The coronavirus emergency, which turned out to be more virulent than expected, has made the masks a necessary personal protection tool but, unfortunately, not always easily available. Or, if you are able to find them, you can buy masks at high prices. That's why many people are making reusable masks at home. Can these DIY masks help? A study published in the ACS Nano journal a few days ago by a team of the University of Chicago (Konda et al, 2020) tries to answer this question.
How the study took place
The novel coronavirus is transmitted through contact with infected droplets, emitted when a person speaks, breathes, sneezes or coughs. The droplets, as several studies have shown, have different sizes, they can be larger with a diameter greater than 5 micrometers but also smaller, forming what is called an aerosol. It may happen that the aerosol can pass through the openings of some fabrics. For this reason, it is important to find out the best materials for masks able to filter properly droplets and aerosol.
The scientists thus recreated droplets of different diameters between 10 nanometers and 6 micrometers. By using a fan, they sent the droplets against different types of fabric at a speed corresponding to that of a person who breathes at rest. Then, they measured the amount of particles in the air before and after the fabric to evaluate its ability to filter droplets. So let's see the results.
Coronaviruses and masks, here are the best filter materials
As a reference to read the results we use masks of the FFP2, or N95, type worn without gaps. These masks filtered 85% of the particles with a diameter of less than 300 nm and 99.9% of particles with a larger diameter. Surgical masks, which are purchased in pharmacies or shops, filter 76% of the smallest particles and 99.6% of the largest ones. As for the DIY mask fabrics, the best results were obtained by combining two materials, a layer of tightly woven cotton and two layers of spandex polyester chiffon. This type of combination filtered 97% of the smallest particles and 99% of the largest ones. Similar results have been obtained by replacing chiffon with silk or flannel. But be careful, the masks must adhere, the presence of gaps significantly reduces the filtering capacity that becomes around 35%. Using only a layer of silk as a mask, as a scarf could be, proves to filter 55% on average of both small and large particles. Four layers of silk instead increase the filtering power that becomes around 87%. And if only cotton is available, research has observed that two layers of cotton 600 TPI have filtered 82% of the smallest particles and 99.5% of the large ones.
The different action between cotton, chiffon and silk
Finally, the scientists also provided an explanation of how the different materials work. Tightly woven fabrics, such as cotton, provide a mechanical barrier to the passage of particles, thus physically opposing them, while materials such as chiffon or silk are more an electrostatic barrier.
