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Original Research: COPD |

Effect of Occupational Exposure to Vapors, Gases, Dusts, and Fumes on COPD Mortality Risk Among Swedish Construction WorkersOccupation and COPD Mortality Risk: A Longitudinal Cohort Study FREE TO VIEW

Kjell Torén, MD, PhD, FCCP; Bengt Järvholm, MD, PhD
Author and Funding Information

From the Sections of Occupational and Environmental Medicine (Dr Torén), University of Gothenburg, Gothenburg, Sweden; Occupational Medicine, Respiratory Diseases and Toxicology (Dr Torén), University of Perugia, Perugia, Italy; and Occupational Medicine, Department of Clinical Medicine and Public Health (Dr Järvholm), Umeå University, Umeå, Sweden.

Correspondence to: Kjell Torén, MD, PhD, FCCP, Section of Occupational and Environmental Medicine, University of Gothenburg, Box 414, S-405 30 Gothenburg, Sweden; e-mail: Kjell.Toren@amm.gu.se


Funding/Support: This study was supported by the Swedish Research Council for Health, Working Life and Welfare (Forte), and the Swedish Heart and Lung Foundation.

Reproduction of this article is prohibited without written permission from the American College of Chest Physicians. See online for more details.


Chest. 2014;145(5):992-997. doi:10.1378/chest.13-1429
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Published online

Objective:  The aim of the present study was to elucidate whether occupational exposure to vapors, gases, dusts, and fumes increases the mortality risk of COPD, especially among never smokers.

Methods:  The study population was a cohort of 354,718 male construction workers; of these, 196,329 were exposed to vapors, gases, dusts, and fumes, and 117,964 were unexposed. Exposure to inorganic dust, wood dust, vapors, fumes, gases, and irritants was based on a job-exposure matrix with a focus on exposure in the mid-1970s. The cohort was followed from 1971 to 2011. Relative risks (RRs) were obtained using Poisson regression models adjusting for age, BMI, and smoking habits.

Results:  There were 1,085 deaths from COPD among the exposed workers, including 49 never smokers. Workers with any occupational exposure to vapors, gases, fumes, and dust showed an increased mortality due to COPD (RR, 1.32; 95% CI, 1.18-1.47). When comparing different exposure groups, there was a significantly increased mortality due to COPD among those exposed to fumes (RR, 1.20; 95% CI, 1.07-1.36) and inorganic dust (RR, 1.19; 95% CI ,1.07-1.33). Among never smokers, there was high mortality due to COPD among workers with any occupational airborne exposure (RR, 2.11; 95% CI, 1.17-3.83). The fraction of COPD attributable to occupational exposure was 0.24 among all workers and 0.53 among never-smoking workers.

Conclusions:  Occupational exposure to airborne pollution increases the mortality risk for COPD, especially among never smokers.

A number of industry-specific cohort studies and general population-based studies have shown associations between progressive airflow decline and exposure to inorganic dusts.1,2 In 2003, the American Thoracic Society published a statement providing a systematic review of the occupational contribution to COPD, concluding that 15% was a reasonable estimate of the work-related burden of COPD.3 Other reviews also concluded that occupational exposure is an important risk factor for COPD,46 as did a 2010 statement from the American Thoracic Society.2 The potential interaction between occupational exposures and tobacco smoking has also been addressed, with studies showing an interaction between these two factors.7,8

It has been recognized that COPD also occurs among never smokers, with up to 25% of all COPD cases occurring among individuals who have never smoked.9 Other risk factors, such as occupational exposures, may, thus, be of increased importance among these never smokers.9 To our knowledge, however, only two prospective, longitudinal studies have addressed occupational exposure and the risk for COPD among never smokers. A follow-up study among the general Swiss population revealed an increased incidence ratio (3.3) for COPD among never smokers exposed to a global measure of occupational exposure (vapors, gases, dusts, and fumes [VGDFs]).10 The risk estimates were based, however, on 11 exposed cases and four unexposed cases. Among never smokers, the population attributable fraction (AF) for VGDFs was 0.51. In 2004, we published a prospective, longitudinal analysis of 317,629 Swedish male construction workers followed from 1971 to 1999. Among never smokers, we found a doubled risk for COPD associated with occupational exposure to gas, dust, and fumes, based on 31 exposed cases and seven control subjects. The fraction of COPD attributable to work among the exposed never smokers was 0.53.

