“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.