A neck fleece turned out to be worse than wearing nothing. Duke University researchers saw more droplets, not fewer, when a speaker pulled one on.
The finding came from a test built out of cardboard sheets, duct tape, a laser and a mobile phone. Led by molecular imaging specialist Martin Fischer, the team compared 14 types of masks and face coverings in a proof-of-concept study reported in Science Advances.
How the test worked
A lens turns the laser beam into a flat sheet of light inside a dark enclosure. Droplets crossing the sheet flash, and the phone films them. Volunteers repeated the phrase “Stay healthy, people” toward the light while wearing each covering.
Fischer said the aim was to answer one question: “how well does a specific mask type prevent droplets from spreading.” Surgical masks have been tested fairly extensively in clinical settings, he noted, but there was no quick, cheap way to show how a wide range of other masks perform. Many people were buying fabric masks online or sewing their own, and most earlier testing had covered surgical and fitted N95 masks rather than loose fabric ones.
The experiment also confirmed something simpler. “When people speak, small droplets get expelled, so disease can be spread by talking, without coughing or sneezing,” Fischer said.
Best and worst
Fitted N95 masks blocked the most droplets from a speaking person. Surgical masks came next, then masks made of polypropylene. Cotton masks and even knitted ones also stopped droplets. So did an N95 with an exhaust valve, though it scored lower than the fitted version.
The valve explains why. Fischer said these valves close when the wearer breathes in but can open during speech, letting unfiltered air out. “They do a great job of protecting the wearer from the outside environment, but a bad job of protecting others from the wearer,” he said, and the second job is the one that matters for slowing the spread of COVID-19.
At the bottom of the table, bandanas were among the less effective coverings. The neck fleece was the worst of all.
“Common sense would dictate that wearing anything is better than wearing nothing – this was not the case here,” Fischer said. The team believes the fleece material breaks large droplets from speech into several smaller ones. Smaller droplets are carried off more easily by air currents, which could put people nearby at greater risk. The researchers said the result likely warrants further examination.
They stressed that the study’s real subject is the low-cost testing method, not the ranking of masks. The same approach could be run more systematically and with more rigor in future work.
