Hair Cycle
During the first of three stages of hair growth, the anagen phase, hair cells produce as normal, extending hair outward at a rate of six inches every year. After two to six years, the hair moves into the catagen phase and the follicle shrinks, cutting off nutrient supplies and preventing the hair from further development. In the final stage, the telogen phase, the human head sheds 100 hairs a day as the follicle prepares to restart the hair cycle. Any pressure applied to the scalp--showering, combing or pulling--can result in the loss of superficially attached hairs.
Genetic Balding
Androgentic alopecia is the loss of hair due to pattern baldness, a hereditary condition believed to be caused by the mother's side of the family. All mammalian species resume the hair cycle slower as they age. However, for some, this process doesn't resume in all regions of the scalp. As a result, thinner hair appears in specific, patterned patches and the telogen phase lasts longer than usual. Eventually no growth occurs at all, resulting in baldness.
Stress Balding
Telogen effluvium is the sudden premature entry of hair into the telogen phase as a result of shock, trauma, emotional distress, fever, drugs or malnutrition. For whatever reason, as the body deals with a stressful event, nutrients divide their attention away from hair roots, sending hair prematurely into a resting state. A few weeks later, it can fall out in large amounts. Unlike pattern baldness, however, most people recover from telogen effluvium as the stressor is removed.
Other Explanations
Certain genetic skin conditions and ailments such as lupus can cause scarring on the scalp, which prevents hair from ever growing back. Without sufficient follicles, hair cells no longer reproduce. This leaves a permanent bald spot on the scarred area. Hair loss also occurs at random due to a genetic condition called alopecia areata, in which the immune system temporarily attacks hair follicles and causes hair to uproot and fall. Chemotherapy, likewise, produces chemicals that kill hair follicles, resulting in temporary hair loss until the anagen cycle can resume. Furthermore, traction--the pulling, parting and chemical bleaching of the hair--can result in temporary injury in which hair follicles return to normal if left alone. Hair loss can occur for many different reasons. The hair cycle, age and environment are leading factors.
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