The term, “dirty bomb” is used to refer to a radiological dispersal device (RDD), a radiological weapon which combines radioactive material with conventional explosives. Though an RDD is designed to disperse radioactive material over a large area, the convential explosive would most likely have a more immediate lethal effect than the radioactive material. A dirty bomb is not a nuclear bomb, as it does not involve a nuclear explosion; radioactive material is simply wrapped around a conventional explosive. Any type of radioactive material could be used in a dirty bomb, but at levels created from most probable sources, not enough radiation would be present to cause severe illness or death.
The most commonly available radioactive materials used in the production of dirty bombs are alpha emitters, including radium, radon, thorium and uranium. Because alpha particles are charged and relatively heavy, they interact intensely with atoms in materials they encounter, giving up their energy over a very short range. In air, their travel distances are limited to no more than a few centimeters. Alpha particles are easily shielded against and can be stopped by a single sheet of paper. Since alpha particles cannot penetrate even the dead layer of skin on a human body, they do not present a hazard from exposure external to the body. However, due to the very large number of ionizations they produce in a very short distance, alpha emitters can present a serious hazard when they are in close proximity to cells and tissues such as the lung. If inhaled, alpha emitters can cause severe internal damage.
Weapons-grade plutonium or uranium, as well as spent nuclear fuel, would be the most deadly, but are also the hardest to obtain due to strict security guidelines that have been established to protect this material at storage and use sites, such nuclear power plants.
A dirty bomb is not a weapon of mass destruction. A detonation would create psychological, not physical, harm through mass panic and terror. It is unlikely that the radioactive material contained in a dirty bomb would kill anyone. Rather, the material would be dispersed into the air and reduced to relatively low concentrations, resulting in low doses to people exposed. The contamination from the radioactive material of a dirty bomb would be based on the size of the explosion, the type of material, the quantity of the material that was used and the weather. Use of a dirty bomb could result in radioactive contamination of an area of a city, up to several city blocks, with low levels of contamination that would require cleanup. Cleanup of the contamination could be costly (conceivably running into the millions) and take weeks to months to complete, rendering affected areas unusable and causing extensive economic damage.