FUKUSHIMA, Japan — Even if the worst nuclear accident in 25 years leads to many people developing cancer, the public may never find out. The ordinary rate of cancer is so high, and medical understanding of the effects of radiation exposure so limited, that any increase in cases from the Fukushima nuclear plant disaster may be undetectable.
Several experts inside and outside Japan told the Associated Press that cancers caused by the radiation may be too few to show up in large population studies, like the long-term survey just getting under way in Fukushima.
That could mean thousands of cancers undetected in a study of millions of people.
Some of the dozen experts the AP interviewed said they believe radiation exposure most Japanese people have gotten fall in a "low-dose" range, where the effect on cancer remains unclear.
The cancer risk may be absent, or just too small to detect, said Dr. Fred Mettler, a radiologist who led an international study of health effects from the 1986 Chernobyl disaster.
That is partly because cancer is one of the top killers of people in industrialized nations, where the average lifetime cancer risk is about 40 percent.
In any case, the 2 million residents of Fukushima province, targeted in the new, 30-year survey, probably got too little radiation to have a noticeable effect on cancer rates, said Seiji Yasumura of the state-run Fukushima Medical University. Mr. Yasumura is helping run the project.
"I think he's right," as long as authorities limit children's future exposure to the radiation, said Richard Wakeford, a visiting epidemiology professor at the Dalton Nuclear Institute at the University of Manchester in England.
Mr. Wakeford, who is also editor of the Journal of Radiological Protection, said he's assuming that the encouraging data he's seen on the risk for thyroid cancer is correct.
The idea that Fukushima-related cancers may go undetected gives no comfort to Edwin Lyman, a physicist and senior scientist with the Union of Concerned Scientists, a group that advocates for nuclear safety.
He said that even if cancers don't turn up in population studies, that "doesn't mean the cancers aren't there, and it doesn't mean it doesn't matter."
"I think that a prediction of thousands of cancer deaths as a result of the radiation from Fukushima is not out of line," Mr. Lyman said.
But he stressed that authorities can do a lot to limit the toll by reducing future exposure to the radiation. That could mean expensive decontamination projects, large areas of condemned land and people never returning home, he said. "There's some difficult choices ahead."
Japan's Cabinet this month endorsed a plan to cut contamination levels in half within the next two years. The government recently announced it plans to study the risk from long-term exposure to the low-dose radiation level used as a trigger for evacuations.
The plant was damaged March 11 by a tsunami triggered by a magnitude-9 earthquake. Japanese authorities estimate it leaked about one-sixth as much radiation as the Chernobyl accident.
It spewed radioactive materials like iodine-131, cesium-137 and 29 others, contaminating the water, soil, forests and crops for miles around. A recent study suggested that emissions of cesium-137 were in fact twice what the government has estimated.
So far, no radiation-linked death or sickness has been reported in either citizens or workers who are shutting down the plant.
A preliminary survey of 3,373 evacuees from the 10 towns closest to the plant this summer showed their estimated internal exposure doses over the next several decades would be far below levels officials deem harmful.
While the Fukushima disaster has faded from world headlines, many Japanese remain concerned about their long-term health. Many don't trust reassurances from government scientists like Mr. Yasumura, of the Fukushima survey.
Consumers worry about the safety of food from Fukushima and surrounding prefectures, although produce and fish found to be above government-set limits for contamination have been barred from the market.
For example, mushrooms harvested in and around Fukushima are frequently found to be contaminated and barred from market.
Controversy also has erupted around the government's choice of a maximum allowed level for internal radiation exposure from food.
Fukushima officials have distributed radiation monitors to 280,000 children at elementary and junior high schools. Many children are allowed to play outside only two or three hours a day.
Schools have removed topsoil on the playgrounds to reduce the exposure, and the Education Ministry provided radiation handbooks for teachers. Thousands of children have been moved out of Fukushima since the March disasters, mainly because of radiation fears.
Many parents and concerned citizens in and around Fukushima carry Geiger counters for daily measurement of radiation levels in their neighborhoods, especially near schools.
Citizens groups also are setting up radiation measuring centers where people can submit vegetables, milk or other foods for tests. Some people are turning to traditional Japanese diets of pickled plum, miso soup and brown rice, based on a belief that it boosts the immune system.
"I try what I believe is the best, because I don't trust the government any more," said Chieko Shiina, who has turned to that diet.
The 65-year-old Fukushima farmer had to close a small Japanese-style inn because of the nuclear crisis. She thinks leaving Fukushima would be safer but says there is nowhere else to go.
"I know we continue to be irradiated, even right at this moment. I know it would be best just to leave Fukushima," she said.
Yuka Saito, a mother of four who lives in a Fukushima neighborhood where the evacuation order was recently lifted, said she and her three youngest children spent the summer in Hokkaido to get away from the radiation.
She tells her children, ages 6 to 15, to wear medical masks, long-sleeved shirts and a hat whenever they go out, and not to play outside.
She still avoids drinking tap water and keeps a daily log of her own radiation monitoring around the house, kindergarten and schools her children attend.
"We Fukushima people are exposed to radiation more than anyone else outside the prefecture, but we just have to do our best to cope," she said. "We cannot stay inside the house forever."
Japanese officials say mental health problems caused by excessive fear of radiation are prevalent and posing a bigger problem than actual risk of cancer caused by radiation.
But what kind of cancer risks do the Japanese really face?
Information on actual radiation exposures for individuals is scarce, and some experts say they can not draw any conclusions yet about risk to the population.
Michiaki Kai, professor of environmental health at Oita University of Nursing and Health Sciences, said that based on tests he has seen on people and their exposure levels, nobody in Fukushima except for some plant workers has been exposed to harmful levels of radiation.
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 Dr. 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.