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Thread: Whole-shop Air Filtration Units Bad?

  1. #1
    Join Date
    Mar 2005
    Ontario, Canada

    Whole-shop Air Filtration Units Bad?

    I was discussing the idea of making a whole-shop air filtration unit with the Oneida rep and engineer. They were very adamant that this was *not* a good idea. The following is an article forwarded to me in support of this argument....

    Steve Aiken
    Belleville, Ontario

    Overhead Shop Air Cleaners Can Increase Airborne Health Hazards in the Woodshop

    The ubiquitous air cleaners that hang on shop ceilings do not improve shop air quality. A scientific look at how they work and the percent of fine material actually filtered indicate that in the best case they do not improve shop air or in the worst-case scenario increase the fine airborne particulate in suspension. Recently a national wood working magazine published 3<SUP>rd</SUP> party filter efficiency tests of these units using a 1-100 micron test material dust. The results were misunderstood.

    The test data actually presents a strong argument as to the ineffectiveness of these units. ASHRAE and other recognized tests use a test powder between 0.3 - 10 microns in size. The ASHRAE test measures the filter efficiency by measuring and counting all the particles that migrate through filter. It is the 1-10 micron particle size range industrial hygienists consider the most damaging to human health. This size has the ability to lodge into the deepest recesses of the lung, and is very difficult for the body to excrete. It is also the predominant size range floating for hours in your shop air. The test results indicate that even the best machine tested did not filter the finest and most lung-damaging material. If a one-micron particle is the size of a " BB" then a 100-micron is a bowling ball. The best filtering machine tested allowed 0.1 grams out of 80 grams through the filter. This might sound good on the surface but assuming a fairly even size distribution of the test dust (no size break down was given) the 0.1 grams represents the entire weight of all of the 1- 15-micron dust in the sample. Actually, calculating by average weights of the size distribution, it is possible that none of the material in the 1-15 micron range was filtered on the most efficient unit tested. It is precisely this range that constitutes the worst health hazard. A 100-micron particle, assuming stoke equivalent or roughly spherical, is one million times heavier than a one-micron particle, and has a settling velocity of about 10 inches a second, about the same as a falling cotton ball. Large particles this size are far too heavy to float up to the ceiling where the units are typically positioned.

    Another misconception in the same article is the idea that the proper size air cleaner will filter all the air in your shop in 6 minutes. The example given: a 15 x 20 x 8 ft shop contains 2,400 cubic feet of air, divide this by 6 to get the minimum CFM required, which would be a 400 CFM air unit. Ventilation engineers use a factor for incomplete mixing which in this case would be a factor of somewhere between 7- 10. In other words, based on this formula the real length of time to filter all the air in the shop would be between 42 to 60 minutes, and this would only be valid if the offending external source of dust emission is shut down. Even assuming an ideal 100% filtration the removal process is much slower than the dust generation process. Meanwhile, you are in the shop breathing contaminated air.

    Commonly woodworkers will comment, "when I look in the filter I see trapped dust, isn't it beneficial to collect at least some dust?" In this case the answer is no. Not with the machines tested here. The dust accumulated on the filter is only a fraction of the total dust drawn into the unit. The remaining dust is passed through the filter and exhausted. The circulating fan keeps this dust suspended and aloft in the air you are breathing. The dust on that filter is evidence that too much dust is in your shop air to begin with. Quoting American Governmental Industrial Hygienists, "When toxic contaminants are evolved in the workroom, recirculation must be avoided." This is why these units are not used in industry.

    A properly designed dust collection system lowers airborne particulate to safer levels no higher than 5mg/M3. It does this by entraining the dust with air near the source of dust emission and then filtering the air to near 100%. Air quality testing in industry is performed routinely where workers wear dust monitors on their collar. It is not uncommon for well-designed dust collection systems to lower airborne dust levels by 10 to 30 times over uncontrolled environments. Get the facts and protect your health.


    Robert Witter

    Oneida Air Systems Inc

  2. #2
    Join Date
    Mar 2003
    SE PA - Central Bucks County
    I have always found this to be more of a mixed message. It is true that air filtration systems have limited or no value in improving air really do need to catch it at the source. But I also love my air filter for it's ability to help keep my shop a little cleaner from settling know, the stuff that doesn't get caught when you are hand-sanding or even spraying water-bourne finishes. It does bubkus for your health, but the layer of fines on those shelves is at least a little bit less. Mine also serves to circulate air around when I'm heating in the cold months for a more comfortable shop in a faster time frame.

