\[FIGffp3mask\]). Thus, whereas typical household fabrics may not provide sufficient protection, industrial materials from which FFP3-masks are made typically have satisfactory filtering qualities.

However, the filtration efficiency of the filter material is not sufficient to assess the protection offered by a mask; rather, the protection can be measured by the *total inward leakage*, which is the percentage of particles that enter the mask through both the filter and the face-seal leakage. Alternatively, the protection factor (PF) can be used, for which holds $\mathrm{PF} = 1/\mathrm{TIL}$ (van der Sande et al., 2008).

Milton et al. (2013) found that surgical masks reduced the number of influenza virus droplet that were emitted by ca. $75\%$ compared to test persons not wearing masks if the droplets were larger than $5\, \mathrm{\mu m}$, but for smaller droplets the surgical masks provided no substantial protection.

Therefore, in the following the focus will be only on FFP-masks, which are typically more effective than surgical or home-made masks (van der Sande et al., 2008). FFP-masks can be subdivided in classes FFP1 to FFP3 with filtration efficiencies of $80\%$ (FFP1), $94\%$ (FFP2) and $99\%$ (FFP3), respectively, and leakage rates of less than $22\%$ (FFP1), $8\%$ (FFP2) and $2\%$ (FFP3) 