The change was prompted by data showing that despite many local municipalities aiming to lower air radiation levels to within 0.23 microsieverts per hour, there have been numerous cases where even with air radiation levels at between 0.3 and 0.6 microsieverts per hour, people's yearly exposure to radiation caused by the disaster has stayed below a limit of 1 millisievert of extra radiation exposure per year.
The data, introduced in the report, came from a survey on around 70,000 people by the cities of Date and Soma in Fukushima Prefecture, where people living in areas with 0.3 to 0.6 microsieverts of radiation per hour were still under 1 millisievert of extra radiation exposure per year. The ministry suggested to local municipalities that they supply personal dosimeters and base decontamination off of gathered readings, and that they announce information on areas with high radiation levels.
Regarding the 0.23 microsieverts-per-hour standard, the report said, "It is not the goal for decontamination work, it is an estimate based on one lifestyle pattern and its meaning was not accurately expressed." The report said that radiation exposure varies greatly depending on lifestyle, and it would be difficult for the national government to set a single, unified goal for decontamination efforts. It is calling a focus on individual radiation exposure amounts and better communication of risk information the pillars of decontamination efforts.
Senior Vice Minister of the Environment Shinji Inoue said, "We want to speed up decontamination work and disaster recovery, using the report as a basis."
While allowing higher radiation levels could speed up decontamination efforts, local municipalities are divided in their response to the idea, and confusion is expected.