Nuclear "dirty bombs" were first investigated during World War II. American nuclear weapons scientists knew that if they could not create a fission bomb (a huge nuclear explosion), they could spread dangerously radioactive material over a large area by packing the nuclear material around explosives and detonating it. But some simple calculations indicated that such a weapon would not be worth the cost. The radiation would not create enough immediate casualties, and would leave a radioactive mess that would have to be cleaned up. There have been several dirty bombs detonated. In 1958, an American bomber accidentally dropped a nuclear bomb in the backyard of a house in South Carolina. The 400 pounds of explosives went off when the bomb hit the ground, spewing over a hundred pounds of highly radioactive uranium and plutonium. The crater was 60 feet wide and 30 feet deep. The house was destroyed, and six people injured. The clean up took six months (cost was never revealed) and the family paid some $300,000 (in current dollars) in compensation. In 1966, two bombs fell in Palomares, Spain. They did not explode, but did break open, releasing nuclear material. Some 1400 tons of soil and vegetation had to be removed. The people of the town were paid some $5 million (in current dollars) in compensation. It's unclear if there were any long term injuries due to increased radiation. The danger of a dirty nuke bomb is additional cancer caused by increased radiation levels. It is also possible to kill people with large amounts of radiation, but this is difficult to pull off with a dirty bomb. You need something highly radioactive, like Plutonium, and the victims have to stay near the radioactive material long enough to get a fatal dose. The more likely result is increased incidence of cancer from the higher radioactivity levels. But even this is a slight risk. The normal incidence of cancer is 2,000 deaths per 10,000 people. Depending on how radioactive the material is, and how much of it is in the dirty bomb, victims will have an increased chance of cancer ranging from 1 (or less) per 100,000 to five or more per 1,000 people. Airline crews that regularly fly high altitude (transoceanic) routes for years pick up this kind (on the low end)  of additional cancer risk from radiation. But radioactivity scares people, and the effect of any dirty bomb would be more terror than termination. Cleaning up the radioactivity in an urban area (like mid-town Manhattan) would cost billions of dollars and close off the area for months. Thus the disruptive and financial effect of such a dirty bomb would be far higher than its impact on peoples health.