An analysis of caesium 137 concentration in the organs of rats highlighted similar conclusions to those observed in the mice experiments. In fact, caesium 137 is detected in concentrations ranging from 0.54 ± 0.14 Bq.g-1 in the blood and up to 13.7 ± 2.9 Bq.g-1 in striated muscles with an increase in the concentration of caesium 137 in parallel to the growth of the animal during the first few months of life.
Two observations recorded during this study should, however, be noted :
The central nervous system does not display any specific feature of caesium 137, other than in the olfactory bulbs in which an increase in caesium 137 concentrations was observed as a function of the duration of contamination, especially in juveniles. This differential accumulation of caesium 137 in olfactory bulbs is probably linked to the sniffing activity of animals and suggests the existence of a direct contamination pathway through the olfactory epithelium, as described for the inhalation of other elements (Monleau et al., 2005).
The thyroid contains a higher concentration of caesium 137than that observed in other organs, especially in adults (Tourlonias et al., 2010). Interestingly, the concentration of caesium 137 in this tissue decreases with the duration of contamination, from 17.9 ± 6.9 Bq.g-1 after 1 month’s contamination down to 4.7 ± 1.5 Bq.g-1 after 9 months’ contamination. This decrease, which depends on the contamination period, was also observed in the juvenile model. This could be explained by the transport of caesium via the Na+/K+ ATPase-dependant pump and a variation in the activity of thyroid follicle cells depending on the age of the animals. Regardless of the mechanism involved, this accumulation of caesium 137 in the thyroid, which is a particularly radio-sensitive organ, should be taken into account in the health risks associated with the chronic ingestion of caesium 137.
It should be noted that this chronic ingestion model of caesium 137 via the drinking water seems to be consistent with the chronic ingestion data observed for humans in contaminated areas. In fact, the daily ingestion of caesium 137 by mice (approximately 80 Bq/day/animal) or by rats (close to 100 Bq/day/animal) is within the low range of estimated daily ingestions in humans (estimated to be between 100 and 2100 Bq/day)(Cooper et al., 1992, Handl et al., 2003). The resulting mean whole body activity ranges from 4 to 15 Bq.g-1 in mice and between 4 and 7 Bq.g-1 in rats. These values should be compared to the values recorded in man, between 1x10-3 and 10 Bq.g-1, depending on the diet and especially on mushroom consumption (Handl et al., 2003). This concentration of caesium 137 in rodent tissue, which is similar to that observed in humans with a lower daily ingestion rate, is probably associated with the faster metabolism of rodents compared to man. However, this comparison shows that the chronic ingestion model via drinking water used in these studies can be considered typical of the situation for populations living in areas contaminated by the Chernobyl accident.
Overall, the results obtained in mice and rats are consistent with current biokinetic models, which suggest the lack of caesium 137 accumulation in the tissues and providing the basis for evaluating the health risks for exposed populations. However, the accumulation of caesium 137 in the muscles during the growth of juvenile animals together with the possible implications on the growth of animals and the accumulation of caesium 137 in the thyroid should be noted. This could increase the risks of cancer in this organ.