Japanese researchers have found very low amounts of radioactivity in the bodies of about 10,000 people who lived near the Fukushima Daiichi power plant when it melted down.

The first published study that measured the radiation within a large number of residents reassured health experts because the numbers reported imply only negligible health risks. The threat appeared to be considerably lower than in the aftermath of the Chernobyl accident, the experts agreed.

“Exposure levels were much lower than those reported in studies even several years after the Chernobyl incident,” said Masaharu Tsubokura of the University of Toyko, lead author of a short paper published in the Thursday issue of the Journal of the American Medical Association.

The study measured radiation in 8,066 adults and 1,432 children in the town of Minamisoma, about 14 miles north of the Fukushima Daiichi power plant. Researchers found an average radiation dose of well under 1 millisievert, which is considered a safe amount.

The residents stood in a full-body radiation counter for two minutes, which allowed the scientists to measure the presence of radioactive cesium isotopes. Those isotopes, which can be taken in from the air or through eating contaminated food, are generally considered to be among the ones posing the greatest health risk from the Fukushima accident. The measurements were taken between September 2011 and March.

Roy Shore, chief of research at the Radiation Effects Research Foundation in Hiroshima, Japan, said in an interview that the measured doses constitute a very low health risk.

“Out of 10,000 people with a dose of 1 millisievert, the radiation would cause two to get cancer during their lifetimes, but about 3,500 would get cancer also without any radiation,” he said. “The jury is still out, but I expect the public health impact from radiation to turn out to be considerably lower than that of Chernobyl.”

But he said radiation is not the only health risk after the Fukushima accident. “The psychological impact has been very great and has caused a lot of anxiety,” he said.

Although the study results appear to be reassuring, they are considered preliminary and come with a number of caveats.

The measurements of radiation were not initiated as a scientific study, for instance, but meant as health checkups for volunteers from the local population. Therefore, there may be a selection bias in the results, although it is not known whether it would lead to an overestimation or underestimation of the measured doses.

“We would expect that people most concerned and therefore with a higher chance of exposure would seek out the screening, yet we cannot know for sure,” said Stuart Gilmour, co-author of the study at the University of Tokyo.

Kiyohiko Mabuchi, head of the Chernobyl Research Unit at the National Cancer Institute, said that only internal radiation from one source — cesium — was measured. Therefore, the study does not address external radiation from contaminated areas, which can be harmful, too.

“External exposure could be as much as or more than our measured internal exposure,” Gilmour said, “but it is difficult to quantify because it can greatly vary even in small areas.”

Radioactive iodine, which was not measured by the new study, has been identified as the likely cause of some of the most serious health effects of the 1986 Chernobyl accident by causing thyroid cancer. Radioactive iodine was taken up primarily by children who drank contaminated milk.

Although cesium isotopes have half-lives of years, radioactive iodine isotopes can be measured for only a short time. As a result, the threat from iodine may not be known for years.

David Weinstock of Harvard University, while aware of the shortcomings of the report, agrees with the authors’ conclusions. He calls the measured doses an “approximately zero risk.” He attributes the results to the public health response in Japan.

“In Chernobyl, there was no response in the beginning and people were left to consume contaminated food, while in Fukushima the response has been to evacuate and to stop food consumption from contaminated areas, and it seems to have been validated,” he said.

The researchers converted the radiation activity from the individuals’ bodies to what is called the effective dose, a measure that reflects the health impact. About one-third of the individuals, 235 children and 3,051 adults, had detectable cesium radiation.

The highest effective dose measured was 1.07 millisieverts. Background radiation from natural sources such as radon gas is typically around 2 to 3 millisieverts per year, and medical applications such as a chest X-ray or a CT scan of the heart can produce 0.1 and 16 millisieverts, respectively. An airline crew flying over the North Pole from Tokyo to New York will generally receive an annual dose of 9 millisieverts.

Evan Douple, associate chief of research at the Radiation Effects Research Foundation, said the study’s results are consistent with published predictions. In a preliminary report, based primarily on estimates rather than direct measurements, the World Health Organization concluded that most residents of Fukushima and the neighboring prefectures received a total combined dose below 10 millisieverts each. This includes internal and external radiation from different isotopes.

The WHO is working on a broad study of the public health effects of the Fukushima accident.