Study of the COVID-19 mask finds layers, a matter of choice of material

Wearing a face mask can protect you and others from COVID-19, but the type of material and how many layers of tissue used can significantly affect the risk of exposure, a study from the Georgia Institute of Technology found.

The study measured the filtration efficiency of submicron particles that pass through a variety of different materials. For comparison, a human hair is about 50 microns in diameter, while 1 millimeter is 1,000 microns in size.

“A submicron particle can stay in the air for hours and days, depending on the ventilation, so if you have a room that is not ventilated or poorly ventilated, these small particles can stay there for a long period of time,” said Nga Lee (Sally ) Ng, associate professor and member of the Tanner faculty at the School of Chemical and Biomolecular Engineering and at the School of Earth and Atmospheric Sciences.

The study was carried out during the spring of 2020, when the pandemic triggered the global shutdown of most institutions. Communities faced a severe shortage of personal protective equipment, which led many people to make their own homemade masks. Georgia Tech quickly set up the study, since it already had “a great system for testing the efficiency of filtration using the instruments in the laboratory,” recalled Ng.

The study’s findings were used to shape the recommendations for homemade facial masks here last April, with the study’s comprehensive findings published on March 22 in the newspaper Aerosol science and technology.

In all, the researchers tested 33 different commercially accessible materials not limited to cloth fabrics, including single layer fabrics, such as cotton and polyester fabric, mixed fabrics, non-woven materials, cellulose-based materials, materials commonly found and used in hospitals, and various filter materials.

“We learned that there was a lot of variability in filtration performance, even on the same type of material,” said Ng.

“We found commercially available materials that provide acceptable levels of submicron particle rejection while maintaining airflow resistance similar to a surgical mask,” said Ryan Lively, associate professor and fellow at John H. Woody College at the School of Chemical Engineering and Biomolecular. “These materials combine the fiber density of the fabric, a labyrinth-like structure and the surface chemistry of the fiber to effectively reject submicron particles.”

The best performing materials for home masks were blackout curtains and sterilization wrap widely used to pack surgical instruments. Both materials are commercially available.

The researchers said that people should avoid using filters like HEPA / MERV or vacuum bags unless they are certified as fiberglass-free, since these filters themselves can often release glass fibers that can be inhaled. Other materials that should be avoided for masks include loose knit fabric, batting fabric, felt, wool or shiny, reusable shopping bags.

Multilayer samples performed much better than single layer samples, but people should pay attention to breathability. The two- and three-layer samples tested show an overall filtration efficiency of about 50% for submicron particles. The fit of the mask is also important, as particles can easily escape through the slits in the nose or the sides of the mask.

The analysis showed that multi-layered and well-fitting masks reject 84% of the particles expelled by a person when a person uses them. Two people wearing these types of masks reduce particle transmission by 96%.

A final conclusion of the research was the importance of the universal use of masks.

“The best way to protect ourselves and others is to reduce the particles exhaled at the source, and the source is our face,” said Ng, adding, “It really amplifies when everyone starts wearing masks.”

She expressed optimism that the findings will motivate people to more widely adopt the use of masks if they are sick and need to be in public.

“Not everyone understands the importance of transmitting the virus through the air and the importance of wearing a mask,” she said. “I hope the practice will continue to help reduce the release of these viral particles into the environment and help protect others.”