    The most expensive tool is the one you buy "cheaply" and often...

  3. #3
    Join Date
    Feb 2003
    Huntsville, AL (The Sun and Fun Capital of The South)
    I can tell you that the air in my shop can be clouded with dust and within minutes of turning on my air cleaner it is very noticeably cleaner.
    If less particles are visable, it must be better air.

    The source of the article is not identified - only the fact that Oneida is using it to promote their own products. So that makes me somewhat suspicious.

    Like Jim said: "But I also love my air filter for it's ability to help keep my shop a little cleaner from settling know, the stuff that doesn't get caught when you are hand-sanding or even spraying water-bourne finishes."
    "If you believe in yourself and have dedication and pride - and never quit, you'll be a winner. The price of victory is high - but so are the rewards" - - Coach Paul "Bear" Bryant
    Ken Salisbury Passed away on May 1st, 2008 and will forever be in our hearts.

  4. #4
    I agree with Jim. When I am sanding, I still wear a filter mask. Also, I don't have any facts to back it up, but I used to use my sander with no vacuum attached and without the dust bag. (I know...I know!!). From the time I quit sanding until the time the air "looked" clean was about 5-6 minutes. Now this was sanding at the other side of the shop from the air filter.

    I use my filter to keep my shop cleaner.
    Jeff Sudmeier

    "It's not the quality of the tool being used, it's the skills of the craftsman using the tool that really matter. Unfortunately, I don't have high quality in either"

  5. #5
    Join Date
    Feb 2003
    Charlottesville, VA

    So, should one leave their Onieda cyclone running...

    ... with an open blast gate to let it filter the air. While its a little noisy, it certainly is moving lots of CFM and exhasting (Onieda certified) filtered air.

  6. #6
    I've heard other people say the same thing about air filters and wondered why there would be less dust on everything if the air cleaner wasn't doing something. I guess the best bet would be to have it exhaust outside, that way the fine dust wouldn't be recirculated. BTW aren't 1 micron dust filters available for the air cleaners?

  7. #7
    Join Date
    Feb 2003
    Southern MD
    I always use mine when finishing. The amount of dust nibs at the end is definitely less. As already mentioned, the dust layer is also noticeably less after a day with the router or cutting up a bunch of MDF on the table saw.

    Jay St. Peter

  8. #8
    Join Date
    Jul 2004
    Tacoma, WA
    When I first saw this it seemed to me that it was comparing the efficacy of a whole shop filter and a point source cyclone system. I would agree that a shop filter does not replace a cyclone but it just doesn't make sense that it wouldn't help along with a cyclone. Not everything can be hooked to my cyclone. This includes some sanders, routing operations, drill press, etc.

  9. #9
    Join Date
    May 2004
    Meridianville, AL


    I too have read these writeup's from the DC companies and have gotten the impression that they are talking using air filters alone to take care of the dust problem. While it makes perfect sense that removing the dust at it's source is the best way to keep the dust down I also believe that one can't have enough of "all types" of dust removal. One of the themes that they keep repeating is how a ceiling mounted unit only gets what is at the upper levels and not at the floor level, that's why I use a large 220v whole house blower in a 25x25 box that sits on the floor using 3 normal filters as prefilters and a hepa type final filter. In the other corner of the shop is a ceiling mounted 16x20 filter using a 1750cfm attic fan, a regular filter and a filtrete final. I now have air movement at all levels of my shop and I use a 2 HP DC that vents to the outside of my shop. In addition if I'm cutting or sanding I wear a quality face mask (not one of those paper types). I have done all of this for around $500 bucks versus the $1500 to 3000 that one of the DC companies wanted to sell me for my 24x20 shop. DC companies are in the business to sell you DC systems and anything that will cut into that is going to be painted as "bad". So bottom line is you have to have a comprehensive plan to attack the dust threat, limiting your self to just one way or the other is short cutting your health.

    Last edited by Jack Wood; 05-03-2005 at 7:28 AM.

  10. #10
    Join Date
    Mar 2005
    Houston, TX
    I read their point as the air filters are causing the fine particles to remain suspended in the air rather than just settle if you were not using anything. This implies that if you do not use their product than you are better off not using anything; clearly this also implies that using their competitors products is worse than doing nothing.

