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Face masks can reduce the spread of airborne particles from respiratory viruses such as COVID-19, but even the most secure professional face coverings don’t eliminate everything, according to a new study published Wednesday in mSphere, an open access journal from the American Society for Microbiology.

Cotton masks, surgical masks and N95 masks reduced the transmission of coronavirus droplets but didn’t completely block all particles, the University of Tokyo research team found. However, the transmission between two people was much less when both people wore masks.

“There was a synergistic effect when both the virus receiver and virus spreader wore masks,” they wrote.

The research team built a secured chamber with two mannequin heads facing each other. One head simulated coughing and expelled actual coronavirus particles. The other head simulated natural breathing, with a collection chamber in the airway to collect virus particles.

When the breathing mannequin had a cotton mask, it reduced transmission between 20% to 40%. An N95 mask blocked 80% to 90% of particles. When the coughing mannequin had a cotton or surgical mask, it reduced transmission by 50%, and the N95 blocked 90%.

A limitation of the study is that the researchers can’t say whether the particles that passed through the masks could result in infection or illness, the researchers wrote. In addition, the stability of the virus in the air changes based on droplet and aerosol size, composition and other factors.

“Considerable viral loads have been detected in the nasal and throat swabs of asymptomatic and minimally symptomatic patients, as well as those of symptomatic patients, which suggests transmission potential,” the researchers wrote. “Accordingly, it is desirable for individuals to wear masks in public spaces.”

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mSphere, “Effectiveness of Face Masks in Preventing Airborne Transmission of SARS-CoV-2.”

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