Measurements by Adachi and colleagues were made by high-school students and their teachers from 12 Japanese high schools (6 in Fukushima Prefecture), 4 high schools from 3 different regions of France, 8 schools from seven regions in Poland and 2 high schools located in Belarus near the Polesky State Radioecological Reserve which was created to enclose the area there most affected by fallout from the Chernobyl disaster in 1986.  The respective locations of the high schools participating in Fukushima Prefecture are shown below. Figures of the other locations can be found in the original paper.

Other schools from Japan were selected from areas known to have higher levels of natural background ionizing radiation from radioactive Uranium/Thorium decay series and Potassium isotopes.

A total of 216 students wore personal dosimeters (D-shuttle) and kept track of their movements and habits over a two week period during the school term.  The D-shuttle is a semiconductor type personal dosimeter that records both natural and artificial (primarily gamma emitting isotopes ¹³⁷Cs and ¹³⁴Cs) ionizing radiation the latter of which are negligible in all areas outside Fukushima and Belarus.

Recorded doses were used to estimate hourly and annual external ionizing radiation doses for the participants in the different study locations.  A comparison of the individual annual doses in mSv yr^-1 are shown in the figure below.

The median of estimated doses for students in Fukushima Prefecture were 0.63 to 0.97 mSv yr^-1 while those in other areas of Japan were 0.55 to 0.87 mSv yr^-1. Median values from France, Belarus and Poland were found in the range 0.51 — 1.10 mSv yr^-1.  For comparison the dose experienced by individuals living in Victoria BC, Canada where I live is ~1.8 mSv yr^-1.  The individual doses of students living in Fukushima Prefecture are not much higher than the other regions considered in the study and significantly lower than my own.

Outliers in the Data and Increase of Dose in Fukushima After the Disaster

According to this study the release of radionuclides from FDNPP following the meltdowns has significantly increased the external doses experienced by residents of Fukushima.  This is demonstrated by and outlier in the data where a teacher who spent 2 hours in Okuma town in the restricted zone of Fukushima Prefecture who experienced 5 μSv h^-1 during that period of time.  Additionally one can compare the annual dose estimates owing to naturally occurring isotopes to those measured with the D-shuttle dosimeters inside and outside Fukushima Prefecture.

The estimated and measured dose rates agree well outside of Fukushima suggesting that natural background radiation is the primary contributor to experienced annual dose.  Within Fukushima the measured dose is higher than the natural background (which is low by Japanese standards).  This indicates that the disaster added (principally through radiocesium deposition) to the natural background, but that the increment was not so great as to increase doses outside the envelope of natural background doses in Japan (which vary region to region).

Conclusions

1. In Fukushima as well as in Belarus the individual annual external dose of ionizing radiation including naturally occurring radiation was below 1 mSv yr^-1 for high-school students
2. Doses experienced by students in Fukushima and Belarus are well within the levels experienced by students in other areas of Japan and regions of France and Poland

Recently published measurements of internal levels of radiocesium contamination in residents of Fukushima have been low to absent after the disaster leading research scientists to suggest that the bulk of ionizing radiation dose from the disaster is likely to occur through external exposure of residents.  The dosimeter measurements reported in the Adachi et al. (2015) study suggest that the external exposure of Fukushima high-school students to ionizing radiation is not higher than external exposures experience by students in other parts of Japan, Belarus, France and Poland.

More research to determine predominant paths and levels of exposure experienced by residents of Japan to the contamination released by the FDNPP disaster will ultimately help to constrain impacts to human health in the future. This study is a powerful example of how independent, citizen science can address important problems in environmental monitoring.  My congratulations to these young scientists and their mentors for such a useful study.

For those interested the full data tables from the study can be accessed here

This post builds on a series reporting on a growing body of scientific evidence regarding the impact of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) meltdowns on environmental and public health.