Triple masking? Scientists say 3 layers provide the most protection against COVID viral droplets

SAN DIEGO, Calif. — With COVID’s Delta variant becoming the new concern during the pandemic, many areas are continuing to insist that the public use face masks. However, just how many masks will do? According to a recent study, three-layered masks are more effective at preventing the spread of COVID viral particles.

Previous studies have shown masks with three layers are more successful at blocking the passage of small respiratory particles however, recent research indicates they are also better at preventing large particles from breaking down into smaller droplets. This makes them the better option for protection.

In a study using a droplet generator, researchers from the University of California-San Diego obtained images of this phenomenon using a high-speed time-lapse camera. They tested surgical masks with varying layers to see how large droplets pass through each layer. The team, including scientists from the Indian Institute of Science and the University of Toronto, added virus emulating particles (VEPs) to simulate the transference of COVID viral particles.

“We do droplet impact experiments a lot in our labs,” says study co-author Abhishek Saha, professor of mechanical and aerospace engineering at UC San Diego, in a university release. “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–that could be a single layer, double, or triple layer, depending on which we’re testing. Simultaneously, we use a high-speed camera to see what happens to the droplet.”

3 layers provides complete protection?

The generator allowed researchers to change the size as well as the speed of the droplets. The team altered droplet size and speed to test the effect of each on particle flow.

Results reveal that the larger droplets disintegrate into smaller particles upon striking the mask with only one layer. Additionally, most of the VEPs were transferred through the mask, similar to large water droplets becoming tiny droplets after being strained. The normal size of a large respiratory droplet is 620 microns, which a single-layer mask only restricts by about 30 percent.

Dynamic images of a droplet impacting on different-layered mask
The droplet impacting on the mask surface is recorded at 20,000 frames per second. These time sequence images of droplet impingement on a single-, double-, and triple-layer masks show the total number count of atomized droplets is significantly higher for the single-layer mask in comparison with the double-layer mask, while only a single droplet penetrates through the triple-layer mask. (Credit: Basu et al, Science Advances, March 5 2021)

On the other hand, the percentage of droplet volume restricted by a double-layer mask increased significantly to 91 percent and almost 100 percent for masks with three layers. Smaller respiratory droplets tend to suspend in the air longer, which means easier transmission to others, even at farther distances. This study is a pivotal step in helping to prevent the spread of viral particles through respiration.

“While it is expected that large solid particles in the 500-600-micron range should be stopped 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. 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,” Saha explains.

The team intends to study the function of alternative mask materials and the effect of masks that are moist or wet on particle attenuation in the future. Researchers say it is better to wear a mask, even if it is only a single layer. Moreover, the team anticipates the production of a more effective mask, given their research covering thickness of mask materials, number of mask layers, and optimal mask pore size.

The study appears in the journal Science Advances.

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