Do material face masks work?
How effective are household materials in preventing the spread of COVID-19?
The topic of wearing masks has become the subject of some debates during the global SARS-CoV-2 pandemic. Opponents of wearing masks often cite the lack of high quality evidence specifically investigating the impact of mask wearing on the spread of COVID-19. However, conducting such experimental studies related to COVID-19 is an ethical challenge as participants are likely to face a higher risk of contracting the virus. Instead, research is initially focused on showing how and why masks (including cloth face masks) work to prevent the spread of the virus that causes COVID-19, rather than showing through experimentation that it does.
A new study published in Extreme Mechanics Letters attempts to do just that (1). The study seeks to examine the effectiveness of various tissues in blocking large droplets at high speeds. It is believed that COVID-19 is mainly spread by large droplets (greater than 10 µm in diameter) as opposed to aerosol droplets, which are typically in the 10 nm to 5 to 10 µm range. Aerosol droplets can be dispersed over greater distances in the air, while larger droplets tend to settle out of the air within short distances. Therefore, a tissue mask that can effectively stop the transmission of large droplets should help dramatically reduce the ability of infected people to spread the coronavirus.
However, droplet blocking efficiency is not the only consideration in determining the optimal face mask fabric. Breathability is also a major concern, and not just from a compliance perspective. A mask with poor breathability will result in greater "leakage" as air will flow around the mask rather than through the fabric if the resistance is too great. As a result, in this study, the researchers considered the suitability of a mask fabric as a compromise between breathability and droplet blocking efficiency.
To test the efficiency of the droplet block, they performed a mechanistic experiment. A standard metered dose inhaler was used to simulate the large droplet size expelled when speaking, sneezing and / or coughing. The inhaler was loaded with a suspension of fluorescent beads that were approximately the same size as the SARS-CoV-2 virus (70-100 nm in diameter). In this way, each dose of the inhaler mimicked the virus being emitted by infectious individuals, with the beads suspended in large droplets.
A petri dish was placed opposite the inhaler to "catch" the ejected droplets and the fluorescent beads could then be observed in this dish. The various substances were placed between the inhaler and the Petri dish to test their blocking ability. The distance between the inhaler and the fabric was adjusted so that the speed droplets hitting the fabric could be varied to mimic both the high speeds observed when coughing / sneezing and the slower speeds observed when speaking.
Breathability, on the other hand, was assessed using an instrument called a plug flow tube. The various swatches of fabric were placed over one end of the tube and compressed air was then forced through the tube. This created a pressure differential with the air inside the pipe at a controlled pressure and the air outside the pipe at normal atmospheric pressure. Breathability was then defined as the rate at which air flow through the fabric changed with changes in the pressure differential.
Do cloth masks work effectively to block droplets?
The researchers tested a number of different household items, including bedsheets, T-shirts, tea towels, and shirts. They also tested the effects of laminates and examined medical masks. In general, most fabrics performed well in blocking droplets. The median values ranged from 71.7% of the droplets blocked by new quilting fabric to 98.7% for a used woven shirt. This increased even further to 98.9% when three layers of fabric were applied. Using two layers of even a permeable fabric like a t-shirt fabric blocked more than 94% of the droplets.
It has been found that breathability depends on the porosity of the fabric used. The more porous the fabric, the more breathable it became. It was also found that porosity has an inverse relationship to blocking efficiency, confirming the trade-off between breathability and blocking efficiency.
Overall, this study clearly highlighted the mechanisms by which fabric references can help prevent the spread of COVID-19. The tests on various fabrics suggest that any number of household substances can cause high levels of droplet blockage. Choosing less porous fabrics and adding a second layer of fabric to the masks are two methods of increasing the functionality of the fabric masks to block droplets.
However, the authors point out caution. This study focuses solely on the transfer of larger droplets. While this is still considered the primary route of transmission for SARS-CoV-2, there is some evidence to suggest that transmission through smaller aerosolized droplets may occur under certain circumstances. Previous mechanistic studies suggest that tissue masks may also have some effectiveness in blocking these smaller particles. However, more research is needed to investigate how often aerosol droplets are involved in COVID-19 transmission and how effective tissue masks are in preventing aerosol transmission.
Written by Michael McCarthy
1. Aydin O., Emon B., Cheng S., Hong L., Chamorro LP, Saif MTA. Performance of fabrics for homemade masks against the spread of COVID-19 through droplets: A quantitative mechanistic study. Extreme Mechanics Letters. 2020; 40: 100924.
Image by Innviertlerin from Pixabay
Comments are closed.