My wife mentioned this to me. I didn’t argue with her, but I wonder what might be something that would do much damage released from that high of an altitude. It would be so dispersed by the time it hit ground.
Radiation? I imagine much more was released from a single test in Nevada in the 50’s than could be carried in a ballon.
A virus? It would be not viable by the time it hit ground.
I suppose there are things, perhaps, but it seems unlikely.
A payload of tiny insect or bird sized drones, released a very wide intervals to prevent notice. Drones with who knows what purpose.