Starting in the 1940s, the world's superpowers conducted extensive nuclear weapons testing - on the ground, underground, and in the atmosphere. In total, more than 500 nuclear devices were detonated between 1945 and 1980, significantly changing the quantity and distribution of global radiation.

The main long-term health concern following exposure to radioactive fallout from nuclear testing is an increased risk of cancer. As Drs. Steven Simon, André Bouville, and Charles Land of NCI write in the January 2006 American Scientist, "The relationship between various forms of radiation exposure and subsequent cancer risk is perhaps the best understood, and certainly the most highly quantified, dose-response relationship for any common environmental human carcinogen." Research has shown that, for most types of cancer, radiation-related risk tends to increase with increasing age over time following exposure. Thus, the authors note that "even though the fallout exposures discussed here occurred roughly 50 to 60 years ago, it is likely that only about half of the predicted total numbers of cancers have occurred so far."

At the request of Congress, NCI has conducted extensive studies of fallout and related doses and risks. The American Scientist article explains that fallout distribution across the United States is largely a result of weather conditions, including wind speed and direction at different altitudes and local rainfall rates. Fallout deposition tends to decrease with distance downwind because of dispersion of the cloud bearing the radioactive particles and radioactive decay over time after detonation. However, local "hot spots" of concentrated fallout can occur when passage of the cloud coincides with rainfall.

Fallout deposited on the ground can lead to external whole-body exposure from x-rays and gamma rays emitted from the radioactive particles, as well as internal radiation following ingestion of the radioactive by-products that can contaminate plants and animals. The largest particles are deposited close to the test site; small, lighter particles tend to remain airborne for many days and are dispersed widely. The smaller particles also are readily available for biological uptake whereby they enter the food chain and can be ingested by people.

An important example of biological uptake is seen with radioactive iodine, which is produced abundantly in radioactive fallout and can be deposited on pasture grass, consumed and concentrated in milk by grazing dairy animals and, because iodine is essential for human thyroid function, further concentrated in the thyroid glands of people who consume the contaminated milk. Most of the radiation from radioactive iodine is restricted to the thyroid gland. Young children exposed to this radiation have an increased risk of developing thyroid cancer later in life.

Fallout from nuclear testing has likely resulted in some increased health risks to populations in the United States and elsewhere. Research on fallout has resulted in findings that may be helpful in dealing with current and future hazards including those related to nuclear terrorism.

By Sharon Reynolds

The Iodine-131 Individual Dose and Risk Calculator is now available on the NCI Web site at http://ntsi131.nci.nih.gov for those who want to estimate their exposure to fallout and risk of thyroid cancer as a result of U.S. nuclear testing at the Nevada Test Site.