Estimated radiation doses in thyroid glands exceeded safe levels in nearly 2,000 people who worked at the stricken Fukushima nuclear plant, more than 10 times the number previously announced, The Asahi Shimbun has learned.
The larger figure was deduced after doubts were raised both at home and abroad over the results of Tokyo Electric Power Co.’s belated first study on the workers’ health.
TEPCO, the nuclear plant’s operator, said in December that radiation doses topped 100 millisieverts--the widely accepted threshold for an increase in the risk of cancer--in 178 people, with a maximum reading of 11,800 millisieverts.
But that figure covered only a fraction of those who have braved the high radiation levels to try to bring the nuclear crisis under control.
The workers themselves say TEPCO has provided little or no information about radiation doses in their thyroid glands. Some have stopped working at the crippled Fukushima No. 1 plant.
The new figure is based on a review of an expanded number of study subjects.
TEPCO and its partner companies not only re-evaluated the readings from thyroid gland dose tests, but they also estimated doses when the amount of radioactive iodine that entered the body was unavailable. These estimates were based on cesium intake amounts, the airborne iodine-to-cesium ratio on the days they worked, and other data.
The latest study showed that doses topped the 100-millisievert mark in 1,973 workers. In one worker, the estimated thyroid gland dose increased by more than 1,000 millisieverts during the review.
A thyroid gland dose reflects the amount of internal exposure to radioactive iodine that has entered the body through inhalation and other processes. The thyroid gland doses received during the early stages of the nuclear disaster, which started in March 2011, account for most of the potential internal damage to the Fukushima plant workers.
Early on in the crisis, health experts warned about the risks of high radiation doses received by the workers. But TEPCO was late in opening a full-scale investigation into the thyroid gland doses.
The utility submitted data about the doses to the World Health Organization. However, TEPCO only released available data for some of the workers in December after it learned that the WHO was planning to disclose the information.
The data concerned 522 workers for whom thyroid gland dose test results were available.
It took TEPCO 28 months since the earthquake and tsunami on March 11, 2011, caused the nuclear disaster to learn that so many workers have been exposed to cancer-inducing levels of radiation doses in their thyroid glands.
Its re-evaluation also came after the U.N. Scientific Committee on the Effects of Atomic Radiation, which received data from TEPCO, questioned the reliability of the company’s thyroid gland dose readings. Japan’s health ministry also ordered TEPCO and its partner companies to review the internal dose readings for the workers.
“We will provide and pay for annual, ultrasound thyroid gland tests to all workers with thyroid gland doses in excess of 100 millisieverts over their lifetimes,” a TEPCO public relations official said. “We have already notified those who are eligible for the checkups.”
But TEPCO said it does not know how many of those eligible workers have actually taken the tests. Sources said only about half of them have received the thyroid gland checkups.
In addition, the utility has not announced a schedule for the thyroid gland checkups for the workers and has yet to explain what it will do when it spots abnormalities during the tests.
Most of the study subjects with thyroid gland doses exceeding 100 millisieverts entered the Fukushima plant site early in the disaster and inhaled radioactive substances. TEPCO employees account for 976 of them, with the remainder employed by the utility’s main contractors and their subcontractors.
Several workers told The Asahi Shimbun that TEPCO has never provided a careful explanation about the risks of radiation exposure in thyroid glands. Some subcontractor workers have already quit their jobs, complaining that they were never told about the radiation doses or received any notification of thyroid gland tests.
The delay in the testing can be partly blamed on a health ministry policy, which says health control for nuclear plant workers should be based solely on whole-body doses.
The ministry has never taken the initiative to investigate thyroid gland doses in the Fukushima workers. It has left the task to TEPCO on a “voluntary” basis.
Some experts have emphasized that enhanced thyroid gland doses do raise the risk of cancer even if the whole-body doses remain modest. But the health ministry has maintained that whole-body dose control is sufficient.
The international consensus for the 100-millisievert threshold for an increased risk of cancer is based partly on studies following the 1986 Chernobyl nuclear disaster.
The international standard for taking iodine tablets to block radiation exposure in thyroid glands, however, is 50 millisieverts.
Some health experts have said the cancer rate began increasing at the 50-millisievert level after the Chernobyl disaster.
Children are believed to be the highest at risk to thyroid gland doses. But a recent study showed the risk of cancer from thyroid gland doses rises even in people over 40, countering the previous belief that older people were far less susceptible to the cancer-inducing effects of radiation.
(This article was compiled from reports by Yuri Oiwa and Toshio Tada.)
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