I wear a respirator just about whenever I'm airbrushing indoors. Occasionally if I'm just doing something very quickly with acrylics, I'll leave it off. I also have a homebuilt spray booth made out of a large plastic storage box that vents out the window above my workbench. Woodworking, especially heavy sanding, I'll wear a N95-type dust mask. Even cleaning my garage every summer, when I move the big stuff out, sweep it and then powerwash the floor, I wear a mask now. That was after one time of cleaning, blew my nose afterward and it came out all dark gray-black...from all the dust, road salt, and who knows what else my sweeping put into the air.
Growing up, I built lots of plastic models and never wore a respirator. I've always wondered if all the things I breathed in over the years will some day come back to haunt me.