Sawmills and other lumber mills are extremely hazardous work environments due to the nature of the process, which involves the movement and cutting of large, very heavy pieces of wood at relatively high speeds. Even when good engineering controls are in place, strict adherence to safety rules and procedures is necessary. There are a number of general factors which may contribute to the risk of injury. Poor housekeeping can increase the risk of slips, trips and falls, and wood dust may pose a fire or explosion hazard. The high noise levels have been a cause of injuries due to the reduced ability of workers to communicate and hear audible warning signals. Many large mills operate on multiple shifts, and the hours of work, particularly changes in shift, can increase the probability of accidents.
Some common causes of fatal or very serious injuries are being struck by mobile equipment; falls from elevated walkways and platforms; failure to de-energize or lockout equipment during maintenance or attempts to remove jams; kick-backs from saws, edgers and planers; and drowning in log ponds or waterways. Newly hired workers are at increased risk. For example, in an analysis of the causes of 37 sawmill fatalities between 1985 and 1994 in British Columbia, Canada, 13 (35%) of the fatalities occurred within the first year of employment, and 5 of these occurred within the first week of employment (4 on the first day) (Howard 1995).
There is also a high risk of injuries which are not life threatening. Eye injuries may result from particles and small pieces of wood or debris ejected from machinery. Splinters, cuts and puncture wounds can result from contact between lumber and unprotected skin. Strains, sprains and other musculoskeletal injuries can result from attempts to push, pull or lift heavy materials during sorting, grading and other operations.
Workers in sawmills and related industries are exposed to a variety of respiratory hazards, including wood dust, the volatile components of wood, airborne moulds and bacteria, and formaldehyde. A number of studies have examined respiratory health among sawmill, plywood, particleboard and strandboard workers. The focus of the sawmill studies has generally been on wood dust, while the focus of the plywood and particleboard studies has primarily been on formaldehyde exposure.
Occupational exposure to wood dust has been associated with a broad range of upper- and lower-respiratory effects. Because of the particle sizes generated by operations in the lumber industries, the nose is a natural site for the effects of wood dust exposure. A wide variety of sino-nasal effects have been reported, including rhinitis, sinusitis, nasal obstruction, nasal hypersecretion and impaired mucociliary clearance. Lower-respiratory effects, including asthma, chronic bronchitis and chronic airflow obstruction, have also been associated with exposure to wood dust. Both upper- and lower-respiratory effects have been associated with both softwood and hardwood tree species from both temperate and tropical climates. For example, occupational asthma has been found to be associated with exposure to dust from African maple, African zebra, ash, California redwood, cedar of Lebanon, Central American walnut, Eastern white cedar, ebony, iroko, mahogany, oak, ramin and Western red cedar as well as other tree species.
Wood is primarily composed of cellulose, polyoses and lignin, but also contains a variety of biologically active organic compounds such as monoterpenes, tropolones, resin acids (diterpenes), fatty acids, phenols, tannins, flavinoids, quinones, lignanes and stilbenes. Because health effects have been found to vary by species of tree, it is suspected they may be due to these naturally occurring chemicals, referred to as extractives, which also vary by species. In some cases specific extractives have been identified as the cause of the health effects associated with exposure to wood. For example, plicatic acid, which occurs naturally in Western red cedar and Eastern white cedar, is responsible for asthma and other allergenic effects in humans. While higher-molecular-weight extractives remain with the dust during woodworking operations, other, lighter-weight extractives, such as the monoterpenes, are easily volatilized during kiln drying, sawing and trimming operations. The monoterpenes (such as α-pinene, β-pinene, d3-carene and limonene) are major components of the resin from many common softwoods and are associated with mouth and throat irritation, shortness of breath, and impaired lung function.
The moulds which grow on timber are another natural, wood-related exposure with potentially harmful effects. Exposure to moulds among sawmill workers appears to be common in regions where the climate is sufficiently damp and warm for moulds to grow. Cases of extrinsic allergic alveolitis, also referred to as hypersensitivity pneumonitis, have been observed among sawmill workers in Scandinavia, Great Britain and North America (Halpin et al. 1994). A much more common, although less serious, effect of exposure to moulds is inhalation fever, also referred to as organic dust toxic syndrome, consisting of acute attacks of fever, malaise, muscular pain and cough. The prevalence of inhalation fever among Swedish wood trimmers has been estimated to be between 5 and 20% in the past, although rates are likely to be much lower now due to the introduction of preventive measures.
Respiratory effects are also possible from exposure to chemicals used as adhesives in the lumber industry. Formaldehyde is an irritant and can cause inflammation of the nose and throat. Acute effects on lung function have been observed and chronic effects are suspected. Exposure has also been reported to cause asthma and chronic bronchitis.
