Around four years ago, a team of security professionals from a Washington think tank played a game of Let's Pretend. The group parked a yellow, 66-passenger school bus on Independence Avenue, on the National Capitol Mall, near an overhead rail line. Congress was in session that morning; people were flocking to national monuments. The weather was also nice that day - sunny, warm, with low humidity - and there was a steady breeze blowing in from the southeast. All the necessary conditions were there. The team from the Center for Strategic and International Studies (CSIS) then imagined that their bus was packed with 4,000 pounds of TNT and a pound and a half of radioactive cesium-137. What would happen to Washington, DC, they wondered, if the bus went ka-boom? Interestingly, most of the pretend casualties were caused by the regular old TNT and flying debris; the damage from the cesium was negligible by comparison. However, notes Phil Anderson, the man who headed the CSIS team, the fact that radiation was involved in the blast "had an enormous psychological impact on the public." The so-called dirty bomb proved to be a weapon of mass hysteria. Radiation is "something unseen and mysterious for most people," says Ivan Oelrich, a nuclear physicist with the Federation of Atomic Scientists. Even if the radiation levels are not particularly high, they will not return to their homes or places of business. Unfortunately, terrorists know this as well and while a nuclear weapon is beyond their grasp, radioactive materials are not. In fact, the modern world is filled with the stuff. Irradiation plants, for instance, use cobalt-60 to kill harmful bacteria in food, while radioisotope thermoelectric generators, which provide electricity to remote areas, contain strontium-90. Even household smoke detectors contain trace amounts of americium-241. One attractive candidate for a dirty bomb is cesium-137, used by hospitals for anti-cancer treatment. How dangerous is cesium? In 1987, four people from Goiania, Brazil lost their lives when they stumbled upon a radiotherapy machine at a junkyard and played with the glowing, blue dust found inside. BOTH CHECHEN terrorists and al-Qaida have built dirty bombs. In 1995, Chechen rebels notified a Russian television station that they buried cesium in Moscow's Ismailovsky Park. This time, they didn't explode anything, but three years later, the Russian-backed Chechen Security Service found a container filled with radioactive materials attached to a mine hidden near a railway line. As far as al-Qaida goes, Muhammad Atef's predecessor, Abu-Ubaydah al-Banshiri, drowned in Lake Victoria in 1996 while on a mission to procure material for a dirty bomb. And, documents captured in 2003 in Afghanistan show, the Taliban helped al-Qaida succeed in building a dirty bomb. Fortunately, even if al-Qaida or another terrorist group was to detonate a dirty bomb in a city and radioactive material would be dispersed over a wide area, it is unlikely that it would be enough to do any real harm. "The so-called safety limits for radiation exposure are exceedingly conservative," explains Fred Singer, physicist and president of the Arlington-based Science and Environmental Policy Project. "You can exceed them by a factor of 10 or 100 and not suffer any damage." In fact, a dangerous concentration of radiation would melt most containers - but try telling that to the public right after a non-conventional terrorist attack. Citizens would clamor for a very expensive clean-up, draining the public coffers; businesses would come to a halt. So how does one overcome the fear? "The answer is more education," Anderson insists.