Scientists examined how aerosol-sized droplets moved around a face shield.
Using a face shield without a mask offers little protection against COVID-19, according to a new study.
Researchers from Florida Atlantic University’s College examined how face shields performed in impeding the spread of aerosol-sized droplets.
For the study, just published in the journal Physics of Fluids, researchers employed flow visualization 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.
Researchers found 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.
“There is an increasing trend of people substituting regular cloth or surgical masks with clear plastic face shields,” said Dr. Siddhartha Verma, one of the studies authors.
“A driving factor for this increased adoption is better comfort compared to regular masks. However, face shields have noticeable gaps along the bottom and the sides”.
Inferior to masks.
“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, aerosolized droplets expelled with the jet are able to move around the visor with relative ease,” added Dr. Manhar Dhanak.
“Over time, these droplets can disperse over a wide area in both lateral and longitudinal directions, albeit with decreasing droplet concentration.”
The researchers say that the key takeaway from this latest study illustrates that face shields are not as effective as regular face masks.
World Health Organisation (WHO) recommends the use of face shields “in the context of non-medical mask shortages” or for those that find wearing a mask difficult but stress they are “inferior to masks” when it comes to preventing droplet transmissions.
The same study also revealed that face masks equipped with an exhalation port allowed a large number of droplets to pass through the exhale valve unfiltered.