- To find out which fabric masks are most effective, tests were conducted in a simulation setting
- Researchers came up with a simple experiment to test the number of droplets expelled from 14 types of fabric masks
- One candidate actually fared worse than no face covering at all
Masks are becoming mandatory in many parts of the world, as there is evidence that face coverings do reduce Covid-19 transmission through exhaled droplets.
However, many of the reusable fabric masks, buffs, scarves, bandanas, neck gaiters and other face coverings available have not been tested to determine to exactly what extent they reduce the spread of droplets.
What about earlier research?
While previous research has determined that some fabrics and designs are more effective than others, these experiments were performed in laboratories with simulation models.
In new research by Duke University, scientists made use of a proof of concept study to test 14 different types of reusable masks. A “proof of concept” study is usually a small procedure to test an idea or assumption.
This differs from previous research performed in laboratories. The full study is published in Science Advances.
What this experiment entailed
To determine how different face coverings fare in practice, Martin Fischer, molecular imaging specialist and lead author of this study, put together a simple experiment where a lens turns an optical laser into a sheet of light.
The light sheet is shone through a dark enclosure, which is made from cardboard sheets and duct tape. People wearing different masks then speak towards the sheet of light, all saying the phrase “stay healthy, people”.
The droplets that pass through the covering are filmed with a mobile phone camera to determine the volume of droplets that pass through each mask.
Droplets expelled even when speaking
According to Fischer, it is clear from these experiments that small droplets that may contain Covid-19 are expelled simply by simply talking, even when there's no coughing and sneezing.
He stated that some face coverings clearly performed better than others. Medical-grade N95 masks remained the most effective at blocking droplets, while loose-fitting bandanas were the least effective.
Other masks in the middle of the spectrum, such as woven cotton masks, were proven to block a significant number of droplets, but surgical masks and masks made from polypropylene were even better.
A reusable version of the N95 mask, which includes an air valve for more comfort, was also tested – even though they are effective when you inhale air, they let out unfiltered air from the mouth when you exhale or speak. This type of mask was discussed in a previous Health24 article, and this experiment backs up the findings.
“These masks do a great job of protecting the wearer from the outside environment, but a bad job of protecting others from the wearer, and it’s the second role that is the important one to reduce Covid-19 spread,” Fischer stated.
One surprising candidate
One would think that any face covering is better than nothing and that even a loose-fitting covering will be able to curb some droplets. But there was one type of face covering that surprised Fischer and his team.
A neck fleece was actually worse than wearing no covering at all, and the researchers think that this type of covering actually causes droplets to proliferate in the air.
"We observed that the number of droplets increased when the speaker put on the neck fleece. We believe that the material of our fleece breaks down large droplets emitted during speaking into several smaller ones. This could make wearing such a mask counterproductive, since smaller droplets have an easier time being carried away by air currents and endangering nearby persons," Fischer stated.
No mask equal or 100% effective
According to the researchers, the aim of the study was to test masks in a more realistic, low-cost setting than a droplet simulation, but caution that their study does have some limitations.
They also reiterate that masks should be combined with other measures such as hand hygiene and keeping a distance of at least 2m from other people in public.
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