After the first 24 hours, when most of the early fallout has settled, the radiation level decreases roughly in proportion to elapsed time. Reducing exposure by leaving the area or remaining in a fallout shelter will greatly reduce the radiation effects on people
Significant levels of radiation (greater than 200 rads will cause radiation sickness) will extend out much farther than other effects such as blast overpressure. Radioactive contamination will hamper rescue, cleanup and recovery efforts as most of the city will have dangerously high levels of radiation for many weeks to come.
