The aim of the Envirhom-Santé programme is to acquire a better understanding of the nature and importance of biological non-cancerous effects following chronic exposure to low doses of radionuclides, to determine the threshold at which these biological responses appear and to establish whether the biological effects observed have obvious harmful effects. It aims to respond to two shortcomings of the radiation protection system: the belief that chronic exposure is the sum of acute exposure and the lack of consideration of non-cancerous diseases to estimate risk. The answers uncovered must allow the robustness of the international radiation protection system for humans, which was designed for workers and extended to population groups, to be adapted, if necessary.
Envirhom-Santé is divided into several lines of research:
1. The location and distribution of radionuclides at tissue and cellular level are studied in specific organs following contamination. Different analytical techniques are used: analytical ion microscopy (SIMS) and transmission electron microscopy (EELS, EFTEM). This line of research also includes the modelling and calculation of absorbed doses, developed in cooperation with IRSN's Internal Dosimetry Department (SDI).
2. The organisms' physiological responses are studied on different physiological systems and organs in order to:
define the nature and extent of non-cancerous biological effects caused by chronic exposure to low concentrations of radionuclides;
determine the threshold(s) at which these biological responses occur;
specify whether the biological effects observed will have obvious harmful effects.
3. The mechanisms of the actions that trigger the physiological responses observed are studied, particularly with regards to the central nervous system, the vascular system, intestine, kidney and liver.
In order to carry out these analyses, the research programme requires a significant amount of experimental data on the effects of chronic exposure on the organism of rodents to be gathered. The main physiological functions of the organism are studied: metabolism, cognitive functions (central nervous system), reproduction, immunity and cardiovascular system.
The experiments are carried out in vivo on rodents (rats and mice) which are chronically exposed through the ingestion of water containing low doses of radionuclides (realistic concentration levels present in the environment) over long periods (up to 18 months of contamination, which is equivalent to 40 years for humans). The contamination conditions thereby aim to reproduce the potential exposure situations of the population living in the vicinity of nuclear installations in normal or post-accident scenarios.
In order to take into consideration the heterogeneity of the population groups exposed to these low environmental doses, several contamination models have been created: an adult model (contamination from the adult stage), a post-natal model (contamination from birth), an in utero model (onset of contamination in the uterus) and different pathological models (hypercholesterolaemia and Alzheimer's disease). Finally, multi-generational models (several successive generations of animals) will also be used to observe whether there is an amplification of the biological effects which could result in the appearance of pathologies through the generations, particularly with regards to population groups living in contaminated areas.
Different radionuclides are being studied under this programme: uranium (in its depleted and natural form and enriched with uranium-235) to reproduce an exposure situation affecting a population living in the vicinity of a fuel cycle installation; caesium-137 and strontium-90 in order to assess the effects of chronic exposure in the case of a post-accident situation. In light of the Fukushima accident, multi-pollution and multi-contamination situations (combination of several radionuclides) are also being studied.
In the programme's initial phase, the rodents are contaminated with drinking water which has a single non-toxic reference concentration level (namely, 40 mg/l for uranium, which is twice the maximum environmental concentration level measured). In the second phase, we have to determine the thresholds at which the biological effects appear in line with their level of impact. Two thresholds are sought: the threshold which triggers no harmful effect (NOAEL, No Observed Adverse Effect Level) and the threshold at which the first harmful effects are observed (LOAEL = Lowest Observed Adverse Effect Level). To estimate these thresholds, so-called dose-effect studies are undertaken with a more extensive range of radionuclide concentrations, using high and low doses to encompass the reference doses used within the Envirhom-Santé programme (40 mg/l for uranium).
The different exposure situations and radionuclides considered in the Envirhom-Santé programme (left-hand side of the figure)
Depending on the results obtained, this data could consolidate and complete the radiation protection system developed to protect man against the effects of radionuclides in the event of chronic exposure. For example, showing the effect of chronic contamination caused by caesium-137 on the cardio-vascular system could lead to the consideration of non-cancerous diseases when estimating the risk of this radionuclide to human health