The new coronavirus is mainly transmitted through inhalation of droplets of saliva and respiratory secretions suspended in the air. Because of this, wearing masks and maintaining social distance are the most effective ways to prevent Covid-19 while there is no vaccine for everyone.
Inexpensive, reusable and available in different colors and prints. Cloth masks are most commonly used by Brazilians. However, the ability to filter aerosol particles with a size equivalent to that of the new coronavirus can vary between 15% and 70%, according to a study at the Institute of Physics at the University of São Paulo (IF-USP).
The work, coordinated by Professor Paulo Artaxo and supported by Fapesp, is part of the (breathe!) Initiative, the aim of which was to ensure that the USP community had access 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 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.
For the test, the scientists used devices that produce aerosol particles of controlled size from a solution of sodium chloride – in this case 100 nanometers (Sars-CoV-2 is around 120 nanometers in size). After the aerosol jet was launched into the air, the particle concentration was measured before and after the mask.
The models that, as expected, proved to be the most effective in the test were the surgical masks and the type PFF2 / N95 – for both professional and certified use – with which between 90% and 98% of the aerosol particles could be filtered. This is followed by TNT (made from polypropylene, a type of plastic), which are sold in pharmacies and whose efficiency was between 80% and 90%. Finally came the fabrics – a group that includes models made from cotton and synthetic materials like lycra and microfiber. In this case the filtration efficiency varied between 15% and 70% with an average of 40%. Several factors have been found to be decisive in increasing or decreasing the level of protection.
“In general, masks with seams in the middle are less protective as the machine drills holes in the fabric that increase air flow. The presence of a nose clip, with the help of which the mask can be fixed on the face, significantly increases the filtration due to the better fit on the face. Some tissue masks are made of metal fibers that inactivate the virus, such as nickel or copper, and therefore provide more protection. And there are also models of electrically charged material that increases particle retention. In all of these cases, however, the efficiency decreases with washing as the material is worn out, ”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.
breath in and breath out
According to Artaxo, the two-layer cotton masks filtered significantly more aerosol particles than those made with just one. From the third shift, however, efficiency increased slightly, while breathability decreased significantly.
“One of the novelties of the study was the assessment of the breathability of the masks, ie the resistance of the material to air penetration. TNT and cotton were the best in this regard. The PFF2 / N95 ones weren’t as comfortable. But the worst was one made with paper. This is an important aspect because if the person can’t even spend five minutes on the mask, it’s useless, ”says Artaxo.
As the authors of the article point out, all masks help reduce the spread of the new coronavirus with varying degrees of effectiveness, and its use – coupled with social distancing – is vital in fighting the pandemic. They also state that the ideal would be to mass-produce PFF2 / N95-type masks that could be freely distributed to the population – something that should be considered when assessing (breathe) the initiative’s coordinator, Vanderley John!, Organized by the USP Innovation Agency and co-author of the study.
“It has already been proven that the main form of contamination is airborne and that wearing masks all the time is one of the best prevention strategies. So keep windows and doors open to ventilate the area as much as possible,” recommends Artaxo.
The article Filtration Efficiency of a Large Set of COVID-19 Face Masks in Use in Brazil can be read at www.tandfonline.com/doi/abs/10.1080/02786826.2021.1915466.