You need to wall off the workshop from everything else (this includes HVAC ducts) AND put in an area filter. In my basement, I have 1/2" sheetrock on the outside of the ship studs, fiberglass batting to fill in the open area between the top of the walls and the joists. Put in a door (without sweep) and a rug as an entrance mat just inside the door to the shop. I also recently bought a Delta 3 speed area filter unit that filters out to
1 micron. I wish I had done this 15 years ago when I first moved in.
I did not have much dust leaking out of the shop due to the fiberglass batting, but the shop air was certainly filled with lots of dust. Have been using a face respirator (more than a paper dust mask) to help. After the Delta filter installation, I really don't need the respirator much anymore, but still use it because I am used to it and I know there are still lots of particles floating around just after using power tools and sanding. R