Hair loss (alopecia) is generally classified as either scarring or nonscarring. The most common types of nonscarring hair loss include:

• Androgenetic alopecia. By far the most common form of hair loss. Caused by heredity, it occurs in women almost as frequently as men. However, men are usually unable to disguise it as effectively as women because they tend to lose more hair and tend to have fewer styling options. Men with this condition may begin losing hair as early as their 20s, while women usually do not experience significant hair loss or thinning until their 40s or later. Androgenetic alopecia in men is distinguished by a receding hairline and the disappearance of hair from the crown (top of head), forming a bald spot. Women tend to experience a general thinning throughout the scalp and may lose hair at the crown as well, though not as significantly as men. People with this form of hair loss have some hair follicles (tiny, tubular structures in the skin through which hair grows) with a shorter than normal hair growth phase, producing hairs that are shorter and thinner than normal. This is thought to occur as a result of hormonal changes in the follicle itself.
  
  
• Telogen effluvium. Widespread hair loss across the scalp. It usually is characterized by temporary or reversible loss of hair, usually following a highly stressful event. Telogen effluvium causes hair throughout the scalp to shed prematurely, in contrast to normal hair loss, which occurs in a staggered fashion. This type of hair loss may not be noticeable until a significant amount of hair is missing. Hair regrowth should begin about three to four months after the triggering event. Common triggers of the condition may include pregnancy, surgery or illness and psychological trauma. In addition, many medical conditions (e.g., nutritional deficiency, syphilis) and medications (e.g., antidepressants) may cause hair loss. In women, this condition can be difficult to distinguish from androgenetic alopecia.
  
  
• Traumatic alopecia. Hair loss that is caused by chemicals (e.g., bleach, hot oils) or styling techniques that apply constant tension to the hair (e.g., hair straightening, tight braids). Habits of consistently altering hair (e.g., twisting, plucking) can cause hair to be distributed unevenly or broken. Traumatic alopecia is usually confined to the area being styled. It is often reversible but may become permanent if the area becomes scarred.
  
  
• Alopecia areata. Patchy baldness. This form of hair loss may also occur on other areas of the body (e.g., face). This form of hair loss may also occur on other areas of the body (e.g., face). Alopecia areata is an autoimmune disease (a condition in which the body's immune system attacks its own cells), which develops in people who are usually in good health otherwise. The cause of alopecia areata is unknown, though research suggests that some people may be genetically predisposed and some type of trigger (e.g., virus, stress) may activate it internally. Patients with alopecia areata often lose and regrow hair repeatedly.

Scarring hair loss occurs at scarred or damaged areas of the scalp and is much less common than nonscarring hair loss. It is usually caused by trauma (e.g., burns, physical injury) but may be caused by disease as well (e.g., skin cancer, lupus). Once an area of the scalp is fully scarred, hair regrowth is highly unlikely.