Photo/IllutrationThe Asahi Shimbun

Radioactive cesium released after the Fukushima No. 1 nuclear power plant's triple meltdown in 2011 is continuing to contaminate the environment through wild mushrooms, scientists say.

It turns out that the fungi absorb cesium and then release it through their spores after concentrating it.

But the amount of cesium in the environment is miniscule and poses no threat to human health, say the researchers, who are primarily with the Meteorological Research Institute of the Japan Meteorological Agency, Ibaraki University and Kanazawa University.

The new findings indicate that cesium is released into the environment again by mushroom spores in mountains and forests in zones designated as difficult to return to because of high contamination levels after the nuclear accident triggered by the March 2011 earthquake and tsunami disaster.

Radiation levels in the air are measured at monitoring posts and disclosed to the public. Those measurements are taken at a designated height to measure radiation from the ground and in the atmosphere.

In a separate effort, a team of scientists from the Meteorological Research Institute and other bodies measured the radioactivity concentration of cesium-137 by collecting airborne particles 1 meter above ground in Fukushima Prefecture.

The team’s survey showed that cesium levels in a mountainous area in the northwestern part of the town of Namie rise five times in summer compared with winter. The region is part of the difficult-to-return zone.

The increased cesium level during summer is equivalent to less than one ten-thousandth of the radiation dose of 2.1 millisieverts, which the average individual is naturally exposed to each year.

The latest findings were in marked contrast to studies covering the prefectural capital of Fukushima and elsewhere that showed cesium levels were higher in winter than summer.

Initially, the researchers considered the possibility of cesium on the ground's surface being kicked up by clouds of dust. But they found no clear association between the cesium level and dust.

Teruya Maki, an associate professor of microorganism ecology at Kanazawa University, analyzed genes of airborne particles gathered in forests and mountains in the northwestern part of Namie from August to September 2015.

The results showed that many of the particles were derived from mushrooms.

Between June and October last year, more than 10 kinds of wild mushrooms were gathered on 10 occasions in the region’s forests and mountains. The radioactivity concentration levels in the spores measured up to 143 becquerels per gram.

When multiplying the cesium concentration per spore by the number of collected spores per cubic meter, the result roughly matched the measured cesium concentration for the area.

“Spores in which cesium was concentrated were likely released into the atmosphere, raising the airborne concentration,” said Kazuyuki Kita, an air environment science professor at Ibaraki University, who was involved in the analysis of cesium levels.

The amount of cesium contained in a spore of sampled mushrooms was extremely small.

“Even if people inhale the air in areas where mushroom spores containing cesium are spreading, that could never affect human health,” said Kazuhiko Ninomiya, a researcher of radiochemistry at Osaka University, who is a member of the research team.

The researchers are also trying to ascertain the extent to which the mushroom spores spread. They are planning more studies to figure out if the distances involved could be several kilometers.

Last summer, airborne cesium concentration levels for mountains and forests in Namie that have yet to be decontaminated were almost the same as those for an area 1 kilometer away that has been decontaminated on a trial basis.

That indicates cesium is likely spreading in the air, according to the scientists.

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