If a so-called "dirty bomb" were detonated in the heart of an American city, it could cause casualties on the magnitude of the 1995 Oklahoma City bomb which killed 168 people but only in the immediate blast area, experts say.
Yet such an attack could also spark a much wider public panic due in part to what some observers see as an overly dramatic fear of radiation that's crept into the national zeitgeist in the past half century.
With US officials saying this week that they thwarted a dirty-bomb attack by arresting alleged Al Qaeda member Jose Padilla, there's renewed concern about such devices. Yet outside the immediate bomb-blast area, experts say a dirty bomb's impact might consist of a statistically insignificant rise in long-term health troubles, depressed real estate prices, and the need to demolish scores of buildings. That's a far cry from a mushroom cloud of destruction erupting over Baltimore in Tom Clancy's "The Sum of All Fears," or the ghostly desolation portrayed in TV's 1983 nuclear-war drama "The Day After."
Yet the public largely doesn't distinguish between these scenarios, observers say. "In a perfect world," people would understand that even after a dirty-bomb attack, the most dangerous part of being in a big city "would be getting mugged, not being exposed to radiation," says Michael Levi of the Federation of American Scientists here.
If a dirty bomb were set off, the casualties would be caused, experts say, by the "bomb" part of the device the fertilizer or other explosive material shattering windows or shredding concrete.
The "dirty" element a radioactive substance strapped onto the explosive material would simply make the area much more complicated to work in and the debris much harder to clean up.
A bomb's actual impact would depend greatly on everything from prevailing winds to the radioactivity of the device. Spent nuclear fuel rods, for instance, would be very dangerous but also extremely difficult for terrorists to obtain and handle.
One recent mock attack envisioned a medium-size dirty bomb being placed in a stolen yellow school bus and detonated outside the Smithsonian's Air & Space Museum. In the scenario, conducted by the Center for Strategic and International Studies here, the "ground zero" area was radioactive enough that emergency workers could only be present for one hour before receiving the maximum allowable radiation dose. Yet outside this several-block area, the scenario involved minimal radiation and nearly undetectable health effects.
Still, the scenario envisioned the bomb sparking a chaotic mass evacuation and a shortage of emergency workers willing to assist at the bomb site. In fact, experts say public panic can actually increase destruction. For instance, harried drivers crashing into each other during an evacuation could boost fatalities.
The more rational approach for people nearby, experts say, would be to take a shower to clean off any radioactive dust and to begin an orderly exit from the city. Staying in the city for hours or days would have little if any health impact, they say.
The scenario's biggest hypothetical impact was economic. Real-estate values plummeted. Tourists stopped coming to the nation's capital. Scores of buildings had to be demolished, since decontamination is so costly.
Indeed, dirty bombs are "weapons of mass disruption not mass destruction," says Matthew Bunn, a researcher at Harvard University. But "the disruption can be pretty massive."
Yet the distinction is lost on many Americans. Instead, images of mushroom clouds begin dancing in peoples' heads. Terrorists count on using this linkage to magnify an attack's impact, says Philip Anderson, a CSIS expert. "It is the fear factor that we've got to get our arms around."
Staff writer Francine Kiefer contributed to this story.