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While some still argue over wearing masks a year after the start of the pandemic, scientists have set out to work out exactly the best strategy – and cotton masks have just received new approval.
Various studies have tested different combinations of materials, and health authorities such as the World Health Organization and CDC recommend sheet masks for the general public, based on their findings. But some of these studies overlooked an important real-world factor – those tissues covering the face end up getting wet from our breath.
Now, a team of researchers have tested mask materials under high humidity conditions that mimic the air expelled from our mouths.
“This new study shows that cotton fabrics work better in masks than we thought,” said Christopher Zangmeister, materials specialist, US National Institute of Standards and Technology (NIST).
Zangmeister and his colleagues tested nine different types of cotton and six types of man-made fibers, including polyester and rayon, in 99% humidity (about the humidity of our breath) and 55% humidity.
This resulted in a remarkably noticeable difference in the performance of cotton.
Cotton masks become better filters when they absorb moisture from your breath, making them even better at slowing 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 performed poorly compared to dry cotton, did not change performance in wet conditions, cotton fabrics increased their ability to capture particles by 33%.
The researchers used salt particles of different sizes as a test surrogate for droplets and aerosol particles carrying the virus, and these appeared to absorb some of the moisture trapped by the water-attracting cotton fibers. The particles swell in volume, making them more difficult to pass through the tissue without being inhibited.
Synthetic fibers, however, repel water, thus not creating the humid environment inside the mask itself for this inhibition to occur. There has also been no change in medical masks – but they are designed to perform at high levels under all conditions (cotton-equivalent levels).
The best performing type of cotton was cotton flannel, according to the results.
Microscopic images of the materials reveal a distinct difference in structure – an orderly weave pattern of synthetic polyester versus the chaotic network of crisscrossing fibers that give flannel its soft feel to the touch.
NIST researchers believe this mess of fibers is what increases the chances that airborne particles passing through the mask collide and adhere to the fabric.
Cotton flannel (left), polyester (right). (EP Vicenzi / Smithsonian Museum / NIST)
However, all of this doesn’t mean that wet masks are better: if your mask gets wet, it needs to be replaced. The amount of liquid present in the masks in these humid conditions is only a few drops, which does not affect the breathability of the material – the team found that the air pressure on both sides of the fabric remained relatively similar.
This is also great news from an environmental point of view. With the assembly of waste disposable surgical masks that remove microplastics, it is 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 most environmentally friendly option to protect you and your loved ones.
While the team says more research is needed to fully appreciate the interactions between masks, humidity, and aerosol particle transmission, their study contributed to the first international standards for sheet masks intended to slow down spread of COVID-19, recently released by standards. -organization in development ASTM International.
“To understand how these materials work in the real world, we need to study them under realistic conditions,” Zangmeister concluded.
This research was published in Nano Applied Materials ACS.
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