While fiberglass dust is nasty stuff, and should be vacuumed out of the way whenever possible, I can't find any authorities that will come out and say that it is actually "carcinogenic". There seems to be a lot of "waffling" saying that is it "possibly" or "potentially" carcinogenic, etc. And examples of listing it, and then de-listing it, etc. I get the sense that much of the concern is based on previous experience with asbestos fibers, but of course, being man-made, fiberglass fibers are orders of magnitude larger than natural asbestos fibers and don't appear have an equivalent method of affecting lung tissue, etc. as asbestos does.
I have on order a CNC machine kit from
http://pdjinc.com/ and, to be sure, I ordered it with the (optional) dust boot to connect to the shop vacuum. Wearing safety eye protection is mandatory, as far as I am concerned, whenever using power tools, and many hand tools as well. And I will certainly wear a dust mask if the vacuum can't keep up with that fine dust. But fiberglass dust doesn't make my top-10 list of possible environmental hazards, at least in my world. I wouldn't sprinkle it on my popcorn, but I wouldn't run to the emergency room if I inhaled a lung-full of it, either.
This thread makes me want to try direct routing PC boards when I get my machine running.