Face masks can filter small airborne particles, including respiratory droplets, and are effective at reducing the spread of COVID-19

The wearing of face masks is one of the public health measures that has been adopted to reduce the spread of COVID-19. Multiple scientific studies have shown that mask-wearing is effective in reducing the number of infections. COVID-19 is transmitted mainly through respiratory droplets from infected people, which are expelled when coughing, talking or breathing. Face masks prevent most of these droplets from being released into the air, thus reducing the release of infectious material that could reach other people. This is known as source control.

Different arguments that questioned the usefulness or safety of face masks have been refuted in previous Health Feedback reviews, such as this one explaining that respiratory droplets are blocked by cloth masks, or this one explaining that face masks don’t prevent the passage of air and therefore can’t cause health problems associated with lack of oxygen or carbon dioxide buildup in the blood.

In late April 2021, a video was published by the YouTube channel Net Nuance, in which immunologist Byram Bridle questioned the efficacy of surgical masks. In the video, Bridle argued that the pores in the fabric of face masks are larger than respiratory droplets, and that these therefore pass through the masks and can spread viral particles. To support his argument, Bridle put on five surgical masks and showed the condensation of water vapor that his breath left on his glasses through the masks. The video accumulated more than 35,000 interactions on Facebook at the time of this review’s writing, including more than 13,000 shares, according to social media analytics tool CrowdTangle.

Other videos have also questioned the effectiveness of face masks, such as this one posted by political commentator Liz Wheeler on 4 August 2021. Wheeler claimed that face masks aren’t effective in preventing COVID-19 infections, citing several studies that found no benefit from the use of face masks. This video received almost 4,000 interactions on Facebook at the time of this review’s writing, including more than 1,200 shares.

However, contrary to what is claimed in these videos, face masks are an effective measure for preventing COVID-19 transmission, as we explain in this review.

How face masks prevent airborne disease transmission

Viruses such as SARS-CoV-2 are transmitted through the air by small respiratory droplets, which are expelled when people cough, talk, sneeze or breathe. These droplets are classified into two groups according to their size and the way they move through the air.

As the website of the U.S. Centers for Disease Control and Prevention (CDC) explains, large droplets are those with a diameter greater than five micrometers. Because of their size and weight, such droplets fall quickly to the ground in seconds to minutes. They can therefore only be transmitted over short distances.

On the other hand, smaller droplets with a diameter of five micrometers (1 millionth of a meter) or less can remain suspended in the air for minutes to hours. Such droplets are known as droplet nuclei and form a small cloud in the air known as an aerosol.

A study published in June 2021 found that the average pore diameters for reusable face masks, surgical masks and N95 masks are 47, 33 and 30 micrometers, respectively, based on X-ray tomography analysis^[1]. Does this mean that face masks don’t prevent the passage of smaller particles? Not so.

Masks prevent the release of most small respiratory droplets into the air by slowing the particles as they pass through the mask, facilitating their interception by fabric fibers^[2]. The smaller a particle is and the slower it moves, the greater the chance that it will be caught by fabric fibers while passing through the mask. This is because small airborne particles collide with surrounding gas molecules, meaning that the airborne particles don’t move in a straight line, but in a random, zig-zag pattern. This behavior is known as Brownian motion or diffusion^[3]. As explained by Pierre Herckes, a professor in the Arizona State University School of Molecular Sciences:

“Tiny particles don’t just move in a straight path—they are subject to Brownian motion—darting about randomly. Due to this—the masks catch more than you would imagine in both directions. They protect and prevent much of the viral particles in droplets from dispersing into ambient air.”

Therefore, masks are able to filter most airborne particles, even if the particle size is smaller than the pores in the fabric, as shown by different studies. Medical-grade face masks can capture most of the smallest respiratory droplets emitted by a person while breathing, talking and coughing. A study with surgical and KN95 masks found a 74 to 90% reduction in the emission rate of respiratory droplets with diameters between 0.3 and 0.5 micrometers^[4].

Even homemade masks have proven to be effective in filtering aerosols from the air. Another study found that homemade masks made from several layers of different commonly used fabrics, such as cotton, gauze or flannel, could filter up to 80% of particles smaller than 0.3 micrometers in diameter^[5].

But if the masks prevent respiratory droplets from passing through, why do Bridle’s glasses fog up when he breathes on them through five surgical masks, as seen in the video? The answer to this lies in a physical phenomenon called condensation, whereby water vapor in a gaseous state present in the air becomes liquid on contact with a cold surface.

