The UN has identified the exposure to radiation through medical tests such as x-rays and CT scans as accounting for 98 percent of all artificial radiation exposure humans receive. The remaining 2 percent comes from residual radiation from the testing of nuclear bombs, fallout from nuclear accidents like the Chernobyl disaster, and byproducts of industrial activity like the burning of coal.
Natural background radiation comes from soil, gas, and cosmic rays, with by far the most coming from radon gas. There is even natural radiation in the human body from food we eat and air we breathe. The average background dose for people in the United States is about 360 millirems (mrem) per year.
Other sources of radiation might be a chest x-ray (14 mrem), a CT scan (110-200 mrem), a dental x-ray (3-4 mrem), a cross-country flight (5 mrem), living in a brick house (75 mrem/yr). Some specialized medical procedures can expose people to higher doses. Cigarette smoking can expose the smoker to over 1300 mrem/year, by far more dangerous than average medical testing.
Uncontrolled or excessive medical use of radiation for unnecessary procedures (repeated CT scans for example) is the major concern of the UN and the International Atomic Energy Agency IAEA), especially in less developed countries where use is unregulated or dangers not well understood. However, for more advanced countries there is growing concern about the increased of radiation from the increased use of CT scans.