The health hazards of low doses of ionizing radiation are unknown and controversial, because the effects, mainly cancer and genetic damage, take many years to appear, and the incidence due to radiation exposure can't be statistically separated from the many other causes of these diseases.
The purpose of the BRET measure is to allow a low level dose to be easily compared with a universal yardstick: the average dose of background radiation, mostly from natural sources, that every human unavoidably receives during daily life.
Background radiation level is widely used in radiological health fields as a standard for setting exposure limits.
[1] Presumably, a dose of radiation which is equivalent to what a person would receive in a few days of ordinary life will not increase their rate of disease measurably.
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.