Before 1963, the United States and other countries conducted more than 500 nuclear weapons tests in the atmosphere. During these tests, radioactive particles and gases were spread in the atmosphere. Depending on the size and type of weapon that was exploded, some of these particles and gases traveled great distances before falling to earth (called fallout) where people could be exposed to the radiation.

Radioactive fallout was deposited all over the world, so many people were exposed to it. Even today, radioactive fallout is present in all parts of the world in small amounts. CDC and NCI, in their study of global fallout, looked only at fallout in the contiguous United States (the 48 states between Canada and Mexico). The study found that any person living in the contiguous United States since 1951 has been exposed to some radioactive fallout, and all of a person’s organs and tissues have received some exposure.