Multilayer masks most effective at preventing aerosol generation: Study

Multilayer masks most effective at preventing aerosol generation: Study

Multilayer More expensive they’re more effective at preventing the generation of aerosols, says a new study by a team led by researchers from the Bengaluru-based Indian Institute of Science (IISc).

The study was carried out in collaboration with scientists from UC San Diego and the University of Toronto Engineering.

According to IISc, when a person coughs, large droplets (> 200 microns) hit the inner surface of a mask at high speed, penetrate the mask fabric, and break or “atomize” into smaller droplets, which are more likely aerosolization. and therefore carry viruses like SARS-CoV-2 with them.

Using a high-speed camera, the team closely followed individual cough-like droplets incident on single-, double- and multi-layer masks, and observed the size distribution of the “daughter” droplets generated after penetration through of the mask fabric, according to an IISc statement on Saturday.

For single- or double-layer masks, most of these atomized daughter droplets were found to be smaller than 100 microns, with the potential to become aerosols, which can remain airborne for a long time and potentially cause infection, according to the study.

“You are protected, but those around you may not be,” says Saptarshi Basu, a professor in the Department of Mechanical Engineering and lead author of the study published in Science Advances.

Triple-layer “even cloth” masks and N95 masks were found to successfully prevent atomization and therefore offer the best protection.

The researchers, however, clarify that when such masks are not available, even single-layer masks can offer some protection and therefore should be used where required by health officials.

Masks can significantly reduce virus transmission by blocking both large droplets and aerosols, but their effectiveness varies depending on the type of material, the size of the pores, and the number of layers.

Previous studies have looked at how these droplets “seep” down the sides of masks, but not how the mask itself can aid secondary atomization into smaller droplets.

“Most of the studies also do not analyze what is happening at the individual drop level and how aerosols can be generated,” adds Basu.

To mimic a human cough, the team used a custom drop dispenser to pressurize a substitute cough liquid (water, salt with mucin, and a phospholipid) and push individual drops into the mask.

“Pressurization increases the velocity of the drop and the [nozzle] the opening time determines the size, ”explains Shubham Sharma, a doctoral student in the Department of Mechanical Engineering and first author of the study. “With this, we could generate droplets ranging from 200 microns to 1.2mm
Size.”

The team used a pulsed laser to cast shadows from the drops and a camera and zoom lens to capture images at high speeds (20,000 frames per second). In addition to surgical masks, some locally sourced cloth masks were also tried on.

What kind of mask are you wearing? (AP Photo / Andy Wong)

The team also investigated the effects of varying the speed at which the drop is ejected and the angle of impact.

They found that single-layer masks could only block the escape of 30 percent of the initial volume of droplets.

Double-layer masks were better (about 91 percent blocked), but more than a quarter of the daughter droplets that were generated were in the aerosol size range. Droplet transmission and generation was negligible or nil for the triple-layer and N95 masks.

The team also dispersed fluorescent nanoparticles the same size as the virus in the artificial cough drops to show how these particles can become trapped in the fibers of the mask, underscoring the importance of disposing of the masks after use. The researchers hope to conduct further studies using a large-scale patient simulator that would also allow multiple droplet tracking.

“Studies are also being done to propose more robust models to understand how this atomization is actually taking place,” says Basu. “This is a problem not only for COVID-19, but also for similar respiratory illnesses in the future.”