A new study shows that Fukushima residents have been exposed to considerably less radiation than had been estimated in the wake of the 2011 incident. The study was the first of its kind, in that it analyzed thousands of individual citizen readings, instead of using the traditional method of taking readings from aircraft hundreds of meters above. The new analysis found that these traditional methods had overestimated the radiation level by a factor of four.

According to Kathryn Higley, a certified health physicist at the School of Nuclear Science and Engineering at Oregon State University:

“The work [these] researchers are doing is extremely important … [because] it is logistically challenging to sample and monitor every potentially exposed person.”

Few of these studies have been completed in the wake of a nuclear disaster, given the high price of distributing personal dosimeters to measure exposure. In other cases, areas are evacuated right away. Most studies like this have only examined a small population, sometimes far from the site or very long after. Testing using aircraft is normally cheaper and more practical, it has been unknown exactly how accurate these airborne methods really are.

However, in this case, local officials in Date, Japan, began a campaign to measure exposure within months of the incident.

Mayor of Date, Japan, a town just 60 kilometers from the Fukushima Daiichi complex, was one of the main proponents of campaign. In 2014, he explained that Date never received an evacuation order, even though surveys had shown it had similar levels of exposure to other towns that were told to evacuate.

“We decided that we should not depend on the national government and that we had to take our own independent actions,” he said.

The city began its own decontamination efforts, and monitoring levels of radiation exposure in individual citizens, in 2011. They began the expensive task of distributing dosimeters, and by 2012, nearly all of the town’s 65,000 residents were monitoring their exposure levels for a period of at least one year.

At the same time, the national government was conducting its own airborne surveys to estimate exposure at ground level.

In the new study, Fukushima Medical University radiologist Makoto Miyazaki, and University of Tokyo physicist Ryugo Hayano, compared the airborne estimates to thousands of data points from the individual measurements from Date. The study, published last month in the Journal of Radiological Protection concluded that actual levels of exposure were at about 15 percent of the estimates from airborne data.

The researchers gave a number of reasons for the inconsistency, primarily that residents were not outdoors for as much of the day as the estimates assumed.

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