The fallout, from more than 400 Cold War atmospheric bomb tests, include the radioactive isotopes strontium 90, which bonds to bones, and iodine 131, which causes thyroid cancer.

A previous study by the National Cancer Institute estimated that 11,300 to 212,000 thyroid cancers resulted from atmospheric testing. Most thyroid cancers are curable. Here's more on thyroid cancer and bomb testing.

Radioactive iodine decays rapidly. Overseas tests, including about 100 that the United States detonated over the Pacific Ocean, would have harmed U.S. residents through longer-lived isotopes, which can travel thousands of miles on the wind before decaying.

Indeed, while the study focused on U.S. citizens, the contamination from fallout was worldwide.

The study follows a similar assessment of fallout at a Soviet nuclear test site.

The preliminary study by the Centers for Disease Control was designed to test the feasibility of getting a better picture of disease caused by atmospheric bomb testing. A more precise body count, however, may never be possible.

Bibliography
Almost All in U.S. Have Been Exposed to Fallout, Study Finds, James Glanz, The New York Times, March 1, 2002.