Measuring Radiation Exposure
Exposure to radiation is measured in "rem" or "millirem" (one thousand millirem equals one rem). In the United States, average background radiation is estimated to be 360 millirem (mrem), with 200 millirem from radon gas, which is a natural source, and 40 millirem from medical x-rays, one of several human-made sources.
By comparison, the dose from one chest x-ray is approximately 10 millirem. Regulatory agencies in both the U.S. and Canada have set limits for worker exposure which are considerably higher than the level of natural background radiation. In the U.S., the annual dose limit for radiological workers is 5,000 millirem (the limit for a pregnant worker is 500), while the dose to the public from nuclear industries is limited to 100.
The U.S. Nuclear Regulatory Limit has published estimates that a whole body dose of 25,000 mrem poses a significant risk of serious health effects and a whole body does of 500,000 mrem is a probable cause of death.
Government regulations also require that the nuclear industry follow a radiation control concept known as "ALARA" or "As Low As Reasonably Achievable". The ALARA objective is to keep worker and public doses as low as possible.
It must be noted that there is considerable technical controversy about individual health effects of any additional exposures above background levels, with a large body of thought subscribing to the view that there is no acceptable level of radiation exposure above background.
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