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. Leg hair – 3 to 6 months, Thigh hair – 1 to 2 months, Moustache hair -1 to 1.5 months.
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.
Hairs in the Anagen phase are more sensitive to the different treatment modalities than those in the Catagen phase.
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. Once Anagen begins the remaining club hair is ejected, and the growth cycle continues. About 25 to 100 telogen hairs are shed normally each day.
Patterns of hair growth vary greatly depending on whether the person is male or female. In addition, ethnicity may also determine normal growth patterns that can be interpreted as abnormal if outside of a person’s native culture. Growth of androgen-sensitive hairs at various regions of the body (beard, axillae, pubis, chest, and shoulders, for example) can arouse suspicion of an underlying organic cause in women or prior to puberty.
Overproduction of testosterone and other androgens in females may cause abnormal growth patterns that can be clinically evident as hirsutism. All women should be evaluated for an underlying cause of the hair growth, whether the unwanted facial hair is to be treated or not.