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Abstract
Introduction The risk of asbestosis, malignant mesothelioma and lung cancer among motor vehicle mechanics is of concern because of potential exposure to chrysotile asbestos during brake, clutch and gasket repair and maintenance. Asbestos has also been used in insulation and exhaust systems.
Methods We examined the long-term risk of incident mesothelioma, lung cancer, asbestosis and other lung diseases and mortality due to mesothelioma, lung cancer, asbestosis and other lung diseases in a nationwide cohort of all men registered as motor vehicle mechanics since 1970 in Denmark. This was compared with the corresponding risk in a cohort of male workers matched 10:1 by age and calendar year, with similar socioeconomic status (instrument makers, dairymen, upholsterers, glaziers, butchers, bakers, drivers, farmers and workers in the food industry, trade or public services).
Results Our study included 138 559 motor vehicle mechanics (median age 24 years; median follow-up 20 years (maximum 45 years)) and 1 385 590 comparison workers (median age 25 years; median follow-up 19 years (maximum 45 years)). Compared with other workers, vehicle mechanics had a lower risk of morbidity due to mesothelioma/pleural cancer (n=47 cases) (age-adjusted and calendar-year-adjusted HR=0.74 (95% CI 0.55 to 0.99)), a slightly increased risk of lung cancer (HR=1.09 (95% CI 1.03 to 1.14)), increased risk of asbestosis (HR=1.50 (95% CI 1.10 to 2.03)) and a chronic obstructive pulmonary disease risk close to unity (HR=1.02 (95% CI 0.99 to 1.05)). Corresponding HRs for mortality were 0.86 (95% CI 0.64 to 1.15) for mesothelioma/pleural cancer, 1.06 (95% CI 1.01 to 1.12) for lung cancer, 1.79 (95% CI 1.10 to 2.92) for asbestosis, 1.06 (95% CI 0.86 to 1.30) for other lung diseases caused by external agents and 1.00 (95% CI 0.98 to 1.01) for death due to all causes.
Conclusions We found that the risk of asbestosis was increased among vehicle mechanics. The risk of malignant mesothelioma/pleural cancers was not increased among vehicle mechanics.
- clinical epidemiology
- asbestos induced lung disease
- lung cancer
- occupational lung disease
- mesothelioma
Data availability statement
Data may be obtained from a third party and are not publicly available. Danish law does not allow researchers to share raw data or datasets, which include individual-level datapoints from the registries with third parties. Information on contents of the DOC*X database used in this study, and on how to gain access, is available at the DOC*X website: www.doc-x.dk/en. Data linked in the DOC*X database are available through online access at Statistics Denmark under standard conditions and can be accessed by researchers through application to Statistics Denmark: https://www.dst.dk/en%23.
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Key messages
What is the key question?
The risk of asbestos-related disease among vehicle mechanics is of concern because of potential exposure to chrysotile asbestos, but evidence from long-term cohort studies is scarce.
What is the bottom line?
This nationwide 45-year cohort study found that the risk of asbestosis was increased but the risk of malignant mesothelioma/pleural cancers was not increased among vehicle mechanics, consistent with previous studies.
Why read on?
This study indicates that the risk of malignant mesothelioma is not increased among vehicle mechanics compared with other workers. However, there is increased risk of asbestosis.
Introduction
The work environment of vehicle mechanics may involve exposure to chrysotile asbestos, in addition to gasoline, diesel fuel, oils, lubricant volatiles, articulate matter with particles smaller than 10 μm, solvents, spray paint, anticorrosive substances, adhesives and particulates and exhaust from gasoline and diesel engines.1–3 Chrysotile asbestos has been used worldwide in automobile brakes since the early 1900s4–6 but is now phased out in most countries.7 During use, chrysotile fibres in brake linings undergo thermal decomposition, as chemically bound water within the chrysotile structure is driven off at temperatures between 150°C and 800°C, and are converted to forsterite, a non-fibrous mineral. Brake dust contains small amounts (0.1%–1.0%) of mostly short (<1 μm) chrysotile fibres.8 Because of the conversion of chrysotile to forsterite, the small amount of chrysotile longer than 5 μm in brake dust and the binding of resin matrix to the fibre surface,5 7 the question has been raised whether the relative toxicity of brake dust is comparable to that of chrysotile.
