Residents’ radiation dosage proves only one-third of gov’t estimate: Minamisoma hospital

28 January 2015

The average level of annual exposure to radiation among selected residents in Minamisoma city, Fukushima Prefecture, stemming from the 2011 nuclear accident has turned out to be only one-third of the central government’s estimate, according to a survey by a team of doctors at a local hospital. The finding was announced on Jan. 27 by the Minamisoma Municipal General Hospital team led by Dr. Masaharu Tsubokura. It was based on the dosage of radiation recorded on film badge dosimeters worn by hundreds of students in the city.

As reasons for the discrepancy, the team cited the children’s shorter hours of outdoor stays than in the estimate and the government’s failure to convert midair radiation levels correctly into effective dosages. Such a difference between the converted data and dosimeter recordings had been pointed out long before.

The survey covered 520 elementary, secondary and high school students in Minamisoma who wore film badge dosimeters for a period ranging from 18 to 30 months after the accident at the Fukushima Daiichi nuclear power plant of Tokyo Electric Power Co. The hospital team compared radiation doses recorded on the dosimeters with data converted from midair radiation readings in front of the children’s residences on the basis of the government’s conversion formula. The average radiation dosage based on the dosimeter data was 0.8 millisieverts on an annual basis compared with 2.4 mSv calculated from the government formula.

The government method is based on the assumption of residents staying outdoors for a total of eight hours per day but in reality the city’s children spent a much shorter period outside the home, with 97% of them doing so for only four hours or less on weekdays and 85% on holidays. It was the first detailed analysis of actual and estimated radiation dosages among individuals, according to the hospital group. Tsubokura and other doctors published a report on the finding in a professional journal issued by the School of Public Health, the Imperial College, London.

(Translated by Kyodo News)