Baldness (alopecia) is a condition that afflicts both men and women. According to Medlineplus, at any one time 85 percent of your hair is in a growing stage, while 15 percent is in a resting or shedding stage. If more than 15 percent of your hair enters the resting stage, baldness may occur. There are several conditions that may cause this to happen.

Traction

Tight ponytails and braids may cause hair to pull free from the follicle, or even cause breakage. These types of hairstyles include cornrows and braided extensions. This causes what is known as traction alopecia. It is normally identified by the appearance of thinning hair or baldness from the hairline.

Scalp Injuries

Injuries to the scalp, or operations that require cutting through the scalp, may cause baldness in those areas. This is known as cicatricial alopecia (scaring alopecia). The hair follicles scar, causing permanent hair loss.

Follicular Damage

The overuse of certain hair products such as perms, relaxers, hairspray, and hair gel may cause follicular damage by clogging hair follicles or breaking the hair. This causes alopecia areata, or spot baldness.

Hormonal Disorders

Hormonal disorders such as menopause, polycystic ovary syndrome, and diabetes may affect hair growth. These disorders may cause a higher level of male hormones (androgens). Areas of hair loss are usually controlled by androgens. This is called male pattern baldness.

Chemotherapy

Chemotherapy is a treatment used to combat cancer by destroying cancerous cells. These cells are fast-multiplying. Since hair cells are also fast-multiplying, chemotherapy will destroy them as well, causing widespread baldness. This baldness is called alopecia totalis. You may also lose hair from other parts of your body.

Source:

Mayo Clinic: Hair Loss Causes

Mayo Clinic: Chemotherapy and Hair Loss

HMC: Cicatricial Alopecia