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
Rad (unit) nuclear applications can survive up to 100 Mrad (1 MGy). Metals creep, harden, and become brittle under the effect of radiation. Foods and medical equipment can be sterilized with radiation. In the 1930s the roentgen was the most commonly used unit of radiation exposure. This unit is obsolete and no longer clearly defined. One roentgen deposits 0.877 rad in dry air, 0.96 rad in soft tissue, or anywhere from 1 to more than 4 rad in bone depending on the beam energy. These conversions to absorbed energy all depend on the ionizing energy of a standard medium, which is ambiguous