The properties of electret filter materials have been investigated. The removal efficiency of electret filter materials used in ventilation filters and in room air cleaners has been measured for particle sizes ranging from 0.1 to 5 μm. The results indicate that the efficiency and reliability of an electret filter depend to a great extent on the stability of the electrostatic charge in the filter material. It has been found that the efficiency of electret filter material can decrease significantly when exposed to diesel fume aerosol or cigarette smoke aerosol. Arizona road dust aerosol, however, causes only a minor decrease in efficiency. The extensive use of electret filter materials in ventilation applications should be avoided until the stability and the reliability of the filtration has been verified by long-term field tests.