Not all face coverings are created equal. While face shields and masks with valves might score high on comfort, breathability, and making communication easier, they don't rank well when it comes to preventing the spread of viral particles.
A visualization study published in Physics of Fluids demonstrated this using a mannequin and an air pump meant to simulate the path of real-life coughs and sneezes.
"We've heard from all sorts of people in the general public, even medical doctors and other scientists as well, that this really helps convey what's happening in reality," lead author Siddartha Verma, assistant professor in the department of ocean and mechanical engineering at Florida Atlantic University, said.
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Aerosols the technical term for smaller droplets that have a diameter of 5-10 microns or less are lighter, allowing them to move through the air more slowly and freely. As such, they are more likely to escape a face shield or a poorly-fitted mask.
The visualization also didn't address how effective face shields are at protecting the wearer. However, a study published in the Journal of Occupational and Environmental Hygiene in 2014 found that health care workers wearing face shields were only protected from 23% of aerosol inhalation.
More recently, the efficacy of face shields was called into question after a coronavirus outbreak at a hotel in Switzerland. The only people infected with the coronavirus were those who wore face shields without masks.
"If a person is sick and wearing one of these exhalation valve masks, then when they exhale, they let out the full breath through the vent," co-author Manhar Dhanak said. "That would potentially contain droplets of all sizes, particularly these aerosol-sized droplets which can then propagate into the room."
Dhanak said valved masks are not intended for medical use, but for people working in industrial settings where they may be at risk of breathing in chemicals. For the purpose of preventing the spread of disease, he said wearing a homemade fabric mask would be more effective than wearing an N95-rated mask with a vent.
"One thing to be mindful of is none of the masks that we tested were 100% effective," Verma said. "There's always some degree of leakage, which is why even when using masks it's important to be maintaining distance from others."
Verma and Dhanak acknowledged that, in certain situations, people may need to wear face shields. The shields may be more comfortable for outdoor exercise, and they allow people who are deaf or hard of hearing to read lips.
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These simulations show how face shields and valved masks allow clouds of respiratory droplets to escape - Business Insider India
