We all notice those hairs in our brush when we brush our hair. And it sometimes seems like we shouldn’t have any hair left, with all that hair coming out. But, in normal situations, we continue to grow hair. So how does hair grow?
By the 22nd week of gestation, a developing fetus has all her hair follicles formed. There are approximately 1 million follicles on the head. 100,000 of these are on the scalp. During your life, you will not get any new follicles.
The growth cycle of a hair lasts approximately 4-7 years. Hair typically grows faster in the summer, which is why those short summer hair cuts need so much trimming! Generally, hair grows .3 to .4 mm per day, or about 6 inches per year. Each hair on your head will be in one of three stages of growth.
Anagen is the active phase of hair growth. During this stage, your hair grows approximately 1/2 inch per month. This phase lasts from 2-6 years. Those people who have longer growth cycles are able to grow longer hair. Approximately 90% of your hair is in the growing stage. Over time, the length of the growing stage decreases, causing weakening and thinning of your hair.
The next phase of growth is called Catagen. During this stage, the active growth of the hair stops, cells in the follicle die, and the follicle shortens. This transitional phase lasts 2-3 weeks and during this time the hair shaft detaches from the follicle. Approximately 1-3% of your hairs are in this stage.
Telogen is the period of rest when the follicle stays shortened. When telogen ends, the hair germ cells start growing a new hair follicle and the next cycle begins.
During the fourth stage, exogen, the old hair shaft is dislodged as the germ cells in the follicle start growing a new hair, whose cycle begins again.