Relative biological effectiveness and dose rate effect of tritiated water on chromosomes in human lymphocytes and bone marrow cells.

Tritiated water (HTO) is a major toxic effluent from the nuclear power industry, that is released into the environment in large quantities. The low dose radiation effect and dose rate effect of HTO on human lymphocytes and bone marrow cells have not been well studied. The present study was therefore undertaken to investigate the HTO dose-response relationship for chromosomal aberrations in human lymphocytes and bone marrow cells at low in vitro radiation doses ranging from 0.1 to 1 Gy. Lymphocytes (G0 stage) and bone marrow cells were incubated for 10-150 min with HTO at a dose rate of 2 cGy/min (555 MBq/ml). The relative biological effectiveness (RBE) of HTO was calculated with respect to 60Co gamma-rays for the induction of dicentric and centric ring chromosomes at low radiation doses. The RBE value for HTO beta-rays relative to 60Co gamma-rays was 2.7 for lymphocytes and 3.1 for chromatid aberrations in bone marrow cells. Lymphocytes were also chronically exposed to HTO for 6.7-80 h at dose rates of 0.5 cGy/min (138.5 MBq/ml) and 0.02 cGy/min (5.6 MBq/ml). There was a 71.5% decrease in the yield of dicentrics and centric rings at the dose rate of 0.02 cGy/min, indicating a clear dose rate effect of HTO. The RBE value for HTO relative to 137Cs gamma-rays was 2.0 at a dose rate of 0.02 cGy/min, suggesting that low HTO dose rates produce no increase of the RBE values and that the values may be constant between 2 and 3 within these dose rates. These results should prove useful in assessment of the health risk for humans exposed to low levels of HTO.