Sampling efficiency is essential in exposure assessments of contaminants in air, as well as other matrices. In the measurement of airborne contaminants, it is critical to collect a sample of air containing representative contaminants in the air of concern, that is, contaminant concentration and size distribution in the sampled air must be the same as that of the air of concern. Typically, mixed cellulose ester (MCE, 0.45 or 0.8 um pore size) and to a much lesser extent, capillary-pore polycarbonate (PC, 0.4 um pore size) membrane filters are used to collect airborne asbestos for count measurement and fiber size analysis. A literature review did not identify any study that compared the fiber retention efficiencies of 0.45 um and 0.8 um pore size MCE or 0.4 um pore size PC membrane filters for asbestos aerosols. In this research study chrysotile asbestos (fibers both shorter and longer than 5 um) were generated in an aerosol chamber and sampled by 25-mm diameter MCE filter media to compare the efficiency of a 0.45 um pore size filters versus 0.8 u pore size filter media. In addition, the effect of plasma etching times on fiber densities was evaluated.