Some see only a "minuscule" rise in cancer rates, while others predict that huge sections of New York or Washington would become uninhabitable if such a bomb were ever to go off.
All the experts stress that a "dirty bomb" is not the same as a nuclear weapon, which generates intense heat and radiation from splitting atoms, according to a statement from Rob Fanney and Jim Tinsley of defense watchdog Jane's Information Group. A dirty bomb packs radioactive material inside or around conventional explosives, which are then detonated to spread the radioactive material.
The radiation wouldn't immediately kill, Naval War College professor William Martel said. "But it'd create huge amounts of terror, havoc, and panic."
The most likely radioactive element in a dirty bomb is cesium-137, according to Phil Anderson, a senior fellow at the Center for Strategic and International Studies. And the "consensus government view," according to a March report in The Washington Post, is that al-Qaida "has probably acquired" the isotope, which has a half-life of 30 years.
Cesium-137 is used to treat cancer and to maintain accurate atomic clocks. And it's created as a byproduct of nuclear reaction -- the splitting of uranium in a nuclear power plant, for example.
As cesium-137 "cools" from its radioactive to its normal state, the isotope emits gamma radiation, waves of ultra-high electromagnetic energy. These rays, while not as toxic as the heavier, alpha particle emitted by uranium, travel further, and are extremely difficult to contain. Only concrete, steel or lead can keep gamma radiation in check.
What's worse, cesium is the most "reactive" metal there is -- in nature, cesium's always found combined with another element. So the isotope becomes easily attached to roofing materials, concrete, and soil, said Fritz Steinhausler, who led the International Atomic Energy Agency's environmental assessment of the disaster at Chernobyl.
"The Russians tried to clean it up for years, and they eventually gave up. It just wasn't economically viable," said Steinhausler, who's currently a physics professor and visiting scholar at Stanford University's Center for International Security and Cooperation.
In Goiania, Brazil, four people died and more than 34,000 people had to be individually screened for contamination after a man in 1987 found an abandoned medical device filled with cesium-137 in a junkyard.
That's because cesium interacts disturbingly well with muscle tissue because of its chemical similarity to potassium, which muscles need to flex.
Fortunately, the body is used to processing these kind of chemicals, and excretes half of the cesium it absorbs within 100 days. (In contrast, radioactive strontium-90, similar to calcium, is absorbed into bone, and can take 30 years for the body to get rid of half.) But the absorbed cesium "would nevertheless cause a radiation dose, potentially increasing the risk for cancer," Steinhausler said.
The risk is actually pretty minimal, replied Steve Koonin, a physics professor at the California Institute of Technology.
"Long exposure to low-level gamma radiation, if you do the numbers, produces a miniscule increase in cancer rates -- one extra cancer per 100,000 people," he said.
Members of the Federation of American Scientists paint a much darker picture.
If a relatively tiny "dirty bomb" -- one containing only ten pounds of TNT and pea-sized amount of cesium-137 -- were detonated in Washington, federation scientists recently told Congress, "The initial passing of the radioactive cloud would be relatively harmless, and no one would have to evacuate immediately."
"However," the scientists continued, "residents of an area of about five city blocks ... would have a one-in-a-thousand chance of getting cancer. A swath about one mile long covering an area of forty city blocks would exceed EPA contamination limits, with remaining residents having a one-in-ten thousand chance of getting cancer. If decontamination were not possible, these areas would have to be abandoned for decades."
In February, a missing medical gauge containing exactly this amount of cesium-137 was discovered in a North Carolina scrap yard. The Nuclear Regulatory Commission said it receives nearly 300 reports of lost or stolen radioactive materials every year.
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