On average, 90 percent of the hairs on your head are in the growing phase while the other 10 percent enter into a resting state, in which the hair follicle dries up and hair loss begins. While hereditary thinning is the most common cause of hair loss, there are many other factors that can cause or accelerate hair loss and breakage, which may resolve on its own or require treatment for regrowth.
Dieting and Malnutrition
Certain eating disorders -- such as anorexia and bulimia, crash diets and fad diets -- may deplete you of vital nutrients needed for hair growth. For example, vegan diets that rule out meat as an option may not provide enough iron, the main transporter of oxygen to the hair follicles. In fact, without enough nutrients such as silica, vitamin B-12, protein, zinc and iron, your hair may start to grow out brittle or possibly not at all.
Chemical Treatments and Hair Care
Chemicals, dyes, straightening products and bleaches can all cause hair to break and fall out. Usually, misuse and overuse of the products contribute to the hair loss as chemicals begin to burn into the scalp, killing hair follicles. Similarly, persistent brushing or combing your hair can split the ends and damage the hair shaft.
Overuse of Heated Appliances
According to CNN research, flat irons and hot rollers--which use extreme temperatures to reshape hair--can cause splitting and frizzing of the hair strands. Each tool relies on heat to break the hydrogen bonds that cause the hair to look curly. Constant usage, however, leads to thinning and the eventual breaking off of hair from the scalp.
Traction alopecia is the loss of hair due to excessive pulling, usually during hairstyling and brushing. A New York Times article says shampoo and conditioner doesn't trigger hair loss, but persistent pulling of hair into a ponytail or bun may. Every time you pull your hair, even as the result of a nervous habit, you cause it to thin. With enough pulling of your hair, it can fall out or break off.
Stress and Hormonal Changes
Stresses such as pregnancy, rapid weight loss, high fever and emotional distress may trigger hormonal imbalances causing hair loss. This form of hair loss, known as telogen effluvium, results when stressful situations send hair cells prematurely into telogen--the resting phase of hair growth. On average, telogen effluvium can take up to three months to show after the initial stressor and can last an additional three months once the stressor is removed.
Genetics and Pattern Baldness
Pattern baldness, or androgenetic alopecia, results when the resting phase of the hair cycle lasts longer and comes sooner than the growth phase. This form of hair loss is inherited and more common for men who have a history of early balding somewhere within their family. Women may experience common balding as well but are much likelier to develop telogen effluvium since androgenetic alopecia depends on the male hormone, androgen. In all cases, hair loss occurs more often with old age.