.
There are other factors that can change the pigmentation of hair,
making it lighter or darker. Scientists have divided them by intrinsic
(internal) and extrinsic (external) factors:
- Genetic defects
- Body distribution
- Chemical exposure
In 2009, scientists in Europe described how hair follicles produce small amounts of hydrogen peroxide. This chemical builds on the hair shafts, which can lead to a gradual loss of hair color. (Wood, J.M et al. Senile hair graying: H2O2 mediated oxidative stress affect human hair color by blunting methionine sulfoxide repair. FASEB Journal, v. 23, July 2009: 2065-2075).
- An average scalp has 100,000-150,000 hairs.
- Hair is so strong that each hair can withstand the strain
of 100 grams (3.5 ounces). An average head of hair could hold
10-15 tons if only the scalp was strong enough!
- Human hair grows autonomously, that is each hair is on
its own individual cycle. If all our hair were on the same cycle,
we would molt!
- Hair has the highest rate of mitosis (cell division).
An average hair grows 0.3 mm a day and 1 cm per month.
Jess M. Hairy science. New York, Planet Dexter, c2000.
(Science fair projects involving hair)
Kiyokazu. Hair follicle: differentiation under electron
microscope: an atlas. Tokyo, New York, Springer, c2005.
Emi K., Scott R. Granter, and David E. Fisher. Mechanisms
of hair graying: incomplete melanocyte stem cell maintenance
in the niche. Science, v. 307, Feb. 4, 2005: 720-723.
Clarence R. Chemical and physical behavior of human
hair. New York, Springer, c2002. 483 p.
Science of hair care. Edited by Claude Bouillon and John
Wilinson. Boca Raton, Taylor & Francis, 2005. 727
Eirikur, Neal G. Copeland, and Nancy A. Jenkins. Melanocyte
stem cell maintenance and hair graying. Cell, v. 121,
April 8, 2005: 9-12.
Desmond J. and R. Paus. Graying: gerontobiology of hair
follicle pigmentary unit. Experimental gerontology, v.
36, 2001: 29-54.
Desmond J. Biology of hair pigmentation. In Skin,
hair, nails: structure and function. Edited by Bo Forslind,
Magnus Lindberg, and Lars Norlen. New York, Basel, Switzerland,
Marcel Dekker, c 2004: 319-363.
more print resources...
Search on "hair" and "Hair
care and hygiene"
in the Library of Congress Online
Hair Dye. From the Prints &
Photographs Division, Library of Congress.
amboline for the hair. From the
Prints and Photographs Division,
Library of Congress.
From Prints and
Division, Library of Congress.
Photo courtesy of the National