As countries around the world experience a steep surge in novel coronavirus (COVID-19) infections, face masks have become increasingly accepted as an effective barrier to the virus’ spread.
When combined with social distancing and frequent hand-washing, wearing face masks is one of the best means for combating the coronavirus’ spread. It is also increasingly common to find people wearing clear plastic face shields and masks with valves, instead of regular cloth or surgical masks, since they can be more comfortable.
Researchers from Florida Atlantic University’s (FAU) College of Engineering and Computer Science used qualitative visualisations to test how face shields and masks with valves perform in impeding the spread of aerosol-sized droplets.
The research was conducted to increase public awareness about the effectiveness of face shields alone as well as face masks with exhalation valves. It found that the widespread public use of these alternatives to regular masks could potentially have an adverse effect on mitigation efforts.
For the study, just published in the journal Physics of Fluids, researchers employed flow visualisation in a laboratory setting using a laser light sheet and a mixture of distilled water and glycerol 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, researchers were able to map out droplet pathways and demonstrate how they performed.
The study’s results show that, although face shields block the initial forward motion of the jet, the expelled droplets move around the visor with relative ease. They are then able to spread out over a large area, depending on light ambient disturbances.
Visualisations for the face mask equipped with an exhalation valve indicate that a large number of droplets are able to pass through unfiltered, which significantly reduces its effectiveness as a means of source control.
“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,” said Dr Manhar Dhanak, Department Chair, Professor, and Director of SeaTech, and co-author of the paper. “Over time, these droplets can disperse over a wide area in both lateral and longitudinal directions, albeit with decreasing droplet concentration.”
Dhanak co-authored the paper with Dr Siddhartha Verma, lead author and an assistant professor, and John Frankenfeld, a technical professional, all with FAU’s Department of Ocean and Mechanical Engineering.
To demonstrate the performance of the face shield, researchers used a horizontal laser sheet in addition to a vertical laser sheet, revealing how the droplets cross the horizontal plane. Not only did the researchers observe forward spread of the droplets, they found that droplets also spread in the reverse direction.
Notably, face shields impede forward motion of the exhaled droplets to some extent, and masks with valves do so to an even lesser extent. However, once released into the environment, the aerosol-sized droplets get dispersed widely depending on light ambient disturbances.
Like the N-95-rated face mask used in the study, other types of masks such as certain commercially available cloth-based masks, also come equipped with one to two exhale ports, located on either side of the facemask.
The N95-rated face mask with the exhale valve used in this study allowed a small amount of exhaled droplets to escape from the gap between the top of the mask and the bridge of the nose.
Moreover, the exhalation port significantly reduced the mask’s effectiveness as a means of source control, as a large number of droplets passed through the valve unfiltered and unhindered.
“There is an increasing trend of people substituting regular cloth or surgical masks with clear plastic face shields as well as using masks that are equipped with exhalation valves,” said Verma. “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, and masks with exhalation ports include a one-way valve which restricts airflow when breathing in, but allows free outflow of air.”
He added, “The inhaled air gets filtered through the mask material, but the exhaled breath passes through the valve unfiltered.”
The researchers note that the key takeaway from this latest study illustrates that face shields and masks with exhale valves may not be as effective as regular face masks in restricting the spread of aerosolised droplets.
Despite the increased comfort that these alternatives offer, they say it may be preferable to use well-constructed, high quality cloth or surgical masks of a plain design. These should be worn instead of face shields and masks equipped with exhale valves.
The widespread public adoption of the alternatives, in lieu of regular masks, could potentially have an adverse effect on ongoing mitigation efforts against the coronavirus.
“The research conducted by professors Dhanak and Verma on the importance of proper face coverings to stop the spread of COVID-19 has literally illuminated the world,” said Dr Stella Batalama, Dean of FAU’s College of Engineering and Computer Science.
“While broad acceptance regarding the need for face coverings has risen steadily, there is an increasing trend of people who are substituting regular cloth or surgical masks with clear plastic face shields, and with masks equipped with exhalation valves,” she added. “This latest research provides important evidence to further support CDC guidelines and inform the public to make better selections in their choice for face coverings for their benefit and for public safety.”
In a study published on 7 August in the journal Science Advances, researchers reported that face masks are effective in reducing droplet emissions during normal wear.
“We confirmed that when people speak, small droplets get expelled, so disease can be spread by talking, without coughing or sneezing,” said Dr Martin Fischer, a chemist and physicist at Duke University, “We could also see that some face coverings performed much better than others in blocking expelled particles.”
According to the paper, the best face coverings were N95 masks without valves, which are hospital-grade coverings used by front-line health care workers. Researchers noted that surgical or polypropylene masks also showed good performance in protection against the novel pandemic.
Moreover, researchers believe that hand-made cotton face coverings provide good coverage, eliminating a substantial amount of the spray from normal speech. On the other hand, bandanas and neck fleeces, such as balaclavas, did not block the droplets much at all.
