Although studies have shown that most local residents were likely not exposed to cancer-causing doses of radiation from the crippled Fukushima nuclear plant, the great fear remains the long-term effects of exposure to radioactive iodine.
Little is known about the health effects of their exposure to radioactive iodine, which can cause cancer in thyroid glands, because the short half-life of iodine is only eight days.
Experts have pointed out a need for establishing a framework to monitor the health impact of radiation over the coming decades. For many residents, the uncertainty and doubts are ever present.
A 60-year-old woman from Iitate, Fukushima Prefecture, who evacuated to the prefectural capital of Fukushima, had her two daughters undergo the prefectural government's ultrasound thyroid gland testing in autumn 2011. The test results, mailed to her home in early February 2012, reported no abnormality in her younger daughter, 17, whereas a small lump, measuring 5 millimeters or less, had been found in her 20-year-old daughter.
The woman said she felt helpless at the time, not knowing why her two daughters, who always lived under the same roof, were diagnosed differently.
She did not tell her daughters about the results, because she did not want to give them cause for concern.
The woman's family stayed for some time in Iitate after the disaster began to unfurl at the Fukushima No. 1 nuclear plant on March 11, 2011. They left the village only in late May 2011, after the central government had designated Iitate a "planned evacuation zone."
While they were still in the village, the woman took care so her daughters did not leave the house too often, except when they went out shopping two or three times a week. But her older daughter walked the family's dog from time to time.
"That may have exposed her to larger doses in the thyroid gland and helped the formation of the lump," her mother said.
She said she felt a little relieved when she attended the village government's pedagogical session on thyroid gland tests and learned that lumps had been found in many other children. But that has not convinced her to tell her daughters about the test results.
"Not a day goes by without concern, although I try not to brood about it too much," the woman said.
Little is known about the effects of exposure to radioactive iodine on the thyroid glands of Fukushima children, as very few test results are available. That has only increased the concerns of their parents and other guardians.
The central government has only studied iodine doses in thyroid glands of 1,080 children in Iitate and elsewhere. Their parents and other custodians have not been notified of their dose readings because of the poor accuracy of the measurements.
During the early phase of the disaster, Shinji Tokonami, a professor of radiology at Hirosaki University, and coworkers tested the thyroid glands of 62 residents who had evacuated to the prefectural capital from their hometowns near the stricken nuclear plant. The test results, compiled in 2012, showed doses of radioactive iodine of up to 33 millisieverts in adults and 23 millisieverts in children.
Because few test readings are available, only estimates have been made of thyroid gland doses in the entire population across the prefecture. The Environment Ministry in 2012 commissioned that task to the National Institute of Radiological Sciences.
To begin with, the NIRS evaluated the thyroid gland doses of radioactive iodine using available data on the whole-body internal doses of radioactive cesium, which has a much longer half-life, under the assumption that the iodine-to-cesium ratio was 3:1 in human bodies.
The institute estimated the thyroid gland doses were 30 millisieverts or less in 90 percent of 1-year-olds in Iitate and Futaba.
The corresponding figures were 20 millisieverts or less for Namie, Okuma and other municipalities, and 10 millisieverts or less for Tomioka, Kawamata and other municipalities.
The NIRS also used a simulated spread of airborne iodine to estimate its time-varying concentrations on a grid of 3 kilometers per side, and evaluated thyroid gland doses in the residents who evacuated from 12 municipalities around the stricken nuclear plant along 18 typical evacuation routes.
The dose estimates were lower than 100 millisieverts in most cases, although it was assumed that the subjects stayed outdoors for 24 hours.
Separately, the World Health Organization used radiation levels in the environment and food test results to evaluate the thyroid gland doses on the assumption that residents of Fukushima Prefecture only consumed food produced locally. The dose estimates for 1-year-olds were 122 millisieverts in Namie, 73 millisieverts in Iitate and 39 millisieverts in the prefectural capital of Fukushima and elsewhere.
While different studies have produced different estimates, a consensus is being formed that the thyroid gland doses seldom exceeded 100 millisieverts, an accepted threshold for increased risk of thyroid gland cancer. But all the estimates contain factors of uncertainty.
Whole-body external doses can be assessed to a certain degree only if the history of an individual's whereabouts is available. The Fukushima prefectural government has so far estimated the whole-body external doses in about 395,000 individuals on the basis of the records of their whereabouts during the first four months of the disaster.
When individuals engaged in radiation-related professions were excluded, the maximum dose estimate was 25 millisieverts, with the estimates being smaller than 2 millisieverts for 95 percent of the subjects studied.
Internal doses in individuals are more difficult to estimate than external doses, because diet and many other factors must be taken into account. Efforts to estimate thyroid gland doses are expected to continue into the next fiscal year, which starts in April.
The health impact of thyroid gland doses is another subject for future studies.
The Fukushima prefectural government plans to conduct ultrasound thyroid gland testing on all the 360,000 children and former children in the prefecture, who were 18 or under when the nuclear disaster occurred, over the course of their lifetimes. It is currently studying their basic conditions so as to enable comparison with future results.
About 133,000 of the eligible subjects have undergone the testing by January. Nodules, or lumps, measuring up to 5 mm--such as the one found in the older daughter of the Iitate woman--and cysts measuring up to 2 centimeters were found in 40 percent of them. Experts have said nodules and cysts of that size are nothing to be concerned about.
To provide control data for comparison, the Environment Ministry is conducting similar tests on about 4,500 children in Nagasaki, Kofu and elsewhere outside Fukushima Prefecture. The results will be forthcoming shortly.
Thyroid gland cancer was found in three tests, and suspected in seven more, of the approximately 38,000 children and former children who were tested in fiscal 2011 in Fukushima Prefecture.
"From a comprehensive viewpoint, it seems unlikely that radiation had any impact, considering that cancer only began to develop four to five years after the Chernobyl disaster," said Shinichi Suzuki, a Fukushima Medical University professor.
In the case of the 1986 Chernobyl nuclear accident, comparison of test results on children born before and after the disaster established an increase in the rate of thyroid gland cancer, which could be attributed to radiation.
"It is essential to compare results from identical areas," said Shigenobu Nagataki, a professor emeritus of radiology at Nagasaki University. "Tests on children born after the Fukushima disaster would establish whether radiation had any effects."
(This article was written by Yuri Oiwa and Teruhiko Nose.)
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