**2.1 Initial state of a medical mask**

When discussing the mechanisms of filtration the initial state of the mask before usage must be defined:


#### **2.2 Virus**

Viruses are smaller than dust particles and microbes and thus the most difficult to filter out. They come in different shapes: spherical, see figure 1, helical, icosahedra, etc. and sizes, which varies between 10nm and 300nm. The average density of a virus is approximated to v = 1200kg/m3. We will focus on the spherical or near spherical viruses.

holds.

Parameters of porosity Outer non-

samples: 1cm2; liquid in the pores: n-butanol

by pattern of dots where the fibres are partially pressed together. In this way, the parts of the other layers are transformed into a foil. Nevertheless, it is the open surface P that allows the airflow, table 5. The inner layer sub-layers are not pressed nor melted together and hence the air can flow through sub-layers and pores that are actually blocked by the outer layers areas transformed into the foil. Hence, 100% of active surface of the inner layer reported in table 4

Novel Theoretical Approach to the Filtration of Nano Particles Through Non-Woven Fabrics 211

Inner nonwoven layer Outer on the subject face

Mask (all three layers)

woven layer

The biggest pore [*μ*m] 305 38 211 30 *dmax* [*μ*m] 275 28 195 26 *dmin* [*μ*m] 15 8 15 9 *dp* [*μ*m] 83.2 12.46 76.3 13.91 *b* 0.6183 0.7313 0.6143 0.7521 *A* 0.249 0.0889 0.2925 0.0921 *P* [%] 27.71 8.43 25.32 8.42 Width of the classes [*μ*m] 13 2 18 2 Number of the classes 13 10 10 10 Number of pores/cm2 3745 68414 4506 45294

Table 5. Parameters of porosity for all three non-woven layers of mask; the surface of

Fig. 2. The inner non-woven layer of the medical mask zoomed 200 times

The mask porosity parameters are presented in table 5. Number of the pores on 1cm2 is 45294. The maximal diameter of pores is 30*μ*m. The open area (free for air flow) is (8.42%. The coefficient *a* (regression equation 2 - flow air through dry sample), is 0.0921. The exponent *b*

Fig. 1. Computer-aided reconstruction of a rotavirus based on several electron micrographs; Wikipedia (2011)
