On Mon, 14 Nov 2005 20:18:37 GMT, ScRaPLeR
wrote:

Reprinted Material. All credit to -Robert Witter
Oneida Air Systems Inc.


What do you guys think. I have one of these systems. Am I hurting
myself and my shop by running it.

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

The ubiquitous air cleaners that hang on shop ceilings don’t 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
don’t improve shop air or in the worst-case scenario increase the fine
airborne particulate in suspension.

Recently a national wood working magazine published 3rd party filter
efficiency tests of these units using a 1-100 micron test material
dust. The results were misunderstood. Actually, the data 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 efficiency of filter by measuring
and counting all the particles that migrate the 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 predominate 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 than 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’s 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.

The second 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 is only 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 woodworker’s 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’s 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.


ScRaPleR


I wear a dust even with both my high effeciecy cyclone dust collector
and an ambient air filter but I feel that my tool and surfaces in the
shop benefit at least a little from the air filter. I also use three
phase filtering on the ambient air system. 1. a 3M type furnace
filter, the 2. 5-20 micron filter then 3. the Hepa sheet and all three
get dirty after about 12 hours of contineous use. So I guess what
still have me puzzled is the fact that I AM capturing a lot of dust to
get out of the shop and is this not a good thing in the long run?