"An apparatus designed to test aerosol filter efficiency at pressures equal to or less than atmospheric and at linear face velocities of 5 to 1000 ft/ min is described. Aerosol collection efficiency studies were completed for IPC paper at 20,000 and 40,000 ft. Data developed in these studies indicate that ia the impaction and diffusion regions collection efficiency increases with altitude for all linear face velocities less than 800 ft/min. A propeller-type flowmeter, calibrated to measure the volume of air moved by a blower in a balloon-borne system, was flown on five separate flights to obtain air flow data. These data sre presented. Flowmeter air flow measurement data were approximately 10% lower than telemetered data."