Three-layer masks are most effective against large breathing drops

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

Although this advice was originally based on studies that showed that three layers prevent small particles from moving through the mask pores, researchers have now shown that three-layer surgical masks are also the most effective way to prevent large drops from coughing or sneezing. to become smaller. droplets. These large cough drops can penetrate through the single and double layer masks and atomize into much smaller drops, which is especially crucial as these smaller drops (often called aerosols) can stay in the air longer. Researchers studied surgical masks with one, two and three layers to demonstrate this behavior.

The researchers presented their results in Scientific progress on March 5th.

The team notes that single and double layer masks do provide protection by blocking some of the fluid volume of the original drop and that it is significantly better than wearing no mask. They hope that manufacturers can use the findings about the ideal pore size of the mask, material thickness and layers to produce the most effective mask motifs.

The team of engineers from the University of California, San Diego, the Indian Institute of Science and the University of Toronto, using a drop generator and a high-speed time-lapse camera, discovered that, counter-intuitively, large respiratory drops which contain virus-emulating particles (VEPs) are actually atomized when they hit 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 pressed through a sieve. For a 620 micron drop – the size of a large drop of cough or sneeze – a single-layer surgical mask limits only about 30 percent of the drop volume; a double-layer mask performs better and limits about 91 percent of the droplet volume; while a three-layer mask has negligible, almost zero droplet ejection.

“Although large solid particles in the 500-600 micron range are expected to be stopped by a single-layer mask with an average pore size of 30 microns, we show that this is not the case for liquid droplets,” Abhishek said. Saha, Professor of Mechanical and Aeronautical Engineering at UC San Diego and co-author of the article. “If these larger breathing drops have enough velocity, which happens to cough or sneeze, they are dispersed and pushed through the smaller pores in the mask when they end up on a single layer of this material.”

This is a problem. Droplet physics models have shown that although these large droplets are expected to fall to the ground very quickly due to gravity, these now smaller, 50-80 micron droplets coming through the first and second layers of a mask will enter the air lingers. where they can spread to people at greater distances.

The engineering team – which also includes Professors Swetaprovo Chaudhuri of the University of Toronto and Saptarshi Basu of the Indian Institute of Science – were well versed in these types of experiments and analyzes, although they studied the aerodynamics and physics of droplets for applications including propulsion systems, combustion or thermal sprays. They focused their attention on respiratory droplet physics last year when the COVID-19 pandemic broke out, and have since studied the transport of these respiratory droplets and their role in the transmission of COVID-19-type diseases.

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

Using the droplet generator, they can 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 depletion.