Certain N95 masks provide the best protection against spreading respiratory droplets that can transmit the novel coronavirus, while using bandannas and neck fleece as masks may be worse than not covering your mouth and nose at all, according to a new study published in Science Advances.
For the study, researchers from Duke University and Duke University School of Medicine used an "optical measurement method" to evaluate the efficacy of 14 different styles of masks or mask alternatives at protecting against the spread of respiratory droplets generated by a wearer during regular speech. The researchers also evaluated the amount of respiratory droplets spread during regular speech while wearing no mask or mask alternatives.
The researchers assessed respiratory droplet transmission using a laser beam, a lens, a box, and a camera on a cellphone in a "simple" setup that "can easily be built and operated by non-experts," the researchers wrote in the study. They explained, "A laser beam is expanded vertically by a cylindrical lens and shined through slits [on the sides of] the enclosure. The camera is located at the back of the box, [and] a hole for the speaker [is located] in the front." The researchers said the setup makes it possible to see and photograph respiratory drops that are transmitted into the box from the speaker, as they demonstrated in a photo, where they used water particles sprayed into the box from a bottle to represent respiratory droplets.
The device allowed the researchers to track the amounts of respiratory droplets that emerged while a person said the phrase "[s]tay healthy, people" into the hole in the front of the box while wearing one of the masks or mask alternatives and while wearing no mask or alternative.
However, the researchers noted several limitations to the study. They wrote that the study was a “proof-of-principle experiment” focused on whether the device could successfully measure droplets, and how those droplets differed between masks, rather than a definitive assessment of quality of the masks themselves. Specifically, the researchers noted that while the study "could inform attempts to improve training on proper mask use and help validate approaches to make existing masks reusable," it "differ[s] in several ways from the traditional methods for mask validation, such as filtration efficiency of latex particles."
Moreover, the researchers pointed out that study featured just "one speaker for all masks and four speakers for selected masks," noting, “"Inter-subject variations are to be expected, for example due to difference in physiology, mask fit, head position, speech pattern, and such."
The researchers found that a fitted, non-valved N95 mask offered the best protection against spreading respiratory droplets, with a droplet transmission rate below 0.1%. The second-best performing mask was a three-layer surgical mask, with a droplet transmission rate between 0.0% and 0.1%.
At the bottom of the spectrum, the researchers found that some mask alternatives performed worse than not wearing a mask at all. The worst performing of the 14 mask types and alternatives that the researchers evaluated was a neck-fleece mask alternative, which had an average droplet transmission rate of 1.1%, followed by a bandanna mask alternative, which had a transmission rate ranging from 0.2% to 1.2%. Not wearing a mask at all had a droplet transmission rate of 1%.
How can some mask alternatives be less protective than not wearing a mask at all?
The researchers explained that some masks, especially the neck-fleece mask alternative, "seemed to disperse the largest droplets into a multitude of smaller droplets … which explains the apparent increase in droplet count relative to no mask in that case." Further, they noted that, because smaller droplets can remain in the air for longer periods than larger droplets, wearing such masks "might be counterproductive."
Martin Fischer, a molecular imaging specialist who was the lead author on the study, said he was "extremely surprised to find that the number of particles measured with the fleece actually exceeded the number of particles measured without wearing any mask." He added, "Common sense would dictate that wearing anything is better than wearing nothing—this was not the case here."
Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security who was not involved in the study, said the research is important for Americans to consider. "People really don't understand that not all face coverings are equal, and that there are some that are going to be more or less effective."
Adalja added that, given the results showing neck fleeces and bandannas could be as unprotective or less protective than not wearing a mask at all, he thinks "they should be abandoned."
"I think that many people are just wearing these face coverings to check a box and not realizing that in order to serve a purpose they need to be effective," he said (Lapin, New York Post, 8/9; Lee, Forbes, 8/9; Haglage, Yahoo Life, 8/10; Dockrill, Science Alert, 8/10; Fischer et al., Science Advances, 8/7; Frieden tweet, 8/9).