The increasing prevalence of infectious diseases in recent decades has posed a serious threat to public health. Routes of transmission differ, but the respiratory droplet or airborne route has the greatest potential to disrupt social intercourse, while being amenable to prevention by the humble medical face mask. Different types of masks give different levels of protection to the user. The ongoing COVID-19 pandemic has even resulted in a global shortage of face masks and the raw materials that go into them, driving individuals to self-produce masks from household items. At the same time, research has been accelerated towards improving the quality and performance of face masks, e.g., by introducing properties such as antimicrobial activity and superhydrophobicity. This review will cover mask-wearing from the public health perspective, the technical details of commercial and home-made masks, and recent advances in mask engineering, disinfection, and materials and discuss the sustainability of mask-wearing and mask production into the future.

1. Introduction Emerging and reemerging infections have emerged as a threat to human health in recent decades [1]. Given how interconnected the world is today, a pathogen capable of human-to-human transmission can spark an outbreak far from where it originated. The virus causing the Middle East Respiratory Syndrome, for example, emerged in the Middle East but caused an outbreak in Korea. The world is in the midst of the COVID-19 pandemic, which is caused by the SARS-CoV-2 virus. Lockdowns and travel restrictions imposed to halt the spread of COVID-19 have led to devastating economic repercussions. The control of an infectious disease is based on knowledge of its mode of transmission. The recent COVID-19 pandemic is caused by the novel coronavirus, SARS-CoV-2, which is transmitted largely by the respiratory route (vide infra) [2, 3].

The best nonpharmaceutical interventions against disease spread via the respiratory route are broadly termed social or safe distancing measures, i.e., reducing close contact between individuals [4, 5]. Where safe distancing is not possible, personal protective equipment (PPE) is the accepted mode of self-protection. Masks and respirators are arguably the most important piece of PPE. They are a physical barrier to respiratory droplets that may enter through the nose and mouth and to the expulsion of mucosalivary droplets from infected individuals [6, 7]. Their role may be particularly important in COVID-19, where infected individuals may be shedding virus while asymptomatic or presymptomatic [8–10].

There are many different types of face masks and respirators offering different levels of protection to users [11–15]. Generally, masks do not fit tightly while respirators do. Masks and respirators may be reusable or disposable. Reusable ones include industrial-use half or full facepiece respirators with cartridge filters attached and homemade or commercial cloth masks; disposable ones include surgical masks, N95 respirators, and KN95 respirators. They all serve the general purpose of providing some form of protection against contaminants in the air, ranging from pollen to chemical fumes to pathogens. The filtering capacity, and hence the level of protection against pollutants and pathogens, depends on the materials used and the engineering design [11–15]. Contaminants in the air differ vastly in size (Figure 1). SARS-CoV-2 has a size ranging from 60 to 140 nm [16], smaller than bacteria, dust, and pollen. Therefore, masks and respirators made of materials with larger pore sizes, such as cotton and synthetic fabric, will not be able to effectively filter these viruses or tiny virus-laden droplets, as compared with those made of materials with much smaller pore sizes. Likewise, masks and respirators made of or coated with water-resistant materials are more effective against large virus-laden respiratory droplets and fluid spills. In addition to filtering capacity, factors such as user comfort and breathability also vary across different models. For instance, although the tight-fitting N95 respirator has filtering capacity superior to surgical masks, they have lower breathability and may cause discomfort after hours of wearing.

Figure 1

Relative size chart of common airborne contaminants and pathogens. Mask-wearing can be effective in the containment of communicable diseases [17, 18] and has thus become a new normal in many societies in the COVID-19 pandemic. The surge in demand for surgical masks and respirators has led to a global shortage of supply and raw materials. As a result, many people have resorted to making their own masks, recycling used masks, or settling for masks offering less protection than actually needed. Researchers and industry players have therefore been working hard to address the issue of shortage, as well as to enhance the protection afforded by existing mask models. These efforts include (i) sourcing and engineering alternative materials with sufficient filtering capacity, (ii) engineering the design of masks and respirators for better protection, breathability, and user comfort, (iii) developing and engineering multifunctional masks and materials with hydrophobic, antimicrobial, self-disinfecting, and even sensing properties, and (iv) exploring new technologies for efficient production and customization of masks, e.g., 3D printing [19].

