The Dirt on Wood Dust
When we work with wood using hand or power tools, we create wood chunks, chips, slivers and dust. Few hobbyists realize just how much fine dust is generated and how damaging the inhalation of this fine dust is to our health.
Under high magnification, the structure of wood resembles big bundles of fine glass tubes. This makes sense because wood gets much of its strength from silica, a material commonly used to make glass. The use of hand tool blades, bits, cutters and sandpaper on wood explode these fine tubes creating visible wood debris and huge numbers of very fine dust particles that are invisible to the human eye. For example, the sharpest hand plane can make a perfect spiral shaving with no visible dust; however, a particle meter reads huge amounts of fine particle materials that have been launched into the air. Inhalation of this fine, invisible airborne wood dust not only affects us in the confines of our garage, basement or workshop, but can harm all those close to us, including our pets, since it is quickly and easily transported on our hair, skin, and clothes.
Wood dust poses both particle and chemical risks. Peer reviewed medical studies show that every exposure to fine dust particle material (PM) causes a measurable loss in respiratory capacity. Some of this loss becomes permanent, with greater health damage associated with higher fine dust exposure levels. A Google search on "PM Health Risks" provides a staggering number of references.
The good news is our bodies typically have a significant amount of extra respiratory capacity and the damage per fine dust exposure is so small that unless you have some other reaction, it takes most people years to notice the aggregate health damage. The bad new is this damage causes serious respiratory problems and worsens age related diseases.
Trees naturally produce harsh chemicals to protect themselves from predators. Processed wood carries other man-made chemicals such as pesticides, insecticides, herbicides, finishes, etc. that can be quite toxic. Alone or in combination, these chemicals can cause irritation, sensitization (i.e. worsening allergic reactions), poisoning, and even increase cancer risks. For a listing of woods and their toxicity rating, please view the Wood Toxicity Table found on Bill Pentz's web page.
Minimum Dust Protection
Careful testing with fine particle meters shows we must expend effort to ensure good fine dust collection in our shops. Fortunately, it is relatively easy to protect ourselves from fine dust. Good fine dust protection can be accomplished with a properly fitted, quality dual cartridge respirator mask (such as the 3M 7500 series) and a good commercial fan. Depending on your shop size, adequate air circulation/venting may be achieved by opening your main shop door a few inches while blowing a commercial fan out a side door or window. Test results from hundreds of small shops found that customers achieved excellent protection when they a) donned their respirator masks and adequately vented their work spaces before they started working wood and b) left both their masks and fans on for about (30) minutes after they stopped generating dust. Particle meter testing shows that properly ventilated shops can be well cleared of hazardous fine airborne dust within approximately one half hour.
Fine Dust Collection
As a result of increased understanding of the importance of good air quality, experts have been refining fine dust collection techniques for large facilities since the 1960s. They have tested and refined what techniques are needed for almost every type and size of tool. Because small shops use some of the same tools as those in larger facilities, we can benefit from this same shared information. For good, small shop fine dust collection, we must comprehend that:
- Most tools need ventilation hoods to control and capture the fast moving, dust filled air streams. Dust collection systems move air at about 45 miles per hour; however, most blades, bits, and cutters launch air streams at over 100 miles per hour. Without hoods to provide fine dust control, our fine dust collection efforts are futile.
- The tool (i.e. the source of the dust) must be surrounded by a large, low pressure area with adequate airflow to ensure that normal room air currents don't scatter the fine dust our hoods do not capture. Decades of testing indicate good fine dust collection for most small shop stationary tools require at least 50 feet per minute (FPM) of airspeed out to a radius of approximately 15.15 inches in all directions from the tool dust source, which equates to an airflow of 1000 cubic feet per minute (CFM).
- Once airborne dust is "controlled", it must be transported to a collection point for future disposal. Based on nearly 100 years of industry dust collection experience, we know that if we do not maintain a ducting speed of at least 3800 FPM, plugging of vertical ducts will occur. Ducting airspeeds of less than 2800 FPM will result in dust piles forming in horizontal ducts. Dust piles in ducts can result in fires, ducting damage, and filter damage when airflow is ultimately restored.
Extensive testing of normal dust collection systems, which move airspeeds of 2800 - 3800 FPM, require ducting that is a minimum of seven inches (7") in diameter to carry dust safely. Because air barely compresses at dust collection pressures, any smaller diameter pipe, sharp bend or other obstruction can seriously reduce airflow below what is needed for good fine dust collection. It should be noted that unlike other small shop vendors, Clear Vue Cyclones uses oversized blowers to generate enough extra pressure to provide the required 1000 CFM for good fine dust collection when using 6" diameter ducting; therefore, allowing the use of less expensive and more readily available standard Sewer and Drain PVC.
- The best way to dispose of collected dust is to blow it outside and leave a door or window open in order to allow for return air. When venting outside, return air prevents the possibility of sucking deadly carbon monoxide backward through flues and vents of fired appliances. Large firms have found that infrared heat dishes will keep workers comfortable in the coldest of weather; however, many live and work in areas where heat, humidity or local codes make venting outside illegal or impractical. In these areas, the air must be filtered.
Medical experts suggest that filtering should provide a minimum of 99.9% separation of 0.5 microns and larger particles, but typically recommend the use of HEPA quality filters, which filter off 99.7% of particles sized 0.3 microns and larger. For these recommended filters, industry guidelines suggest that at least one square foot of fine filter area is required for every four cubic feet of air movement. Regardless of the guideline, many filter manufacturers, such as Wynn Environmental, advocate the tighter specification of one square foot of fine filter area for every two cubic feet per minute of airflow. This means that with 1000 CFM airflow, most small shop stationary tools will need about 500 square feet of filter area to prevent rapid filter clogging or wear-out.
Leading firms, which provide air quality advice to large facilities who use traditional cyclone separators, recommend replacing fine filters approximately every three months. The filter life of polyester fiber filters can be extended approximately four fold if they are lightly cleaned whenever they start passing too much fine dust. Workers should use either a pressure gauge or a particle meter to determine when it is time to clean or replace filters.