After the Second World War physics enjoyed a new found status and authority born of its vital role in the nuclear enterprise and its importance for national defence. For some physicists, however, the close association of their discipline with nuclear weapons and the commitment of governments to the development of nuclear weapons engendered deep dilemmas of conscience. This followed in the wake of unease amongst scientists internationally about the use of nuclear bombs against Japan in 1945, and amid mounting concerns in the 1950s about nucler weapons proliferation and intensifying programmes of nuclear bomb testing. This paper focuses on the problem of ‘fallout’ from nuclear bomb tests as this became the subject of highly charged political debate, bringing the dangers of ionising radiation - leukaemia and genetic damage - sharply into focus. Fallout was an integral part of the nuclear age and Cold War politics, and was also important in the development of a new environmental consciousness. It remains, however, under-explored and poorly understood. This paper seeks to contibute new understanding of the fallout issue.

Organized into three parts, the paper begins by highlighting divisions amongst scientists on the dangers of fallout, examined here through the different responses of British scientists to the ‘fallout’ problem. Secondly, it emphasizes ‘fallout’ as a key animating force in the inception of the Pugwash movement in 1957. The Pugwash Conferences on Science and World Affairs sought to further the principles, values and aims set out in the Russell-Einstein manifesto of July 1955 which called for an end to nuclear weapons, the arms race and nuclear bomb tests. Thirdly, the paper analyses the unique role of Pugwash as a transational actor which sought both to shape policy on testing and fallout, and inform the public about this problem. It argues the significance of Pugwash for scientists as a distinctive form of activism and as a means to express their sense of social responsibility. The paper concludes with some reflections on the fallout issue as a site to explore the sometimes fraught relationships within the scientific community in the ‘atomic age’ and between scientists and the state in the early Cold War.