Air expelled when breathing contains water vapor, which is a gas. Face masks can’t prevent the passage of gas molecules, as these are many times smaller than the filter size of any mask. The diameter of a water molecule is about 0.00028 micrometers. Upon contact with the surface of the glass, which is cold, the water vapor condenses and forms water droplets. These remain on the glass until they are heated to room temperature and evaporate again. Therefore, what is presented in the video as proof of respiratory droplets passing through five layers of surgical masks, is just hot air passing through the masks. This is also the phenomenon that causes glasses to fog up if face masks are not properly fitted.

Current evidence supports the use of face masks to prevent COVID-19 transmission

Apart from the video by Bridle, another video by Liz Wheeler called mask effectiveness into question by citing various studies that found no benefit associated with face mask use in relation to transmission of COVID-19 or other flu-like respiratory illnesses. However, most of the sources cited by Wheeler were either updated later or had significant limitations acknowledged by their authors, as were based on then-existing evidence on COVID-19 transmission.

Wheeler first cited a commentary by two researchers at the University of Illinois, who did not find a benefit related to the daily use of cloth face masks. In the original article published in March 2020, the authors stated:

“We do not recommend requiring the general public who do not have symptoms of COVID-19-like illness to routinely wear cloth or surgical masks because: There is no scientific evidence they are effective in reducing the risk of SARS-CoV-2 transmission Their use may result in those wearing the masks to relax other distancing efforts because they have a sense of protection We need to preserve the supply of surgical masks for at-risk healthcare workers.”

In an update published in July 2020, the authors acknowledged that their initial conclusions were affected by the limited data available at the beginning of the pandemic for assessing the efficacy of cloth face masks in preventing COVID-19 transmission. They clarified:

“Despite the current limited scientific data detailing their effectiveness, we support the wearing of face coverings by the public when mandated and when in close contact with people whose infection status they don’t know. However, we also encourage everyone to continue to limit their time spent indoors near potentially infectious people and to not count on or expect a cloth mask or face covering to protect them or the people around them.”

Wheeler also cited a Perspective article published in The New England Journal of Medicine in April 2020. The authors discussed possible limitations of face masks in preventing the spread of COVID-19. An editor’s note was added on June 2020, which reads:

“This article was published on April 1, 2020, at NEJM.org. In a letter to the editor on June 3, 2020, the authors of this article state ‘We strongly support the calls of public health agencies for all people to wear masks when circumstances compel them to be within 6 ft of others for sustained periods.’”

Another study cited by Wheeler was a viewpoint published in July 2020 by the Oxford Center for Evidence-Based Medicine. Wheeler claimed that the authors found “no evidence for the effectiveness of face masks against virus infection or transmission”. The viewpoint’s authors found 12 trials studying the benefits of face masks, which had various limitations:

“The design of these twelve trials differed: viral circulation was usually variable; none had been conducted during a pandemic. Outcomes were defined and reported in seven different ways, making comparison difficult. It is debatable whether any of these results could be applied to the transmission of SARS-CoV-2.”

Another study cited by Wheeler was a randomized control trial published in October 2020 in the journal PLOS One^[6]. The results of the trial didn’t find a positive effect from the use of face masks, but the authors acknowledged that limitations in their study made their results inconclusive:

“This trial was unable to provide conclusive evidence on face mask efficacy against viral respiratory infections most likely due to poor adherence to protocol [by participants in the study].”

Similarly, Wheeler cited another randomized control trial, published in Annals of Internal Medicine in November 2020^[7]. Results on benefits of face masks were not significant, and the authors mentioned the following limitations of their study:

“Inconclusive results, missing data, variable adherence, patient-reported findings on home tests, no blinding, and no assessment of whether masks could decrease disease transmission from mask wearers to others.”

Another cited study was a review published in November 2020 by the non-profit medical research organization Cochrane^[8]. Authors found little to no benefit of different types of face masks related to the transmission of flu-like illnesses. However, the following limitations were noted:

“Our confidence in these results is generally low for the subjective outcomes related to respiratory illness, but moderate for the more precisely defined laboratory-confirmed respiratory virus infection, related to masks and N95/P2 respirators. The results might change when further evidence becomes available. Relatively low numbers of people followed the guidance about wearing masks or about hand hygiene, which may have affected the results of the studies.”

Thus, in contrast to the numerous studies demonstrating the efficacy of face masks in preventing COVID-19 infections, Wheeler cited studies conducted with preliminary data at the onset of the pandemic or studies that had limitations that could affect the results obtained, as the authors acknowledged.

