a beard a few hours after a close shave) causes that $2\%$ to $8\%$ of the inhaled air flows unfiltered through the gap, depending on the width of the gap (here: between $2.5\, \mathrm{cm}$ and $10\, \mathrm{cm}$).

The results also show that the gap height $H_{\mathrm{g}}$ has the largest influence: increasing gap height $H_{g}$ by a factor of $2$ can increase the flow rate $F_{\mathrm{g}}$ through the gap by a factor of up to $10$. The gap width had a comparatively small influence: increasing gap width $B_{g}$ by a factor of $2$ increased the flow rate $F_{\mathrm{g}}$ through the gap by a factor of up to ca. $2$.

Flow rate $F_{\mathrm{t}}=30\, \mathrm{L/min}$\
![As Fig. \[FIGFt0Fg\], except for logarithmic vertical axis[]{data-label="FIGFt0FgLOG"}](images/Lg1Cm2000/compareFt-0.0005_Fg0FtLOG.png "fig:"){width="\factor\linewidth"}\
Flow rate $F_{\mathrm{t}}=95\, \mathrm{L/min}$\
![As Fig. \[FIGFt0Fg\], except for logarithmic vertical axis[]{data-label="FIGFt0FgLOG"}](images/Lg1Cm2000/compareFt-0.001583333_Fg0FtLOG.png "fig:"){width="\factor\linewidth"}

Figure \[FIGdp\] shows that the pressure drop across the mask filter can decrease by more than $75\%$ when the gap height is increased to $H_{\mathrm{g}}=1\, \mathrm{mm}$. This makes breathing easier, which could tempt mask wearers not to tighten the mask to the face, with the consequence that up to ca. $70\%$ of the air flows unfiltered through the gap into the mask. Figure \[FIG2DpvelVecs\] shows pressure 