The cohort included 161,534 men and 23,967 women had experienced a health assessment from 1993 onwards and had a job title recorded. There were 4,555 deaths among men and 196 deaths among women over 1,953,032 and 238,437 person-years, respectively.
Table 1 summarizes the distribution of workers in this analysis, by Work Categories. As shown, 48% of men and 61% of women had their first assessment in or after 2010, but the proportions vary somewhat by Work Category. About half of these workers only had one assessment (n = 98,023; 52.8%) and 29.2% (n = 37,094) had two or three assessments.
Not all members of the cohort had recorded more than one assessment at least five years after their initial assessment. Only 11% of Administration workers had more than one assessment. About 40% of male workers in Maintenance jobs, 60% of male Production workers had more than one assessment. The Production workers more than one assessment included 81% of CHPP workers (Coal Handling and Preparation Plant workers), 86% of ERZ/Deputies (Explosion Risk Zone controller) and 96% of OCE (Open Cut Examiner) and Dragline operators. The proportions with more than one assessment were lower for female workers.
Over a third of men (38%) had held at least one job in Maintenance, 34% had a job in the Production, while 31% of women had only worked in Administration and 27% of women had held one or more jobs in the Production. About two-thirds of workers across all Work Categories had other job titles within the same category, the highest proportions were reported in the Maintenance (71.4%) and Construction (70.3%) Work Categories (data presented in the supplementary file). Half of the workers who were ever Labourers (53.6%) or ever Truck Drivers (50.2%) had only been in that job group, whilst 27% of Supervisors only had that job group recorded.
Median age at first examination (after 1993) ranged from 27 years (Labourers in the Unclear Work Category) to 41 years (Administration workers and for Truck Drivers in the Unclear Work Category) among men and 26 (Occasionally exposed workers) to 37 (Supervisors) years in women (Table 1).
Smoking status was recorded for more than 99% of participants. Approximately half the cohort had ever smoked with differing proportions between Work Categories (Table 1). About half of the ever smokers reported quitting i.e. were ex-smokers.
All-cause mortality comparisons for men and women
Figure 2 illustrates the all-cause mortality SMRs compared to the general Australian population, by Work Categories. There were few women in some Work Categories, so mortality rates were compared to population rates only for selected Work Categories. The confidence intervals for All Cause SMRs overlapped for men and women in most Work Categories, although the confidence intervals were wider for women.
Summary of standardised mortality ratios for all causes of death combined, by sex and Work Category; comparison with the general Australian population rates. The Labourer, Cleaner, Supervisor and Truck Driver groups refer those in the Unclear Work Category [Note that no estimates are provided for women in the Exploration Driller, Construction, Labourer, Supervisor or Truck driver groups because of small numbers]
Comparisons within the cohort after adjusting for age, era and smoking status, showed increased all-cause aRMRs for men in the Unexposed Non-office, Production, and Construction Work Categories and for male Labourers, male Truck Drivers and female Cleaners in the Unclear Work Category (Table 2). Of these, only the findings for male Labourers and Truck Drivers were statistically significant. Male workers in the following Work Categories had significantly reduced overall relative mortality: only Administration, only Occasionally Exposed jobs, ever Maintenance, and Supervisors (Table 2).
Major causes of death for female coal mine workers
Increased risks of accidental death and suicide were seen for women in some Work Categories: excess suicides in Production workers (SMR 201, 95%CI 108–374; aRMR 3.48, 95%CI 1.25–9.67 n = 10) and deaths from accidents among Cleaners (SMR 276, 95%CI 138–552); RMR 3.13, 95%CI 1.30–7.57, n = 8) (Tables 3 and 4). For most categories of death, the rates were similar to the Australian and Queensland rates. However, the SMR compared to Queensland (SMRQ) for suicides among women cleaners was somewhat attenuated (SMRQ 177, 95%CI 95–329). Deaths in this group from accidents were still increased (SMRQ 263, 95%CI 132–526) (Table 3). Deaths from malignancies were not increased for women in any Work Category.
Major causes of death for male coal mine workers
Mortality from specific major causes of death (except from accidents and suicides) within Work Categories among men were either lower than, or similar to, the age-standardised population rates (Fig. 3; Table 5). Comparisons within the cohort, RMRs adjusted for age, era and smoking status (aRMR), showed differences in risk by Work Category (Fig. 4). Figures 3 and 4 show that Supervisors had the lowest SMRs and RMRs for all major causes of death. The SMRs for all cancers combined showed some variability between Work Categories (Fig. 3) although the confidence intervals overlapped, however the inter-category variability was reduced for RMRs after adjusting for smoking (Fig. 4).