There is a clear need for further large, prospective, longitudinal studies aimed at detecting the effect of occupational airborne exposure on COPD, especially among never smokers. Here, we report the results of a further follow-up of 314,293 male construction workers followed from 1971 to 2011 regarding mortality from COPD. We also separately report the findings for the new follow-up period 2000 to 2011.

Since 1968, Swedish construction workers have been invited to medical examinations at intervals of 2 to 5 years. Although the program is voluntary, at least 80% of eligible workers have participated at least once. Data from the health examinations between 1971 and 1993, including occupational titles, smoking habits, BP, height, and weight, were registered in a central database established in the early 1970s, as previously described.11,12

Using the personal identity number and a linkage with the National Cause of Death Register, it was possible to identify subjects who had died (underlying cause) from COPD. We used the COPD diagnoses from the International Classification of Diseases, 10th revision13 (diagnosis codes J43-J44), and the 9th revision14 (diagnosis codes 491, 492, and 496).

Smoking habits were categorized according to information from the first health examination. If information on smoking was lacking in the records of the first examination, we used information from the second or third visit. In a small proportion of participants, smoking habits were unknown. The current smokers were stratified according to smoking habits at the time of examination as light (< 15 cigarettes/d) or heavy smokers (≥ 15 cigarettes/d). Based on smoking habits at baseline, subjects were consequently classified as never smokers, ex-smokers, light smokers, heavy smokers, or people with unknown smoking habits.

The occupational title at the time of the first health examination was used. Most workers remained in the same occupation throughout the study period. Among the workers who participated in repeated examinations, 74% reported the same occupation. A job-exposure matrix (JEM) was developed for selected exposures, as previously described.11 The JEM was based on exposure estimations from the 1970s, in which each occupation was studied during visits to approximately five different sites in different geographic regions of Sweden. The estimations in the 1970s were performed by industrial hygienists. Exposures to asbestos, asphalt fumes, cement dust, concrete dust, diesel exhaust, epoxy resins, diisocyanates, manmade mineral fibers, metal fumes, organic solvents, quartz dust, and wood dust were assessed, with focus on exposure during the mid-1970s. Each occupation was placed into one of two groups, exposed or not exposed. Common occupations in the not exposed group were certain carpenters, roofers, reinforcement workers, crane operators, and supervisors. For the purpose of this study, the specific exposures were further merged into four broader exposure categories: exposure to inorganic dust (asbestos, cement dust, concrete dust, manmade mineral fibers, or quartz), gases and irritants (organic solvents, epoxy resins, or diisocyanates), fumes (metal fumes, asphalt fumes, or diesel exhaust), and wood dust. Workers could be classified as having several exposures, so there was some overlap among the four major exposure groups. In particular, workers classified as being exposed to inorganic dust were also exposed to fumes and to gases and irritants (Table 1).

Table Graphic Jump Location
Table 1 —Data on the Overlap With Other Exposure Groups, 1971-2011

Data given as %.

The basic cohort consisted of 389,132 individuals. Only a few women had been exposed to dusts, fumes, or gases, and so the analysis was restricted to men (N = 369,714). Men who were aged < 15 years or > 65 years (the previous retirement age) at the first examination were excluded (n = 1,726). Men with a BMI < 18 or > 35 kg/m2 were excluded, as were those for whom data for BMI calculation were not available (n = 6,816). Men with an occupation at the first examination indicating replacement were also excluded (n = 6,454), as were 40,749 men with incomplete data. In total, 354,718 men were included in the final analysis (Table 2); 196,329 exposed men and 117,964 men were assessed as not exposed. Among these were 11,222 white-collar workers (office workers), who, for the purpose of this study, were included in the analysis. The subjects followed 2000 to 2011 are described in Table 3. The study was approved by the Committee of Ethics at Umeå University (2010/326-32M).

