Biological Effectiveness of Ionizing Radiation: Acute vs.Protracted Exposures

This letter refers to the current discussion around re-evaluation of the dose and dose rate effectiveness factor (DDREF) equal to 2, presently recommended by the International Commission on Radiological Protection. The topics of the threshold, hormesis and DDREF are interrelated with the linear no-threshold theory (LNT). The LNT does not take into account that DNA damage and repair are permanent processes in dynamic equilibrium. Given the evolutionary prerequisite of best fitness, it would be reasonable to assume that living organisms have been adapted by the natural selection to the background levels of ionizing radiation. Accordingly, there must be an optimal exposure level, as it is for many environmental factors. Several studies cited in the literature in support of the LNT and lowering of the DDREF down to 1 are discussed here. In the author's opinion, the dose-effect relationships with non-neoplastic diseases found in certain exposed populations call in question dose-effect relationships with cancer. Self-selection and other biases in epidemiological studies are discussed. The dose-response relationships should be clarified in largescale experiments involving different animal species. In conclusion, the LNT and under-estimation of DDREF tend to exaggerate radiationrelated health risks at low radiation doses and dose rates.