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The efficiency of masks

Tests reveal how and to what extent different types of masks limit the airborne spread of infectious diseases

Used correctly, surgical masks have a filtration capacity of 89%

Léo Ramos Chaves

Almost two years ago, an accessory previously restricted to a few specific professional environments, such as hospitals, construction sites, and mines, became a mandatory part of daily life, seen on the streets all over the world. As a result of the severity and duration of the COVID-19 pandemic, respiratory protection masks have become ubiquitous and will likely continue to be used for some time to come. Researchers have been studying the many different types of masks to determine their ability to stop the spread not only of SARS-CoV-2, but also other infectious diseases, such as the recent wave of H3N2 flu spreading across several Brazilian states.

One of the most recent studies was published in December by the Max Planck Institute in Germany and confirmed the efficient antiviral role played by masks. An article published in the scientific journal Proceedings of the National Academy of Sciences (PNAS) found that even at a distance of 3 meters (m) from an infected person not wearing a mask, someone who is not vaccinated against COVID-19 and also not wearing a mask can be infected with the novel coronavirus within five minutes.

The good news is that if these same two people were wearing properly fitted PFF2/N95 masks, the chance of contagion in four times the contact time (20 minutes) would be just 0.1%—a one in a thousand chance. Another conclusion of the study, led by Eberhard Bodenschatz, is that PFF2/N95 masks (filtering facepiece respirators) provide 75 times greater protection than surgical masks, which for their part reduce the risk of contagion to a maximum of 10% if properly fitted.

One of the most comprehensive investigations into the efficiency of face masks was carried out by Brazilian researchers. The group, from the Institute of Physics (IF) at the University of São Paulo (USP) and the Institute for Energy and Nuclear Research (IPEN), evaluated 227 different masks, from those made with cutting-edge technology, such as PFF2/N95 masks, to surgical masks, synthetic or cotton fabric masks commonly sold in stores, and homemade face coverings. The results were published in the journal Aerosol Science and Technology in April 2021. “At the beginning of the pandemic, USP’s School of Medicine [FM] contacted Vanderley John, a professor from the Civil Construction Engineering Department at the Polytechnic School [POLI-USP], saying that they only had enough masks for another three weeks, both at the Hospital das Clínicas and the University Hospital. Since the market was unable to meet the sudden high demand, FM suggested testing alternative fabrics for making masks,” recalls IF doctoral student Fernando Morais, the study’s lead author.

The initiative became the Respire! (“Breathe” in English) project, led by John, who asked his IF colleagues to help test mask filtration capacities. To do so, the group adapted equipment usually used to monitor air quality in the Amazon and São Paulo to measure the volume of particles in the atmosphere.

“Our team studies atmospheric aerosols and their impacts on the environment and climate. We already had the instrumentation—which was actually funded by FAPESP—needed to measure nanoparticles in the order of 10 nanometers [nm], which is 100 times smaller than the width of a hair. We also had aerosol generators capable of creating particles of the size we needed, of between 60 and 300 nm,” explains IF physicist Paulo Artaxo, one of the coauthors of the study and Morais’s PhD cosupervisor.

Tiago Cardoso

The particle filtration capacity (which prevents the virus from being breathed in or out) and the breathability (which measures to what extent they facilitate or hinder the passage of air) were measured and combined to establish a quality factor (QF) for each mask evaluated (see infographic). “Our study was a world first in combining filtration and breathability, which is important because lots of people don’t like wearing masks that make it difficult to breathe,” says Artaxo.

The results confirmed what we already knew—the importance of face masks to the epidemiological control of airborne diseases—and provided details on the efficiency of different types. Considering filtration alone, the masks that performed best were the PFF2/N95 types—as expected—which blocked 98% of the particles in the sizes tested (from 60 to 300 nm).

Surgical masks had a filtration capacity of 89% and nonwoven fabric masks with three layers filtered 78% of particles. Cotton masks proved less effective (between 20% and 60%) because there is more space between the threads. If the mask has a seam, the risk of penetration by the virus is greater still.

In terms of breathability, the standout was nonwoven fabric, followed closely by surgical masks. The PFF2/N95 and cotton masks had a lower breathability index, although that does not mean they are difficult to use, even for long periods. “When breathability is extremely low, it is difficult for air to get through the mask. This is bad because people want to take their mask off as soon as they can,” says Artaxo.

