Confusion and misunderstanding ensue, following the reporting on the Ministry of the Environment's plan (yet to be officially announced) to raise the radiation target level after decontamination in Fukushima from the current 0.23 microsievert/hour to 0.4-0.6 microsievert/hour.
The Ministry of the Environment (supposedly) says the additional exposure from the radiation under the new target level will be still under 1 millisievert per year.
The empirical data so far collected in Fukushima Prefecture (glass badges worn by residents, monitoring posts, actual measurements before and after decontamination, for example) seem to endorse the position by the Ministry, but this particular ministry unfortunately lacks credibility after having been headed by Goshi Hosono (DPJ) and now by Shintaro Ishihara's son who once wanted to ban the use of personal survey meters by citizens.
The Ministry and the municipal governments involved seem to be doing this for very wrong reason, if what Fukushima Central TV (FCT)'s reporting is correct.
To them, it is about cost-performance - too costly and time-consuming to decontaminate to the 0.23 microsievert/hr level. But again, this is according to the TV station reporting, and the official word from the Ministry is not expected for two to three months, according to Asahi Shinbun.
About this yet-to-be-official change in the government's decontamination policy, from Fukushima Central TV (FCT) (6/6/2014):
Raising the target [radiation] level after decontamination to about twice the current level is being discussed
FCT has found that the Ministry of the Environment has been discussing with the municipal governments [in Fukushima] on the new policy on decontamination which is considered to be prerequisite for recovery from the nuclear accident.
The new policy would raise the target ambient radiation level that decontamination should achieve from the current 0.23 microsievert/hour to about twice as high as the current level.
In the decontamination guideline by the Ministry of the Environment, the target ambient radiation level after decontamination is set at 0.23 microsievert/hour.
This number is calculated from the additional annual radiation exposure [target] of 1 millisievert.
The Ministry of the Environment has been discussing with the municipal government involved to raise the target level to 0.4 to 0.6 microsievert/hour, which is about twice as high as the current level.
Some municipal governments in Fukushima Prefecture have requested the Ministry of the Environment to revise the target radiation level to something more realistic. The current target of 0.23 microsievert/hour is too difficult to achieve, according to these municipal governments.
Officials at the Ministry of the Environment say the actual additional radiation exposure per year will still be under 1 millisievert even if the ambient radiation level after decontamination exceeds the current target level of 0.23 microsievert/hour. They will have a meeting on June 15 with the municipal governments involved to discuss raising the target level for decontamination.
The effective dose measured by glass badges is shown to be about half to one-third of the dose calculated by the Ministry of the Environment from the ambient radiation levels indicated by monitoring posts.
Here's a chart from the report by Date City in November 2013. Date City is located in Nakadori (middle third) of Fukushima Prefecture with elevated ambient radiation levels in western part of the city:
(English labels are by me)
The calculation used by the government (Ministry of the Environment) is as follows:
予測年間追加被ばく線量（mSv）＝空間線量率（μSv/h）× （8h ＋ 16 h × 0.4） × 365 (日)÷1000
Expected annual additional radiation exposure (mSv) = (ambient radiation - background radiation of 0.04 μSv/h) x (8h + 16h x 0.4) x 365 / 1000
In other words, expected annual additional radiation exposure in millisievert is calculated by:
(1) Subtract background radiation level of 0.04 μSv/h (pre-accident Fukushima average) from the current ambient radiation level.
(2) Multiply (1) by 8 hours as number of hours one stays outdoors per day.
(3) Multiply (1) by 16 hours as number of hours one stays indoors per day, then multiply by the coefficient 0.4 to account for shielding effect of the buildings (houses, offices, etc.).
(4) Add (2) and (3), then multiply it by 365 to come up with the annual additional exposure dosage in microsievert.
(5) Divide (4) by 1000 to come up with the number in millisievert.
Instead of saying "Decontaminating until the ambient radiation level drops to 0.23 microsievert/hr is too costly and near-impossible to achieve," the Ministry of the Environment could admit its mistake in setting the coefficient at 0.4 (to account for one- or two-story buildings made of wood, according to National Institute for Radiological Science).
Date City's result indicates that the coefficient of 0.2 (to account for one- or two-story concrete buildings) would be closer to the actual effective radiation dose. Children and adults in white-collar jobs may be spending the bulk of their time in buildings that are more than three-story high and made of concrete (schools, office buildings), and the coefficient for such buildings is 0.1.
We will have to wait for two to three months until Shintaro Ishihara's son (Minister of the Environment) officially announces the change, but Japanese Twitter is already full of people thinking that the national government is raising the additional radiation exposure per year allowed for Fukushima to 2 millisieverts.