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Nonmedical masks reduce aerosols spread by coughing

15 July 2020

Visualizations of leakage from various nonmedical face coverings show their differing efficacies.

Face masks have emerged as a critical tool in mitigating the spread of the novel coronavirus. Months into the pandemic, though, medical-grade personal protective items—in particular, coveted N95 masks—are in short supply and largely reserved for health-care workers. That’s left everyone else seeking alternative face coverings. But whereas N95 masks fit tightly around the wearer’s nose and mouth and must filter at least 95% of particulates larger than 0.3 μm, the performance of nonmedical masks varies widely and is often unknown.

To get a sense for how well some readily accessible face coverings prevent the spread of respiratory droplets, Siddhartha Verma, Manhar Dhanak, and John Frankenfield at Florida Atlantic University imaged droplet clouds passing through four types of masks: a bandana, a folded handkerchief, a homemade cotton mask, and a store-bought cone-style mask. Their experiments showed that even though the masks leaked a bit when subjected to an artificial cough, all four reduced how far droplets traveled.

Experimental set up
Credit: Adapted from S. Verma, M. Dhanak, J. Frankenfield, Phys. Fluids 32, 061708 (2020)

The researchers simulated a person’s cough by creating a burst of microscopic droplets using a smoke generator and a manual pump. The droplets rushed through a mannequin head, shown in the first figure, whose interior contained cavities that mimicked the volume of adult nasal passages. The water–glycerol droplets were 1–10 μm in diameter, similar to the smallest droplets expelled in a real cough, and were imaged using a laser sheet.

With no mask, the aerosol jet extended an average of 8 feet from the head. A handkerchief folded according to instructions from the US Surgeon General reduced that distance to 1 foot 3 inches. A store-bought cone-style mask performed better, with the jet only extending 8 inches. The best result came from a mask stitched out of two layers of quilting cotton (second figure), which permitted only a 2.5-inch jet. All three designs showed droplets escaping up from the unsealed top edge of the mask and forward through the barrier, as indicated in the second and third panels. Masks that minimized flow through them caused more significant leakage at the top edge.

Four panels showing mask and aerosol leakage
Credit: Adapted from S. Verma, M. Dhanak, J. Frankenfield, Phys. Fluids 32, 061708 (2020)

A single-layer bandana with a higher thread count than the quilting cotton—85 rather than 70 threads per inch—performed much worse than any of the masks, with droplets spreading 3 feet 7 inches away. The result suggests that rather than a higher thread count, mask construction, including layering, may be more important for reducing droplet spread.

Although detailed quantitative studies are needed to fully characterize the wide range of face coverings now being used as personal protective equipment, the visualizations presented by Verma and coworkers can provide a quick assessment of a mask’s efficacy and help convey the importance of mask wearing. (S. Verma, M. Dhanak, J. Frankenfield, Phys. Fluids 32, 061708, 2020.)

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