While some still complain about wearing masks a year after the pandemic, scientists have continued to work on exactly which strategy is best, and cotton face masks have just received another approval.
Several studies have tested different combinations of materials and health authorities such as the World Health Organization and the CDC recommend cloth masks for the general public, based on their findings. But some of these studies missed an important real-world factor: These face-covering fabrics end up damp from our breath.
Now, a team of researchers has tested mask materials in high-humidity conditions that mimic the air expelled from the mouth.
“This new study shows that cotton fabrics actually perform better in masks than we think,” said materials scientist Christopher Zangmeister of the US National Institute of Standards and Technology (NIST).
Zangmeister and his colleagues tested nine different types of cotton and six types of synthetic fibers, including polyester and rayon, with 99 percent humidity (roughly how wet our breath is) and 55 percent humidity.
This resulted in a remarkably visible difference in cotton yield.
Cotton masks become better filters when they absorb moisture from your breath, making them even better at slowing down COVID-19 than we think: https://t.co/qbGtnC4a42 pic.twitter.com/91hNVIvlTX
– National Institute of Standards and Technology (@NIST) March 9, 2021
While synthetic fabrics, which also performed poorly compared to dry cotton, did not change their performance in wet conditions, increased its ability to capture particles by 33 percent.
The researchers used salt particles of various sizes as a test substitute for aerosol particles and droplets that carry viruses, and these appeared to absorb some of the moisture trapped by the water-attracting cotton fibers. The particles increase in volume, making it difficult to pass through the fabric without inhibitions.
However, synthetic fibers repel water, so they do not create the moist environment within the mask for this inhibition to occur. There were no changes to medical masks either, but they are designed to perform at high levels in all conditions (levels equivalent to cotton).
The best performing cotton type was cotton flannel, according to the results.
Microscopic images of the materials reveal a marked difference in structure: a neat weave pattern in synthetic polyester compared to the chaotic web of criss-cross fibers that give flannel its soft touch.
NIST researchers believe that this mess of fibers is what increases the possibility that airborne particles that pass through the mask will collide and adhere to the fabric.
However, all this does not mean that wet masks are better: if your mask gets wet, you should replace it. The amount of liquid present in the masks in these humid conditions amounts to only a few drops, which does not alter the breathability of the material; The team found that the air pressure on both sides of the fabric remained relatively similar.
This is great news from an environmental perspective too. With waste build-up from disposable surgical masks that remove microplastics, it’s comforting to know that there is a safe and reusable option.
Research suggests that owning a bunch of reusable masks that can be machine washed together is the greenest option for keeping you and your loved ones safe.
While the team says more research is required to fully appreciate the interactions between masks, moisture, and the transmission of aerosol particles, their study has contributed to the first international standards for fabric masks intended to slow the spread of COVID- 19, recently published by the standards-developing organization ASTM International.
“To understand how these materials work in the real world, we must study them under realistic conditions,” concluded Zangmeister.
This research was published in Nano materials applied by ACS.