NOTE: The following article is an abridged version of a more complete and referenced article published in the Science Partner Journal August 7, 2020.
[Original article: https://spj.sciencemag.org/journals/research/2020/7286735/]
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 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.
There are many different types of face masks and respirators offering different levels of protection to users. 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.
Contaminants in the air differ vastly in size.
SARS-CoV-2 has a size ranging from 60 to 140 nm , 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.
How Do Masks Protect Us against Airborne Diseases
The Respiratory Route of Transmission
A respiratory pathogen may be transmitted via three routes—contact, droplet, and airborne spread. Contact transmission may be direct (i.e., transfer of virus via contaminated hands) or indirect (i.e., via fomites). 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.
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. 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. 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”. 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.
Mechanistic Effect of Wearing a Mask
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. This suggests that surgical face masks can reduce the release of coronavirus and influenza from an infected person.
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.
Advantages and Caveats of Wearing Masks
Mask usage, in addition to other nonpharmaceutical interventions, can be an effective containment measure in an epidemic. Face masks can prevent dispersal of droplets when infected persons talk, sing, cough, or sneeze. The rate of emission of particles correlates with voice loudness during speech or other vocal activities. A physical obstruction that prevents the wearer from touching the face, a mask may lead to better hand hygiene. The reverse is also true—an increased tendency for wearers to touch their faces, such as when adjusting their masks. Even with the right mask, wearers can still be infected if droplets enter via the eyes, thus highlighting the importance of additional protection.
Masks also reduce the risk of environmental contamination by respiratory droplets. Hence, mask usage must be complemented by other behavioral changes for effective infection prevention. Finally, the universal use of face masks prevents discrimination of individuals who wear masks when unwell because everybody is wearing a mask. Universal mask-wearing can create new social norms, motivating individuals to wear masks at the initial onset of symptoms without fear of being stigmatized.
Understanding Performance of Commercial Mask
3-Ply Surgical Mask
The 3-ply surgical mask is commonly used in the COVID-19 pandemic. The 3-ply surgical mask is made up of 3 different layers of nonwoven fabric with each layer having a specific function, as shown in the graphic above. The outermost layer (typically blue) is waterproof and helps to repel fluids such as mucosalivary droplets. The middle piece is the filter, which prevents particles or pathogens above a certain size from penetrating in either direction. The innermost layer is made of absorbent materials to trap mucosalivary droplets from the user. This layer also absorbs the moisture from exhaled air, thus improving comfort. Together, these 3 layers effectively protect both the user and the surrounding people by limiting the penetration of particles and pathogens in both directions.
Masks Made from Household Materials
The surge in demand worldwide for commercial face masks during the COVID-19 pandemic has led to a global shortage of supplies for both physical products as well as raw materials. In this circumstance, making a mask at home can be a life-guarding action. Homemade masks may vary from the commercial ones in terms of structural integrity and effectiveness, but they are cheap and accessible. Wearing a simple cloth mask is far better than wearing no mask to safeguard the wearer and the others’ health.
Using commonly available household materials, it is easy to fabricate simple masks that may block respiratory droplets from the wearer. A lot of household materials have been used to fabricate masks and tested accordingly. These typically include cotton fabrics, clothing, silk, tissue paper, kitchen towels, pillowcase, and tea cloths.
In addition to material, other factors, including the design, the velocity, the fitness to the wearer’s face (sealing issue), and the properties of the particles to which it will be exposed, also affect the overall performance of a homemade mask.
Historically, cloth masks have been used to protect healthcare workers (HCWs) from respiratory infections, yet it is only in recent years that researchers started to systematically study their efficacy. By combining different commonly available fabrics, for instance, cotton-silk, cotton-chiffon, cotton-flannel, and filtration efficiency for particles <300 nm and >300 nm can be as high as >80% and >90 %, respectively. The high efficiency comes from the synergistic effect of mechanical filtration from cotton and electrostatic filtration from the other layer like silk. It was also highlighted in this work that for the same material such as cotton, there are other factors that critically and significantly affect the overall performance when used as a mask. These include the layer number, the layer density (threads per inch, TPI), and the facial fitness (openings and gaps between the mask edge and the facial contours). Therefore, future mask development should consider the above factors while taking into consideration the breathability, washability, and reusability.It was also highlighted in this work that for the same material such as cotton, there are other factors that critically and significantly affect the overall performance when used as a mask. These include the layer number, the layer density (threads per inch, TPI), and the facial fitness (openings and gaps between the mask edge and the facial contours). Therefore, future mask development should consider the above factors while taking into consideration the breathability, washability, and reusability.
NOTE: The above article is an abridged version of a more complete and referenced article published in the Science Partner Journal August 7, 2020.
[Original article: https://spj.sciencemag.org/journals/research/2020/7286735/]