The goal of this manuscript is to define the role of dose rate and dose protraction on the induction of biological
changes at all levels of biological organization. Both total dose and the time frame over which it is delivered are
important as the body has great capacity to repair all types of biological damage. The importance of dose rate has
been recognized almost from the time that radiation was discovered and has been included in radiation standards as a Dose, Dose Rate, Effectiveness Factor (DDREF) and a Dose Rate Effectiveness Factor (DREF). This
manuscript will evaluate the role of dose rate at the molecular, cellular, tissue, experimental animals and humans to demonstrate that dose rate is an important variable in estimating radiation cancer risk and other biological effects. The impact of low-dose rates on the Linear-No-Threshold Hypothesis (LNTH) will be reviewed
since if the LNTH is not valid it is not possible to calculate a single value for a DDREF or DREF. Finally, extensive
human experience is briefly reviewed to show that the radiation risks are not underestimated and that radiation
at environmental levels has limited impact on total human cancer risk.