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Cancer incidence among workers exposed to softwood dust in Lithuania
  1. Giedre Smailyte
  1. Lithuanian Cancer Registry, Institute of Oncology, Vilnius University, Vilnius, Lithuania
  1. Correspondence to Dr Giedre Smailyte, Lithuanian Cancer Registry, Institute of Oncology, Vilnius University, P. Baublio g 3B, Vilnius LT-08406, Lithuania; giedre.smailyte{at}vuoi.lt

Abstract

Objectives The aim of this study was to assess cancer incidence in a cohort of woodworkers exposed to softwood dust in a Lithuanian wooden joinery products factory.

Methods The study population consisted of 1518 workers (1080 men and 438 women) employed in the factory for at least 1 year between 1947 and 1996 and living in Lithuania on 1 January 1978, when the follow-up for cancer incidence began. The follow-up period for cancer was 1978–2007. Cancer risk was assessed by standardised incidence ratios (SIR) with reference to the national population.

Results Overall cancer incidence was not increased among woodworkers. However, the number of mouth and pharynx cancer cases among male woodworkers was significantly increased compared with expected numbers (SIR 2.19, 95% CI 1.17 to 3.74). A higher risk was found for cancer of the buccal cavity than for pharyngeal cancer (SIRs 2.83 and 1.45, respectively). The SIR for larynx cancer was also elevated (SIR 1.39, 95% CI 0.64 to 2.64) among men, while the number of lung cancer cases was higher than expected only among women (SIR 2.07, 95% CI 00.57 to 5.31).

Conclusions This results of this study support the hypothesis that exposure to softwood dust may increase the risk of oral and pharyngeal cancer. No support was found for an increased risk of other respiratory cancers among workers exposed to softwood dust.

  • Cancer incidence
  • softwood dust
  • occupational exposure
  • cohort study
  • cancer
  • epidemiology
  • wood dust
  • asbestos
  • radiation

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What this paper adds

  • Wood dust has been classified as a human carcinogen due to a marked increase in cancers of the nasal cavities and paranasal sinuses among workers predominantly exposed to hardwood dust.

  • Most epidemiological studies have examined the risk of cancer among woodworkers exposed to mixed or unspecified wood dust, not softwood dust alone.

  • The available epidemiological data are insufficient to determine an association between wood dust exposure and an increased risk of cancer other than nasal cancer.

  • This study of woodworkers exposed to softwood dust in a Lithuanian wooden joinery products factory supports the hypothesis that exposure to softwood dust may increase the risk of oral and pharyngeal cancer.

Introduction

Wood dust has been classified as carcinogenic to humans (group 1) by the International Agency for Research on Cancer (IARC), primarily on the basis of the observation of a marked increase in cancers of the nasal cavities and paranasal sinuses among workers exposed predominantly to hardwood dusts.1 However, about two-thirds of the wood used commercially worldwide is softwood. Unfortunately, the epidemiological data did not distinguish clearly between the different types of wood to which workers were exposed, and there is no separate assessment of the carcinogenic risk related to hard- or softwoods.

Only two cohort studies on workers exposed to softwood dust have been published.2 3 One of these studies was conducted in Finland among 1223 sawmill workers followed up from 1945 to 1980.2 Cancer incidence was not in excess overall, and no cases of nasal cancer were found (0.3 expected). In the study from the USA, cancer mortality among 2283 workers employed in plywood mills for at least 1 year was examined.3 Increased cancer risks for multiple myeloma and Hodgkin's disease were observed, but no nasal cancers were found (expected 0.4 cases). Excesses of various other cancers in these studies were observed, but all results were based on very small numbers.

The available epidemiological data are not sufficient to allow definitive assessment of wood dust exposure and increased risk for cancers other than nasal cancer.4 Exposure to softwood dust has been reported to also increase the risk of upper aerodigestive tract and respiratory cancers in some case–control studies.5–7 A census and register based study in five Nordic countries identified an increased risk of nasal cancer among woodworkers mainly exposed to softwood.8

Since it is unclear to what extent softwood dust is associated with cancer risk, cancer incidence was investigated in a cohort of woodworkers exposed to softwood dust in a Lithuanian wooden joinery products factory.

Methods

The factory manufactured joinery products such as wooden windows, doors and floorboards. Pine was the main wood used for production, together with some spruce. Exposure data by different departments were only available for the years 1983 and 1989–1991. The reported means of total dust concentrations in wood-processing departments varied from 1.7 to 23.4 mg/m3.

Information for the study population was obtained from employment records. Personal identification data and work history were abstracted for each worker. The inclusion criteria were at least 1 year's employment in the factory between 1947 and 1996 and residence in Lithuania on 1 January 1978, when the follow-up for cancer incidence began.

Information on vital status was obtained from the Residents' Register Service. The cohort consisted of 1518 employees (1080 men and 438 women). By the end of the follow-up on 31 December 2007, 54.8% of workers were alive, 41.3% had died, 3.1% had emigrated and 0.8% had been lost to follow-up.

Cancer cases in the cohort were identified in the Lithuanian Cancer Registry by linkage procedures using sex, name and date of birth. Cancer diagnoses were coded according to the WHO 9th Revision of International Classification of Diseases (ICD-9).

