Multilayer masks offer best protection: IISc. study

‘Droplet transmission and generation negligible or zero for triple-layer and N95 masks’

A team of researchers from the Department of Mechanical Engineering at the Indian Institute of Science (IISc.) with the help of a high-speed camera to track the impact of individual cough-like droplets on protective wear, has concluded that multilayer masks are the most effective.

When a person coughs, large droplets, greater than 200 microns, hit the inner surface of a mask at a high speed, penetrate the fabric and break up or atomise into smaller droplets, said IISc. in a press release. These droplets have a greater chance of aerosolisation, where they are converted into smaller particles that are light enough to be carried into air, along with the virus load.

Tracking impact of droplets

The team used a custom droplet dispenser to mimic a human cough. “We could generate droplets ranging from 200 microns to 1.2 mm in size,” said Shubham Sharma, a PhD. student in the department and first author of the study, in the release.

They then tracked the impact of the droplets on single-, double- and multilayer masks. They noted the size distribution of the smaller droplets generated after they entered the mask fabric. For single- and double-layer masks, most of these atomised droplets were found to be smaller than 100 microns, with the potential to become aerosols, which can remain suspended in the air for a long time and potentially cause infection, said the release.

“You are protected, but others around you may not be,” says Saptarshi Basu, Professor, Department of Mechanical Engineering and senior author of the study, which was published in the peer reviewed journal, Science Advances.

Single-layer masks could only block 30% of the initial droplet volume from escaping. “Double-layer masks were better, with about 91% blocked,” said the release. But more than a quarter of the atomised droplets that were generated were in the size range of aerosols. Droplet transmission and generation was either negligible or zero for triple-layer and N95 masks.

“Triple-layer masks ‒ even those made of cloth ‒ and N95 masks were found to successfully prevent atomisation, and therefore offered the best protection,” said the researchers in the press release.

However, they noted that in the absence of such masks, people should use even a single-layer mask, as they may offer some protection.

Previous studies

According to the release, previous studies have looked at how these droplets ‘leak’ from the sides of masks, but not at how the mask itself can aid in secondary atomisation into smaller droplets. “Most studies also don’t look at what is going on at the individual droplet level and how aerosols can be generated,” said Prof. Basu.

The study was carried out in collaboration with scientists in the UC San Diego and the University of Toronto Engineering. “Studies are also going on to propose more robust models to understand how this atomisation is actually taking place,” he said in the release. “This is a problem not just for COVID-19, but for similar respiratory diseases in the future as well.”

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