French regulations set at 1 mSv (milliSievert) per year maximum admissible effective dose resulting from human activities outside the natural radioactivity and expositions for medical reasons. This is a "whole body dose". For exposures of an organ or tissue, equivalent doses are considered. For instance, he equivalent dose limits for the crystalline lens and the skin are set respectively to 15 mSv / year and 50 mSv / year.
The limit of 1 mSv / year is for the general public. It compares with the average exposure (outside medical and natural radioactivity) which was of 0.060 mSv / year in France, where exposure to nuclear that concerns the public represents only a part. For people who work with ionizing radiation, the limit is 100 mSv for a set of 5 consecutive years, where the maximum for one year must not exceed 50 mSv.
These limits apply to the total effective dose (or total equivalent dose) received by “ordinary” individuals during a year. The same goes for total equivalent doses. Their surpassing is unacceptable in principle. These limits, however, lend themselves to some confusion. The fact that the natural and medical exposures are excluded is generally forgotten or omitted.
In a developped european country like Frnance, the French population is exposed each yar to an averge effective dose of 3.7 mSv per capita. These 3.7 mSv are divided into 2.5 mSv of natural radioactivity, 1.1 mSv of medical origin and 0.06 mSv of radiation linked to other human activities including nuclear.
The limit of 1 mSv per year might seem excessive, compared with 0.06 mSv due to human activities once ruled out the medical part and even more to 0.002 mSv for the impact of a nuclear power plant. A dose of 1 mSv is considered as a radiation dose low or very low (*).
On the other hand, exposure to radiation of natural and medical origin varies considerably from person to person. These variations, especially because of diagnoses and medical treatments, blithely exceed the limit for the third leading cause of exposure... If one applied a limit of 1 mSv to these two causes, we could not undergo a scan, we should abandon aircraft, mountain climbing, avoid living in Brittany or in Corsica.
The cells of our body do not differentiate between rays of a natural or medical origin and that of a nuclear power plant. If fluctuations in the exposure of a person to person exceed 1 mSv because of habitat or medical diagnostics and treatments that prolong our lives, it seems illusory to reduce an already low limit since the third source exposure is minor in comparison. The limit makes sense only if very low doses trigger cancers: dose reduction of the third type then remove some cancers. But the effect of low doses is highly uncertain and it is quite possible that they have no effect. However because of the uncertainty on the low dose effects, one applies the precautionary principle.
The principle of limitation in the event of a radiological accident
Finally, one must provide exceptional measures for radioprotection of the population in case of accident or radiological emergency. Actions and counter measures are implemented according to the nature and extent of exposure. In the case of nuclear accidents, intervention levels expressed in terms of doses are used as benchmarks for governments to decide on a case by case, the actions to put in place:
- Sheltering the population in a safe place, if the predicted effective dose exceeds 10 mSv;
- Evacuation, if the predicted effective dose exceeds 50 mSv;
- Administration of stable iodine, when the thyroid dose is likely to exceed 100 mSv.
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