Radioactive iodine has a half-life of just eight days, and it was not thought to be present outside the power plant in concentrations high enough to cause immediate health problems. But the isotope was concentrated by cows in milk, and children who drank contaminated milk or ate affected dairy products are particularly at risk.

An international team of researchers led by the National Cancer Institute, a federal agency based in Bethesda, Md., has been monitoring the health effects of the Chernobyl accident for years. In the study released Thursday, the team screened 12,500 people who were under 18-years-old at the time of the 1986 accident and lived in one of three provinces near the accident site. The subjects’ thyroid glands were measured for radioactivity within two months of the accident…

Some of the participants in the Chernobyl study lived as far as 90 miles from the accident site, demonstrating the risks of eating or drinking contaminated foods among people who were exposed to little or no radioactive iodine from the immediate fallout.