Employee exposure to airborne fiber and total particulate matter in two mineral wool facilities

Two facilities, one producing rock wool by retort melting with subsequent fiber formation by spinning (Plant A), and the other utilizing slag, glass scrap, and fly ash in a reverbatory furnace to form a melt which is spun (Plant B), were surveyed for dust exposures of employees. A variety of products was manufactured in each plant. The surveys were performed by dividing each facility into “dust zones” on the basis of the processing operations, ventilation methods, or employee jobs. Representative men in each job title associated with each “dust zone” wore personal dust sampling filters and air pumps for an entire work shift. A total of 63 samples was collected in Plant A and 72 in Plant B. Samples of dust on filters were analyzed to determine weight of total dust in the air, expressed as milligrams per cubic meter, and fiber concentrations, expressed as fibers per cubic centimeter in two diameter ranges, greater than 1 μm and less than 1 μm. Phase contrast and electron microscopic methods were utilized for the latter two analyses, respectively. Fiber length and diameter distributions were also determined. Electron microscopic analyses followed ashing of samples; each fiber observed was subjected to an intense beam to determine crystalline or amorphous structure by the presence or absence of a diffraction pattern. The ranges of Total Suspended Particulate Matter (TSPM) concentrations in Plants A and B were 0.53–23.64 and 0.045–6.88 mg/m³, respectively. In Plant A, Tile Finishing and Warehouse were the dustiest zones; in Plant B, Maintenance and Main Plant zones were the dustiest. Average fiber concentrations (>1 μm diameter) determined by phase contrast microscopy ranged from 0.20–1.4 fibers/cm³ in Plant A and from 0.011–0.43 fibers/cm³ in Plant B. The average concentrations of fibers less than 1 μm diameter, as determined by electron microscopy, ranged from 0.0056 to 0.16 fibers/cm³ in Plant A and from 0.0059 to 0.089 fibers/cm³ in Plant B. The percentage of respirable fibers, those less than 3 μm diameter, as determined by phase contrast microscopy, was approximately 75% of all airborne fibers in both plants. Estimates of fiber weight suggest that fibers contribute from 0.11 to 4.3% of TSPM for samples collected in Plant A, and from 0.8 to 57.8% of TSPM for samples collected in Plant B. Thus, fibers, defined to be particles with at least a 3:1 aspect ratio, represent a small fraction of airborne particles to which employees in these plants are exposed. In both facilities the weight concentration of total airborne dust, expressed as milligrams per cubic meter, was a poor indicator of airborne fiber concentration, expressed as fibers per cubic centimeter and determined by either phase contrast microscopy (fibers > 1 μm diameter) or electron microscopy (fibers < 1 μm diameter).