Duke University Face Masks Study: Which Work the Best?

Face Masks U.S.

Getty Customers wear face masks as they line up to enter a Costco Wholesale store April 16, 2020, in Wheaton, Maryland.

With many states reopening more fully, including the return of kids to school in some areas, wearing face masks is becoming more and more important to reduce the spread of COVID-19. Researchers at Duke University have determined through a simple study that some face masks are much more effective than others when it comes to preventing an infected person from spreading the virus through respiratory droplets — and what’s worse, some masks are essentially useless while others may actually enhance transmission.

The idea for the study came about when a professor at Duke University’s School of Medicine was asked by the community which masks would be best to buy in bulk, and the professor wanted to recommend the most effective option. The results of the study were published on Friday.


The Study Compared 14 Commonly Available Masks, Including Surgical Masks, Cotton Masks & Bandanas

The study tested 14 different face masks, including the N95 mask used mostly by health care professionals. Each mask was tested 10 times, and each time the test began with the person speaking without a mask for comparison. Here are all the different masks used:

Duke University Face Masks

Emma Fischer, Duke UniversityPictures of face masks under investigation.

The study found that the most effective mask when it comes to reducing the transmission of respiratory droplets was the professionally fitted, non-valved N95. Three-layer surgical masks and cotton masks were quite effective as well. Surprisingly, a few masks were found to be quite ineffective, specifically folded bandanas and knitted masks. Valved N95 masks also “decrease protection of persons surrounding the wearer” because of “the exhalation valve, which opens for strong outwards airflow.”

Neck fleeces, also known as gaiter masks, were the least effective of all and were actually found to produce more respiratory droplets compared to not wearing a mask because they appeared to “disperse the largest droplets into a multitude of smaller droplets.” The study indicates that because they create more particles and smaller particles that are airborne longer than larger ones, these masks may actually be counterproductive. Fischer told CNN, “We were extremely surprised to find that the number of particles measured with the fleece actually exceeded the number of particles measured without wearing any mask. We want to emphasize that we really encourage people to wear masks, but we want them to wear masks that actually work.”


The Study Was Simple & Cost-Effective to Run, Using Only a Laser Beam & a Cell Phone

The physics department of the university used a low-cost method that’s “simple and can easily be built and operated by non-experts,” with only a laser beam and cell phone required. Martin Fischer, one of the researchers, told CNN the process is simple: “We use a black box, a laser, and a camera. The laser beam is expanded vertically to form a thin sheet of light, which we shine through slits on the left and right of the box.” The cell phone camera is placed at the back of the box.

A person then speaks into a hole at the front of the box and the camera records the laser beam light that gets scattered by respiratory droplets when the person talks. The number of droplets seen in the video is then counted by a computer algorithm to determine which masks are most effective. Here is a photo of the setup:

Duke University Study Face Masks

Martin Fischer, Duke UniversityA schematic of the experimental setup.

Although the study is quite simple, Fischer said he doesn’t recommend that people try it at home due to the risks with mishandling a laser beam. The study does indicate that other researchers or companies should test masks under different conditions, including talking, sneezing and coughing.

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