Scientists from Duke University have demonstrated a simple technique designed to test the efficacy of different face mask fabrics and designs in reducing the spread of respiratory droplets during normal speech. The research strikingly suggests some alternative face mask options not only offer little protection, but could be more harmful than wearing no facial covering at all.
Face masks have become a mandatory public health necessity in many parts of the world as the COVID-19 pandemic continues to spread. With dwindling supplies of surgical masks and N95 respirators, which are rightfully being diverted to health care environments, the general public is often urged to use whatever they can to cover their faces when out in shared spaces.
The genesis of the new study came when Eric Westman, from the Duke University School of Medicine, was trying to figure which face masks to buy for a non-profit organization helping at-risk local communities. He quickly realized the market was full of products making extraordinary claims but there was no testing process to verify the efficacy of these masks.
“We were trying to make a decision on what type of face covering to purchase in volume, and little information was available on these new materials that were being used,” says Westman.
So Westman turned to his colleague Martin Fischer, director of Duke’s Advanced Light Imaging and Spectroscopy facility, for help. Using a number of common lab materials, all commercially available, the researchers developed a cheap and simple system to test how effectively different face mask materials blocked droplets during normal speech.
“We wanted to develop a simple, low-cost method that we could share with others in the community to encourage the testing of materials, masks prototypes and fittings,” explains Fischer. “The parts for the test apparatus are accessible and easy to assemble, and we’ve shown that they can provide helpful information about the effectiveness of masking.”
As a proof-of-concept testing the new technique, the researchers trialled a number of common masks and mask alternatives. The test involved a speaker wearing a mask repeating the phrase “Stay healthy, people” for ten seconds while a laser illuminated any droplets coming through the mask.
“We confirmed that when people speak, small droplets get expelled, so disease can be spread by talking, without coughing or sneezing,” says Fischer. “We could also see that some face coverings performed much better than others in blocking expelled particles.”
Unsurprisingly, a fitted N95 mask resulted in the most effective reduction in droplet emissions, with a surgical mask relatively close behind. However, most homemade cotton masks tested delivered strong results, blocking droplet emissions at rates not far off what was seen in the surgical mask tests.
The notion that ‘anything is better than nothing’ didn’t hold true
But not every type of facial covering was effective in reducing droplet emissions, unfortunately. Knitted fabrics and bandanas were notably weak in reducing droplet volumes from a speaker. But it was the results from testing neck fleeces, also known as gaiter masks, that really surprised the researchers.
“The notion that ‘anything is better than nothing’ didn’t hold true,” says Westman, discussing the results of the neck fleece test. Westman says the number of particles emitted through the neck fleece resembled the volume seen in baseline tests with no mask at all.
“We attribute this to the fleece, the textile, breaking up those big particles into many little particles. They tend to hang around longer in the air and get carried away easier in the air,” he explains.
The research concludes that wearing this kind of mask may be ultimately counter-productive, causing greater risk of transmission than wearing no mask. But this conclusion is still resolutely hypothetical and the research does not explicitly prove wearing a neck fleece heightens viral transmission. Instead the research suggests the frequent adage “something is better than nothing” may not be true.
It is important to note these kind of droplet transmission studies only investigate the physical properties of droplets exiting our mouths while speaking. There is no evidence so far to suggest these studies can be used to generate conclusions regarding viral transmission.
Another limitation of this particular study is the inability for this test set-up to detect the smallest of aerosol particles. The commonality of the materials used in the test, including a smartphone camera as a recording device, means small aerosol droplets cannot be measured. While the science is still unclear on whether aerosol emissions are a significant form of SARS-CoV-2 transmission, this novel technique only offers data on what kind of face mask material effectively reduces larger droplet emissions.
When it comes to exhalation valves, the study found little difference in droplet emissions between a N95 mask with an exhalation valve and a non-medical cloth mask, making it less effective than an N95 mask without the valve. This suggests that valved N95 masks, while protecting the wearer, are not especially safe for persons surrounding the wearer because the exhalation valve generates strong outwards airflow – a finding recently echoed in updated CDC mask guidelines urging against the wearing of masks with exhalation valves.
Fischer suggests a lot more work is needed to understand what kinds of masks are optimal in our new COVID-19 reality. In the short term he hopes this simple method developed by the Duke University team can be easily appropriated by mask manufacturers to test the products they are developing.
“This was just a demonstration – more work is required to investigate variations in masks, speakers, and how people wear them – but it demonstrates that this sort of test could easily be conducted by businesses and others that are providing masks to their employees or patrons,” says Fischer.
The new study was published in the journal Science Advances.
Source: Duke Health