Hair is made up of proteins called keratin. These grow out from a hair follicle. The average person's head has about 100,000 hairs. Hair has a growth cycle. It grows actively for 2 to 8 years. This is followed by a resting phase, during which the hair follicle shrinks and degrades. This second phase lasts 2 to 4 weeks. The hair is shed during its final phase, and a new hair takes its place.
Monthly Growth
The hair on a person's head grows, on average, about ½ inch each month. This rate of growth will slow with age. Some 90 percent of a person's hair is growing at any given time. The 10 percent of hair that is not growing is in the second "resting phase."
New Hair
Most people lose 50 to 100 hairs every day. This rate of hair shedding is part of the normal hair cycle. A new hair from the same follicle replaces the hair that was shed. The growing cycle restarts at this time. New hairs start with a tapered end but then have a constant diameter as they continue to grow.
Cycle Names
The three hair growth phases each have a technical name. Anagen is the growth phase. Catagen is the name for the degradation phase. Telogen is the resting phase.
Layers
Hair follicles have multiple layers. The outside layer is continuous with the epidermis. The innermost layer is the cortex. It is surrounded its cuticle, which gives rise to the visible hair shaft that appears. Hair growth happens in a bulb that's at the base of the follicle. This has cells that divide rapidly. A person does not form any new hair follicles after birth. Hair's size, its growth cycle and its color, however, can change throughout life. The head has the highest density of follicles on the body.
Hair Loss
Hair loss that is greater than expected in a hair growth cycle can have many causes. Medicines, skin conditions, an autoimmune issue, chemical hair treatments that are not used properly, underlying health concerns like diabetes and heredity can all contribute to hair loss. These factors can impact the hair growth cycle at any age.
Damage
A person's hair shafts are very porous structures. These and soften quickly when wet. Dry hair is the most brittle and prone to injury. Sunlight and chemicals can damage a person's hair shafts, especially if the person has light-colored hair.
Androgens
Androgens or male hormones, namely testosterone and dihydrotestosteron, are the most important factors that control hair growth. The body needs androgens to grow underarm, beard and pubic hair. Hair on the head is not dependent on androgens, but these hormones play a role in male pattern baldness. Eyebrows and eyelashes are not regulated by androgens.
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