Dirty bomb - don't panic
I don't want to panic you, but it won't be long before someone explodes a "dirty bomb".
I really mean that I don't want to panic you unduly, because a Radiological Dispersal Device isn't actually significantly any more dangerous than conventional explosives (which admittedly are not exactly "safe".)
An RDD wouldn't cause radiation sickness. The threat would be more long-term. If the area was not decontaminated, the risk of contracting fatal cancer due to long-term exposure would range from the utterly insignificant upwards, depending on the nature of the radioactive material. This would be a major economic problem.
For those not caught in the explosion itself, the personal physical risk in the short-term would come from the havoc caused by mass panic.
So, should the day come when a dirty bomb is used, there's no need to panic unduly. The "unconventional" factors can be dealt with after the attack.