COVID-19: the choice of mask materials is important – it can reduce the transmission of particles by 96%

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Wearing a face mask can protect yourself and others from COVID-19, but the type of material and how many fabric layers used can significantly affect exposure risk, finds a study from the Georgia Institute of Technology.

The study measured the filtration efficiency of submicron particles passing through a variety of different materials. For comparison, a human hair is about 50 microns in diameter while 1 millimeter is 1,000 microns in size.

“A submicron particle can stay in the air for hours and days, depending on the ventilation, so if you have a room that is not ventilated or poorly ventilated then these small particles can stay there for a very long period of time,” said Nga Lee (Sally) Ng, associate professor and Tanner Faculty Fellow in the School of Chemical and Biomolecular Engineering and the School of Earth and Atmospheric Sciences.

The study was conducted during spring 2020, when the pandemic triggered a global shutdown of most institutions. Communities faced massive shortages of personal protective equipment, prompting many people to make their own homemade masks. Georgia Tech quickly set up the study since it already had “a great system for testing filtration efficiency using existing instruments in the lab,” Ng recalled.

The study’s findings were used to shape homemade face mask recommendations here last April, with the comprehensive study findings published on March 22 in the journal Aerosol Science and Technology.

In all, the researchers tested 33 different commercially accessible materials not limited to cloth fabrics, including single-layer woven fabrics such as cotton and woven polyester, blended fabrics, nonwoven materials, cellulose-based materials, materials commonly found and used in hospitals, and various filter materials.

“We learned there was a lot of variability in filtration performance even in the same type of material,” Ng said.

“We found commercially available materials that provide acceptable levels of submicron particle rejection while still maintaining air flow resistance similar to a surgical mask,” said Ryan Lively, an associate professor and John H. Woody Faculty Fellow in the School of Chemical and Biomolecular Engineering.

“These materials combine fabric fiber density, a maze-like structure, and fiber surface chemistry to effectively reject submicron particles.”

The best-performing materials for homemade masks were blackout drapery and sterilization wrap widely used for packing surgical instruments. Both materials are commercially available.

The researchers said people should avoid using filters such as a HEPA/MERV or vacuum bags unless they are certified to be fiberglass-free since often such filters on their own may release glass fibers that can be inhaled. Other materials to avoid for masks include loose-knitted material, batting fabric, felt, fleece, or shiny, reusable shopping bags.

Multilayered samples performed much better than single-layer samples, but people should pay attention to breathability. The two-layered and three-layered samples tested show an overall filtration efficiency of about 50% for submicron particles. Mask fit is also important since particles can easily escape through gaps at the nose or through the sides of the mask.

The analysis showed that properly fitted and multilayer masks reject 84% of particles expelled by a person when one person wears it. Two people donning these types of masks reduces particle transmission by 96%.

A final takeaway of the research was the importance of universal mask wearing.

“The best way to protect ourselves and others is to reduce exhaled particles at the source, and the source is our face,” Ng said, adding, “That really gets amplified when everyone starts wearing masks.”

She expressed optimism that the findings will motivate people to more widely embrace mask wearing if they are sick and need to be in public.

“Not everyone understands the importance of airborne virus transmission, and the importance of wearing a mask,” she said. “I hope that the practice will continue to help reduce the release of these viral particles into the environment and help protect others.”


Face masks protecting from infections

Respiratory masks (RM) are protective devices covering a part of the face. They are designed to protect both the person who wears them and the immediate environment from breathable pollutants (respiratory poisons or bacterial/viral pathogenic organisms). Different masks can be classified as I) full masks (normed following EN 136) and II) half and quarter masks (EN 140) (Figs. 1, ​,2,2, ​,33 and ​and4).

While a full mask covers the whole face, a halfmask fits from under the chin to above the nose, a quarter mask fits from the top of the nose to the top of the chin. The breathing resistance varies proportionally to the density of the mask material.

