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Friday, Nov. 4, 2011
Science far from conclusive on low-level radiation risks
Experts just don't know the effect on humans below 100 millisieverts
The March 11 nuclear accident at the Fukushima No. 1 plant has transformed what used to be a long-standing academic debate into an urgent issue for millions of ordinary people: Will long-term exposure to low-level radiation cause any health problems?
Experts have long been at odds over whether low-level radiation doses of less than 100 millisieverts are damaging.
Following are questions and answers on the effect of low-level radiation on human health:
How much risk does exposure to a cumulative radiation dose of 100 millisieverts present?
According to the International Commission on Radiological Protection, it could increase the cancer mortality risk by about 0.5 percent, meaning about 50 out of 10,000 people exposed to that level could die of cancer caused by the radiation.
The ICRP is an organization of scientists whose guidelines have served as the basis for radiation regulations in many developed countries, including Japan.
How afraid should we be of radiation at that level?
It depends on individual perceptions. If the ICRP is correct, it would increase your chances of dying of cancer by only 0.5 percent, but that could be a significant increase for a government responsible for protecting hundreds of thousands of people living around a nuclear plant.
For individuals, there are innumerable other factors in everyday life that can cause cancer, and doctors say improved lifestyle choices can easily offset the increased risk posed by exposure to 100 millisieverts of radiation.
In fact, various forms of cancer account for as much as one-third of all deaths in Japan, with major factors including smoking, insufficient consumption of vegetables and lack of exercise.
What about the risks of cumulative radiation doses of less than 100 millisieverts?
Results of tracking surveys of survivors of the Hiroshima and Nagasaki atomic bombings suggest exposure of 100 millisieverts does increase the cancer mortality risk, but it is unclear whether there is a link between doses below 100 millisieverts and cancer.
Scientists are split on the question of whether there are any risks below the 100-millisievert mark. Some believe 100 millisieverts is the "threshold" level under which radiation won't harm humans. Others argue otherwise, saying there is still a risk, however small it may be.
What is the government's assessment of the 100-millisievert risk, and how is it reflected in food restrictions?
The Food Safety Commission, a government panel consisting of independent experts, concluded on Oct. 27 that radiation above a cumulative exposure of 100 millisieverts poses a significant risk to humans, but it is difficult to draw any conclusions on the health effects from radiation less than that level.
Based on the commission's interpretation, the Health, Labor and Welfare Ministry will lower the acceptable annual intake in food products to 1 millisievert per year from the current provisional limit of 5 millisieverts.
Is there any scientific research on radiation exposure of less than 100 millisieverts?
Fifteen nations, including Japan, the U.S., the U.K., Canada, some European countries and South Korea, have conducted research on workers at nuclear power plants for about a decade. Also, scientists studied people living in Kerala, India, well-known for its high background radiation from radioactive thorium in monazite sand.
In short, there was no statistically significant evidence to conclude exposure to less than 100 millisieverts increases the cancer risk.
Why are those studies inconclusive?
Because there are confounding — or outside — factors in getting cancer, such as smoking.
Cigarettes for example have a much stronger effect than radiation. The risk of lung cancer for smokers is 2.3- to 22.4-fold higher than that for nonsmokers, according to the Japan Health Promotion and Fitness Foundation, which cites separate studies by Japanese and U.S. groups. Those levels are far higher than the theoretical 0.5 percent risk posed by 100 millisieverts of radiation exposure.
What were the results of the research in Japan on nuclear plant workers?
The Radiation Effects Association researched 277,000 employees of nuclear plants, most of whom had been exposed to less than 100 millisieverts over an extended period.
The study found that those who were exposed to higher radiation, though still below the 100-millisievert level, saw higher cancer mortality rates, which may indicate that even low-level radiation doses pose a lethal cancer risk.
But many of those with cancer were also smokers, which may have pushed up the risk. This makes it difficult for researchers to draw a clear conclusion on the risks of low-level radiation exposure.
"Past research (on smoking, drinking and the workers' other habits) shows those with high radiation doses tended to smoke more," the study's research paper concluded.
"Taking these facts into account . . . confounding bias by smoking and other habits is undeniable," it stated.
The 204,000 men in the study had an average cumulative radiation dose of 13.3 millisieverts. Of them, 14,224 had died as of March 2009, including 5,711 who succumbed to cancer.
What was the result of the Kerala research?
In short, no correlation was found between the ambient radiation and the risk of cancer, but there was one for cigarettes. Smoking was one of the factors studied.
The study "showed no HBR- (high background radiation) related excess of malignant tumors. Although the statistical power of the study might not be adequate due to the low dose, these findings suggest it unlikely that estimates of cancer risk at low doses are substantially greater than currently believed," a research paper stated in the January 2009 edition of Health Physics magazine.
Seiichi Nakamura of the Health Research Foundation, under the science ministry, said Kerala's radiation level is about the same as in Iitate, Namie and other municipalities in Fukushima Prefecture that the government ordered evacuated because of the nuclear crisis. Nakamura was on the team that researched Kerala residents.
So radiation of less than 100 millisieverts won't harm the human body?
Just because studies haven't found a statistically significant increase in risk doesn't necessarily prove there is no chance of an increase in the cancer rate. Some scientists argue there is a risk, even if it hasn't been confirmed in epidemiological surveys.
The Committee on the Biological Effects of Ionizing Radiation, comprised of radiology scientists in the United States, wrote a report titled "BEIR VII" in 2006 that says "the current scientific evidence is consistent with the hypothesis that there is a linear, no threshold (LNT) dose-response relationship between exposure to ionizing radiation and the development of cancer in humans."
Sentaro Takahashi, another researcher at the Kyoto University Research Reactor Institute, said the report is conservative because it is intended to provide a standard for protection from radiation.
He also said the report doesn't mention whether the linear relation is statistically significant because the aim of the committee is to come up with an overall conclusion based on individual studies, each of which mention the statistical significance of their theories.
Is cancer the only health risk associated with low radiation doses?
Scientists and medical doctors say yes. Nakamura is aware of some blogs stating there has been an increase in the Kanto and Tohoku regions of heart attacks resulting in death, birth defects and cases of childhood diseases, but it is scientifically impossible any of this was caused by radiation from the Fukushima No. 1 plant because the leakage of radiation has not been high enough, he said.