Sure, you won’t keel over dead instantly if you breathe in some sanding dust or the excess spray paint that doesn’t make its way onto your lawn furniture (or nearby parked car). But wearing a proper respirator can add years to your life. To get the full benefit, you’ll need to do a little research to see what’s in the crap to which you’re exposing yourself, and that’s the point where most people just say “screw it” and plow on, sans protection. There’s another option, though: Pick up a cheap-but-decent model like the one above, offered for $17 from TP Tools.
According to TP, the “SAS respirator offers protection against paint mists (lacquers and enamels), dusts, fumes, mists free of oil, and other particulate and organic vapor hazards.” That’s not everything in the world, but it sounds like it’d catch most of the most common stuff around the shop. TP sells replacement pre-filters (for $5), but they categorize the whole thing as disposable.
Then again, if you’re looking for something more permanent, you’ll probably want to do your research and buy a serious mask with cartridges designed to fully protect you from the toxic stuff you plan to use. At around $25 shipped, I’m thinking about just having one of these in the cabinet on hand.