Comprehensive measurements of asbestos fibres in ambient air during vehicle maintenance operations found that 8-hour time-weighted average (TWA) asbestos exposures ranged from <0.002 to 0.68 f/cc, with a mean of 0.04 f/cc, among mechanics servicing automobiles and light trucks. Exposures ranged from 0.002 to 1.75 f/cc, with a mean of 0.2 f/cc, among mechanics servicing heavy trucks and buses.4 Task-specific and 8-hour TWA exposures for brake mechanics were found to be similar worldwide during the same time periods. Estimates of mean lifetime cumulative asbestos exposure among European brake mechanics range from 0.54 to 2.6 f/cc-year.1 9 10 This exposure ranged from 0.16 to 0.41 f/cc-year in the USA in the absence of dust control procedures (during the 1970s) and from 0.01 to 0.012 f/cc-year when engineering controls were used (during the 1980s).6 The lung burden of asbestos fibres in brake workers with malignant mesothelioma was found to be within reference limits in one study,11 but its conclusion was challenged by others who claimed that the concentration of tremolite fibres in the lungs indicated excessive chrysotile exposure from friction products.12 However, the original authors showed that lung levels of commercial amphiboles in the general population were significant predictors of tremolite concentrations in the brake workers’ lungs and that the duration of employment as a brake worker was unrelated to lung levels of tremolite.8
Because exposure to chrysotile asbestos can cause asbestosis, malignant mesothelioma and lung cancer in some exposure circumstances,13–15 risks of these conditions among vehicle mechanics are of concern.15–18 Regarding asbestosis, it is claimed there is a threshold of 25–100 f/cc-year for clinical asbestosis and 10–20 f/cc-year for pathological evidence of asbestosis.13 19 20
A recent meta-analysis of 16 studies found no increased risk of mesothelioma among vehicle mechanics,21 and studies since then have provided consistent results.22 23 The meta-relative risk of mesothelioma in vehicle mechanics was 0.80 (95% CI 0.46 to 1.25).21 An earlier meta-analysis of vehicle mechanics reported the meta-relative risk of lung cancer was 1.09 (5% CI 0.92 to 1.28).24 Long-term cohort studies of vehicle mechanics are particularly valuable because of historic employment records and the opportunity to evaluate risks of multiple lung diseases such as asbestosis, mesothelioma and lung cancer. Thirty years ago, a historical cohort study with 10-year follow-up for cause-specific mortality among vehicle mechanics was conducted in Denmark,25 based on linkage among a nationwide census carried out in Denmark in 1970, the Danish Central Personal Registry and the Danish National Register of Causes of Death. Persons who worked as vehicle mechanics were compared with other skilled workers who were not exposed to asbestos, petrochemical products or paint pigments. Overall, mortality due to all causes has 1.21-fold increased (standardised mortality ratio (SMR)=1.21; 95% CI 1.12 to 1.31) among vehicle mechanics, most increased for ischaemic heart disease (1.21-fold) and external causes (1.31-fold). Mortality due to all cancers combined was modestly elevated among vehicle mechanics (SMR=1.15; 95% CI 0.97 to 1.36), with an SMR close to unity (1.01; 95% CI 0.72 to 1.37) for lung cancer. One death due to mesothelioma was reported.25
To examine long-term risks of mesothelioma, lung cancer, asbestosis, other lung diseases and death, we conducted a new cohort study among Danish vehicle mechanics. The current study allowed for up to 45 years of follow-up time, compared with 10 years of follow-up in the previous study, and also included complete data on incident cancer and lung disease diagnoses through linkage with the Danish Cancer Registry and the Danish National Patient Registry.
Methods
Study population
This study took advantage of the new nationwide DOC*X Project research database, which has been approved by the Danish Data Protection Agency.26 The members of our study cohort were identified from the files of the nationwide census carried out in Denmark on 9 November 1970, in which self-reported occupation, trade, industry and employment on the day of the census were recorded for all Danish inhabitants older than age 14 years.25 Additional cohort members were identified using data on annual personal labour market affiliation, available in Danish registers starting in the late 1970s.27 The unique Danish Civil Personal Registration (CPR) number28 permitted individual-level census data and labour market data to be linked to other Danish population-based registries. These registries cover the entire population of Denmark (5.8 million inhabitants). All Danish residents have universal access to tax-financed primary and secondary healthcare services, which are provided free at the point of delivery.29
The study comprised skilled workers and apprentices, focusing on men aged between 10 and 74 years who were occupationally active (ie, employed) on the day of the census in 1970.25 Also, the study comprised all men registered with relevant jobs between 1980 and 2012, identified using the labour market data.
