Montana Earth Science Picture of the Week

From 1951 to 1958 the U.S. government conducted 90 nuclear bomb tests in the Nevada desert, sending particles contaminated with radioactive iodine-131 across much of the country. According to a 1997 report from the National Cancer Institute, the county with the highest per capita dose of radioactive Iodine 131 from the bomb tests was Montana's Meagher County located between Bozeman and Great Falls (White Sulphur Springs area). Furthermore 15 of the 25 counties with the highest doses of Iodine-131 are in Montana. Those “hot spots’ are the brightly-shaded counties on the map shown above. The distribution of fallout from each of the bomb tests was dependent on the strength and direction of winds, especially the jet stream, at the time of the explosion.

Casualties of the Cold War . . .
Iodine-131 is a radioactive isotope that decays rapidly in the environment. In fact its radioactivity is reduced by half every eight days (half-life), so it is only dangerous for about two months. But if it is ingested before it breaks down, it concentrates in the thyroid gland, which is made up of the only cells in the body able to absorb iodine. There the I-131 can cause thyroid cancer, which may not appear for several decades. The National Cancer Institute estimates that exposure to fallout from the bomb tests could produce over 200,000 excess cases of thyroid cancer.

Got milk? . . .
The primary way that people were exposed to I-131 was by drinking contaminated milk. The radioactive iodine was carried over Montana as part of the fallout, and fell on pastures where it was eaten by cows, contaminating their milk. Smaller amounts contaminated other dairy products and leafy vegetables. Children of that era (people ranging in age from late 40s to mid 60s now) are at higher risk for a couple reasons.

1. Children generally drink more milk than adults.

2. Children have smaller thyroids.

A little good news . . .
Thyroid cancer is a very slow growing cancer and accounts for only 1 percent of all cancers in the United States. An estimated 16,100 cases will be diagnosed this year with 1,230 being fatal. It is a highly curable cancer with the five-year survival rate at 95 percent.

Terms: isotope, half-life