1.3.1 The individual response to ionising radiation The effects of ionising radiation on personal health vary from one individual to the next. As early as 1906, Bergonié and Tribondeau stated for the first time that a given dose does not have the same effect when received by a growing child or by an adult. The variability in individual radiosensitivity is observed at high doses of ionising radiation, notably in terms of tissue responses. It has been well documented by radiation oncologists and radiobiologists. High levels of radiosensitivity have been observed in subjects suffering from genetic diseases affecting the repair of DNA and cellular signalling. Such abnormal responses are also observed in people suffering from neurodegenerative diseases. This variability in radiosensitivity at low and moderate doses, particularly at cellular level, is increasingly documented, as is the fact that radiosensitivity at a given dose level does not necessarily imply radiosensitivity at other dose levels. Thanks to the lowering of detection thresholds, some recent methods of immunofluorescence of molecular targets for signalling and repairing DNA damage enable the effects of ionising radiation at low doses to be better documented. The results of the research work conducted using these new investigation methods must still be confirmed in the clinical environment before being integrated into medical practices. The work of the European research group on low doses (Multidisciplinary European Low Dose Initiative – MELODI) and in the medical field (European platform for research activities in medical radiation protection – Euramed) is continuing on this subject. The ICRP task group (TG111) dedicated to this subject has published a review of the state of knowledge on individual radiosensitivity and the possibilities of predicting it with a view to developing international radiation protection recommendations. At this stage however, no valid biomarker allowing such a prediction has been identified. The individual response to ionising radiation remains an important subject of research and application in radiobiology and radiation protection (Euratom 2021-2022). 5. The radioactive dose rate determines the absorbed dose (energy absorbed by the material per unit mass and time). It is measured in Gray per second (Gy/s) in the International System of Units (SI). It is used in physics and radiation protection. 1.3.2 Effects of low doses The linear no-threshold relationship The linear no-threshold relationship is a model used in radiation protection to estimate the probability of risk associated with an exposure to ionising radiation taking into account the principle of precaution. According to this relationship, there would be a risk from the first exposure, proportionate to the radiation dose received. There are nevertheless many uncertainties. This is why some feel that the effects of low doses could be higher, while others believe that these doses could have no effect below a certain threshold, and some others even assert that low doses have a beneficial effect. Research in molecular and cellular biology is progressing, as are epidemiological surveys of large cohorts. The ICRP considers that the hypothesis of this relationship, used to model the effect of low doses on health (see point 1.2), constitutes a cautious basis for managing the risks due to exposure to ionising radiation. It is the obvious option for decision-makers given the uncertainties that subsist faced with the complexity of the phenomena of DNA repair and mutation and the methodological limits of epidemiology, despite the progress of research in molecular and cellular biology. Dose, dose rate and duration of exposure The epidemiological studies performed on individuals exposed to the Hiroshima and Nagasaki bombings have given a clearer picture of the effects of radiation on health, concerning exposures due to external irradiation (external exposure) received in a few fractions of a second at high dose and high dose rate(5) of ionising radiation. The studies carried out in the countries most affected by the Chernobyl accident (Belorussia, Ukraine and Russia) were also able to improve our understanding of the effects of radiation on health caused by exposure through internal contamination (internal exposure), more specifically through radioactive iodine. Studies on nuclear industry workers have given a clearer picture of the risk associated with chronic exposures at low doses established over many years, whether as a result of external exposure or internal contamination. The ICRP, which published new recommendations for the calculation of effective and equivalent doses (publication 103) in 2007, is gradually updating the values of the effective dose coefficients for internal and external exposure. Its Publication 137 (2017) focuses on 14 radionuclides, including radon. ICRP Publication 115 (2010) updated the risk of lung cancer associated with radon exposure on the basis of new epidemiological studies. The ICRP had concluded that the risk of death from lung cancer in adults having been chronically exposed to low concentrations of radon was nearly two times higher than that estimated on the basis of the knowledge available in 1993 (Publication 65). These coefficients were based on an epidemiological approach. In its Publication 137, the ICRP proposes new coefficients based on a dosimetric approach, in the same way as for the other radionuclides. For an equal given level of exposure to radon and its progeny, they lead to a significant increase in the annual effective dose received by workers exposed to radon (nearly two times higher). The Order of 16 November 2023 defining the methods of calculating the effective doses and equivalent doses resulting from the exposure of persons to ionising radiation updated the dose coefficients for radon as of 1 January 2024. This update changes the calculation of the average effective dose received by the population in France, which thus increases from 3.5 to 6.5 millisieverts per year (mSv/year)(*), with exposure to radon now representing 54% of the overall exposure (compared with 33% previously). * Exposure of the French population to ionising radiation – Results for 2014-2019, IRSN, 2021. ASSESSMENT OF EXPOSURE DUE TO RADON: THE RECOMMENDATIONS OF THE INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION 102 ASN Report on the state of nuclear safety and radiation protection in France in 2023 • 01 • Nuclear activities: ionising radiation and health and environmental risks
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