Americium is pretty stuffty as far as fissionable material goes. It's generally safe to handle if you do not ingest absolutely any of it.
Well the point of a dirty bomb is to just spread radioactive material over an area. Its more of an aerial denial and fear weapon. Hense very low-grade dirty bomb, because of the low radiation levels.
You'd probably do more damage by putting a stink bomb in a ventilation system and spraying black licorice odor all over the place.