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Explained: Study zeroes in on most effective face mask to prevent Covid-19

Coronavirus (COVID-19) face mask: The best face covering for preventing spread of droplets were N95 masks without valves while fleece coverings and bandanas were the least effective

Written by Abhishek De , Edited by Explained Desk | New Delhi | Updated: August 18, 2020 11:53:11 am
A study by Duke University in the US has thrown light on which masks are the most effective at preventing the spread of droplets

At a time when face coverings of different types and sizes have flooded the markets, a study by Duke University in the US has thrown light on which masks are the most effective at preventing the spread of droplets emitted by people when talking — a crucial aspect in reducing the transmission of Covid-19.

For the study, published in the journal Science Advances, the Duke University researchers (Emma Fischer, Martin Fischer, David Grass, Isaac Henrion, Warren S Warren, and Eric Westman) devised a low-cost laser sensor device and used it to compare 14 different types of masks and face coverings.

The study revealed that the best face mask for preventing spread of droplets were N95 masks without valves while fleece coverings and bandanas were the least effective, with researchers finding them “worse than not wearing any kind of facial covering”.

“Surgical masks are commonly worn by medical personnel and have received a fair amount of testing in clinical settings. But there was no quick, easy, and cost-effective way to demonstrate the effectiveness of such a wide variety of other mask types, like loose fabric masks or face coverings,” Fischer said.

How was the experiment on masks conducted?

For the study, the researchers devised an easy-to-make, inexpensive laser experiment to measure the amount of respiratory droplets that escape from different coverings whenever the wearer speaks. The setup consisted of a black box (dark enclosure), a laser, a lens, and a mobile phone camera — all of which costs about $200. The laser beams were used to create a sheet of light inside the box.

For the test, a person was asked to wear each mask and made to stand in the dark enclosure. Then the person was asked to say the phrase “stay healthy, people” five times in the direction of the laser beam, which scattered light from the droplets released during the speech. Using a mobile camera, the droplets were recorded and a simple computer algorithm counted them.

“Companies and manufacturers can set this up and test their mask designs before producing them, which would also be very useful,” the researchers said.

The masks were ranked on the basis of relative droplet count on a scale of 1.0, representing the number of droplets that hit the sheet when the person had no face covering, to 0.0, when minimal or no droplets were spread.

The different types of face masks used for the study by Duke University researchers (Source: Science Advances)

What did the study reveal?

The experiment revealed N95 masks without exhalation valves, worn by frontline healthcare workers, were the best at blocking droplets and had a relative droplet count of zero. N95 masks with valves was ranked seventh, registering a relative droplet count ranging from 0.1 to 0.2.

“Fitted, non-valved N95 masks performed far superior to valved N95 masks because the exhalation valve opens for strong outward airflow…it can decrease protection of persons surrounding the wearer,” the study said.

The three-layer surgical mask came second and had a more variable relative droplet count that ranged from zero to 0.1. At the third and fourth spot were the ones with polypropylene: the cotton-polypropylene-cotton mask and the 2-layer polypropylene apron mask.

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Four different two-layer cotton pleated masks and one one-layer cotton pleated mask occupied spots from fifth to eleventh, having relative droplet count range from zero to 0.4.

At ninth spot was a one-layer Maxima AT mask while knitted masks stood at 12th position, having a large relative droplet count range of around 0.1 to just under 0.6.

The study shows that the last two masks could actually be worse than wearing no mask at all. At number 13, the bandanna had a range of 0.2 to 1.1 while the fleece mask finished at number 14, with an average relative droplet count of 1.1. Simply put, this means a person wearing a fleece mask generated more droplets than when his or her nose and mouth had no covering at all. The researchers said this was likely because these types of masks break down large droplets into smaller particles, allowing them to slip out the sides of the covering more easily.

Droplet transmission through face masks. (A) Relative droplet transmission through the corresponding mask. (B)The time evolution of the droplet count (left axis) is shown for representative examples, marked with the corresponding color in (A): No mask (green), Bandana (red), cotton mask (orange), and surgical (blue – not visible on this scale). The cumulative droplet count for these cases is also shown (right axis). (Source: Science Advances)

Limitations of the study

However, the study has its own limitations. It didn’t test all possible versions of the different masks and one cannot gauge the quantity virus is being spread through the droplets. Each droplet may not contain enough of SARS-CoV2 to infect others. Moreover, the study only measured the droplets being emanated from the front of the speaker’s mouth and didn’t factor in the droplets released from the sides of the mask. “This was just a demonstration — more work is required to investigate variations in masks, speakers, and how people wear them,” Fischer said.

What type of masks has the Health Ministry, WHO, CDC recommended?

The Health Ministry has encouraged the public to use home-made face masks or coverings made of cloth, preferably cotton, which can be tied or fastened over the nose and mouth. The World Health Organisation, in its June revised guidelines, has suggested that everyone should wear fabric masks (non-medical) in public. Fabric masks should contain at least three layers of different materials. The US Centers for Disease Control and Prevention, in its latest guidelines, said masks made of tightly-woven fabric should be used but they should not have valves or vents.

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