WASHINGTON (AP) - Fallout from 1950s nuclear bomb tests exposed millions of children across the country to radioactive iodine, raising the possibility that 10,000 to 75,000 of them might develop thyroid cancer, the National Cancer Institute said Friday.
But government doctors emphasized they have no proof this radioactive substance causes thyroid cancer, so their estimate is a worst-case scenario. Nobody was tested in the NCI study.
"We do not feel that we have the data to support the idea that there was a large risk. On the other hand, we cannot rule it out," said Dr. Richard Klausner, NCI's director.
Independent thyroid experts immediately urged caution, noting that even if a link is proved, thyroid cancer grows very slowly and is highly curable.
"What we don't want to have happen is mass hysteria about this," said Dr. Stanley Feld, past president of the American Association of Clinical Endocrinologists.
If the estimate is accurate, 30 percent of the radiation-related cancers already would have been diagnosed in the 40 years since the blasts, the NCI said.
Anyone worried about fallout exposure during childhood should get a thyroid exam, the NCI advised.
The government asked the prestigious Institute of Medicine to determine within six months the health risks raised by the radiation study and recommend whether people need routine thyroid testing.
Everyone living in the 48 contiguous states between 1951 and 1958 received some fallout from 90 nuclear bomb tests in Nevada, the NCI study found.
People who lived directly downwind of the tests already were known to have been heavily exposed, especially in southwest Utah, where some people have been compensated by the government. But wind and rain can carry radiation far afield, so NCI spent 14 years studying county-by-county fallout.
Average national exposure was 2 rads, about five times the radiation delivered by a modern mammogram.
But 24 counties - in Montana, Utah, Idaho, Colorado and South Dakota - were exposed to an average of 9 to 16 rads, enough to be called hot spots. Dozens of other counties throughout the Farm Belt and Northwest received above-average fallout as well.
Children living in the most-exposed areas received five to seven times the average fallout, the NCI said.
Adults at the time have little risk, because the radioactive iodine-131 concentrates most in the thyroids of children and is spread mostly by drinking contaminated milk. Particularly risky was goats' milk or fresh milk from backyard cows. Processed milk allowed more time for the iodine to dissipate; it was gone within two months of each Nevada blast.
The NCI study did not actually test any person. But the government already recommends precautionary medical monitoring for people exposed to more than 10 rads of iodine-131.
And NCI doctors separately concluded that if the radiation proves cancer-causing, then 10,000 to 75,000 thyroid cancers might develop from the fallout. That number would be in addition to the almost half a million cases of thyroid cancer that would normally occur among the 95 million Americans who were children during the 1950s blasts.
"The increase ... from these very rough calculations, would be a maximum 10 (percent) to 15 percent increase," Klausner added. "That is significant, that's a lot of thyroid cancer," but it's only a rough estimate, he stressed.
Iodine-131's effect is still unclear. External radiation, from such sources as strong X-rays, is known to cause thyroid cancer. But the evidence that ingesting radioactive iodine is harmful is "suggestive but not conclusive," the NCI said.
Children downwind of the 1986 Chernobyl nuclear explosion in the former Soviet Union do have increased thyroid cancer, but the iodine exposures there ranged up to 10 times higher than U.S. exposures - so Klausner emphasized that no one knows what a risky dose is. It will be three to five years before Chernobyl studies answer that question.
"The experts tell us if you grew up east of the Nevada test site and drank fresh milk, get your thyroid checked," said Bob Schaeffer of the watchdog Military Production Network, which criticized the NCI for not releasing the information more quickly.
The NCI has had much of the fallout data since 1994, but did not release estimates of average county exposures until Friday. It will release the full, 100,000-page study by Oct. 1.