Attempts to enhance the mask will pivot on understanding the basics of mask technology. The fundamental questions, to our mind, are as follows: (i) how do masks (and the mask materials) protect us from pathogens; (ii) what are the existing models and materials of mask available in the market; (iii) how do they perform and how is their performance benchmarked against others; (iv) what are their limitations; (v) how can their performance be improved; (vi) what are some new features that can be incorporated into existing materials and models? This review seeks to address the above questions.

1. How Do Masks Protect Us against Airborne Diseases 2.1. The Respiratory Route of Transmission A respiratory pathogen may be transmitted via three routes—contact, droplet, and airborne spread [20]. Contact transmission may be direct (i.e., transfer of virus via contaminated hands) or indirect (i.e., via fomites) [20]. Fomites are objects or materials that may carry infection, and spread by fomites means spread by touch. Viruses do survive for some time on inanimate objects, although the viral load declines dramatically [21]. If we touch a contaminated surface and then touch our eyes or nose, we may inoculate the virus into our mucosal surfaces. The role of touch in the spread of a respiratory virus is best exemplified by studies of the Respiratory Syncytial Virus (RSV) [22, 23]. The spread of SARS-CoV-2 via fomites has been elegantly demonstrated by real-world contact tracing, aided by closed-circuit cameras [24].

Droplet spread and airborne spread are different modes of transmission of the virus through the air. Viruses released when an infected person coughs, sneezes, sings, talks, or merely exhales may be found in particles of varying sizes [17]. Generally, particles larger than 5 μm were thought to fall to the ground within 1 metre. More recently, however, the “gas cloud” hypothesis has been proposed [25]. Coughing, sneezing, or even exhaling produces mucosalivary droplets that exist as part of a cloud that “carries within it clusters of droplets with a continuum of droplet sizes” [25]. In combination with environmental factors, the “cloud” may be propelled up to 7–8 m. Wind speed, in particular, has been shown to play a role in determining the distance travelled by these particles [26].

Airborne spread occurs with pathogens found in exhaled  μm in diameter. These particles remain afloat for some time and are able to travel long distances. Respiratory viruses accepted as being capable of spread via the airborne route include measles and varicella zoster (chickenpox). These viruses have a large , a feature thought to characterise spread by the airborne route. Interestingly, influenza, coronavirus, and rhinovirus RNA, generally thought to be transmitted by the droplet route, can be found in exhaled particles smaller or larger than 5 μm [17, 27]. Further, viable influenza is present in particles smaller than 5 μm. Hence, even viruses thought to be transmitted primarily by the respiratory droplet route may have the potential for airborne spread. Concern that SARS-CoV-2 may spread by the airborne route rose when it was shown to be viable for 3 hours in a drum that artificially kept particles afloat for several hours [21].

It might be less well known that more basic processes like talking can also lead to the release of potentially infectious droplets and aerosols. Using laser light scattering, it was found that there were average emissions of about 1000 droplet particles per second during speech, with high emission rates of up to 10,000 droplet particles per second [28]. By fitting the time-dependent decrease in particle detected to exponential decay times, the droplet particle sizes and estimated viral load could be calculated. The authors estimate that 1 min of loud speaking generates greater than 1000 droplets containing viruses [29]. Alternatively, respiratory particles of between 0.5 μm and 5 μm could be imaged by aerodynamic particle sizing. When participants made the “Aah” sound, there were emissions of up to 330 particles per second [30]. Taking into account that aerodynamic particle sizing measures particles under the detection limit of laser light scattering, these two methods can be seen to be complementary, and the total number of particles emitted could be even higher. In a separate study, droplet particle emission was shown to be directly proportional to loudness, with the number of particles emitted increasing from 6 particles per second when whispering to 53 particles per second at the loudest talking. The number of particles generated varied greatly across individuals, raising the possibility of superspreaders who could be the primary spreaders of viruses by talking [31].

