Most sandblast cabinets use either silica sand, glass beads (which are actually identical to silica sand in chemical composition), walnut hulls, plastic, and silicon carbide. When you sandblast with silica materials the silica dust comes out the back of the cabinet unless you have a dust extractor system. This dust is really fine silica. It can be so fine that it settles in the lungs and is so fine that the body can not get rid of it. Silica in the lungs causes silicosis. You get steadily more short of breath, the lungs scar and eventually you cannot breathe at all. Silicosis is the chief real cause of black lung. You can see them under the microscope. Little clear needles. The carbon in coal is actually pretty nontoxic. The silica in the coal is what does the nasty. The body does a good job of getting rid of large particles, but below a certain size the body can not get rid of them.
I sandblast and use either an air fed hood or a cabinet with a dust extractor. In addition to using the dust extractor, I have a second hose that leads from the outlet side of the dust extractor to the outside of the building.
If you run a hose from the cabinet or room to the outside, you are pumping that fine dust into the atmosphere. That dust can be thick enough to be visible. You run the risk of having a visit from the EPA or the county health department. The filter bag on a shop vac is not fine enough to trap the fine dust from a blast cabinet. Clean your shop and use a shop vac for a while and I think you will find that there is a fine dust on all flat horizontal surfaces such as machine ways.