Guys,
I have an engineer friend who designs dust collectors for wood.
There is some specific information out there very relevant to particulate size.
For a good intro see:
http://billpentz.com/woodworking/cyclone/index.cfm
The summary on the above page is a good start, too large to paste into here, but a fairly quick read
But, IIRC, my friend's information is a "quantum level" better in its scientific and health aspects.
mjb