“頭屑去無蹤，秀發更出眾”，這樣的一句廣告詞似乎已經很鮮明地表示了人們對頭皮屑的態度。頭皮屑是不好的，是不受歡迎的。那么這些白色的小碎屑是怎么產生的呢？

Dandruff (also called scurf and historically termed Pityriasis capitis) is due to the excessive shedding of dead skin cells from the scalp(頭皮). As it is normal for skin cells to die and flake off, a small amount of flaking 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 also be accompanied by redness and irritation. Most cases of dandruff can be easily treated with specialized shampoos. Dandruff is not an organism like lice(虱子); it is just dead skin that accumulates in the scalp. Dandruff is unlikely to be the cause of hair loss.

Excessive flaking can also be a symptom of seborrhoeic dermatitis(脂溢性皮炎), psoriasis(牛皮癬), fungal infection(霉菌感染) or excoriation(剝皮) associated with infestation of head lice.

Dandruff is a global phenomenon and many people find that dandruff can cause social or self-esteem problems. Treatment may be important purely for psychological reasons.

As the epidermal layer(表皮層) continually replaces itself, cells are pushed outward where they eventually die and flake off. In most people, these flakes of skin are too small to be visible. However, certain conditions cause cell turnover to be unusually rapid, especially in the scalp. For people with dandruff, skin cells may mature and be shed in 2 - 7 days, as opposed to around a month in people without dandruff. The result is that dead skin cells are shed in large, oily clumps, which appear as white or grayish patches on the scalp, skin, and clothes.

Dandruff has been shown to be the result of three required factors:

Skin oil commonly referred to as sebum(皮脂) or sebaceous secretions

The metabolic(新陳代謝的) by-products of skin micro-organisms (most specifically Malassezia yeasts(秕糠狀鱗斑癬菌))

Individual susceptibility

Common older literature cites the fungus Malassezia furfur(秕糠狀鱗斑癬菌頭皮屑) (previously known as Pityrosporum ovale(皮屑芽孢菌)) as the cause of dandruff. While this fungus is found naturally on the skin surface of both healthy people and those with dandruff, it was discovered that a scalp specific fungus, Malassezia globosa(球形馬拉色菌), is the responsible agent. This fungus metabolizes triglycerides(甘油三酸酯) present in sebum by the expression of lipase(脂肪分解酵素), resulting in a lipid byproduct oleic acid (OA油酸). Penetration by OA of the top layer of the epidermis, the stratum corneum(角質層), results in an inflammatory response in susceptible persons which disturbs homeostasis(自動動態平衡) and results in erratic cleavage of stratum corneum cells.

Rarely, dandruff can be a manifestation of an allergic reaction to chemicals in hair gels/sprays, hair oils, or sometimes even dandruff medications like ketoconazole(酮康唑).

There is no convincing evidence that food (such as sugar or yeast), excessive perspiration, or climate have any role in the pathogenesis(致病原因) of dandruff.

（來源：wikipedia 英語點津 Helen 編輯）

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