People are constantly exposed to radiation on earth. Background radiation is the ionizing radiation that is constantly present in the natural environment, and comes from various natural and artificial sources. Natural sources include radioactive substances in the earth’s crust, cosmic rays from outer space that constantly bombard the earth, and very small amounts of radioactivity in the body acquired mainly through water and food. Although natural sources of background radiation make up the majority of the ionizing radiation to which people are exposed, there are artificial sources such as medical x-rays, nuclear weapon tests, nuclear power stations, and scientific research materials that account for a fraction of the background radiation. Figure 5 shows a breakdown of background radiation.