The variation of the most penetrating particle size of electret filter media during usage under different humidity levels

Electret filter media are widely used in building ventilation systems to improve indoor air quality because they offer high filtration efficiency with low energy consumption. However, their efficiency declines during usage under different humidity conditions, and existing studies evaluate performance limits through fixed particle sizes or total efficiency, overlooking the dynamic migration of the most penetrating particle size (MPPS). To address this gap, we performed dust‐loading experiments on electret filter media using KCl and Al₂O₃ aerosols at 30 %, 50 %, 70 %, and 80 % relative humidity (RH), monitoring efficiencies and tracking the MPPS. Results indicated that dust deposition attenuated electrostatic effects and reduced efficiency. For KCl at low RH (30 %, 50 %), the MPPS shifted from 36.5 nm to 305.3 nm as branched chains shielded charges, causing efficiencies at MPPS to decrease by 20 % and to be lower than those at 0.4 μm. At around 9 g/m² loading, dendritic structures boosted mechanical filtration, shifting the MPPS to 284.1 nm with efficiencies rebound. However, at high RH (70 %, 80 %) with KCl, deliquescence led to dispersed deposition, the MPPS climbed to 305.3 nm and efficiencies declined without recovery. Experiments with Al₂O₃ at all RH followed similar results with KCl at low RH, underscoring humidity's selective impact on hygroscopic particles. The Cumulative MPPS shift index was introduced to quantify MPPS migration and its impact on filtration performance. This study revealed the interactions among electrostatic decay, particle deposition and humidity, providing novel insights for efficiency evaluation and performance optimization of electret filter media.