SCIENCE FILE / An exploration of issues and trends affecting science, medicine and the environment.

Q: Why does the hair on our heads keep growing longer, but the hair on our arms grows to a certain length and stops?

A: All hair on the body goes through a cycle in which it grows for most of its life, then stops growing and falls out, according to scientists at San Francisco’s Exploratorium. After it falls out, a new cycle begins and a new hair shaft starts growing. The length a hair grows to depends on the length of that cycle, which varies with location on the body. The cycle for hair on the head lasts for a few years whereas hair elsewhere on the body usually grows for only a few months.