Electret filters find many applications since they offer a high initial filtration efficiency combined with a low pressure drop. They carry electric charges on the fibers, which interact either directly with charged particles by Coulomb attraction (electrophoresis), or by interaction with induced dipoles in both charged and uncharged particles (dielectrophoresis). A well-known disadvantage of electret filters is that the charge stored on the fibers can drastically degrade with time under exposure to particles, gases or humidity. In order to assess this effect, the testing of particle filters for general ventilation according to EN 779 required the immersion of the filter media in liquid isopropyl alcohol (IPA). In contrast, ISO 16890 which recently replaced EN 779 requires discharging of the filters in IPA vapor saturated air. So far, there is only little knowledge about the comparability of the two methods.

In order to gain new insights, various filter types were immersed in liquid IPA and discharged with IPA saturated air. Before and after discharging, the fractional filtration efficiencies were determined with a sodium chloride (NaCl) aerosol. For several of the filter types, it turned out that...