Radiation Units

Activity

Curie (Ci) = 3.7 x 10¹⁰ disintegrations per second
Becquerel (Bq) = 1 disintegration/sec

Absorbed Dose

Gray (Gy) = 1 joule / 1 kilogram of tissue
1 Rad = 0.01 Gray

Equivalent Dose

Sievert (Sv) = 1 Gray of X-ray radiation, weighted for other types of radiation
1 Rem = 0.01 Sievert

Radiation Dose Limits

The present NRC limits are 5,000 millirem (mrem) per year for radiation workers, and 100 mrem per year for members of the general public. As noted below, typical background radiation levels are about 360 mrem per year.

Background Radiation Levels

  Source                   millirems

  Natural
   Radon                   200
   Other than Radon:       100
    Cosmic           27
    Terrestrial      28
    Internal         39
   Total                   300

  Nuclear Fuel Cycle       0.05
  Consumer Products*       10

  Medical
  Diagnostic X-rays        39
  Nuclear Medicine         14
   Total                   53

  Total   about 360 mrems/year

  *Includes building material, television receivers, luminous watches, 
smoke detectors, etc.

Source: National Council on Radiation Protection and Measurement (NCRP 
93)

There is a nice graph from the University of Michigan.