SNS Science News Release, May 2020
Fabrics Charge Up Filtering Efficiency of Cotton Used in Face Masks
Multiple layers of specific combinations of every-day fabrics were found to provide greater filtration efficiency than N95 respirators when it came to the smallest aerosolized particles emitted during speech. The new study indicates that, in the right combinational layers, cloth face masks help protect the wearer--not just protect others from the wearer. Scientists with the University of Chicago and Argonne National Laboratory in Lemont, Illinois, authored the study published April 24, 2020, which is made available openly to support Covid-19 mitigation efforts.
Researchers and contact tracers note viral transmission is heightened when individual carriers are in close proximity indoors for tens-of-minutes to hours, such as when people congregate at parties and shop. Sensitive laser lights have been used by other scientists to observe thousands of invisible oral fluid droplets emitted per second during speech. Particles smaller than 300 nanometers become aerosolized, or temporarily airborne, when emitted during speech. (continued below)
Worse performing filtration efficiency, researchers noted, were "fabrics that are porous, which should be avoided". This would include thin or single-layered coverings of loosely woven cotton materials. Even quilters cotton, marked 80 threads per inch (TPI), provided less than 50% filtration when double layered.
The enhanced performance of the hybrid sample comprising one layers of high density weave cotton at 600 TPI and two layers of chiffon is likely due to the combined effect of mechanical and electrostatic-based filtration, according to the researchers. The corresponding author for the study is Dr. Supratik Guha at University of Chicago's Pritzker School of Molecular Engineering.
Another option the authors considered good at filtering airborne particles (by 96%) was cotton quilt defined as two layers of 120 TPI cotton with a filling that was about 1/2 cm thick made up of 90% cotton, 5% polyester, and 5% other fibers.
The researchers summarized that cloth masks can potentially provide significant protection against the transmission of particles in the aerosol-size range. Other combinations they found to provide good filter efficiency (FE) one layer of 600 TPI cotton combined with 1 layer of flannel comprised of 35% polyester and 65% cotton--that layering provided 95% filtration efficiency. One layer of 600 TPI cotton with 2 layers of silk provided 94% filtration efficiency.
The best performers have in common the addition of some polyester. "Polyester woven fabrics can retain more static charge compared to natural fibers or cotton due to their lower water adsorption properties," according to the study. Thus, instead of working with 100% cotton flannel, they chose flannel that included 35% polyester.
To test the 19 fabric-combination samples, the researchers used an aerosol generator that is widely used for testing face respirators in compliance with NIOSH.
There has been limited data on the performance of various commonly available fabrics used in making cloth masks. The issue is increasingly important as the CDC, starting April 3, recommends everyone wear face masks in public to help mitigate the pandemic. Individuals who lack symptoms, while actively infected, can transmit the virus to others, particularly when indoors, at close proximity, for tens of minutes, to half hour, and longer.
The research findings were published by the American Chemical Society (ACS) in ACSNANO April 24, 2020 and are made available via the ACS COVID-19 subset for unrestricted research re-use and analysis in any form during the pandemic.
Further cloth mask research should focus on non-gap fitting, the role of repeated washing, and humidity arising from exhalation, according to the researchers.
The Study's Supporting Information is available free of charge at:
Further technical specifications on fabrics used in the study can be found in the study's Table S2, or the last page (7) at:
To read more about this research, go to:
"The researchers limited their measurements to droplet
particles sized from 1/100th-of-a-micron wide to 1
0-microns wide--a size range particularly relevant for
respiratory virus transmission. For scale,
one thousand microns equals one millimeter,
or about half the side of a nickel.
PARTICLES GO AIRBORNE AT 300 NM
FOR 12 MINUTES AND MORE
"The researchers limited their study measurements to droplets sized from 10 nanometers wide to 10,000 nanometers wide, which is the respiratory virus transmission range.
Droplets go temporarily airborne when particles are sized from 10 nm to 300 nm wide in diameter.
To help relate, one millimeter is about half the side of a nickel--you can find that measure on a ruler in your drawer!
Still, 1 nanometer is 1/1 millionth of a millimeter--now that's floaty! Time to reach for a face mask.
The researchers found that using two layers of 600-thread-count cotton combined with two layers of chiffon provided filtration efficiency of 97% for airborne particles less than 300 nanometers (nm) wide and 99.2% for bigger particles greater than 300 nm wide. In comparison, N95 respirators are engineered to filter particles larger than 300 nm in diameter, which are mostly non aerosolized--by a rate of 95% or more, thus the name.
The researchers limited their measurements to droplet particles sized from 10 nm wide to 10,000 nm wide, which is the respiratory virus transmission range.
To achieve the best filtration efficiency found in the study, mask makers would use two layers of chiffon (made from 90% polyester and 10% spandex) covered by at least one layer of 600-thread-count cotton or higher (commonly used in fine sheets and pillowcases). Critically, all masks, regardless of makeup, must have no gaps around them while worn--just one gap in any mask diminishes efficiency by as much as 50%. See video on tips to custom fit a mask, and make darts that remove gaps in a cloth mask [to come].