The March 2011 meltdowns at the Fukushima Daiichi Nuclear Power Plant caused extensive human suffering—evacuations, emotional trauma and premature deaths, disrupted jobs and schooling. What they have not caused, so far, is radiation-related illness among the general public, and few specialists expect dramatic increases in cancers or other ailments. The reactors spewed just a tenth of the radiation emitted by the Chernobyl disaster, winds blew much of that out to sea, and evacuations were swift. Yet one wave of illness has been linked to the disaster—the ironic result of a well-intentioned screening program.

Months after the disaster, Fukushima Prefecture set about examining the thyroids of hundreds of thousands of children and teens for signs of radiation-related cancers. The screening effort was unprecedented, and no one knew what to expect. So when the first round of exams started turning up thyroid abnormalities in nearly half of the kids, of whom more than 100 were later diagnosed with thyroid cancer, a firestorm erupted.

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One result, says Kenji Shibuya, a public health specialist at University of Tokyo, was “overdiagnosis and overtreatment,” leading dozens of children to have their thyroids removed, perhaps unnecessarily. Activists trumpeted the findings as evidence of the dangers of nuclear power. The large number of abnormalities appearing so soon after the accident “would indicate that these children almost certainly received a very high dose of thyroid radiation from inhaled and ingested radioactive iodine,” antinuclear crusader Helen Caldicott wrote in a post on her homepage.

Scientists emphatically disagree. “The evidence suggests that the great majority and perhaps all of the cases so far discovered are not due to radiation,” says Dillwyn Williams, a thyroid cancer specialist at University of Cambridge in the United Kingdom. In journal papers and in a series of letters published last month in Epidemiology, scientists have attacked the alarmist interpretations. Many acknowledge that baseline data from noncontaminated areas were needed from the outset and that the public should have been better educated to understand results and, perhaps, to accept watchful waiting as an alternative to immediate surgery. But most also say the findings hint at a medical puzzle: Why are thyroid abnormalities so common in children? The “surprising” results of the screening, Williams says, show that “many more thyroid carcinomas than were previously realized must originate in early life.”

Memories of Chernobyl got Japanese authorities worrying about thyroid cancer. The fallout from that April 1986 accident included radioactive iodine, which settled across swathes of Belarus, Russia, and Ukraine, contaminating pastures grazed by dairy cows. Children who drank the tainted milk accumulated the radioactive iodine in their thyroids. (Adult thyroids absorb less iodine.) A 2006 World Health Organization (WHO) study found that in the most contaminated areas, there had been about 5000 thyroid cancer cases among those who were under 18 at the time of the accident, though the report noted that more cases could emerge over time. The United Nations in 2006 attributed 15 childhood thyroid cancer deaths to Chernobyl. Caught early, the cancer is almost always cured by removal of the thyroid gland.

With that in mind, Japanese authorities set out to screen the thyroids of all 368,651 Fukushima residents who were 18 and under at the time of the accident. Most experts were not anticipating a bumper crop of thyroid problems. For starters, the potential radiation exposure of Fukushima residents was slight compared with Chernobyl victims. Moreover, the day after the meltdowns, Japanese authorities evacuated some 150,000 people living within 20 kilometers of the plant, and a week later they started screening for contaminated food. Also, a limited number of Fukushima residents were offered iodine tablets after the accident to block absorption of radioactive iodine from breathing contaminated air or eating contaminated food.

In 2013, WHO estimated that the 12 to 25 millisieverts (mSv) of exposure in the first year after the accident in the hardest hit areas might result in minuscule increases in cancer rates. (Worldwide, people receive on average 2.4 mSv per year from background radiation; a medical chest x-ray delivers about 0.1 mSv.) WHO noted that females have a 0.75% lifetime risk of developing thyroid cancer; it estimated that the highest exposures in the Fukushima area raised that risk by an additional 0.5%.

The initial round of thyroid screening, started in late 2011, was simply to provide baseline data, as any radiation-induced tumors were not expected to emerge for at least 4 years. Children with nodules larger than 5.0 mm or cysts bigger than 20.1 mm underwent a second, more detailed examination and, if necessary, fine needle aspiration. After the initial screening, children will have their thyroids examined every 2 years until age 20 and every 5 years after that.

