By Jay T. Cullen

The purpose of this post is to relate findings published (open access) in the peer-reviewed journal PLOSOne by Akiyama and colleagues who investigated internal radiocesium contamination in residents living south of the Fukushima Dai-ichi Nuclear Power Plant. This post is the most recent in a series dedicated to sharing the results of scientific research into the impact of the nuclear power plant disaster on ecosystem and public health. The Akiyama study builds on recent work by this group using a whole body counter (WBC) to look for internal contamination of residents living near (north and west) the reactors at the time of the meltdowns. Measurements were made on 9,206 affected individuals, including 6,446 school children (4-15 years) living south of the reactors or evacuated to Iwaki city following the bulk of radionuclide releases. Measurements began one year after the accident and continued for two years. The authors conclude that, despite lack of data for the first year following the disaster, internal doses experienced by residents to the south of the FNPP were minimal. This suggests that efforts to mitigate foodborne exposure have been largely successful and that nearly all the annual effective doses for this segment of the population more than one year after the disaster are likely due to external exposure.Study Location
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 FNPP and air dose rate resulting from the background and radionculides from the disaster are shown on the map below:

.

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:

Results

Children

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.

Adults

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:

1. 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
2. 1.3% of adults (35 of 2,760) had detectable internal contamination but only for ¹³⁷Cs (range 250-859 Bq body^-1)
3. only one of these adults still had detectable internal contamination after the first year
4. other than naturally occurring ⁴⁰K no other gamma emitting isotopes were detected
5. 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.