Human exposure to radiation is measured in mrem or μSv, that is millirem or microsievert (1 mrem = 10 μSv). This measurement takes into account the human tissue which is exposed to the radiation. The dose of radiation also is weighted differently depending on the type of radiation; alpha (α) particles are weighted 20 times higher in dose than the same amount of gamma (γ) rays.
We are surrounded by many sources of natural radiation. Cosmic radiation comes from outer space and terrestrial radiation comes from the earth. About half of our radiation exposure is due to natural sources. The other half is attributed to man-made sources such as medical procedures. A person living in a brick house is exposed to more radiation than a person living in a wood house. A person living in Denver is exposed to more cosmic radiation than a person living in Miami.
Here are some typical doses of radiation:
The average annual radiation exposure from natural sources – cosmic and terrestrial - is about 310 millirem
Dental xRays – 0.5mrem
Mammogram - 300mrem
Dose from living near a nuclear power station: 0.01–1 mrem/year
Average dose to people living within 10 miles of the Three Mile Island accident – 8 mrem
Standing in front of the granite of the United States Capital building – 85mrem/year
Dose from smoking 30 cigarettes a day - 1300-6000 mrem/year
A daily dose of greater than 25,000 mrem will produce nausea and loss of appetite; bone marrow, lymph nodes, spleen damage. Higher doses cause much more severe and perhaps permanent damage. A dose of 600 -1000 rem (1,000,000 mrem) within one day will cause incapacitation and death.
For more information on radiation exposure consult the USNRC Fact Sheet on Biological Effects of Radiation.