Development of a Fiber Size-Specific Job-Exposure Matrix for Airborne Asbestos Fibers

Abstract

Objectives: To develop a method for estimating fiber size-specific exposures to airborne asbestos dust for use in epidemiologic investigations of exposure-response relations. Methods: Archived membrane filter samples collected at a Charleston, SC asbestos textile plant during 1964-1968 were analyzed by transmission electron microscopy (TEM) to determine the bivariate diameter/length distribution of airborne fibers by plant operation. The protocol used for these analyses was based on the direct transfer method published by the International Standards Organization (ISO), modified to enhance fiber size determinations, especially for long fibers. Procedures to adjust standard PCM fiber concentration measures using the TEM data in a job-exposure matrix (JEM) were developed in order to estimate fiber size-specific exposures. Results: A total of 84 airborne dust samples were used to measure diameter and length for over 18,000 fibers or fiber bundles. Consistent with prior studies, a small proportion of airborne fibers were longer than >5 µm in length, but the proportion varied considerably by plant operation (range 6.9% to 20.8%). The bivariate diameter/length distribution of airborne fibers was expressed as the proportion of fibers in 20 size-specific cells and this distribution demonstrated a relatively high degree of variability by plant operation. PCM adjustment factors also varied substantially across plant operations. Conclusions: These data provide new information concerning the airborne fiber characteristics for a textile operation used for previous exposure-response analyses. The TEM data demonstrate that the vast majority of airborne fibers inhaled by these workers were shorter than 5 µm in length, and not included in current PCM fiber counts. A new fiber size-specific JEM was developed for use in fiber size-specific risk assessments.