Exposed and comparison cohorts
We identified the exposed cohort of motor vehicle mechanics based on a combination of the occupational title/classification code for ‘mechanic’ and the industrial classification code for ‘motor car and motor cycle repair shops’ in the 1970 census. This is similar to the approach used by Hansen25 (online supplemental appendix table S1). We also selected all men who were registered as motor vehicle mechanics and/or as working in auto repair shops in the annual labour market registries between 1980 and 2012. Exposure was fixed. For instance, a man registered as a motor vehicle mechanic in the 1970 census year (whether he recently started working as a mechanic or had decades of work experience) stayed in the mechanic exposure category from 1970 until the end of the follow-up period. A man registered as a motor vehicle mechanic in the first year of the annual labour market registry year of 1980, but not in the 1970 census, stayed in the mechanic exposure category from the end of the year 1980 until the end of follow-up. We identified a total of 1 38 559 men registered as motor vehicle mechanics for the first time between 1970 and 2012.
Supplemental material
In the same way, we identified a matched comparison cohort of men not occupationally exposed to asbestos based on their occupational title/code. Following the exposure characterisation approach of Hansen,25 we first included as eligible comparators all men who were registered as an instrument maker, dairyman, upholsterer or glazier in the 1970 census or in the labour market registries between 1980 and 2012. Unlike Hansen, we did not include electricians or carpenters in the comparison group, as persons in these two occupations may be exposed to asbestos and at increased risk of mesothelioma, as has been reported previously.30–32 In addition, we included other male workers who were likely to be asbestos-unexposed. While they were not all skilled workers, their socioeconomic status was similar to that of vehicle mechanics (butchers/bakers, drivers, farmers, any workers in the food industry, trade, public services, office workers and clerks) (see online supplemental appendix table 1 for codes). Comparison workers were ineligible if they had had a job as a motor vehicle mechanic recorded at any point in time. For each member of the mechanic cohort, on the date of first registration as a mechanic, ten comparators were randomly selected (with replacement) among all eligible workers on that date. The ten comparators were matched to an individual mechanic by year of birth (±1 year) and by calendar year of first-time registration in the occupation in question. The matching resulted in 845 480 unique male workers who (due to matching with replacement) functioned as 1 385 590 comparators to the 138 559 vehicle mechanics.
Outcomes
Mortality
For each 1970 census cohort member alive on 31 December 1970 and for each labour market registry cohort member beginning on 1 January 1980, we ascertained the date of death (if any) up to and including the year 2015 from the Danish Civil Registration System (CRS). The CRS has recorded each resident’s unique CPR number, address and date of death since 1968.28 We also ascertained causes of death up to and including the year 2013 from the Danish Register of Causes of Death,33 including death due to asbestosis (International Classification of Diseases, Revision 8 (ICD-8) code 515.2; ICD-10 code J61), death caused by any lung disease due to external agents (ICD-8 codes 515–517; ICD-10 codes J60–J70), death due to lung cancer (ICD-8 162.1; ICD-10 C34) and death due to pleural cancer or mesothelioma (ICD-8 code 158 (peritoneal cancer) and 163 (pleural cancer), ICD-10 code C38.4 (pleural cancer) and ICD-10 code C45 (mesothelioma)).
Morbidity
In addition, for each census cohort member alive on 1 January 1977 and for each labour market registry cohort member beginning on 1 January 1980, we ascertained all hospital contacts up to and including the year 2015 for asbestosis (ICD-8 code 515.2; ICD-10 code J61), as well as contacts for any lung disease due to external agents (ICD-8 codes 515–517; ICD-10 codes J60–J70) and contacts for chronic obstructive pulmonary disease (COPD) (ICD-8 codes: 490–492; ICD-10 codes: J40-J44). Hospital contacts were retrieved from the Danish National Registry of Patients, which contains individual-level information on all hospitalisations since 1977 and on all hospital outpatient clinic visits since 1995.29 Patients with a previous diagnosis of any of these conditions recorded in the Danish National Registry of Patients were excluded from the analysis.
