And the beauty of bio and dirty bombs is that the psychological value is so high that the weapon effectiveness can be nearly nil.
It simply has to be good enough to set off the radiation detectors. The panic and 24 hour news cycle will do the rest.
Just like the Cuban Missile crisis.
Soviets had no intention of actually firing the weapons just set them up to create mass panic in the US as people fled the area which could be hit.
Completely overwhelm the system to take care of the refugees.
Same with dirty bombs. You would have a 100 million or more fleeing the cities.