Silica Exposure Assessment in a Mortality Study of Vermont Granite Workers

Abstract

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.

Keywords:

• gravimetric concentration
• impinger counts
• job exposure matrix
• respirable dust
• respirable free silica (α-quartz)
• retrospective exposure elements

ACKNOWLEDGMENTS

This study was supported by the Crystalline Silica Panel of the American Chemistry Council (ACC) through a contract with University of Vermont (contract CS-2004–6.0-UV-Vacek). ACC had no role in data collection, statistical analysis, or interpretation of results. We are grateful to Chief Engineer Don Murray and Safety Director Alan Garceau, both with Rock of Ages Corporation, for their help and cooperation during the study. We also thank Mel Freiberg for his insight into past conditions in the Vermont granite industry.

Notes

^A Location: 1 = shed; 2 = quarry.

^B Trend applied to quarry using jackhammer and driller data (quarry workers).

^C Values extrapolated based on similarity to shed exposure.

^A Location: 1 = shed; 2 = quarry.

^B Trend applied to quarry using jackhammer and driller data (quarry workers).

^C Values extrapolated based on similarity to shed exposure.