For each census cohort member alive on 1 January 1977 and for each labour market registry cohort member beginning on 1 January 1980, we identified cancer outcomes up to and including the year 2015 from the Danish Cancer Registry, which was established in 1943.34 The Cancer Registry has coded malignancies according to the ICD-10 system since 1977. We focused specifically on cancer of the lung and bronchus (ICD-10 C34) and on pleural cancer or mesothelioma (ICD-10 codes C38.4 and C45). For each cancer type, patients with a previous diagnosis of the same cancer type in the Cancer Registry were excluded from the analysis.
Statistical analysis
1970 census cohort
Each census cohort member and each comparison cohort member in the 1970 census year was followed for occurrence of a study outcome from 31 December 1970 (all-cause or cause-specific mortality) or for incident cases (morbidity) from 1 January 1977 (start of availability of hospital diagnoses and ICD-10 Cancer Registry outcomes) until the date of death, emigration or end of the period of data availability, whichever came first. Data availability ended on 31 December 2015 for incident cancers, on 31 December 2013 for cause-specific deaths, on 31 December 2015 for all-cause deaths and on 31 December 2015 for hospital diagnoses.
1980 onwards labour market registry cohorts
Beginning in 1980, each labour market registry cohort member and comparison cohort member were followed from 31 December in the year that the first occupational exposure was registered until the date of death, emigration or end of the period of data availability (see above), whichever came first.
We computed the cumulative incidence of each outcome (treating death as a competing risk) during the total follow-up period (ie, asbestosis, other lung diseases due to external agents, COPD, pleural cancer/mesothelioma and lung cancer). We similarly estimated cumulative all-cause and cause-specific mortality, considering death from other causes as a competing risk when assessing cause-specific mortality outcomes. Incidence rates (IRs) were calculated as the number of events divided by 100 000 person-years at risk. We applied Cox proportional hazards regression analyses to compute age-adjusted and calendar-year-adjusted HRs for all outcomes, comparing the exposed cohort of vehicle mechanics with the cohort of other male workers. HRs were cause-specific HRs where time was censored at time of the event, death or loss to follow-up due to either death, emigration or end of study period whichever came first. The proportional hazard assumption was evaluated by visual inspection of the log cumulative hazard function plotted against time.
Additional analyses
First, analyses were run separately for vehicle mechanics and comparators in the 1970 census cohort and then for the 1980 population, for the 1981–1990 population and for the 1991+ population, to assess the effect of successively shorter exposure versus early exposure with long follow-up. Second, we examined longer-term persistent exposure by focusing on individuals who were registered as holding a vehicle mechanic or comparison job over a long time period, that is, those who had a record of being a mechanic or other worker in the 1970 census and had similar records after 1990.
Since the study did not involve patient contact or any sort of intervention, approval and involvement of the Danish Scientific Ethical Committee were not required according to Danish legislation.
Results
We identified 138 559 vehicle mechanics, with a median age at cohort entry of 24 years (quartile range 19–37 years) and median follow-up time of 20 years (maximum 45 years). We ascertained 845 480 unique male workers with comparison jobs, who had a similar median age at cohort entry of 25 years (quartile range 19–37 years) and median follow-up time of 19 years (maximum 45 years); these workers provided 1 385 590 comparison profiles in the 1:10 matched sets (table 1).
Characteristics of 138 559 male vehicle mechanics and 1 385 590 matched male comparison workers
Incidence of study outcomes
The cumulative incidence, based on hospitalisation and cancer registry data, of pleural cancer/mesothelioma during the entire follow-up period was 0.07% (n=47) among vehicle mechanics, compared with a slightly higher cumulative incidence of 0.09% (n=628) among comparators. (table 2). The corresponding pleural cancer/mesothelioma IRs per 100 000 person-years were 1.71 among vehicle mechanics and 2.39 among comparators. IRs for lung cancer among vehicle mechanics were 66.02 versus 62.90 among comparators. For asbestosis, we estimated a cumulative incidence of 0.07% (n=47) among vehicle mechanics and 0.04% (n=313) among comparators, with an IR of 1.71 among vehicle mechanics and 1.19 among comparators. For other lung diseases due to external agents, IRs were similar (17.24 among vehicle mechanics vs 16.93 among comparators). Differences in IRs between vehicle mechanics and comparators for cause-specific deaths from the above diseases were generally similar to differences in IRs for occurrence of the diseases (table 2). Cumulative incidence function curves treating death as a competing risk are shown in figure 1.
