If you are doing this daily in a high volume lab environment, I might consider a respirator, but probably not.
Well, depending on what one considers high-volume, what they call "engineering controls" are preferred. These would be ventilation systems, or maybe fume hoods. Plus protective gear for the mix operator, but it's seen more as a secondary protective stage.
FWIW, in the US an agency known as OSHA has oversight over this sort of thing (in the so-called workplace, and only in the workplace). Something not well-known is that it's not permissible to just stick some employee into a respirator. My former QC department, or later a spin-off of same used to oversee this sort of thing. First thing was a doctor's note that it was permissible for the specific person to actually use a respirator (some people might have a physical problem with the slight suction, or perhaps a claustrophobic reaction to the mask). After this they had to have training on the fitting and use of a respirator, including how to clean and sanitize afterwards. (Fwiw no facial hair allowed in the sealing area.) In places where we didn't think a respirator was necessary we would actually do air sampling - the person doing the work would wear a battery powered "sampler" on their belt; a small tube comes up near their "breathing zone," not unlike those tiny microphones that entertainers sometimes wear. At the end of the day the captured "whatever" is analyzed by a specialist lab, etc.
I dunno how other places would do it, but this was our operation. Same thing in our three "satellite" labs. Including one in Canada where essentially the same laws existed, just under a different authority.
All this not really applicable to a guy at home. Personally I would probably just hold my breath while pouring in the powder, then go to the next room to breathe again. Then wait a while for the airborne dust to settle a bit. Actually, I would probably read the MSDS (material safety data sheet) to see what IT says, although it's probably overkill for an occasional home user.
FWIW a guy named Richard Henry wrote a book about "Controls in Black and White Photography," or something similar. He also had concerns about breathing in chemical dust. His method fir D-76, as I recall, was to make a diagonal cut in the corner of the envelope, then submerge that corner whilst pouring in the powder. So... why not?
Too much info? Yeah, probably. But what the heck.