Table Graphic Jump Location
Table 2 —Basic Data on the Cohort of Swedish Male Construction Workers Followed Between 1971 and 2011a
a 

Note that an individual can occur in more than one category.

Table Graphic Jump Location
Table 3 —Basic Data on the Cohort of Swedish Male Construction Workers Followed Between 2000 and 2011a
a 

Note that an individual can occur in more than one category.

Statistics

Relative risk (RR) was calculated with the person-year method15 using the age distribution in the reference group as control. Person-years were calculated from the year after the first examination starting in 1971 until emigration, or to December 31, 2011, whichever came first. We also made a separate analysis for the new follow-up (ie, from January 1, 2000, through December 31, 2011). The analyses were stratified by death, by 13 5-year age groups, by five groups according to smoking habits, and by three groups according to BMI (18-24, 25-29, and 30-35 kg/m2).

The influence of the different exposures, smoking, and age was analyzed in Poisson regression models, with mortality from COPD as the dependent variable. Exposure was either handled as any exposure or separated into four different exposures (inorganic dust, gases and irritants, fumes, and wood dust). The analyses were restricted to subjects 50 to 84 years of age, as very few cases occur below the age of 50 years. The models were adjusted for smoking, age, and BMI. The 95% CIs of the RRs were calculated using Wald estimates. The SAS program page PROC GENMOD (SAS Institute Inc) was used in the calculations. The AF was calculated as AF = [(RR − 1) / RR].

The total cohort followed from 1972 to 2011 included 196,329 exposed workers. Of these, 151,072 were exposed to inorganic dust, 50,886 were exposed to gases and irritants, 68,084 were exposed to fumes, and 29,892 were exposed to wood dust (Table 2). There was overlap among the four major exposure groups. During this period, there were 1,085 deaths from COPD among the exposed workers and 432 deaths from COPD among the nonexposed control subjects. There was an increased RR for dying of COPD (RR, 1.32; 95% CI, 1.18-1.47) among workers with any occupational exposure to air pollution (Table 4). When comparing the different exposure groups, the highest risk was among those exposed to fumes (RR, 1.20; 95% CI, 1.07-1.36). Among never smokers, the mortality due to COPD among workers with occupational exposure to airborne pollutions was further increased (RR, 2.11; 95% CI, 1.17-3.83) based on 49 deaths due to COPD. The highest risk was seen among those exposed to inorganic dust (RR, 1.82; 95% CI, 1.03-3.21).

Table Graphic Jump Location
Table 4 —Mortality From COPD in Subjects Aged 50-84 Y: 1971-2011

RR = relative risk.

a 

Poisson regression analysis, adjusting for smoking, age, and BMI.

b 

Poisson regression analysis, adjusting for age and BMI.

c 

Also adjusting for other exposures.

In the same period (1972-2011), there were 44,032 all-cause deaths among the exposed workers and 21,015 among the nonexposed control subjects. Any occupational exposure to air pollution was associated with increased total mortality (RR, 1.17; 95% CI, 1.13-1.21). The same was observed among never smokers (Table 5).

Table Graphic Jump Location
Table 5 —Mortality From All Causes in Subjects Aged 50-84 Y: 1971-2011

See Table 4 legend for expansion of abbreviation.

a 

Poisson regression analysis, adjusting for smoking, age, and BMI.

b 

Poisson regression analysis, adjusting for age and BMI.

c 

Also adjusting for other exposures.

For the subcohort followed from 2000 to 2011, there were 607 deaths from COPD among the exposed workers and 254 deaths from COPD among the nonexposed control subjects. The Poisson regression analysis showed an increased RR of dying of COPD (RR, 1.44; 95% CI, 1.24-1.67) among workers with occupational exposure to particulate air pollution (Table 6). There were no statistically significant increased risks among never smokers, but the power was low due to a low number of exposed cases (n = 21).