Standardised incidence ratios (SIR) were calculated as ratios between the observed and expected numbers of cancers.9 The expected numbers were based on sex-specific incidence rates in 5-year age groups and 5-year observation periods for the total Lithuanian population. Follow-up for each individual started after 1 year of total employment for those with first entry after 1 January 1978 and on 1 January 1978 for workers who had reached 1 year of employment before that date. Workers who died or emigrated were followed until the date of these events, while all others were followed until the end of 2007. For workers lost to follow-up during the observation period, person-years were calculated up to the date of discharge from the factory. The cohort contributed 50 444 person-years to the study (34 456 for men and 15 988 for women). Stata V.11.0 was used for analysis.

Ethics approval was obtained from the Lithuanian Bioethics Committee.

Results

Among male woodworkers, a total of 157 cases of cancer were observed compared to 193.93 expected (SIR 0.81, 95% CI 0.69 to 0.95) (table 1). The number of mouth and pharynx cancer cases among male woodworkers was significantly increased compared to the expected numbers (SIR 2.19, 95% CI 1.17 to 3.74). A higher risk was found for oral cavity cancers than for pharyngeal cancers (SIRs 2.83 and 1.45, respectively). The SIR for larynx cancer was also elevated (SIR 1.39, 95% CI 0.64 to 2.64), whereas the number of lung cancer cases was no higher than expected (SIR 0.85, 95% CI 0.60 to 1.18). No cancers of the nose and sinuses were observed in this group of male woodworkers (0.47 cases expected).

Table 1

Observed (Obs) number of new cancer cases and standardised incidence ratio (SIR) among woodworkers during follow-up (1978–2007)

For female workers, the overall incidence of cancer was not increased (SIR 0.88, 95% CI 0.65 to 1.16). The observed numbers of cancers at specific sites were based on a few cases only, however there was one case of oral cavity cancer and four cases of lung cancers rare among women, giving SIRs of 3.75 (95% CI 0.09 to 20.60) and 2.07 (95% CI 00.57 to 5.31), respectively. In addition, an elevated SIR of nose and sinus cancer was observed (SIR 13.88, 95% CI 0.35 to 77.31) because of one case of squamous cell carcinoma of the maxillary sinus (0.07 expected).

For respiratory cancers, dose–response analyses were conducted using employment duration as an indicator of exposure. No trend for mouth and pharynx, lung and laryngeal cancers among men was observed with increasing duration of employment. The highest risk of lung and laryngeal cancers was found among those employed for <5 years (SIR 1.09, 95% CI 0.72 to 1.59 and SIR 2.01, 95% CI 0.87 to 3.96), while the risk of mouth and pharynx cancers was not significantly elevated in all groups with different employment duration (SIRs were 2.09, 1.97 and 2.80 among those employed for <5, from 5 to 10, and for >10 years, respectively).

Discussion

The present investigation among workers in a joinery production factory showed no excess in overall cancer risk, which corresponds with the results of other studies.2 3 10 The reduced number of cancer cases in this cohort might reflect the healthy worker effect, which is common in occupational studies.11 As only 0.47 cases of nasal cancer were expected among men in this cohort and 0.07 cases among women, this study should be considered as uninformative for nasal cancer. An increased risk of oral cavity and pharyngeal cancers was found among Lithuanian woodworkers exposed only to softwood dust, but the risks of laryngeal cancer among men and of lung cancer among women were not significantly elevated.

The present study included workers who had experienced a specific type of occupational exposure. Over 50 000 person-years were available for the examination of cancer risk in a cohort of 1518 workers exposed only to softwood dust. The limitations of the present study were the small cohort size and high loses to follow-up. Due to its retrospective design, the study has no information on potential confounding exposures, including smoking and alcohol consumption. This is an important limitation and the study results may be influenced by different prevalences of smoking and alcohol consumption in this cohort and in general population.

The most interesting finding of the study was the increased risk of oral cavity and pharyngeal cancers. In western countries, tobacco and alcohol consumption are well known risk factors for the development of head and neck cancers, whereas the aetiological relationship between occupational exposure and the development of the oral cavity and pharyngeal cancers is not clear. An insignificantly higher risk of lip, mouth and pharynx cancer was observed in a softwood exposed cohort from Finland.2 In a cohort of workers exposed to softwood dust from North America, the risk of oral cavity and pharynx cancer was not elevated.3 In a pooled analysis of cancer mortality among workers in wood-related industries, the risk of oral cavity and pharynx cancer was not elevated, but an increase was observed for nasopharyngeal cancer.10 According to the study by Vaughan and Davis, exposure to softwood dust increases the risk of nasopharyngeal squamous cell cancer.12 No increased risk of upper respiratory and lung cancer was found among Finnish woodworkers exposed to wood dust, mainly from pine, spruce and birch, at a level of about 1 mg/m3.13 Another Finish study identified an elevated risk of nasal and lung cancer.5 A case–control study performed in Washington State found exposure to softwood dust did not increase the risk of lung cancer.14

Although smoking and alcohol consumption can explain some of the results of the present study, the increased risk found for oral cavity and pharyngeal cancers may partly be explained by work-related exposures.

Conclusions

The results of this study support the hypothesis that exposure to softwood dust may increase the risk of oral and pharyngeal cancer. No support was found for an increased risk of other respiratory cancers among workers exposed to softwood dust.

References

Footnotes

  • Funding This study was supported by a grant from the Lithuanian State Science and Studies Foundation (grant No. T—38/06).

  • Competing interests None.

  • Ethics approval This study was conducted with the approval of the Lithuanian Bioethics Committee (No. 71, 2004-12-22).

  • Provenance and peer review Not commissioned; externally peer reviewed.

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