Face shield or face mask to stop the spread of COVID-19?
Visualization study illustrates why face shields alone don't work

Date:
September 1, 2020
Source:
Florida Atlantic University
Summary:
If CDC guidelines aren't enough to convince you that face shields
alone shouldn't be used to stop the spread of COVID-19, then maybe
a new visualization study will. Researchers simulated coughing and
sneezing from a mannequin's mouth using a laser light to visualize
droplets expelled. They tested a plastic face shield and found
that they block the initial forward motion of the exhaled jet,
however, aerosolized droplets are able to move around the visor
with relative ease.



FULL STORY ==========================================================================
If the United States Centers for Disease Control and Prevention (CDC) guidelines aren't enough to convince you that face shields alone shouldn't
be used to stop the spread of COVID-19, then maybe a new visualization
study will.


==========================================================================
To increase public awareness about the effectiveness of face shields alone
as well as face masks with exhalation valves, researchers from Florida
Atlantic University's College of Engineering and Computer Science used qualitative visualizations to test how face shields and masks with valves perform in impeding the spread of aerosol-sized droplets. 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 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. They visualized 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, they were able to map out the paths of droplets
and demonstrate how they performed.

Results of the study show 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.

Visualizations 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.

"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," said Manhar Dhanak, Ph.D., department chair, professor, and director of SeaTech, who co-authored the paper with
Siddhartha Verma, Ph.D., lead author and an assistant professor; and
John Frankenfeld, a technical professional, all within FAU's Department
of Ocean and Mechanical Engineering. "Over time, these droplets can
disperse over a wide area in both lateral and longitudinal directions,
albeit with decreasing droplet concentration." 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 this study, other types of masks
such as certain cloth-based masks that are available commercially 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 had a small amount of exhaled droplets that escaped from the gap
between the top of the mask and the bridge of the nose. Moreover, the exhalation port significantly reduced the effectiveness of the mask 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. The inhaled air gets filtered through the mask material, but the
exhaled breath passes through the valve unfiltered." The researchers
say 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 aerosolized 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
that are of a plain design, instead of face shields and masks equipped
with exhale valves. Widespread public adoption of the alternatives,
in lieu of regular masks, could potentially have an adverse effect on
ongoing mitigation efforts against COVID-19.

"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 Stella Batalama, Ph.D., 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. 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."

========================================================================== Story Source: Materials provided by Florida_Atlantic_University. Original written by Gisele Galoustian. Note: Content may be edited for style
and length.


========================================================================== Related Multimedia:
*
YouTube_video:_Face_Shield_or_Face_Mask_to_Stop_the_Spread_of_COVID-19? ========================================================================== Journal Reference:
1. Siddhartha Verma, Manhar Dhanak, John Frankenfield. Visualizing
droplet
dispersal for face shields and masks with exhalation valves. Physics
of Fluids, 2020; 32 (9): 091701 DOI: 10.1063/5.0022968 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200901112231.htm

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