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Fig. 1
FFP (filtering face piece) mask without valve
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Fig. 2
FFP (filtering face piece) mask with valve
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Fig. 3
Homemade face mask for everyday use
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Fig. 4
Surgical mask (MNP)

FFP masks (filtering face piece) are classified as half masks. Their use is required to prevent the entry of pathogens through the airway and have the role of protecting both the wearer and the surrounding people. They are different from medical MNC, (often referred to as “surgical masks”), and from “self-made” masks for everyday use. MNCs and self-made masks are not “leak-proof” and do not provide complete respiratory protection since air can escape through them. FFP masks come without (Fig.)1) or with (Fig. 2) a valve.

FFP (filtering face piece) masks with valves provide an air flow from the inside to the outside of the mask. FFP 1 masks are dust masks and mainly used for this purpose. They do not prevent COVID-19 infections. FFP1 masks are suitable for work environments in which only non-toxic dusts are found. FFP2 masks are suitable for work environments where there are pathogens and mutagens in the air composition.

In the context of SARS-CoV-2 the following types of masks are available (WHO, 2020):

  1. Masks for everyday use (temporary masks made from fabric, etc.; Fig. 3): These masks grant no protection for the user from being infected. However, it is safe to assume there is a small risk reduction for droplet transmission, especially during exhalation, resulting in a reduction of potential viral spread. These masks should not be used in the health care system, but are commonly recommended for the general population for walking, shopping, or using public transportation.
  2. MNP (= medical mouth–nose protection; Fig. 4): often referred to as a “surgical mask”. The industrial production of MNP abides to strict rules to provide protections against infection. The filtering capability is like the one for everyday use masks and they are intended to protect patients. They are approved for medical staff use, warrantying only patient-protection, specifically aimed against aerosols.
  3. FFP2-mask (= face filtering piece)/N95-mask: FFP2-masks fulfil a set of stricter protective norms. They protect the person wearing them, as > 95% of particles and droplets are held back when inhaling. FFP2-masks also effectively protect the environment as long as there is no exhaling valve. In contrast, masks with an exhaling valve let exhaled air pass out unfiltered, with contamination of the immediate environment.
  4. FFP3-mask: FFP3-masks protect the user even more effectively than FFP2, as > 99% of droplets and particles are filtered when inhaling. FFP3-masks also protect the environment in the absence of an exhaling valve.

A full face mask in a level-3 biosafety lab is shown in Fig. 5.

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Fig. 5
Full face mask in a level-3 biosafety lab (source: Wikipedia https://en.wikipedia.org/wiki/Face_masks_during_the_COVID-19_pandemic)

The WHO states that the declared protective effect of these masks recommended during the SARS-CoV-2 pandemic can be severely reduced by their inappropriate use, such as improper donning or doffing, insufficient maintenance, long or repeated use of disposable masks, no dry cleaning of fabric masks, or using masks made of non-protective material [2].

During an epidemic/pandemic crisis every possible risk reduction strategy is useful. It is likely that the risk of infection and its severity depends on the viral load entering the body. This was the rationale for the Robert Koch Institute (RKI) to recommend the use of masks starting from March 2020. Specifically, they looked at the availability of the resources and tailored the supply to the risk of infection. Healthcare workers were considered essential workers at high risk of infection, therefore prioritized to the use of FFP2/3 masks, while MNC or masks for everyday use were to be made available for the general population.

Current decree on wearing a mouth and nose covering
Due to the German Federalism, the Federal Minister of Health can only make health recommendations, which are then reinforced by the Infection Protection Act of the different Federal States. In the current situation of a pandemic crisis, nearly all measures are taken to prevent an exponential increase of new SARS-CoV-2 infections.

As of June 1st, 2020, the Netherlands considers the public use of protective masks unnecessary. This is based on the assumption that SARS-CoV-2 is only transmitted as a droplet infection via the nasopharynx pathway, which mostly occurs during coughing or sneezing. These droplets do not stay in the air, but rather drop to the ground within a 1.5 m radius if larger than 5 µm [3].

