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Pere Renom

“We have given you, O Adam, no visage proper to yourself, nor endowment properly your own, in order that whatever place, whatever form, whatever gifts you may, with premeditation, select, these same you may have and possess through your own judgement and decision […] We have made you a creature neither of heaven nor of earth, neither mortal nor immortal as the free and proud shaper of your own being, fashion yourself in the form you may prefer.”

Giovanni Pico della Mirandola

Mask physics

published on 17.11.2020

There are 4 basic types of masks, classified based on pore size: textile, surgical, FPP2 and FPP3. The textile is not standard and, therefore, does not have a single pore size, the surgical one has pores of 2 microns, the FFP2 (N95, in American nomenclature) of 0.3 microns and the FFP3 of 0.023 microns. Consequently, the filtration capacity increases 10 times from one type of mask to the next.
In some way, the masks could be compared to a set of sieves used in geology to make granulometries, that is, measurements of the sediment particles. Depending on the size of the pore, a sieve retains, in decreasing order, pebbles, gravel, sand or silt. It is clear that a sieve for pebble does not retain silt, nor does a surgical mask retain particles as small as an FFP3 mask. However, all masks protect viruses reasonably well, although most have larger pore sizes. How is it possible? Well, because viruses travel associated with saliva particles of variable size and because static electricity comes into play at that microscopic scale. As the French physicist Charles-Augustin de Coulomb (1736-1806) described, the force with which two electrically charged particles attract or repel each other depends on the electric charge and is inversely proportional to the square of the distance. When we breathe out or breathe in, the microdroplets of saliva are forced to pass extremely close to the mask tissues and are retained by physical filtration and electrostatic charge.
However, the filtration capacity of a mask reduces with use. The textile mask should be washed daily at 40-60ºC, and has a durability of 25-40 washes depending on the manufacturer. The surgical one has a durability of 4 hours, although used with care it can be used throughout the day. The FFP2 has a durability of 8h, but it can be used for about 3 days, and the FFP3, in general, is reserved only for healthcare personnel.
Apart from these masks, the Catalan company Mar Màgic has managed to homologate a mask with a transparent window that allows deaf people to read lips. Masks should hinder viruses, but not communication!

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