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Enhancing Nuclear Safety


Research

The effects on health of chronic contamination with a low dose of radionuclides

(juillet 2011)

RESEARCH allowing the evaluation of radionuclide-related risks and their impact on health

The ENVIRHOM-Santé experimental research programme or Understanding the effects on health of chronic contamination with a low dose of radionuclides

The brain: A new target organ to uranium

Is the xenobiotic detoxification system affected by chronic exposure to uranium ?

Chronic ingestion of caesium 137 in a post-accident situation

Chronic exposure to caesium 137: Experimental and epidemiological studies

Metabolomics: Application in radiotoxicology

The perspectives of the ENVIRHOM-Santé programme

Chronic ingestion of caesium 137 in a post-accident situation

The biokinetics of caesium in rodents


The behavior of caesium 137 in man following chronic ingestion is currently predicted by extrapolating the data for potassium given the similarity in the biological behaviour of these two elements (Leggett et al., 2003). Moreover, biokinetic models of ICRP (ICRP 1989) currently used to calculate the health risk postulate that chronic exposure situations are assimilated to a succession of acute exposure at low concentration. However, earlier results from our lab on the biokinetics of uranium (Monleau et al., 2005) have already indicated that these hypotheses could be wrong. The first studies of caesium 137 therefore sought to specify its biokinetic profile following chronic ingestion in two rodent models – rats and mice.


An in-utero contamination model followed by contamination via ingestion up to the age of 20 weeks was used in mice (Bertho et al., 2010). The parents were contaminated via drinking water with a single concentration of 20 KBq.l-1 of caesium 137, two weeks prior to mating. With this caesium 137 concentration, a daily intake of 76.5 ±18.9 Bq per day and per animal was obtained. According to the literature, the two-week interval allows a balance to be reached between ingestion and excretion although this point has yet to be checked in a chronic ingestion situation. Confirmation of this balance was obtained by measuring caesium 137 levels in the muscles and livers of the dams at the time of weaning the offspring.

The caesium 137 concentration was then measured by gamma spectrometry in offspring’s organs at various ages (from birth up to 20 weeks). The measured concentrations of caesium 137 varied from 1.8 ±0.3 Bq.g-1 in the blood up to 27.8 ± 3.3 Bq.g-1 in the muscleswithout any significant difference between males and females. This variability shows that caesium 137 accumulates mainly in the muscles (see figure above) as well as in the kidneys (11.8 ±3.1 Bq.g-1 at 20 weeks) and the heart (9.6 ±3.2 Bq.g-1 at 20 weeks). The other organs (lungs, spleen, bone, thymus and skin) all showed an average caesium 137 concentration of approximately 5 Bq.g-1. It should be noted that, in most of the organs, this concentration reaches a plateau soon, generally at the age of 3 weeks, i.e. when the offspring are being weaned. Only the kidneys and muscles show a continuous accumulation of caesium 137 up to the age of 6 and 12 weeks, respectively. This variability shows that caesium 137 accumulates essentially in the muscles (see figure above) as well as in the kidneys (11.8 ±3.1 Bq.g-1 at 20 weeks) and the heart (9.6 ±3.2 Bq.g-1 at 20 weeks).

The other organs (lungs, spleen, bone, thymus and skin) all showed an average caesium 137 concentration of approximately 5 Bq.g-1. It should be noted that, in most of the organs, this concentration reaches a plateau soon, generally at the age of 3 weeks, i.e. when the offspring are being weaned. Only the kidneys and muscles show a continuous accumulation of caesium 137 up to the age of 6 and 12 weeks, respectively. This variability shows that caesium 137 accumulates essentially in the muscles (see figure above) as well as in the kidneys (11.8 ±3.1 Bq.g-1 at 20 weeks) and the heart (9.6 ±3.2 Bq.g-1 at 20 weeks).


Other biokinetic aspects of caesium 137 in situations of chronic ingestion have been evaluated in rats (Tourlonias et al., 2010). A chronic ingestion model in the adult (with onset of contamination at 3 months) was compared to a juvenile model with contamination initiated at birth or on weaning. The results show an increase in the total whole body activity in the juvenile model. This is associated with the increased ingestion after withdrawal and weight gain as a function of age. Conversely, the total whole body activity in the adult rats was not modified by the duration of chronic ingestion. These results confirm that caesium 137 did not accumulate over time in animals following chronic ingestion (outside the growth period of the juvenile animals) and that a balance between the quantity of caesium 137 ingested on a daily basis and the total whole body activity was reached within a few weeks.

figure 1

figure 1

Changes in caesium 137 concentration in the muscles, kidneys and liver in male and female mice according to age. No significant difference was observed according to gender. However, a significant increase in caesium-137 concentrations as a function of the animals’ age was observed in the muscles (up to 12 weeks) and the kidneys (up to 6 weeks) but not in the other organs such as the liver shown here as an example (two-parameter ANOVA test).


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.

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