Summary of standardised mortality ratios for selected causes of death (ICD10 codes included) among male coal mine workers by whether they were ever in the selected Work Categories. The Labourer, Supervisor and Truck Driver groups refer those in the Unclear Work Category. [Exploration Drillers and Cleaners omitted because of the small number of deaths], comparison with the general Australian population rates
Summary of relative mortality ratios (adjusted for era, age and smoking status) for selected causes of death (ICD-10 codes included) among male coal mine workers by whether they were ever in the selected Work Categories. The Labourer, Supervisor and Truck Driver groups refer those in the Unclear Work Category. [Exploration Drillers and Cleaners omitted because of the small number of deaths] compared to all other categories
Higher lung cancer mortality was observed among male Production (SMR 115, 95%CI 100–132; aRMR 1.14, 95%CI 0.93–1.40, n = 194) and Construction workers (SMR 173, 95%CI 121–248; aRMR 1.60, 95%CI 1.09–2.34, n = 30) but not in other Work Categories including Maintenance (SMR 106, 95%CI 75–109; aRMR 0.87, 95%CI 0.69–1.09). Within Production, lung cancer mortality was higher for those who ever worked as Drillers (general) (SMR 210, 95%CI 128–342, n = 16) or Operators (SMR 118, 95% 101–139, n = 153) compared to the general population. Lung cancer mortality showed a monotonic increase with time since first assessment among Production workers. Male coal mine workers aged under 65 and those over 65 had similar patterns of overall mortality, however lung cancer deaths were higher than expected for older men (65+) but not for younger men (< 65 years) in Production and Construction Work Categories (Table 6).
The non-malignant respiratory disease mortality rate, across all Work Categories, was either comparable to or lower than the expected rates when compared to the national population (Fig. 3). On the other hand, within cohort comparisons showed that the risk of non-malignant respiratory disease was raised (but did not reach statistical significance) for Maintenance workers (aRMR 1.33, 95%CI 0.99–1.79, n = 66) and Truck drivers (aRMR 1.37, 95% CI 0.84–2.22, n = 80) (Fig. 4).
The rates of COPD in the cohort were comparable to the general population rates and differed little by Work Category: Production (SMR 86, 95% CI 66–112; aRMR 0.94 95%CI 0.64–1.37, n = 54), Maintenance (SMR 99, 95% CI 74–135; aRMR 1.45, 95%CI 0.99–2.12, n = 42) and Construction (SMR 126, 95% CI 63–251; aRMR 1.51 95% CI 0.73–3.11, n = 8). Results for lower exposed workers are presented in Table 7.
Deaths from mesothelioma were increased among Maintenance Workers (SMR 168, 95%CI 101–278; aRMR 2.35, 95%CI 1.11–4.96, n = 15) and somewhat among Construction workers (SMR 228, 95%CI 73–706; aRMR 2.00 95%CI 0.61–6.59, n < 6), when compared with rates in the national population and the rest of the cohort. There were no other Work Categories with an excess of mesothelioma, there were fewer than six mesotheliomas among Production workers.
Increased mortality from all circulatory diseases was only observed among Labourers (Figs. 3 and 4). Ischaemic heart disease mortality was higher in Unexposed Non-office workers compared with the general Australian population (SMR 153, 95%CI 102–230, n = 23). After adjusting for age, era and smoking status, the within-cohort comparisons suggested that male Administration and Maintenance workers had a reduced risk of death from circulatory disease (Table 7; Fig. 4). However higher risks were seen for male Production workers, Unexposed Non-Office workers, Labourers and Truck Drivers (Table 7; Fig. 4).
Digestive disease mortality was raised for Construction workers compared to the rest of the cohort (aRMR 2.66, 95%CI 1.50–4.73, n = 14), mainly from liver disease (SMR 133, 95%CI 75–234, n = 12).
Stomach cancer mortality was higher in Occasionally Exposed workers (SMR 227, 95%CI 102–505, n = 6) while colorectal cancer mortality was higher in the Unexposed Non-Office group (SMR 229, 95%CI 123–426, n = 10) compared with the national population. Numbers of stomach and colorectal cancers were too small for meaningful internal comparisons.
Suicides were increased compared to the national population for men working in Production, Construction and as Truck Drivers (Fig. 3). However, this excess risk was attenuated (but still elevated when the cohort was compared with Queensland suicide rates) (Table 5). Internal comparisons within the cohort showed that the risk of suicide was similar across Work Categories (Fig. 4; Table 7).
An increased risk of mortality from accidents was seen in Cleaners, Truck drivers, Labourers, Exploration Drillers, Production and Construction workers when compared to Australian national population rates (Fig. 3; Table 5). However, comparison with Queensland-specific rates attenuated the increased risk for all these groups. The highest risks were for Cleaners, Truck drivers, Labourers, Exploration Drillers, Production and Construction workers (Table 5). Internal comparisons showed that male Administration, Occasionally Exposed and Maintenance workers were less likely to die from accidents than other coal mine workers, but the risk was higher for Production workers and Truck drivers (Table 7; Fig. 4).