Starting May 1, Illinois will require everyone over age 2 to wear a mask when they can’t maintain a 6-foot social distance in public. N-95 masks, which are in short supply, are best reserved for health care workers, who come into direct contact with COVID-19 patients. So what fabric or combination of fabrics is best for homemade masks?
A new study conducted by University of Chicago professor Supratik Guha and colleagues at Argonne National Laboratory in Lemont looked at more than 15 common household fabrics to see which were best for protecting against the coronavirus that causes COVID-19.
More specifically, the study investigated the fabric’s filtration efficiencies against the tiny droplets that are how COVID-19 and other respiratory illnesses are spread. Wearing a mask or a cloth facial covering reduces the transmission of these respiratory droplets from an infected person, Guha says.
Guha and his team completed the study over two weeks after he started seeing news reports that the Center for Disease Control and Prevention and different states had begun recommending the use of cloth masks.
“I saw that this was coming,” he says. “At that point, we also realized that there was very little scientific data on the performance of (cloth) masks. … We went through seven or eight days of intense experiments and analysis that went on through the night; the data would be taken during the day. I think we felt that we wanted to get this data out in public quickly. That was the key goal, and that’s what we did.”
His team also tested an N-95 respirator and surgical masks for comparison. They also looked into the effectiveness of multiple layers of a single fabric and a mixture of multiple fabrics.
They found the most effective fabrics to be cotton, natural silk and chiffon; synthetic silk and satin did not provide as much protection. Hybrid combinations, such as high thread cotton, along with silk, chiffon or flannel also supplied broad filtration coverage.
The study notes, “Fabric with tight weaves and low porosity, such as those found in cotton sheets with high thread count, are preferable. For instance, a 600 TPI (thread per inch) cotton performed better than an 80 TPI cotton. Fabrics that are porous should be avoided.”
Despite silk and chiffon’s sheerness, the two fabrics were amazingly effective, particularly four layers of silk, such as a folded scarf.
Guha says chiffon and other materials that have electrostatic properties can actually act as a barrier to the tiny droplets.
”What we found was that some of these materials are pretty good,” he said. ”Using a combination of cotton and these materials is the best. A quilt, a mixture of polyester and cotton also had excellent filtration.”
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Two chambers at Argonne were used to conduct the study. In the first chamber, Guha and his team generated aerosols with dry particles of sodium chloride, a standard method in respirator testing. From there, a PVC pipe led to the collection chamber, which is where the fabric was held in place by clamps. The collection chamber had a fan that sucked the air, so it flowed from the generation chamber to the collection chamber. Guha said they used specialized equipment that measured the density of the particles upstream and downstream of the fabric.
“What was unique in our work was the equipment used to measure particles of 10 nanometers, which is about a few thousand atoms,” he says. “We were able to measure the filtration efficiency at different particle sizes, going all the way from a few thousand atoms to 6 micrometers range. A human hair is roughly about 75 micrometers in diameter, so 6 is a little less than one-tenth of that.”
Something that surprised Guha during this study was the effect of gaps in masks.
“Our studies also imply that gaps (as caused by an improper fit of the mask) can result in over a 60% decrease in the filtration efficiency, implying the need for future cloth mask design studies to take into account issues of ‘fit’ and leakage, while allowing the exhaled air to vent efficiently,” the study says.
He says that if a mask doesn’t fit properly, it’s not much use. Masks should fit with minimal gaps, but exhaled breath must be able to come out or else the user will breathe in carbon dioxide.