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.
The Cosmic Microwave Background Radiation. Perhaps the most conclusive, and certainly among the most carefully examined, piece of evidence for the Big Bang is the existence of an isotropic radiation bath that permeates the entirety of the Universe known as the cosmic microwave background (CMB).