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Multilayer masks effective to prevent aerosol generation: IISc

As triple-layered and N95 masks offer the best protection from Covid, they must be used where health officials have made it mandatory to prevent aerosol generation, according to a study by the Indian Institute of Science (IISc)on Sunday.

“You are protected, but others around you may not be. For single and double-layered masks, most of the droplets were found to be below 100 microns, with the potential to become aerosols that remain suspended in the air for a long time and potentially cause infection,” said IISc’s mechanical engineering professor Saptarish Basu in a statement here.

Triple-layered masks, even those made of cloth, and N95 masks were found to prevent atomisation and therefore offered the best protection,” the study revealed.

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Using a high-speed camera, the institute’s research team tracked individual cough-like droplets impinging on single, double, and multi-layered masks.

“When a person coughs, large droplets (less than 200 microns) hit the inner surface of a mask at a high speed, penetrate the mask fabric and break up or “atomize” into smaller droplets, which have a chance of aerosolization and carry viruses like Covid with them, said the premier institute’s study, published in “Science Advances”.

Face masks can reduce virus transmission by blocking both large droplets and aerosols, but their efficiency varies with the type of material, pore size, and the number of layers.

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“Most studies don’t look at what is going on at the individual droplet level and how aerosols can be generated,” Basu asserted.

Previous studies looked at how these droplets “leak” from the sides of masks, but not at how the mask can aid in secondary atomization into smaller droplets.

To mimic a human cough, the team used a custom droplet dispenser to pressurize a surrogate cough liquid (water, salt with mucin, and a phospholipid) and eject single droplets onto the mask.

“The pressurization increases a droplet’s velocity and the [nozzle] opening time determines the size,” noted research student Shubham Sharma on the occasion.

The team also used a pulsed laser to cast shadows of droplets, and a camera and zoom lens to capture images at high speeds (20,000 frames per second).

Apart from surgical masks, locally sourced cloth masks were also tested.

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The study was carried in collaboration with scientists in UC San Diego and University of Toronto engineering.

The researchers plan to do more studies using a patient simulator that also allows tracking of multiple droplets.

“Studies are on to propose robust models to understand how this atomization is actually taking place. This is a problem not just for Covid, but for similar respiratory diseases in the future as well,” added Sharma.



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