  The use of face masks for the general public has been suggested in literature
as a means to decrease virus transmission during the global COVID-19 pandemic.
However, literature findings indicate that most mask designs do not provide
reliable protection. This paper investigates the hypothesis that the impaired
protection is mainly due to imperfect fitting of the masks, so that airflow,
which contains virus-transporting droplets, can leak through gaps into or out
of the mask. The fluid dynamics of face masks are investigated via analytical
and numerical computations. The results demonstrate that the flow can be
satisfactorily predicted by simplified analytical 1D-flow models, by efficient
2D-flow simulations and by 3D-flow simulations. The present results show that
already gap heights larger than 0.1mm can result in the mask not fulfilling
FFP2 or FFP3 standards, and for gap heights of ca. 1mm most of the airflow and
droplets may pass through the gap. The implications of these findings are
discussed and improvements to existing mask designs are suggested.
