The average human scalp is covered by approximatey 100,000 hair follicles. Each hair undergoes hair cycle and normally 50-100 hairs randomly fall out a day, which is unnoticeable because lost hair is replaced by as many new hairs springing up daily. Hair loss results from the fall out of hair from the hair follicle. Alopecia or excessive, premature hair loss is the condition caused by many factors. Loss of hair itself does not pose critical health problems because biological role of human hair is relatively marginal. Hair on our scalp protects the head from mechanical shock, heat loss, and exposure to UV-light. The eyelashes and eyebrowes protect the eyes, and hair in the ear canal or the nasal passages help filter out particles and pathogens, thus protecting our internal organs. However, hair does play important social role: it is one of the major determinants of our appearance and identity in daily life. Fullness of hair also implicates or manifests physical integrity and youthfulness of the person. Losing hair could have more than just emotional impacts on individuals. The hair is a unique organ that goes through a characteristic cycle consisting of an immature phase, a growing phase called anagen, a transitional phase between the growing phase and the resting phase called catagen, and finally a resting phase called telogen in which the hair stops growing, waiting to fall out. 85-90% of hairs on our body are in anagen phase or growing phase, which lasts anywhere from two to five years. This phase is followed by a short regression phase, or catagen, which lasts 2-3 weeks. Approximately 1% of hair follicles are in catagen. Approximately 10-15% of hair follicles are in the resting phase, the telogen, which lasts about 3-5 months. Hair follicles typically goes through 10-20 asynchronous cycles during the lifetime. Persistent loss of more than 150 hairs would consist a state of hair loss, or alopecia, albeit it could be temporary. How long can hair grow before it stops growing eventually if it does? Given that the hair growth rate is quite uniform and constant, somewhere between 0.3-0.5 millimeters per day, it's believed that the difference in the length of anagen, the growth phase, among individuals is the major determinant to the maximum hair length. For some individuals, anagen may last ten years. Of course the length of the anagen is governed by genes, and the genetic background of the individuals. Non-genetic factors such as nutritional condition, weather, seasonal changes (hair may grow a bit faster during winter), taking medications, health condition may of course influence the rate of hair growth as well as hair loss. The shape of the hair, straight or curly, is dependent on the shape of the follicle. A circular or round hair follicle would generate straight hair, while the follicle with oval or elliptical shapes (in its cross-section) would produce a curly hair. |