While there is scientific consensus that wearing masks helps limit the spread of COVID-19, there are unanswered questions about which masks are most effective.
Now, a group of scientists released a paper earlier this week which answers that question by considering an oft-overseen element in the mask equation: humidity. Exhaled human breath contains about 5% water vapor, meaning a large volume of water filters through one’s mask over time.
As scientists at the National Institute of Standards and Technology (NIST) and the Smithsonian’s Museum Conservation Institute explained in a paper published for the journal ACS Applied Nano Materials, masks made out of cotton fabrics appear to work better in humid air. In their research, cotton fabric masks increased their filtration efficiency by 33% when exposed to humid conditions.
On the other hand, masks composed of synthetic fabrics generally did not perform as well as their cotton counterparts — their effectiveness remained the same under humidity. Medical masks also did not become more effective filters under humidity, although they did perform in approximately the same range as cotton masks.
Salon reached out to NIST’s Christopher D. Zangmeister, who worked on the paper, to learn more about why cotton becomes more effective at filtering out potential COVID-19 particles under humidity.
“We had a prior paper that came out and found the most effective masks that we saw just in general were made out of cotton materials,” Zangmeister explained, referring to a NIST study published in June. “They performed better than things that are synthetic like polyester, nylon, or combinations of synthetics. Cotton materials worked the best.”
That said, the original paper measured different mask materials under dry conditions, which they realized meant the findings did not take into account how human breath contains a lot of water vapor. As such, they decided to test the same materials under humid conditions.
“It turns out that the cotton masks that worked really well in our first study, even worked better when they’re exposed to high humidity, and the reason for that is that they absorb water and the other materials, the synthetic materials, do not,” Zangmeister explained. “They’re not designed to absorb water. That’s why swimsuits and things like that are made out of synthetics. They’re made to repel water. These cotton materials, they absorb water, and as a small particle goes through them, some of that water gets bound to the particle and gets stuck to the particle itself.”
To visualize how this works, imagine a small particle moving through any type of filter. If the filter forces a particle to bend, weave and go up and down, it is less likely that it will be able to penetrate that barrier. Cotton masks contain tiny fibers that create precisely that sort of “maze,” as Zangmeister put it, which already makes them effective as filters. Their absorbency makes them even more effective.
While Zangmeister said that cotton is the most effective fabric, he emphasized that this does not mean you should not wear other types of masks if that is what you have available. All of the major fabrics used to create masks ultimately protect yourself and other people from infection.
“Things like N95 masks, they may not have synthetics. You may have a polypropylene mask and they do not absorb water, but they still work very, very well. I would encourage everyone to wear a mask,” Zangmeister told Salon. “It’s important to get out that everyone should wear a mask.” As he pointed out, the whole reason why masks work is that they stop droplets and particles from entering the environment from the wearer because they get stuck in the mask.
“From that perspective, all masks worked very, very well for that,” Zangmeister told Salon.
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