2.1.5　Hair

Hair is skin appendage，consisting of the hair shaft outside the skin and the hair root that penetrates into the skin.In addition to providing protection，hair also transmits psychosocial and gender signals.Though hair diseases scarcely endanger a life，people attach more and more great attention to hair health.Common hair diseases include folliculitis，androgenetic alopecia（AGA），alopecia areata（AA），etc.

Hair grows periodically.There are three phases of hair cycle：growth stage（anagen），regression stage（catagen）and resting stage（telogen）（Figure 2-4）.Each hair follicle is growing independent of its neighbors.The duration of each stage varies from regions.The hair cycle of scalp is the longest，with the anagen lasting for up to 5 years，catagen for about 2 weeks and telogen for about 3 months.The scalp contains an average of 100000 hairs averagely，in which about 85％are normally in the anagen，10％～15％in the telogen，and less than 1％in the catagen.Shedding less than 100 hairs a day is considered normal.Excessive hair loss or abnormal hair loss are considered as pathologic alopecia，which may be due to various factors，including genetic factors，hormone levels，mental stress，medication，and so on.The most common type of alopecia is androgenetic alopecia，affecting at least 80％of men and at least 42％of women，and its pathogenesis remains obscure.Genetic factors（single nucleotide polymorphisms of gene loci and hormone levels）could lead to increase of sensitivity to androgen of scalp hair follicles，and the terminal hair follicles turn into vellus hair，with hair follicles gradually miniaturize and atrophy.There are many treatments for androgenetic alopecia，such as oral medication，topical medication，phototherapy，hair transplantation and platelet rich autologous plasma injection therapy（PRP）；however，none of them achieves a complete cure.Finasteride（selective type Ⅱ 5α-reductase inhibitors）and minoxidil are still the best choice in the treatment of AGA for male，and minoxidil is the best choice in the treatment of AGA for female，with high level of evidence.However，the drug treatment often needs a long course of treatment.How to reduce the sensitivity to androgen of hair follicles and reverse the miniaturization of hair follicles remain challenges currently.(https://www.daowen.com)

Except for the treatment of hair diseases，hair health management include daily care of hair.Hair care is as much in demand as skin care.A variety of shampoo，conditioner and germinal products emerge in endlessly，hair daily care has a very broad market prospect.