[Image: Human Respiratory System]Introduction
Four critical factors may influence the effect of airborne particulate matter on the health of individuals exposed to sawdust (Lee 1998).
- dust composition
- size of the particles
- duration of exposure time
- airborne dust concentration
All of these four factors are interrelated in such a way that no one factor can be considered independently of the others.
Dust Composition
Dust composition refers to the type of dust, e.g. organic or inorganic (Alpaugh 1988), or to its health effects. Many hardwood dusts are considered sensitizers, i.e. they make persons hypersensitive or reactive to an antigen, which stimulates the production of antibodies, especially by repeated exposure (Grantham 1992).
The Size of the Particles
Particle size is critical in determining where particulates will settle in the lungs. Larger particles will settle in the bronchi and the bronchioles and will not tend to penetrate to the smaller airways found in the alveolar region (Figure 1). These larger particles are termed inspirable particles. The smaller-sized particles that can penetrate the gas exchange region of the lungs, the alveolus, are termed respirable particles. All dust particles (<50 μm) are inspirable but are classified as being thoracic if they measure between 10 and 4.0 μm and respirable if they are smaller than 4.0 μm.
Duration of Exposure Time
The duration of exposure to humans may be acute, i.e. minutes or hours, but usually no longer than a day or two at the very most; or it may be chronic, in that the duration of the exposure may be measured in months, years or over a full working lifetime (Lee 1998).
Airborne Concentration of Dust
The concentration of dust to which a person is exposed is also critical to the impact on the health of that person. This is measured in the breathing zone of the person, which is an imaginary hemisphere of approximately 30 cm, extending in front of the face and measured from the midpoint of an imaginary line joining the ears.
The concentration of dust to which a person is exposed is measured in the breathing zone of the worker. The dust might consist of airborne contaminants or be consistently present in different exposure concentrations. An airborne contaminant is a potentially harmful substance that is present in an unnaturally high concentration and to which workers may be exposed in their working environment.
An exposure standard represents an airborne concentration of a particular substance in the worker's breathing zone. According to current knowledge (OSHA), such exposure should not cause adverse health effects nor cause undue discomfort to nearly all exposed workers. The exposure standard can be of three forms: time-weighted average (TWA), peak or short-term exposure limit (STEL).
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Exposure standards - average: Where a "peak" or STEL is not specified for an exposure standard, and provided that the eight-hour TWA exposure standard is not exceeded, short-term exposures should not exceed three times the TWA exposure standard for more than a total of 30 minutes per eight-hour working day. Under no circumstances should the short-term values exceed five times the exposure standard.
- Exposure standard - peak: A maximum or peak airborne concentration of a particular substance is determined over the shortest analytically practicable period of time which does not exceed 15 minutes.
A recent report examined the respiratory health of wood processing industry employees (Glindmeyer et al. 2005) by analyzing, over time, the effects of the nature of the dust in question, the duration of exposure time, and the airborne concentration of the dust in the breathing zone of the exposed person. Results of this exhaustive study showed that there were no statistically significant adverse effects to any wood solid (e.g. sawdust) exposure.
Objective
The objective of this study was to determine the smallest size wood dust particles produced in a hardwood sawmill and correlate those findings with the impact those particles might have on the human breathing system.
Materials and Methods
To begin the process of determining the smallest size sawdust particles produced in a hardwood sawmill, one pound samples was collected from 5 workstations in a local, mixed hardwood mill that produces pallet parts, crossties and grade lumber. Table 1 describes the source of the sawdust for each workstation.
Table 1. Source of sawdust for each of the workstations in study sawmill.
| Workstation |
Sawdust Source |
| W1 |
Sawdust was collected from general dust pile outside of mill (i.e. from all sawdust emitting sources) |
| W2 |
Sawdust was collected at the main saw (5/16-inch kerf circle saw) |
| W3 |
Sawdust was collected from a 10-inch table saw in workshop |
| W4 |
Sawdust was collected from a 2-inch-wide thin kerf (<1/8-inch) ripping band saw |
| W5 |
Sawdust was collected from an 18-inch trim saw (where lumber is crosscut to length) |
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Table 1 indicates the source of sawdust for each workstation. W1 is a combination of all the sawdust produced from each workstation. The sawdust from W2 comes from the main saw, i.e. where each log is processed. This workstation produces the greatest amount of sawdust in the mill. W3 was used as a control to compare sawdust from industrial machinery with sawdust from machinery found in a woodshop. W4 sawdust came from a thin kerf bandsaw that should generate a lesser sawdust volume because of its geometry. The last workstation (W5) was the trim saw, which crosscuts each board to the appropriate length.
