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.
Asahi