might be able to transmit the virus during face-to-face conversation with an asymptomatic infected individual.

Fabian et al. (2008) found that humans exhale $>500$ droplets per liter of air, of which $99.9\%$ had diameters between $0.3\, \mathrm{\mu m}$ and $ 5 \, \mathrm{\mu m} $, and $87\%$ were smaller than $ 1\, \mathrm{\mu m}$. That most exhaled particles have diameters below $1\, \mathrm{\mu m}$ has been confirmed by other authors (e.g. Fairchild and Stampfer, 1987; Papineni and Rosenthal, 1997; Edwards et al., 2004). Thus this work focuses on particles smaller than $ 5 \, \mathrm{\mu m} $.

To reduce the risk of inhaling such droplets, self-made-masks, surgical masks and so-called respirators, such as Filtering Face Piece (FFP) masks, are under discussion (cf. Steinle et al., 2018).

The performance of face masks is typically assessed via experiments that determine the *filtration efficiency* (FE) and the *total inward leakage* (TIL). The *filtration efficiency* is the percentage of particles that do not pass through the filter if the mask is tightly fitted. For example, Mueller et al. (2018) reported average filtration efficiencies of handkerchiefs ($\mathrm{FE}=22.7\%$), T-shirts ($\mathrm{FE}=42.5\%$), surgical masks ($66.2\% \leq \mathrm{FE} \leq 88.7\%$) and FFP3-masks ($\mathrm{FE}=99.3\%$, corresponding to the mask in Fig. 