Carcinogenicity

Wood dust is known to be a human carcinogen based on sufficient evidence of carcinogenicity from studies in humans.

Properties

Wood is an important worldwide renewable natural resource. Forests cover about one third of the earth’s total land mass (about 3.4 million square kilometers). An estimated 12,000 species of trees each produce a characteristic type of wood, and the species of trees harvested vary considerably among different countries and even among different regions of a country. However, even in countries with high domestic production of wood, some wood may be imported for specific uses, such as furniture production (IARC 1995).

Most of the 12,000 tree species are broad-leaved deciduous trees, or hardwoods, principally angiosperms. Only about 800 species are pines, firs, and other coniferous trees, or softwoods, principally gymnosperms. The terms “hardwood” and “softwood” refer to the species, and not necessarily the hardness of the wood. Although hardwoods generally are denser than softwoods, the density varies greatly within each group, and the hardness of the two groups overlaps somewhat. The composition of softwood tissue is simpler than that of hardwood, consisting of mainly one type of cells, tracheids. Hardwoods show more detailed differentiation among stabilizing, conducting, and storage tissue. Although most trees harvested worldwide are hardwoods (58% of volume), much of the hardwood is used for fuel. Softwood is the major wood used for industrial purposes (69%); however, the percentage varies from region to region (IARC 1995).

Wood dust is a complex mixture generated when timber is processed, such as when it is chipped, sawed, turned, drilled, or sanded. Its chemical composition depends on the species of tree and consists mainly of cellulose, polyoses, and lignin, plus a large and variable number of substances with lower relative molecular mass. Cellulose is the major component of both softwood and hardwood. Polyoses (hemicelluloses), which consist of five neutral sugar units, are present in larger amounts in hardwood than in softwood. The lignin content of softwood is higher than that of hardwood. The lower-molecular-mass substances significantly affect the properties of wood; these include substances extracted with nonpolar organic solvents (fatty acids, resin acids, waxes, alcohols, terpenes, sterols, steryl esters, and glycerols), substances extracted with polar organic solvents (tannins, flavonoids, quinones, and lignans), and water-soluble substances (carbohydrates, alkaloids, proteins, and inorganic material). Wood dust is also characterized by its moisture content: “dry” wood has a moisture content of less than approximately 15%, and “moist” wood has a higher moisture content. Woodworking operations using dry wood generate more total dust and a larger quantity of inhalable dust particles than do those using moist wood (IARC 1995).

Use

Wood dust is produced in woodworking industries as a by-product of the manufacture of wood products; it is not usually produced for specific uses. One commercial use for wood dust is in wood composts (Weber et al. 1993). “Industrial roundwood” refers to categories of wood not used for fuel, which include sawn wood (54%), pulpwood (21%), poles and pit props (14%), and wood used for other purposes, such as particle board and fiberboard (11%) (IARC 1995).

Production

Wood dust is created when machines or tools are used to cut or shape wood materials. Industries in which large amounts of wood dust are produced include sawmills, dimension mills, furniture industries, cabinetmaking, and carpentry (IARC 1995). In 1990, total estimated production of wood used in U.S. industry was 311.9 million cubic meters of softwood and 115 million cubic meters of hardwood (Demers et al. 1997).

Exposure

Exposure to wood dust occurs when individuals use machinery or tools to cut or shape wood. When the dust is inhaled, it is deposited in the nose, throat, and other airways. The amount of dust deposited within the airways depends on the size, shape, and density of the dust particles and the strength (turbulence and velocity) of the airflow. Particles with a diameter larger than 5 μm (inspirable particles) are deposited almost completely in the nose, whereas particles 0.5 to 5 μm in diameter (respirable particles) are deposited in the lower airways (IARC 1981; 1995).

Wood dust usually is measured as the concentration of airborne dust, by particle size distribution, by type of wood, and by other characteristics of wood. Total airborne dust concentration is reported as mass per unit volume (usually milligrams of dust per cubic meter of air). Wood dust generally is collected by a standard gravimetric method, whereby a sampling pump is used to collect a known volume of air through a special membrane filter contained in a plastic cassette. Some sampling studies reported that the particle size distribution varied according to the woodworking operation, with sanding producing smaller particles than sawing, but others found no consistent differences (IARC 1995). The majority of the wood-dust mass was reported to be contributed by particles larger than 10 μm in aerodynamic diameter; however, between 61% and 65% of the particles by count measured between 1 and 5 μm in diameter (IARC 1995).

Exposure to wood dust also occurs through handling of compost containing wood dust. One study measured dust concentrations resulting from handling of compost material consisting of successive layers of chopped leaves, bark, and wood; visible clouds of fine particles were easily generated when the compost material was agitated. The reported background concentration of respirable dust of sampled upwind of the compost pile was 0.32 mg/m³. During loading and unloading of compost, samplers in the breathing zone detected inspirable dust at 0.74 mg/m³ and respirable dust at 0.42 mg/m³. Samples collected directly from the visible clouds of particles generated by compost agitation contained inspirable dust at 149 mg/m³ and respirable dust at 83 mg/m³ (Weber et al. 1993).

