Hundreds of residents in Fukushima Prefecture checked for radiation exposure after the nuclear accident there had levels exceeding what the government says is the safe annual limit.

In total, some 1,730 residents from three municipalities were examined. The checks showed they were exposed externally to radiation at a maximum level of 37 millisieverts and slightly above 1 millisievert on average.

About half of the examinees were exposed to more than 1 millisievert of radiation, which is the annual limit in normal situations, during the first four months of the crisis, according to an analysis by the Fukushima prefectural government. It is the first time that levels of external exposure have been disclosed.

The prefectural government plans to shortly release its findings to the public and inform the examinees of the results by mail.

The estimates relate to cumulative levels of external exposure to radiation in the four months after the crisis unfurled at the Fukushima No. 1 nuclear power plant following the March 11 Great East Japan Earthquake. Natural background radiation was not included.

Nobody is thought to have been exposed to as much as 100 millisieverts, a level above which cancer can result, even taking into account internal exposure through ingestion and inhalation.

The impact on human health from low-level radiation has, however, not been sufficiently explained.

On the basis of estimated external exposure levels, the prefectural government plans to conduct follow-up health surveys, which will cover all of the approximately 2 million residents, for at least 30 more years. The latest estimates were produced by the Fukushima Medical University and the National Institute of Radiological Science using original analysis software.

Three areas--Iitate village, the Yamakiya district of Kawamata town and Namie town--were designated as targets of a pilot study in the prefecture's health survey program. Of the total 29,000 residents, some 1,730 have now undergone checks for radiation exposure.

The first batch of analysis was made for people who, as a general rule, provided a detailed record of their whereabouts--data indispensable to the estimation--and were the fastest to submit that information to the prefectural government.

The results showed that the level of exposure was less than 1 millisievert for about half of the examinees, and less than 5 millisieverts for most of the remainder.

Researchers said that about 40 people had been exposed to levels ranging from between 5 and 10 millisieverts. About 10 individuals had been exposed to more than 10 millisieverts. The maximum level was 37 millisieverts.

Those with high levels likely stayed for a prolonged period in the "planned evacuation zone"--the area outside the 20-kilometer no-entry radius of the stricken nuclear facility where air radiation levels were high--or in other areas affected by radioactive plumes.

The International Commission on Radiological Protection says annual exposure of 1 millisievert is not harmful. The figure does not include normal background radiation found in nature nor exposure due to medical treatment such as X-rays.

The levels were estimated on the basis of records filled out by examinees on their whereabouts during the four months after the nuclear disaster.

For a two-week period from March 11, the examinees were asked to provide minute-by-minute records of their whereabouts, including whether they were "outdoors," "indoors" or "on the move." If they were indoors, they were asked to say if they were in a wooden building or a reinforced concrete structure.

For the period from March 26 through July 11, the examinees were asked to provide information on where they lived, where they went on a regular basis and the average number of hours they spent outdoors and indoors each day.

On the basis of this data, the doses of external exposure were calculated using time-series data on air radiation levels in various locations. The information was evaluated using the science ministry's monitoring survey results and the System for Prediction of Environmental Emergency Dose Information (SPEEDI), a simulation system operated by the government.

The examinees' ages were also taken into account, because children are more susceptible to the effects of radiation than adults.

The levels of external radiation exposure among Fukushima residents are lower than those for inhabitants of areas near the Chernobyl nuclear power plant.

According to the U.N. Scientific Committee on the Effects of Atomic Radiation and other sources, residents evacuated after the 1986 Chernobyl disaster had average external exposure readings of between 20 and 30 millisieverts.

The amount of topical internal exposure in thyroid glands was estimated at between 300 and 1,100 millisieverts. Some 6,000 residents in surrounding areas, mostly children, developed thyroid cancer and more than 10 people died.

So far, no spike has been detected in other incidences of cancer and other diseases.

In the case of Fukushima, the impact on human health of radioactive iodine, which causes thyroid cancer, remains unknown. Radioactive iodine, which has a short half-life, was no longer detectable when the health survey started more than three months after the accident.

The health impact of low-level exposure below 100 millisieverts remains unverified scientifically. All that can be done at present is to continue monitoring people's health over the long term and find ways to treat any anomaly as soon as possible.

The prefectural government plans to provide regular and lifelong thyroid checkups to approximately 360,000 children aged 18 or less. It will also provide health checkups to residents suspected of having been exposed to high levels of external radiation.

(By Yoshinori Hayashi and Yuri Oiwa, The Asahi Shimbun, 09/12/2011)