As Covid-19 cases mount in several states, and the U.S. as a whole still is still posting frighteningly high numbers, some local leaders are finally requiring face masks in public. But there has been no federal level directive on the matter. Masks have been strangely politicized, while numbers of cases and hospitalizations are rising in many areas. A new paper out in the journal the Physics of Fluids helps visualize exactly why masks are important in reducing the spread, and which types of mask appear to be most effective.
“While there are a few prior studies on the effectiveness of medical-grade equipment, we don’t have a lot of information about the cloth-based coverings that are most accessible to us at present,” said study author Siddhartha Verma in a statement. “Our hope is that the visualizations presented in the paper help convey the rationale behind the recommendations for social distancing and using face masks.”
The team used a manikin (a medical mannequin) to model coughing and sneezing. The spray was replicated using a pump of water and glycerin solution (“respiratory jets”) paired with fog machine particles (“tracer particles”) to help visualize the activity. Lasers captured the behavior of the particles as they escaped the masks through gaps or the fabric itself. The team repeated the experiment with a variety of mask types—a bandana, a mask made from a folded handkerchief using instructions from the U.S. Surgeon General, a stitched mask made of two layers of quilting cotton, and a drugstore brand cone face mask.
The stitched mask and store-bought masks were the most effective (see images below). The stitched mask was able “to arrest the forward motion of the tracer droplets almost completely,” the authors write. There was little leakage through the material itself—most of it came from gaps above the nose. The leakage traveled 2.5 inches on average.
The cone mask was also quite good, though the team notes a little more leakage than the homemade mask. Jets traveled about 6 inches.
The handkerchief mask was mediocre, with an average jet distance of 1.25 feet. The team writes that “while the forward motion of the jet is impeded significantly, there is notable leakage of tracer droplets through the mask material.” Some droplets also escaped through the top of the mask. The bandana (not pictured) performed considerably worse, with average jet distance of 3.5 feet.
The team also looked at the spray of a cough and sneeze when the manikin was wearing nothing over its face: This, of course, spewed droplets far and wide, with particles reaching 12 feet—twice the distance that social distancing guidelines say to maintain. In fact, another study out today in the same journal analyzed the aerodynamics of droplets as they move through the air or evaporate and fall—they traveled up to 13 feet. The findings are also in line with earlier work, suggesting that droplets can linger in the air for many minutes.
It is, of course, important to point out that the new study was just a small observational study, with manikins rather than people—and the coughs and sneezes generated by the pump were simply a model of the human versions.
Still, the new research, along with previous studies, suggest that wearing a mask in public is a relatively simple and cheap way to reduce the spread of the virus. It’s not a stand-alone and it’s not foolproof, but it does seem to help. Masks, along with the other known strategies—social distancing and hand-washing—all work in concert to reduce the risk. Financial models also suggest that a mask mandate could save the economy considerably, as it might obviate the need for further lockdowns.
It’s also worth mentioning that masks are worn as much or more for other people than for yourself—that’s why everyone needs to wear one to be effective. “We have witnessed some aversion to using face masks, and hopefully the study will help convey that using masks is primarily an effort to protect the most vulnerable members of our society (i.e., the elderly, or people with underlying health conditions),” said Verma in an email. “This is crucial since current estimates suggest that one in three infected people (35%) do not show overt symptoms, and could potentially infect such vulnerable individuals inadvertently.”
These realities are all the more important as people return to work and school in the coming months, and second waves are possible, or probable. “Promoting widespread awareness of effective preventative measures is crucial,” the team concludes, “given the high likelihood of a resurgence of COVID-19 infections in the fall and winter.”