Numerous articles of used clothing, old blankets, and outgrown bed sheets that had been collecting dust in 2019 have found new life in 2020 as DIY homemade face masks. But, just how effective are homemade masks at mitigating the spread of COVID-19?
More than one recent study has concluded that DIY masks constructed using extra household materials are indeed capable of getting the job done. However, the vast majority of those earlier studies only focused on the transfer and blockage of tiny aerosol particles produced when an individual breathes normally. Whenever a person speaks, coughs, or sneezes, larger (potentially virus-carrying) particles are usually released.
With this oversight in mind, a team of researchers at the University of Illinois at Urbana-Champaign set out to conduct a more comprehensive assessment of the droplet-stopping abilities among various household materials typically used to create homemade face masks.
In short, the study produced reassuring results. In all, 11 different common household fabrics were tested by the research team, and every single material showed the ability to effectively stop both small aerosol particles and larger droplets.
Tested materials included new and used clothes, bedsheets, kitchen towels, and quilted cloths. Although, that being said, the research team also recommends using at least two to three layers when it comes to particularly permeable pieces of fabric, like a t-shirt.
“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,” comments study leader Taher Saif, a mechanical science and engineering professor at UI, in a university release. “With two or three layers, even the more permeable fabrics, such as T-shirt cloth, achieve droplet-blocking efficiency that is similar to that of a medical mask, while still maintaining comparable or better breathability.”
A small aerosol particle is usually less than five micrometers in size (hundreds of nanometers). Meanwhile, larger droplets that are often expelled when someone coughs, speaks, or sneezes can reach a full millimeter in diameter. Obviously, these larger droplets represent a bigger viral threat, and researchers speculated they may be capable of seeping through flimsier face masks, breaking up into smaller aerosols, and eventually making their way into nearby air.
Besides just droplet-blocking capabilities, breathability is another important aspect of any good face mask. No one wants to wear an uncomfortable mask all day.
“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 contours of a face, defeating the purpose of the mask and providing a false sense of protection,” Saif explains. “Our goal is to show that many common fabrics exploit the trade-off between breathability and efficiency of blocking droplets – large and small.”
Using a standard medical face mask as a reference point, the research team analyzed both the droplet-stopping effectiveness and breathability of face masks constructed using 11 household materials. More specifically, each fabric’s weight, thread count, fiber content, construction, water-absorption rate, and porosity was assessed.
“Testing the breathability of these fabrics was the easy part,” Saif says. “We simply measured the rate of airflow through the fabric. Testing the droplet-blocking ability is a bit more complicated.”
To test droplet-blocking capabilities, the nozzle of an inhaler was filled up with some water containing very easy-to-see fluorescent colored particles measuring exactly 100 nanometers across (coronavirus particles are roughly this size). Each time the inhaler was puffed it sprayed out some high-velocity droplets (containing the pseudo-coronavirus particles) onto a nearby, strategically placed plate.
So, all the research team had to do to test each fabric’s effectiveness was place each DIY mask, one at a time, in front of the plate before puffing the inhaler.
“We 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,” Saif adds.
In addition to all that, a high-speed video was used to measure velocity and establish the size of particles being expelled from the inhaler.
Regarding droplet speed, droplets left the inhaler at a rate of roughly 17 meters per second, which falls within average cough/sneeze velocity rates (10-40 meters per second). Similarly, each full droplet was roughly 0.1 to 1 millimeter in diameter, matching the average size of droplets released when sneezing or coughing.
All of that led the research team to conclude each one of the 11 tested fabrics is indeed capable of blocking coronavirus particles within high-velocity, larger droplets. Moreover, all of these DIY options provide just as much, if not more, breathability than medical masks.
“Our experimental platform offers a way to test fabrics for their blocking efficiency against the small – and now – larger droplets that are released by human respiratory events,” Saif concludes.
The full study can be found here, published in Extreme Mechanics Letters.