Dirty bombs (high explosives coated with radioactive material) are an ideal weapon for terrorists. Not because they would kill more people than chemical or biological weapons, but because anything associated with the word, "radioactive" is more terrifying to people. Terrorists are more interested in scaring you than killing you.

What makes dirty bombs particularly troublesome is that radioactivity, like fire, is something we deal with on a daily basis. For example, there is a US government standard of 5,000 mrem (a measurement of radiation) a year for those working with nuclear material. People cleaning up after a dirty bomb would be monitored (usually via a measuring device carried by each person), and once they hit 5,000 mrem (for the last year), they could not work in a highly radioactive area until the next year began. Actually, the workers would also have do limit how many mrem they were exposed to in an hour or day, for it is now known that radiation is much less harmful if exposure is spread out, rather than absorbed in a short period. 

The whole concept of how much radiation people acquire naturally is still not fully understood. As more people are monitored over a longer time, the picture is becoming more clear. Two trends are apparent; people get more natural and lifestyle radiation than was previously thought, and the amount of radiation needed to cause cancer or other health problems appears to depend more on how much radiation is received in a short period of time.

For a long time, it was thought that the average annual radiation exposure in the U.S. was about 160 mrem per person. Then we came to know more about radon (a naturally occurring radioactive gas that is present everywhere, but in very dense concentrations in some areas.) This, and greater amounts of lifestyle radiation, has increased the average to about 360 mrem a year. This is considered way below the level at which damage is done. 

A lot of mrem in a short time will kill you. When the Russian Chernobyl nuclear power plant had a fire and explosion in 1986, 134 firefighters and plant workers got from 70,000 to 1,340,000 mrem over a week or so. Of these, 28 soon died from radiation sickness and the rest are expected to have shorter life spans as a result. Hundreds of thousands of people got doses of several thousand mrem over a longer period, causing the cancer rate to increase ten times, especially among those who were young children in 1986. Chernobyl was the first time since 1945 (Hiroshima and Nagasaki) that there large numbers of people exposed to a wide range of radiation doses. Unlike 1945, there was more, and better, radiation measuring equipment in 1986. Much more was known about radiation, and the Chernobyl radiation victims are being carefully monitored (if not adequately treated) over the years. This is important, as some of the studies of Japanese radiation victims were perplexing. For example, overall, radiation victims seem to be living longer than those not exposed to radiation. This may be because radiation victims got better media care right after the war, or for other, as yet not understood, reasons.

Lifestyle radiation has become a major source of exposure. This is the radiation that we can avoid. Much we cannot, like the 30 mrem a year we get from the sun, or the 40 mrem a year we get from what we eat and drink. Another 25 mrem or so come from building materials, particularly stone. But if you choose to live inside a stone building, add another 50 mrem a year. Want, or have to, fly 100,000 miles a year? That's another 67 mrem. A chest x-ray is about 5 mrem. Other types of x-rays or medical tests using radioactive material can give you hundreds of mrem (or more) a year. When these levels get that high, the doctors are supposed to take the higher radiation levels into account. If the tests are a matter of life and death, then the decision is clear. But at other times, it's more of a life style decision. Some parts of the country have a lot more radon, and if you dont ventilate your basement continuously, the radon gas will build up and you will pick up hundreds (or even thousands) of additional mrem each year. 

Which brings us back to dirty bombs. The easiest to steal radioactive material is the low level stuff found in hospitals, labs, universities and factories (that use nuclear material as part of their manufacturing process.) The heavy duty stuff (plutonium and uranium) is much more heavily guarded. It's much more likely that low level would be used and it would be vaporized by an explosion and spread over a wide area if there was enough wind blowing. The material would also disperse as it spread from the spot where the bomb went off. Thus hundreds, or thousands, of acres might be contaminated. 

The media won't zero in on the degree of contamination, because headlines screaming "Downtown is a Radioactive Wasteland" are too tempting (and lucrative). There won't be much of a wasteland, as  the "hottest" area might be generating 50 mrem an hour, while at the fringes of the hot zone, it's one mrem an hour or less. Now you don't want to live in an area that is giving you an extra one mrem an hour. Even if you just work there, that's an extra 2,000 or so mrem a year. You have to clean the place up. But a lot of that can be done with high pressure water (which flushes the radioactive material into the sewer system, or catch basins, depending on what the stuff is). Where the terrorists win big time is when the public health people have a hard time convincing a terrified public that an additional .001 mrem an hour is "acceptable" (it is, but not if you got a real bad case of radiation phobia.)

The U.S. and Russian government have gotten together and actually tested dirty bombs (apparently in some remote part of Russia). The idea was to get a better idea about just what kind of radiation could be spread using various types of radioactive material and what clean up methods work best. The results have been classified (lest the terrorists obtain useful information), but the rumors are that there were no surprising discoveries. However, to deal with public fears over dirty bombs, there is a case to be made about being more forthright in explaining exactly what they are, what they can do and how the clean up will proceed. Waiting until a dirty bomb goes off to share this information just gives the terrorists an other advantage. Terrorist love ignorant and uninformed victims. Makes it much easier to terrorize them. And that's what terrorists do.