Apparently a friend of his found him selenium toning entirely maskless & 
issued dire warnings, saying RESPIRATOR, not just dust maks required.
I checked whatshername's Overxposure, finding the only mention of selenium
in any context a warning about irritation by powder, as when mixing
selenium from scratch, and not the direst warning either, though I suspect
that's what friend's friend was speaking from.  What I normally tell
students is simply keep your hands out of it, don't mix it with funny
things, and don't leave to evaporate into your breathosphere...
Guess I'll add that I have found most photo "safety" books safely 
useless, since they treat everything as worst case scenario. Suspect 
that's what friend did here.....But any facts?
Judy