Rochester, NY (March 12, 2002)-The latest post-9/11 disaster scenario making news headlines is the "dirty bomb." The theoretical situation occurs when terrorists get hold of radioactive material from a hospital or food-irradiation plant, attach it to an explosive, and detonate the bomb in an urban area. The explosion spreads the radioactive material all over a city and exposes the population to radiation. Yet according to a health physicist, the biggest health risk from a dirty bomb would not, reassuringly, be cancer, but something more preventable: panic.

A dirty bomb "would probably not lead to many, if any, cancer deaths," says Andrew Karam, radiation safety officer of the University of Rochester in Rochester, NY. But if the public receives unreliable or exaggerated information about dirty bombs, Karam worries that "the use of a radiological weapon would result in many deaths in traffic accidents as people flee the scene, and possibly stress- and anxiety-induced heart attacks."

The radiation dose from a dirty bomb would likely be relatively small, says the Rochester health scientist. Even a potent dirty bomb, consisting of a radioactive cobalt-60 rod used for food irradiation, for example, would deliver an average dose of a few tenths of a rem for people within a half-mile radius, he says. (A rem is a unit of radiation dose.) This compares to the 0.3-0.4 rem average dose per year that a person receives from natural sources, and 5 rem, the typical annual dose limit for nuclear and radiation workers (most radiation workers receive less than 1 rem of exposure annually).

Some recent news accounts have predicted that dirty bombs would cause a small amount of additional cancer cases. However, Karam says these estimates are all based on a faulty assumption.

"They are based on the use of a concept called 'collective dose,' the concept that exposing a large number of people to very low levels of radiation will result in a certain number of cancer deaths," he explains. "By analogy, we can say that throwing one small stone at each of a million people will result in crushing one or two people because the combined weight of all the stones adds up to a ton, which is enough to crush someone."

Karam notes that the Health Physics Society, a professional organization comprised of over 6,000 radiation safety professionals, has advised against calculating risk from exposure to low levels of radiation (less than10 rem). The International Council on Radiation Protection (ICRP) has similarly advised against calculating risk to populations when the highest-exposed person in that population receives a small dose.

"The radiation exposure to people in the wake of a radiological attack is much less than the doses to which both the Health Physics Society and the ICRP refer," Karam says.

Why do these organizations advise against making such estimates for low doses? The only comprehensive data for calculating radiation-based cancer risks comes from the Hiroshima and Nagasaki populations surviving the two atomic bombs dropped by the U.S. These populations were exposed to relatively large radiation doses. The problem occurs when people try to extrapolate the cancer risks from these doses-tens to hundreds of Rems-to the relatively low dose of a dirty bomb.

Extrapolating in this way from high-dose to low-dose assumes that cancer risk is linear-in other words, the lower the dose, the lower the cancer risk-and has no threshold-that is, the risk of cancer can occur no matter how low the radiation dose.

However, researchers have been finding evidence against this linear, no-threshold hypothesis, according to Karam. In fact, some evidence exists that there is a threshold-a minimum radiation dose-for causing cancer, he says.

Government radiation exposure standards, such as those from the Environmental Protection Agency, are currently based on the linear, no-threshold hypothesis, because it is the most pessimistic scenario. Because of these standards, mass evacuations and large cleanup costs would result from a dirty bomb. But in the unfortunate event of a dirty bomb, he says, the biggest enemy is fear. Radiation officers would be able to measure radiation levels fairly quickly and assess the situation. According to all imaginable scenarios, the public would suffer no measurable health risks by taking a few extra moments to evacuate the affected area in an orderly fashion.

"The best way to protect ourselves against radiological terrorism is to make sure the public and emergency responders are provided with the best information," Karam says.

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For more information:

Ben Stein, Inside Science News Service
301-209-3091

Andrew Karam, Rochester Institute of Technology
585-475-1473

Health Physics Society Papers (select "Radiation Risks in Perspective")