Not that I am any expert in this, but I have worked with experts in the field of environmental air quallity, specializing in closed air / life support systems. (Submarine, aerospace etc...), and I saw an article I wish like crazy I could find now.... That listed the paticle sizes between 1 and 2.5 microns as being the hazzards, the sub micron stuff being absorbed into the body, and flushed out through the normal filtering mechanisms in the body, the liver, kidneys etc...

I work in a server farm / cube farm environment, and I would almost be willing to bet I should be wearing a respirator at the office too... I am pretty sure there are things like Asbestos, and ultra fine silicon powder (sub micron sized) blowing through our air systems...