This section is devoted to present the key messages to come out of the research that has been performed in the ENVIRHOM program since its launching in late 2000. Our focus is on results obtained during the last 2-year period avoiding redundancy with the first synthesis that was presented to the last scientific committee (ENVIRHOM 2003). Chronic low-level exposure to radionuclides and induced biological responses at various organizational levels, from subcellular level to individual level (and to ecosystem level for the environmental aspect), has become a major issue in environmental and health science research and policy. From a scientific perspective, a series of lessons has been learnt during this period, some of them being specific to the element studied (e.g. uranium, selenium), others being relevant to more general issues concerned with the impact of radioactive substances on environmental and human health.
The results we have selected within this report are presented in a consistent and integrated way whatever the radionuclide studied, the biological model and the organizational level. This presentation highlights the common points between the environmental and the human aspects, both on the methodological side and on the acquired knowledge side. Obviously, the two fields are also interesting per se and the outcome present specific operational issues in each domain i.e. environmental and human radioprotection, as explained below.
Health aspects were evaluated by the mean of experimental contamination and follow-up of rodents (human model). The experiments carried out in this field aimed to verify if the biokinetic and toxicity data already established for acute exposures to radionuclides are transposable to situations of protracted exposure. The first radioelement studied was uranium, since it can be present at very high concentrations in underground waters of certain areas such as Finland, New Mexico in USA and Canada. The corresponding studies were therefore undertaken on rats and mice contaminated experimentally with uranium added to the drinking water. They were carried out in two parts, centered respectively on the comparison of biokinetics and on the biological effects of uranium after acute or chronic exposure. The first aspect comprised the description of the general kinetics of accumulation and excretion of radionuclides, of the influence of their speciation on their absorption, of the different ways of absorption in the gastro-intestinal tract and of their microdistribution after translocation. The second part was centered on the toxicology of uranium. It attempted to describe the effects of uranium on variousorgans (the kidneys, the liver, the intestine, the central nervous system, the lungs) and on some metabolisms such as that of the drugs, the vitamin D or the cholesterol.
From an environmental policy perspective, results on chronic effects of low-level radionuclides exposure of living organisms may contribute to the derivation of more robust safe levels for ecosystems and their sub organizational levels. They also contribute to propose extrapolation rules to deal with the quantification of the main sources of uncertainties associated with these safe/acceptable criteria. Factors/key extrapolations issues that are known to influence the proposed values are numerous, the most important being extrapolations over time (acute vs. chronic), irradiation pathways (external vs. internal), taxa, level of biological organization, stressors (Garnier-Laplace, Gilek et al. 2004). From a human radioprotection perspective, results presented here demonstrated for the first time that biokinetics and toxicity of radionuclides after chronic exposure may not be simply extrapolated from data acquired after acute exposure (Paquet, Houpert et al. in press). Moreover, they showed that many deterministic effects may be induced after ingestion of small amounts of radionuclide (Houpert, Lestaevel et al. 2004; Houpert, Lestaevel et al. 2005; Lestaevel, Bussy et al. 2005; Souidi, Gueguen et al. 2005), although main concern in this range of dose was on cancer induction. These data are too sparse to be already incorporated in the current system of radioprotection but emphasize the interest to get more specific data for these particular -although widely represented- situations of exposure.