"Low Level
Radiation Health Effects: Compiling  the Data"

Revision 2
March 19, 1999

by Radiation, Science, and Health, Inc.,
Edited by J. Muckerheide

1.3
Animal & Plant
Biology

1.3.1
Mammals

References

"BACKGROUND: There is epidemiological evidence that suggests there are beneficial effects of ionizing radiation at low doses. Some experimental studies confirmed this hormetic effect with doses of about 1 cGy/day, but no data concerning very low dose rates are available."

OBJECTIVE: The aim of this study was to determine the life span of mice exposed to very low doses of ionizing radiation."

METHODS: Six hundred female C57BL/6 mice, 1 month old, were exposed to chronic gamma irradiation at very low dose rates of 7 or 14 cGy/year. These doses are about 25 or 50 times higher than background, but much lower than the doses of about 1 cGy/day used in previous experiments. Three hundred mice living in the same room were used as controls."

RESULTS: The life span, after the beginning of the experiment, determined by the survival time of 50% of each population, is increased in irradiated mice: 549 days in controls, 673 days in both irradiated groups. The differences are significant between the control and the irradiation mice. Differences between mice irradiated with 7 or 14 cGy are not significant."

CONCLUSIONS: These results confirm the possibility of a non-harmful effect (hormesis) of ionizing radiation. They demonstrate that the paradigm, which states that low-dose effects can be predicted high-dose effects, cannot be systematically applied in radiation biology in general and gerontology in particular."

"It is well known that exposure to ionizing radiation can induce a shortening of the life span in man and animals. However, the survival time can be prolonged after exposure to low doses. This effect was reported, for the first time, by Lorenz et al. (1955) in the 1950s: mice exposed to 0.11 rad for 8 h/day had a longer mean survival rate, nearly 2 months. According to Sacher and Grahn (1964), exposure to 0.4 or 5 cGy/day of 60Co gamma rays results in about a 10% increase of the life span. Similar results were reported in mice exposed to 1 cGy/day for many generations (Searle 1964) or after exposure to 0.8 cGy/day for 1 year (Bustad 1965). An increased life span was also noted in 2-month-old mice continuously irradiated at 0.7 or 6 cGy/day (Spalding et al 1982)."

"More recently, it was shown that the mean survival time was significantly prolonged from 283 ± 3 days in controls to 316 ± 10 days in mice irradiated with 1 5-cGy X-rays twice a week (Ishii et al 1996). With X-ray or neutron single irradiation in mice performed on days 7 or 21 of age, survival was reduced with high doses but increased in groups receiving the lowest doses (0.125 Gy neutrons or 0.5 Gy X-ray) (Maisin et al 1996). It can be pointed out that an increase in life span occurred, in most experiments, when a chronic irradiation was performed at a dose rate of about 1 or several cGy/day."

"Over the past few years, we have conducted in our laboratory large-scale experiments in single-cell organisms of the effects of background and very low dose radiation. In particular, we have demonstrated that ionizing radiation, under these conditions, can stimulate cell proliferation (Planel et al 1987) and affect the clonal life span in Paramecium tetraurelia (Tixador et al 1981)."

"As shown in figure 2, it is obvious that the survival capacity is higher in both irradiated groups than in controls. The survival curves can be fitted by a logistic model such as the Nelder curve. Comparison by c2 test of the survival frequencies observed on 12 different dates demonstrates that differences between irradiated mice and controls are significant."

                              Figure 2

"Life lengthening in irradiated mice is shown in Table 3 which indicates the survival time (days) of 50% of the population (ST50). ST50 values are very similar using weighted probits or weighted logits (Table 3)."

                                   Table 3

"Exposure to chronic low-dose gamma irradiation results in a significantly increased life span expressed by the survival time of 50% of the population. The response is the same in mice exposed to the two dose rates."

"Changes in longevity can be ascribed to radiation, as besides the irradiation, the three groups lived in an identical environment."

"In both cases, the low-dose effects could be due to a radiation-induced stimulation of immune functions. Indeed, with high doses, the immune system can be activated: 20 Gy of gamma 60Co rays induces an enhanced interleukin 2 production by human T lymphocytes (Gerber et al 1985). In mice exposed to 75 mGy of X-rays, the T cell function was increased by 212% when compared to controls (Liu et al 1987). In China, where human populations are exposed to a high level of background radiation, the reactivity of lymphocytes, measured by [3H]thymidine incorporation, is higher than in populations living in areas with a background radiation three times lower (Liu et al 1985). Furthermore, in the previous work (Planel et al 1992), we have reported an enhancement in immune reaction. This is expressed by changes in cell populations of the thymus and the spleen of mice exposed to a chronic gamma irradiation at a dose rate of 74 mGy/ year (Planel et al 1992). These results are in good agreement with those of previous investigations James and Makinodan 1988) which have shown a higher proliferation of splenic T cells in mice exposed to 20 days of gamma irradiation at doses from S to 40 mGy/day. Additional experiments are in progress in our laboratory in order to confirm this assumption.

"On the other hand, changes in the life span after exposure to very low doses could be considered adaptive responses (Olivieri et al 1984): small doses can stimulate the DNA repair process and increase the resistance to high doses given a few hours later. In our experiment, changes in life span could be related to an enhancement in the repair of damaged DNA related to aging. Supporting this hypothesis is evidence of stimulated expression of a number of genes even with doses as low as 0.06 Gy (Fornace 1992).

"Furthermore, the increase in life span reported in this paper supports the hormesis concept, i.e., there is a possible nonharmful effect of ionizing radiation (Luckey 1994). Radiation hormesis is currently reported after exposure to low doses, about 1 Gy. However, it can be observed with much lower doses and, in particular, with background radiation: in paramecia and a blue alga, Synechoccocus lividus, protection against natural ionizing radiation is followed by a lower cell growth rate (Planel et al 1987). The results presented here show that hormesis, induced by very low doses, can also be observed in more complex organisms. Furthermore, they demonstrate that the radiation paradigm (Sagan 1987), which states that low-dose effects can be predicted from those observed after high-dose exposure, cannot be systematically applied in radiobiology and gerontology."