Dealing with wood dust.

For most types of wood dust, the permissible exposure level allowed by OSHA is 5 mg/[m.sup.3]. Yet depending on the size, not all dust particles are harmful, although some types may be small enough that the body cannot rid itself of the inhaled dust. Big particles are considered to be between 2.5 end 10 microns in size, while small particles are 2.5 microns and smaller. To give you a point of reference, 2.0 microns is 50 times smaller than a typical human hair.

Dust particles 10 microns in diameter and larger are considered to be large enough to either be caught in the nasal passages, or brought back up out of the upper lungs by cilia, the hair-like media in the upper lungs and bronchial passages. Particles 2.5 micron and smaller are believed to settle in the lower area of the lungs. Once there, they may dissolve and go into the blood stream to be filtered by the liver. Some of the particles, however, may not be dissolvable and may stay in the lungs, possibly causing problems such as allergies, respiratory problems, lung diseases and even cancer.

Dust masks work for nuisance dust 10 microns and larger. However, testing has shown that wearing a mask the entire workday can result in a productivity drop of up to 40 percent because the user has to work harder for every breath of air. Thus, dust collection systems often are the preferred method for wood dust filtration and removal.

Central system chip/dust collectors work well and can be hooked directly to the equipment, thereby pulling the dust at the source and preventing it from traveling into the worker's breathing zone. When dust is properly captured at the source, it prevents the 1 to 50 micron particle dust--the sizes that cannot be seen individually by the naked eye--from floating in the air.

Dust collection hoods also help prevent dust from escaping into the breathing zone of the operator. Other tools on the market have a vacuum built in, so when the tool is turned on, the vacuum will turn on also. A portable vacuum source capture system is recommended for use on job sites.

Ceiling mounted air cleaners are good for getting rid of air contaminates after they have passed the operator's breathing zone. However, if overhead collectors are more than 12 feet above the floor, they may lose their effectiveness. In the case where overhead collectors are used, the best results use the filtered exhaust air to push the ambient dirty air toward the next overhead unit. With the use of four big units using the push-pull system, they will keep the area in a 40-foot by 60-foot room fairly clean. For best results, the units need to be approximately 20 to 30 feet apart.

--Excerpted from a presentation by Dennis Rayon, Denray Machine Inc.