Table Graphic Jump Location
Table 6 —Mortality From COPD in Subjects Aged 50-84 Y: 2000-2011

See Table 4 legend for expansion of abbreviation.

a 

Poisson regression analysis, adjusting for smoking, age, and BMI.

b 

Poisson regression analysis, adjusting for age and BMI.

c 

Also adjusting for other exposures.

The joint associations between smoking and occupational exposure are shown in Table 7. Of a total of 1,517 cases, only 63 occurred among nonsmokers (4.2%). The fraction of COPD attributable to occupational exposure was estimated to be 0.24 among all workers (RR, 1.32) and 0.53 among never-smoking workers (RR, 2.11).

Table Graphic Jump Location
Table 7 —Joint Associations Between Occupational Exposures to Dust, Gases, and Fumes and Smoking Habits in Subjects Aged 50-84 Y: 1971-2011

Ref = reference. See Table 4 legend for expansion of other abbreviation.

The results of this follow-up of a large cohort of Swedish construction workers indicate that occupational exposure to air pollution increases COPD mortality. When the analyses were restricted to never smokers, the risk was considerably increased. These results gives further support to the existing evidence that occupational exposure is important for the development of COPD.16 Moreover, our results show that it is also necessary to consider occupational exposure to gas, dusts, and fumes among never smokers as a risk factor for COPD.

Among the strengths of our follow-up are the following: The exposure was based on expert assessment, the outcome was underlying cause of death, and the design was prospective. The study was very large, covering almost all Swedish construction workers, making it possible to have an internal, unexposed control group, which limited the influence of selection bias (ie, the nonexposed construction workers had similar socioeconomic backgrounds).

The design has some weaknesses that should be discussed. One important limitation is that the study only included men. Another limitation is that the exposure assessment did not include analyses of dose-response relationships, as the exposure was dichotomized as exposed or nonexposed. The presence of dose-response relationships is important for making causal inferences; the absence of this aspect is an important weakness of the present study. Further, it is likely that the workers in the control group had some particulate air pollution exposure, albeit at a lower level than in the exposed group, since (1) there is a general dust exposure at most construction sites, so no one in the cohort was really unexposed and (2) our exposure assessment only took the first recorded job title into account, and the worker may later have changed occupations. This may have caused underestimations of the risks. It should be noted, however, that Swedish construction workers largely remain in the same occupation within the construction industry. As both the exposed workers and those in the control group were construction workers, there were only small differences between them regarding socioeconomic factors and lifestyle. Finally, the classification of smoking habits in the study was not time dependent, but we did differentiate at baseline between smokers with high and low daily tobacco consumption. Still, the results for the whole group may not be entirely accurate, as the exposed group may have had different cumulative exposure to tobacco smoke (ie, different numbers of pack-years).

We previously presented the results for the period 1974 to 1999,11 and the new analyses covering the period 1971 to 2011 showed similar results. In these new analyses, we excluded men younger than 50 years, as very few men below 50 years of age die of COPD.

There are several studies showing that occupational exposure to gas, dust, and fumes is associated with increased risk of COPD. However, most studies are cross-sectional with retrospective assessment of exposure and assessment of COPD outcome at the same time. The findings from the cross-sectional, general population-based studies have been confirmed in one longitudinal analysis from Switzerland.10 Compared with our results, Mehta et al10 observed higher risks associated with occupational VGDF exposure among never smokers with the most severe COPD according to GOLD (Global Initiative for Chronic Obstructive Lung Disease) II.

Our outcome was underlying mortality due to COPD, meaning that the individuals with COPD in this study formed a group with very severe COPD. Very few studies have differentiated the occupational risk according to severity of COPD, and there is a lack of consistency in the findings of those that have.9,10 Never smokers seem to have less severe COPD than smokers.9 When we restricted the analyses to never smokers, the mortality increased, indicating that occupational exposure is an important risk factor for COPD among never smokers. In the current analyses, the AF among the never smokers was 0.53, in line with our previous analysis, in which it was 0.51.11 Due to the limited numbers of cases, it was not possible to analyze the importance of different exposures among never smokers. It was also not possible to confirm the findings from other studies that exposure to organic dust seems to be associated with an increased risk among never smokers.9,10 In the analyses of joint associations between smoking and occupational exposure, these factors showed neither multiplicative nor additive interaction (Table 6), as observed in other studies.7,8 Still, despite our large study sample, the numbers are quite small, and our results should be interpreted with caution.

