A surgical mask – in German OP-Maske – was originally made out of cotton. With new materials discovered in the 1960s, it became practice that certified medical masks are made out of non-woven microfiber material (synthetic), where multiple layers of microfiber with different characteristcs – usally 3 layers – get combined.
The colored outward facing side with a fulid-repellant layer an inside white water absorbing layer and a middle layer with filtration characteristics. The middle layer is a melt-blown nonwoven fabric (mostly polypropylene) that can hold back bactarias due to physical behavior of the fabric (best described a melt-blown fabric production line producer Geotex, but as well very nicely by a german filter technic comany EMW):
- Sieve effect – large particles get caught between the fibers
- Inertia effect – larger particles do not follow the airflow and stick to fibers
These two effects are fine to keep back larger aerosol drops in the size of 5-10 µm, but not for a virus like the corona Virus that has a size of 0.12µm. Here two other effects help:
- Diffusion effect – very small viruses do not follow the airflow and eventually adhere to fibers
- Electrostatic capture – as with a duster, an electrical charge draws tiny particles to the fibers.
Due to these two effects an electrostatic melt-blown fabric captures a virus in its fabrics with a high probrobability, even though that the virus is smaller than the space between the fabric fibres. This effect is very good explained on the webpage of several chinese suppliers:
Why do you need to add electret masterbatch to meltblown cloth? Meltblown cloth without electret has only mechanical barrier function, and the filtration efficiency is generally less than 50%. Only the electret melt blown cloth will be charged, so that it will have an efficient adsorption effect on the same charged 0.3 micron virus particles.
The filter performance is mesured in BFE (cacterial filtration efficiency). As of Jan 2022 most surgical masks sold in Germany are of EN 14683:2019 type IIR . As a final remark, surgical masks do not sit tight on your face and therefore you only get a partial filter effect – thats why FFP2 or N95 respiratiors are considered safer than surgical masks.
Finally I add my own example of a M+G medical mask which is of Type IIR and enables a BFE of >98%. It consist of 3 layers. I cut it into pieces – you can see the 3 layers, where the middle one is the melt-blown fabric.