Wheeler also cited the European Centre for Disease Prevention and Control (ECDC)’s advice against the use of KN95 masks for the general public. This is taken out of context, as a technical report by the ECDC regarding the use of face masks states the following:

• In areas with community transmission of COVID-19, wearing a medical or non-medical face mask is recommended in confined public spaces and can be considered in crowded outdoor settings.
• For people vulnerable to severe COVID-19, such as the elderly or those with underlying medical conditions, the use of medical face masks is recommended as a means of personal protection in the above-mentioned settings.

Regarding the use of respirators, including KN95 masks, the ECDC advises:

“The very limited scientific evidence regarding the use of respirators in the community does not support their mandatory use in place of other types of face masks in the community. Although respirators would not be expected to be inferior to non-medical or medical face masks, the difficulties to ensure their appropriate fitting and use in community settings as well as potential adverse effects related to lower breathability should be taken into account.”

Hence, the ECDC recommends the use of face masks to prevent the transmission of COVID-19. Its guidance on KN95 masks simply means that respirators are no less effective than other types of masks, based on available evidence, but that such masks aren’t recommended for the public because they aren’t as easy to use or comfortable to wear.

Lastly, Wheeler cited a CDC response to a Freedom of Information Act request asking for the randomized control trials the CDC uses to support face mask mandates, which was shared in this tweet, dated 5 April 2021.

The CDC responded by stating it wasn’t aware of any randomized control trials showing face mask use to be effective against COVID-19 transmission. But there is a reason why randomized controlled trials on the efficacy of face masks were scant during the COVID-19 pandemic. As explained by PolitiFact, in order to conduct such a trial, some of the participants would be required not to wear a face mask for the duration of the trial. As there is evidence that face masks prevent the transmission of flu-like diseases, conducting such a trial during a pandemic places non-mask-wearing participants at risk for illness and complications, and thus creates ethical concerns.

However, evidence can also be gathered using other types of studies besides a randomized controlled trial. Indeed, in response to the FOIA request, the CDC provided a list of references to other types of scientific studies showing the efficacy of face masks in reducing COVID-19 transmission^[9]. Those studies supported the CDC’s conclusion that “experimental and epidemiological data support community masking to reduce the spread of SARS-CoV-2”.

A review published in January 2021 found that both medical and non-medical face masks acted as source control to reduce transmission of COVID-19 in the community^[10]. Many studies arrived at similar conclusions—here is a list of 22 scientific studies published in 2020, which concluded that wearing a face mask reduces one’s risk of being exposed to the disease.

Masks don’t completely prevent the emission of small respiratory droplets, which is why physical distancing is important even when masks are used in order to reduce the risk of contamination even more. This is highlighted by CDC guidelines, which recommend keeping a distance of more than two meters (six feet) from other people in poorly ventilated areas, where aerosols can remain suspended in the air for a longer period of time.

Conclusion

There is ample scientific evidence that face masks are capable of filtering small airborne particles which carry viral particles, even if these particles are smaller than the pore size of the mask fabric. Bridle’s demonstration of glasses fogging is due to hot air passing through face masks, not respiratory droplets. There is also evidence that face masks are effective in preventing the transmission of COVID-19, supporting the recommendation by various public health agencies for the general public to wear masks.

REFERENCES

• 1 – Iacoviello et al. (2021). Microstructure analysis and image-based modelling of face masks for COVID-19 virus protection. Communications Materials.
• 2 – Tcharkhtchi et al. (2021). An overview of filtration efficiency through the masks: Mechanisms of the aerosols penetration. Bioactive Materials.
• 3 – Liu (2010). Chapter 1 – Particle Deposition onto Enclosure Surfaces, in Developments in Surface Contamination and Cleaning.
• 4 – Asadi et al. (2020). Efficacy of masks and face coverings in controlling outward aerosol particle emission from expiratory activities. Scientific Reports.
• 5 – Konda et al. (2020). Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks. ACS Nano.
• 6 – Alfelali et al. (2020). Facemask against viral respiratory infections among Hajj pilgrims: A challenging cluster-randomized trial. PLOS ONE.
• 7 – Bundgaard et al. (2021). Effectiveness of Adding a Mask Recommendation to Other Public Health Measures to Prevent SARS-CoV-2 Infection in Danish Mask Wearers. Annals of Internal Medicine.
• 8- Jefferson et al. (2020). Physical interventions to interrupt or reduce the spread of respiratory viruses. Cochrane Database of Systematic Reviews.
• 9 – Brooks and Butler (2021). Effectiveness of Mask Wearing to Control Community Spread of SARS-CoV-2. JAMA.
• 10 – Howard et al. (2021). An evidence review of face masks against COVID-19. PNAS.