The National Occupational Exposure Survey (conducted from 1981 to 1983) estimated that nearly 600,000 workers were exposed to woods (NIOSH 1990). Teschke et al. (1999) analyzed 1,632 measurements of personal time-weighted-average airborne wood-dust concentrations in 609 establishments on 634 inspection visits that were reported to the Occupational Safety and Health Administration Integrated Management Information System between 1979 and 1997. Exposures ranged from less than 0.03 to 604 mg/m³, with an arithmetic mean of 7.93 mg/m³ and a geometric mean of 1.86 mg/m³. Exposure levels decreased significantly over time; the unadjusted geometric mean was 4.59 mg/m³ in 1979 and 0.14 mg/m³ in 1997.Occupations with high exposure to wood dust included sander in the transportation equipment industry (unadjusted geometric mean = 17.5 mg/m³), press operator in the wood products industry (12.3 mg/m³), lathe operator in the furniture industry (7.46 mg/m³), and sander in the wood cabinet industry (5.83 mg/m³). High exposures occurred in the chemical, petroleum, rubber, and plastics products industries, in sanding, pattern making, and mill and saw operations. The lowest exposures occurred in industrial pattern-making facilities, paper and paperboard mills, schools and institutional training facilities, and veneer and plywood mills.

Use of hand-held electric sanders has been identified as a particularly dusty process that leads to dust exposure. Wood-dust concentrations vary with type of dust extraction, amount of wood removed, and type of sander (Thorpe and Brown 1994). For electric belt sanders used to sand dowels, total dust concentrations ranged from 0.22 mg/m³ with external dust extraction to 3.74 mg/m³ without extraction, and concentrations of respirable dust ranged from 0.003 mg/m³ with extraction to 0.936 mg/m³ without extraction. Rotary sanders tested with flat wood samples produced total dust concentrations ranging from 0.002 mg/m³ with extraction to 0.699 mg/m³ without extraction; concentrations of respirable dust ranged from 0.001 mg/m³ with extraction to 0.088 mg/m³ without extraction. Comparable decreases in dust concentration were observed when dust extraction was used with electrical orbital sanders.

Regulations

Guidelines

References

1. Demers PA, Boffetta P, Kogevinas M, Blair A, Miller BA, Robinson CF, Roscoe RJ, Winter PD, Colin D, Matos E, et al. 1995. Pooled reanalysis of cancer mortality among five cohorts of workers in wood-related industries. Scand J Work Environ Health. 21(3):179–190. 10.5271/sjweh.26 [PubMed: 7481605] [CrossRef]
2. Demers PA, Teschke K, Kennedy SM. 1997. What to do about softwood? A review of respiratory effects and recommendations regarding exposure limits. Am J Ind Med. 31(4):385–398. 10.1002/(SICI)1097-0274(199704)31:4<385::AID-AJIM3>3.0.CO;2-V [PubMed: 9093652] [CrossRef]
3. IARC. 1981. Wood. In Wood, Leather and Some Associated Industries. Lyon, France: International Agency for Research on Cancer. p. 49–197.
4. IARC. 1995. Wood dust. In Wood Dust and Formaldehyde. Lyon, France: International Agency for Research on Cancer. p. 35–215.
5. NIOSH. 1990. National Occupational Exposure Survey (1981-83). National Institute for Occupational Safety and Health; https://www​.cdc.gov/noes/noes1/94220sic​.html.
6. NTP. 2000. Report on Carcinogens Background Document for Wood Dust. Atlanta, GA: U.S. Department of Health and Human Services, National Institute of Environmental Health Sciences, National Toxicology Program. https://ntp​.niehs.nih​.gov/ntp/newhomeroc/roc10/WD.pdf.
7. Teschke K, Marion SA, Vaughan TL, Morgan MS, Camp J. 1999. Exposures to wood dust in U.S. industries and occupations, 1979 to 1997. Am J Ind Med. 35(6):581–589. 10.1002/(SICI)1097-0274(199906)35:6<581::AID-AJIM5>3.0.CO;2-I [PubMed: 10332511] [CrossRef]
8. Thorpe A, Brown RC. 1994. Measurements of the effectiveness of dust extraction systems of hand sanders used on wood. Ann Occup Hyg. 38(3):279–302. [PubMed: 8048789]
9. Weber S, Kullman G, Petsonk E, Jones WG, Olenchock S, Sorenson W, Parker J, Marcelo-Baciu R, Frazer D, Castranova V. 1993. Organic dust exposures from compost handling: Case presentation and respiratory exposure assessment. Am J Ind Med. 24(4):365–374. 10.1002/ajim.4700240403 [PubMed: 8250057] [CrossRef]