Objectives: To develop a job-exposure matrix (JEM) for historical asbestos fiber exposures in three asbestos textile plants and to develop fiber size-specific estimates of exposure to airborne asbestos dust for use in epidemiological investigations of exposure-response relations.
Methods: Historical samples of asbestos dust collected in three North Carolina, USA asbestos textile plants were obtained. Plant specific samples were used to express impinger dust concentrations as fiber concentrations by phase contract microscopy (PCM). Mixed models were used to estimate PCM exposures by plant, department, job, and calendar time. Archived membrane filter samples collected in these plants were analyzed by transmission electron microscopy (TEM) to determine the bivariate diameter/length distribution of airborne fibers by plant and operation. Previously published procedures to adjust standard PCM fiber concentration measures using the TEM data were applied to the PCM estimates to produce fiber size-specific exposures.
Results: A total of 3240 samples were used in the mixed model and the resulting model accounted for approximately 64% of variability in PCM fiber levels. Estimated PCM fiber levels were very high in these plants in the 1930’s, with some operations in excess of 200 fibers/cc, and decreased appreciably over time, with most operations < 15-20 fibers/cc by 1970. A total of 77 airborne dust samples were used to measure diameter and length for over 22,776 fibers or fiber bundles by TEM. A small proportion of airborne fibers were measured by PCM (>0.25 µm in diameter and >5 µm in length) and the proportion varied considerably by plant and operation (range 2.9% to 10.0%). The bivariate diameter/length distribution of airborne fibers was expressed as the proportion of fibers in 28 size-specific cells and this distribution demonstrated a relatively high degree of variability by plant and operation. PCM adjustment factors also varied substantially across plants and operations.
Conclusions: These data provide new information concerning airborne fiber levels and characteristics in three historically important asbestos textile plants. PCM concentrations were high in early years and TEM data demonstrate that the vast majority of airborne fibers inhaled by the workers were shorter than 5 µm in length, and thus not included in the PCM-based fiber counts. Both the PCM and TEM size-specific exposures will add new information when linked with an ongoing cohort study for exposure-response analyses.
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