Nonuniform airflow in inlets and its effect on filter deposits in the fiber sampling cassette were investigated. Proper sampling of aerosols demanded that the aerosol enter the sampler and be deposited on the filter without loss or modification. Air flow patterns around a sampler were critical. Three experimental systems were used to observe air flow patterns. The first system consisted of an open faced 25 millimeter (mm) cassette sampler with a 50mm long plastic cowl that served as a special inlet. The second system consisted of a vertical chamber, 1.8 meters long, 24 centimeters in diameter, with particle deposition on a 25mm sampler filter, and aerosol generated using a vibrating orifice monodisperse aerosol generator (VOMAG). All samples were taken using an asbestos filter sampler with a conductive cowl, and a 25mm, 0.8 micrometer pore size cellulose ester filter. In the third system a tungsten-carbide aerosol was generated, and the aerosol was sampled using the 25mm cassette with conductive cowl at several angles to the air flow in the chamber. Air flow rates were about 1 liter per minute. Results showed that vortices created a complex nonuniform pattern on the filter deposit, especially when particle settling or electrostatic interactions between sampler and particles were present. Other factors causing nonuniformity of particle deposit such as inertial effects could be counteracted by changes in samples placement. The authors conclude that the variations noted may explain some reports of high variability on asbestos (1332214) fiber samples. Although only compact particles were used in the study, the air flow patterns and forces will probably apply to other fibers of similar aerodynamic diameter.