The transmission of the new coronavirus occurs mainly by inhaling saliva droplets and respiratory secretions suspended in the air and, for this reason, wearing masks and maintaining social distance are the most effective ways to prevent Covid-19 while there is no vaccine for everyone .
Cheap, reusable and available in different colors and prints, fabric masks are among the most used by Brazilians. However, its ability to filter aerosol particles with size equivalent to that of the new coronavirus can vary between 15% and 70%, as revealed by a study conducted at the Institute of Physics of the University of São Paulo (IF-USP).
Coordinated by Professor Paulo Artaxo and supported by Fapesp, the work is part of the initiative (breathe !, whose objective was to guarantee the accessibility of the USP community to safe masks. The results were published in the journal Aerosol Science and Technology.
“We evaluated the filtration efficiency of 227 models sold in Brazil, whether in pharmacies or in popular stores. Our goal was to find out to what extent the population is really protected with these different masks, ”says Artaxo to Agência Fapesp.
To do the test, the scientists used equipment that produces, from a solution of sodium chloride, aerosol particles of controlled size – in this case 100 nanometers (Sars-CoV-2 is approximately 120 nanometers). After the aerosol jet was launched into the air, the particle concentration was measured before and after the mask.
The models that proved to be most effective in the test, as expected, were the surgical masks and the PFF2 / N95 type – both for professional and certified use -, which managed to filter between 90% and 98% of the aerosol particles. Following are the TNT (made of polypropylene, a type of plastic) sold in pharmacies, whose efficiency ranged from 80% to 90%. Finally came the fabric ones – a group that includes models made with cotton and synthetic materials, such as lycra and microfiber. In this case, the filtration efficiency varied between 15% and 70%, with an average of 40%. And some factors have proved to be critical in increasing or decreasing the degree of protection.
“In general, masks with stitching in the middle protect less, as the machine makes holes in the fabric that increase the air passage. The presence of a nasal clip, which helps to fix the mask on the face, considerably increases the filtration, due to the better fit on the face. Some fabric masks are made with metal fibers that inactivate the virus, such as nickel or copper, and therefore protect more. And there are also models of electrically charged material, which increases particle retention. In all of these cases, however, efficiency decreases with washing, as there is material wear and tear ”, comments Fernando Morais, PhD student at IF-USP and at the Institute for Energy and Nuclear Research (Ipen), who is the first author of the article.
Inhale and exhale
According to Artaxo, the two-layer cotton masks filtered considerably more aerosol particles than those made with just one. But from the third layer onwards, efficiency increased slightly, while breathability decreased considerably.
“One of the novelties of the study was to evaluate the respirability of the masks, that is, the resistance of the material to the passage of air. TNT and cotton were the best in this regard. Those of the type PFF2 / N95 were not so comfortable. But the worst was one made with paper. This is an important aspect, because if the person cannot stand to spend even five minutes with the mask, it is useless ”, says Artaxo.
As the authors in the article point out, although with varying efficiency, all masks help to reduce the spread of the new coronavirus and its use – associated with social detachment – is essential in controlling the pandemic. They also state that the ideal would be the mass production of masks of the type PFF2 / N95 to distribute free to the population – something that “should be considered in future pandemics”, in the evaluation of Vanderley John, coordinator of the initiative (breathe!, Organized by USP’s Innovation Agency, and co-author of the study.
“Today it is already proven that the main form of contamination is by air and wearing masks all the time is one of the best prevention strategies, so keeping windows and doors open to ventilate the environments as much as possible,” recommends Artaxo.
The article Filtration efficiency of a large set of COVID-19 face masks commonly used in Brazil can be read at www.tandfonline.com/doi/abs/10.1080/02786826.2021.1915466.
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