However, whereas walking is a low-impact activity because one foot is always in contact with the ground, in jogging and in running, impact is introduced. This is a high-impact exercise that places strain on the body, notably the joints of the knee.
Most background radiation comes from natural sources, while most artificial radiation comes from medical examinations, such as x-ray photographs. Radiation can damage cells and make them cancerous.
Radiation the emission and propagation of energy through space or tissue in the form of waves. It usually refers to electromagnetic radiation, classified by its frequency: radio, infrared, visible, ultraviolet, X-ray, gamma ray and cosmic rays. Natural background radiation --.