About 98% of the atoms in the human body are renewed each year. This surprising fact is discussed by Dr. Paul C. Aebersold of Oak Ridge in the latest Annual Report of the Smithsonian Institution. Dr. Aebersold based his conclusion on experiments with radioisotopes, which trace the movements of chemical elements in and out of the body.

The fastest-changing component, says Dr. Aebersold, is water. It forms about 70% of the body, and about half the water molecules are replaced every eight days. Other fleeting elements are carbon, sodium and potassium. The calcium and phosphorus in bones and teeth stay put longer,...