The risk assessment for population’s exposures to low doses and low dose-rates of ionizing radiation is still subject to clear uncertainties. The issue has outstanding societal importance in relation to radiologic occupational safety, medical applications of radiation, effects of the natural background radioactivity and the future of nuclear power, due to its particular influence on the public acceptance of this form of energy.

This review article analyzes, in a critical, historical and bibliographical manner, the worldwide accepted hypothesis of linearity without a threshold dose (LNT model). As well known, it rejects, from its first proposal in 1946 by American geneticist and Nobel laureate Hermann J. Muller, the concept of zero-risk for exposures to any dose level of ionizing radiation.

The starting point is the dose-effects relationship provided by this model and related risk’s excess graphic curve. The biological and physical motivations for the linearity assumption are argued and challenged by the explanation of human body’s natural defense mechanisms and its repair capacity of the radiation damage.

Furthermore, the historical and political truthfulness of the LNT model is also contested by the review of a recent investigation by Prof. Edward Calabrese, regarding the lack of scientific sources behind Muller’s Nobel Prize Lecture. Calabrese’s inquiry demonstrates that Muller, at the moment of his declaration on LNT model’s validity, had experimental proofs contradicting his conclusions about the unacceptability of a threshold dose. This finding is of historical importance since Muller’s Nobel Lecture is a turning point in the acceptance of the linearity model in risk assessment by the major regulatory agencies till today.

Finally, the results of many epidemiological and statistical studies are shown specifically. They give further evidences concerning the inapplicability of the LNT model and its overestimation of the risk for various cases of exposures to low doses of ionizing radiation in different fields. By that, hormesis model is also discussed, with its assumption of possible benefits for the organism following low dose exposures: a dose-response model characterized by low-dose stimulation and high-dose inhibition, which has been frequently observed in the aforementioned studies.

The argumentations and the experimental evidences provided here challenge the validity of the LNT model. We contest the fact that its establishment is principally based on a cautionary philosophy on nuclear public safety, rather than on actual scientific comprehension of the phenomenon. As such, it implies an exaggerated conception of the radiological hazard. In particular, this article calls attention to the need for a deeper understanding of the biological impact of low doses of ionizing radiation and the development of further specific and exhaustive researches.

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