You could, but you’d have to have a lot of it blown airborne. you’d have to have it blown in your face to realistically inhale any of it.

Anthrax and other effective mail-delivered substances were dangerous since they were much more airborne or were hard to properly ensure is cleaned off your person. Fentanyl won’t be ingested if it gets on your clothes or a trace remains on your hands, but invisible, long-lasting bacterium spores? Lethal.