A "dirty bomb" combines a conventional explosive with radioactive materials. Technically known as radiological dispersion devices (RDD), they use mechanical and/or explosive means to spread highly radioactive materials over an area with the intent of contaminating it for years. The explosion itself is not nuclear, only the material dispersed by the explosion. Although the detonation of a RDD would cause a number of deaths, the primary effect would be long-term contamination of people and property in the dispersal area. Three radioactive isotopes--americum-241, cobalt-60, and cesium-137--are particularly suited for use in a RDD because of their potential for conversion into a dispersible form, accessibility, and the length of time it takes for the radioactivity to subside. These isotopes have been widely used throughout the world in hospitals, factories, and laboratories. Apparently, only Iraq has successfully detonated a RDD test device, but it abandoned the program because the radioactive fallout was not sufficient to make it an effective military weapon. In the late 1990s, al Quaida failed in an attempt to produce and detonate a RDD during training in the Sudan; however, intelligence information suggests this organization may have another RDD. Countermeasures should include limiting access to radiological materials, increasing port and border security, improving training for first responders in case of a RDD detonation, and having an adequate supply of appropriate medication to treat casualties. 112 notes and discussion questions