UK-based researchers from HH Wills Physics Laboratory, University of Bristol, released the findings of their review of the efficacy of the face coverings and masks in preventing transmission of coronavirus disease 2019 (COVID-19) on the preprint server medRxiv*. Their study is titled, “How effective are face coverings in reducing transmission of COVID-19?”
The COVID-19 pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has infected over 65.67 million people around the world, and has killed over 1.51 million people. The pandemic was detected late in 2019 in Wuhan, China, and on the 11th of March 2020 was declared a pandemic by the World Health Organization (WHO).
To date, no drugs are effective against the virus and, until an effective vaccine is available for mass use, the risk of contracting the infection remains. Lack of pharmacological measures to prevent and cure the infection has mandated non-pharmacological measures, such as physical distancing, hand hygiene and one of the most important measures – wearing a face covering or a face mask that covers both the nose and mouth.
Aerosols and masks
The researchers write, “The prevailing view presupposes a binary classification between large and small droplets (also called aerosols), which respectively transmit disease via the droplet and airborne routes.”
Controversy regarding face coverings
Around the world, there has been a lot of controversy around wearing a face cover appropriately. Despite this, wearing masks and avoiding large gatherings remain the main agenda of public health campaigns by all major health bodies worldwide. Masks remain one of the most effective ways to prevent the spread of this airborne infection, write the researchers, and this study was conducted to show the efficacy of wearing face coverings correctly in preventing SARS-CoV-2 infection.
The team looked at the underlying physics that prevents the entry of particles with diameters > 1 micron into the nose and mouth of the wearer. As well as analyzing the efficacy of simple cotton or surgical masks, the study attempted to answer several questions regarding the viral load of airborne exhaled particles and the infectious dose from the infected person to the healthy person.
Type of mask
The team wrote, “Agencies like the Centers For Disease Control and Prevention recommend members of the public wear reusable fabric coverings, whereas disposable surgical masks are more common in East Asian countries such as China.” They note that the wide use of surgical masks will lead to a huge amount of plastic waste, so one of the goals of this study was to compare the efficacy of fabric coverings and surgical masks in preventing the infection. In order to compare the two, mathematical modeling studies were used to assess the relative efficacy.
They added, “We argue that face coverings do not eliminate the risk of secondary transmissions, especially in high-risk settings such as in hospitals or amongst large crowds, but should significantly reduce the risk in most settings.”
Personal air filters
The team says that masks can act as a personal air filter by effectively filtering out particles. The size of the particle it can filter out determines its efficacy. The authors of the study concluded from their study:
- Droplets and coarser aerosols ( diameter greater than, or equivalent to, 1 µm) contain a significant amount of the virus. These are easily filtered because they are less mobile. The filtering efficacy for these particles 5 µm in diameter or larger is 100%.
- Finer aerosols that are between 0.1 and 1 µm are transported around the fibre by the gas flow. Thus, they cannot be stopped. Filtering efficiency is low (30 to 60%) for these particles.
They add, “Both surgical and cotton masks are thus only partially effective at filtering out sub-micrometre aerosols. However, their efficiency rapidly increases as the size increases beyond a micrometre.”
When more effective masks are considered, N95/KN95/FFP2 masks, for example, the filtering efficiency rises. These have “electret fibres which can sustain considerable electrostatic charge.” This raises their efficiency in filtering 0.1 to 1 µm particles.
Which aerosols contain the virus?
The researchers explained that expiratory particles containing the virus could be determined based on their site of origin. Larger droplets, for example, get deposited in the respiratory tract and smaller droplets emerge from lower in the respiratory tract. Droplets from the larynx and lower respiratory tract are between around 0.1 and 10 µm. Those from the oral cavity are between around 10 to 1000 µm. The larger ones are most likely to contain the majority of the viral microbes, but transmission mainly occurs with the smaller droplets.
The team concluded, “For the moderately large viral load of 108 ml−1 the majority of viral aerosols extend into the micron regime greater than, or equivalent to, >1 µm. Only for extremely large viral loads of 1010 ml−1 do the submicron droplets begin to contain significant numbers of virus.”
Can masks prevent community transmission of the infection?
This study shows that masks remove the majority of viral aerosols except for the submicron size particles. These masks are also “more effective with decreasing viral loads”.
Thus, masks are highly effective at reducing transmission rates from the vast majority of people infected with SARS-CoV-2, the team writes.
Conclusions and implications
The team explains that both masks made from simple cotton fabrics and surgical masks can reduce the risk of transmission of respiratory viruses. Cloth masks provide the advantage of being washable and reusable. They are, therefore, preferable to disposable surgical masks because they generate less plastic waste. However, the fit of cloth masks is generally poorer and more studies are needed to see if they can prevent the transmission of all respiratory viruses, the researchers wrote. “Transmission of respiratory viruses is complex and poorly understood,” they explained.
Some COVID-19 patients have viral loads thousands or millions of times higher than others. Thus, if there is a 50% reduction in viral load dose due to mask-wearing, the risk of infection in the non-infected person may vary. They write, “As typically the viral load of an infectious person will not be known, other forms of interventions may be warranted in addition to masking.”
They concluded that the finest aerosols (< 1 micron) are unlikely to contain the virus and they suggest that masks are “effective at reducing the risk of airborne transmission in most settings.”
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.