The exposure assessments for the JEM were based on exposure estimations from the 1970s. These originated from an extensive exposure-assessment project involving several industrial hygienists with experience from the construction industry. Hence, we regard the JEM as valid, although no formal validation has been performed. Based on our personal experience, the exposure to dust among Swedish construction workers is still rather high and similar to previous decades. This is also supported by a Swedish investigation from 2004, in which the threshold limit values for silica dust and total dust were exceeded in 65% of the measurements.11 Hence, the findings in the present study may well reflect a risk still existent in this workforce.

Occupational exposure to air pollution was also associated with increased all-cause mortality, especially among those exposed to inorganic dust, and this was also observed among never smokers. When analyzing total mortality, cardiovascular diseases constituted the largest group. An earlier study from this cohort showed an increased mortality due to ischemic heart disease among subjects exposed to occupational air pollution, especially inorganic dust.17 There were 44,031 deaths among the exposed individuals, of which 1,085 (approximately 2.5%) were due to COPD. In 2009, the total number of deaths in the Swedish population was 43,753, of which 1,208 deaths were due to COPD (2.8%).18

In conclusion, occupational exposure to air pollution, such as inorganic dust, gases, and fumes, increases the risk of COPD, especially among never smokers. These findings give further support to previous observations and underscore the importance of occupational exposures and COPD among never smokers.

Author contributions: Dr Järvholm is the guarantor of the manuscript and takes responsibility for the integrity of the dataset and the accuracy of the analysis.

Dr Torén: contributed to study design, drafting of the manuscript, and finalizing the last version of the manuscript.

Dr Järvholm: contributed to study design, statistical analyses, and finalizing the last version of the manuscript.

Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

Role of sponsors: The sponsors had no role in the design of the study, the collection and analysis of the data, or the preparation of the manuscript.

AF

attributable fraction

JEM

job-exposure matrix

RR

relative risk

VGDF

vapor, gas, dust, and fumes

Torén K, Balmes J. Chronic obstructive pulmonary disease: does occupation matter? Am J Respir Crit Care Med. 2007;176(10):951-952. [CrossRef] [PubMed]
 
Eisner MD, Anthonisen N, Coultas D, et al; Committee on Nonsmoking COPD, Environmental and Occupational Health Assembly. An official American Thoracic Society public policy statement: novel risk factors and the global burden of chronic obstructive pulmonary disease. Am J Respir Crit Care Med. 2010;182(5):693-718. [CrossRef] [PubMed]
 
Balmes J, Becklake M, Blanc P, et al. ATS statement on occupational contribution to the burden of airway disease. Am J Respir Crit Care Med. 2003;167(5):787-797. [CrossRef] [PubMed]
 
Meldrum M, Rawbone R, Curran AD, Fishwick D. The role of occupation in the development of chronic obstructive pulmonary disease (COPD). Occup Environ Med. 2005;62(4):212-214. [CrossRef] [PubMed]
 
Blanc PD. Occupation and COPD: a brief review. J Asthma. 2012;49(1):2-4. [CrossRef] [PubMed]
 
Naidoo RN. Occupational exposures and chronic obstructive pulmonary disease: incontrovertible evidence for causality? Am J Respir Crit Care Med. 2012;185(12):1252-1254. [CrossRef] [PubMed]
 
Blanc PD, Iribarren C, Trupin L, et al. Occupational exposures and the risk of COPD: dusty trades revisited. Thorax. 2009;64(1):6-12. [CrossRef] [PubMed]
 
Boggia B, Farinaro E, Grieco L, Lucariello A, Carbone U. Burden of smoking and occupational exposure on etiology of chronic obstructive pulmonary disease in workers of Southern Italy. J Occup Environ Med. 2008;50(3):366-370. [CrossRef] [PubMed]
 
