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.
Arno Penzias and Robert Wilson of Bell Labs shared in the 1978 award; while trying to explain faint background noise in a sensitive antenna, they had discovered the cosmic microwave radiation that has coursed throughout the universe since the Big Bang.