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When radiation deposits energy in a person, he or she receives a radiation dose. Radiation doses are measured in units of rem or millirem (mrem). One thousand millirem is equal to one rem (1000 mrem = 1 rem).

The primary risk associated with radiation exposure is an increased risk of cancer. The degree of risk depends on the amount of radiation dose received, the time period in which the dose is received, and the body parts that receive the radiation dose. Although scientists assume that low-level radiation doses increase one’s risk of cancer, studies have not demonstrated any adverse health effects in individuals who are chronically exposed to small radiation doses over a period of many years (e.g., a total of up to 10,000 mrem above the average background dose).

The increased risk of cancer from occupational radiation exposure is small when compared to the normal cancer rate in today’s society. For example, the current risk of dying from all types of cancer in the United States is approximately 25 percent – while a person who receives a whole-body radiation dose of 25,000 mrem over his or her lifetime has a risk of dying from cancer of 26 percent – a one percent increase.  This table shows the likely effects of total body radiation doses (measured in rem) to the humans.



Likely effects from total body radiation dose

1000 rem  An acute dose would cause immediate illness and subsequent death within weeks.
100 rem  Acute dose could cause illness such as nausea, and cancer in 5% of persons within several years.
5 rem  MPD allowed by the NRC for occupational exposure over one year. No likely effects at this level.
0.3 rem  Estimated yearly exposure to all individuals from natural sources such as radon and cosmic rays, commonly referred to as background radiation. There are no likely effects at this exposure level.