Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
1. People and Animals
How do we have hair?
Some of us don’t. And most of us don’t have hair on the bottoms of our feet, on our lips, on the palms of our hands, or on our eyelids, except along the very front edge, where we have eyelashes.
Each hair on your body grows in three stages. There is a growth stage, usually lasting two to three years (but sometimes as many as eight). This is followed by a two- to three-week period during which the hair stops growing and is cut off from its blood supply and from the cells that make new hair. After this comes a resting stage, lasting about three months. Then the cycle begins again, and a new hair pushes the old, dead hair out, and it falls away. Each hair has its own cycle, so only a few hairs are shed each day.
The timing of these three stages determines how long your hair will be. If the hair grows for a long time before it rests, the hair will be longer. The hair on your eyebrows only grows for about four to seven months and rests for about nine months, so the hairs there are not as long as those on the top of your head. The same timing of the growth and rest phases determines the length of the other hairs on your body.
Each person has different hair growth and rest cycles, determined by genetics and environmental factors. Some people can grow their hair very long, while others will find that their hair never gets much past their shoulders, even if they never cut it. Women can usually grow longer hair than men, because the hair growth cycles are affected by sex hormones.