(cf. Seidler et al., 2004; Lee et al., 2016; European norm EN 149:2001+A1:2009). Thus technically it would appear that FFP3-masks provide satisfactory protection, and indeed they are widely used in hospitals. As outlined in the following, though, literature suggests that many FFP3-masks do not provide reliable protection, especially if not properly fitted.

Steinle et al. (2018) investigated FFP2-masks with median filtration efficiency of $\mathrm{FE}\geq 98\%$ and found that the total inward leakage varied between $0\% \leq \mathrm{TIL} \leq 84.4\%$. Cherrie et al. (2018), Lee et al. (2016) and Jung (2014) reported similar values and argued that, regardless of the quality of the filter materials, FFP-type masks may not provide reliable protection if the mask does not fit tightly. Lee et al. (2017) reported face-seal leakage surrounding the chin and the cheeks for FFP-masks, because the masks did not fit all wearers. Furthermore, they observed that the percentage of particles that penetrated through face-seal leaks increased for low air intake and for particle diameters below $0.1 \, \mathrm{\mu m}$. Children were found to be less protected by FFP-type masks, which was attributed to inferior fitting of the masks on smaller faces (van der Sande et al., 2008).

Moreover, filtration efficiencies for 