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Abstract
Tritium is a low energy beta emitter and is discharged into the aquatic environment primarily in the form of tritiated water (HTO) from nuclear power plants or from nuclear fuel reprocessing plants. Although the biological effects of HTO exposures at significant doses or dose rates have been extensively studied, there are few reports concerning the biological effects of HTO exposures at very low dose rates. In the present study using a hyper-sensitive assay system, we investigated the dose rate effect of HTO on the induction of mutations. Confluent cell populations were exposed to HTO for a total dose of 0.2 Gy at dose rates between 4.9 mGy/day and 192 mGy/day by incubating cells in medium containing HTO. HTO-induced mutant frequencies and mutation spectra were then investigated. A significant inflection point for both the mutant frequency and mutation spectra was found between 11 mGy/day and 21.6 mGy/day. Mutation spectra analysis revealed that a mechanistic change in the nature of the mutation events occurred around 11 mGy/day. The present observations and published experimental results from oral administrations of HTO to mice suggest that a threshold dose-rate for HTO exposures might exist between 11 mGy/day and 21.6 mGy/day where the nature of the mutation events induced by HTO becomes similar to those seen in spontaneous events.
Highlights
Tritium is a radionuclide which is discharged into the aquatic environment, primarily in the form of tritiated water (HTO), by nuclear power plants and nuclear fuel reprocessing plants
Clarifying the biological effects of low dose or low dose rate HTO exposures, as well as providing strong evidence for the effects of such exposures is necessary in order to make any decisions on how to deal with contaminated water
It is clear that ionizing radiation induces DNA damage, even when the doses are quite low, because many reports shows that radiation induced gamma-H2AX or 53BP1 foci can be detected even at several milli-Gray exposures [16,17,18]
Summary
Tritium is a radionuclide which is discharged into the aquatic environment, primarily in the form of tritiated water (HTO), by nuclear power plants and nuclear fuel reprocessing plants. A dose threshold for radiation can be seen for deterministic effects but cannot be seen for stochastic effects. Stochastic effects such as carcinogenesis are thought to be caused by genetic alterations such as somatic mutations. As the DNA damage yield increases in a dose dependent manner, the ‘linear non-threshold’ (LNT) model is considered to be an appropriate model to use to estimate the incidence of stochastic effects [1]. In this case, health risks would be expected to appear, even when the exposure dose or dose-rate is quite low. It is difficult to distinguish radiation-induced events from spontaneous events, especially in the case of low-dose or low-dose-rate radiation exposures
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