Process
Each hair takes two to six years to complete its growth cycle, report doctors at the American Academy of Dermatology. At any one time, about 90 percent of the hair on the body is growing. The other 10 percent is in the resting phase. After resting, the hair falls out naturally. Once the hair is shed, the process in the hair follicle begins again. Hair on the head from the scalp grows about half an inch every month. As people age, the growth process is even slower. Healthy people lose between 50 and 100 hairs per day in the normal hair growth process.
Role of Hair
The role of hair is to prevent heat from leaving the body. Air is trapped next to the skin by hair, providing an insulating layer of protection. Hair also gives the skin a small layer of protection from ultraviolet light and minor abrasions. Hairs on certain parts of the body serve additional roles. Eyebrows and eyelashes keep dust and other particles from getting in the eyes. Hair also helps the body with sweat and promotes evaporation of moisture accumulated on the skin. Some hair follicles are sensitive to touch and can provide another layer of protection when necessary. Finally, hair is an integral part of the identity of many humans, which has created a multibillion-dollar industry to serve those purposes.
Growth
Researchers at Keratin report that hair follicles begin forming in the fourth month of a fetus's development. The soft, fine hairs developed in the womb typically are shed by the time a baby is born. Many babies do retain some of the fetal hair at birth but shed that hair within a month. Hair growth varies in newborns as the follicles continue to grow and produce permanent hair. The hair shaft, when finally developed, contains the pigment that will give hair its color. Genetic influences are present in the hair follicle and determine a number of different hair factors, ranging from the color and thickness of the hair to the genes for male-pattern baldness and other forms of hair loss later in life.
Phases
There are three phases of hair growth that occur in each follicle during its growth cycle. Anagen is the phase in which hair remains 90 percent of the time in active growth and continues for three to six years. During catagen, the second stage, the hair follicle is in a state of controlled regression and hair growth stops. During this phase, the hair is very vulnerable to outside influences such as brushing, pulling and use of chemicals. The catagen stage lasts about two to three weeks. In the final stage, telogen, the hair rests and eventually falls out. The telogen phase can last up to three months.
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