Cumulative incidence of outcome events (with 95% CIs). (A) Mesothelioma or pleural cancer. (B) Cancer of the lung and bronchus. (C) Asbestosis. (D) Lung disease due to other external agents. (E) Chronic obstructive pulmonary disease (COPD). (F) Death due to pleural cancer or mesothelioma. (G) Death due to cancer of the lung or bronchus. (H) Death due to asbestosis. (I) Death from lung disease due to external agents. (J) All-cause death.
Risk of outcome events among vehicle mechanics versus comparators
Adjusted HRs of study outcomes
The vehicle mechanics’ age-adjusted and calendar-year-adjusted HR for pleural cancer/mesothelioma was lower than that for other male workers: HR=0.74 (95% CI 0.55 to 0.99) (table 2). Compared with other workers, vehicle mechanics had a slightly increased risk of lung cancer (HR=1.09; 95% CI 1.03 to 1.14), whereas the relative risk of COPD was close to unity (HR=1.02; 95% CI 0.99 to 1.05). Vehicle mechanics were at increased risk of asbestosis morbidity (HR=1.50; 95% CI 1.10 to 2.03) and mortality (HR=1.79; 95% CI 1.10 to 2.92), while the HR for morbidity due to other lung diseases due to external agents was 1.03 (95% CI 0.94 to 1.14). For all-cause death, the HR in vehicle mechanics was the same as that of other male workers (HR=1.00; 95% CI 0.98 to 1.01) and slightly higher for death from lung cancer (HR=1.06; 95% CI 1.01 to 1.12), but not for deaths due to pleural cancer/mesothelioma (HR=0.86; 95% CI 0.64 to 1.15).
In analyses restricted to the 21 102 vehicle mechanics and 211 020 comparators in the 1970 census cohort (who had the longest follow-up), IRs for pleural cancer/mesothelioma were 2.17 per 100 000 in vehicle mechanics versus 2.11 in comparators, resulting in an HR of 1.07 (95% CI 0.62 to 1.82). IRs of recorded asbestosis were clearly higher in vehicle mechanics included in the 1970 census (2.47 per 100 000) than in the 1970 comparators (0.68 per 100 000), resulting in an asbestosis HR of 3.86 (95% CI 2.20 to 6.76). Of the 21 102 vehicle mechanics in the 1970 census, 10 534 were still recorded as vehicle mechanics in the 1990–1999 labour market database. When focusing on these individuals who were likely employed for >20 years, with start of follow-up in 1995, IRs for pleural cancer/mesothelioma were 3.83 for vehicle mechanics and 3.00 for other workers, and the corresponding HR was 1.15 (95% CI 0.53 to 2.50). In contrast, IRs for asbestosis were higher at 4.38 and 0.55, respectively, with an HR of 7.05 (95% CI 2.88 to 17.30). These additional analyses reflect the data in figure 1C, showing an identical incidence of asbestosis in vehicle mechanics and comparators for about 25 years of follow-up after which the risks abruptly diverged. We further evaluated the temporal pattern of asbestosis by focusing on the 1970 cohort (figure 2), which was of particular interest because the elapsed time from first exposure (pre-1970) to end of follow-up (2015) in this group was at least 45 years. Figure 2A shows a significant excess of inpatient asbestos among vehicle mechanics beginning in the mid-1980s and continuing through 2015. Figure 2B shows that in the mid-2000s, there was an abrupt increase in outpatient diagnoses of asbestosis among vehicle mechanics but not among comparisons.
Cumulative incidence (with 95% CIs) of asbestosis morbidity among vehicle mechanics and comparison workers recorded in the 1970 cohort, showing (A) inpatient and (B) outpatient diagnoses by calendar year.
