Half nuclear weapon, half conventional explosive, a dirty bomb is cause for concern because it could potentially spread radioactive material over a broad region. But dirty bombs aren't as scary as you might think.
Basically, a small amount of radioactive material is surrounding by a conventional explosive, such as TNT. When the bomb detonates, the radioactive material becomes airborne and scatters across the surrounding area.
A dirty bomb is not "nuclear" in the sense of an atomic or hydrogen bomb, or even a nuclear power plant. There is no fission (splitting of the atomic nucleus) or fusion (combining atomic nuclei). Instead, highly unstable versions of some radioactive elements decay rapidly, emitting rays of energy, such as gamma rays that damage living cells, in the process.
It's the radiation dosage that determines how much damage is likely to be done. The Hiroshima and Nagasaki survivors of nuclear bombs were exposed to enormous doses of radiation -- exactly how much depended on how far they were from the center of the blast. In contrast, small amounts of radiation reach us every day from the Sun, and we are sometimes exposed to carefully calculated doses of radiation when we get an X-ray or a radiation treatment.
Dirty bombs, while dangerous, are unlikely to emit radiation in highly concentrated amounts. That's because the bigger the conventional bomb that distributes the radioactive material, the better it will spread. More people will be exposed to a lower dose of radiation in a larger explosion; a smaller explosion would expose a few people to a very high dose. It's still cause for concern, but would not necessarily lead to a sharp increase in cancer deaths, for example.
The most likely source of radioactive material for a dirty bomb would be the radioactive isotopes used to sterilize food and medical equipment, or to treat cancer. The most highly radioactive are cobalt-60 and cesium-137. But handling these materials may also kill the bomb maker. Unshielded cesium-137, for example, could kill the handler in minutes unless it is shielded with lead. This type of bomb would be large and unwieldy, and easily detectable by security systems.
The American Meteorological Society contributed to the information contained in the TV portion of this report.