2.2. Mechanistic Effect of Wearing a Mask Masks and other PPE items serve as a physical barrier to respiratory droplets. With imaging using laser light scattering, it was found that the number of flashes, which corresponds to the number of respiratory droplets, could be kept at background levels by covering the speaker’s mouth with a slightly damp washcloth [28]. An in vitro model with source and receiver mannequins was created to test the effect of the mask on filtering away radiolabelled aerosol emitted from the source. Masking at the source mannequin was consistently more effective at lowering radio-labelled aerosols reaching the receiver mannequin, whereas the only experimental setup where the receiver mannequin could be equally well protected was if the receiver mannequin wore an N95 mask sealed with Vaseline [32]. Therefore, masks can act as a physical barrier and seem to be more effective when worn by the droplet emitting person.

Masks have generally shown an effect in reducing virus emission from infected patients. The surgical mask was tested for its ability to block the release of various viruses by studying the amount of virus present in the exhaled breath of patients. The investigators were able to collect particles separated by size (> or <5 μm). A significant drop in coronaviruses in both larger and smaller particles was observed with the mask on. The mask reduced influenza viruses found in larger but not smaller particles. After wearing a mask, no coronavirus was detected in all 11 patients, while influenza was detected in 1 patient’s respiratory particles (out of 27). The mask did not lower rhinovirus counts in larger or smaller particles [17]. This suggests that surgical face masks can reduce the release of coronavirus and influenza from an infected person. In an earlier study for influenza, participants were induced to cough, and with both surgical masks and N95 masks, there was no influenza that could be detected by reverse transcriptase-polymerase chain reaction (RT-PCR) for 9 infected patients [33]. When the exhaled influenza virus was separated into the fractions based on size, it was found that surgical masks were highly effective at removing influenza from the larger coarse fraction (≥5 μm) but less effective from the fraction with smaller particles [34].

Wearing masks has also been shown to protect individuals coming into contact with an infected person. In a survey of 5 hospitals in Hong Kong during SARS, hospital staff were asked about the protective measures they took and this information was correlated with whether they were infected by SARS. It was found that wearing masks was the single most important protective measure in reducing the chance of getting infected (), and the people who wore either surgical masks or N95 masks were not among the 11 infected staff. There were however 2 instances of people who wore paper masks being infected, suggesting that the type of masks was also important [35]. A study compared the effectiveness of N95 and surgical face masks against viral respiratory infections in healthcare workers. Healthcare workers had no significant difference in influenza infection outcomes when wearing N95 and surgical masks, suggesting that both types of medical masks could protect similarly [36]. A meta-analysis was performed on clinical studies to explore the protective effect of masks. The risk ratio was calculated for the incidence of infection in the protected group vs. the unprotected group, where risk suggests a reduced risk. Wearing a mask protected individuals against influenza-like illness, showing a risk ratio of 0.34, with a 95% confidence interval between 0.14 and 0.82. Similar to the study above, surgical masks and N95 masks showed little difference in protection, with a risk ratio of 0.84 and a 95% confidence interval of 0.36-1.99 suggesting no significant difference in risk [37].

Recently, a modelling study performed by Eikenberry et al. based on COVID-19 infection data obtained in New York and Washington suggested that the broad adoption of face mask by the general public can significantly reduce community transmission rate and death toll [18]. As shown in Figure 2, based on data obtained from 20th February to 30th March, the cumulative death rate was projected to be reduced to a greater extent as more people wear masks over the next 2 months. Therefore, the study concludes that community-wide adoption of face mask has great potential to help curtail community transmission and the burden of the COVID-19 pandemic.