    One flaw in their logic, even given that suspended dust is a bad thing, don't most people save the nastiest dust jobs for last then turn on the filter with a 4 hour timer then leave the shop?

  11. #11
    Join Date
    Apr 2005
    Colfax, Iowa

    Interesting discussion

    I had intended to built a ceiling mount filter.
    But when I looked at the size of the salvaged Furnace Blower my brother-in-law gave me, It was large enough I could not figure out where to hang it out of the way (8ft ceiling).
    So changed my plans and am now working on a downdraft sanding table/air filter.
    My intent was to run it anytime I was doing any process that generated dust.
    Is there some reason that would be a bad practice?

  12. #12
    Join Date
    Dec 2004
    Herndon, VA
    I would agree, this is a mixed message. I have a great deal of respect for Oneida. They produce high quality products and provide great customer service and advice. I have both a DC and AFS. I always run the DC when tools are on and run the AFS when I 'm doing fine dust emitting work like sanding or sawing. My feeling is every little bit counts. I always run the AFS for 2 hours after I leave the shop to clean things up and help reduce the amount of dust going to the rest of the house.

    I think as long as you wear a NIOSH approved mask and use dust collection you are probably well protected. Also consider that most of us are hobbyest and only get in the shop a few hours a week.

    My 2 cents....


  13. #13
    Join Date
    Mar 2005
    Ontario, Canada
    Good discussion and good points. I've read and re-read this article a good number of times. The article seems to be making three main points: (1) there were some problems with the magazine article; (2) even though it looks cleaner, the air may be harmful; and (3) Oneida cyclone DC are best.

    I am put off by the initial dogmatism,
    The ubiquitous air cleaners that hang on shop ceilings do not improve shop air quality. A scientific look at how they work and the percent of fine material actually filtered indicate that in the best case they do not improve shop air or in the worst-case scenario increase the fine airborne particulate in suspension.
    Personally, I'd like a little more detail on this "scientific look."

    I've got to hunt down this mystery magazine article to better understand the context of this article. Does anyone know what magazine article they're talking about?

    So, I've got a little more research to do on this.


    It is an important point that we don't see the most dangerous of the dust. The biggest thing I got out of this article is the need to continue to use a respirator.

    I'm still thinking about making a whole-shop filtration unit from an old HVAC blower. But I'm going to think a little longer before I start building. The Oneida rep and engineer recommended opening a couple of blastgates and running the DC for 15 to 20 minutes to clean the air. The advantage of this practice is that the air is filtered to 99.9% of all particles between 0.2 and 2.0 microns. The question I have about this method is 'how do I know that all the air in the shop gets filtered?'

    The question I have about shop-made filtration units is 'how effective are the filters?' Many of the shop-made units I've seen use furnace filters. What are the specifications on these filters? Is there any independent testing done on them? Etc, etc. If you can find a filter medium that is a good as what Oneida uses in their DC systems, then how can a shop-made filter be bad?

  14. #14
    Join Date
    Jan 2005
    St. Ignatius, MT
    First, I make every effort to collect dust at it's source. However, I have been operating a shop built (with a furnace blower) air filter system for several years. Everyone who walks into my shop comments on the lack of dust. My air filter system has four filters in it, in progressing orders of 'fineness'. I change outer two filters every other month or so, sometimes more often. My impression is that my unit moves a lot more air than most of the commercially available ones designed for home shops. It actually moves paper and tools on the wall behind it, from the circulation of the air in front of it. Without resorting to a hugely expensive scientific experiment, I'm a total believer in my dust system.
    Alan & Lynette Mikkelsen, Mountain View Farm Gardens & Fine Woodworking, St. Ignatius, MT. Visitors Welcome!

  15. #15
    Onedia saying to leave blast gates open is ludicris! There you are just hoping that you are circulating air! If you set up a air filter with good filters, you will filter to the same amount you get with your cyclone.

    I can kinda buy that they say particles are in suspension longer, okay the air is moving instead of still. However, them saying no don't use something designed as an air filter, use our cyclone to do it, just goes to show you the point of their article. (To sell more cyclones!)
    Jeff Sudmeier

    "It's not the quality of the tool being used, it's the skills of the craftsman using the tool that really matter. Unfortunately, I don't have high quality in either"

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