If you recall what transpired last month, education board officials overseeing schools in the Fukushima Prefecture jacked up permissible doses for school children from the standard 1 milliSievert/year to a very difficult to accept 20 milliSievert/year. Angry parents have been protesting ever since, and have succeeded in forcing the Ministry of Education to acquiesce to their demands. From the NYT:
"The education minister, Yoshiaki Takaki, said Friday that the government would, for the time being, revert to the original limit of 1 millisievert a year. Mr. Takaki said the government would pay for local schools with radiation levels above that limit to remove contaminated topsoil from their grounds.It was a tough call. Education officials - weighing radiation safety vs Japan's high educational expectations - facilely cited a 2009 recommendation from the International Commission on Radiological Protection (ICRP) that addresses this sort of scenario. During emergencies, the ICRP Publication 109 states that populations can be exposed to 20 to 100 mSv per year for short periods of time. The education ministry calculated that children could spend 8 hours a day in a schoolyard exposed to as much as 3.8 microsieverts per hour, and 16 hours a day indoors exposed to 1.52 microsieverts per hour. Under these circumstances, annual doses would come in just under the 20-mSv limit.
“We will provide financial support to schools for measures to deal with soil in schoolyards as a way to lower radiation levels for children,” Mr. Takaki said at a news conference."
Mext had recently conducted measurements at 58 public schools, and said none of them exceeded 3.8 microSeiverts per hour. After the removal of topsoil, municipality officials are stating that doses in some areas will be reduced to just 0.6 microSievert/hour.
Here's a note of concern: ICRP recommendations, along with official exposure measurements are taken "in air", and are likely read with the equipment some distance from the ground. If activity is as high as 600,000 becquerels per square metre in some areas like the IRSN recently reported, foot and ankle doses could be higher than official numbers by a factor of 2-4 due to beta contribution and principles of inverse square law.
If you measure the distance from ground to gonads for younger children (Kinder through Grade 6), you begin to appreciate why they should make the distinction. In addition to the generic 'in air' reading, it's helpful to see the exposure numbers taken at 5cm, 20 cm and 1 meter levels from the soil.