Researchers at Duke University in Durham, North Carolina, have tested the effectiveness of a variety of masks, using a simple droplet experiment to determine which coverings offer the best protection against the spread of COVID-19.

According to the study, published in the American Association for the Advancement of Science on Friday, neck fleeces or gaiter masks (a tube of fabric that can be pulled up to cover the face), knitted masks, and bandanas proved to be least effective of the face coverings that were tested.

The researchers found that wearers of neck fleeces transmitted more droplets than people who were not wearing any mask. These coverings had a transmission rate of 110%.

Duke University researchers tested 14 different face masks or mask alternatives. 1) Surgical mask, 3-layer; 2) N95 with exhalation valve; 3) Knitted mask; 4) 2-layer polypropylene apron mask; 5) Polypropylene-cotton mask; 6) 1-layer Maxima AT mask; 7) 2-layer cotton, pleated-style mask; 8) 2-layer cotton, Olson-style mask; 9) 2-layer cotton, pleated-style mask; 10) 1-layer cotton, pleated-style mask; 11) Gaiter-type neck fleece; 12) Double-layer bandana; 13) 2-layer cotton, pleated-style mask; 14) N95 mask, no exhalation valve. – Image: Emma Fischer, Duke University

“We attribute this increase to the neck fleece dispersing larger droplets into several smaller droplets, therefore increasing the droplet count,” the report said.

Folded-over bandanas reduced the droplet rate by a factor of two.

Conversely, the fitted N95 mask had a 0.1% transmission, making it the safest mask to wear. Due to limited supplies, many countries have sought to keep these masks for medical professionals.

Cotton masks were also found to be quite effective against droplet transmissions, showing the least number of droplets outside of the medical-grade masks.

The report, authored by researchers Emma P. Fischer, Martin C. Fischer, David Grass, Isaac Henrion, Warren S. Warren and Eric Westman, looked at the effectiveness of 14 different masks, including three-layer surgical masks, valved N95 masks, cotton masks and bandanas.

Best 

  • Fitted N95 without valve
  • Surgical mask
  • Polypropylene-cotton mask
  • 2-layer cotton, pleated style mask (various types)
  • N95 mask with exhalation valve
  • 2-layer cotton, Olson-style mask
  • 1-layer Maxima AT mask
  • 1-layer cotton, pleated style mask

Worst

  • Neck fleece
  • Bandana
  • Knitted mask

 

 

 

In Cayman, masks are mandated inside public spaces.

The World Health Organization recommends the wearing of masks to guard against the spread of COVID-19.

However, WHO Director-General Tedros Adhanom Ghebreyesus has said that “masks alone will not protect you from COVID-19″.

“People can infect themselves if they use contaminated hands to adjust a mask or repeatedly take it on or off,” he said in a 5 June report.

(CLICK TO ENLARGE) This graphic shows the relative droplet count for each tested mask. The solid dots represent the outcome of the same speaker testing all masks; the points and error bars represent the mean value and distribution standard deviation, respectively, of the total droplet count normalised to the control trial (no mask). – Graphic: Duke University

The Duke University researchers, who created a device harnessing the power of a laser and a cellphone camera for their experiment, said, “In effect, the mask acts as a temporal low pass filter, smoothens the droplet rate over time, and reduces the overall transmission.”

How they did it

In the experiment, users wore a face mask and spoke in the direction of an expanded laser beam inside a dark enclosure.

“Droplets that propagate through the laser beam scatter light, which is recorded with a cellphone camera. A simple computer algorithm is used to count the droplets in the video. The camera was used to record a video of approximately 40 [seconds] length to record droplets emitted while speaking,” the report said.

The first 10 seconds of the video served as baseline.

In the next 10 seconds, the mask wearer repeated the sentence “Stay healthy, people” five times, after which the camera kept recording for an additional 20 seconds for observation.

Schematic of the experimental setup. A laser beam is expanded vertically by a cylindrical lens and
shined through slits in the enclosure. The camera is located at the back of the box, a hole for the speaker
in the front. The inset shows scattering for water particles from a spray bottle with the front of the box
removed. – Photo: Martin Fischer, Duke University.

For each mask and for the control trial, the protocol was repeated 10 times and a computer algorithm was used to count the number of particles within each video.

In explaining their findings, the researchers said for the bandana the droplet rate is merely reduced by a factor of two and the repetitions of the speech are still noticeable.

“The effect of the cotton mask is much stronger. The speech pattern is no longer recognizable and most of the droplets, compared to the control trial, are suppressed. The curve for the surgical mask is not visible on this scale,” the report said.

Read the full report here.

 

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