Defusing The Terror

--WHAT IS A "DIRTY BOMB"?

It's a lot less than you might think. In a genuine nuclear bomb, enriched uranium or plutonium is explosively compressed, fracturing atomic bonds and releasing a cataclysmic blast of intense radiation. A dirty bomb is little more than a pipe bomb with radioactive rubbish packed into it.

--DO THE BOMBS WORK?

Not really, at least not in the way most people think. Because a limited amount of low-level radiation is dispersed over a wide area, a fatal dose is very difficult to deliver. Virtually all the fatalities would be caused by the explosion--tragic enough but nothing compared with a nuclear blast. The genius of a dirty bomb is the psychological terror it would trigger in a population conditioned to panic at the mere mention of radiation. The actual danger, however, has been overstated. According to the Federation of American Scientists, fallout from a bomb exploding in New York City that contained a 12-in. pencil-shaped rod of cobalt (like those used in food-irradiation machines) might increase the long-term risk of death from cancer in Manhattan by 1 in 100. This may seem a scary number until you consider that you face roughly a 1 in 4 risk of dying from some form of cancer anyway. Essentially, the danger jumps from a 25% risk to 26%. Over most of the contaminated area, the increased cancer risk would be lower still, ranging from 1 in 1,000 to 1 in 10,000.

--IS THERE A LOT OF RADIOACTIVE MATERIAL OUT THERE?

Yes and no. More than 20,000 licensees in the U.S. control millions of individual radiation sources--most of which are designed for use in industrial and medical equipment. The most highly radioactive material, however, is hidden deep inside the machinery, heavily shielded by lead and not something you could slip under a coat and walk off with. There is plenty of radioactive material that is more accessible, but it is less toxic and would pose less of a risk if it were stolen and planted in a bomb. Nuclear waste from military and power-plant operations is another source of worry, but that material is kept fiercely guarded, never more so than since Sept. 11.

--ARE THERE PROTECTIVE STEPS I CAN TAKE?

Potassium-iodide pills are becoming to dirty bombs what Cipro was to anthrax. Potassium iodide can help prevent thyroid cancer following exposure to a nuclear explosion, by saturating the gland with benign iodine, preventing the thyroid from soaking up radioactive iodine, I-131, released by the blast. But that won't help in the case of a dirty bomb, as a homemade explosive is unlikely to contain radioactive iodine. The half-life of the I-131 isotope is only eight days, making it a poor choice for a weapon that counts on radioactivity for its effectiveness. If a terrorist obtained some, most of it would be gone by the time a bomb could be assembled and detonated.

--WHAT SHOULD I DO AFTER A BOMB GOES OFF?

Move away from the blast, staying upwind if possible. In a car, keep the windows and vents closed. If your home is near the blast, close the windows, remove your clothing and leave it outside. Shower thoroughly. Don't go to a hospital unless you were near the blast and were injured or directly exposed to radioactivity. If you have inhaled or ingested radioactive material, doctors may prescribe treatment to help eliminate the contamination from your system.