“头屑去无踪，秀发更出众”，这样的一句广告词似乎已经很鲜明地表示了人们对头皮屑的态度。头皮屑是不好的，是不受欢迎的。那么这些白色的小碎屑是怎么产生的呢？

     Dandruff (also called scurf and historically termed Pityriasis capitis) is due to the excessive shedding of dead skin cells from the scalp(头皮). As it is normal for skin cells to die and flake¹ off, a small amount of flaking² is normal and in fact quite common. Some people, however, either chronically³ or as a result of certain triggers, experience an unusually large amount of flaking, which can also be accompanied by redness and irritation⁴. Most cases of dandruff can be easily treated with specialized⁵ 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(牛皮癣), fungal infection(霉菌感染) or excoriation(剥皮) associated with infestation⁶ 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 purely⁷ for psychological reasons.

     As the epidermal⁸ layer(表皮层) continually replaces itself, cells are pushed outward where they eventually die and flake off. In most people, these flakes⁹ of skin are too small to be visible. However, certain conditions cause cell turnover¹⁰ 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 clumps¹¹, 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 secretions¹²

     The metabolic(新陈代谢的) by-products of skin micro-organisms (most specifically Malassezia yeasts(秕糠状鳞斑癣菌))

     Individual susceptibility

     Common older literature cites the fungus¹³ Malassezia furfur(秕糠状鳞斑癣菌头皮屑) (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(球形马拉色菌), 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). Penetration¹⁴ by OA of the top layer of the epidermis¹⁵, the stratum¹⁶ corneum(角质层), results in an inflammatory response in susceptible¹⁷ persons which disturbs homeostasis(自动动态平衡) and results in erratic¹⁸ cleavage of stratum corneum cells.

     Rarely, dandruff can be a manifestation¹⁹ of an allergic²⁰ 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 perspiration²¹, or climate have any role in the pathogenesis(致病原因) of dandruff.