Influence of the dose rates on mices cancers
Decreasing the dose rate, by spreading an intensive dose on a rather long time reduces the risk. There is quantitative data only for animals, like this comparison of the incidences of cancers on mices between an intense, brief exposures to X or gamma rays and prolonged daily exposures (the total dose is the same). Experience suggests a reduction factor for low rate situated between two and ten. The value of two was used, for prudence, to set acceptable limits for humans.
IN2P3 (Source IRSN)