Impact on ambient dose rate in metropolitan Tokyo from the Fukushima Daiichi Nuclear Power Plant accident

A car-borne survey was made in metropolitan Tokyo, Japan, in December 2014 to estimate external dose. This survey was conducted for all municipalities of Tokyo and the results were compared with measurements done in 2003. The ambient dose rate measured in the whole area of Tokyo in December 2014 was 60 nGy h−1 (23-142 nGy h−1), which was 24% higher than the rate in 2003. Higher dose rates (>70 nGy h−1) were observed on the eastern and western ends of Tokyo; furthermore, the contribution ratio from artificial radionuclides (134Cs and 137Cs) to ambient dose rate in eastern Tokyo was twice as high as that of western Tokyo. Based on the measured ambient dose rate, the effective dose rate after the accident was estimated to be 0.45 μSv h−1 in Tokyo. This value was 22% higher than the value before the accident as of December 2014.