A dirty bomb is a radiological weapon but unlike a nuclear bomb, its purpose is to contaminate rather than destroy. It uses normal explosives to disperse radioactive materials in the local environment, creating a hazard to health that could last for years unless cleaned up. The relative ease of making such a bomb means it is a potent terrorist weapon but Horizon's investigation shows that the risk to health from most such devices need not be great. It also underlines the need for governments to act to secure radioactive sources from falling into criminal hands. Horizon deliberately avoids outlining the production process in any detail. Horizon publishes the results of specially commissioned research, modelling two possible dirty bomb scenarios: attacks on either London or Washington DC. The main conclusion is that the health risks from a dirty bomb explosion are localised to people who are close to the incident or are in contact with the contamination. Although the modelled attack scenarios could have wide-ranging economic repercussions, the majority of the population of either capital city would have only a negligible increase in their risk of developing cancer.