The Forestry Agency believes cedar pollen next spring contaminated by cesium fallout from the Fukushima No. 1 nuclear plant will be well below the legal safety limit.
The exposure from inhaling cesium-contaminated cedar pollen circulating from Fukushima Prefecture will have a maximum radiation reading of 0.000132 microsievert per hour, the agency said, based on a recent calculation of fallout affecting cedar needles and leaves.
In June, the education and science ministry studied cedar leaves in the town of Kawamata, located about 45 km from the Fukushima No. 1 nuclear plant, and determined the cesium-134 and -137 levels ranged from 54,300 to 177,600 becquerels per kilogram.
The Forestry Agency used those results to estimate the radiation exposure from pollen grains. If the level of contamination was 177,600 becquerels per kilogram and the concentration of pollen grains — a gauge of pollen density that shows how many grains are floating in 1 cu. meter of air — was 2,207, the exposure would be equal to 0.000132 microsievert per hour.
On average, the concentration of pollen grains is 89 in the Kanto region, but the calculation used 2,207, the highest figure recorded in the region in the past eight years.
The legal radiation exposure limit is 1 millisievert per year. If someone is exposed to 0.12 microsievert per hour for 24 hours over 365 days, it would equal about 1.05 millisieverts per year.
However, the entire mechanism of radiation transfer through cedar pollen remains a mystery.
Yoshihisa Matsumoto, an associate professor at the Tokyo Institute of Technology and a radiation expert, said he is unaware of any research regarding the transfer of radioactive materials on cedar leaves to pollen.
But he said it would be hard to imagine that the contamination level would be higher when the cesium is transferred to pollen.
Because the male cedar flower, which has pollen grains, becomes mature around this season, the agency will conduct tests on how cesium transfers to pollen from this month to January.
Fukushima has about 185,000 hectares of cedar forests.
Hiroki Matsumoto, an official at the agency, said cedar pollen is so light it can fly hundreds of kilometers, meaning it can reach densely populated Tokyo and surrounding areas.
He said cedar pollen that reaches Tokyo usually is carried by wind from the east, which means generally from Gunma, Saitama, Yamanashi and Shizuoka prefectures. But weather conditions could bring pollen from Fukushima, he added.
Matsumoto of the Tokyo Institute of Technology said even if the pollen is contaminated, people can use regular pollen-preventive tools to protect themselves.