As scientists learn more about how the new coronavirus (COVID-19) spreads, we move from the CDC’s non-mandatory recommendation of facial masks to companies and even states that require its use in public, all in a effort to stop the pandemic. Commercially available bandanas, neck gaiters, home cloth face masks, and cone masks are acceptable, but the most effective form of non-medical mask has not yet been investigated.
A new study in Fluid Physics Now he has done exactly that, and has gone a step further, visualizing the effectiveness of different masks through an amazing set of images (available on the magazine’s website). To perform the experiment, the researchers used a mannequin head and a fog machine to emulate coughs and sneezes. They tied different types of face masks to the mannequin and tested how far and fast the emulated “breath jet” of droplets and particles could travel.
How do drops travel in an open cough?
First, the researchers wanted to find out what happens during an emulated, exposed cough. They observed that the droplets constantly traveled beyond the six-foot line currently recommended for social distancing. The drops flew an average of eight feet away, but were found to travel up to 12 feet within a 50-second window.
“These observations, combined with other recent studies, suggest that current guidelines for social distancing may need to be updated,” the researchers wrote. They noted that a “large majority” of drops will fall to the ground before reaching the 12-foot mark, and that “both the number and concentration of the drops will decrease with increasing distance.” Air flow also had a major impact on dispersion patterns, demonstrating how important adequate indoor ventilation will be as the pandemic continues through the summer.
What type of mask is more effective?
The researchers then repeated the experiment using four different types of face masks, all non-medical grade masks that are available to the general public. These were the results, listed from most effective to least effective:
- Double-layer stitched cotton mask: The emulated respiratory jet traveled 2.5 inches on average.
- Store-bought cone-style mask: 8 inches
- Folded cotton scarf (also known as a seamless mask): 1 foot, 3 inches
- Handkerchief: 3 feet, 7 inches
- Without signal: 8 feet
From their experiments, the researchers concluded that slightly folded kerchief masks and bandana-style liners “provide minimal stopping ability for the smallest aerosol respiratory drops.” The droplets seeped through the mask material as well as out to the sides, although both were still much more effective than either cover.
Tapered-style masks and well-fitting home masks were the most effective. “These masks were able to significantly reduce the speed and range of the breathing jets, albeit with some leakage through the mask material and small gaps along the edges,” the researchers observed.
None of the masks was perfect, and the study noted that wearing masks for an extended period can lead to saturation and a possible decrease in effectiveness. Still, his conclusion was clear: Even the least effective coverage (a scarf) had a big impact, reducing droplet dispersion by an average of four and a half feet. And despite varying amounts of leaks, nonmedical masks are effective in stopping larger drops, which “make up a large fraction of the total volume of respiratory fluid expelled.”
Takeaway: wear a mask, any mask (here is a variety you can buy online); do it well if you can; and maintain social distancing.
