The main purpose of the study was to assess and characterize the exposure to dust and selected dust constituents in the Norwegian silicon carbide industry from 1913 to 2005 and construct a retrospective job-exposure matrix for use in epidemiological studies. The dust constituents were selected based on their known or suspected lung carcinogenicity and presence in the SiC industry.
Materials and methods
An exposure assessment based on repeated random personal sampling within a priori defined job groups was performed in the three Norwegian silicon carbide plants in 2001-2003. Total dust was sampled in parallel with respirable dust or fiber. Total dust and respirable dust was analysed gravimetrically and fibers were counted by phase contrast microscopy. The respirable dust was analysed for the content of quartz, cristobalite and silicon carbide by X-ray diffractometry. To characterize the morphology and chemical composition of the fibers, additional samples were analysed using a scanning electron microscope. Information on tasks performed and other possible determinants of exposure was collected and linear mixed effect models were used to identify predictors of reduced or increased exposure. Exposure measurements and information on process and technological changes for the retrospective exposure estimation were obtained from available sources. As the majority of exposure measurements were of total dust, these were used as the basis for the retrospective exposure assessment. Linear regression models were developed to estimate total dust exposure for the time periods with exposure measurements (1967-2005). The exposure estimates were extrapolated backwards to periods without total dust measurements by applying multiplicators for relative changes in exposure due to process related changes and changes in working hours. The parallel sampling from the current study enabled us to construct linear mixed effect models to estimate the content of respirable dust, fiber, quartz, cristobalite and silicon carbide in total dust for the job groups and plants. These models were then applied to the total dust JEM. The performance of the models was evaluated with available historical exposure measurements of crystalline silica and fibers. PAH exposure was assessed semi-quantitatively and asbestos qualitatively.
The fiber characterization in the furnace department showed that the silicon carbide fibers could be divided into eight groups based on morphology, in addition to cleavage fragments of silicon carbide. More than 90 % of the fibers in the furnace department were silicon carbide fibers, with less than two percent of cleavage fragments. In the processing department 82 % were silicon carbide fibers, of which cleavage fragments constituted 57 %. Exposure to sulphur dioxide, fiber, quartz and cristobalite was mainly restricted to job groups in the furnace department. Exposure to silicon carbide and total dust was significantly higher in the processing department than in the furnace and maintenance departments. The cleaner operator, charger and charger/mix operators were generally the highest exposed job groups in the furnace department, and the refinery crusher operator was the highest exposed job group in the processing department. More than 3300 historical total dust exposure measurements were available from 1967-2005 and the current study added another 702 total dust measurements. The linear regression models of total dust described historical exposure best in the furnace department (R2adj = 0.49-0.74). Models in the other departments explained less variance (R2adj = 0.12-0.32). Exposure determinants and total dust exposure explained a substantial portion of the between- (70-100 %) and within-worker (8.0-54 %) variance in the mixedeffect models. The relative bias between the available historical dust measurements and the estimated exposure to dust components varied between -39 % (fiber) and 40 % (quartz). However corrections were not considered necessary due to limitations in the historical data. The rPearson correlation coefficient for the exposure estimates were below 0.7 for all pairs with the exception of total dust and respirable dust (rPearson = 0.84) and total dust and cristobalite (rPearson = 0.72). Job group was a strong determinant of exposure for all agents, explaining between 43-74 % of the between-worker variance. Determinants associated with increased exposure in the furnace department were performing the sorting of the crude silicon carbide inside the furnace hall, and the tasks cleaning, assisting in assembling and filling of furnaces and manual sorting. Filling and changing pallet boxes were important tasks related to increased exposure to total dust, respirable dust and silicon carbide in the processing department. Work in control rooms, laboratories, fresh air ventilated crane cabins, offices and maintenance outside the furnace hall and processing department were predictors of decreased dusts exposure. For maintenance workers, increased exposure to fiber was associated with maintenance in the furnace department and increased exposure to SiC was related to maintenance in the processing department.
Workers in the silicon carbide industry are exposed to a mixture of several agents, including silicon carbide fibers, quartz, cristobalite, non-fibrous SiC and sulphur dioxide. The current exposure levels are generally below the current Norwegian OELs, however, high exposure to fibers and respirable dust still occur in the furnace department. The increased number of total dust measurements and the comparative exposure study using parallel sampling of total dust, fibers and respirable dust with consecutive statistical modeling, made it possible to develop a new and improved JEM. Uncertainties remain in the exposure estimates, especially earlier than 1967 and for certain job groups without exposure measurements. The component specific metrices were sufficient different from each other to be used in component specific epidemiolocial analysis with the exception of total dust and respirable dust and total dust and cristobalite. Job group was a strong determinant of exposure for all agents. Several tasks were associated with increased exposure, indicating possibilities for exposure control measures. Recommendations for exposure reduction based on this study are to (1) separate the sorting area from the furnace hall, (2) minimize manual work on furnaces and in the sorting process, (3) use remote controlled sanders/grinders with ventilated cabins, (4) use closed systems for filling pallet boxes, and (5) improve cleaning procedures by using methods that minimize dust generation.
List of papers. Papers I, III and IV are removed from the thesis due to copyright restrictions.
Asbjørn Skogstad, Solveig Føreland, Erik Bye and Wijnand Eduard
Airborne Fibres in the Norwegian Silicon Carbide Industry.
Ann Occup Hyg. 2006;50(3):231-40
Solveig Føreland, Erik Bye, Berit Bakke and Wijnand Eduard
Exposure to Fibres, Crystalline Silica, Silicon Carbide and Sulphur Dioxide in the Norwegian Silicon Carbide Industry.
Ann Occup Hyg. 2008;52(5):317-36
doi:10.1093/annhyg/men029Creative Commons Attribution-Non Commercial licence
Solveig Føreland, Merete Drevvatne Bugge, Berit Bakke, Erik Bye and Wijnand Eduard
A novel strategy for retrospective exposure assessment in the Norwegian silicon carbide industry
J Occup Environ Hyg. 2012;9(4):230-41
Solveig Føreland, Berit Bakke, Roel Vermeulen, Erik Bye and Wijnand Eduard
Determinants of Dust Exposure in the Norwegian Silicon Carbide Industry
Ann Occup Hyg. Advance Access First published online: December 1, 2012.