Best fabrics for homemade masks are cotton, natural silk and chiffon, study says
CHICAGO — In many parts of the country, everyone over age 2 will be required 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 in protecting against the coronavirus.
More specifically, this study investigated the fabric's filtration efficiencies against the tiny droplets that are how the coronavirus and other respiratory illnesses spread.
Wearing a mask or a cloth facial covering reduces the transmission of these respiratory droplets from an infected person, Guha said.
The most effective fabrics are 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 and 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.”
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.”
Tiny testing
Two chambers at Argonne were used to conduct the study. In the first chamber, Guha and his team produced 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 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. He says that if a mask doesn't fit properly, it's not much use. Masks should fit with minimal gaps, but not too tightly because the exhaled breath must come out or else you'll breathe in carbon dioxide.