Results were released as screening progressed, and right from the start there were surprisingly high rates of abnormalities. Findings from the initial round of screening, completed in April 2015 and released in August 2015, showed that nearly 50% of the 300,476 subjects had solid nodules or fluid-filled cysts on their thyroids. Smaller studies elsewhere had hinted that tiny thyroid cysts and nodules were common in all ages. But “specialists did not know whether the frequency [in the Fukushima results] was high or low,” says Noboru Takamura, a radiation health scientist at the Atomic Bomb Disease Institute at Nagasaki University in Japan.

As the number of confirmed cancers rose, worries grew about a link to radiation—and those concerns gained a high-profile proponent. In 2013, Toshihide Tsuda, an environmental epidemiologist at Okayama University in Japan, started presenting analyses at international conferences claiming the number of thyroid cancers in the Fukushima screening was unusually high. Last October, he published his results online in Epidemiology, concluding that the first round of screening indicated cancer incidence rates ranging from 0 to 605 cases per million kids, depending on location, but overall “an approximately 30-fold increase” over the normal childhood cancer rate. That claim fed alarming headlines.

Other scientists were swift and severe in their criticism. A fundamental error, according to several epidemiologists, is that Tsuda compared the results of the Fukushima survey, which used advanced ultrasound devices that detect otherwise unnoticeable growths, with the roughly three cases of thyroid cancer per million found by traditional clinical examinations of patients who have lumps or symptoms. “It is inappropriate to compare the data from the Fukushima screening program with cancer registry data from the rest of Japan where there is, in general, no such large-scale screening,” Richard Wakeford, an epidemiologist at the University of Manchester in the United Kingdom, wrote on behalf of 11 members of a WHO expert working group on Fukushima health consequences. Theirs was one of seven letters to Epidemiology published online last month that blasted Tsuda’s methodology and conclusions.

To see what comparable screening would find in a population not exposed to radiation, Takamura’s team used the Fukushima survey protocol to examine 4365 children aged 3 to 18 from three widely separated prefectures. They found similar numbers of nodules and cysts and one cancer, for a prevalence of 230 cancers per million people, as they reported in Scientific Reports in March 2015. Other Japanese studies reported thyroid cancer rates of 300, 350, and even 1300 per million. “The prevalence of thyroid cancer detected by advanced ultrasound techniques in other areas of Japan does not differ meaningfully from that in Fukushima Prefecture,” Takamura wrote in Epidemiology. In a letter to Epidemiology, Tsuda claims to have addressed the screening effect by adjusting the number of cancer cases to account for the lag time between when an ultrasound examination would catch the cancers and when they could be clinically identified. He did not address other criticisms or respond to repeated requests for comment from Science.

Although many scientists disagree with the spin Tsuda and activists have put on the findings, they endorse the screening effort. “A thyroid screening program would be expected to save lives by detecting cancers early, whether or not the cancers were caused by radioactivity,” says Timothy Jorgensen, a radiation health physicist at Georgetown University in Washington, D.C.

Yet it has become clear the public and even many doctors did not have the background to put the results in perspective. Even though the vast majority of thyroid abnormalities are safe to ignore, “finding small lesions causes patients anxiety,” says Seiji Yasumura, vice director of the Fukushima Prefecture Health Management Survey. Virtually all of those diagnosed with thyroid cancer have had the glands removed, even though accumulating evidence suggests in many cases it might have been better to wait, the University of Tokyo’s Shibuya adds. “Careful observation would be the best option.”

South Korea offers a cautionary tale. In 1999, the South Korean government initiated a health program in which care providers offered ultrasound thyroid screening for a small additional fee—and thyroid cancer diagnoses exploded. In 2011, the rate of thyroid cancer diagnosis was 15 times what it was in 1993, yet there was no change in thyroid cancer mortality, Heyong Sik Ahn of Korea University in Seoul and colleagues reported in The New England Journal of Medicine in November 2014. Virtually all those diagnosed underwent total or partial thyroid removal. Most required lifelong thyroid-hormone replacement therapy. To stem this “epidemic,” Ahn and others discourage routine thyroid cancer screening.

Williams says the evidence suggests that thyroid growths among children are far more common than previously thought and must be considered normal. The Fukushima survey, he says, promises a “better understanding of the origins and development” of such growths and may lead to better treatment protocols.

*Correction, 11 March, 10:40 a.m.: A previous version of this story implied that iodine tablets were offered to all Fukushima residents; they were actually available to only a limited number of residents.