Each sample was stored in a sealed plastic bag, and the bags were identified with the location of origin. To simplify handling and to reduce any shrinking and swelling due to moisture change, all samples were air dried to reach equilibrium moisture content with ambient air.
Particle size was measured using a Bausch & Lomb microscope fitted with a Filar micrometer eyepiece and using the 10x magnifying lens. The Filar micrometer eyepiece is a Kellner-type eyepiece that has a measuring line controlled by a micrometer sc[Image: Sawdust Macroscopic Particles]rew knob. The micrometer was calibrated using a precise reference ruler (Fig. 2). After careful calibration, it was determined that each unit on the micrometer equaled 8.403 μm.
The light source was directed onto the samples because this gave the highest contrast and made the edges easier to define. Light from the reflective mirror created shadows and made it difficult to differentiate small sawdust particles from lens impurities.
Results and Discussion
Sawdust sub-samples were collected by dipping a glass slide in the sample bag and then tapping the slide to remove larger particles. Figure 3 shows micrometer view of wood particles. Each line represents 0.1 mm.
Although the image in Figure 3 is somewhat out of focus, one can see measuring lines and the white circles surround smallest particles.
Table 2 indicates the particle size of twelve (12) sawdust samples per workstation.
Workstation/
Sample No. |
W1 |
W2 |
W3 |
W4 |
W5 |
| 1 |
33.6 |
57.2 |
33.6 |
33.6 |
16.8 |
| 2 |
16.8 |
37.8 |
12.6 |
16.8 |
25.2 |
| 3 |
33.6 |
54.6 |
16.8 |
16.8 |
21.0 |
| 4 |
21.0 |
54.0 |
25.2 |
25.2 |
21.0 |
| 5 |
16.8 |
53.0 |
29.4 |
33.6 |
12.6 |
| 6 |
33.6 |
25.2 |
21.0 |
25.2 |
25.2 |
| 7 |
33.6 |
33.6 |
16.8 |
25.2 |
33.6 |
| 8 |
16.8 |
50.4 |
21.0 |
23.1 |
16.8 |
| 9 |
16.8 |
25.2 |
21.0 |
16.8 |
25.2 |
| 10 |
12.6 |
29.4 |
25.2 |
16.8 |
16.8 |
| 11 |
16.8 |
21.0 |
29.4 |
16.8 |
21.0 |
| 12 |
12.6 |
21.0 |
21.0 |
16.8 |
33.6 |
| Average |
22.1 |
42.7 |
22.8 |
22.2 |
22.4 |
| Stdev |
8.8 |
14.4 |
6.1 |
6.5 |
6.5 |
It is interesting to note that the average minimum wood particle sizes are similar for all workstations except W2. The head rig (W2) generated the largest average sawdust particle size. This is noteworthy because the largest volume of sawdust is generated at that workstation.
The minimum size observed was 12.6 μm, which indicates that wood particle sizes coming from a sawmill are fairly large. The particles may settle in the bronchi and the bronchioles but will not tend to penetrate to the smaller airways found in the alveolar region (EPA 2006). Particles this large (>12.6 μm) also indicate that the sawdust will not travel very far because of their size and related weight.
Conclusion
Average minimum wood particle size produced at the hardwo[Image: Sawdust Particles Viewed in Microscopic]od sawmill was about 22 μm at all the workstations except at the head rig, which generated 43 μm-sized particles. The minimum wood particle size was measured at 12.6 μm, which in terms of dust particles is fairly large. Because particles that are large are not respirable, they are unlikely to negatively impact the health of workers exposed to them. These findings should be reassuring to hardwood sawmill operators who are faced with environmental pressure concerning air quality.
References
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Alpaugh, E.L. (1988). "Particulates," Chapter 7 of Fundamentals of Industrial Hygiene, 3rd Edition, Ed. Barbara A. Plog, Publ. National Safety Council, USA.
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American Conference of Industrial Hygienists (1997) 1997 TLVs® and BEIs®.
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Glindmeyer, H.W., R.J. Rando, J.J. Lefante, L. Freyder, J.A. Brisolara, and R.N. Jones. (2005). “Respiratory Health Study of the Wood Processing Industry.” Final Report. Tulane University. 39 pages.
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Grantham, D. (1992). "Dusts in the Workplace," Chapter 5 of Occupational Health and Hygiene Guidebook for the WHSO, Publ. D.L. Grantham, January 1992, Brisbane.
- Lee, M. (1998). Introduction To Toxicology. Also, see PDF below.