The oneida points are good ones and should be taken into account. Hoewever, I think there are still advatages to air filtering. FWIW here are my thoughts on the issues.

Valid points for the Oneida statement:

  1. Collect as much as you can at the source. The less dust that even hit the air the better.
  2. A shop air filtration unit keeps dust suspended, so there's more to breath. True, and somtimes if you happen to be between the source ot the dust and the intake, it can actually be drawing all of this right past you. You may be filtering as much dust as the filration unit! (Except that you are of course wearing a mask)
  3. Many commercial units filter larger particles and actually allow the finer (more dangerous stuff) through and recirculate it. So you can end up swirling the worst particles around and around. Incidentally don't rely on visibility. the stuff you nedd to woryy about, you can't see.
  4. Turn over rate are innaccurate as you are not intaking equally from the whole shop. You are mostly grabbing the air near the intake. Points further will get there, but it takes time, so Oneida factoring is relavent and 40- 60 minutes is often more assurate than the 4-6 claimed.
Still I am a much in favor of air filtration. So the key may be to take those point into account and solve them in your filtration system:

Have intakes from as many points as possible. If buying or making the box type this may mean using two (2 small are better than one big) I read a good article which showed that one along each of opposing longer walls (assuming a rectangular shop) facing opposite directions creates a circle of air movement. Ideally they would be place 2/3 distans along the wall (intake toward the 2/3, output toward the 1/3). This kind of turns your shop into a bigger, slower version of a cyclone. The turnover is greatly improved as this little tornado forces particles to the outside where the filters are, so you get the dust to the filter pretty quick.

If you can build you own system; an old furnace fan (free) and ducting allow you to put intakes around the shop and one or more outputs. Now you're talking 6-8 min. turnover times.

The filters supplied are mediocre to keep the CFM numbers up. Again if you can build your own, just plan a good HEPA like filter into it. Upgrade filters on the commercial units to near HEPA levels.

Realize that air filtration systems are related more to Cleaning the Shop than Protecting your Lungs. I run my filtration system most often as I'm done working, and let it run (on a timer) for an hour or so after I leave. I come back to a nice clean shop with little if any dust settled on the floor, benches, and other surfaces. I will run it during esp. dusty operations like sanding, but I'm wearing a mask at the time too.

So I think that there are ways to solve the issues of air filtration and I find it a valuable addition to my shop, when done in this manner.

Remember no matter what else you do a good mask/respirator is the single best protection you can get ( and its the cheapest). I know its kind of a pain, but get a comfortable one and wear it as much as you can at least.