Background radiation equivalent time small area in India as high as 30 mSv (3 rem). Using the 2.4 mSv value each BRET unit equals 6.6 μSv. BRET values for diagnostic radiography procedures range from 2 BRET for a dental x-ray to around 400 for a barium enema study. Background radiation equivalent time Background Radiation Equivalent Time, or BRET, is a unit of measurement of ionizing radiation dosage. One BRET is the equivalent of one day worth of average human exposure to background radiation. The unit is also referred to as BERT (Background Equivalent Radiation Time). BRET units are used as a measure of low
Rad (unit) in the latest NIST definition. Even where the standard medium is fully defined, the ionizing energy is often not precisely known. In 1940, British physicist Louis Harold Gray, who had been studying the effect of neutron damage on human tissue, together with William Valentine Mayneord and John Read published a paper in which a unit of measure, dubbed the ""gram roentgen"" (symbol: gr) defined as "that amount of neutron radiation which produces an increment in energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one roentgen of radiation" was proposed.