Guidance for Selecting Exposure Rate

Each exposure can be defined as acute or chronic. Based on this input information, the calculator determines whether to adjust the risk estimates by a "dose and dose-rate effectiveness factor" (DDREF). Since most underlying risk models in NCI RadRAT are based on studies of the Japanese atomic bomb survivors who were exposed to a short-term/acute radiation dose, the models are adjusted by the DDREF when applied to longer-term (chronic) exposures. This adjustment accounts for the possibility that the risk from chronic exposures is different than the risk observed from acute exposures.

To classify each exposure as acute or chronic, one can rely on the criteria recommended by the UNSCEAR (2003): chronic exposures are those where the dose rate is lower than 6 mGy per hour averaged over the first few hours. This recommendation was adopted by Land et al., (2003) and is similar to that of EPA (1999).

In general, an exposure received over a short period of time (less than 1 day) may be considered acute, while prolonged (or repeated) exposures that span more than 1 day may be considered chronic.


  1. EPA (Environmental Protection Agency). Estimating Radiogenic Cancer Risks. EPA Report 402-R-00-003. Washington, DC: Environmental Protection Agency, May 1999.
  2. Land C, Gilbert E, Smith JM, Hoffman FO, Apostoaei I, Thomas B, Kocher DC. NCI-CDC working group to revise the 1985 NIH radioepidemiological tables. Washington, DC: National Institutes of Health, National Cancer Institute; NIH Publication No. 03-5387; 2003.
  3. UNSCEAR. Sources, Effects and Risks of Ionizing Radiation. No. E.94.IX.2. New York: United Nations, 1993.