Hair Loss: The Science of Hair
Hair Growth Cycle
Hair on the scalp grows about .3 to .4 mm/day or about 6 inches per year.
Unlike other mammals, human hair growth and shedding is random and not seasonal
or cyclical. At any given time, a random number of hairs will be in one of
three stages of growth and shedding: anagen, catagen, and telogen.
Anagen is the active phase of the hair. The cells in the root of the hair
are dividing rapidly. A new hair is formed and pushes the club hair (a hair
that has stopped growing or is no longer in the anagen phase) up the follicle
and eventually out.
During this phase the hair grows about 1 cm every 28 days. Scalp hair stays
in this active phase of growth for two to six years.
Some people have difficulty growing their hair beyond a certain length
because they have a short active phase of growth. On the other hand, people
with very long hair have a long active phase of growth. The hair on the arms,
legs, eyelashes, and eyebrows have a very short active growth phase of about 30
to 45 days, explaining why they are so much shorter than scalp hair.
The catagen phase is a transitional stage and about 3% of all hairs are in
this phase at any time. This phase lasts for about two to three weeks. Growth
stops and the outer root sheath shrinks and attaches to the root of the hair.
This is the formation of what is known as a club hair.
Telogen is the resting phase and usually accounts for 6% to 8% of all hairs.
This phase lasts for about 100 days for hairs on the scalp and longer for hairs
on the eyebrow, eyelash, arm, and leg. During this phase, the hair follicle is
completely at rest and the club hair is completely formed. Pulling out a hair
in this phase will reveal a solid, hard, dry, white material at the root. About
25 to 100 telogen hairs are shed normally each day.
Published on March 1, 2010