I think that depends a lot on what density of particles is required, and how they are distributed in the atmosphere. Intuitively actually turning the whole atmo

I think that depends a lot on what density of particles is required, and how they are distributed in the atmosphere. Intuitively actually turning the whole atmosphere into something like smog would also be pretty difficult, so I would not be surprised if they could have an effect while being pretty diffuse, and that would tend to reduce the health effects, perhaps to a trivial level.