A dirty bomb is an explosive device which is used to spread radioactive material. Because people tend to fear radiation, thanks to a widespread cultural knowledge about the effects of radiation exposure, a dirty bomb could cause widespread fear and panic in addition to generating a big mess to clean up. For this reason, some governments believe that dirty bombs could potentially be used by terrorist organizations as a tool for disrupting a society and/or government.
It is extremely important to differentiate a dirty bomb from an atomic bomb. Although dirty bombs do contain nuclear material, they are in no way as powerful or as devastating as a nuclear bomb. In fact, the explosives in a dirty bomb would be far more dangerous than the radioactive material, with most scenarios attributing the bulk of casualties to people in close proximity to the explosion, rather than to radiation damage. While a dirty bomb is certainly nothing to treat lightly, the threat of a nuclear bomb is far more severe.
The effect of a dirty bomb varies, depending on how it is constructed, the type of materials involved, and the predominant conditions on the day when the device is detonated. For example, severe winds could carry the radioactive materials in the bomb across a wide swatch of area, generating more or a mess to clean up. Or, a large amount of explosives could be used, creating chaos at the site of detonation while also ensuring a wide dispersal.
In studies conducted on dirty bombs, most governments have included that the risk of radiation damage from a dirty bomb is actually pretty minimal. However, the detonation of such a device could be a powerful tool for creating fear in the populace, as many people have a deep-seated fear of radiation. The fear and panic created by a dirty bomb could create chaos, along with demands for an intensive cleanup of the site, which could be extremely costly.
Dirty bombs are one form of Radiological Dispersal Device (RDD). Concern about such devices has led most governments to tightly control radioactive material, with the goal of ensuring that it does not fall into the wrong hands. However, a fair amount of technical know-how is required to build a dirty bomb, and as a result these devices are much less of a threat than improvised explosive devices (IEDs), suicide bombs, and other more crude forms of terrorism which are easy to plan, construct, and execute.
If you are exposed to a dirty bomb, you should shower, discard clothing worn at the time of the explosion, and seek medical attention. In the event that a dirty bomb explodes in a major urban area, such medical attention will probably be readily available, as disaster response teams will undoubtedly descend en masse to the site.