A: None of the polishing compounds, inhaled, are good for you. The silica based ones may be worse, but in general, you want to not be breathing lots of any of this stuff. You nose actually does block a fair bit. That’s part of its job, after all, to filter the air you breathe. But that doesn’t affect what you inhale through your mouth, and some will still get past your nose. So then: first, make sure that whatever mask you use actually fits. There are a number of types. Any of the N95 masks are capable of filtering these materials, but you need it to fit well, and apparently, the ones you’re using don’t. The simple surgical doctors’ masks often aren’t well sealed, being more intended to stop what you might cough out from getting away. The 3M N95 particulate masks come in several styles, some with added valves to make breathing a bit easier and reduce things like fogging your glasses. But try several types to find one that fits. They fit different people’s faces differently, so I don’t know that any one single type would fit everyone.

Also, seriously consider getting a polishing setup equipped with a dust collector and a face shield. The shield (those simple plexiglass sheets sitting between your face and the wheel on many better quality machines) stop a lot of the compound from getting anywhere near your face in the first place, as well as improving the air flow from the suction fans. If you cannot afford a commercially-made polishing machine with its dust-collecting fans, etc, you can build something almost as effective with plywood, some inventiveness, and a shop-vac for the suction. Get a suitable HEPA filter for the shop vac so it actually stops the fine stuff instead of just blowing it back into the room.

Far better than figuring what mask fits you best is keeping the compound from ever getting that close to your face in the first place. That’s the reason for the proper polishing machine setups, be they commercial units or home-made. If, when you’re done polishing, you look in the mirror and find your face covered in compound, then you need to find a mask that fits better, and should consider what you can do to reduce that exposure.

As to actual safety of the compounds, it’s not like they’re instantly toxic. They’re not. Most are pretty benign. But that is just on your skin. None of these dusts are good to breathe, even if they happen to be something your lungs can eliminate over time. Some, like the silica-based tripoli and white diamond, might offer some of the risks of crystalline silica flour, such as is found in casting investment. Depending on the nature of the material, sometimes the stuff occurs as rather sharp tiny (invisible) crystals which the lungs cannot get rid of. The result, after long-term chronic exposure, is called silicosis. Contracting it does usually require chronic exposure over time, rather than a little bit once in a while. But still, it’s a risk to avoid. Check the MSDS sheets about the polishing compounds you use for specific information on each. But as I said, rather than trying to figure out which are safe to breathe, instead try to arrange it so you’re not breathing any of them in on any sort of regular basis.

Oh, and in addition to looking in the mirror at your face, look at the inside of the mask when you’re done working with it. It should remain clean. If it’s not, it’s not fitting well enough.