Washington ? In a “dirty bomb,” terrorists attach radioactive material to run-of-the-mill explosives. The bomb spreads fear and contamination, but doesn’t add much real health risk, experts say.

It’s relatively easy to get the needed radioactive material from medical devices or irradiated food preparation systems.

But a dirty bomb is far different than a nuclear bomb.

When a dirty bomb explodes, the immediate killing comes from the traditional explosive device, not the added radiation. There wouldn’t be radiation illnesses, although hospitals would be flooded with people who think they are sick but aren’t. There may be some long-term increases in cancer deaths over decades, but it wouldn’t be much and may never be noticeable, health experts say.

What would be noticeable and what terrorists count on are lingering contamination, economic cost and fear.

“The impact of a dirty bomb is almost completely psychological, not a health risk,” said University of Georgia toxicologist Cham Dallas, who is studying the health effects of dirty bombs for the Department of Defense. “It’s a very inefficient weapon for hurting people.”

Over the long term, a dirty bomb would add at most one new case of cancer out of every 100 people exposed, according to the Federation of American Scientists, a Washington research and advocacy group.

Unlike a nuclear bomb or a nuclear plant fire like the one at Chernobyl, a dirty bomb will produce radioactive material that’s either too small or too big to stay in people’s lungs, said Dallas, who spent a decade studying the health effects of the 1986 Chernobyl nuclear accident.

Someone can spend hours unprotected in the aftermath before reaching a dangerous dose, but “you wouldn’t want to spend days there,” Dallas said.

The Center for Strategic and International Studies in March conducted a mock dirty bomb attack in Washington, targeting the Smithsonian Air and Space Museum. It found “the impacts of the attacks are far more psychological than they are real real in terms of loss of life,” said CSIS homeland security director Phil Anderson.

Once an area is decontaminated, few people will want to return despite official assurances of safety, said Gary Ackerman, a research associate for the Center for Nonproliferation Studies, a Monterey, Calif., security oriented organization. That fear is going to cost businesses and government lots of money to relocate.