About Human Hair

Hair--or a lack thereof--is an identifying feature of people. Our hair defines us because it is often the first thing that people see. The average scalp contain between 100,000 and 150,000 strands of hair, according to the Library of Congress. Hair color is a genetic condition, but only two pigments--called phaeomelanin and eumelanin--determine the wide array of human hair colors.

Growth

Hair grows about 1/2 inch each month, according to the National Institutes of Health. A strand of hair typically grows for between two and six years before it reaches a resting stage and then falls out. At any given time, about 85 percent of a person's hair is growing, while the remaining 15 percent is in the resting stage. The growth rate of hair slows as a person gets older.

Shedding

On average, a person sheds between 50 and 100 hairs every day, according to the Ohio State University Medical Center. Once a strand of hair is shed or otherwise lost, a new hair begins to grow out of the original follicle.

Pattern Baldness

Pattern baldness can occur in both men and women, according to the National Institutes of Health. The condition, also called andogenetic alopecia, results when a strand of hair is shed normally but a new hair fails to grow in its place. In men, pattern baldness generally begins at the hairline, which gradually recedes, and hair on the crown of the head also may thin out. In women, pattern baldness most often begins with a widening of the natural part. The hair may also thin out along the front and crown of the head, but the hairline generally remains the same. Women also do not typically experience total baldness or near baldness, although this is common in men.

Hair Loss

Some excessive hair loss is not associated with male or female pattern baldness but with an underlying medical condition. Hormone imbalances or a change in hormone levels can cause hair loss, as can illnesses such as syphilis, vitamin deficiencies, burns and trauma to the scalp, according to the Ohio State University Medical Center. Psychological issues such as stress and traumatic experiences can also cause hair to fall out.

Graying

The process by which hair turns gray is not fully understood. The graying of hair is thought to be a genetic condition in which the amount of pigment allotted for each individual hair follicle is depleted, according to the Library of Congress. Once a hair follicle is out of pigment, it will turn gray. Not everyone's hair turns gray, and not everyone grays at the same rate. This is likely because of the amount of pigment available for their hair and the rate at which it is shed.