It has been postulated that for SARS-CoV-2—in contrast to other respiratory-driven infections—the droplets in the aerosols are of little relevance for a COVID-19 outbreak. Therefore, securing a 1.5-m social distance is assumed to be an essential and sufficient preventive measure. However, recent data published in 2020 using high-speed cameras show that small droplets of saliva and mucus can fly up to 8 m [4], requiring critical reconsideration of the above-mentioned assumption.

We conducted a Medline survey to scientifically justify this approach with the key words SARS-CoV-2, face masks, COVID-19, pandemic.

Leung and colleagues [5] screened more than 3000 individuals and identified 123 patients suffering from a viral respiratory infection. The viral load in the exhaled aerosol and droplets were different depending on the etiology of the infection, but was exponentially reduced by wearing surgical masks (cat. no. 62356, Kimberly-Clark).

More viral particles were released through coughing. Generally, the authors reported a notably higher viral load in nose swabs compared to throat swabs. This data applied to influenza, corona, and rhino virus. No data are available for SARS-CoV-2 yet.

In general, droplets, and hence SARS-CoV-2, can be transferred via direct contact or smear transfection modality when the hands are contaminated from touching the nose or the face and then come in direct contact with others, e.g. by handshaking. For this reason, not only the “cough etiquette”, but regular and thorough handwashing are a significant and mandatory hygienic rule (6).

As a result of scientific data combined with daily routine, the RIVM (Rijksinstituut voor Volksgezondheid en Milieu, the Dutch equivalent of the RKI) has mandated to wear masks while using public transportation, due to the inability of maintaining enough protective distance, especially when riding during rush hour. This rule does not apply to other public spaces yet.

Summarizing the arguments in favour of wearing a mask

  • Wearing a mask in areas where sufficient distance is not feasible, such as public transportation, most likely reduces the spread of virus-loaded droplets and therefore the risk of transferring SARS-CoV-2.
  • It is indisputable that infected patients can transfer SARS-CoV-2 to other people, starting few days before manifesting clinical symptoms or during the incubation period. However, there is no reliable data concerning the amount of virus particles that can be spread by an asymptomatic person, when keeping a minimum safe distance.

Main arguments against wearing a mask

  • If there is a limited supply of protective masks, they should be reserved for health care workers in hospitals and care facilities. This applies for surgical masks and for FFP2 and FFP3 masks.
  • Masks give a false sense of security. The main role of MNC is the protection of people standing nearby. MNC do not protect the wearer.
  • It is essential to wear the mask correctly. It must fit airtight to the skin, otherwise its effect is lost. Doffing of the mask needs to be properly done as well. The outside of the mask should not be touched. When supply is not an issue, surgical masks should be used only once.
  • The lack of nonverbal communication when wearing a mask may make people feel insecure, disheartened or even psychologically troubled. This may be particularly true for people suffering from mental illness or hearing impairment.
  • Breathing dampens the mask. If there is excessive moisture, the masks become airtight. Therefore, air is inhaled and exhaled unfiltered around the edges, losing the protective effect for both the wearer and the environment.
  • If masks are not exchanged regularly (or washed properly when made of cloth), pathogens can accumulate in the mask. When improperly used, the risk of spreading the pathogen—including SARS-CoV-2—might be critically increased.

Conclusion of the studies

Currently, most of the literature available on this topic is from experimental investigations. As expected, all the studies demonstrated an increase in protective effects in the following order: masks for everyday use–MNP–N95/FFP–PPE. Masks for everyday use can have a small protective effect for the wearer.

MNP offers a greater protective effect since it was originally designed to decrease droplet elimination, therefore protecting the user’s surroundings. Unfortunately, due to ethical reasons, there is a lack of randomized controlled studies on the protective role of masks in the prevention of SARS-CoV-2 infections when compared to a control group with no masks. Since the Netherlands lack of a law to wear protection masks in public except for public transport since May 2020, it could serve as the control in future studies that compare the infection rates of different countries with different approaches to tackling the pandemic.

