A dirty bomb is an explosive designed to spread dangerous radioactive material over a wide area. When people hear "bomb" and "radioactive" in the ­same sentence, their minds jump to nuclear war pretty quickly. But it turns out that a dirty bomb's primary destructive power would probably be panic, not radiation damage.

A dirty bomb is much closer in power to an ordinary explosive than it is to the widespread destructive force of a nuclear bomb. But the fear of contamination could be debilitating, in the same way that 2001's anthrax scare in the United States terrorized much of the American populace, even though only a few people were infected.

In this article, we'll find out what dirty bombs are and what they do. We'll also explore what might happen if one actually went off in a public area, and consider some of the consequences of this sort of attack.

­Conceptually, a dirty bomb (or radiological dispersion bomb) is a very simple device. It's a conventional explosive, such as TNT (trinitrotoluene), packaged with radioactive material. It's a lot cruder and cheaper than a nuclear bomb, and it's also a lot less effective. But it does have the combination of explosive de­struction and radiation damage.

High explosives inflict damage with rapidly expanding, very hot gas. The basic idea of a dirty bomb is to use the gas expansion as a means of propelling radioactive material over a wide area rather than as a destructive force in its own right. When the explosive goes off, the radioactive material spreads in a sort of dust cloud, carried by the wind, that reaches a wider area than the explosion itself.

The long-term destructive force of the bomb would be ionizing radiation from the radioactive material. Ionizing radiation, which includes alpha particles, beta particles, gamma rays and X-rays, is radiation that has enough energy to knock an orbital electron off of an atom. Losing an electron throws off the balance between the atom's positively charged protons and negatively charged electrons, giving the atom a net electrical charge (the atom becomes an ion). The free electron may collide with other atoms to create more ions. (See How Atoms Work for more information on subatomic particles.)

If this happens in a person's body, the ion can cause a lot of serious problems, because an ion's electrical charge may lead to unnatural chemical reactions inside cells. Among other things, the charge can break DNA chains. A cell with a broken strand of DNA will either die or the DNA will develop a mutation. If a lot of cells die, the body can develop various diseases. If the DNA mutates, a cell may become cancerous, and this cancer may spread. Ionization radiation may also cause cells to malfunction, resulting in a wide variety of symptoms collectively referred to as radiation sickness. Radiation sickness can be deadly, but people can survive it, particularly if they receive a bone marrow transplant.

In a dirty bomb, the ionizing radiation would come from radioactive isotopes (also called radioisotopes). Radioactive isotopes are simply atoms that decay over time. In other words, the arrangement of protons, neutrons and electrons that make up the atom gradually changes, forming different atoms. This radioactive decay releases a lot of energy in the form of ionizing radiation. (See How Nuclear Radiation Works for details on radiation and radioactive isotopes.)

We're exposed to small doses of ionizing radiation all the time -- it comes from outer space, it comes from natural radioactive isotopes, it comes from X-ray machines. This radiation can and does cause cancer, but the risk is relatively low because you only encounter it in very small doses.

A dirty bomb would boost the radiation level above normal levels, increasing the risk of cancer and radiation sickness to some degree. Most likely, it wouldn't kill many people right away, but it could possibly kill people years down the road. ­