After combining the filtration data and the breathability data, the quality factor of each type of mask was compared to the standard recommended by the World Health Organization (WHO). “The QF needs to be greater than 3 for a mask to meet the minimum filtration and breathability requirements proposed by the WHO,” explains Morais.

In the USP/IPEN study, the PFF2/N95, surgical, and nonwoven masks all achieved WHO approval, with QFs of 13.2, 15.9, and 24.9 respectively. Homemade cotton masks, considering an average breathability of 40%, did not reach the recommended level, scoring just 1.4. “Even though some types of masks have a very low quality factor, it’s important to remember that any mask is better than none at all. Although some are better than others, even the worse ones make a difference,” says biomedical engineer Vitor Mori, a member of Brazil’s Covid-19 BR Observatory.

Tiago Cardoso

One significant aspect not addressed by Artaxo, Morais, and John was mask adhesion. Adjusting masks to the face to avoid gaps around the edges is essential to ensuring that filtration in real-world situations is as effective as observed in the lab. Mori explains that there are two factors that contribute to a better seal. “If the mask has a nose clip and elastic to secure it around the back of the head, it is easier to fit it snugly to the face. With a better seal, all incoming and outgoing air passes through the filter, maximizing particle blockage,” he says. According to Artaxo, a well-fitted mask is essential to ensuring good filtration. The group’s article makes a recommendation in this regard.

Another Brazilian study on masks, involving a sample group of people infected with the novel coronavirus, revealed another interesting aspect: the pathogen was only found on the inner layer of the masks, suggesting transmission was being prevented. Carried out by researchers from the Oswaldo Cruz Foundation (FIOCRUZ), the research analyzed 45 facial coverings used in everyday situations by 28 patients, 30 made of two or three layers of cotton, and 15 of the surgical type.

The results, published as a preprint (papers that have not yet undergone formal peer review) on the medRxiv repository, suggest that various mask types are highly effective at hindering or preventing virus expulsion, reinforcing the idea that respiratory masks first and foremost prevent infected individuals from spreading the disease.

“Several data indicate that the presence of multiple layers is important to the level of protection, as well as the porosity of the fabric, which must not be too high,” Andreza Salvio, a doctor of parasitic biology from FIOCRUZ’s Oswaldo Cruz Institute (IOC) and one of the members of the team responsible for the study, said in a press release. “It is also essential to realize that masks are just one of several measures that should be adopted to contain the spread of COVID-19, alongside social distancing and vaccination, for example,” he added.

No interference
Another study by researchers from FM at USP, funded by FAPESP and published as a preprint on medRxiv, indicates that although masks can cause some discomfort, they do not significantly interfere with breathing patterns or cardiovascular physiology, even during physical exercise at moderate to high intensities.

The group subjected 17 men and 18 women, all fit and healthy, to treadmill ergospirometry tests—used to assess cardiopulmonary responses through the exchange of expired and inspired gases during exercise—lactate collection (an indicator of physical intensity and muscle fatigue), and constant cardiac monitoring during various degrees of effort. The volunteers performed one session while wearing a three-layer fabric mask and another without it, to enable a comparison.

“The study found that the fabric mask did not substantially affect physiological or metabolic variables during aerobic physical exercise,” says Natália Guardieiro, an exercise and sports physician, doctoral student at FM, and lead author of the article. “Even during the most intense exercise, the effects of the mask were minimal, as the body establishes physiological compensation. Masks should therefore not be a barrier to practicing physical activity,” she concludes.

Scientific articles
MORAIS, F. G. et al. Filtration efficiency of a large set of COVID-19 face masks commonly used in Brazil. Aerosol Science and Technology. Apr. 26, 2021.
GHOLAMHOSSEIN, B. et al. An upper bound on one-to-one exposure to infectious human respiratory particles. PNAS. Dec. 2, 2021.
GUARDIEIRO, N. M. et al. A cloth facemask causes no major respiratory or cardiovascular perturbations during moderate to heavy exercise. medRxiv. Dec. 16, 2021.
MELLO, V. M. et al. Effectiveness of face masks in blocking the transmission of SARS-CoV-2: A preliminary evaluation of masks used by Sars-CoV-2-infected individuals. medRxiv. June 30, 2021.