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The effect of low dose X-irradiation (0.1∼1.5 Gy) on the peripheral blood cells was investigated in mice. The numbers of leucocytes and lymphocytes were significantly decreased during 1∼10 days after irradiation with 0.5, 1.0 and 1.5 Gy (P<0.001, P<0.001 and P<0.0001, respectively). A significant decrease in the number of platelets was observed until 15 days after irradiation with 0.5, 1.0 and 1.5 Gy (P<0.01, P<0.01 and P<0.01, respectively), and thereafter the number returned to normal limits. A significant decrease in the number of male erythrocytes was shown at 25 days after irradiation with 0.5 Gy (P<0.05) and at 10 days after irradiation with 1.0 and 1.5 Gy (P<0.01 and P<0.05, respectively) in male and female mice. These results suggest that the threshold of irradiation is 0.5 Gy, which is lower than the value reported by ICRP and UNSCEAR.
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