Which homemade masks are most effective?

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.

What visualization revealed

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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