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.
There are other forms of radiation from nuclear decay that  are not ionizing, such as neutrinos and low energy photons, and  other sources for ionizing radiation, such as cosmic rays.    Ionizing radiation comes in two basic forms, particles and waves.