I just found this out not too long ago from watching a BBC documentary called the power of nightmares. I've done some googling today and found something on the US Nuclear Regulatory Commission site saying the same thing.
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Basically, the principal type of dirty bomb, or Radiological Dispersal Device (RDD), combines a conventional explosive, such as dynamite, with radioactive material.
In most instances, the conventional explosive itself would have more immediate lethality than the radioactive material. At the levels created by most probable sources, not enough radiation would be present in a dirty bomb to kill people or cause severe illness. For example, most radioactive material employed in hospitals for diagnosis or treatment of cancer is sufficiently benign that about 100,000 patients a day are released with this material in their bodies.
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http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/dirty-bombs.htmlThe documentary was exceptional, and while it not only dispelled the danger posed by a dirty bomb, it also outlined the rise of the neoconservatives in America and fundamentalist Muslim radicals in Egypt. For your reading pleasure, this is a link to and and excerpt from an article from the UK's Guardian:
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Starting next Wednesday, BBC2 is to broadcast a three-part documentary series that will add further to what could be called the dirty bomb genre. But, as its title suggests, The Power of Nightmares: The Rise of the Politics of Fear takes a different view of the weapon's potential.
"I don't think it would kill anybody," says Dr Theodore Rockwell, an authority on radiation, in an interview for the series. "You'll have trouble finding a serious report that would claim otherwise." The American department of energy, Rockwell continues, has simulated a dirty bomb explosion, "and they calculated that the most exposed individual would get a fairly high dose , not life-threatening." And even this minor threat is open to question. The test assumed that no one fled the explosion for one year. <snip>
http://www.guardian.co.uk/terrorism/story/0,12780,1327904,00.htmlI think that it's both outrageous that there is so much hype regarding the danger of a dirty bomb. It seems to be about as dangerous as a pipe bomb. While there is a real danger of panic resulting from such an attack, I think that this is exacerbated by the media repeatedly reporting on the threats posed by what now seems to me a very ineffective weapon that would likely kill no-one at all.