The National Cancer Institute revealed this past fall that as many as 75,000 Americans were exposed to radioactive fallout from bomb tests in the 1950s and 1960s at levels high enough to cause thyroid cancer. Each of the 90 aboveground tests in Nevada sent clouds of radioactive iodine (I-131) and strontium (S-90) into the sky. In the weeks following each test, the drifting chemical haze settled as dry particles or rain across the country, especially in the states immediately downwind: Colorado, Idaho, Montana, South Dakota, and Utah.
Scientists worry most about those who grew up on farms in those states. When cows graze on grass doused with I-131, the chemical enters their milk. Children under six, who have small thyroid glands and consume lots of milk, received three to seven times higher doses of I-131 than adults did, according to the nci—and they were especially likely to get a high dose if they drank milk from a family cow. I-131 has a half-life of only eight days, which means that milk that was shipped to a dairy and then to the grocery store had enough time for most of the radioactivity to fade. But if you’re living on a farm, the milk from your backyard cow may be just 12 hours old when you drink it, says Joseph Lyon, an epidemiologist at the University of Utah in Salt Lake City. That fresh milk still has a lot of radioactivity in it.
Kids who grew up during the Nevada bomb test years are just now entering their fifties, an age at which cancer rates skyrocket. Estimates of the number of thyroid cancers caused by the tests range anywhere from 10,000 to 75,000, of which perhaps 30 percent, according to the nci, have already been diagnosed. When detected early, the disease can usually be cured.