Efficiency of sampling and analysis of asbestos fibers on filter media: Implications for exposure assessment
To measure airborne asbestos and other fibers, an air sample must represent the actual number and size of fibers. Typically, mixed cellulose ester (MCE, 0.45 or 0.8 ?m pore size) and, to a much lesser extent, capillary-pore polycarbonate (PC, 0.4 ?m pore size) membrane filters are used to collect airborne asbestos for count measurement and fiber size analysis. In this research study, chrysotile asbestos (fibers both shorter and longer than 5 ?m) were generated in an aerosol chamber and sampled by 25 mm diameter MCE filter media to compare the fiber retention efficiency of 0.45 ?m pore size filters vs. 0.8 ?m pore size filter media. In addition, the effect of plasma etching times on fiber densities was evaluated. This study demonstrated a significant difference in fiber retention efficiency between 0.45 ?m and 0.8 ?m pore size MCE filters for asbestos aerosols (structures longer than or equal to 0.5 ?m length). The fiber retention efficiency of a 0.45 ?m pore size MCE filter is statistically significantly higher than that of the 0.8 ?m pore size MCE filter. However, for asbestos structures longer than 5?m, there is no statistically significant difference between the fiber retention efficiencies of the 0.45 ?m and 0.8 ?m pore size MCE filters. The mean density of asbestos fibers (longer than or equal to 0.5 ?m) increased with etching time. Doubling the etching time increased the asbestos filter loading in this study by an average of 13%. The amount of plasma etching time had no effect on the filter loading for fibers longer than 5 ?m. Many asbestos exposure risk models attribute health effects to fibers longer than 5 ?m. In these models, both the 0.45 ?m and 0.8 ?m pore size MCE filter can produce suitable estimates of the airborne asbestos concentrations. However, some models suggest a more significant role for asbestos fibers shorter than 5 ?m. Exposure monitoring for these models should consider only the 0.45 ?m pore size MCE filters as recommended by the U.S. Environmental Protection Agency Asbestos Hazard Emergency Response Act (AHERA) protocol and other methods.