Facial coatings made of layered cotton fabric are likely to decrease the spread of COVID-19 better than synthetics

The researchers have completed a new study on how well a variety of natural and synthetic tissues filter particles similar in size to the virus that causes COVID-19. Of the 32 fabric materials tested, three of the five most effective at blocking particles were 100% cotton and had a visible high fiber or nap, such as that found on flannels. Four of the five lowest performing were synthetic materials. Testing also showed that multiple layers of fabric could further improve the effectiveness of cotton. None of the materials came close to the efficiency of the N95 masks.

Although the sample size was relatively small, the researchers noted that tighter knit fabrics generally filtered better than knitted and loose fabrics. 100% cotton fabrics with many embossed fibers also seemed to filter better than cotton fabrics that lacked this feature. The raised fibers often form network-like structures similar to those of medical grade masks.

Three researchers from the National Institute of Standards and Technology (NIST), Christopher Zangmeister, James Radney and Jamie Weaver, teamed up with Edward Vicenzi of the Smithsonian Institution Museum Conservation Institute to evaluate materials and determine both their ability to filter out particles and their breathability. Their results appear in the magazine. ACS Nano.

The US Centers for Disease Control and Prevention (CDC) recommends that people wear cloth face covers in public places where social estrangement is difficult, primarily to prevent a person who does not know that is infected spread the virus.

The virus that causes COVID-19 is transmitted primarily through respiratory droplets that are expelled when a person sneezes, coughs, or even speaks. However, some research also suggests that the virus can be spread through much smaller aerosols, smaller than 1/100 the width of a human hair, which are also expelled and can remain in the air much longer than drops. .

“It turns out that the materials available on the market provide some protection against aerosols if you use multiple layers of fabric and a face covering fits perfectly,” said Zangmeister. “But none is as good as an N95 mask.”

The project measured a common way of determining how well a material captures particles, called filtration efficiency. Zangmeister and Radney, experts in aerosol measurement, organized a relatively simple experiment that relied on extremely sensitive equipment to size and count aerosol particles.

The experiments used fabric swatches, or swatches, instead of full masks. “Basically, we take a sample of material and flow a stream of particles of a known size,” said Zangmeister. “We count the amount of particles in the air before and after it has passed through the fabric. That tells us how effective the material is at capturing particles.”

Instead of actual (and dangerous) samples of the SARS-CoV-2 virus, the team used table salt or sodium chloride (NaCl), the recommended substitute for virus particles by the National Institute for Occupational Safety and Health (NIOSH). from the CDC), which sets test standards for N95 and other skins. The air flow rates used in the experiments were also from the NIOSH test recommendations.

The researchers tested each material against particles ranging from 50 to 825 nanometers (nm) to plot their relative performance.

Meanwhile, Weaver, a textile chemist with experience in textiles, and Vicenzi, an expert in microscopy, studied each piece of fabric to determine its count, tissue and yarn mass in the hope of establishing a relationship between these characteristics and the capacity of the cloth to filter particles.

The SARS-CoV-2 virus particles are approximately 110 nm in diameter. N95 masks are rigorously tested to ensure they block at least 95% of the particles in this size range. A HEPA (High Efficiency Particulate Air) filter like the ones you can find in an air purifier blocks 99.97% of the particles that are about 300nm in size, and an even higher percentage of smaller particles. Of the fabrics tested in the NIST study, the best performing single fabric layer blocked 20% of the particles in the virus size range.

While Zangmeister and Radney conducted the aerosol experiments on the NIST campus in Gaithersburg, Maryland, Weaver and Vicenzi were able to carry out their imaging work in the home where they have been working since mid-March.

“We intentionally use digital microscopes and free low-cost software to do our part of the research from home,” Weaver said. “One of the motivations for this was to develop imaging methods that would allow citizen scientists to better study fabrics for relatively low startup costs.”

In addition to the fabrics, the team examined materials including a HEPA filter, an N95 mask, a surgical mask, and even coffee filters, which have been suggested for use in home facial coatings, for comparison. The team also tested combinations of fabrics (one layer of cotton and one synthetic), which did not show greater effectiveness.

By combining images and aerosol measurements, the team found that some fabrics that filter most of the particles are also the most difficult to breathe, and some don’t even meet health and safety recommendations for breathability.

“Texture turned out to be one of the most useful parameters to look at because we found that most high thread count cotton fabrics tend to filter better,” Weaver said. “Our findings suggest that a fabric’s ability to filter particles is based on a complex interaction between material type, fiber and fabric structures, and thread count.”

This research builds on fabric and filtration knowledge dating back to the 1918 influenza pandemic that killed approximately 20 to 50 million people worldwide and sparked the first investigation into cloth masks and their potential to protect against virus. It also supports further research suggesting that cloth filters would not be suitable for healthcare settings.

But despite decades of research on the subject, the team found that the lack of standard testing methods and the wide range of materials tested made it difficult to directly compare the results of previously published studies. They hope that their work provides a method for quickly evaluating materials.

“We didn’t know the answer when we started this project,” said Zangmeister. “But the bottom line is that none of these fabrics is as good as an N95 mask. However, fabric face liners can help curb the spread of the coronavirus. We hope this research will help manufacturers and fans determine the better fabrics for the job and serve as the basis for further research. “

The team plans to start another round of testing on a new set of materials in the near future. Weaver and Vicenzi have updated their imaging hardware and plan to employ more sophisticated texture analysis for the next round of fabrics.