During the Cold War era, there was an escalation of atmospheric testing of nuclear weapons. After the atomic bombings of Hiroshima and Nagasaki in 1945, testing continued and the scale increased with the first hydrogen bomb in 1952. Soon after the United States tested the hydrogen bomb, the Soviet Union followed, in 1953. The mushroom clouds that occurred from the explosions released radioactive isotopes in mass quantities.
The first comprehensive study of the problem of radioactive isotope release began in spring 1949 with a one-man project called GABRIEL, conducted by Nicholas M. Smith Jr. at Oak Ridge National Laboratory. Smith produced his first report in 1949. 
Project GABRIEL was revived in mid-1951 because bombs that were dropped had brought up concerns people had about the dangers of strontium-90. United States Atomic Energy Commission (AEC) was interested by GABRIEL's report though they said it was lacking in hard data and needed independent confirmation of the tests.
After reviewing Project GABRIEL in 1953, it was given first priority status. The secret project would define "practical limits" for using atomic weapons. A task team was assembled and the codename used was "Project HORN." In 1954,  AEC argued that fallout was harmless because there was not enough evidence to prove that fallout would harm humans, animals, or crops. The  AEC campaign persuaded the public that the worldwide fallout was harmless. This claim was later disputed when scientists announced publicly that there was no safe level of radiation. This was confirmed in a confidential report by a geneticist for the AEC.  By 1954 and the Castle Bravo incident, it was obvious that radioactive fallout was dangerous to humans. The United States Atomic Energy Commission Division of Biology and Medicine dealt with efforts directed towards experimental and field studies and the correlation of data dealing with Project GABRIEL.
The RAND Corporation, Laboratories at Columbia University, AEC's New York office, the University of Chicago, an exclusive group of scientists, UCLA, and the United States Air Force were all involved in collection and testing of samples from around the world for radioactive fallout.