Hair grows everywhere on the human body except on the palms of our hands and the soles of our feet, but many hairs are so fine they're virtually invisible. Hair is made up of a protein called keratin (the same protein in nails) produced in hair follicles in the outer layer of skin; as follicles produce new hair cells, old cells are being pushed out through the surface of the skin at the rate of about six inches a year. The hair you can see is actually a string of dead keratin cells. The average adult head has about 100,000 to 150,000 hairs and loses up to 100 of them a day; so finding a few stray hairs on your hairbrush is not necessarily cause for alarm.
At any one time, about 90% of the hair on a person's scalp is growing. Each follicle has its own life cycle that can be influenced by age, disease, and a wide variety of other factors. This life cycle is divided into three phases:
Anagen -- active hair growth. Lasts between two to six years.
Catagen -- transitional. Lasts two to three weeks.
Telogen -- resting phase. At the end of the resting phase (two to three months) the hair is shed and a new hair replaces it and the growing cycle starts again.
As people age, their rate of hair growth slows.
There are many types of hair loss, also called alopecia:
Gradual thinning of hair with age is a natural condition known as involutional alopecia. More and more hair follicles go into a telogen, or resting, phase, and the remaining hairs become shorter and fewer in number.
Androgenic alopecia is another form of hair loss. It's a genetically predisposed condition that can affect both men and women. Men with this condition can begin suffering hair loss as early as their teens or early 20s, while most women don't experience noticeable thinning until their 40s or later.
In men, the condition is also called male pattern baldness. It's characterized by a receding hairline and gradual disappearance of hair from the crown. In women, androgenic alopecia is referred to as female pattern baldness. Women with the condition experience a general thinning over the entire scalp, with the most extensive hair loss at the crown.
Patchy hair loss in children and young adults, often sudden in onset, is known as alopecia areata. This condition may result in complete baldness, but in about 90% of cases the hair returns, usually within a few years.
With alopecia universalis, all body hair falls out.
Tearing out one's own hair, a psychological disorder known as trichotillomania, is seen most frequently in children.
Telogen effluvium is hair thinning over the scalp that occurs because of changes in the growth cycle of hair. A large number of hairs enter the resting phase at the same time, causing shedding and subsequent thinning.
What Causes Hair Loss?
Doctors do not know why certain hair follicles are programmed to have a shorter growth period than others. Although a person's level of androgens -- male hormones normally produced by both men and women -- is believed to be a factor, hair loss has nothing to do with virility. An individual's genes, however -- from both male and female parents -- unquestionably influence that person's predisposition to male or female pattern baldness.
Telogen effluvium is temporary hair loss that can occur within a few months after a high fever, a severe illness or extreme stress, and in women following childbirth.
Drugs that can cause temporary hair loss include chemotherapy drugs used in cancer treatment, blood thinners, retinoids used to treat acne and other skin problems, beta-adrenergic blockers used to control blood pressure, and birth control pills.
Hair loss can also be caused by burns, X-rays, and scalp injuries. In such cases, normal hair growth usually returns once the cause is eliminated. Ringworm caused by a fungal infection can also cause hair loss.
The causes of alopecia areata, a disease that often strikes children or teenagers, remain unexplained. It is thought to be an autoimmune disease, meaning that the immune system revs up for unknown reasons and affects the hair follicles. In most cases the hair grows back, although it may be very fine and possibly a different color before normal coloration and thickness return.
Although shampooing too often, perms, bleaching, and dyeing hair do not cause baldness, they can contribute to overall thinning by making hair weak and brittle. Tight braiding and using rollers or hot curlers can damage and break hair, and running hair picks through tight curls can scar hair follicles. In most instances hair grows back normally if the source of the problem is removed, but severe damage to the hair or scalp sometimes causes permanent bald patches.