A
new study investigates how effective face masks of different designs are at
inhibiting the transfer of airborne droplets from coughs and sneezes.
Due to the scarcity and
cost of medical-grade face masks, many people are using homemade masks to avoid
spreading or contracting SARS-CoV-2, the virus that causes COVID-19.
There has been relatively little official
guidance about which mask designs provide the strongest barriers to infection,
leading amateur mask-makers to improvise.
Now, a new study has used laser visualization
experiments to demonstrate the effectiveness of homemade masks of various
designs.
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For the best
protection against the new coronavirus, a mask should be well-fitting and
contain multiple layers of quilted fabric.
These masks are roughly as good at preventing
the spread of infection as commercially produced cone-shaped masks, researchers
from Florida Atlantic University (FAU), in Boca Raton, report in a Physics
of Fluids study paper.
Lead author Siddhartha Verma, an assistant
professor at FAU, notes, “While there are a few prior studies on the
effectiveness of medical-grade equipment, we don’t have a lot of information
about the cloth-based coverings that are most accessible to us at present.”
“Our
hope is that the visualizations presented in the paper help convey the
rationale behind the recommendations for social distancing and using face
masks.”
–
Prof. Siddhartha Verma
Fluid mechanics tell the story
The researchers used a laser sheet setup,
which is commonly employed to study liquid mechanics, to observe the behavior
of airborne respiratory droplets that could, outside the lab, contain
SARS-CoV-2.
As these droplets were “coughed” and
“sneezed” from the head of a mannequin, the researchers mapped their
trajectory.
“The main challenge is to represent a cough
and sneeze faithfully,” says Prof. Verma. “[For] the setup, we have used a
simplified cough, which in reality is complex and dynamic.”
Prof. Verma compares
visualizing the airborne droplets to observing dust particles in a beam of
sunlight.
The researcher acknowledges that while
further quantitative analysis is warranted to confirm his team’s observations,
the visualization technique was nonetheless instructive.
Still, it is important to note that
SARS-CoV-2 may become aerosolized into fine airborne particles
and that the present study did not document the behavior of such particles.
At the outset, Prof. Verma says, it is
“important to understand that face coverings are not 100% effective in blocking
respiratory pathogens.”
“This is why it is
imperative that we use a combination of social distancing, face coverings, hand
washing, and other recommendations from healthcare officials until an effective
vaccine is released.”
Nonetheless, the researchers established
that:
- Without face masks, droplets
were projected up to 12 feet (ft) from their source, well beyond the
often-employed 6-ft social distancing margin. The average distance was 8
ft.
- The droplets hung in the air
for up to 3 minutes before falling.
- Commercially produced,
off-the-shelf cone masks reduced the average droplet projection to just 8
inches (in), though the researchers saw significant leakage of droplets
from the tops and sides of these masks.
- Simple homemade face masks
somewhat reduced the forward projection of droplets. However, they also
exhibited significant side and top leakage.
- When the team tested a bandana
made from elastic T-shirt fabric, the average forward travel was 3 ft, 7
in. When they used a folded cotton handkerchief mask, it was 1 ft, 3 in.
- The
most effective homemade masks were constructed of stitched layers of
cotton quilting. These reduced the forward travel of droplets to just 2.5
in, a shorter distance than the researchers observed with commercial face
masks.
- For anyone hoping that fabrics
with higher thread counts produce more effective coverage, the
visualizations suggested otherwise: The tested mask with the highest
thread count was the bandana, which was the worst at impeding the travel
of droplets.
Prof. Verma reports that his team is
interested in continuing their study and incorporating factors that affect the
real-world dissemination of respiratory droplets, such as evaporation, ambient
airflow, and properties of respiratory fluids that may impact their behavior when
airborne.
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