Partial or complete loss of hair is called alopecia.

Hair loss usually develops gradually and may be patchy or all over (diffuse). You lose roughly 100 hairs from your head every day. The average scalp contains about 100,000 hairs.

Each individual hair survives for an average of 4 1/2 years, during which time it grows about 1/2 inch a month. Usually in its 5th year, the hair falls out and is replaced within 6 months by a new one. Genetic baldness is caused by the body's failure to produce new hairs and not by excessive hair loss.

Both men and women tend to lose hair thickness and amount as they age. Baldness is not usually caused by a disease. It is related to aging, heredity, and testosterone. Inherited or "pattern baldness" affects many more men than women. About 25% of men begin to bald by the time they are 30 years old, and about two-thirds are either bald or have a balding pattern by age 60.

• Typical male pattern baldness involves a receding hairline and thinning around the crown with eventual bald spots. Ultimately, you may have only a horseshoe ring of hair around the sides. In addition to genes, male-pattern baldness seems to need the male hormone, testosterone. Men who do not produce testosterone (because of genetic abnormalities or castration) do not develop this pattern of baldness.
• Some women also develop a particular pattern of hair loss due to genetics, age, and male hormones (which tend to increase in women after menopause). The pattern is different from that of men. Female pattern baldness involves a thinning throughout the scalp. The front hairline generally remains intact.

A sudden physical or emotional stress may cause one-half to three-quarters of the hair throughout your scalp to shed (called Telogen effluvium). You will notice hair coming out in handfuls while you shampoo, comb, or run your hands through your hair. You may not notice this for weeks to months after the episode of stress. The hair shedding will decrease over 6 - 8 months.

Cause of this type of hair loss are:

• High fever or severe infection
• Childbirth
• Major surgery, major illness, sudden blood loss
• Severe emotional stress
• Crash diets, especially those that do not contain enough protein
• A number of medications, including retinoids, birth control pills, beta-blockers, certain antidepressants, NSAIDs (including iburpofen) and calcium channel blockers

Some women ages 30 - 60 may notice a thinning of the hair that affects the entire scalp. The hair loss may be heavier at first, and then gradually slow or stop. There is no known cause for this type of hair loss.

Other possible causes of hair loss, especially if it is in an unusual pattern, include:

• Alopecia areata -- bald patches that develop on the scalp, beard, and, possibly, eyebrows. Eyelashes may fall out as well.
• Autoimmune conditions such as lupus
• Burns
• Certain infectious diseases such as syphilis
• Excessive shampooing and blow-drying
• Thyroid diseases
• Nervous habits such as continual hair pulling or scalp rubbing
• Radiation therapy
• Tinea capitis (ringworm of the scalp)
• Tumor of the ovary or adrenal glands