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