While some are still arguing about wearing masks a year after the pandemic, scientists have begun to determine exactly which strategy is best – and cotton facial masks have just received another tick.
Various studies have tested different combinations of materials and health authorities, such as the World Health Organization and the CDC, recommend cloth masks to the general public, based on their findings. But some of these studies have overlooked an important factor in the real world – these face-covering fabrics become damp from our breathing.
Now, a team of researchers has tested the mask materials in conditions of high humidity, which mimics the air expelled from our mouths.
“This new study shows that cotton fabrics actually perform better in masks than we thought,” said material scientist Christopher Zangmeister of the US 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, at 99% humidity (how wet our breath is) and 55% humidity.
This has led to a remarkably visible difference in cotton performance.
Cotton masks become better filters when they absorb moisture from the breath, making them even better at slowing down COVID-19 than I 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 grown their ability to capture particles by 33%.
The researchers used salt particles of various sizes as a test substitute for the particles of droplets and aerosols that carry the virus, and they appeared to absorb some of the moisture trapped by the cotton fibers that attract water. The particles swell in volume, making it harder for them to pass through the uninhibited fabric.
Synthetic fibers, however, repel water, so it does not create the moist environment inside the mask for this inhibition to happen. There have also been no changes to medical masks – but they are designed to work at high levels in all conditions (levels equivalent to cotton).
The most efficient type of cotton was cotton flannel, according to the results.
Microscopic images of the materials reveal a special difference in structure – an orderly pattern of synthetic polyester fabric compared to the chaotic network of cross-fibers that gives the flannel a soft touch.
NIST researchers believe that this mess of fibers increases the chance that airborne particles passing through the mask will collide and stick to the fabric.
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 should be replaced. The amount of liquid present in the masks in these wet conditions amounts to only a few drops, which does not change the breathability of the material – the team found that the air pressure on both sides of the fabric remained relatively similar.
This is also good news from an environmental perspective. With the installation of waste from disposable surgical masks that spill microplastics, it is comforting to know that there is a safe and reusable option.
Research suggests that having a pile of machine washable reusable masks 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, moisture and aerosol particle transmission, their study contributed to the first international standards for fabric masks designed to slow the spread of COVID-19, recently launched by standards – ASTM International development organization.
“To understand how these materials work in the real world, we need to study them in realistic conditions,” Zangmeister concluded.
This research was published in Nano materials applied ACS.