Given the LD50 of Fentanyl, 118 lbs can poison a water reservoir and cannot be filtered out.  If delivered like anthrax, it can fill the HVAC system of a building, plane, etc. It also was used by Russian Spetsnaz when the stormed the opera house seized by Chechnyan terrorists: https://www.theguardian.com/world/2002/oct/30/russia