The day before the federal government issued new recommendations that Americans wear cloth face coverings to help slow the spread of the coronavirus, Yang Wang, a professor of environmental engineering who studies how fine particles like aerosols are transmitted, decided to test a few common household materials — pillowcases, scarves, furnace filters — “out of curiosity.”
His early results, which he shared on Twitter in April, attracted the interest of do-it-yourselfers, fellow engineers and scientists, and the news media, including The New York Times and the Today Show.
After seeing posts on Twitter about whether scarves would sufficiently block aerosols, Wang decided to test a variety of household materials — including scarves, bandannas, pillowcases and household air filters — to see how well they might prevent the spread of aerosols.
Wang and his Ph.D. student, Weixing Hao, used a scanning mobility particle sizer, which measures particle size and concentration, then compared the “filtration efficiency” of multiple layers of each material against aerosol particles ranging in size from a few nanometers to over 400 nanometers.
They found that layers of scarves and bandannas did a poor job of filtering out aerosols. Pillowcase fabric fared somewhat better, depending on thread count. The higher the thread count, the better the filter, the researchers determined.
But the best aerosol-blocking material of those Wang tested comes from commercially available household air filters. The multilayered air filters work almost as well as N95 medical masks to block aerosols, especially smaller particles, according to Wang’s initial findings.
As more layers of filter materials are stacked, however, a change in air flow through the materials, or “pressure drop,” becomes larger. This pressure drop increase can make it more difficult to breathe. Wang and his team are also looking for a combination of materials that produces the highest filtration efficiency, but the lowest pressure drop.
This off-the-cuff study was more than just a passing curiosity for Wang, however. He is one of several S&T faculty, students and staff members who came together to help local physicians and medical staff by 3-D printing masks and face shields.
To protect health care workers adequately, these reusable masks require a filter to block airborne particles that may spread the coronavirus or other diseases. That’s where Wang comes in.
Wang and Hao continue to test different materials for the masks. While a furnace filter may be much more efficient at filtering out aerosols than a bandanna, its components could pose risks.
“There are so many different types of fabric to consider,” says Wang, who recently won an international award for his research on aerosols. “Even for T-shirts, there are different types of materials. We plan to look at different types of pillowcases, bed sheets and other fabrics with different thread counts as part of the testing,” he adds.
“This is not a new field of study,” Wang says. “People in volcanic regions have studied the filtration qualities of various fabrics for years. I have received information about some of these studies. I find it all very helpful.”