The irritant or allergenic effects of wood dust, formaldehyde and other exposures are not limited to the respiratory system. For example, studies reporting nasal symptoms have often reported an increased prevalence of eye irritation. Dermatitis has been found to be associated with dust from over 100 different species of trees including some common hardwoods, softwoods and tropical species. Formaldehyde is also a skin irritant and can cause allergic contact dermatitis. In addition, a number of the anti-sapstain fungicides used on softwoods have also been found to cause eye and skin irritation.
Workers in sawmills and other lumber industries have a high risk for noise-related hearing loss. For example, in a recent survey in a United States sawmill, 72.5% of workers exhibited some degree of hearing impairment at one or more audiometric test frequencies (Tharr 1991). Workers in the vicinity of saws and other wood processing machinery are typically exposed to levels above 90 or 95 dBA. Despite this well recognized hazard, attempts to reduce noise levels are relatively rare (with the exception of planer mill enclosures), and new cases of noise-induced hearing loss continue to occur.
Work in the lumber industries may entail exposure to both known and suspected carcinogens. Wood dust, the most common exposure in the lumber industries, has been classified as a human carcinogen (International Agency for Research on Cancer (IARC) - Group 1). Very high relative risks of sino-nasal cancer, particularly sino-nasal adenocarcinoma, have been observed among workers exposed to high levels of dust from hardwoods, such as beech, oak and mahogany, in the furniture industry. The evidence for softwood dust is less conclusive, and smaller excess risks have been observed. There is evidence of an excess risk among workers in sawmills and related industries based on a pooled re-analysis of the raw data from 12 sino-nasal cancer case-control studies (IARC 1995). Sino-nasal cancer is a relatively rare cancer in almost all regions of the world, with a crude annual incidence rate of approximately 1 per 100,000 population. Ten per cent of all sino-nasal cancers are thought to be adenocarcinomas. Although associations between wood dust and other, more common, cancers have been observed in some studies, the results have been much less consistent than for sino-nasal cancer.
Formaldehyde, a common exposure among workers in the plywood, particleboard and related industries, has been classified as a probable human carcinogen (IARC - Group 2A). Formaldehyde has been found to cause cancer in animals, and excesses of both nasopharyngeal and sino-nasal cancer have been observed in some studies of humans, but the results have been inconsistent. Pentachlorophenol and tetrachlorophenol pesticides, until recently commonly used in the wood industries, are known to be contaminated with furans and dioxins. Pentachlorophenol and 2,3,7,8-tetrachlorodibenzo-para-dioxin have been classified as possible human carcinogens (IARC - Group 2B). Some studies have found an association between chlorophenols and the risk of non-Hodgkin lymphoma and soft-tissue sarcoma. The results for non-Hodgkin lymphoma have been more consistent than for soft-tissue sarcoma. Other potential carcinogenic exposures which may affect some workers in the lumber industries include asbestos (IARC - Group 1), which is used for insulation of steam pipes and kilns, diesel exhaust (IARC - Group 2A) from mobile equipment, and creosote (IARC - Group 2A), which is used as a wood preservative for railroad ties and telephone poles.
Relatively few studies of cancer among workers specifically employed in sawmills, plywood mills or related board manufacturing industries have been performed. The largest was a cohort study of over 26,000 Canadian sawmill workers conducted by Hertzman and colleagues (1997) in order to examine the risk of cancer associated with exposure to chlorophenol pesticides. A twofold excess of sino-nasal cancer and a smaller excess of non-Hodgkin lymphoma were observed. The excess of non-Hodgkin lymphoma appeared to be associated with exposure to chlorophenates. The remaining studies have been much smaller. Jäppinen, Pukkala and Tola (1989) studied 1,223 Finnish sawmill workers and observed excesses of skin, mouth and pharyngeal cancers, and lymphomas and leukaemias.
Blair, Stewart and Hoover (1990) and Robinson and colleagues (1986) conducted studies of 2,309 and 2,283 US plywood mill workers, respectively. In an analysis of pooled data from the two plywood cohorts, excesses were observed for nasopharyngeal cancer, multiple myeloma, Hodgkin’s disease and non-Hodgkin’s lymphoma. It is unclear from the results of these studies which, if any, occupational exposures may have been responsible for the excesses observed. The smaller studies have lacked the power to examine the risk of rare cancers, and many of the excesses were based on very small numbers. For example, no sino-nasal cancers were observed, but only 0.3 were expected in the smaller sawmill study, and 0.3 and 0.1 were expected in the plywood mill studies.