"Good question. Anyone?"
If it was chemical agent the effects would more than likely be close to the source and downwind. A small package couldn't hold that much and if the explosive is used is of an appropriate quantity, it can likely destroy any agent in the blast. Also, only a non-persistent agent (Sarin) would pose much downwind hazard, and it is rendered harmless due to air column mixing and evaporation fairly quickly.
Radiological is another issue, spreads it out, not necessarily a killing item, but could inflict some minor radiation injuries, and a mess to clean up.
Bio agent and toxins, fireball destroys a large percentage, anthrax spores is a different story, could be transported downwind and in the local area.
It seems to me that the package should be contained in something where tests could be run on chemical contents after it is exploded.
I'm no EOD person, but I understand they do this stuff under things like a Kevlar containment tent, especially in a populated area and when dealing with unknowns.
Thanks again to you and others who offered explanations.