What Is It?

This type of bomb is the most accessible nuclear device for terrorists.

It's a regular explosive laced with lower-grade radioactive material. A "dirty bomb" is a conventional explosive such as dynamite packaged with radioactive material that scatters when the bomb goes off. It kills or injures through the initial blast of the conventional explosive and by airborne radiation and contamination, hence the term "dirty." Such bombs can be miniature devices or as big as a truck bomb. It's not much more difficult to make a dirty bomb than it is to make a conventional bomb.

A dirty bomb isn't a nuclear bomb because nuclear weapons involve a complex nuclear-fission reaction and are thousands of times more devastating.

As far as fatalities, one example given — the detonation of a truck bomb containing 100 pounds of one-year-old spent nuclear fuel — the actual acute physical health threat might be confined to a radius of a few city blocks, plus areas under a narrow wind-borne cloud.

The bomb's destructive power depends on the amount, type and size of conventional explosives and radioactive material used.

How Is It Spread?

Depending on the sophistication of the bomb, wind conditions, and the speed with which the area of the attack was evacuated, the number of deaths and injuries from a dirty bomb explosion might not be substantially greater than from a conventional bomb explosion.

Three types of radiation-induced injury can occur: external irradiation, contamination with radioactive materials and incorporation of radioactive material into body cells, tissues or organs. These types can happen in combination with each other and can be complicated by illness.

What Are the Symptoms of Exposure?

Some effects may not be seen for years after exposure to radiation. They can range from mild effects, such as skin reddening, to serious effects such as cancer and death. The symptoms are determined by the amount of radiation absorbed by the body, type of radiation, route of exposure and length of time a person has been exposed.

Acute radiation syndrome (ARS) — radiation sickness — is usually caused when a person gets a high dose of radiation in mere minutes. Symptoms of ARS include nausea, vomiting and diarrhea; later, bone marrow depletion may lead to weight loss, loss of appetite, flu-like symptoms, infection and bleeding. For those who do survive, recovery could take a few weeks to two years.

How Is It Treated?

People exposed to radiation should remove their clothes as soon as possible and place them in a sealed container or bag. Taking a shower and thoroughly washing the body is recommended, to remove much of the radiation.

Taking potassium iodide can protect a person's thyroid gland from exposure to radioactive iodine only. It must be taken prior to exposure or immediately after exposure in order to be effective. Hospitals will take blood tests on victims and monitor radiation levels to determine what other steps to take.

Who Has It?

It was reported in March 2002 that the Bush administration believes that Al Qaeda probably was in possession of often-stolen radioactive contaminants such as strontium 90 and cesium 137, which could be used to make a dirty bomb.

In May 2002, the United States arrested an alleged Al Qaeda terrorist for plotting to build and use a dirty bomb. Also, according to a U.N. report, Iraq tested a one-ton radiological bomb in 1987 but gave up on the idea because the radiation levels it generated were not deadly enough.

The IAEA notes that virtually every country has radioactive substances that could be used to make dirty bombs, and some countries don't guard those materials very well. Experts are particularly concerned about the security of nuclear facilities in Pakistan, India and other developing countries. But reports have also cited weak protection of spent fuel at U.S. nuclear facilities.