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Preprint Open Access

Accumulation of water in face masks during respiration

Lipfert, Susanne; Günther, Michael; Rockenfeller, Robert; Renjewski, Daniel

When humans exhale, they discharge liquid particles containing solid components (e.g. bacteria and virions). These droplets and aerosols are primarily composed of water. In this study, water absorbed from exhaled human breath was measured (precision scale) in vivo (3 adults, 2 children) for 3 different types of face masks (2 FFP2/N95, 3 surgical, 2 cloth) intended for protection from the SARS-CoV-2 virus (diameter 0.1 - 0.15 μm). Accumulation of water in the tested face masks was also assessed when exposed to steam as well as when submerged in a basin of water. Measured water absorption for each mask was compared to the mask’s estimated pore volume, i.e. the amount of water possible to be absorbed in-between the mask’s fibers. Additionally, a simple physics-based model was used to predict how much water can be absorbed by a face mask and how much time this absorption process will take. The key observation of this study indicates that absorption of water by face masks during human breathing is minuscule to none. A shrinkage
mechanism is discussed to explain the discrepancy between these findings and the widely accepted belief that virions (usually bound to water) can be filtered by face masks. Basic energetic considerations on electrostatic attraction between mask material and charged particles are regarded and put into context with the current state of knowledge on physical properties of particles in exhaled human breath. It is inferred that one-fold positively charged particles of diameter 0.7 μm or larger will escape the electrostatic attraction at the mask’s surface as their kinetic energy within the typical air stream for quiet breathing (1 m/s) exceeds the potential energy (6.4·10e−17 J) at the mask’s surface. There is a lack of empirical evidence and mechanistic understanding of virion-mask interaction that would support the filtering of particles of smaller sizes. It remains open whether an underlying filtering mechanism to reduce transmission of virions, like SARS-CoV-2, exists at all, and if so, to what extent, and with what harm-to-benefit ratio.

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