People may be exposed to ionizing radiation from 3 main sources:
Natural background radiation comes from cosmic rays from our solar system and radioactive elements normally present in the soil. This is the major contributor to worldwide radiation exposure.
Medical radiation comes in the form of diagnostic x-rays and other tests, as well as from radiation therapy. Radiation therapy is currently used to treat some types of cancer and involves dosages many thousand times higher than those used in diagnostic x-rays.
Non-medical, man-made radiation can come from workplace and other sources, and is also a result of above ground nuclear weapons testing that took place before 1962.
Natural background radiation
We are all exposed to some amount of radiation just from being on this planet. This is known as background radiation. For most people, background radiation accounts for most of their exposure to ionizing radiation during the year. It comes from several different sources.
Cosmic rays: Cosmic rays are radioactive particles that hit the earth from outer space. They come from the sun and from other stars. The earth's atmosphere blocks a portion of these rays, but some of them reach the ground. Because the atmosphere blocks some cosmic rays, exposure is greater at higher altitudes. For example, people who live in Denver, Colorado, which is at a high elevation, are exposed to slightly more cosmic rays than people living at sea level. People are also exposed to higher levels of cosmic rays during airplane flights. Airline pilots and flight attendants, who spend many hours at high elevations, are exposed to more of these rays, but it is not clear if they have an increased risk of cancer because of it.........