I got the idea reading Glen Fryxell's e-book. He mentions walnut sawdust specifically. I burn walnut and black locust, also known for its resistance to bugs and rot, and the chemical properties of it's wood. I don't huff the smoke. Arsenic is toxic too and is in a few casting alloys. If the ventilation is good, why would walnut shells be worse than used motor oil or paraffin or pine rosin or any number of other weird things people routinely use to flux? Personally, I like the no-stink, low-smoke powder flux, but the advantages of sawdust in breaking down tin oxides got my attention. Never would have considered it otherwise, nor any other oil, wax, rosin, pitch, etc. flux, because of the smoke. Casting may produce fumes, but not much smoke aside from fluxing/smelting.
Essay W