While some are still arguing about wearing masks a year into the pandemic, scientists have begun to work out exactly which strategy is best – and cotton face masks have just gotten another tick.
Several studies have tested different material combinations and health authorities such as the World Health Organization and the CDC recommend cloth masks to the general public, based on their conclusions. But some of these studies have overlooked an important factor in the real world – this face-covering material ends up damp out of our breath.
Now a team of researchers has tested mask material under high humidity conditions that mimics the air expelled from our mouths.
“This new study shows that cotton fabric actually performs better in masks than we thought,” said materials scientist Christopher Zangmeister of the U.S. National Institute of Standards and Technology (NIST).
Zangmeister and colleagues tested nine different types of cotton and six types of synthetic fibers, including polyester and rayon, in 99 percent humidity (how moist our breath is) and 55 percent humidity.
This resulted in a noticeable visible difference in the performance of cotton.
Cotton masks become better filters as they absorb the moisture from your breath, making it even better to delay COVID-19 than we thought: https://t.co/qbGtnC4a42 pic.twitter.com/91hNVIvlTX
– National Institute of Standards and Technology (@NIST) March 9, 2021
While synthetic fabrics, which also perform poorly compared to dry cotton, did not change performance under humid conditions increased their ability to capture particles by 33 percent.
The researchers used salt particles of different sizes as a test substitute for transporting droplets and aerosol particles by virus, and it apparently absorbed some of the moisture trapped by the water-absorbent cotton fibers. The particles swell in volume, making it more difficult to move uninhibitedly through the dust.
However, synthetic fibers repel water and therefore do not create the moist environment inside the mask to allow this inhibition to take place. There was also no change in medical masks – but it was designed to work at all levels at high levels (equivalent to cotton).
According to the results, the cotton was the best performing cotton flannel.
Microscopic images of the materials show a clear difference in structure – an orderly weave pattern in synthetic polyester compared to the chaotic network of cross fibers that gives flannel its soft feel.
NIST researchers believe that this fiber through the fibers increases the chance that air particles moving through the mask will collide and stick to the dust.
Cotton flannel (left), polyester (right). (EP Vicenzi / Smithsonian Museum / NIST)
However, all this does not mean that wet masks are better: if your mask gets wet, it needs to be replaced. The amount of liquid present in the moist conditions in the masks is only a few drops, which does not change the respiration of the material. The team found that the air pressure on either side of the fabric was relatively the same.
This is also good news from an environmental perspective. With increasing waste of disposable surgical masks shedding microplastics, it is comforting to know that there is a safe, reusable option.
Research suggests that owning a pair of reusable masks that can be machine washed is the most environmentally friendly option to keep you and your loved ones safe.
While the team says more research is needed to fully appreciate the interactions between masks, humidity and aerosol particle transfer, their study contributed to the first international standards for dust mask intended to slow the spread of COVID-19, which recently released by the standards. development organization ASTM International.
“To understand how these materials perform in the real world, we need to study them under realistic conditions,” Zangmeister concluded.
This research was published in ACS Applied Nano Materials.