Lamprecht B, McBurnie MA, Vollmer WM, et al; BOLD Collaborative Research Group. COPD in never smokers: results from the population-based burden of obstructive lung disease study. Chest. 2011;139(4):752-763. [CrossRef] [PubMed]
 
Mehta AJ, Miedinger D, Keidel D, et al; SAPALDIA Team. Occupational exposure to dusts, gases, and fumes and incidence of chronic obstructive pulmonary disease in the Swiss Cohort Study on Air Pollution and Lung and Heart Diseases in Adults. Am J Respir Crit Care Med. 2012;185(12):1292-1300. [CrossRef] [PubMed]
 
Bergdahl IA, Torén K, Eriksson K, et al. Increased mortality in COPD among construction workers exposed to inorganic dust. Eur Respir J. 2004;23(3):402-406. [CrossRef] [PubMed]
 
Torén K, Qvarfordt I, Bergdahl IA, Järvholm B. Increased mortality from infectious pneumonia after occupational exposure to inorganic dust, metal fumes and chemicals. Thorax. 2011;66(11):992-996. [CrossRef] [PubMed]
 
World Health Organization. International Classification of Diseases, 10th Revision. Geneva, Switzerland; World Health Organization; 2007.
 
World Health Organization. International Classification of Diseases, 9th Revision. Geneva, Switzerland; World Health Organization; 1978.
 
Berry G. The analysis of mortality by the subject-years method. Biometrics. 1983;39(1):173-184. [CrossRef] [PubMed]
 
Blanc PD, Torén K. Occupation in chronic obstructive pulmonary disease and chronic bronchitis: an update. Int J Tuberc Lung Dis. 2007;11(3):251-257. [PubMed]
 
Torén K, Bergdahl IA, Nilsson T, Järvholm B. Occupational exposure to particulate air pollution and mortality due to ischaemic heart disease and cerebrovascular disease. Occup Environ Med. 2007;64(8):515-519. [CrossRef] [PubMed]
 
The National Board of Health and Welfare. The National Board of Health and Welfare website. http://www.socialstyrelsen.se/register/dodsorsaksregistret. Accessed September 1, 2013.
 

Figures

Tables

Table Graphic Jump Location
Table 1 —Data on the Overlap With Other Exposure Groups, 1971-2011

Data given as %.

Table Graphic Jump Location
Table 2 —Basic Data on the Cohort of Swedish Male Construction Workers Followed Between 1971 and 2011a
a 

Note that an individual can occur in more than one category.

Table Graphic Jump Location
Table 3 —Basic Data on the Cohort of Swedish Male Construction Workers Followed Between 2000 and 2011a
a 

Note that an individual can occur in more than one category.

Table Graphic Jump Location
Table 4 —Mortality From COPD in Subjects Aged 50-84 Y: 1971-2011

RR = relative risk.

a 

Poisson regression analysis, adjusting for smoking, age, and BMI.

b 

Poisson regression analysis, adjusting for age and BMI.

c 

Also adjusting for other exposures.

Table Graphic Jump Location
Table 5 —Mortality From All Causes in Subjects Aged 50-84 Y: 1971-2011

See Table 4 legend for expansion of abbreviation.

a 

Poisson regression analysis, adjusting for smoking, age, and BMI.

b 

Poisson regression analysis, adjusting for age and BMI.

c 

Also adjusting for other exposures.

Table Graphic Jump Location
Table 6 —Mortality From COPD in Subjects Aged 50-84 Y: 2000-2011

See Table 4 legend for expansion of abbreviation.

a 

Poisson regression analysis, adjusting for smoking, age, and BMI.

b 

Poisson regression analysis, adjusting for age and BMI.

c 

Also adjusting for other exposures.

Table Graphic Jump Location
Table 7 —Joint Associations Between Occupational Exposures to Dust, Gases, and Fumes and Smoking Habits in Subjects Aged 50-84 Y: 1971-2011

Ref = reference. See Table 4 legend for expansion of other abbreviation.

