Three-layer masks most effective against large respiratory drops

If you are going to buy a face mask to protect yourself and others from COVID-19, make sure it is a three layer mask. You may have heard this recommendation, but researchers have now discovered an additional reason why three-layer masks are safer than one- or two-layer alternatives.

Although this advice was originally based on studies that showed that three layers prevented small particles from passing through the pores of the mask, researchers have now shown that three-layered surgical masks are also more effective in preventing large drops of a cough or sneeze from atomizing. in small drops. These large cough droplets can penetrate through single and double layer masks and atomize into much smaller droplets, which is particularly important, since these smaller droplets (usually called aerosols) are able to remain in the air for longer periods. long. The researchers studied surgical masks with one, two and three layers to demonstrate this behavior.

The researchers reported their results in Advances in Science on March 5th.

The team notes that single and double layer masks provide protection by blocking part of the liquid volume from the original drop and are significantly better than using no mask at all. They hope their findings about the mask’s ideal pore size, material thickness and layers can be used by manufacturers to produce the most effective mask designs.

Using a droplet generator and a high-speed time-lapse camera, the team of engineers at the University of California at San Diego, the Indian Institute of Science and the University of Toronto found that, counterintuitively, large respiratory droplets containing particles virus emulators (VEPs) are actually atomized when they reach a single layer mask, and many of these VEPs pass through that layer. Think of it as a drop of water breaking into smaller drops as it is squeezed through a sieve. For a 620 micron drop – the size of a large drop from a cough or sneeze – a single-layer surgical mask restricts only about 30 percent of the drop’s volume; a double layer mask performs better, restricting about 91 percent of the drop volume; while a three-layer mask has negligible, almost zero drop ejection.

“While large solid particles in the 500-600 micron range are expected to be disrupted by a single layer mask with an average pore size of 30 microns, we are showing that this is not the case for liquid droplets,” said Abhishek Saha, professor of mechanical and aerospace engineering at UC San Diego and co-author of the article. “If these larger respiratory droplets are fast enough, which happens with coughs or sneezes, when they land on a single layer of this material, it disperses and squeezes through the smaller pores of the mask.”

This is a problem. Physical droplet models have shown that, although these large droplets are expected to fall to the ground very quickly due to gravity, these now smaller droplets, 50-80 microns in size, passing through the first and second layers of a mask will remain in the air, where they can spread to people over greater distances.

The team of engineers – which also includes Professors Swetaprovo Chaudhuri from the University of Toronto and Saptarshi Basu from the Indian Institute of Science – was well versed in this type of experiment and analysis, although they were used to studying aerodynamics and droplet physics for applications that include propulsion systems, combustion or thermal sprays. They turned their attention to the physics of respiratory droplets last year, when the COVID-19 pandemic broke out, and since then, they have been studying the transport of these respiratory droplets and their roles in the transmission of COVID-19 diseases.

“We do a lot of droplet impact experiments in our labs,” said Saha. “For this study, a special generator was used to produce a relatively fast moving drop. The drop was then allowed to land on a piece of mask material – which could be a single, double or triple layer, depending on which one we are testing. At the same time, we use a high-speed camera to see what happens to the gout. “

Using the droplet generator, they are able to change the size and speed of the droplet to see how it affects the flow of the particle.

In the future, the team plans to investigate the role of different mask materials, as well as the effect of damp or wet masks, on particle friction.