A new study demonstrates that the fabrics typically used in homemade face masks effectively block droplets that could contain SARS-CoV-2.
New research has found that the typical materials in homemade facemasks effectively block droplets that may carry SARS-CoV-2.
The research, which appears in the journal Extreme Mechanics Letters, reinforces current studies on the efficacy of homemade face masks for blocking aerosols that could transmit SARS-CoV-2.
Masks
Until scientists produce an effective vaccine, researchers recommend that various behavioral changes are key to reducing the spread of SARS-CoV-2.
As well as maintaining physical distance from others and regularly washing hands, the Centers for Disease Control and Prevention (CDC) and World Health Organization (WHO) recommend people wear masks.
Mask-wearing is important as SARS-CoV-2 is spread by people projecting saliva and other respiratory liquids when they speak, cough, or sneeze.
This projected liquid potentially carries the virus, which another person can pick up if they touch a surface that it lands on, or if people directly inhale the liquid.
This liquid can expel as either droplets or an aerosol, and scientists believe that both can transmit the virus.
Aerosols are tiny particles that can hang in the air, potentially traveling many meters, whereas droplets are larger and fall to the floor more rapidly.
Experts say people can reduce this type of transmission by wearing masks. However, in justifying their work, the authors argue that while there has been much research into the effect of masks on reducing aerosol transmission, there are fewer studies that look at droplet transmission.
In the present study, the researchers wanted to focus on the role of homemade face masks in blocking droplet transmission. This is important because demand for medical face masks regularly outstrips supply, potentially preventing medical professionals and caregivers from accessing this vital kit.
As a consequence, both the CDC and WHO provide advice on making face masks at home.
People can create masks with various materials, so understanding how effective they are — and how many layers are necessary for them to be effective — is important.
11 material types
To help answer their questions, the research team examined 11 typical materials of homemade face masks, with a medical mask to act as a control. The fabrics included used clothes, quilted cloths, dishcloths, and bedsheets.
While these materials need to block droplets, it is also crucial that they are breathable and therefore, comfortable.
Study lead Prof. M Taher A Saif from the University of Illinois at Urbana-Champaign explains that “[a] mask made out of a low-breathability fabric is not only uncomfortable, but can also result in leakage as the exhaled air is forced out around the contours of a face, defeating its purpose and providing a false sense of protection.
“Our goal is to show that many common fabrics exploit the trade-off between breathability and efficiency of blocking droplets — large and small.”
“Testing the breathability of these fabrics was the easy part. We simply measured the rate of airflow through the fabric. Testing the droplet-blocking ability is a bit more complicated.”
To investigate, the researchers used an inhaler to project water as droplets through the fabrics. The liquid contained 100-nanometer diameter fluorescent particles, around the same size as SARS-CoV-2.
According to Prof. Saif, “[w]e count the number of nanoparticles landing on the dish using a high-resolution confocal microscope. We can then use the ratio of the number collected with and without the fabric to give us a measure of droplet-blocking efficiency.”
Testing fabrics
The study results may help guide people making their masks at home. As Prof. Said explains:
“We found that all of the fabrics tested are considerably effective at blocking the 100-nanometer particles carried by high-velocity droplets similar to those that may be released by speaking, coughing, and sneezing, even as a single layer.
“With two or three layers, even the more permeable fabrics, such as T-shirt cloth, achieve droplet-blocking efficiency similar to that of a medical mask, while still maintaining comparable or better breathability.”
Prof. Said and his co-authors also note homemade masks have the added benefit of easing the environmental burden that medical masks generate. These masks are typically made with plastic, meaning people cannot easily clean them, and therefore they need to be thrown away after use.
By contrast, homemade masks typically consist of materials that can absorb water, which means a person can wash and decontaminate them.
Limitations to consider
While these results could help people choose a fabric to make a mask, there are limitations when transferring the information into real-world situations.
For instance, the mechanical setup of an inhaler propelling droplets does not account for how different fabrics fit on the face. If a mask does not fit snuggly, it will be considerably less effective, regardless of its material.
More research is needed to help evaluate the impact of real-world use of homemade face masks. However, until then, they remain essential to reduce the spread of COVID-19.
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