Theres plenty of research into this kind of matter. As part of the nuclear weapons development program they deliberately 'fizzled' a number of devices in order to see what would happen in the event of a miss-fire. Smaller scale industrial accidents involving 'dirty bomb grade' material has produced ample evidence to the effectiveness of the various materiel that might comprise a dirty bomb and the biggest of these industrial accidents such as The Chernobyl disaster have produced mountains of data on the long term effects of such contamination and its spread on a near global scale.

Depending on the size and type of material used. A dirty bomb could be anything from a local nuisance to a national disaster. car-bomb packed with mixed nuclear junk, is on the low end of danger & the authorities could contain & remediate within months anything like that. carefuly chosen isotopes ground to borderline areosol size, used via a sophisticated delivery device, you could seriously harm a double digit percentage of an entire state/country.