4 Mars 2013
March 4, 2013
Of the many routes taken by evacuees in the aftermath of the 2011 nuclear disaster one stands out as having posed a greater health risk to those fleeing.
The Environment Ministry asked researchers at the National Institute of Radiological Sciences and the Japan Atomic Energy Agency to estimate radiation doses that may have accumulated in the thyroid glands of people who took the 18 most common evacuation routes.
Researchers made their calculations on the premise that evacuees had spent 24 hours outdoors.
Depending on the period and route taken in evacuating, there were differences of more than 100 times in the radiation dose of individuals fleeing from an area within a 20-kilometer radius of the stricken Fukushima No. 1 nuclear power plant.
The highest estimated thyroid radiation dose was found for the evacuation route taken by residents who remained at the Tsushima activation center in Namie until the morning of March 23 and then evacuated to the Adachi gymnasium in Nihonmatsu, some 60 kilometers from the nuclear plant where reactor meltdowns had occurred.
The estimated doses were 104 millisieverts for a 1-year-old child, 89 millisieverts for a 10-year-old child and 53 millisieverts for an adult.
The accepted scientific wisdom is that the risk of thyroid cancer increases when individuals are exposed to more than 100 millisieverts of radiation.
All other routes had estimates under 100 millisieverts.
The estimates are meant to serve as reference points for evacuees who can calculate the health risks they were exposed to by using the figures for the evacuation route they took.
In the event an individual is diagnosed with cancer, it should be possible to estimate the causal effect of the nuclear disaster through exposure to radiation depending on the evacuation route taken.
However, there are almost no actual measurements of radiation doses because the radioactive iodine that accumulates in the thyroid has a short half-life.
Based on data about the estimated volume of radioactive materials emitted from the Fukushima plant and wind direction, estimates were made of the changes in the concentration of iodine in the atmosphere for areas of 3 kilometers square for the period between March 12 and April 30 of 2011.
Those concentrations were used to estimate thyroid radiation doses for children aged 1 and 10 years old as well as adults along 18 representative evacuation routes taken by residents in the 12 municipalities in the vicinity of the crippled nuclear plant.
The evacuees were presumed to have inhaled the iodine in the atmosphere.
The lowest estimated dose was for the evacuation route from Katsurao to the Azuma gymnasium in Fukushima city taken on March 14. The estimates were less than 1 millisievert for both a 1-year-old child and an adult.
Because the estimates were based on the presumption that evacuees were outdoors for 24 hours, the actual radiation doses were likely much lower because very few people likely stayed outdoors for that long.