Discussion
The results of our long-term population-based follow-up study show increased risk of asbestosis among vehicle mechanics, do not show increased risk of malignant mesothelioma/pleural cancers and show a small increased risk of lung cancer. The results for mesothelioma incidence and mortality are consistent with case–control, cohort and standardised mortality registry studies published over the past four decades (meta-RR=0.80; 95% CI 0.61 to 1.05).21 Because of their long follow-up since first exposure, subjects who were recorded as vehicle mechanics in the 1970 census are the most informative group with respect to mesothelioma risk, and they were not at increased risk (HR=0.90; 95% CI 0.47 to 1.72). The groups who were likely employed for >20 years (those who were recorded in the 1970 census and again in the 1990–1999 labour market database), with start of follow-up in 1995, are also of interest because we have greater confidence that they were employed for at least 20 years. They were not at any significantly increased risk of mesothelioma (HR=1.15; 95% CI 0.53 to 2.50). However, insofar as asbestos-containing brakes were phased out in the 1980s, this latter group is less informative since they were younger (being still employed in 1990–1999), had more recent exposure (through 1990–1999) and were followed only after 1995.
The relative risks of lung cancer incidence and mortality were also consistent with other studies published since 1980 (meta-RR=1.09; 95% CI 0.92 to 1.28).24 Studies to date, including our current study, were largely unable to control for confounding by smoking. Thus, it cannot be concluded that the weak association between working as a vehicle mechanic and lung cancer is unrelated to smoking. However, the graph of cumulative incidence of COPD (figure 1E) suggests that there were no major differences in smoking habits between vehicle mechanics and comparison workers. Confounding by other respiratory carcinogens (Polycyclic aromatic hydrocarbons (PAHs) and diesel exhaust) cannot be ruled out with respect to lung cancer.
Our findings for asbestosis incidence and asbestosis mortality indicate an association with work as a motor vehicle mechanic. The cumulative incidence graphs (figure 1C,H) demonstrate that asbestosis morbidity and mortality were comparable between vehicle mechanic and comparison workers for the first 25 years of follow-up and then abruptly changed, showing higher risks among vehicle mechanics thereafter. The 1970 vehicle mechanics cohort showed an excess of inpatient asbestosis beginning in the mid-1980s (figure 2A) and an abrupt increase in outpatient asbestosis beginning in the mid-2000s (figure 2B). There was no similar pattern for other outcomes (mesothelioma, lung cancer, lung disease due to other external agents and COPD). As discussed above, clinical asbestosis occurs above 25–100 f/cc-year, and pathological asbestosis can be seen at cumulative exposures of 10–20 f/cc-years.19 Asbestosis shows no specific clinical, physiological or radiological features that differentiate it from other forms of diffuse interstitial fibrosis, and asbestosis may be misdiagnosed as idiopathic fibrosis. Pathological confirmation remains the gold standard for diagnosis,19 35 but lung biopsy is not routinely recommended in Denmark. Assessment of a potential occupational exposure history (including work as vehicle mechanic) thus becomes key in labelling interstitial fibrosis as asbestosis. Increased attention to asbestosis in Denmark in the 2000s led to closer assessment by occupational physicians, and diagnostic bias (ie, detecting and coding a larger proportion of true asbestosis cases among vehicle mechanics than among other people) may therefore have contributed to the apparently abrupt increase in outpatient asbestosis incidence in vehicle mechanics in the mid-2000s. Still, the excess of both inpatient and outpatient diagnoses in the 1970 cohort, as well as the increased risk of asbestosis morbidity and mortality in the entire cohort, supports a conclusion that there is increased risk of asbestosis in vehicle mechanics in Denmark.
Plato reported that the cumulative asbestos exposure among 103 car and bus mechanics employed for a mean of 29 years between 1938 and 1986 was 2.6 f/cc-years (range 0.1–11.6) and that the annual cumulative asbestos exposure was highest for Swedish truck mechanics in the early 1960s (peaking at 0.41 f/cc in 1959–1964).36 Rohl reported short-term exposures of 3.8 f/cc (range 1.7–7.0) during grinding of used brake linings and 37.3 f/cc (range 23.7–72.0) during beveling new linings during truck brake service in New York City in the 1970s.37 Denmark banned asbestos in 1980 with an exception for asbestos cement products and brake linings. In 1986, a total ban of all asbestos import and consumption was put in force. Thus, it is possible that Danish truck mechanics exposed in the 1950s–1970s may have had sufficient cumulative exposure to develop pathological asbestosis. However, previous clinical studies of car and truck mechanics have not reported increased risk of interstitial lung disease by clinical or radiological findings.36 38–43 More recently, Ameille evaluated 103 automobile mechanics who had no other source of occupational exposure to asbestos for asbestos-related disease using high-resolution computed tomography.44 Pleural plaques were observed in five cases, and interstitial abnormalities consistent with asbestosis were observed in one case.
