Looking for the minimum efficiency of fibrous air filters during their service life

Electret fibrous filter media achieve high efficiencies while maintaining low air flow resistance by incorporating electrostatic charges on their fibers. However, captured ultrafine particles reduce electrostatic effects. Existing test methods specify preconditioning to detect the minimum efficiency by eliminating electrostatic effects. ASHRAE 52.2 exposes media to nanoparticle KCl aerosols. ISO/TS21220 and EN779 immerse media in isopropyl alcohol (IPA). These approaches have some problems: - Nanoparticle generation is fairly complicated and needs to be kept under control; - The structure of some media may be changed by liquid immersion; - Soaking full scale air filters requires large amounts of IPA. A new procedure, exposure to IPA vapor, has been shown to be effective. We summarize these studies and ISO/TC142 activities related to them.