Dandruff is flakes of dead skin that form on the scalp. As it is normal for skin cells to die and flake off, a small amount of dandruff is normal and in fact quite common. Some people, however, either chronically or as a result of certain triggers, experience an unusually large amount of flaking, which can be not only a visual nuisance, but is also often accompanied by redness and irritation. Most cases of dandruff can be treated with the proper shampoo.

Dandruff can also be a symptom of a more serious condition. Care should be taken not to confuse a simple case of dandruff with other conditions that also cause flaking of the scalp, such as seborrheic dermatitis or psoriasis. With all forms of dandruff, as any other rash, excessive scratching should be avoided. Scratching to the point of causing breaks in the skin can increase the risk for infections, particularly from staph and strep bacteria.

Causes
As skin grows, epidermal cells are pushed outward where they eventually die and flake off the body. In most people, these flakes of skin are too small to be visible. However, certain conditions cause cell turnover to be unusually rapid, especially common in the scalp. In people with dandruff, skin cells may die and be replaced about once every two weeks, as opposed to around once a month in healthy people. The result is that dead skin cells are shed in large clumps, which appear as small, unsightly white or grayish patches on the scalp.

Dandruff is understood to be the result of a combination of factors. Some of these factors are well studied, whereas others have not been thoroughly investigated.

The most common cause of dandruff is probably the fungus Malassezia furfur (previously known as Pityrosporum ovale). This fungus is found naturally on the skin surface of both healthy people and those with dandruff. The fungus likes fat, and is consequently found most on skin areas with many sebaceous glands: on the scalp, face and upper part of the body. When Malassezia furfur grows too rapidly, the natural renewal of cells is disturbed and dandruff appears with itching. Other fungi may have a similar role in causing dandruff, as may certain bacteria.

There is a common misconception that dandruff is caused by a dry scalp, or the drying effect of excessive shampooing. In fact, the opposite is true: people with oily scalps tend to suffer most from dandruff. According to Joseph P. Bark, M.D., chairman of dermatology at St. Joseph's Hospital in Lexington, KY, "this may be due to an oily scalp supporting the growth of yeast in the scalp, which is thought to be instrumental in the development of scaling and scalp irritation." He adds that "a large preponderance of males have dandruff, which may suggest some role of androgen hormones in dandruff."

Mild dandruff may be caused by overactive sebaceous glands. Other causative factors include family history, food allergies, excessive perspiration, use of alkaline soaps, yeast infections, and stress. Even the season of the year can contribute to the problem: Cold, dry winters are notorious for bringing on dandruff or making it worse. Symptoms of dandruff can also be aggravated by exposure to dust, UV light, harsh shampoos, and hair dyes.

Dandruff is sometimes the symptom of a more serious condition. If dandruff flakes are greasy and yellow, the probable cause is the skin condition known as seborrheic dermatitis. Joseph Bark notes that "Redness and itching is actually seborrheic dermatitis, and it frequently occurs around the folds of the nose and the eyebrow areas, not just the scalp." Dry, thick lesions consisting of large, silvery scales may be traced to the less common psoriasis of the scalp.

Seasonal changes, stress, and certain diseases seem to affect seborrheic dermatitis. The cold, dry air of fall and winter often triggers a flare-up. Emotional stress can worsen the condition as well.

There is speculation that dandruff is linked with hair loss, as dandruff may indicate an unhealthy scalp.