That bandanna might make you look like a cool outlaw from an old Western movie but it’s largely ineffective in protecting you from the coronavirus, according to a new study.
A group of researchers at Duke University tested 14 different types of common face masks to determine which ones work best to stop the transmission of respiratory droplets during regular speech - and which ones are practically useless.
Fitted N95 masks, which are usually reserved for health care professionals, came out on top of the rankings, while three-layer surgical masks and homemade cotton masks also performed well.
But the study published Friday found that folded bandannas and knitted masks did not offer much protection, while neck fleece -- which are often used by runners - do more harm than good.
People who wear the fleece masks, also called gaiter masks, are likely to release a higher number of respiratory droplets into the air than those who do not wear a face covering at all, according to the researchers, because the material appears to break down larger droplets into aerosols.
“Considering that smaller particles are airborne longer than large droplets, the use of such a mask might be counterproductive,” the study said.
The virus that causes COVID-19 is most commonly spread through respiratory droplets produced when an infected person sneezes, coughs, or talks. These aerosols can remain suspended in the air for up to three hours and be transmitted up to 26 feet, based on one study from MIT.
Masks block, or at least limit, your exposure to these contagious viral droplets and particles. Because many coronavirus cases are asymptomatic, you should also wear a face covering to protect others.
In the proof-of-concept study using a box outfitted with a cellphone camera and laser, the Duke researchers deliberately came up with a simple method of measuring particles emitted by people speaking while wearing a variety of face coverings.
The goal was to allow companies to replicate the low-cost test before manufacturing new products.
“We confirmed that when people speak, small droplets get expelled, so disease can be spread by talking, without coughing or sneezing,” Martin Fischer, lead researcher and molecular imaging specialist, said in a followup Q&A. “We could also see that some face coverings performed much better than others in blocking expelled particles.”
While the results showed that fitted N95 masks are most effective in blocking droplets, there is an exception. Valved N95 respirators with a one-way vent ranked much lower on the list.
“These valves are closed when breathing in, but can open when speaking, hence letting out unfiltered air,” Fischer said. “In other words, they do a great job of protecting the wearer from the outside environment, but a bad job of protecting others from the wearer, and it is the second role that is the important one to reduce COVID-19 spread.”
Beyond the fitted N95s and surgical masks, the face coverings that performed best were made out of a three-layer combination of cotton and polypropylene. That was followed by a two-layer swath of polypropylene. Another top performer was a two-layer pleated cotton.
A separate study found that silk is even more effective than polyester or cotton at repelling droplets.
“As a protective barrier and face covering, silk is more effective at impeding the penetration and absorption of droplets due to its greater hydrophobicity [water-repelling ability] relative to other tested fabrics,” biologists at the University of Cincinnati wrote in a separate paper published for peer review.
The studies come in time for World Mask Week, a global initiative to encourage and normalize the use of face coverings to help stop the spread of the coronavirus pandemic.
Nearly 300,000 people are projected to die of COVID-19 in the U.S. by December, but that number that could drop by 70,000 if universal mask use was adopted, according to a new data project from the Institute for Health Metrics and Evaluation at the University of Washington’s School of Medicine.
The Duke project was conducted at the request of physician Eric Westman, who works with a local non-profit to provide free masks to at-risk and under-served populations. He wanted to make sure the masks they distributed were effective in curbing the spread of the virus.
“About half of infections are from people who don’t show symptoms, and often don’t know they’re infected,” Westman said. “If everyone wore a mask, we could stop up to 99% of these droplets before they reach someone else.”
The findings were echoed by a new observational study by Australian researchers published in the journal Thorax. Their goal was to evaluate the effectiveness of the homemade one- and two-layer cloth face coverings recommended by the Centers for Disease Control and Prevention.
They concluded that while a double-layer cloth mask made from a folded piece of a t-shirt and hair ties is preferable to a single layer covering, a manufactured surgical mask was significantly better in reducing some droplet spread.
The researchers recommended that guidelines for homemade cloth masks should stipulate multiple layers (at least three), but noted, “Even a single-layer face covering is better than no face covering.”
Aidin Vaziri is a San Francisco Chronicle staff writer. Email: avaziri@sfchronicle.com
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August 11, 2020 at 05:56AM
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Does your coronavirus mask work? New study separates the worthy from the worthless - San Francisco Chronicle
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