Answer to Question #2819 Submitted to "Ask the Experts"

With regard to dose units, it is important to remember that one of the fundamental reasons for expressing dose quantities is to give some indication of possible biological effects that could occur. The evolution of dose quantities throughout the history of health physics has largely been the result of this desire to be able to express a dose in such a way that the biological results of the radiation exposure could be evaluated. Some dose quantities are physical quantities that are independent of biology (e.g., exposure, kerma). Others involve the application of factors that attempt to take into account (1) the differing abilities of radiation to cause a stated biological effect and (2) the differing radiosensitivities of the human tissues with regard to the stated biological effect.

In the current dose limitation system in place, the whole-body dose limits are based on stochastic effects, most notably the induction of a fatal cancer. This means that the absorbed dose is modified by the radiation weighting factor as well as the tissue weighting factor when effective dose equivalent is calculated. A complication occurs because radiation weighting factors are based on experimental data involving biological endpoints other than cancer and were generally carried out under low-dose, low-dose-rate conditions. In addition, radiobiological experiments tend to show that at high dose rates, all radiation types seem to produce similar biological effects (especially in terms of cell death) for the same amount of absorbed dose. The result of these two facts is that for potentially high-dose, high-dose-rate situations (where nonstochastic effects would become predominant), it is more appropriate to use absorbed dose instead of dose equivalent for assignment of dose and for evaluation of radiological conditions for posting purposes. The exact point at which the crossover occurs is open for debate; however, both the NRC (the Nuclear Regulatory Commission) and DOE (the Department of Energy) have specific requirements for posting areas with high dose rates in terms of absorbed dose.