: The most harmful particles are the smallest ones, in the range of 1 micron in
: size. They get deep into your lungs and stay there. Most paper respirators
: filter out what are commmonly referred to as "nuisance
: particles", which are the larger ones that you can easily see. If
: you're going to do a lot of sanding, invest in a good respirator with
: replaceable dust cartridges. Most will also have organic vapor cartridges
: available for them so you can use one respirator for sanding, finishing,
: gluing, etc.

Ditto what Brian said. The finer the particle, the deeper into your lung it gets and the more of a problem it is. Your body is pretty good at filtering out the big chunks, but you don't really want the microscopic stuff in alveoli. Sanding with an ROS produces lots of very fine dust. Furthermore, if you use a shop vac to collect the dust but don't have a HEPA filter ($30) on it, your vacuum is literally a "dust pump" blasting the fine stuff back into the air.

For vapours (varnish & other chemicals) the masks designed to remove particulate will not help. You will need something with carbon to remove the chemicals from the air. Usually this is a mask with removable cartridges. Keep the cartridges (or the whole mask) in a plastic ziplock when not in use or it will use up the carbon much faster. You will also need to replace the cartridge every once in a while because the carbon does get used up (the binding surfaces all get occupied). Also, if you can smell through your mask, it isn't working.

Cheers & have fun making dust!
Bryan S.