masks may be different depending on the particles used. Penconek et al. (2013) found that commercially available FFP2- and FFP3-masks did not provide sufficient protection against diesel exhaust fumes. Filtration efficiency is typically tested with NaCl particles or paraffin oil droplets (Penconek et al., 2013), but water droplets appear to be rarely used.

Performing activities such as exercising, nodding or shaking was found to affect total inward leakage, although the total inward leakage varied mostly by factor $2$ or less (van der Sande et al., 2008).

However, how tightly does a mask have to fit and how large may gaps between mask and face be before the mask ceases to provide the promised protection? The aim of the present work is to answer these questions via analytical and numerical flow computations for a generic mask. From these findings, recommendations for the design of more effective industrial and self-made masks are derived, with focus on re-usability of the masks.

Theory {#SECtheory}
======

To investigate the airflow through face masks, the problem can be reduced to one-dimensional (1D) flow for a simplified geometry (cf. Fig. \[FIG2DtheoryGeom\]) by the following assumptions. The fluid is considered as incompressible because the Mach number $Ma$ is 