Exposure. We talk about low doses when radiation doses are below 100 millisieverts. A brief overview.
When are we exposed to low doses?
There are several different types of exposure to low doses of radioactivity:
- chronic external irradiation : For example, cosmic rays that bombard the Earth
- pinpoint external irradiation : For example, a radiological test
- chronic internal contamination : For example, inhalation of radon contained in the air
- pinpoint internal contamination : For xample, ingestion of contaminated water.
There are several sources of exposure to radioactivity. Natural, for example coming from granite which contains minute traces of uranium which, when it disintegrates, produces radon, a radioactive gas emitted from rock and naturally present in our atmosphere. Other sources are artificial. Thus, nuclear industry workers and health professionals are exposed. So is any person who undergoes medical imaging tests, such as an X-ray or scan.
Where does this figure of 100 mSv (millisieverts) come from?
Epidemiological studies conducted for over sixty years on nearly 90,000 survivors of the Hiroshima and Nagasaki bombings showed with certainty that the risk of cancers significantly increases for people who have received a dose of ionizing radiations of over 100 mSv. The deterministic effect (burns, etc.) observed after massive and short-lasting radiation was also evaluated.
However, over 100 mSv, the studies did not establish indisputably the existence of a link between radiation dose and risk of cancers or non-cancerous diseases, mainly because of the uncertainties which accompany this kind of study. This is why the value of 100 mSv was chosen as an outline definition of the “low dose” area.
Do low doses present risks?
The absence of discernable effects in the epidemiological studies hitherto conducted does not mean that the existence of risks to living beings can be dismissed. Further research is essential. To identify these biological and/or pathological risks , complicated and necessarily long research needs to be carried out in a combination of epidemiological, clinical and experimental studies. Only diverse but complementary approaches will provide further knowledge.
What difficulties are faced by researchers working on low doses?
Exposure to natural radioactivity comes on top of possible occupational, medical or accidental exposure. It is therefore difficult to identify the respective impact of each. Moreover, several pathologies, and especially cancers, arise throughout life for genetic or environmental reasons unrelated to exposure to radiation.
Why is an increasing amount of research being done into this field?
Extrapolating the effects of high doses to those of low doses does not reflect reality at all. It is therefore essential to study the effects associated with chronic low exposure rates. This is why the European Commission is now financing this research and an increasing number of teams is starting programs.
What will be the consequences of better knowledge of the effects of low doses?
Currently the Inter national Commission on Radiological Protection (ICRP) recommends that the general population not be exposed to more than 1 mSv per year in addition to natural exposure. For workers, the recommended maximum threshold is 20 mSv per year.
These thresholds have been established so as to manage the risks in an optimum manner at an acceptable cost. By testing the suitability of the current radioprotection system in scientific studies based on ascertained results, it will be possible to assess whether the standards need to be adjusted. It is essential for the protection measures to be based primarily on scientific knowledge obtained from experimental, clinical or epidemiological studies.
1- Distinction is made between biological effects, which are transient or permanent changes in the functioning of the organism without this being harmful, and pathological effects which may be serious to a greater or lesser degree.
1 sievert = 1,000 millisieverts
The sievert (Sv), in honour of the Swede Rolf Sievert and his research on the biological effects of radiation, is the unit which quantifies the risk associated with exposure to ionizing radiation.
Whereas the becquerel (Bq) measures the activity of nuclear matter and the gray (Gy) measures the dose physically absorbed by matter, the sievert enables the impact of radiation on living matter to be evaluated, thus enabling the comparison of the effects of the same dose delivered by different types of radiation on organisms, organs or tissues which do not have the same sensitivity to radiation.