Silicosis prevalence and risk factors in black gold miners

Background  
 
Silicosis is an incurable but preventable disease, with a number of recognised complications including tuberculosis, loss of lung function, progressive massive fibrosis and lung cancer.   Recent studies have shown a high prevalence of silicosis among ex-goldminers. Although black goldminers have historically held the highest exposure jobs on the goldmines, there have remarkably been no studies of silicosis exposure response relationships in employed black South African goldminers.  
 
Objectives
 
The objectives of this study were:   
1. To measure  the prevalence of silicosis among  in-service black goldminers;   
2. To measure the exposure response relationship between silica dust exposure and silicosis in this group.  
 
Population and sample  
 
The setting was a gold mine in the Free State. The intention was to include black mineworkers over 40 years of age to secure a longer service sample in whom enough silicosis was present to examine exposure response associations.  In the final sample the youngest age was 38 years. This is also a suitable group in which to examine prevalence as the burden of silicosis falls on older workers.  
 
Methods  
 
A consecutive sample of 520 mineworkers returning from annual leave underwent medical examinations, including a questionnaire and chest x-ray. Among these 520 men, 85 different occupations were represented.
  
A cumulative exposure index was calculated for each individual by incorporating all jobs worked by that individual on the mine and an 8 hour  time weighted average (TWA) respirable dust and quartz concentration assigned to each job.  
 
TWA dust concentrations came from two different sources. Gravimetric measurements were carried out on a sample of 112 workers on the same mine (who were not part of the x-ray study) across a range of occupations.    Quartz fractions were calculated by a contracted laboratory using the method of x-ray diffraction. TWA quartz concentrations were derived by applying the quartz fractions to the corresponding respirable dust concentrations.
 
Of the 85 different occupations in the group undergoing chest x-rays, only 26 were represented in the dust measurement study. The exposure data obtained during the project were thus augmented with exposure data from routine exposure surveys taken at the mine [Integrated Risk Management System (IRMS) database].  After entering the data from these two sources into a job exposure matrix, the 85 different occupations were eventually reduced to 23 occupational groups by grouping together occupations with similar exposures based on the expert opinion of two occupational hygienists familiar with the mine.  To each occupational group, a time weighted average (TWA) respirable dust concentration and TWA quartz concentration were assigned by taking a simple average of all concentrations available for that category.   
An average intensity of exposure for each individual was calculated by dividing the cumulative exposure by the length of mining service of that individual.
 
Prevalence of silicosis
 
All 520 chest x-rays were read by two experienced readers using the International Labour Organisation Classification of Radiographs of the Pneumoconioses (“ILO”).  Agreement between the two readers was very high. For example using ILO profusion 1/1 and above as the threshold for silicosis (dichotomous scale), the percent agreement was 93.5 percent and the kappa statistic 0.79, indicating excellent agreement. Most of the films were of good or acceptable quality. For  simplicity of description, only the results of reader one are described further.  
Out of the total of 520, only 226 chest x-rays (43.5 percent)  were read as completely normal.    A further 150 (28.9 percent) were read as abnormal but as having no parenchymal abnormalities on the ILO scale (i.e. were equivalent to 0/0).  
Using ILO profusion >1/0 as the definition of silicosis, the prevalence was 23.9  percent.  When ILO profusion >1/1 was used as the cut point, the prevalence of silicosis was 18.3  percent.   
The first finding is thus that in among in-service black goldminers over 38 years of age, almost one in five have evidence of silicosis at the 1/1 level of profusion, and almost a quarter at the 1/0 level.  
In interpreting this prevalence derived from a cross-sectional study, one has to take into account a number of factors.  The group studied excluded workers under the age of 38 years, who would have a lower prevalence of silicosis in keeping with their shorter service.  On the other hand, the workforce is a “healthy survivor” cohort in that workers found to have complicated pneumoconiosis (including silicotuberculosis) are required by law to be barred from further risk work and would be selected out of the workforce.  The prevalence in current workers is thus an underestimate.  Finally, quartz exposure carries a substantial lifelong risk of incident (newly appearing) silicosis even after exposure has ceased, so that the study underestimates the lifetime risk for any individual.  In an earlier study of white South African goldminers, the onset of over half of the radiological silicosis cases occurred after the worker had left exposure. 
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