The question that must be addressed is how our study found increased risk of asbestosis yet no increased risk of mesothelioma among vehicle mechanics. The appropriateness of our control population is supported by the fact that the IR of mesothelioma in our comparison workers was 2.39 per 100 000 person-years, which is 31% of the IR among Danish men of the same median age (7.76 per 100 000 person-years),45 suggesting that our controls were less likely to have been exposed to asbestos than the general population of Danish men. The relationship between mesothelioma risk and asbestosis risk differs for chrysotile and commercial amphiboles. Churg reported that for workers exposed to commercial amphiboles (amosite and crocidolite), mesothelioma appears at levels far below those at which asbestosis appears, whereas for workers exposed to chrysotile, mesothelioma appears only at levels sufficient to produce asbestosis.46 Other studies report that there is no significant correlation between pulmonary fibrosis scores and chrysotile fibre content in the lung.47 48 Lacquet reported that, among asbestos cement workers exposed to chrysotile, no cases of asbestosis were seen below 100 f/cc-years and 16 cases were seen at exposure levels between 100 and 800 f/cc-years, yet no cases of mesothelioma were seen below 800 f/cc-years.49 Ferrante reported on asbestosis and mesothelioma among workers at the Balangero chrysotile mine.50 Among workers with cumulative exposure below 27 f/cc-years, there were no cases of asbestosis and one case of pleural malignancy (SMR=2.92; 95% CI 0.07 to 16.27), and among workers with exposures between 27 and 345 f/cc-years, there were eight cases of asbestosis (SMR=225.2; 95% CI 97.2 to 443.6) and two cases of pleural malignancy (SMR=3.68; 95% CI 0.44 to 13.28). These studies collectively suggest that asbestosis occurs at cumulative chrysotile exposure levels where mesotheliomas are rare or none were observed and that asbestosis does not correlate well with measures of cumulative chrysotile exposure. Our results are consistent with this pattern. Conversely, animal studies indicate that inhalation of brake dust alone or brake dust with added chrysotile does not produce interstitial fibrosis in rats.51 Although the scientific literature suggests that asbestosis may occur at cumulative chrysotile exposures where mesotheliomas have not been seen, our findings on asbestosis need confirmation in future studies. Since smoking is also associated with increased risk of interstitial fibrosis,52 it is possible that confounding by smoking may have biased our results for asbestosis. It is also possible that with longer follow-up, increased risk of mesothelioma might be observed.
Strengths and limitations
The study’s strength was our ability to draw on unique population-based databases that included all persons who were skilled workers and apprentices in Denmark in 1970 and their complete, prospectively collected medical data for all hospital contacts, cancer and death. Several potential limitations must be noted. As data on hospital contacts and cancers coded according to ICD-10 were not available for approximately the first 6 years of follow-up (from November 1970 to 1976), subjects who died early could not be included in analyses of disease incidence. Also, if exposure (being a vehicle mechanic) had led to early death from outcome conditions (lung disease and cancers), the associations may have been underestimated. Still, mortality within 10 years was low in our study: only 274 of 21 105 vehicle mechanics (1.3%) died. Apart from these deaths, there was virtually no loss to follow-up in our cohort study, and the risk of selection bias thus was small.
As the study was based on administrative coding, it was dependent on the validity of registry data. However, the positive predictive values for cancer and for major diseases in the registries are high,29 and the completeness and accuracy of the Danish Cancer Registry with respect to mesothelioma are considered to be high.45 Finally, unmeasured confounders, that is, risk factors for lung disease and cancer, with distributions that are skewed between vehicle mechanics and other skilled workers, may have affected our risk estimates. These potential confounders include unmeasured exposure to chemical products; lifestyle habits including tobacco smoking, diet, alcohol intake and physical activity; anthropometrical measures; and socioeconomic and educational factors, as previously discussed by Hansen.25 It is possible that vehicle mechanics were less likely to work in other asbestos-exposed jobs (either before or after working as vehicle mechanics) than were comparisons, which could explain why there was no increased risk of mesothelioma, but this possibility is incompatible with the observed excess of asbestosis. We did not control for geography in our analyses and cannot rule out that vehicle mechanics might have preferentially lived in different areas, where there were differences in background risks (due to shipyards or asbestos manufacturing industries), than comparators. Our results for COPD morbidity suggest that there is no appreciable difference in smoking habits between vehicle mechanics and comparison workers. If individual-level data on smoking were available, assessment of confounding by smoking for lung cancer, asbestosis and other non-malignant lung diseases could be useful.
