Making your own mask? Researchers say some fabrics can filter nearly as well as an N95

By Matt Berg Globe Correspondent, Updated April 30, 2020, 3 hours ago

MacGyver would be proud.

Fabric from a cotton sheet combined with material from a prom dress, an old tie, or a pair of pajamas could allow people to make a mask with a filter that’s nearly as good as the N95 mask, according to new research.

A new study from researchers at the University of Chicago says that a combination of cotton with silk, chiffon, or flannel can create a filter whose performance nears that of the much-talked-about N95.

The researchers and other experts said there’s a big caveat, though. If your mask allows a bunch of leakage around the edges, unlike the tightly fitted N95, your homemade filter will lose its effectiveness.

“We found that many common fabrics can be surprisingly good at filtering particles,” said Supratik Guha, a University of Chicago professor and author of the study, which was published Friday in the journal Nano.

Tighter-woven cotton alone was found to be effective, particularly two layers of 600 thread-per-inch cotton. So was a cotton quilt made of two 120 thread-per-inch cotton sheets, with a 0.5-centimeter batting of cotton, polyester and other fibers. Four layers of silk — imagine someone really bundled up in a scarf — also performed well.

But the best overall filtration was provided by a sandwich of one layer of the tighter-woven cotton sheet plus two layers of silk, or two layers of chiffon, or one layer of flannel — because of the electrostatic filter created by the non-cotton layers.

“We speculate that the enhanced performance of the hybrids is likely due to the combined effect of mechanical and electrostatic-based filtration,” the study said. “Combining layers to form hybrid masks … may be an effective approach.”

“The benefit of having an electrostatic layer is that it can trap liquid droplets and other particles based on charge rather than the pore size,” said Jill Crittenden, a research scientist at the Massachusetts Institute of Technology and member of N95 Decon, a group of scientists who review and publish information about N95 mask decontamination.

The big problem is around the edges.

“The bad thing about using masks that have really high filtration is that, if there are any gaps in the mask, the air is just going to go through those gaps than through the filter,” Crittenden said. “It’s important to have a mask that has high breathability and high airflow, and that’s not typical of cloth masks.”

Mask makers must ensure that a mask is fitted properly, or risk severely affecting the mask’s efficacy, the UChicago researchers said. In some cases, gaps decreased mask efficacy up to 60 percent.

“You could have the best fabric material around, but if you’re not planning to minimize those gaps, the effectiveness really drops,” Guha said. “That’s something cloth mask makers need to keep in mind.”

That comes as no surprise to many a homemade mask-wearer whose glasses have fogged up because of leakage when they breathe.

In the weeks since the pandemic unfolded, many people have been scrambling for ways to craft masks at home. The US Centers for Disease Control and Prevention recommended wearing masks in public but officials have discouraged people from buying N95 masks needed by health care workers and first responders.

The Boston Globe posted a video on Instagram on how to make a facial covering using just two hair ties and a piece of cloth. The Washington Post and New York Times are also among the many outlets that have posted articles and videos detailing the mask-making process.

In early April, the US Surgeon General posted a video on Twitter demonstrating several ways to make a mask out of household materials. From pillow cases to bandanas, the search for effective household materials is on.

Even with a cotton and silk, chiffon, or flannel combination paired with a proper fit, a homemade mask likely wouldn’t hold a candle to the coveted N95 mask, which has been tested for decades and is used in hospitals by medical professionals working directly with infected patients, researchers said.

“I wouldn’t go as far to say they’re as good as N95 masks,” Supratik said.

That doesn’t mean, though, that people should refrain from wearing a mask in public, he said.

“Our results support the notion that using cloth masks as opposed to not using anything is a big improvement,” he said.

Scientists believe that the virus can be transmitted through talking, sneezing, or even breathing. Masks can protect someone from inhaling the virus, but also, officials have said, they can prevent someone who has the virus from transmitting it, including those who feel fine but unwittingly are spreading the virus.

“It’s widely accepted that wearing a mask of any type will stop droplets from going far,” Crittenden said. “Any cloth mask is very beneficial.”

Matt Berg can be reached at Follow him on Twitter @mattberg33.


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