In 2016, Smith et al. [10] concluded that possible advantages of wearing a mask were difficult to apply to the social “day-to-day” situation. Konda et al. (12) highlighted the inability to discriminate between the protective effects of the mask on the environment, when worn by an infected person, versus the general protective effect within a given population.

This would not have a significant health benefit if only a small percentage of individuals were infected. Only a study done in infected people with and without masks would allow a clear conclusion on the role of masks on the spread of the infection. Finally, a lesson learnt from the COVID pandemic shows significant educational gaps and lack of basic training that need to be addressed. The state should guarantee mask supply for everyone and educate on the proper use.

Mass means of communication could be used for this purpose. A commercial broadcast before the daily news about the correct donning and doffing of the mouth and nose protection and its disinfection could reach a vast audience. In addition to public law, private and digital media, as well as healthcare providers such as doctors, pharmacists and nursing staff could also play an important role in education.

Consequences of the use of protective masks on the wearer—pathophysiologic considerations

Wearing a mask has its own advantages and indisputable protective effects against infections. However, there are also potential risks and side effects that require attention. This specifically applies to the use in the general population.

From a medical standpoint, there is a theoretical possibility of an airflow obstruction when wearing a mask. A subjective feeling of strained breathing rarely occurs when wearing surgical masks. When wearing very dense masks without valves (N95/FFP2-3), breathing occurs against an air flow resistance. Theoretically, an increase in work of breathing can occur, especially during physical exertion.

Depending on the design, masks can increase the lung’s dead space. In extreme cases, carbon dioxide retention (hypercapnia) can occur with side effects. Only few investigations are available and addressing this medical problem. The available literature examined different types of N95 masks in the industrial setting in detail [14–16], and found relevant effects on the wearer.

In this context, Kim et al. [17] studied the role of N95 masks on lung function and heart rate during low-to-moderate exercise/physical work load. Only healthy subjects seem to tolerate wearing such a mask. Studies conducted on employees in advanced stages of pregnancy showed a good tolerance for masks.

The results of this study, even though specific to this population, are valuable for the daily use of MNP as a general mean of protection [18]. Finally, the role of N95/FFP-2 masks was tested in 97 patients with advanced COPD while undergoing a 6-min walk test. Seven patients did not tolerate the test and stopped prematurely.

The respiratory rate, oxygen saturation and CO2 levels changed significantly while wearing N95/FFP2 masks. These results demonstrated the potential risks of wearing this type of mask in the presence of advanced COPD [19]. Their use should be recommended with caution in this patient population, a questionably relevant recommendation, since the use of these masks is limited to health care workers in direct contact with COVID patients. Finally, people with hearing impairment rely on lip reading to understand others. This is not possible when wearing a mask.

Conclusion
Measures to prevent infections are necessary in the current pandemic. Face masks have been considered a first step to prevent and contain the spread of the disease. Different types of masks are available on the market for this purpose.

Simple masks covering mouth and nose are primarily used to prevent transmission by holding back droplets. This is useful when the recommended minimum distance of 1.5 m is not feasible. The masks provide only limited self-protection for its wearer and this is only when they are used properly.

High-quality FFP2/3 masks are a more reliable protection from infections. They should always be available for medical staff and people at risk. When used by the general population, specific groups at risk for complications related to the mask use should be educated on what to expect. For example, patients with severe COPD can experience a deterioration of their respiratory parameters. Therefore, patients must be individually educated by their general practitioner about the risk of wearing MNC.

Finally, it is imperative that the user is educated on the different types of masks available, how and when to wear them and, above all, how to handle them correctly, similar to the safety instructions given before take off in an aircraft.

Our results are consistent with the ones recently reported by Chu et al. in Lancet [20]. These publications will help guide the decisions of politicians and caregivers on when and where to use the available tools to fight a viral pandemic.

reference link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7422455/


More information: Taekyu Joo et al, Evaluation of Particle Filtration Efficiency of Commercially Available Materials for Homemade Face Mask Usage, Aerosol Science and Technology (2021). DOI: 10.1080/02786826.2021.1905149

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