Temporary hair loss can occur in conjunction with a high fever, a severe illness, thyroid disorders, iron deficiency, general anesthesia, drug treatments, hormonal imbalance or extreme stress, and in women following childbirth. In these conditions, collectively known as telogen effluvium, a large number of hair follicles suddenly go into a resting phase, causing hair to thin noticeably. Drugs that can cause temporary hair loss include chemotherapeutic agents used in cancer treatment, anticoagulants, retinoids used to treat acne and skin problems, beta-adrenergic blockers used to control blood pressure, and oral contraceptives.

Hair loss can also be caused by burns, X-rays, scalp injuries and exposure to certain chemicals -- including those used to purify swimming pools, and to bleach, dye and perm hair. In such cases, normal hair growth usually returns once the cause is eliminated. Ringworm can also cause hair loss.

The causes of alopecia areata, a disorder that often strikes children or teen-agers, remain unexplained. In most cases the hair grows back, although it may be very fine and possibly white before normal coloration and thickness return.

Although too-frequent washing, permanent waves, 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 stress is removed, but severe damage to the hair or scalp sometimes causes permanent bald patches.

Prevention
Before men begin with attempts to reverse natural balding, you should take steps to protect your hair from damage that may eventually lead to thinning. Some people, women in particular, put their hair under tremendous stress in the pursuit of beauty. Hair dryers, hot curlers, dyes, bleaches, hair straightening, permanent waves and chemical-laden cosmetics may eventually result in dry, broken and thinning hair. People who leave their hair its natural color and texture will end up with healthier hair. Use a basic shampoo designed for your hair type. If you must curl your hair, use sponge rollers and let it air-dry whenever possible.

Treatment
Hormone therapy may be prescribed for certain types of hair loss, but not without some risk. Although most cases of alopecia areata are resolved naturally, some doctors try to speed recovery with corticosteroids applied topically or injected in the scalp. The treatment may be painful and may cause permanent scarring in the scalp brought on by skin atrophy. Cortisone taken orally may stimulate new hair growth, but the effect is likely to be temporary. Prednisone, another orally administered steroid, has proved effective for alopecia areata patients, but its potential side effects include weight gain, metabolic abnormalities, acne and menstrual problems. Hormone therapy in the form of birth-control pills may reverse baldness due to hormonal imbalances in young women.

Hair transplantation involves the relocation of plugs of skin from parts of the scalp containing active hair follicles to bald areas. A patient may need several hundred plugs -- implanted 10 to 60 at a session. The transplanted hair may drop out, but new hair usually begins to grow from the transplanted follicles within several months.

A form of cosmetic surgery called scalp reduction involves tightening the scalp so that hair-bearing skin from the back and sides of the head is pulled toward the crown. Hair may then be transplanted to the remaining bald area at the top of the head. Like hair transplants, the process is painful and expensive, and it does nothing to retard genetic or age-related hair loss.