Brake repairs for trucks and buses entail higher exposure to chrysotile asbestos than passenger car brake repairs.4 53 Also, tasks such as machine grinding of new brake linings without local exhaust and cleaning using compressed air may confer brief, high exposure.53 As we were unable to identify brake mechanics who specialised in repair of trucks and buses, an increased risk of asbestos-related disease, if any, in this subgroup of vehicle mechanics may have been hidden in the overall risk estimate. Auto repairs may also involve exposure to other asbestos-containing parts such as gaskets, clutches, insulation and exhaust systems.
Some authors question whether studies of vehicle mechanics are the appropriate setting to examine risks of mesothelioma from chrysotile exposure.54 55 Because chrysotile is capable of causing mesothelioma, lung cancer and asbestosis, it is important to study settings in which cumulative exposures are low in order to characterise the risk in those settings, as we have done. Those authors also assert that many motor vehicle mechanics have done no brake work and that, among those who had, most would have done so infrequently as part of a broad array of activities.54 The published estimates of cumulative chrysotile exposure among motor vehicle mechanics take into account the percentage of time spent in brake repair versus other activities and the distribution of job tenure among vehicle mechanics.6 Thus, the risks we report among vehicle mechanics recognise that their exposures vary, and the risks we report are the group average across all cumulative exposures. It is possible that some individuals may have substantially higher cumulative exposure than the group mean and may have higher risks—this is why we studied vehicle mechanics who were employed before 1970 (when asbestos-containing brakes were widely used) and those who were employed as vehicle mechanics for more than 20 years. We also believe that having individual-level historic data on chrysotile exposure in vehicle repair jobs would improve our exposure assessment, but we are not aware of any such data. Job-title-based analyses may not reflect the risks in subgroups that are at substantially higher exposure than the group mean. It is also important that the exposures of vehicle mechanics are not fixed in time; future studies should include proportional hazard models that take into account the time-varying nature of exposures.
Conclusions
This nationwide study found that the risk of asbestosis was elevated in vehicle mechanics. Malignant mesothelioma/pleural cancer was not increased among vehicle mechanics compared with other demographically similar workers. Future studies should examine whether this association is based on clinical evidence or pathological evidence from biopsies and autopsies. Risk of lung cancer was weakly increased in vehicle mechanics, possibly related to asbestos, other occupational exposures, tobacco smoking or other adverse lifestyle factors.
Data availability statement
Data may be obtained from a third party and are not publicly available. Danish law does not allow researchers to share raw data or datasets, which include individual-level datapoints from the registries with third parties. Information on contents of the DOC*X database used in this study, and on how to gain access, is available at the DOC*X website: www.doc-x.dk/en. Data linked in the DOC*X database are available through online access at Statistics Denmark under standard conditions and can be accessed by researchers through application to Statistics Denmark: https://www.dst.dk/en%23.
Ethics statements
Patient consent for publication
Ethics approval
The study was approved by the Danish Data Protection Agency. By Danish law, informed consent is not required for a register-based study.
References
Supplementary materials
Supplementary Data
This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.
Supplementary Data
This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.
Footnotes
Twitter @dr_rwt
Correction notice This article has been corrected since it was published Online First. An author first name was incorrectly added to their last name.
Contributors RWT, DHG, JPEB and HTS conceived and designed the study. EMF, JPEB and AHR retrieved the data. AHR analysed the data. All authors participated in the interpretation of the results of the data analysis. RWT drafted the initial manuscript. All authors revised the manuscript critically and approved the final manuscript for submission.
Funding This study was supported by a research grant from EpidStat Institute, USA, to Aarhus University.
Competing interests DHG has served as the president of EpidStat Institute and has served as an expert witness in matters involving mesothelioma, lung cancer and asbestosis among vehicle mechanics on behalf of defendants including motor vehicle manufacturers; brake, clutch and gasket manufacturers; and motor vehicle parts distributors. EpidStat received payment for DHG’s services as an expert witness.
Provenance and peer review Not commissioned; externally peer reviewed.
Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.