National Geographic Magazine
NCI Fallout Map

Iodine-131 Fallout From the Nevada Test Site

In 1997 the National Cancer Institute (NCI) released the first nationwide study on exposure to radioactive iodine 131 (I 131), from 100 atomic bombs tests detonated above ground at the Nevada Test Site during the 1950s and 1960s.

Rain, wind, and the food supply spread I 131 from these tests across the United States, with the largest deposits immediately downwind of the test site and the lowest on the West Coast, upwind of the site. Exposure to released iodine occurred mainly during the first two months following a test. After that I 131 disintegrated to harmless levels.

Because I 131 accumulates in the thyroid gland, the NCI estimates that the fallout may have caused up to 212,000 cases of thyroid cancer in people who were exposed. The average cumulative thyroid dose to approximately 160 million people who lived in the country during testing was about two rads, about five times the radiation dose emitted by a mammogram. A rad is the measurement unit for the amount of radiation the body absorbs. The federal government recommends medical monitoring for people who have been exposed to ten rads or more.

Americans were exposed to varying levels depending on their residence, age, and food consumption. People who lived in Western states to the north and east of the site, such as Colorado, Idaho, Montana, South Dakota, and Utah, had the highest per capita thyroid doses, ranging from 9 to 16 rads. And children between three months to five years old in these high fallout areas probably received three to seven times the average dose for the population in their county because they had smaller thyroids and tended to drink more milk than adults.

Milk was a major exposure vehicle because I 131 fell on pasture grasses and then was consumed by cows. But an estimated 20,000 people who drank goats' milk during testing were at an even greater risk because it concentrates I 131 more than cows' milk. Thyroid doses to these individuals could be 10 to 20 times greater than to residents of the same county, who were the same age and gender, and drank an equal amount of cows' milk. Other pathways included inhaling contaminated air or ingesting tainted leafy vegetables, cottage cheese, and eggs.

However, the relationship between I 131 and thyroid cancer still isn't fully known. It makes up less than one percent of cancer cases nationwide each year and cancer registries do not indicate that fallout has caused an epidemic, although record keeping didn't start until the early 1970s.

— Miki Meek

© 2002 National Geographic Society. All rights reserved.

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