Three years after the Fukushima Daiichi nuclear disaster, reports are surfacing of a "cancer cluster." The Japanese government has reportedly tested 254,000 of the 375,000 children and adolescents in Fukushima Prefecture and found 33 cases of thyroid cancer. In Japan, the rate of this disease in 10- to 14-year-olds is typically one or two per million.

The Japanese government is investigating the matter, but it has already stated that the high prevalence is not a direct result of the radiation released during the meltdown. (See: "Pictures: A Rare Look Inside Fukushima Daiichi Nuclear Plant.")

To learn more, we interviewed Norman Kleiman, who is on the faculty of the Department of Environmental Health Sciences at the Columbia University Mailman School of Public Health in New York City.

Do the reports of 33 cases of thyroid cancer give you concern?

If you're going to screen that many children, you're going to find more cases than you normally [would], because you're looking for something. I suspect if you took the same number of children in Montana and did the same [screening], you'd probably find a similar ratio.

But there was definitely a "cancer cluster" of thyroid cases after the 1986 Chernobyl nuclear disaster, right?

There were somewhere between 5,000 and 7,000 cases [among Chernobyl's children]. But children there were the most heavily exposed population of individuals, except for the people who worked in the plant. (Read "Inside Chernobyl" in National Geographic magazine.)

View Images This nursery in Prypiat, Ukraine, was evacuated a day after the Chernobyl nuclear disaster. PHOTOGRAPH BY GERD LUDWIG, NATIONAL GEOGRAPHIC

Why were those children exposed to so much radiation?

One of the great failures of Chernobyl was that the government of the Soviet Union did not immediately take steps to protect the public, especially the vulnerable—children and pregnant mothers—from potential radioactive fallout. (See: Pictures: "'Liquidators' Endured Chernobyl 25 Years Ago.")

The large explosion at Chernobyl sent a plume of material way up in the sky containing a variety of radioactive isotopes, including iodine 131. Iodine is required for thyroid function, and if you have radioactive iodine in a food or water supply, it's going to go to your thyroid.

Iodine 131 has a short half-life—eight days—but it fell on the ground. Cows, sheep, and goats ate the grass and ingested small amounts of radioactive iodine, and dairy products made from their milk were given to young children for about a month after the accident. So children were drinking and eating radioactive-contaminated dairy products, and had an enormous concentration of radioactive iodine in their thyroids.

Were Fukushima kids also consuming contaminated food?

The lesson learned from Chernobyl was to immediately shut down food and water sources after the Fukushima accident. Any animal that could be a source of dairy products was banned from food supplies, and water systems were tested for radioactive contamination. So there really was no exposure of children to these high levels of radioactivity.

Are there other precautions for people who live near a nuclear plant?

Public health authorities in the United States have distributed iodine pills to people who live near certain nuclear power plants. In the first 24 to 48 hours [after an accident], if you take a pill and your thyroid is saturated with iodine, radioactive iodine is less likely to get to your thyroid.

Should the Japanese government even be testing children for thyroid cancer?

They wanted to reassure everyone that they're doing everything they can. Many of these [detected] cancers would have gone undiagnosed, or might not have progressed. The same thing happens with prostate cancer: Most men over a certain age are going to have cancerous cells in their prostate [if they are tested]. But that doesn't mean there's any clinical significance.

Is three years after a nuclear accident a reasonable time to test for thyroid cancer?

In Chernobyl, the earliest cases were found about four years after the accident. That's not where the peak was but where they started noticing a small increase, started saying, "Uh-oh, something's going on." Based on Hiroshima and Nagasaki survivors, we would expect this kind of solid [thyroid] tumor to appear somewhere in the four- to six-year range [after an event].

In Chernobyl, did the children with thyroid cancer recover?

I think the mortality was less than one percent.

Will the Japanese kids who have been diagnosed receive treatment?

I'm not a pediatric endocrinologist, and I don't want to speculate. But I understand a lot of this will be watchful waiting.

I've read reports that parents in the Fukushima area don't let their children go outside and play because they fear the children will be exposed to radiation.

There is no more radioactive iodine. Small amounts of long-lived isotopes, such as cesium or strontium, are of some concern. They have a half-life of 30 years, and they have health effects: Cesium concentrates in fatty tissues, strontium in bones.

But levels in Fukushima are minimal, and no [one there is] living near any serious concentration. That's not to say there couldn't be a place where a little bit of radioactivity fell on the ground. But by the same token, [radiation] surrounds us all the time. We live in a radioactive environment, from natural sources and medical/diagnostic exposures. If you fly in a plane over the Poles, you get a higher than average dose. If you have granite countertops in your kitchen, you get a little bit of radiation. If you like bananas, you get a little.

Do you have any predictions of what might happen in Fukushima?