I used milky spore on a windy day and didn't use any respiratory protection. I had a tiny touch of cold symptoms for a few days after. It was not at all serious, but I did worry a bit.
So I guess It might be best to use it when the soil is physically damp, and the air is still. And to apply it so that it doesn't float through the air much. Maybe use a cheap respirator while spreading it around.
