Accumulation of water in face masks during respiration

Human exhalation releases droplets and aerosols primarily composed of water, which can contain bacteria and virions. This study measured water absorbed from exhaled breath (in vivo, 3 adults and 2 children) by three types of face masks (FFP2/N95, surgical, and cloth) intended to protect from SARS-CoV-2 (virion diameter: 0.06 - 0.14 μm). Absorption was compared to estimated mask pore volumes, and a physics-based model predicted absorption rates and durations. Results show minuscule to no water absorption by masks, with only 1.3% of the exhaled humidity being absorbed (until saturation after about 25 minutes), challenging the notion that water-bound virions are effectively filtered. A proposed shrinkage mechanism highlights discrepancies between observations and the accepted view of mask filtration. Energetic considerations suggest that one-fold positively charged particles ≥0.7 μm evade electrostatic attraction due to their kinetic energy during quiet breathing (air stream velocity 1 m/s ) surpassing the mask’s surface electrostatic energy (6.4e−17 J). The findings reveal a lack of empirical evidence and mechanistic understanding of virion-mask interaction supporting filtration of smaller particles by face masks. This underscores the need for further research to provide empirical evidence for an underlying filtering mechanism and to evaluate the harm-to-benefit ratio of maskwearing in mitigating SARS-CoV-2 transmission.