About 90 percent of the hair on most people's scalps is in a two- to six-year growth (anagen) stage at any given time. The other 10 percent is in a two- to three-month resting (telogen) phase, after which time it is shed. Most people shed 50 to 150 hairs a day. Once a hair is shed, the growth stage begins again as a new hair from the same follicle replaces the shed hair. New hair grows at a rate of approximately one-half inch each month.
Hair loss may lead to baldness when the rate of shedding exceeds the rate of regrowth, when new hair is thinner than the hair shed or when hair comes out in patches.
What causes androgenetic alopecia
Androgenetic alopecia is caused by heredity. Although it's most common among men, it can also affect women. A history of androgenetic alopecia on either side of your family increases your risk of balding. Heredity also affects the age at which you begin to lose hair and the developmental speed, pattern and extent of your baldness.
What causes alopecia areata
Alopecia areata is classified as an autoimmune disease, but the cause is unknown. People who develop this type of baldness are generally in good health. Some scientists believe that some people are genetically predisposed to develop alopecia areata and that a trigger, such as a virus or something else in the environment, sets off the condition. A family history of alopecia areata makes you more likely to develop it. With alopecia areata, your hair generally grows back, but you may lose and regrow your hair a number of times.
Other causes of temporary hair loss include:
Disease. Diabetes, lupus and thyroid disorders can cause hair loss.
Poor nutrition. Having inadequate protein or iron in your diet or poor nourishment in other ways can cause you to experience hair loss. Fad diets, crash diets and certain illnesses, such as eating disorders, can cause poor nutrition.
Medications. Certain drugs used to treat gout, arthritis, depression, heart problems and high blood pressure may cause hair loss in some people. Taking birth control pills also may result in hair loss for some women.
Medical treatments. Undergoing chemotherapy or radiation therapy may cause you to develop alopecia. After your treatment ends, your hair typically begins to regrow.
Recent high fever, severe flu or surgery. You may notice you have less hair three to four months after events such as an illness or surgery. These conditions cause hair to shift rapidly into a resting phase (telogen effluvium), meaning you'll see less new hair growth. A normal amount of hair typically will appear after the growth phase resumes.
Infancy. Newborns often lose hair during the first several months of life. This baby hair (vellus) is eventually replaced by more permanent hair. It's also common for babies to lose a patch of hair on the back of their heads from rubbing against mattresses, playpens and car seats. Hair will grow back once a baby begins to spend more time sitting up.
Childbirth. Some women experience an increase in hair loss several months after delivering a baby. This is because during pregnancy the hair is shifted into an active growth state that then goes back to baseline soon after delivery. This increased hair loss usually corrects itself.
Hair treatments. Chemicals used for dying, tinting, bleaching, straightening or perming can cause hair to become damaged and break off if they are overused or used incorrectly. Excessive hairstyling or hairstyles that pull your hair too tightly also can cause some hair loss. This is known as traction alopecia.
Scalp infection. Infections such as ringworm can invade the hair and skin of your scalp, leading to hair loss. Once infections are treated, hair generally regrows. Ringworm, a fungal infection, can usually be treated with a topical or oral antifungal medication.