These large cough droplets can penetrate through the single- and double-layer masks and atomize to much smaller droplets, which is particularly crucial since these smaller droplets (often called aerosols) are able to linger in the air for longer periods of time. Researchers studied surgical masks with one, two and three layers to demonstrate this behavior. They reported their results in Science Advances.
The team notes that single and double-layer masks do provide protection in blocking some of the liquid volume of the original droplet and are significantly better than wearing no mask at all. They hope their findings on ideal mask pore size, material thickness, and layering could be used by manufacturers to produce the most effective masks designs.
Using a droplet generator and a high-speed time-lapse camera, the team of engineers from the University of California San Diego, Indian Institute of Science and University of Toronto found that, counterintuitively, large respiratory droplets containing virus emulating particles (VEPs) actually get atomized when they hit a single-layer mask, and many of these VEPs pass through that layer. Think of it like a water droplet breaking into smaller droplets as it is being squeezed through a sieve. For a 620 micron droplet - the size of a large droplet from a cough or sneeze - a single-layer surgical mask only restricts about 30 percent of the droplet volume; a double-layer mask performs better, restricting about 91 percent of the droplet volume; while a three layer mask has negligible, nearly zero droplet ejection.
"While it is expected that large solid particles in the 500-600-micron range should be stopped by a single-layer mask with average pore size of 30 micron, 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 a co-author of the paper. "If these larger respiratory droplets have enough velocity, which happens for coughs or sneezes, when they land on a single-layer of this material it gets dispersed and squeezed through the smaller pores in the mask."
MEDICA-tradefair.com; Source: University of California - San Diego
