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Especially in connection with the COVID-19 Face mask pandemic, the question arose to what extent the various masks provide protection against viral infection for the wearer. The state of research on this question was relatively weak until the beginning of the pandemic.
The evaluation of everyday masks is difficult and difficult to generalize, among other things, because the production and the requirements are not subject to standardization and the properties of different everyday masks thus differ considerably from each other. The Swiss Covid-19 task force has published recommendations for minimum specifications for community masks for Swiss manufacturers in 2020. [9] According to the German Society for Pneumology and Respiratory Medicine, masks made from fabrics probably have a protective effect, but this has not yet been proven by clinical studies. The filtration performance of different substances varies considerably. [10]
Medical mouth-nose protection (MNS), N95 masks and FFP masks are all standardized (see table above), which allows for better comparability.
In terms of pure separation efficiency, medical mouth-nose masks (NMS type I), N95 masks and FFP-2 masks are essentially identical: All masks have a bacterial filtration capacity (BFE) of at least 95%; depending on the standardization, a particle separation of 95 % must also be achieved with a particle size of 0.1 μm (corresponds to 100 nanometers). [11] In terms of deposition efficiency, MNS masks type II with a BFE of 98% (see classification of mouth-nose protection) are even more efficient than FFP-2 masks and N95 masks, but less efficient than FFP-3 masks with a BFE of 99%.
The main difference between FFP and N95 masks compared to MNS and other mouth-nose coverings (MNB) is the sealing of the mouth-nose area – provided that the mask is applied correctly and the user is not a beard wearer. This significantly reduces the flow of air when inhaling and exhaling. This has been confirmed by various studies: In 2008, for example, an experimental study commissioned by the Dutch Ministry of Health came to the conclusion that the effectiveness of aerosol filtering depends mainly on the type of protective mask. FFP2 masks, medical mouth-nose protection (MNS, surgical mask) and homemade makeshift masks made from tea bag material were examined. The effectiveness of these masks was considered at particle sizes from 0.02 to 1 μm. FFP2 masks, which ensure a sealing fit around the mouth and nose of the wearer, were found to be about 25 times more effective than medical mouth-nose protection (despite identical filtering efficiency compared to the FFP-2 masks studied) and 50 times more effective than makeshift masks. [12]
In April 2020, the Max Planck Institute for Chemistry tested the deposition efficiency of a wide variety of materials used in commercial and self-made masks. Different mechanisms of action with regard to particle size and pressure difference were investigated. Particles in the size range of 100 to 500 nm diameter were generally stopped with the lowest efficiency (Most Penetrating Particle Size). This can be explained by the different mechanisms of action (interception, impaction, diffusion separation, electrostatic deposition) of a filter, which in combination are the least efficient in this size range. The study showed that in the range of the size of SARS-CoV-2 viruses, the deposition efficiency of the materials used for everyday masks is significantly lower than that of mouth-nose masks; however, since the viruses are transported on droplets of very different sizes, this finding is only of limited significance in terms of practical significance. In particular, particles with a diameter of 5 μm and larger were deposited very efficiently by all materials investigated in the study. Droplets produced during coughing and sneezing are mainly found in this size range, while virus-carrying aerosol particles emitted when speaking and singing can also be significantly smaller. [13]