Radiobiology > Background > Medical Radiation in Context (cont.)

Medical Radiation in Context Background Exposure, continued


  • Radon is a radioactive (half life about 3 days) noble gas formed during the decay of uranium in soil and water. Some of its degredates may adhere to motes of dust that, inhaled, may deposit in deep airways. Some of these progeny emit alpha particles.
  • Radon exposure is approximately 55% of effective dose in the U.S. (double the effective dose of diagnostic medical x-rays). Approximately 48% of natural background exposure worldwide (UNSCEAR report, 2000).
  • The EPA estimates lifetime risk of developing lung cancer in never-smokers with home radon levels 20 pCi/L is 3.6% (36 in 1000 exposed) compared with 0.73% (73 in 10,000 exposed) for levels of 4 pCi/L (the current action level for radon in the U.S.)

Cosmic Radiation

  • Exposure clearly increases with altitude so, it is highest for astronauts and pilots.
  • A single commercial roundtrip transcontinental flight in the U.S. is associated with an effective dose of 0.05 mSv (i.e. the equivalent of 2.5 chest x-rays).

Occupational Exposures

  • In 1975, the mean annual occupational dose for hospital radiation workers was 350 mRem (3.5 mSv).
  • During fluoroscopic procedures, a rule of thumb for an occupational exposure is ~1/1000 the patient dose at a distance of 1 meter (Radiology 1991; 180: 865). 
  • Over a 30-year career, a diagnostic radiologist may have an occupational exposure of 60 rem (0.6 Sv).

One mission aboard the International Space Station exposes astronauts to a dose of 0.1 Sv (10 rem).
Photo: NASA Gemini 4, by James McDivitt


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