Measuring radiation levels accurately is difficult for laypersons and they shouldn’t panic if their devices show much higher levels than the figures announced by the government, radiation experts say.
“Cheap and easy-to-handle devices sold on the Internet can sometimes show abnormally high radiation levels. Figures change on such devices even if you hold them still, and thus margins of error by 20 or 30 percent would be no surprise,” said Genichiro Wakabayashi, a professor of radiology at Kinki University.
“The important thing is to keep monitoring at the same place over a long period of time to check changes in radiation levels. Thus, the figures from the Education, Culture, Sports, Science and Technology Ministry are, after all, reliable,” he said.
Wakabayashi stressed that parents should feel safe letting their small children play in the sandboxes at their neighborhood parks everywhere except Fukushima Prefecture, where they should heed the advice of local authorities regarding the ever-changing fallout situation stemming from the crippled nuclear plant.
Wakabayashi is part of a group of radiation experts who use the Internet to publish radiation levels at various places across Japan.
The group aims “to prevent false rumors from being spread by nonexperts who have monitored radiation levels on their own,” according to its website. “Alarming the public (by challenging the credibility of) the government’s announcements is not our purpose, as some media apparently are attempting.”
Magazines and Internet content, including personal blogs, articles and postings of monitoring results by individuals sometimes slam the science ministry for publishing results deemed meaningless.
But Wakabayashi said monitoring over short periods is meaningless. The science ministry measures radiation levels every day, while Wakabayashi’s group has been taking measurements at least every other day since late March, and he has concluded that people can live normal lives.
“If people are worried, they can ask municipalities and universities to check radiation levels at parks or other locations in order to feel relieved,” he said.
The science ministry currently publishes two sets of monitoring results on its website.
For one, it has been gauging radiation levels at one point at least 3 meters above the ground in all 47 prefectures since 1957 with large, expensive and high-quality equipment, ministry official Hirotaka Oku said.
For the other set, it has been taking measurements since March 30 at 54 points locations 1 to 1.5 meters above the ground in 40 prefectures. These readings are taken with small, less-expensive equipment in cooperation with universities and other educational institutions, Oku said.
“To check effects of radiation on people, monitoring points should be lower” than 10 meters, he said, explaining why the ministry began monitoring 1 to 1.5 meters above the ground.
Radioactive substances come in the form of particles and travel on the wind. They eventually fall to the ground and thus radiation levels are considered higher at places near the ground. Parents are worried because small children play on the ground and may inhale or put in their mouth dust or sand contaminated with radioactive substances.
Oku said the ministry had to place equipment as high as 10 meters off the ground when it began monitoring in 1957 in order to have the equipment work correctly. This was during the time when the U.S. and the Soviet Union were conducting atmospheric nuclear tests.
“We wanted to know radiation levels in normal times so that we would know as soon as something abnormal happens. Therefore, we set the equipment at a high place where it can show stable results,” he said.
Big and expensive monitoring equipment determines radiation levels more precisely than small, cheap devices, said a spokesman for Hitachi-Aloka Medical Ltd., a maker of large equipment.
Large devices cost at least several million yen, while hand-held devices of lower quality are much cheaper, the spokesman said.
There are even cheaper devices, sold online for ¥30,000 to ¥100,000, which experts say are no better than toys.
The science ministry’s two monitoring methods yield roughly similar results, though a simple comparison may not be appropriate due to the differences in the two methods.
For example, the spot at an elevation of 18 meters in Shinjuku Ward, Tokyo, had a maximum hourly reading of 0.07 microsievert of radiation in the 24 hours leading to 9 a.m. Monday. With its other method, the ministry had figures for five places at 1 to 1.5 meters in Tokyo indicating 2 or 3 microsieverts per day, which would be interpreted as 0.08 or 0.13 microsievert per hour, for the 24 hours to 2 p.m. Sunday.
The group of volunteer radiation researchers publishes on the Internet radiation levels in 19 different places, 1 meter above ground, in Tokyo as well as many places in other prefectures.
A point in Katsushika Ward shows the highest level in Tokyo. A volunteer has been checking there since March 25 and the level has been roughly between 0.3 and 0.5 microsievert per hour and has not exceeded 0.4 so far this month. That compares with other places in Tokyo that have marked roughly 0.1 microsievert in May.
Assuming that the figure will be 0.4 microsievert per hour constantly in the next 12 months and a person stays at that spot, which is outdoors, 24 hours a day, the cumulative exposure would be 3.5 millisieverts a year, exceeding the government’s recommended maximum intake of 1 millisievert a year during normal times, but smaller than the 20-millisievert recommendation in times of a nuclear accident.
Wakabayashi said even Katsushika is safe. The government’s recommendation is conservative and the standard 20 millisieverts a year is in line with the International Commission on Radiological Protection, he said.
To be sure, some radiation experts differ from Wakabayashi, insisting children should not be exposed to more than 1 milisievert a year.
The ICRP recommends the target level of human exposure to radiation can be between 1 millisievert and 20 millisieverts per year in case of nuclear accidents, with the long-term goal of reducing it to 1 millisievert.
“The Katsushika Ward Office consulted with me and I told them it’s absolutely safe,” he said.
Also, radiation levels are unlikely to increase for now, Wakabayashi said.
Radioactive substances that were blown into the air by hydrogen explosions and fires at reactors 1, 2, 3 and 4 at the Fukushima No. 1 plant have probably already fallen to the ground, he said. Radioactive substances keep being released into the air from fuel rods no longer covered in water, but the amount is far smaller than when the explosions occurred, he said.
For more information see the website of the Education, Culture, Sports, Science and Technology Ministry.