Citizen science usually isn’t this personal. In 2011, roughly 65,000 Japanese citizens living near the crippled Fukushima Daiichi Nuclear Power Plant started measuring their own radiation exposure in the wake of the Tōhoku earthquake and tsunami. That’s because no one, not even experts, knew how accurate the traditional method of estimating dosage—taking readings from aircraft hundreds of meters above the ground—really was. Now, in a first-of-its-kind study, scientists analyzing the thousands of citizen readings have come to a surprising conclusion: The airborne observations in this region of Japan overestimated the true radiation level by a factor of four.

“The work [these] researchers are doing is extremely important … [because] it is logistically challenging to sample and monitor every potentially exposed person,” says Kathryn Higley, a certified health physicist at the School of Nuclear Science and Engineering at Oregon State University in Corvallis.

It’s rare to monitor individual radiation exposure after a nuclear power plant accident. In some cases, regions are simply evacuated. In others, the cost and difficulty of handing out personal sensors—called dosimeters—is just too much. The few studies that have gathered individual readings in places like Chernobyl in Ukraine have also suffered limitations. Most target only small populations, and many are conducted far from the disaster site or long after the accident. Using aircraft is often easier, cheaper, and faster.

But in Date, Japan—just 60 kilometers from the six-reactor Fukushima Daiichi complex—local officials started a radiation-monitoring campaign within a few months of the accident. Mayor Shoji Nishida was one of the main supporters. He explained at a 2014 meeting of the International Atomic Energy Agency that his city was never ordered to evacuate, despite surveys revealing radiation levels similar to those in nearby towns that were evacuated. “We decided that we should not depend on the national government and that we had to take our own independent actions,” Nishida said at the meeting. He ordered Date to begin its own decontamination efforts and to monitor individual radiation exposure, allocating 1 billion yen of city funds to the project in May 2011.

Under Nishida’s mandate, pregnant women and children younger than 16 were the first to receive a dosimeter, a candy bar–sized sensor that measures gamma rays. These high-energy electromagnetic waves, emitted by radioactive elements like cesium, can damage DNA and cause cancer. After roughly 9000 dosimeters had been distributed to children and expectant mothers, Date officials expanded the monitoring project: By 2012 almost all of Date’s roughly 65,000 inhabitants had been given one. Residents returned the dosimeters every 3 months for analysis; more than 52,000 residents participated in the survey for at least one year.

Meanwhile, six airborne radiation surveys of Fukushima Prefecture were conducted by the national government. Sensors attached to helicopters measured radioactive cesium on the ground, and researchers used scaling laws to convert that data to expected doses at ground level. Because many people spend most of their time indoors, protected by radiation-absorbing buildings, government scientists further assumed that just 60% of that radiation actually reached most subjects. That estimate was based on the standard assumption that people spend 8 hours outdoors and 16 hours indoors each day.

Now, Makoto Miyazaki, a radiologist at Fukushima Medical University, and Ryugo Hayano, a University of Tokyo physicist, have taken the thousands of data points from the Date dosimeters and compared them with the ground-level estimates from the helicopter data. The scientists concluded that actual radiation doses were roughly 15% of what the helicopters were measuring, scaled to ground level, they reported last month in the Journal of Radiological Protection. That’s four times less radiation than what the Japanese government was previously assuming.

The researchers give several reasons for the large difference. Chief among them: “Residents [are] not staying outdoors for 8 hours each day,” Miyazaki says. He hopes these results will help other researchers better predict actual radiation doses—and therefore potential health effects—based on rapid airborne surveys. A better estimate of individual radiation doses might also allow displaced people to return to their homes sooner, Higley notes.

For Date residents, it’s good news that radiation levels are lower than expected. But the result comes with a less-than-silver lining: Some of the region’s expensive, time-consuming decontamination efforts—such as the removal of topsoil and tree bark—might not have been necessary.