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Radiation generally raises cancer risk in proportion to its amount. At low-dose exposures, many experts and `regulators embrace the idea that this still holds true. But other experts say direct evidence for that is lacking, and that it’s not clear whether such small doses raise cancer risk at all.

“Nobody knows the answer to that question,” says Mettler, an emeritus professor of radiology at the University of New Mexico and the U.S. representative to the United Nations Scientific Committee on the Effects of Atomic Radiation, or UNSCEAR. If such low doses do produce cancers, they’d be too few to be detected against the backdrop of normal cancer rates, he said.

To an individual the question may have little meaning, since it deals with the difference between no risk and small risk. For example, the general population was told to evacuate areas that would expose them to more than 20 millisieverts a year. A millisievert measures radiation dose and 20 mSv is about seven times the average dose of background radiation Americans get in a year. A child exposed to 20 mSv for a year would face a calculated risk of about 1 in 400 of getting cancer someday as a result, says David Brenner of Columbia University. So that would add 0.25 percent onto the typical lifetime cancer risk of about 40 percent, he said.

And the average dose among the 14,385 workers who worked on the plant through July was 8 mSv, according to the Japanese government. The average lifetime risk of cancer to an individual from that dose alone would be calculated at about 0.05 percent, or 1 in 2,000, Brenner said.

Brenner stresses that such calculations are uncertain because scientists know so little about the effects of such small doses of radiation.

But in assessing the Fukushima disaster’s effect on populations, the low-dose question leads to another: If a lot of people are each exposed to a low dose, can you basically multiply their individual calculated risks to forecast a number of cancers in the population?

Brenner thinks so, which is why he believes some cancers might even appear in Tokyo although each resident’s risk is “pretty minuscule.”

But Wolfgang Weiss, who chairs the UNSCEAR radiation committee, said the committee considers it inappropriate to predict a certain number of cancer cases from a low-dose exposure, because low-dose risk isn’t proven.

Nuclear accidents can cause cancer of the thyroid gland, which can absorb radioactive iodine and become cancerous. That disease is highly treatable and rarely fatal.

After the Chernobyl disaster, some 6,000 children exposed to radioactive fallout later developed thyroid cancer. Experts blame contaminated milk. But the thyroid threat was apparently reduced in Japan, where authorities closely monitored dairy radiation levels, and children are not big milk drinkers anyway.

Still, the new Fukushima survey will check the thyroids of some 360,000 young people under age 18, with follow-ups planned every five years throughout their lifetimes. It will also track women who were pregnant early in the crisis, do checkups focused on mental health and lifestyle-related illnesses for evacuees and others from around the evacuation zone, and ask residents to fill out a 12-page questionnaire to assess their radiation exposure during the first weeks of the crisis.

But the survey organizers are having trouble getting responses, partly because of address changes. As of mid-October, less than half the residents had responded to the health questionnaire.

Some residents are skeptical about the survey’s objectivity because of mistrust toward the government, which repeatedly delayed disclosing key data and which revised evacuation zones and safety standards after the accident. Also, the government’s nuclear safety commission recommended use of iodine tablets but none of the residents received them just before or during evacuation, when the preventive medicine would have been most effective.

Some wonder if the study is using them as human guinea pigs to examine the impact of radiation on humans.

Eisuke Matsui, a lung cancer specialist and a former associate professor at Gifu University School of Medicine, criticized the project. He said it appears to largely ignore potential radiation-induced health risks like diabetes, cataracts and heart problems that have been hinted at by some studies of Chernobyl.

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