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.
We are all exposed to natural radiation to a greater or lesser extent, and for most. 5Nuclear Energy in Everyday Life people it is the major source of radiation exposure. Background radiation is that which is naturally and inevitably present in our environment. Figure 3: Penetrating Power of RadiationFigure 4: Sources of natural radiation.