So why should you listen to my ramblings on dust control and personal protection?  Well I design respiratory protection products for a living, but no reason other than that.  So, I have some knowledge in the field but: Use this advice at your own risk.  I am not a Certified Industrial Hygienist (CIH), but have learned a lot over the years and hope to clear up some of the bunk I hear all the time.  If you run a business and have more detailed questions than you find answered here, you need to seek out a CIH or other safety professional to ensure you are in compliance with federal and state regulations.  These pages are aimed at the hobbyist woodworker, not businesses.

This page contains my ramblings on selected topics on dust & dust collection.
Info on respiratory protection can be found here.
Also click here for Folklore's DustFAQ.

Pet Peeve:  "5 (or 20 or 3...) Micron Dust Collector Bags"

I see this a lot and it really bugs me.  Manufacturers call a dust collector filter bag a "3 micron bag" or "20 micron bag".  These terms are meaningless marketing mumbo-jumbo and here's why.  All filters have characteristic filtration curves that vary with particle size and face velocity (the air speed through the filter).  This curve typically looks like the figure below.  One quick definition - Filter Efficiency is given as a percentage.  This percentage indicates what proportion of particles in the incoming airstream are removed.  The opposite of efficiency is penetration (the percent that gets through).  The terms are often used interchangeably.

In this graph, you will notice that filter efficiency does not steadily increase or decrease with particle size, but rather a minimum filter efficiency is reached at some particle size, typically around 0.3 microns.  Most people assume that efficiency decreases steadily as particle size decreases.  The reason it does not is that there are 3 or 4 different mechanisms by which particles are filtered from an airstream and each of them comes into play to a greater or lesser extent depending on particle size.  The minimum point of the curve is called the "most penetrating particle size".  There is a lot more technical detail behind this, but most people are probably asleep already so I'll skip it, but there is one more key point to raise.  This curve is shown for one given face velocity.  If the air is moving faster or slower, the efficiency values will be different (filters are less efficient at higher face velocities) although the basic shape of the curve will remain similar to the above.  This may lead you to one quick conclusion:  An easy way to improve the filtration of your system is to increase the filter surface area as the same amount of air will be pushed through a larger area, thus lowering the face velocity and improving efficiency.

So how does this apply to a "3 micron bag"?  Well it doesn't, or rather trying to imply that a bag filters out 3 micron particles is silly.  Every filter will remove particles of a given size - the question is what proportion of them? Even a 2x4 in an air duct will remove some 3 micron particles, but not very many.  Without data that show the efficiency of the filter against particles of a given size at a given face velocity, you really know nothing about the filter performance.  Furthermore, you don't know how Brand A's "3 micron bag" compares to Brand B's "3 micron bag" since the term is meaningless.  The one exception I've seen to this problem is Oneida Air Systems.  They will actually give you real info on their filter bags and are the only one I've seen that will, but then they are almost an industrial supplier so the expectations their customers have are higher than your average woodworker hobbyist.  If you visited my woodworking buddy Brook's page, you will note his dust collection system is partly from Oneida.  He was quite happy with them.

So the question you might have now is, "What bag should I buy for my DC?"  Unfortunately, I can't give you a good answer to this because the manufacturers don't share the data that you need to make the decision.  The best you can do is know the types of filter cloth that are used and get a bag made from the better materials.  The least efficient bags use canvas (either cotton or polyester, or a blend).  The better material is felted polyester.  Oneida claims their polyester felt is further enhanced by being "flame singed" on the inside to reduce caking of the filter.  I haven't seen another bag that claims to be singed.  As a filter bag is used, more and more of the filter gets plugged with particles ("cakes").  As the filter cakes,  the pressure drop of the filter increases, greatly decreasing the amount of air the system can push through the filters, so it is important to keep your filter from caking for best performance.  Somewhat ironically, the filter efficiency actually improves as the caked filter is actually better at trapping small particles.  This can be offset by the increased pressure in the filter forcing dust past seals and through small leaks in the filter as the air seeks the path of least resistance so you may not get any net improvement in dust trapping.  The real problem with filter caking is loss of airflow.  If it goes too far, you will lose enough flow so that the air speed in the ducts drops below the limit for keeping dust entrained in the airstream.

What about those ceiling-mounted air filtration units?

These have become quite popular in the past couple of years as awareness of the hazards of wood dust has increased.  In reality they are a mixed bag:  they do some some good, but mostly they don't address the real problem.  The basic tenet of Industrial Hygiene (the science of health and safety in the workplace) regarding dusts is: "Catch it at the source."  If you can't catch it all at the source, then you should be wearing respiratory protection.  A unit that filters the ambient air will remove particles in the air, of course, but you are simultaneously breathing that same contaminated air, if you aren't wearing a respirator.

So do they do any good?  Some, but not what you may expect.  There is a small benefit to filtering the ambient air that you are breathing, but that isn't the biggest plus that these units have going for them.  One of the biggest benefits you will see when using a filtration unit like this is that everything in your shop won't be coated with as much dust the day after you were slicing and dicing wood.  This is nice to help clear the air when you are not in the shop.  If you do not have a separate finishing room, then running a filter unit for a few hours prior to finishing will help keep dust out of your finish.  A good procedure would be to sweep up and clean the shop to get the dust up (cover your project with a sheet when doing this), run the filter unit, come back and wipe down your project again, let the filter run some more, and then start finishing.  This is especially important if you are using a slow-curing finish like traditional varnish or polyurethane where dust can easily imbed in the film and cause you lots of grief.

In a nutshell, not a lot of benefit to your personal safety and protection, but it has some side benefits on general shop dustiness/cleanliness.

Onward to topics on respiratory protection ... Or to the DustFaq