Project 4.1

Project 4.1 was the designation for a medical study conducted by the United States of those residents of the Marshall Islands exposed to radioactive fallout from the March 1, 1954 Castle Bravo nuclear test at Bikini Atoll, which had an unexpectedly large yield. Government and mainstream historical sources point to the study being organized on March 6 or March 7, 1954, six days after the Bravo shot.

In the wake of the Bravo accident, a new research section was added to the Castle Bravo Weapons Effects research section. Program 4, "Biomedical effects," was to include one project, Project 4.1, titled "Study of Response of Human Beings exposed to Significant Beta and Gamma Radiation due to Fall-out from High-Yield Weapons." Eugene P. Cronkite of the National Naval Medical Center was designated as Project Officer. Cronkite's instructions stressed the importance of secrecy surrounding the project:

...the project is classified SECRET RESTRICTED DATA. Due to possible adverse public reaction, you will specifically instruct all personnel in this project to be particularly careful not to discuss the purpose of this project and its background or findings with any except those who have specific "need to know."

The purpose of the project, as a 1982 Defense Nuclear Agency report explained, was both medical as well as for research purposes:

The purposes of [Project 4.1] were to (1) evaluate the severity of radiation injury to the human beings exposed, (2) provide for all necessary medical care, and (3) conduct a scientific study of radiation injuries to human beings.

As a Department of Energy Committee writing on the human radiation experiments wrote, "It appears to have been almost immediately apparent to the AEC and the Joint Task Force running the Castle series that research on radiation effects could be done in conjunction with the medical treatment of the exposed populations." The DOE report also concluded that "The dual purpose of what is now a DOE medical program has led to a view by the Marshallese that they were being used as 'guinea pigs' in a 'radiation experiment.'"

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