Background radiation equivalent time BRET value corresponding to a dose of radiation is the number of days of average background dose it is equivalent to. It is calculated from the equivalent dose in sieverts by dividing by the average annual background radiation dose in Sv, and multiplying by 365: The definition of the BRET unit is apparently unstandardized, and depends on what value is used for the average annual background radiation dose, which varies greatly across time and location. The 2000 UNSCEAR estimate for worldwide average natural background radiation dose is 2.4 mSv (240 mrem), with a range from 1 to 13 mSv. A
Radiation exposure Radiation exposure Radiation exposure is a measure of the ionization of air due to ionizing radiation from photons; that is, gamma rays and X-rays. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. The SI unit of exposure is the coulomb per kilogram (C/kg), which has largely replaced the roentgen (R). One roentgen equals ; an exposure of one coulomb per kilogram is equivalent to 3876 roentgens. As a measure of radiation damage exposure has been superseded by the concept of absorbed dose which takes