N95 face mask under increasing solids loading and moisture

Submicron particles can be pollutants from engine emissions (0.02-1 μm), or airborne contagious virus (0.05 -0.2 μm). The N95 face mask has been employed to remove the finest particulates in the submicron range. In our filtration experiments, exhaust gas from a diesel engine laden with submicron particles was used to challenge the test N95 face mask with a gas velocity of 0.05 m/s. Measurements were taken over an extended 6 h period under heavy solids loading. Among the three layers only the middle 'filtration' layer of the N95 face mask was used in the test. Over the 6 h period when solids loading generated from the engine emission increased linearly from zero to 2.83 g/m2, the pressure drop across the filter increased nonlinearly from 19.6 Pa at no loading condition to 36.0 Pa. The ratio of downstream particles (penetrating the filter) to those upstream stayed relatively constant for 0.3 μm particles, whereas for 0.05 μm particles this penetration ratio decreased rapidly from 30 to 0.2 %. The reduction in penetration of the fine particles (less than 0.1 μm) with solids loading is quite interesting. The quality factor qfwhich measures benefit versus costs showed that during extended use of the N95 face mask, the qfreduced for particles greater than 0.1 μm due to constant capture efficiency yet increasing pressure drop, whereas for particles less than 0.1 μm, qf increases due to lower penetration (i.e. much higher capture efficiency) but only at modest increase in pressure drop. Unless there is excessive sweating causing the face mask to be fully wetted with liquid for which the electrical charges of the fibres are neutralized, the capture efficiency of the mask may only be reduced slightly due to the loss of electrostatic charges in attracting particles with opposite charges.