William G. B. Graham

Mortality in Vermont granite workers and its association with silica exposure

Objectives To assess mortality in Vermont granite workers and examine relationships between silica exposure and mortality from lung cancer, kidney cancer, non-malignant kidney disease, silicosis and other non-malignant respiratory disease. Methods Workers employed between 1947 and 1998 were identified. Exposures were estimated using a job–exposure matrix. Mortality was assessed through 2004 and standardised mortality ratios (SMRs) were computed. Associations between mortality and exposure to silica were assessed by nested case–control analyses using conditional logistic regression. Results 7052 workers had sufficient data for statistical analysis. SMRs were significantly elevated for lung cancer (SMR 1.37, 95% CI 1.23 to 1.52), silicosis (SMR 59.13, 95% CI 44.55 to 76.97), tuberculosis (SMR 21.74, 95% CI 18.37 to 25.56) and other non-malignant respiratory disease (SMR 1.74, 95% CI 1.50 to 2.02) but not for kidney cancer or non-malignant kidney disease. In nested case–control analyses, significant associations with cumulative exposure to respirable free silica were observed for silicosis (OR 1.13, 95% CI 1.05 to 1.21 for each 1 mg/m3-year increase in cumulative exposure) and other non-malignant respiratory disease (OR 1.10, 95% CI 1.03 to 1.16) but not for lung cancer (OR 0.99, 95% CI 0.94 to 1.03), kidney cancer (OR 0.96, 95% CI 0.84 to 1.09) or non-malignant kidney disease (OR 0.95, 95% CI 0.84 to 1.08). Conclusions Exposure to crystalline silica in Vermont granite workers was associated with increased mortality from silicosis and other non-malignant respiratory disease, but there was no evidence that increased lung cancer mortality in the cohort was due to exposure. Mortality from malignant and non-malignant kidney disease was not significantly increased or associated with exposure.

Vermont granite mortality study: an update with an emphasis on lung cancer.

This mortality study extends the period of observation of an article published in 1988 1 of 5414 workers in Vermont granite sheds and quarries to assess whether previously reported reductions in silicosis and tuberculosis mortality were maintained. The relationship between lung cancer and quartz exposure is also examined by comparing mortality in workers hired before and after 1940, when dust controls were introduced and exposures were reduced by 80% to 90%. Before 1940, general stone shed air contained 20 million particles/cubic foot (mppcf) (approximately equivalent to 0.2 mg/m2 of quartz), and pneumatic chisel workers were exposed on average to 60 mppcf (approximately equivalent to 0.6 mg/m2 of quartz). Other workers had variable exposures. After 1940, a period of decline occurred in dust levels and then stabilized in approximately 1955, when average dust levels were 5 to 6 mppcf (equivalent to 0.05–.06 mg/m2 of quartz). Dust exposures in the Vermont industry is considered to be free of confounding occupational substances such as arsenic, although cigarette smoking was common. By the end of 1996, 2539 workers, or 46.9% of the cohort, had died. There were no silicosis deaths in workers hired after 1940 who were exposed only in the Vermont granite industry, illustrating the effect of lowering quartz exposures. Tuberculosis caused 2 deaths in those hired after 1940 (standardized mortality ratio [SMR] = 0.52; not significant). Overall lung cancer mortality was elevated in shed workers who had been exposed both to high levels of quartz before 1940 and to the lower levels prevailing after 1940 (SMR = 1.32; P < 0.01). Quarry workers did not show an excess of lung cancer (SMR = 0.73; not significant). When shed workers with high and low exposure histories (before and after 1940) but with comparable latency and tenure were contrasted, lung cancer mortality was similar. Differing levels of quartz exposure, which resulted in large differences in the mortality experience from silicosis, did not result in differences in lung cancer mortality. The results do not support the hypothesis that granite dust exposure has a causal association with lung cancer.

Radiographic abnormalities in long-tenure Vermont granite workers and the permissible exposure limit for crystalline silica.

This study was undertaken to assess the prevalence of radiographic abnormalities consistent with silicosis in a group of 600 retired granite workers who were receiving pensions. Files of regional clinics and hospitals were searched for chest radiographs taken on these men, and 470 x-ray films suitable for interpretation were located. After exclusions (women, and men who had worked in the granite industry elsewhere), 408 x-ray films were independently read by three experienced readers using the 1980 International Labour Office scheme. Dust exposures were estimated for workers hired after 1940, when the dust-control standard of 10 million particles per cubic foot (mppcf) (equivalent to 0.1 mg/m3) was put in place and monitored by the Vermont Division of Industrial Hygiene. Dust levels were initially high but were gradually reduced from 1940 to 1954, after which average quartz exposures stabilized to a mean of approximately 0.05 to 0.06 mg/m3; however, about 10% to 15% of samples after 1954 exceeded 0.1 mg/m3. Of the 408 x-ray films, 58 were taken on workers hired before dust controls were instituted in 1940, and 25.9% showed abnormalities (a profusion score of 1/0 or greater). A total of 350 x-ray films were taken on workers hired in 1940 or after, and the prevalence in this group was 5.7%. The radiographic changes in workers hired after 1940 are likely due to excessive exposures during the first 15 years of dust control. We conclude that if the exposure standard of 0.1 mg/m3 is rigorously observed in the workplace, radiographic abnormalities caused by quartz dust in long-term workers will be rare.

Silica Exposure Assessment in a Mortality Study of Vermont Granite Workers

A study of past silica and respirable dust exposures in the Vermont granite industry was conducted to develop a job exposure matrix (JEM) that used 5204 industrial hygiene measurements made from 1924–2004. The construction of the JEM involved data entry from several original sources into an Excel database that was reviewed later to ensure accuracy. Exposure measurements by job or location were grouped in two broad categories of quarry or shed and then into 22 job classes. Missing exposure data by time period were computed, taking into account improvements in dust control and periods of significant reduction in dustiness. Percent free silica (α-quartz) in respirable dust was estimated to be 11.0% based on previous published studies in Vermont and on data in the current database. About 60% of all measurement data (primarily from years prior to 1972) were obtained using the impinger and expressed in millions of particles per cubic foot (mppcf), which were converted to equivalent respirable free silica concentrations using the conversion of 10 mppcf = 0.1 mg/m3 of respirable silica. For impinger data, respirable dust was calculated by multiplying respirable silica by a factor of 9.091 to reflect that the respirable silica was 11.0% respirable dust. This JEM has been used in a recent epidemiologic study to assess mortality in Vermont granite workers and to examine the relationships among mortality from silicosis, lung cancer, and other nonmalignant respiratory diseases.