When the pandemic started, a team of NIST scientists began studying how well different cloth mask materials filter particles. But although their initial research showed that tightly woven cotton fabrics filter most efficiently, their work was conducted in a lab with dry air and using pre-pandemic standards for measuring mask filtration.
“Those methods don’t incorporate what I consider to be real-world conditions,” says Christopher Zangmeister, a research scientist at NIST who led the study. So he and his colleagues changed their approach. In their work, published in the journal ACS Applied Nano Materials, they made new measurements at 99 percent relative humidity. That’s similar to the humidity generated by a person’s breath.
The scientists tested 32 different kinds of cloth, humidifying two-layer swatches of each before putting them in front of a pipe that exhaled air that contained salt particles about the same size as those contained in human breath. Then they counted the number of particles that made it through.
When cotton was humidified, it filtered significantly more particles than when it was dry. The filtration efficiency of woven, 100 percent cotton flannels increased from 12 percent to 45 percent during the tests.
“When particles go into the humid environment of the mask, they swell up and become much easier to filter,” Zangmeister explains. “It was quite surprising.”
The team didn’t see the same effect for synthetic fabrics, even cotton-poly blends. Since synthetic fabrics repel water, they don’t absorb breath as well as cottons, and their filtration didn’t rise under humid conditions. N95s and medical procedure masks performed similarly under humid and dry conditions.
Electron microscope imagery of the different cloths revealed another reason cotton filters better than synthetics. While polyester materials are woven in consistent patterns, cotton flannel’s fibers look chaotic beneath a microscope. The researchers think that makes them better filters because particles passing through have a better chance of colliding with, and sticking to, one of the disorganized fibers than with those with a predictable pattern.
The images show the strange beauty of the fabrics that help protect people from the coronavirus, which causes the disease covid-19.
Materials aren’t the only thing to consider when choosing a mask, researchers warn. If the mask doesn’t fit tightly, it won’t be as effective.
Zangmeister says the work has helped him appreciate the humble cotton mask as a tool to help stop the pandemic. “When you exhale into a simple cotton mask, one that’s two layers, it works at least as well as a single-layer surgical mask,” he says. “It may even be better.”