References

Torén K, Balmes J. Chronic obstructive pulmonary disease: does occupation matter? Am J Respir Crit Care Med. 2007;176(10):951-952. [CrossRef] [PubMed]
 
Eisner MD, Anthonisen N, Coultas D, et al; Committee on Nonsmoking COPD, Environmental and Occupational Health Assembly. An official American Thoracic Society public policy statement: novel risk factors and the global burden of chronic obstructive pulmonary disease. Am J Respir Crit Care Med. 2010;182(5):693-718. [CrossRef] [PubMed]
 
Balmes J, Becklake M, Blanc P, et al. ATS statement on occupational contribution to the burden of airway disease. Am J Respir Crit Care Med. 2003;167(5):787-797. [CrossRef] [PubMed]
 
Meldrum M, Rawbone R, Curran AD, Fishwick D. The role of occupation in the development of chronic obstructive pulmonary disease (COPD). Occup Environ Med. 2005;62(4):212-214. [CrossRef] [PubMed]
 
Blanc PD. Occupation and COPD: a brief review. J Asthma. 2012;49(1):2-4. [CrossRef] [PubMed]
 
Naidoo RN. Occupational exposures and chronic obstructive pulmonary disease: incontrovertible evidence for causality? Am J Respir Crit Care Med. 2012;185(12):1252-1254. [CrossRef] [PubMed]
 
Blanc PD, Iribarren C, Trupin L, et al. Occupational exposures and the risk of COPD: dusty trades revisited. Thorax. 2009;64(1):6-12. [CrossRef] [PubMed]
 
Boggia B, Farinaro E, Grieco L, Lucariello A, Carbone U. Burden of smoking and occupational exposure on etiology of chronic obstructive pulmonary disease in workers of Southern Italy. J Occup Environ Med. 2008;50(3):366-370. [CrossRef] [PubMed]
 
Lamprecht B, McBurnie MA, Vollmer WM, et al; BOLD Collaborative Research Group. COPD in never smokers: results from the population-based burden of obstructive lung disease study. Chest. 2011;139(4):752-763. [CrossRef] [PubMed]
 
Mehta AJ, Miedinger D, Keidel D, et al; SAPALDIA Team. Occupational exposure to dusts, gases, and fumes and incidence of chronic obstructive pulmonary disease in the Swiss Cohort Study on Air Pollution and Lung and Heart Diseases in Adults. Am J Respir Crit Care Med. 2012;185(12):1292-1300. [CrossRef] [PubMed]
 
Bergdahl IA, Torén K, Eriksson K, et al. Increased mortality in COPD among construction workers exposed to inorganic dust. Eur Respir J. 2004;23(3):402-406. [CrossRef] [PubMed]
 
Torén K, Qvarfordt I, Bergdahl IA, Järvholm B. Increased mortality from infectious pneumonia after occupational exposure to inorganic dust, metal fumes and chemicals. Thorax. 2011;66(11):992-996. [CrossRef] [PubMed]
 
World Health Organization. International Classification of Diseases, 10th Revision. Geneva, Switzerland; World Health Organization; 2007.
 
World Health Organization. International Classification of Diseases, 9th Revision. Geneva, Switzerland; World Health Organization; 1978.
 
Berry G. The analysis of mortality by the subject-years method. Biometrics. 1983;39(1):173-184. [CrossRef] [PubMed]
 
Blanc PD, Torén K. Occupation in chronic obstructive pulmonary disease and chronic bronchitis: an update. Int J Tuberc Lung Dis. 2007;11(3):251-257. [PubMed]
 
Torén K, Bergdahl IA, Nilsson T, Järvholm B. Occupational exposure to particulate air pollution and mortality due to ischaemic heart disease and cerebrovascular disease. Occup Environ Med. 2007;64(8):515-519. [CrossRef] [PubMed]
 
The National Board of Health and Welfare. The National Board of Health and Welfare website. http://www.socialstyrelsen.se/register/dodsorsaksregistret. Accessed September 1, 2013.
 
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