Nearly 15 years after the Fukushima nuclear disaster, air radiation doses have dropped significantly in Fukushima Prefecture, with most of the land showing levels below the required decontamination.

But the Nuclear Regulation Authority’s secretariat also said while residential areas saw a steep decline in radiation doses over the years, that was not the case with mountain regions.

The Japan Atomic Energy Agency (JAEA) is set to collect comprehensive data on radiation levels in the forests, which make up the large majority of the so-called difficult-to-return zone, where entry is prohibited due to relatively higher radiation levels.

Hironori Funaki, a scientist with the JAEA’s Fukushima Research and Engineering Institute, said close monitoring of the forests will be necessary to come up with solutions to lower radiation doses there.

“We should identify what measures should be implemented toward the lifting of the evacuation orders,” he said, noting forests account for about 90 percent of the difficult-to-return zone. “Close monitoring of the mountain regions will provide us with clues about what we should do next.”

The triple meltdown at the Fukushima No. 1 nuclear power plant in March 2011 released massive amounts of radioactive materials into the environment.

While radioactive fallout has contaminated broader areas of the prefecture, the passage of time has contributed to a sharp decrease in air dose rates.

This is largely due to the fact that most radioactive materials released in the accident have half decay periods of about two years, such as cesium-134, or shorter.

A half decay period, or a half life, refers to the time it takes for half of a sample of a radioactive substance to decay.

But cesium-137 remains to pose safety risks for a much longer period, given its half life being about 30 years.

The JAEA, commissioned by the Nuclear Regulation Authority’s secretariat, has produced the distribution maps of dose rates periodically since the accident.

The map is based on figures recorded at about 5,000 fixed-point observation spots as well as measurements by foot and car.

Helicopters were also deployed to cover large expanses of land.

The JAEA’s surveys showed a steady decrease in radiation doses year after year.

In July 2011, the share of land with a reading of 0.2 microsieverts per hour or lower stood at about 44 percent of Fukushima Prefecture.

The share rose to about 91 percent in December 2024, meaning the greater part of the prefecture falls under the hourly dose of less than 0.23 microsieverts, a baseline for requiring cleanup work set by local governments.

The JAEA cited three reasons for decreasing radiation levels: most radioactive materials have passed their half decay periods; radioactive materials were washed away by rain and blown away by wind; and widespread cleanup operations conducted in residential areas, school grounds and other key locations.

Experts said radiation doses tend to go down more quickly in urban centers and land along the streets than in others because decontamination of such sites had been prioritized.

But readings tend to remain high in the forests, where radioactive materials stay for a longer period after they have been attached to fallen leaves and soil.

With doses sharply down in living areas, local authorities and residents are now calling for a study to obtain a detailed picture of what radiation doses are like in farmlands and privately owned forests in mountainous regions.

A plot with an estimated annual dose of up to 20 millisieverts, or 3.8 microsieverts per hour, can qualify the possible lifting of the evacuation order set by the central government.

A survey in 2024 detected similar radiation levels in a mountain region in the difficult-to-return zone in Futaba, a town co-hosting the embattled nuclear plant.

Residents from the region who are under the evacuation order are not currently allowed to return.

Yet it is a challenge to collect data on radiation doses across the forests because of the difficulty involved in the measurement.

Manned helicopters had been flown to cover the area, but obtaining reliable figures was hard due to the distance from the ground surface.

From October, the JAEA started a pilot program in the difficult-to-return zone in Katsurao village, about 20 kilometers northwest from the nuclear plant, involving an unmanned helicopter capable of measuring at a low altitude.

The agency also sent surveyors to the zone to perform the task on foot, making it possible to identify the exact locations and doses there combined with data collected through the airborne survey.

Data on air radiation by year and other information is available at the Nuclear Regulation Authority’s official website at (https://radioactivity.nra.go.jp/cont/ja/updates/Environmental_radioactivity_level_at_monitoring_posts_in_Fukushima_Prefecture.pdf ）