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Wednesday, June 1, 2011
Radiation-linked cancer an intangible numbers game
By MIZUHO AOKI
With contaminated produce continuing to be detected beyond Fukushima Prefecture, public concern over the health effects of radiation exposure continues to mount.
Experts agree that exposure to more than 100 millisieverts in total increases the risk of cancer. However, scientists have yet to achieve consensus about the degree of risk of contracting cancer below that level.
"What we know today is that there is a risk of cancer incidence and mortality from exposure to more than 100 millisierverts in total. Above that level, the percentage of cancer risk increases in proportion to exposure level," said Masayori Ishikawa, a professor in the department of applied molecular-imaging physics at Hokkaido University and a radiation therapy expert.
The best epidemiological study on health effects from radiation exposure in the world is that of the survivors of the Hiroshima and Nagasaki atomic bombings.
A followup study on some 100,000 survivors found that the occurrence of cancer increases in proportion to doses above 100 millisieverts, but "the study so far failed to detect any statistically significant increase in the incidence and mortality of cancer at doses below 100 millisieverts," explained Otsura Niwa, professor emeritus of Kyoto University.
According to the study, lifetime cancer mortality is estimated to rise 0.5 percent for anyone exposed to a dose of 100 millisieverts, said Niwa, an expert on radiobiology.
In Japan, about one out of every two people develop cancer during their lifetime, and about 30 percent of deaths in Japan are attributed to cancer, Ishikawa said.
Below 100 millisieverts, "the chances are too small to get statistically significant data," he said. With such a low risk, it would be difficult to have statistically significant data, even if information on about 1 million people were available, he added.
However, Niwa said that the lack of evidence does not mean there is no risk.
"It means that if there is an increase in the health risk, it is below the level detected by the best study in the world," Niwa said.
The International Commission on Radiological Protection has drawn up recommendations on radiation protection based on such hypotheses as that there is a small risk of contracting cancer even when the level is below 100 millisieverts. ICRP's recommendations have been used worldwide, including in Japan, for setting guidelines to set limits for the general public as well as for nuclear plant workers.
The ICRP's 2007 recommendations set three different dose limits, depending on the situation.
Normally, it advises not exceeding an upper level of 1 millisievert per year for the general public. In times of emergency, it calls for upper limits of between 20 and 100 millisieverts. For a postaccident period, it urges setting the limit to between 1 and 20 millisieverts, which the government is now using as a base figure in setting the 30-km evacuation zone around the Fukushima No. 1 plant, as well as for the use of school grounds in the rest of the prefecture.
Although there are arguments over the degree of risk below 100 millisieverts, a panel of experts at the U.S. National Academy of Sciences also supports the hypothesis that a small dose has the potential to increase the risk of cancer in humans. It predicts that approximately 1 in 1,000 people will develop cancer after being exposed to a total of 10 millisieverts.
But even based on such a hypothesis, the chances of developing radiation-induced cancer are very small compared with the risks associated with a high salt diet or lack of exercise, Niwa said.
According to a report by the National Cancer Center, based in Tokyo, the risk of cancer incidence from low exposure to radiation is much smaller than that from smoking or obesity. The NCC notes, however, that the report should be considered a reference to get an idea about the level of cancer risks due to radiation exposure because the percentage changes depending on the individual and also the duration of studies.
Its report was based on several studies, including about 40 years of followup data on atomic bomb survivors by the Radiation Effects Research Foundation, and a 10- to 15-year followup study by the NCC on Japanese citizens aged between 40 and 69.
The report said cancer risk from exposure to between 100 and 200 millisieverts is 1.08 times higher when compared with people who weren't exposed, while the cancer risk of people whose body mass index was between 30 to 39.9 was 1.22 times higher than a group of people whose body mass index was between 23 and 24.9. The cancer risk was 1.6 times higher for a group of people who smoke, when compared with nonsmokers, it said.
"The risk of cancer incidence is not zero even at low doses. . . . But the levels we are now exposed to are not something people have to worry deeply about," said Ikuro Anzai, a professor emeritus at Ritsumeikan University who has criticized the safety of nuclear power plants for decades.
"Many people get scared simply by hearing the word radioactivity. But we have to base our worries on reality. It is very difficult, but we need to have rational fears," said Anzai, an expert on radiation protection.
But when it comes to children, experts say extra care is needed, because the younger they are, the more vulnerable they are to radiation.
"Children have higher rates of cell division than adults. So when they are exposed to radiation, it can result in more damage than adults," Ishikawa of Hokkaido University said.
Experts estimate that children have two to three times more risk of cancer mortality than adults.
Children's vulnerability was manifested in the 1986 Chernobyl accident.
Among people exposed to radiation while they were age 18 or under, more than 6,000 cases of thyroid cancer were diagnosed. Of those, 15 people had died as of 2005, according to a 2008 report by the U.N. Scientific Committee on the Effects of Atomic Radiation.
The thyroid cancer cases were believed to be caused by drinking milk contaminated with high levels of iodine-131. When ingested, iodine-131 accumulates in the thyroid gland. Because children's thyroids are smaller and because they absorb iodine much more actively than adults, they have a much higher chance of developing thyroid cancer, experts say.
Considering these factors, the annual upper limit of 20 millisieverts set by the government for schoolchildren in Fukushima Prefecture is too high, experts say.
"Twenty millisieverts a year is a level that should prompt a country to start instituting safety measures. It is too high, especially for children," said Ishikawa of Hokkaido University.
In response to strong criticism from parents and activists about the 20-millisievert annual upper limit, the education ministry last Friday set a new nonbinding target to reduce radiation exposure of children in Fukushima Prefecture while they are at school to 1 millisievert or less per year. However, the ministry has not changed the binding upper limit of 20 millisieverts for Fukushima children both in and outside schools.
Anzai of Ritsumeikan believes what people should do now is make an extra effort to reduce the intake of radioactive materials as much as possible, especially among children, rather than keep discussing dose limits.
As the level of radioactive materials detected in the air across Japan has declined and stabilized since hitting a peak in mid-March shortly after the hydrogen explosions at the Fukushima plant, what the public should look out for now is how much cesium-137 fell from the sky. That has a much longer half-life than iodine-131, Anzai said.
Removing surface soil from school playgrounds, as has been done in Fukushima Prefecture, is a very effective way of reducing exposure, both internally and externally, he said.
As for food, people should not be too worried, because foodstuffs contaminated with radioactive materials that exceed recommended levels are currently not being sold to consumers, experts say. Vegetables, including cabbage and turnips, and milk and "konago" sand lance fish from Fukushima Prefecture, excluding some areas, as well as spinach from Kitaibaraki and Takahagi in Ibaraki Prefecture, were still banned as of Monday, according to the agriculture ministry. Any bans on using tap water had been lifted as of Monday, according to the health ministry.
If people want to take extra precautions, washing vegetables or boiling them can remove some radioactive materials. For rice, most of the radioactivity can be removed by milling the grains, Anzai said.
"Each person also needs to get updated information about contamination levels," Anzai said.
What people can also do is check if vegetables and other produce being sold in stores is really safe by setting up consumer groups and confirming whether local governments are really checking levels of contamination, Anzai said.