Radiographic Abnormalities in Vermont Granite Workers Exposed to Low Levels of Granite Dust

doi:10.1378/chest.100.6.1507

Chest

Volume 100, Issue 6, December 1991, Pages 1507-1514

https://doi.org/10.1378/chest.100.6.1507 Get rights and content

The issue of whether low levels of granite dust exposure lead to radiographic abnormalities after a lifetime of exposure has not been settled. In 1983, we carried out a radiographic survey of the Vermont granite industry, consisting of quarry and stone shed workers who had been exposed to the low dust levels prevailing in the industry since 1938 to 1940. Films were read by three “B” readers, using the ILO classification system, which requires the identification of both rounded and irregular opacities, as well as combinations of both. X-ray films were taken of 972 workers, out of a total work force of approximately 1,400. Of these films, 28 (3 percent) were interpreted by either two or three of the three readers as showing abnormalities consistent with pneumoconiosis. Only seven films (or 0.7 percent of the entire cohort) showed nodular or rounded opacities of the type typically seen in uncomplicated silicosis. The remainder of the abnormal x-ray films showed irregular opacities, largely in the lower lung zones, which are of uncertain significance, but may be related to heavy cigarette smoking and aging, and possibly dust inhalation. In addition, total gravimetric dust concentrations in the workplace were measured; 417 respirable-size mass samples showed concentrations of 601 μg/cu m±368 μg/cu m. Using previously published estimates of 10 percent quartz in granite dust, the average quartz concentration was 60 pg/cu m. Twelve percent of the samples exceeded 100 pg/cu m, the current OSHA standard for quartz. We conclude that control of quartz exposure in the Vermont granite industry to levels which are on average less than the current OSHA standard has essentially eliminated definite radiographic changes of silicosis. The significance of the irregular opacities in the lower lung zones seen on a majority of the 28 x-ray films judged to be abnormal is not clear.

Section snippets

MATERIALS AND METHODS

All workers employed in 1983, including quarry, stone shed, and office workers, were offered 14 × 17-in chest roentgenograms, which were taken in a mobile van provided by the Appalachian Laboratory of Occupational Safety and Health. Work histories which had been recorded in previous surveys were updated. Workers not previously logged into the files had complete occupational histories taken, including the type of job (such as cutter, polisher, sawyer, etc), the name of the shed where employed,

RESULTS

Out of a total of approximately 1,400 granite workers in the work force, 972 had x-ray films taken; 31 of those were women, all but one of whom was an office worker. Of the workers who did not have x-ray films taken, 102 were absent on the day of the survey, and the remainder refused.

Of the x-ray films, 28 (3 percent) were interpreted by either two or three of the three readers as showing abnormalities (either rounded or irregular) consistent with pneumoconiosis, in a profusion score of 1/0 or

DISCUSSION

These results indicate that radiographic abnormalities in the Vermont granite work force are occurring at a low level, that the changes observed are of low grades of profusion, and that the predominant change is of the irregular or reticular type (stu), which is not thought to be typical of early silicosis. This occupational disease is generally manifested by small nodulations or rounded densities which tend to be located in the upper lobes.⁹ Standard texts on occupational lung disease and on

ACKNOWLEDGMENTS

The Appalachian Laboratory for Occupational Safety and Health, Morgantown, WVa, kindly loaned us a mobile x-ray van. L. Bizzozero and B. Clark provided expert secretarial and logistical support. R. Stewart collected and analyzed dust samples, and M. Rigatti did additional x-ray diffraction work.

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Chronic inhalation exposure of workers to crystalline silica can result in silicosis. The general public can also be exposed to lower levels of crystalline silica from quarries, sand blasting, and entrained fines particles from surface soil. We have derived an inhalation chronic reference exposure level for silica, a level below which no adverse effects due to prolonged exposure would be expected in the general public. Incidence of silicosis and silica exposure data from a cohort of 2235 white South African gold miners yielded a reference level of 3 μg/m³ for respirable silica (particle size as defined occupationally) using a benchmark concentration approach. Data from cohorts of American gold miners, Chinese tin miners, diatomaceous earth workers, and black South African gold miners yielded similar results with a range of 3–10 μg/m³. Strengths of the chronic reference exposure level include the availability of several large long-term studies of inhalation in workers at varying exposure concentrations, adequate histopathological and radiologic analysis, adequate follow-up of exposed workers, a dose–response effect in several studies, observation of a No Observed Adverse Effect Level in the key study, and the power of the key study to detect a small effect. Uncertainties include the general underestimation of silicosis by radiography alone and the uncertainties in exposure estimation.
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The results of all the studies have concurred that occurrence of obstructive disease is significantly greater among individuals with both silica and tobacco exposure in comparison with either one separately. Our study supports the initial results of the Vermont granite workers study, which found a decrement in pulmonary function results within the existing allowable air levels for silica,452223 which were not confirmed by later findings from other investigators.32324 We have presented our results using both the prevalence of PFT results below those commonly used as clinical cutoffs of normal and abnormal, and as differences in the means of the pulmonary functions.
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Analysis of spirometry data (FVC, FEV₁, FEV₁/FVC) only using the test results that met American Thoracic Society criteria for reproducibility and acceptability shows decreasing percent-predicted FVC and FEV₁ and decreasing FEV₁/FVC in relationship to increasing silica exposure among smokers. Logistic regression analyses of abnormal FVC and abnormal FEV₁ values (where abnormal is defined as < 95% confidence limit for predicted using the Knudson prediction equations) show odds ratios of 1.49 and 1.68, respectively, for occurrence of abnormal result with 40 years of exposure at the Occupational Safety and Health Administration (OSHA)-allowable level of 0.1 mg/m³. Longitudinal analyses of FVC and FEV₁ measurements show a 1.6 mL/yr and 1.1 mL/yr, respectively, decline per milligram/cubic meter mean silica exposure (p = 0.011 and p = 0.001, respectively). All analyses were adjusted for weight, height, age, ethnicity, smoking status, and other silica exposures. Systematic problems leading to measurement error were possible, but would have been nondifferential in effect and not related to silica measurements.
There is a consistent association between increased pulmonary function abnormalities and estimated measures of cumulative silica exposure within the current allowable OSHA regulatory level. Despite concerns about the quality control of the pulmonary function measurements use in these analyses, our results support the need to lower allowable air levels of silica and increase efforts to encourage cessation of cigarette smoking among silica-exposed workers.
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Supported by NIOSH grant RO1 OHO1035-04.

Manuscript received October 12, 1990; revision accepted April 4.

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Chest

Clinical InvestigationsRadiographic Abnormalities in Vermont Granite Workers Exposed to Low Levels of Granite Dust

Section snippets

MATERIALS AND METHODS

RESULTS

DISCUSSION

ACKNOWLEDGMENTS

Silicosis and dust control

Public Health Rep

Twenty-six years experience with dust control in the Vermont granite industry

Ind Med Surg

Pulmonary function and roentgenographic changes in granite dust exposure

Arch Environ Health

Dust exposure in the Vermont granite sheds

Arch Environ Health

Statistical methods for rates and proportions

Estimation of long term dust exposures in the Vermont granite sheds

Am Ind Hyg Assoc J

Silicosis

Occupational lung diseases

Clinical Investigations
Radiographic Abnormalities in Vermont Granite Workers Exposed to Low Levels of Granite Dust