The study by Akiyama et al. can be found here as an open-access, peer reviewed publication in PLOSOne for anyone wishing to access the original paper and data. The Jyoban Hospital, Iwaki city was the site of WBC measurements and its proximity to the FDNPP and air dose rate resulting from the background and radionculides from the disaster are shown on the map below:
Areas to the south of the Fukushima Daiichi nuclear power plant
The number of visitors to the hospital, their age and whether they had detectable levels of 137Cs (detection limit 250 Bq body-1) are shown in the figure below:
Monthly visitors and those with detectable levels of 137-Cs (detection limit 250 Bq/body).
Two children among the 6,446 children examined showed Cs-137 contamination levels that were over the detection limit, but Cs-134 levels were not found above the detection limit.
A total of 6,446 children were tested, and only two boys (11 and 15 years old) were found to have detectable internal radiocesium (see Figure above). 137Cs in these two were just over the detection limit: 251 and 259 Bq body-1. This level of contamination would result in internal annual doses of 0.018 and 0.011 mSv year-1. 134Cs levels were below the detection limit of 220 Bq body-1. 99.9% of the children had no detectable internal Cs contamination and no other radionuclides except naturally occurring 40K were detected. Maximum annual effective doses from radiocesium, assuming conservatively that the children had a level corresponding to the detection limit of the WBC, were calculated as 0.03, 0.02, and 0.01 mSv year-1 for children aged 6, 10, and 15 years, respectively.
Of the 2,760 adults examined, 35 (1.26%) had very low levels of Cs-137 contamination. Cs-134 levels were below the detection limit.
Unlike the children a larger proportion of the adults (1.26% or 35/2760) had detectable levels of internal Cs contamination. Detectable radiocesium contamination was seen in adults throughout the screening period (ending in November 2014). 137Cs found in this 1.26% were in the range of 250 to 859 Bq body-1 with a median value of 318 Bq body-1. This level of contamination would mean doses of 0.008 to 0.028 mSv year-1, with a median dose of 0.011 mSv year-1.
Summary and Conclusions
Akiyama and colleagues found that:
- 2 children had 137-Cesium (137Cs, half life ~30 years) above the detection limit (250 Bq body-1) but no detectable 134-Cs (134Cs, half life ~ 2 years) early in the program
- 1.3% of adults (35 of 2,760) had detectable internal contamination but only for 137Cs (range 250-859 Bq body-1)
- only one of these adults still had detectable internal contamination after the first year
- other than naturally occurring 40K no other gamma emitting isotopes were detected
- conservative maximum effective doses experienced by the schoolchildren and adults from internal radiocesium were 0.029 and 0.028 mSv year-1 respectively and far below the 1 mSv year-1 set by the Japanese Government
These findings suggest that the internal Cs contamination in residents of areas south of the nuclear plant is minimal. It appears that programs that encourage consumption of only inspected foods and/or commercially distributed food from unaffected areas has been successful in controlling radio-contamination and that public education about radio-contamination and mitigation strategies have been similarly successful. Whole body measurements of radio-contamination suggest that annual effective doses for this segment of the population more than one year after the disaster are likely to be due to external exposure. Further work to monitor the health of affected individuals and their exposure to artificial radionuclides through time will be invaluable toward determining the impact of the disaster on public health.
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