In this July 27, 2017 file photo, contaminated water storage tanks are seen on the Fukushima No. 1 nuclear plant grounds, in Okuma, Fukushima Prefecture. (Mainichi)

Airborne radiation in "difficult to return" zones around the Fukushima No. 1 nuclear plant was as high as around 8.48 microsieverts per hour as of summer last year, according to data presented by the government nuclear watchdog on Jan. 17.

The Nuclear Regulation Authority (NRA) released the results of the July-September 2017 measurements at a regular meeting on the day. The highest reading was taken in Futaba, Fukushima Prefecture -- one of the municipalities hosting the Fukushima No. 1 plant.

Following the March 2011 triple meltdown, the government set a long-term radiation exposure limit of 1 millisievert per year, which breaks down to an hourly airborne radiation dose of 0.23 microsieverts.

The NRA took airborne radiation readings in the Fukushima Prefecture towns of Futaba, Okuma, Namie and Tomioka, and the village of Katsurao. The highest reading registered in the previous year's survey was 8.89 microsieverts per hour, in Katsurao.

Some of the NRA members at the Jan. 17 meeting pointed to study results showing that human exposure doses are relatively small compared to airborne doses. Regarding the calculation that an annual dose of 1 millisievert is equivalent to hourly exposure of 0.23 microsieverts, NRA Chairman Toyoshi Fuketa stated, "That was decided right at the start of the nuclear disaster, so it can't be helped that it's a cautious number." He added, "If we don't revise (that calculation) properly, it could hinder evacuees' return home."