Chennai: Researchers including those of Indian origin have found that face masks with exhale valves and face shields may not be as effective as regular masks in restricting the spread of aerosol droplets, and could potentially hamper Covid-19 mitigation efforts.
Researchers from Florida Atlantic University in the US used qualitative visualisations to test how face shields and masks with valves perform in impeding the spread of aerosol-sized droplets.
They found that widespread public use of these alternatives to regular masks could potentially have an adverse effect on mitigation efforts.
For the study, published in the journal Physics of Fluids, the researchers employed flow visualisation in a laboratory setting using a laser light sheet, and a mixture of distilled water and glycerin to generate the synthetic fog that made up the content of a cough-jet.
They visualised droplets expelled from a mannequin’s mouth while simulating coughing and sneezing.
By placing a plastic face shield and an N95-rated face mask with a valve, the researchers including Siddhartha Verma, lead author and an assistant professor at Florida Atlantic University, were able to map out the paths of droplets and demonstrate how they performed.
The research shows that although face shields block the initial forward motion of the jet, the expelled droplets move around the visor with relative ease and spread out over a large area depending on light ambient disturbances, the researchers said.
Visualisations for the face mask equipped with an exhalation port indicate that a large number of droplets pass through the exhale valve unfiltered, which significantly reduces its effectiveness as a means of source control, they said.
Manhar Dhanak, a professor, and co-author of the paper, said, “from this latest study, we were able to observe that face shields are able to block the initial forward motion of the exhaled jet, however, aerosolised droplets expelled with the jet are able to move around the visor with relative ease.”