Predicting the effects of a dirty bomb is difficult because of the many variable factors, such as the radioactive materials used and weather.
The Federation of American Scientists' examples illustrated here assume calm weather.
In the Washington scenario a fraction of a gram of caesium-137 exploded with 10lbs of TNT would leave people in a five-block radius facing a one in 1,000 chance of cancer if they stayed put.
In the New York example nine grams of cobalt-60, with 10lbs of TNT, exploded at the tip of Manhattan would render the whole borough uninhabitable with a 1 in 100 cancer risk, assuming no decontamination and continued residence for 40 years.
Both scenarios would require decontamination of wide areas, which would be difficult if the materials used chemically bonded to building materials.