Lots of bad fumes and dust.

Can’t recall where I saw it, but I have run across a welding helmet that doubled as a respirator for exactly those conditions. Looked kinda like a full-face respirator only with a permanent clear plastic front and a welding mask that was in front of that and could be flipped up. Capable of working with beards, positive-pressure so no shroud needed over the nose and mouth that would otherwise muffle the voice.

Now granted: stupidly expensive. But the filter was on the helmet, not on the back or the waist (no hoses to get in the way). And it had a normal power-tool battery slot with adapters for the common ones like Milwaukee or Bosch.